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Copyright © 1979 American Telephone and Telegraph Company T H E BELL SYSTEM TECHNICAL JOURNAL

Vol. 58, No. 1, January 1979 Printed in USA.

Advanced Mobile Phone Service:

Cell-Site Hardware

By N. EHRLICH. R. E. FISHER, and T. K. WINGARD

(Manuscript received August 1, 1978)

The hardware facilities of the AMPS cell site connect the mobile radio customer to the land telephone network and perform actions necessary for R F radiation, reception, and distribution; voice and data communications and processing; equipment testing, control, and reconfiguration; and call setup, supervision, and termination. Cell-site operational control is achieved partially through wired logic and partially through programmable controllers. This paper describes the cell-site functional groups, their physical characteristics and design, and the ways they interface with the rest of the AMPS system.

I. INTRODUCTION

In the A M P S system, the interface between the land te lephone network and the radio pa ths to the mobiles occurs a t the cell sites. In addit ion to performing functions needed for t runk terrnination and for radio t ransmission and reception, the cell site handles many semiau-tonomous functions under the general direction of the Mobile Telephone Switching Office (MTSO ) . Figure 1 is a block diagram of the major A M P S subsystems.

Cell sites have facilities to: ( i ) Provide R F radiation, reception, and distribution.

( ii ) Provide da ta communicat ions with the M T S O and mobiles. (Hi) Locate mobiles. (iv) Perform remotely ordered equ ipment testing. (v) Perform equ ipment control and reconfiguration functions.

(vi) Perform voice-processing functions. (vii) Perform call setup, call supervision, and call terminat ion

functions.

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(viii) Handoff or receive from ano the r cell site any mobile which has moved out of the normal service area of the cell site carrying the call.

Cell-site operat ions are controlled part ial ly by wired logic and partially by programmable controllers. Control functions a re r edundan t and can be reconfigured as needed to overcome a localized failure. A ba t te ry p lan t assures main tenance of service in case of commercial power outage. Facilities dependen t upon traffic requi rements in each cell coverage a rea a re modular so t h a t addit ional uni ts may be installed as needed to m a t c h busy-hour traffic levels. Th i s will ensure t h a t plant inves tment can grow sensibly as a function of ant icipated revenues.

154 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1979

Figure 2 is an isometric view of a typical cell site with a capacity of 48 voice channels . T h e precise n u m b e r of frames at each site is a function of the voice channel requi rements for t ha t site. T h e r e are four frame codes, and the smallest size cell site requires one of each code. E a c h radio frame h a s a max imum capacity of 16 radios. When the n u m b e r of voice radios grows beyond 16, ano ther radio frame mus t be added. Each line supervision frame (LSF) can handle 48 voice channels and, when this n u m b e r is exceeded, ano ther L S F is added. A single da ta frame (DF ) and a single main tenance test frame (MTF ) a re necessary regardless of the n u m b e r of voice radios in t h e cell site. T h e max imum size of a cell site is 144 voice radios, which would require a total of 14 frames: nine radio frames, th ree line supervision frames, one da ta frame, and one main tenance test frame.* T h e discussion in this paper of the functional design of the cell site parallels the organization of these frames.

Sect ion II describes the da ta frame, which serves as the mas ter control center for the cell site. Section III describes the line supervision frame, which interfaces the four-wire voice t runks (originating a t the M T S O ) with the cell-site voice-radio transceivers. Section IV describes the radio frame, which is composed of two bays. Section V describes the main tenance and test frame, which gives the cell site the abi l i ty—through the use of ano ther p rogrammable controller—to test for t roubles in the radio and audio equipment . Section VI describes the power system. Section VII describes the physical design of a cell site.

II. DATA FRAME

T h e da ta frame (see Figs. 3 and 4) contains the equ ipment for major cell-site control functions, which include communicat ion with the M T S O , control of voice and da ta communicat ion with mobiles, and communicat ion with the controller in t h e main tenance test frame. Communica t ion between controllers is necessary for request ing performance of specific tes ts and for receiving results. T h e D F contains bo th hardwired logic and programmable controllers. Only one set of hardwired logic and one controller is needed per cell site regardless of the n u m b e r of voice radios. Because of the critical functions performed in the D F , r edundancy of all subassemblies is provided to assure cont inuat ion of service in the presence of a failure. T h e D F can reconfigure itself under the direction of the M T S O , which mainta ins service by permit t ing any malfunctioning subassembly to be replaced with an off-line r edundan t unit.

T h e da ta frame (see Fig. 4) contains five major subsystems:

* In addition to these transmission and control frames, four additional W E 111A power system frames and an associated battery system are required for the maximum size cell site.

CELL-SITE HARDWARE 155

156 THE BELL SYSTEM TECHNICAL JOURNAL. JANUARY 1979

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( i ) Land da ta Links, described in Section 2.1. ( i i ) Controllers described in Section 2.2.

(Hi) S e t u p radio communicat ion, described in Section 2.3. (iv) Locating radios, described in Section 2.4. (v) Voice-channel da ta communicat ions, described in Section 2.5.

2.1 Land data links

D a t a communicat ion between the M T S O and each cell site takes place over two redundan t da ta links connecting Western Electric 201D da t a sets a t each terminat ion. T h e 201D da ta set operates a t a 2.4-k b / s ra te and supplies T T L level ou tpu ts so t ha t no buffering is required between it and the D F logic. T h e da ta set controller converts the 32-bi t serial messages into 16-bit parallel words for transfer to the controllers. T h e 201D also can configure itself for loop-around testing unde r remote control of the M T S O . Th i s feature is essential for maintenance because t h e cell si tes will normally be unmanned . T h e da ta sets and t h e da t a set interfaces also opera te in t h e reverse direction to t ake and t ransmi t da ta from the controllers to t h e M T S O .

2.2 Controllers

T h e controller of the da ta frame (see Fig. 5) consists of a P R O C O N , a writable s tore uni t (wsu) , a par i ty generator and checker, and a da ta bus to connect the P R O C O N to the numerous peripherals with which it mus t communicate . All uni ts are provided redundant ly to assure cont inuat ion of service in the presence of failure.

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developed a t Bell Laboratories, designed to have sequencing and control functions with very high reliability. I t is self-checking; an A S W (all seems well) signal indicates the presence or absence of a failure. T h e r edundan t P R O C O N recognizes this indication and repor ts failure to the M T S O . T h e M T S O will then use the properly functioning P R O C O N

for further cell-site control and will pr int ou t in a maintenance center a reques t to dispatch a craftsperson to t h e cell site to correct t he problem.

P R O C O N processes 16-bit parallel da ta words bu t uses 24-bit words for program instructions. I t contains da ta manipulat ion uni ts ( D M U ) ,

a control uni t (cu-16), and program storage uni ts ( P S U ) . T h e D M U contains instruction decoders, internal registers, logical /ar i thmetic capability, and peripheral communicat ion logic. T h e control uni t contains program-addressing logic, clock distribution, and fault-detection circuits. Each P S U board contains 2048 (2K) 24-bit words of read-only memory (ROM ) . T h e P R O C O N contains 4000 words of R O M and accesses an addit ional 4000 24-bit words of random-access memory (RAM ) in its associated w s u . Th i s increases its effective program store capacity to 8000 words. T h e w s u also provides 2000 18-bit words of da ta memory ( D M ) for P R O C O N access. Sixteen bits of each D M word contain da ta , and t h e remaining two bits are used for pari ty.

T h e P R O C O N ou tpu t is linked to a 16-bit da ta bus, which connects it to all peripherals, both within the D P and in o ther cell-site frames.


Separa te control signals from the P R O C O N indicate the address of the peripheral to be connected to t h e da ta bus to receive a part icular message. In a similar manner , per ipherals are connected to the da ta bus to allow the controller to read information from a peripheral 's ou tpu t register when the peripheral is act ing as a sensor. Par i ty is added a t the source for all da ta messages placed on the bus and is checked by the uni t receiving the message before it is used.

2.3 Setup radio communication

Se tup radios, as described in Ref. 1, t ransmi t only data, and are used in t h e initial phase of "set t ing u p " t h e call prior to the es tabl ishment of a voice p a t h for communicat ion. T h e y are for the general (shared) use of the cell site in communicat ing with all mobiles within its zone. In addit ion, the se tup radios also t ransmi t overhead messages to assure t h a t idle mobiles within the cell coverage zone are ready and able to communica te should a call be init iated to or from t h e mobile.

In the forward direction (land to mobile), referred to as the forward se tup channel , messages may be ei ther one or two words in length. Each word consists of da ta bi ts t ransmi t ted serially a t a ra te of 10 k b / s and encoded before transmission to provide 28 message bits and 12 B C H error detect ion/correct ion bits for a to ta l of 40 bits per word. In t h e reverse se tup direction, the mobile t ransmi ts—at the same da ta rate—48-bit words, with 36 of these bits available for message informat ion and 12 bits used for the error detect ion/correct ion code. T h e words in the reverse direction vary in number . T h e n u m b e r of words needed is t r ansmi t ted as par t of the message information. In each direction, each word is repeated five t imes to allow a majority voting of t h e detected word to protect the integrity of the transmission against t h e effects of noise, mul t ipa th fading, and interference. T o minimize t h e effects of noise t h a t comes in bursts , t he five repeats of each message in t h e forward direction are interleaved with similar messages addressed to ano ther mobile. Th i s group of two words, each t ransmitted five t imes, is preceded by 10 bits of dot t ing (al ternate ones and zeros) for bit synchronization and 11 bits of Barker Code* for word synchronizat ion (see Fig. 6). T h e bit-and-word synchronization permits the mobiles to frame t h e forward se tup messages and determines when each word and each sequence of t h e five-word repea ts begin and end. Each mobile will look a t only one of the two interleaved sets of words in t h e message s t ream, depending on whe ther the last digit of t h e mobile 's te lephone number is odd or even.

An addit ional bit, called the busy-idle bit, is inserted immediately following t h e bit sync, the word sync, and every 10 bits of each message word. If t he bit is a 1, t he reverse se tup channel of the part icular cell

* Barker Code consists of a bit sequence that is highly unlikely to be reproduced by rhythmic or random noise. It is 11100010010.


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B 5 • FIFTH OF FIVE REPEATS DF WORD 8 (40-BIT MESSAGE WORD)

Fig. 6—Forward set-up channel message stream. A given logic unit reads only one of the two interleaved messages.

site t ransmit t ing the message s t ream is idle, and any mobile desiring to init iate a call or to respond to a page m a y t ransmit . If t he bit is a 0, t h e reverse se tup channel is being used by another mobile t ransmit t ing a call origination or a page response. A mobile wishing to t ransmi t on t h a t channel mus t wait a shor t t ime interval and moni tor the channel again unti l idle bits are observed.

T h e r e is no essential difference between a voice radio and a se tup radio, per se. In fact, the identical radio equipment codes may be used in ei ther position. T h e differences in practice between the se tup radio and the voice radio are the frequency channel to which each is assigned and the interface circuits t h a t control the operat ion of t h e radio.

In the case of the se tup radio t ransmit ter , four circuit packs,* three designated as se tup t ransmi t te r interface and one as setup t ransmi t te r controller, t ake the information from the controller and prepare it in a form appropr ia te to sending the da ta message over the se tup channel . T h e th ree se tup transmit ter- interface packs behave as one functional unit . T h e y latch and hold the da ta received from t h e controller, de te rmine the appropr ia te t ime to load this word into a shift register, check the word for pari ty, inhibit t he transmission of a word if par i ty does not check properly, shift the da ta out of the register one bit a t a t ime to convert the word from parallel to serial form, and convert the da ta into Manches te r coding.f T h e se tup t ransmi t te r controller determines which of the se tup radios will be used on line and which will be retained as the r edundan t spare. T h e se tup t ransmi t te r controller h a s the capability of controlling up to five se tup radio t ransmit ters . In the Chicago developmental system, however, t he anticipated traffic levels

• T h e physical design of these circuit packages is discussed in Section 7.2. t See Ref. 2 for a discussion of Manchester coding.


during t h e equ ipment and service tes ts will no t require more t han one set-up t ransmi t te r and one spare.

2.4 Locating radios

T o main ta in signal s t rength sufficient for good-quality voice and da ta transmission, each mobile mus t communicate with an appropriately located cell site within the M S A . W h e n a call is initially set up, locating the appropr ia te cell site is done by the mobile as it scans all s e tup channels and selects the one with t h e highest signal level for use in t ransmi t t ing t h e reverse se tup messages. After the call h a s been established, t h e mobile m a y move out of the a rea of sufficient signal s t rength . I t t hen becomes necessary to rou te the call th rough ano ther cell s i te whose location provides be t te r signal quali ty to t ha t mobile. Reference 1 describes how t h e system implements a handoff.

After t h e handoff event has been completed, the call cont inues until ano the r handoff is required or until ei ther par ty t e rmina tes the call.

T o de termine when and if a handoff is necessary, locating measuremen t s are m a d e once every few seconds on each active voice channel . T w o techniques for locating are provided in the A M P S systems. T h e pr imary me thod is signal-strength measurement . T h e a l te rnate me thod is called phase-ranging and is described in Section III .

Signal measurements for locating are performed by equ ipment consisting of a locating radio receiver ( L R B ) , a tunable synthesizer, and a locating receiver interface (LRI ) . T h e r e a re four L R R S with their associated synthesizers and interface circuit packs per cell site. T h r e e sets are required to handle the busy-hour traffic load. T h e fourth is a spare to assure main tenance of service by reconfiguration should any of the on-line equipment become defective.

T h e cell-site controller (Section 2.2) keeps t rack of all calls which the cell is serving and makes a locating measurement on each call every few seconds. T h e controller sends, via the da ta bus to the L R I , a message containing a 10-bit binary n u m b e r represent ing the channel code. T h e L R I t hen directs t h e associated synthesizer to tune its local oscillator to t h e frequency of the selected channel . T h e L R R develops an ou tpu t voltage which is a function of the carrier signal quality. After a period of t ime to allow for settling, this voltage is held fixed by a t rack-and-hold circuit, while an analog-to-digital converter in the LRI converts the voltage represent ing an input signal range between —110 and —30 d B m into an 8-bit binary n u m b e r and places it in the ou tpu t register. Concurrent ly, a "Ready Outpu t flag" is set to signal the controller t h a t the measuremen t is available for readout . Because the controller has s tored the channel n u m b e r for which the measurement reques t was made , it is unnecessary to include any channel identificat ion in t h e ou tpu t word. Only the signal s t rength value is re turned to the controller.


T h e M T S O considers voice channel signal quali ty information from the controllers in the serving cell and in adjacent cells. A handoff process is init iated to transfer the mobile as it moves between cells so t h a t it will again be served by t h e cell site receiving the best signal quali ty. T h e process of executing t h e handoff is described in Ref. 1.

2.5 Voice channel data communications

After a call has been set up , it mus t be moni tored to de termine when it is necessary to send various orders to the mobile, such as an order to tu rn off the mobile 's t ransmi t te r a t t he te rminat ion of the call, or an order following a user reques t for one of the optional vertical services. T h e me thod of monitor ing the call (referred to as call supervision) is described in Section III for all features except locating, which has been discussed above. Orders and requests for vertical services mus t be t ransmi t ted so as no t to interfere significantly with voice conversations. T h e y are sen t in the form of binary da ta messages over the voice channel by momentar i ly mut ing the voice and inserting a binary da ta sequence, t hen restoring the audio capability. T h e da ta sequence requires approximately a t en th of a second. Th i s technique, called blank-and-burst , is discussed in more detail in Refs. 1 and 2. Below is a brief summary of the me thod of implement ing this technique.

T h e da ta messages over the voice channel in t h e direction from the cell site to the mobile are referred to as forward blank-and-burst . Those from the mobile to the cell site are called reverse blank-and-burst . T h e forward blank-and-burs t order is init iated by the M T S O ,

which sends an appropr ia te message over the da ta link to the controller in t h e cell site. T h e controller then sends the required message to t h e voice t ransmi t te r da ta interface (VTDl), a single function spread over th ree circuit packs; it also directs the L S F controller to set up t h e required connection in the L S F be tween the V T D I and t h e voice radio channel assigned to the addressed mobile. T h e V T D I accepts the order from the controller in th ree successive parallel 16-bit words and converts t h e m into a single 40-bit serial word t h a t is sent a t a 10-kb/s ra te to the voice radio t ransmi t te r via an electronically switched connection in the Line Supervision F rame (LSF ) . T h e message format is shown in Fig. 7. T h e V T D I also precedes the da t a word with the bit sync and t h e word sync and repeats this grouping of bit sync, word sync, and t h e 40-bit da ta word 11 t imes before t h e L S F restores the channel to the voice mode. T h e use of 11 repeats ensures t ha t there will be a sufficient number of properly received words to permi t accura te word decoding by the mobile 's logic uni t in the noisy or interference-limited environment of A M P S .

If the mobile cus tomer has subscribed to vertical service features, his reques t for a specific vertical service (such as th i rd-par ty add-on to a call) mus t be t ransmi t ted as a da ta word via the blank-and-burst


technique. T h e implementa t ion of blank-and-burst in the reverse direction (mobile to cell site) is somewhat different from tha t of the forward direction.

T h e customer init iates his reques t for vertical service by entering a specific number sequence (including the te lephone number of a th i rd par ty , if applicable) via t h e TOUCH-TONE® calling pad into a register within the mobile logic unit . T h e n the customer depresses t h e S E N D but ton* which is analogous to operat ing the switch-hook to obtain an operator ' s a t tent ion. T h e S E N D bu t ton causes the signalling tone (ST ) f

to be t ransmi t ted over t h e voice channel for abou t 0.5 second. T h e L S F , recognizing t h a t a signaling tone has been detected, opera tes a relay on the t runk switching unit to pu t the t runk into t h e on-hook s ta te . W h e n t h e M T S O , which moni tors the on-hook, off-hook condition of each t runk, detects an on-hook condition of 0.5-second durat ion, it sends a message to t h e cell site telling the request ing voice channel to t ransmi t da ta .

T h e voice receiver da ta (VRD ) group in the data frame consists of a voice receiver da ta radio, a tunable frequency synthesizer, two interfacing circuit packs, and a da ta m o d e m consisting of four circuit packs (clock initialization, clock acquisition system, Barker sequence detector and bit decoder, and majority voter) . One V R D group is used for the ent ire cell site because traffic levels on it are expected to be low and t h e messages handled are not time-critical. It is backed up by a r edundan t spare . T h e working V R D must be tuned, therefore, to the frequency of t h e channel expecting a reverse blank-and-burs t message. Upon receipt of t h e M T S O message indicating the channel number of a mobile t ha t had "flashed," the P R O C O N orders the synthesizer associa ted with the da ta receiver to tune to the designated channel . When the P R O C O N de tects the "in lock" flag (which indicates tuning is complete) , it orders a forward blank-and-burs t message to be sent to t h a t mobile directing it to t ransmi t a reverse blank-and-burs t message. T h e mobile t hen t ransmi ts over the voice channel the da ta message corresponding to the request which the cus tomer had init iated via t h e TOUCH-TONE calling pad and S E N D bu t ton .

T h e reverse blank-and-burs t message format is d iagrammed in Fig. 7 and consists of 100 bits of "dot t ing" bit sync (alternate Is and 0s), 11 bits of word sync (Barker code), and 48 bits of message data , of which 36 are information bits and 12 are error-detect ing/correct ing bits. Th i s grouping of bit sync, word sync, and message is repeated four more t imes, for a to ta l of five consecutive transmissions, except tha t in the last four the bit sync is limited to 37 bits of dot t ing ra ther t han 100. T h e Barke r sequence detect ion and bit decoder, t he clock initialization,

* Other features of the S E N D button are discussed in Ref. 3. f T h e signaling tone is an out-of-voice-band 10-kHz tone detectable within the LSF.

T h e function of the signaling tone and the operation of the LSF in detecting various states of the call are discussed in Section III.


and the clock acquisition system circuit packs detect the dot t ing and develop from it a clock signal synchronized with the clock in the mobile to facilitate detect ion of the Barker code and the da ta message.

T h e five t ransmissions of the message are each delayed within the majority voter shift registers long enough to cause t h e m to enter the bit summing network (voter) in bit synchronism as shown in Fig. 8. As a result, a voted ou tpu t occurs, one bit a t a t ime, according to the detected value of each bit t ha t occurred on a t least t h ree of t h e five

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t ransmissions. Th i s word, made u p of majori ty voted bits, is then converted in an interface circuit pack from a 48-bit serial word to th ree successive 16-bit parallel words and sent over the da ta bus to the P R O C O N . T h e P R O C O N tes ts t h e B C H error de tec t ion/correc t ion coding, reformats t h e message, and sends the information over t h e da ta Link to t h e M T S O . T h e M T S O performs the necessary act ions to comply with the cus tomer ' s reques t for a vertical service. T h e cus tomer ' s reques t is received by the M T S O within a second after the "flash" message is received a t t h e cell site.

III. LINE SUPERVISION FRAME

T h e line supervision frame (LSF ) , shown in Fig. 9, provides the perchannel audio-level speech-path interface between t h e M T S O - c o n -trolled te lephone t runk network and the radio frame t h a t t ransmi ts the radio frequency voice communicat ion to and from the mobile unit . In addi t ion to this principal function, the L S F also performs the following system functions:

( i ) Enables t ransmission of forward blank-and-burs t da ta messages by connecting t h e V T D I circuits to the appropr ia te voice t ransmit ter .

(ii) Provides line supervision and control th rough monitoring of the supervisory audio tone (SAT ) and the signaling tone.

(i'i() T u r n s t ransmi t te r s and receivers on and off as requested by the M T S O via the D F according to the level of mobile te lephone traffic.

(iv) Provides range measuremen t s on each mobile by measur ing the phase delay of the received S A T .

(v) Enables voice t runk main tenance tes ts to be performed by switching t runks into loop-back configurations.

T h e audio circuits in the L S F are supplied in modules. Thus , a single L S F can suppor t from 1 to 48 separa te voice channels . As m a n y as th ree L S F S can be connected to a single D F , allowing the max imum capacity of a cell site to be 144 voice channels .

T h e L S F h a s two functional par ts : t he voice channel circuits and the frame controller. T h e voice channel circuits are modular; t h e quant i ty supplied varies according to the n u m b e r of voice t runks terminat ing in the frame. Th i s n u m b e r depends on the traffic requi rements for the cells, bu t it cannot exceed 48 in a single L S F . T h e controller is installed complete , wi th redundancy, independent of the number of t runks te rminat ing in the frame.

Each voice channel circuit consists of a group of eight pr in ted circuit boards and six jacks used for test ing and monitor ing the t runk /vo ice channel circuits and the voice channel circuits/voice radio interfaces.


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3.1 Vole» Channel circuits

T h e voice channel circuit performs all t he baseband signal processing for a single voice radio. Before the transceiver baseband signals can interface with the te lephone network, certain control signals mus t be added on the t ransmi t te r pa th , and other control signals mus t be removed from the receiver pa th . T o obtain these control signals, each line circuit has access to several busses carrying both analog and digital information. Each line circuit is permanent ly wired to the 6-kHz supervisory audio tone (SAT) bus and to the 10-kHz clock bus. Access to t h e t runk-main tenance bus, serial-data bus, and phase-range bus is controlled by signals from the L S F controller. T h e s ta te of each line circuit is indicated by a group of s t a tus signals t h a t may be read by the L S F controller. T h e five s t a tus signals are: ( i ) t ransmi t te r power o n / off, (ii) main tenance relay s ta te (normal or loop-around), (Hi) line control logic fade timing, (iv) line control logic t imed out, and (v) off-hook. Each line circuit contains a logic circuit t ha t controls dc line supervision on the MTSO -cel l -s i te pa th and shuts off the cell-site t ransmi t te r if mobile-to-cell-site t ransmission is in terrupted for more t han 5.5 seconds.

T h e audio processing section serves to interface the four-wire, voice-grade, te lephone t runks with the cell-site transceivers. A syllabic compandor reduces audio noise in the transmission system. T h e compandor is composed of two sections. A compressor a t the t ransmit t ing end reduces variat ions in speech input power levels by a factor of 2 (in decibels). An expandor a t t he receiving end performs the inverse operation. T h e loss of the expandor mus t complement t h e gain of the compressor so t h a t the end-to-end relative signal levels are unaffected. T h e overall effect of these circuits provides an improved signal-to-noise rat io for the received speech. Both t h e mobile and the cell-site audio circuits mus t contain similar speech compressors and expandors. (See Ref. 2 for a more complete discussion.)

Figure 10 is a block diagram of t h e audio processing circuits. T h e P C 7 t ransmit -audio compressor circuit pack contains the compressor half of the compandor circuit. T h e audio from t h e voice t runk feeds into the compressor; t h e compressed audio ou tpu t feeds into the audio filter. T h e p e l t ransmit -audio filter contains a sharp 300- through 3000-Hz bandpass filter, which band-l imits the audio from the compressor. One of t h e th ree possible cell-site S A T frequencies* is selected a t t h e S A T cross-connect panel, added a t the ou tpu t of the low-pass filter, and combined with the audio signal in an operat ional amplifier summing circuit. T h e ou tpu t is the composite audio and S A T signal, which is passed th rough the da ta /voice switch in the P C 2 bit encoder

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and data-voice switch to t h e t ransmi t te r in the radio frame when t h e switch is in normal or voice position.

W h e n the cell site mus t send shor t burs ts of high-speed da ta (during the t ime t h e mobile is tuned to t h e voice channel) , it uses the blank -and-burs t mode. While da t a are being sent, a framing pulse switches the data-voice switch to t h e da ta mode. T h e framing pulse inhibits t he audio, selects for use one of t h e two redundan t da ta busses, disconnects the voice t r ansmi t t e r from the audio system, and connects it to the signaling sys tem bit encoder. Serial da ta from the selected da ta bus are gated into t h e Manches te r encoder. T h e da ta and the 10-kHz clock are exclusively NOR-gated to give a Manches ter -encoded format. T h e da ta then pass th rough a Beesel shaping-filter, which removes t h e high-frequency components . T h e serial da ta message, Manches ter -encoded, is t hen passed to the t ransmi t t e r in t h e radio frame and t ransmi t t ed to the mobile. See Ref. 5 for more detai ls on da ta t ransmission.

Communica t ions in the o ther direction—from the mobile—are received by t h e associated voice receiver in the radio frame. T h e s e signals contain audio plus t h e S A T and on occasion the 10-kHz signaling tone (ST) . While da t a in the form of blank and burs t messages are also sent from t h e mobile over the voice channel , those messages a re not processed th rough the voice radios or the Line supervision frame. Instead, they are received by the voice da ta radio receiver in the da ta frame and processed through its associated modem.

T h e ou tpu t of t h e voice receiver 's discriminator is sen t to the PC4 receive audio processor, basically a combinat ion bandpass filter and frequency modula t ion de-emphasis filter. T h e overall transfer characteristic is a 6-dB/octave slope in the voiceband and a sha rp 24 -dB/ octave fall-off in the region outside the voiceband. T h e ou tpu t is fed to t h e audio expandor circuit. An ou tpu t ahead of the filter, containing t h e S A T and the 10-kHz S T , is connected to the S A T and S T detectors , respectively. T h e audio expandor circuit is moun ted on t h e P C 5 line control circuit card. T h e input to t h e expandor is from the P C 4 receive audio processor circuit pack and the ou tpu t is connected to the operat ing company voice t runk.

In addit ion to the expandor circuit, t h e P C 5 line control circuit conta ins logic to detect t h e voice channel s ta tus , to control t h e on/off s t a tus of the voice t ransmi t t e r and receiver, and to t ransmi t s t a tus indications to the da ta frame controller. Voice channel s t a tus is developed from the t ransmi t t e r power ar ray in the L S F controller (see Sect ion 3.3) and from the ou tpu t s of the S A T and S T de tectors for the following s ta tus reports : t iming, t imed out, off-hook, and t ransmi t t e r power on. T h e transmitter is tu rned off to prevent radiat ion of power on any channel no t in use. Similarly, t he receiver is disconnected from


t h e t runk to prevent receiver noise (which is maximum in the absence of a detected carrier) from entering the land line t runk when the channel is unoccupied. T h e mut ing circuits to disconnect the receiver from the t runk are located in PC4 .

T h e S A T , which is added to the t ransmi t te r baseband signal a t the ou tpu t of the audio bandpass filter, is t ransponded a t the mobile and detec ted in P C 3 of the cell-site receiver signaling system. It monitors the continuity of the cell-site-to-mobile pa th and furnishes ranging information. T h e S A T de tector ou tput is a t logic 1 as long as the correct S A T frequency is detected and the carrier-to-noise rat io is greater t han 7 d B . If S A T is not detected, the line control goes into a t iming condition. If recovery is not made in 5.5 seconds, the call is considered lost and the circuit will t ime out and shut off the cell-site t ransmit ter .

A phase-locked loop detector performs an es t imate of the distance between t h e cell site and the mobile by comparing the phase of the t ransmi t ted and received S A T signals. T h e mobile-to-cell-site distance is a linear function of this phase difference. T h e difference in phase is converted into a dc analog signal, which is connected via the phase-range switch and the phase-range bus to an analog-to-digital converter in the L S F controller.

T h e mobile may autonomously t ransmit a 10-kHz signaling tone as pa r t of its disconnect sequence or as an acknowledgment of the receipt of certain orders. T h e P C 6 contains a detector circuit, which is an active 10-kHz (ST) bandpass filter followed by a full-wave rectifier, low-pass filter, and level comparator . T h e S T ou tpu t is a logic 1 when the tone is present . I t is fed to the P C 5 Une control circuit. T h e Une control circuit monitors bo th the S A T and S T logic ou tpu t s generated by the tone detectors in the signaling system and uses t h e m to control the D C supervision current (off-hook signal) in the M T S O to ceU-site t runk and the t ransmi t te r on-off s ta tus . When the mobile par ty disconnects, the mobile sends the 1.1-s, 10-kHz S T . T h e Une control circuit, via control of the t runk switching unit , removes the off-hook signal from the land t runk. T h e M T S O de tects the t runk on-hook transi t ion and sends a blank and burs t release order to t h e mobile to shu t off its t ransmit ter .

For maintenance aids, the voice t runks from t h e M T S O connect to a set of tes t jacks for each t runk. T h e r e are six jacks per t runk:

( i ) T ransmi t t e r network out: Disconnects the t runk and connects to t runk output .

( i i ) T ransmi t t e r compressor in: Disconnects the t runk and connects to the audio compressor input.

( iii ) T ransmi t t e r monitor: Monitors the t ransmi t t runk. (iv) Receiver network in: Disconnects the t runk and connects to

the t runk input.


(ν) Receiver out: Disconnects the t runk and connects to audio processor output .

(vi) Receiver monitor: Moni tors the receive t runk.

3.2 Trunk switching unit

T h e t runk switching uni t (TSU ) consists of the t runk maintenance switch and t h e loop signaling switch for one t runk. It contains two relays mounted on a pr inted circuit board. In the normal s ta te , each t runk connects through its T S U to its associated audio processor. T h e main tenance relay signal from the L S F controller opera tes the maintenance relay to disconnect the t runk from the audio processors and connect it to the tes t t runk. A main tenance relay s ta tus signal is re turned to t h e L S F controller to indicate t h a t the relay has operated. T h e off-hook signal from the line control circuit opera tes the second relay to provide a closure for t h e loop signal current to operat ing company equipment .

3.3 Line supervision frame controller

T h e L S F controller receives da ta words from the D F cell-site controllers and examines each word to de termine the voice circuit to be accessed and the function to be performed. T h e L S F controller consists entirely of wired logic and contains r edundan t circuitry, designated side A and side B . Each side may accept da ta words from ei ther cell-site controller, t he choice being determined by the load signal used. Bo th sides can access any of the voice circuits. A block diagram of one side of the L S F controller is shown in Fig. 11. T o avoid complexity, this figure omits all interact ions with t h e r edundan t side.

T h e t ransmi t t e r power control circuit consists of two ar ray access circuit packs, one for each side of the L S F controller, and two t ransmi t t e r power ar ray circuit packs, which are common to both sides of the controller. T h e ar ray access circuit pack contains the control to set and reset t h e selected flip-flop in a 48-element a r ray contained in t h e two t ransmi t te r power ar ray circuit packs. T h e inputs consist of t h e radio address, t h e frame address, and the t ransmit ter -on and t rans-mitter-off signals. T h e ou tpu t controls the on/off s ta te of each voice t ransmi t t e r and is sent to the t ransmi t te r via its associated PC5 line control circuit.

T h e main tenance selector circuit is similar to the power control circuit. I t consists of two ar ray access circuit packs, one on each side of t h e L S F controller, and two maintenance ar ray circuit packs which are common to both sides of the controller. T h e maintenance ar ray consists of 48 flip-flops t h a t are set or reset by signals from the ar ray access circuits. T h e ou tpu ts go to the 48-trunk switching uni ts to opera te the main tenance relays.

T h e da ta /vo ice selector consists of a demultiplexer circuit pack on each side of t h e L S F controller. I ts purpose is to select the voice channel


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Fig. 11—Line supervision frame controller block diagram.

το VOICE CHANNEL CKT'S

and control t he flow of serial da ta for the forward blank-and-burs t function. T h e da ta /vo ice selector contains an eight-bit register to store t h e radio address , t he frame address, and the V T D I select bit . T h e V T D I

select bit chooses t h e voice t ransmi t te r da t a interface it will use as the serial da t a and framing pulse source. T h e selected serial da ta a re placed on a bus t h a t drives all t he bit encoder and da ta voice switches in t h e 48 voice circuits. T h e address ou tpu t of the register is used to drive the da t a voice demultiplexer. I t is a l-out-of-48 decoder, which delivers t h e framing pulse to the selected bit encoder and da ta voice switch t h a t receive t h e serial da ta .

T h e phase-range selector circuit consists of a phase-range demult i plexer circuit pack in each side of the L S F controller. T h e phase-range demult iplexer is a l-out-of-48 decoder, which receives t h e radio and frame address from the input register and is enabled by t h e set phase-range switch signal from t h e operat ion code decoder. T h e analog-to-digital conver ters change the phase-range analog-voltage ou tpu t of t h e phase-ranging circuit to an eight-bit binary code, which is t r ansmi t ted to the cell-site controller.


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Fig. 13—Typical channel transmitter module unit with covers removed.

IV. RADIO FRAME 4.1 Overview

Figure 12 is a photograph of a 16-channel radio frame.*" As s ta ted earlier, each radio frame is composed of two bays. T h e transceiver (TR) bay contains 16 pai rs of voice channel t r ansmi t te r s and receivers. A companion power amplif ier /combiner ( P A / C ) bay amplifies and combines the ou tpu t s of the voice t ransmit ters .

T h e radio frame interfaces with the radio t ransmission envi ronment th rough th r ee an tennas : one for t ransmit , t he o thers for two-branch space-diversity receive. W h e n the cell site equ ipment is configured for omnidirect ional coverage, these an tennas are omnidirectional (in the az imuthal plane) with 10-dB gain. Alternatively, when the cell site functions in the directional mode, one radio frame services each face (direction) via th ree 120-degree directional an t ennas each with 10-dB gain.f

T h e radio frame interfaces with the L S F via 16 four-wire, balanced bidirectional t runks , one servicing each voice channel . "Transmi t te r -on" control signals originate within the L S F . Finally, dc power is supplied from the +24 V ba t te ry system as described in Section VI.

Each duplex voice channel (see Fig. 12) is served by a " rad io" consisting of a set of four modules located within t h e radio frame.

* When more than 16 voice radio channels are required at a cell site, additional radio frames are added.

t Additional antenna gain, easily obtained in the directional mode, is not required.


(i) A channel - t ransmi t te r module (see Fig. 13) produces a 1-watt carrier, which is phase-modula ted by vo ice /SAT or frequency-modula ted wi th 10 k b / s da t a provided by a t ransmi t channel circuit within the L S F . A 666-channel frequency synthesizer, located within t h e t r ansmi t t e r module , generates the correct channel frequency, which is also t h e local oscillator for the companion receiver.

(ii) A power-amplifier module (see Fig. 14) boosts the 1-watt angle modula ted carrier, from t h e t ransmi t t e r module , to 45 wat ts .

(Hi) T h e channel mult iplexer combines t h e 16 45-watt carriers, from the power-amplifier modules, onto one coaxial t ransmission line, which goes to a t r ansmi t an tenna .

(iv) A channel-receiver module receives a two-branch diversity input derived from the two receiving an tennas feeding an ar ray of b roadband amplifiers and hybrid power spli t ters. F rom these inputs and from a local oscillator signal, derived from t h e companion t r ansmi t t e r module , t h e receiver demodula tes a baseband v o i c e / S A T or da t a signal, which is delivered to a receive-channel circuit within the L S F .

A radio frame need not be fully loaded with modules; any n u m b e r of sets, from 1 to 16, are used depending upon the required channel

Fig. 14—Typical power amplifier module.


capacity. T h e channel multiplexer, as presently designed, mus t provide for all 16 channels; the unused inputs are te rmina ted by 50-ohm loads. A brief design overview of each radio module follows.

4.2 Channel transmitters

Figure 15 is a block d iagram of a 16-channel radio frame. T h e blocks marked T R A N 0 to T R A N 1 5 a re 1-watt output , P M v o i c e / S A T or F M da ta t ransmi t te r modules . T h e channel frequency for each t ransmit ter , s i tuated in the 870- to 890-MHz band, is generated within i ts self-contained frequency synthesizer. A digital program plug inserted into the front panel of each t ransmi t te r module selects the desired channel . T h u s , each voice t r ansmi t t e r resides pe rmanen t ly on one selected radio channel .

Figure 16 is a block diagram of t h e frequency synthesizer, which uses the indirect frequency synthesis me thod to generate any one of 666 stable carriers upon digital command from 10 parallel b inary program lines. Each carrier, a t one-quar ter t he final ou tpu t frequency, is s table to within ± 1 pa r t per million over a 0°C to +40° C t empera tu re range. A relatively unstable, varactor- tuned, voltage-controlled oscil-

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la tor (vco) generates the synthesizer ou tpu t frequency f0. A port ion of t h e v c o ou tpu t power enters a mixer, where it is he terodyned against fi = 228.02250 MHz, which is derived from a quar tz crystal-controlled oscillator located within the M T F (see Section V). T h e ou tpu t difference frequency f\ — fo (between 5.5 and 10.5 MHz) is "divided down" by a selected integer N, in a programmable digital frequency divider. T h e specific combinat ion of dc voltages on the 10 parallel b inary program lines determines the division factor N, which can range between 737 and 1402. A stable 7.500-kHz reference oscillator ( f2) is compared with t h e divider ou tpu t frequency [ ( / i — fa)/N], nominally near 7.5 kHz, in t h e phase detector . Any phase error is fed back to the v c o in the form of a dc control voltage, keeping t h e tota l loop in phase-lock. W h e n in lock, t h e ou tpu t frequency is given by fo = fx — Nf2. Therefore, fo will have the same long-term frequency stabili ty as the two stable reference oscillators f, and f2, yet can be varied in integer s teps of 7.5 kHz, by assigning different values to N. Since f0 is in the 217.5- to 222.5-MHz band, which is one-quar ter the ou tpu t frequency, the 7.5-kHz frequency s teps are multiplied to 30-kHz steps, the final channel spacing, in a subsequent X4 frequency multiplier.

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T h e synthesizer ou tpu t is qui te pure . W h e n the ou tpu t frequency is quadrupled, the result ing audio noise in a 0.3- to 3.0-kHz band (after F M detect ion, deemphasis , and C-message weighting) is 40-dB below a reference 1-kHz tone with ±8-kHz peak frequency deviation.

Figure 17 shows the t ransmi t te r circuits following the frequency synthesizer. Power enter ing a t a specified frequency in the 217.5- to 222.5-MHz band (from the frequency synthesizer) is first split, one port ion going to a low-power-transistor frequency quadrupler which generates the 870- to 890-MHz local oscillator (LO) for the companion receiver. Since the L O equals the t ransmi t frequency, the duplex-receive frequency will be 45 M H z lower (or higher) if t he first intermedia te frequency (IF) of the receiver is 45 MHz. For example, if a

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specific t r ansmi t t e r is p rogrammed to t ransmi t on channel 134, which is centered a t 870.030 MHz, then its companion receiver will receive the duplex channel , located 45 M H z lower a t 825.030 MHz.

T h e o the r port ion of the ou tpu t power (from the frequency synthesizer) enters the phase modulator , which is a two-stage, varactor-diode, reflection-type circuit. Balanced audio (or data) originating within t h e L S F modula tes t h e dc bias on the varac tor diodes. T h e modula tor provides a peak phase deviation of ±12 radians (after subsequent x 4 multiplication) with less t h a n 5-percent audio distortion.

T h e resu l tan t phase-modula ted carrier enters a four-stage t ransis tor amplifier, where it is boosted to abou t 3 wat ts . Th i s power drives a varactor-diode frequency quadrupler . After passing through a ferrite isolator, t he quadrupler ou tpu t appears as a 1-watt phase-modula ted carr ier in t h e 870- to 890-MHz t ransmi t band. Th i s ou tpu t power is delivered to a companion 45-watt power amplifier located in t h e adjacent power amplif ier /combiner frame.

4.3 Power amplifier

In Fig. 15, the blocks marked A M P 0 to A M P I S are Class C power amplifier modules, which boost t h e 1-watt input from a companion t r ansmi t t e r to approximately 45 wa t t s ou tput . T h e power amplifiers, which consume mos t of the dc power in a cell site, are designed to be powered directly from the " raw" 24-V ba t te ry supply whose voltage can vary between +21 and +28 V, depending upon the ba t te ry ' s s ta te of charge. Thus , a significant cost savings is achieved by avoiding a requ i rement for voltage regulation of these major loads. All o ther equ ipment within the radio frame is powered from regulated (dc-to-dc converter) voltage sources.

4.4 Channel multiplexer

T h e 45-watt ou tpu t signal from each power amplifier module is delivered into a channel mul t ip lexer , 5 , 6 which is an array of 16 cavity resonators each functioning as a nar rowband filter feeding a common load, the t ransmi t an tenna . T h e multiplexer combines these 16 signals wi th a maximum of 3 d B loss per channel . T h e min imum channel- to-channel isolation is 18 d B . Figure 18 is a photograph of the cavity multiplexer. No te t h a t the cavities a re arranged in a radial a r ray abou t a 16-branch stripline feeder assembly contained within the center section. Power en ters each cavity from a coaxial connector (and coupling loop) a t t ached to the back of the cavity. T h e combined power exits the mult iplexer by a coaxial connector connected to a "load po in t " a t t h e back of t h e assembly. T h e coupling to each cavity is de te rmined by an acceptable compromise between transmission loss and off-channel isolation. T h e length of each stripline to each cavity feedpoint, from t h e common load point, is approximately % wavelength. T o mee t the 3-dB loss per channel , t he channels are spaced


Fig. 18—Sixteen channel multiplexer.

630 kHz, or 21 channel frequencies, apar t . In termodula t ion is controlled by ferrite isolators, providing 30-dB reverse loss, contained within the ou tpu t section of each 45-watt power amplifier. With th ree channels excited, the measured intermodulat ion products are a t least 55 d B down from t h e desired signals.

4.5 Directional coupler poet-transmit filter

T h e combined 16-channel group leaving the multiplexer (see Figs. 2 and 15) en ters a t ransmi t filter panel a t tached to the wall of the cell-site building. Here the channel group first passes through a dual directional coupler where samples (30 dB down) of the forward and reflected wave are taken. Th i s sampled power feeds via two coaxial cables to the main tenance and test frame (Section V), where appropr ia te t ransmi t te r tes ts are made and analyzed.

Finally, t he channel group passes through a low-loss, 870- to 890-MHz, bandpass filter, where out-of-band harmonics and spurious signals are removed. Th i s interdigital filter is an eight-resonator s t ructure t ha t exhibits an inband loss of about 0.5 dB . T h e channel group reaches t h e an tenna via a run of 1-% inch o.d. coaxial cable having a loss of abou t 0.66 dB/100 ft.

T h e t ransmi t te r system is designed to provide a power of a t least 10 wat t s per channel a t the t ransmit an tenna .

4.6 Receiver filter/preamplifier/divider

T h e receive signals from each of the an tennas first enter the receive filter-divider panel (see Figs. 2 and 15). T h e a r rangement of radio hardware and signal distribution on both t ransmi t and receive ends


was conceived with a basic modular i ty of 16 in mind. T h e t ransmi t te r channel multiplexer, though providing low loss, is relatively expensive. T h u s , in the receive chain, the l-to-16 demultiplexer was chosen to be a 16-way broadband hybrid-power spli t ter which is low in cost bu t unfor tunately inserts a 10 log 16 = 12-dB loss into each receive pa th . T o recover this loss, a low-noise preamplifier (see Fig. 15) is s tat ioned ahead of t h e power divider. Composed of two commercially available 25-dB gain low-noise amplifiers "parallel-coupled" via two 3-dB quadr a tu re hybrids , ' th is preamplifier provides redundancy and also reduces by 9 d B the generat ion of in termodulat ion spurious signals. T h e noise figure of this amplifier-hybrid combinat ion is 2.5 dB , and the th i rd-order in termodulat ion products a t t he ou tpu t are greater t han 65 d B down from two R F signals which are —35 d B m at the input.

T h i s U H F preamplifier is preceded by an interdigital bandpass filter giving a t least 55-dB rejection to signals arriving from the 870- to 890-M H z t ransmi t band. T h e total sys tem noise figure, measured a t the antenna , should not exceed 10-dB.

4.7 Receiver

Following the receive-filter preamplifier and 16:1 divider (see Fig. 15) a re 16 two-branch diversity receivers labeled R C V R 0 to R C V R ) 5 .

Figure 19 shows a detailed block diagram of the receiver module. T h e R F receive band is 825 to 845 MHz. T h e t ransmit and receive frequencies a re separa ted by 45 MHz, and the frequency synthesized for each t ransmi t channel is used as the first conversion local oscillator frequency in the receiver. T h e voice receiver noise figure is abou t 11-dB. A two-resonator 825- to 845-MHz bandpass filter in the feed to each voice-receiver module prevents leakage of L O out of each module into o ther modules and helps suppress the "half - iF" response in Mixer A. T h e half- iF response results from the second harmonic of an incoming signal beat ing against the second harmonic of the mixer 's local oscillator signal. For such a response to fall a t t he I F frequency, the incoming signal mus t be displaced, in frequency, one-half t h e I F frequency away from the local oscillator frequency.

In t h e voice-receiver module, t h e channel to be detected is first mixed down to 45 M H z in Mixer A, which is a Schottky-diode, single-balanced mixer. T h e conversion loss is abou t 6 dB. A P I N diode a t tenuator , ahead of Mixer A, is driven by an au tomat ic gain control (AGC ) bus and provides u p to 40-dB of a t tenuat ion . Th i s reduces the dynamic range of the signals entering the diversity combiner. A one-stage, 12-dB gain, 45-MHz I F amplifier with a two-resonator, 30-kHz bandwidth , quartz-crystal filter a t bo th its input and ou tpu t performs prel iminary channel filtering.

T h e 45-MHz first I F is next down-converted to a 1.8-MHz second I F by Mixer B, a balanced F E T type, which is driven by a 43.2-MHz


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Fig. 1 9 — A M P S cell-site voice receiver block diagram.

crystal-controlled second local oscillator. A 1.8-MHz I F amplifier with a two-resonator , 30-kHz bandwidth , L-C ( inductor-capacitor) filter a t bo th inpu t and ou tpu t performs final channe l filtering. T h e combined gain of t h e second mixer and 1.8-MHz I F amplifier is abou t 43 dB , which is adjustable . T h e overall frequency response of the voice receiver is essentially eight-pole* (eight-resonator) , with four poles result ing from t h e 45-MHz quartz-crysta l filters and four poles from t h e L-C double- tuned circuits in t h e 1.8-MHz section of t h e I F amplifier.

T h e second I F frequency was m a d e as low as t he first I F image rejection would permi t to simplify t he design of t he diversity combiner .

T h e two-branch , equal-gain diversity combine r uses a technique

* Right poles appear in the low-pass prototype of this eight-resonator filter.


originally proposed by Gran lund . 8 T h e complete theory of operat ion of this sys tem has been described by H a l p e m 9 and J a k e s 1 0 ; a simplified explanat ion is presented in the appendix to this paper.

T h e practical l imitat ions in the combiner design have resulted in its having a dynamic range of 50 d B . T h e gain and A G C in the m i x e r / i F were de te rmined to suit these l imitations.

T h e 400-kHz ou tpu t signal from the diversity combiner en ters a conventional l imiter and a "quadra tu re coil" discriminator, bo th contained in one integrated-circuit package. T h e result ing baseband au-dio /sAT or da t a are then delivered over a balanced line to a receiver voice channel circuit in the L S F .

V. MAINTENANCE AND TEST FRAME

T h e M T F (see Fig. 20) contains t h e oscillators and frequency dividers to genera te the mas te r clock signals and t h e S A T for o ther equipment in t h e cell site. It also permi ts test ing of the cell-site radios, the associated R F t ransmission circuits, and the voice t runks connecting t h e cell s i te t o the M T S O .

T h e frame is digitally controlled by t h e main tenance test frame control ler ( M T F C ) , which is opera ted as a per ipheral uni t to the cell-site controller located on the D F . T h e M T F C ' S main function is to interface t h e cell-site controller with the various circuits and test ins t ruments on t h e M T F . T h e r e is also a manua l capabili ty of loading commands into the M T F C locally, independent ly of the cell-site controller.

T h e M T F m a k e s it possible to moni tor the functioning of t h e cell site unde r the overall direction of the M T S O . When a local failure occurs, t h e M T F furnishes t h e information necessary to "main tenance busy" a faulty voice channel , or to reconfigure active and r edundan t circuits for mainta in ing service while a craftsperson goes to t h e cell site to replace the faulty unit .

5.1 Oscillator section

T h e mas t e r oscillator set genera tes a high-frequency reference (228.02250 MHz) and a low-frequency reference (7.500 kHz) for all t he frequency synthesizers in the cell site (see Section 4.2). T h e 228-MHz oscillator is crystal-controlled and enclosed in a temperature-controlled oven. I t has a frequency stabili ty of ± 1 pa r t per million per year . T h e frequency is dis t r ibuted via coaxial cable to all radios in t h e radio frames and in the D F , and to t h e test synthesizer within the M T F . T h u s , individual precise frequency sources for each of the radios are not required.

T h e 7.5-kHz clock signal is derived from a separa te oven-controlled, 10-MHz oscillator, whose frequency is first divided by 4000 to 2.5 kHz


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and t hen multiplied by 3. T h e signal is then dis tr ibuted to the radios in a way similar to t h e 228-MHz clock signal distr ibution. T h e divider chain is t apped a t 1 M H z for the P R O C O N clock and a t 10 kHz for the serial da ta clock. A r e d u n d a n t mas te r oscillator set will also be switched into operat ion automat ical ly in the event of a lost ou tpu t signal or a gross frequency change.

T h r e e S A T frequencies—available a t 5.97, 6.00, and 6.03 kHz—are each derived from a separa te oscillator and dis tr ibuted to the audio-processing circuits in the L S F . T h e 22-Hz (nominal) clock is generated by dividing the 10-kHz da t a clock by 456 to obta in 21.93 Hz and then sen t to t h e L S F for use in fade t ime-out measurements . All clocks are r edundan t and can be tes ted by the counter within the M T F .

5.2 Test equipment

T h e M T F radio tes t equ ipment consists of a tes t receiver tunable to any t ransmi t t e r channel and a tes t generator tunab le to any receiver channel . A tes t frequency synthesizer for channel tuning and a tes t audio processor, in conjunction with the tes t radios, furnish controlled s imulat ion of a mobile transceiver. T h e r e a re also a digital vol tmeter and a counter , bo th remotely controllable, and a s tandard 1-milliwatt, 1000-Hz reference oscillator. T h e s e uni ts can isolate a t rouble condition in t h e cell site, via remote control from the M T S O , to a single radio t ransmi t t ing or receiving channel (or group of channels) . T h e channel can t h e n be shu t down and a craftsperson sent to replace or repair any faulty un i t of the channel .

T h e test receiver measures the appropr ia te signals to compute t h e following pa ramete r s of each t r ansmi t t e r channel for comparison against specified main tenance limits:

(i) Incident power to the an tenna , (a) Reflected power from the an tenna .

(Hi) T ransmi t t e r frequency. (iv) T ransmi t t e r deviation. (v) Modula t ion quali ty ( S I N A D * ) .

T h e tes t generator injects known signals to allow measu remen t of t h e following pa ramete r s for each dual-diversity receiving channel:

( i ) Sensit ivity (noise quieting wi th a low-level R F input) . (ii) Audio ou tpu t quali ty a t an R F input above threshold.

5.3 Maintenance and test frame controller

M u c h of t h e equipment in the M T F is used to facilitate r emote test ing of t h e cell-site radios, the mas te r oscillator equipment , and the interconnect ing t runks . T h e M T F C serves as the digital control interface

signal + noise + distortion • S I N A D = : :

noise + distortion


between the cell-site controller and the M T F oscillators and test equipment . I t also serves as a da ta interface to the various ins t ruments on the M T F . Since mos t of these ins t ruments require several seconds to complete the i r measurements , the M T F does the waiting and raises a flag when a sequence of measurements is completed. Th i s saves tying up t h e cell-site controller in a long-wait loop.

T h e M T F C consists of a P R O C O N and a wri table s tore unit, similar to those in the D F , and a group of logic and m o d e m cards. T h e P R O C O N

controls t h e operat ion of the test equ ipment in the M T F and formats and t ransmi t s the responses to each requested test measurement . I t opera tes unde r the direct control of the P R O C O N in the D F which, in turn , is commanded by the M T S O . T h e logic cards provide the necessary interface buffering, while the modem, test receiver, test generator, and S A T t r ansponder s imulate t h e action of a mobile to permi t measuremen t s of ( t ) the da ta messages t ransmi t ted or received by any radio and (ii) t he performance of the S A T detect ion and phase-range measurement circuits.

A l amp and display panel provides a manua l capabili ty to load commands into the M T F C and to observe its bit-and-flag s ta tus . Th i s panel can manual ly reset the M T F C for manua l error recovery and sys tem testing.

5.4 Typical test operation All tes ts a re controlled by the M T S O , which also has to opera te on

some of t h e da ta to arrive a t t he desired measurement . T h e following sequences show the test procedures but do not necessarily correspond to specific M T S O test algori thms.

5.4.1 Transmission power and frequency tests

T o measure t ransmi t ted power, t he power from t h e forward-power coupling por ts of the directional couplers is summed and routed via a switch to the mixer input of the test receiver. Power from t h e reflected-power coupling por ts of the directional coupler is summed and connected th rough a second position of the switch to the mixer input . After determining t h a t the channel fequency to be tested is not in use on any of the an tennas , the M T S O uses the test synthesizer to tune the test receiver to the desired frequency and energizes the appropr ia te t ransmit ter . T ransmi t t e r forward or reflected power, depending on t h e position of the switch, is read by means of a cal ibrated vol tmeter and t ransmit ted to the M T S O . T h e application of appropr ia te scale factors permi ts calculating the power into the an tenna , and re turn loss. T h e s e numbers are then compared against s tored limits to de te rmine whe the r performance is satisfactory or faulty.

With the system configured as for the power measurement , t he frequency t ransmi t ted on t h e channel under test may also be deter-


mined. A frequency counter is connected via a switch to measure a subharmonic of the test-receiver local oscillator (LO / 4 ) and t h e I F frequency from t h e test receiver. T h e counter display may be read locally or t r ansmi t ted to t h e M T S O . T h e t ransmi t t e r frequency may be calculated as 4 x (LO / 4 ) + I F .

Main tenance of t h e clock sys tems also requires measuremen t s of the mas te r oscillator distr ibution bus frequency (228 MHz) , the low-frequency group of clocks, and the S A T frequencies. Any such frequency m a y be individually measured on command from the M T S O .

5.4.2 Other radio measurements

T o measure phase deviation, t h e S A T t h a t is continuously modulat ing the t ransmi t t e r channel is measured. T h e modula ted discriminator ou tpu t of t h e test receiver is measured locally using the M T F vol tmeter . T h e vol tmeter measu remen t s a re re tu rned to the M T S O and compared to fixed, prede termined tolerance numbers .

T o test whe the r the receiver sensitivity is within limits, the M T S O ,

after ascertaining t ha t the channel frequency to be tested is not in use, uses the synthesizer to tune the test generator to the desired receiver frequency. T h e ou tpu t of t h e test generator passes th rough a variable a t t enua to r and a switch to the directional coupler of e i ther diversity input of the receiver unde r test . T h e M T F C switches the a t t enua to r to i ts higher a t t enua t ion position. Noise-quieting of the receiver under tes t is verified a t t he M T S O . T h e ou tpu t of the tes t generator is then switched to the o ther diversity input and the noise-quieting verified again. T h e two measuremen t s a re compared against a s tored limit as a go/no-go test for each diversity section of the receiver.

T o measure the audio ou tpu t quali ty of the receiver, the M T S O

applies a s tandard test tone to modula te the test generator over the voice t runk. T h e a t t enua to r is switched to i ts lower a t t enua t ion s ta te . T h e audio ou tpu t from the receiver under tes t is verified for presence a t the M T S O .

5.4.3 Data radio Interface measurements

T h e tes t receiver and generator can receive and t ransmi t serial highspeed data . Th i s capabil i ty allows simple tes ts to be performed on the se tup radio interfaces and voice radio da ta interfaces. T o check the forward se tup channel interface, a special 200-bit serial da ta message is t r ansmi t t ed via the se tup t ransmi t te r and its interfaces to the test receiver. T h e test receiver sends the received da ta to the M T F C , where it is s tored in t empora ry memory. T h e controller then does a bit-by-bit comparison of t h e message with an identical message stored in its p rogram memory and generates an "all-seems-well" message if t he two messages check. T h e reverse se tup channel is tested in the same m a n n e r except t h a t the roles a re changed—i.e., t he tes t generator


t ransmits a message to the se tup receiver and its interfaces where it is received, reformatted, and sent via the controller and the da ta link to the M T S O for checking. T h e voice radio da ta interfaces are checked in a similar fashion.

VI. POWER SYSTEM

In the Chicago developmental trial, t he pr imary power system for each cell site is the Western Electric type 111A. T h e input to this system is commercial three-phase, four-wire, 208-volt, 60-Hz power. I ts ou tpu t is a nominal +24 volts with a capacity of u p to 800 amperes . A J87123 bat tery p lant floats across the rectifier ou tpu t s and provides an emergency power source in case of loss of commercial power.

T h e electronic equipment operates mainly from dc voltages a t t he levels of + 5 , ±15, and +24 volts. Commercial 60-Hz ac power is used for cooling fans in the radio and da ta frames and for the commercial vol tmeter and counter in the M T F . An inverter can develop the necessary 110-volt, 60-Hz power from the ba t te ry plant during commercial power failure so t h a t system operat ion and test can continue.

T h e +24-volt ba t te ry supply is dis t r ibuted to all cell-site frames. T h e +24-volt loads are powered directly from the nominal +24-volt* ba t t e ry busses. T h e + 5 - and ±15-volt loads derive their power from dc-to-dc converters . T h e total 24-volt load amoun t s to 300 amperes for a sys tem with only one radio frame, 430 amperes with two radio frames, and 560 amperes with three radio frames. In all cases, another 100-ampere capacity has been included in the power plant for ba t te ry charging. Where the radio frames are not fully loaded with radios, or when all radio t ransmi t te rs are not operating, these loads will, of course, be less.

VII. PHYSICAL DESIGN

T h e A M P S cell site is functionally and physically divided into frames of radio control and transmission equipment , a power system, an tenna interface equipment connecting the radios to the outside antennas , an tennas , cables, and support ing mast and s t ructure . T h e cell site equipment mus t be capable of being located in a variety of places. T h e Chicago developmental systemf includes (i) small self-contained buildings with a dedicated an tenna mast, (ii) small self-contained buildings adjacent to an existing microwave tower to make maximum use of tower facilities, and (lit) a portion of the top floor of a large downtown central office building.

Rented building space of many types may be necessary for future growth. In addition, designs must consider the visual appearance of

* Which can vary between 21 and 28 volts, depending on the battery's state of charge. f T h e developmental system layout is described in Ref. 11.


buildings; t h e an tenna mas t assembly, because of its height, will be especially visible and may draw the a t tent ion of local zoning boards.

Since the A M P S is a new service using largely new equipment , the re was little to guide design decisions. Thus , there is a need to learn how the equipment will function and how people will use the service. Product ion will be low in volume for the early years, relative to o ther te lephone equipment . For these reasons, the physical design concept chosen for the Chicago trial equipment sought to fill several objectives. T h e design had to be flexible to accommodate many ant icipated early changes and to make maximum use of existing general-purpose hardware to avoid the expense, t ime, and tooling necessary to generate a customized equipment technology. T h e equipment was par t i t ioned into smaller uni ts t h a n will be ul t imately op t imum so t ha t the system would be more flexible and responsive to changes. Th i s section contains a general physical description of the equipment designed to accommoda te these considerations.

T h e cell-site equipment m a y be housed ei ther in a dedicated building or in an appropriately located existing building. T h e approximate floor area required for the trial equipment is 22 by 23 ft with a vacant wall or ceiling required for the p lacement of an tenna interface equipment such as the filter divider panels. T h e cell sites should take advantage of existing facilities where possible to mee t operat ional and economic objectives.

T h e outside equipment consists initially of omnidirectional, vertically polarized t ransmit and receive an tennas mounted on a frees tanding mast , or o ther tower. These an tennas mus t be located and installed with part icular a t tent ion to height and diversity spacing requirements . As the system grows into a directional configuration, the an tenna ar ray will also require directional t ransmi t and receive an tennas . T h e an tennas are connected to their respective filter panels within the building via coaxial cable feedlines t ha t pass into t h e building through a cable ha t ch plate . An effective grounding system is required to minimize voltage potent ials generated by lightning. T h e building and mas t m u s t be surrounded by an external ring ground, and the interior of the building mus t contain ano ther ring ground with all equipment frames and meta l cabinets connected to it.

A typical interior equipment layout is shown in Fig. 2. T h e an tenna interface equipment is supported by a wall and located as near as possible to t h e ha t ch plate. T h e radio control and transmission equipm e n t is housed in s tandard Bell Sys tem Electronic Switching Sys tem ( E S S ) frames, 7 ft high, 2 ft, 2 in. or 3 ft, 3 in. wide, and 18 in. deep. T h e end guards selected are 24 in. deep to protec t the equipment wiring. T h e E S S cable t rays on top of the equipment bays a re used for frame interconnect ion pa ths , and most of t h e interbay cables are equipped with connectors. T h e frames are sufficiently modular in design so t h a t


different channel capacity requi rements of the various cell sites can be easily accommodated .

T h e remaining equipment , which may be considered as suppor t equipment , consists of the power plant, ba t te ry s tand and batteries, inverter, fuse panel , and air pressurization equipment .

7.1 Technologies 7.1.1 Components

Circuits for t h e A M P S cell-site equipment use bo th conventional discrete components (transistors, diodes, capacitors) and silicon integrated circuits (ics). Analog and digital ics with 5-volt and 15-volt power a re used. Wes te rn Electric, commercial, and K S specification dual-in-line packages are employed, with most i c s having 16 pins. For ongoing designs beyond t h e trial hardware , addit ional emphasis is expected to be placed on using the highest-reliability devices available a t acceptable costs. M a n y of the radio devices for use in the 900-MHz radio band are technologically new, and the technology is rapidly changing due in pa r t to t h e considerable interest in this band generated by several new radio services, including A M P S .

7.1.2 Circuit boards In addit ion to t h e conventional pr inted wiring board design, boards

wi th wire-wrap interconnected socket pins on Vio-in. centers are used in m a n y of the plug-in logic circuit packages to give max imum flexibility for changes introduced during t h e trial. These will be replaced by double-sided or multi layer epoxy-glass pr inted boards on subsequent product ion of additional systems. Where circuits are replicated m a n y t imes or high confidence existed t h a t no changes would be made , conventional double-sided printed-wiring boards were chosen to save space and reduce cost.

7.1.3 Backplane wiring

T h e next level of interconnection is between circuit packs to merge t h e m into panels or groups of panels. T h e logic boards are connected th rough t h e W E 947 backplane connector with an array of wire-wrap pins of V6-in. centers . Mos t of the power wiring is pr inted on the backplane. Where possible, a ground plane is also printed on the backplane to minimize noise. Most of the panel-level signal wiring for these panels is 30-gauge and is wrapped with an automat ic wiring machine . Connect ions t h a t require twisted pair or twisted-shielded pair for noise or impedance-matching considerations are manual ly installed after machine wrapping is completed. Figure 21 is a photograph of some of t h e backplane wiring.

T h e boards in t h e analog circuit sections of the frames generally use a lower-density connector, and the wiring between connectors is man-


Fig. 21—Backplane wiring showing both local wiring and interpanel cabling.

ually wrapped, R F signals within a frame are rou ted using 0.141-in. semirigid coaxial cable te rmina ted in S M A or type-N connectors. T h e semirigid cable minimizes spurious radiat ion and pickup and provides low signal loss in constrained space.

7.1.4 Frame wiring

T h e next level of wiring is between panels, or between the top of the frame and panels . For logic and audio signals, this wiring is generally twisted pair or twisted-shielded pair, depending on the sensitivity of the signal and the length of the run. Power wiring from the power modules uses large-gauge wire or laminated bus bars where t h e amount of cur ren t is large and space is limited. Semirigid coaxial cables are used for radio frequency signals.

7.7.5 Interframe cabling

Most wiring between equipment frames is via connectorized cable be tween interconnection panels a t t he top of the frames. S tandard twisted pair cable is normally used, with twisted-shielded pair being used where extra shielding is required. T h e R F signals between frames, and between filter panels and frames, are routed on RG-214 coaxial cables fitted with type-N connectors. T h e filter panels are connected to the an tennas with l-5/e-in. semirigid coaxial cable between t h e t ransmi t te r filter panel and the t ransmi t an tennas and %-in. semirigid coaxial cable be tween the receiver filter panel and the receive an tennas .


7.2 Equipment and apparatus mechanical design

7.2.1 Circuit package mechanical design

T h e individual circuit packages for t h e cell-site equ ipment a re all appara tus-coded and are of th ree general types . T h e mos t used are F-coded (a t empora ry manufactur ing code for use during a trial period) packages t h a t use e i ther pr in ted or wire-wrap boards for component interconnect ion and are fitted to the 946A circuit pack connector (see Fig. 22). T h e s e cards are moun ted into 80A appara tus housings and connected to 947C backplane connectors. T h e F-coded circuit packages a re used primari ly for logic and control circuitry in the da t a frame and in t h e controllers of the line supervision and main tenance and test frames. T h e y also contain some analog circuitry associated with t h e controllers. T h e main audio and signaling da ta circuits t ha t a re required on an individual channel basis are implemented on seven PC codes. T h e s e codes are also mounted in 80A appa ra tus housings bu t use gold fingers on 0.060-inch-thick, pr inted double-sided, epoxy-glass boards fitted directly into a backplane connector to reduce cost relative to the F-code boards (see Fig. 23). T h e th i rd type of board used primari ly for analog circuits is an A-code board also on epoxy glass bu t

Fig. 22—F code circuit package.


Fig. 23—PC code circuit package.

0.090-inch thick, wi th gold fingers t h a t fit into a 36B appara tus housing containing 905B type connectors (see Fig. 24).

7.2.2 Mechanical design of the radio modules

Radio modules in t h e da ta frame perform setup, locating, and signaling functions. Other radio modules in the radio frame perform voice channel functions, a capabil i ty which can be increased on a module-per-voice-channel basis. Tes t radio modules are also housed in the main tenance and tes t frame, along wi th t h e reference frequency equipment . T h e t ransmi t te rs , receivers, and synthesizers were designed by Bell Laborator ies and manufactured unde r K S specifications. T o maximize early design flexibility and to minimize early tooling costs, a flexible packaging technique using th ree special extrusions was developed. T h e radio subassemblies could be developed individually and la ter packaged together with a m i n i m u m of circuit interact ion. Figure 13 shows t h e vo ice /da ta t r ansmi t t e r wi th i ts covers removed to i l lustrate t h e packaging technique. Th i s same general design approach was used for the vo ice /da ta t ransmi t te rs and receivers, t h e frequency synthesizers, t h e test radio modules, and the reference frequency leveler amplifiers.

T h e o ther major radio module is t h e power amplifier unit . I t is used on an individual radio channel basis and was manufac tured to a K S specification (Fig. 14). T h e hea t sink for t h e power amplifier was specified to be compat ible wi th a forced-air cooling sys tem t h a t is pa r t of t h e radio frames. Power dissipation requi rements and long-life considerat ions indicated forced-air cooling as the bes t way to get a cost-effective design for t h e 900-MHz output -power device.


Fig. 24—A code circuit package.

7.2.3 Channel multiplexer T h e channel multiplexer (see Fig. 18) is a 16-to-l R F multiplexer,

which uses individually tuned high Q cavities to combine efficiently the R F power from 16 t ransmi t te rs into a single t ransmi t an tenna . T h e individual cavities mus t remain physically s table over a wide range of ambien t t empera tu re and varying power dissipation. T o accomplish this, the cavities a re made of invar, a steel formulation with a very small coefficient of thermal expansion. T h e cavities are plated with a t least one-half mil of copper to provide good electrical conduction in the skin region a t R F . A final thin layer of gold plate on the copper mainta ins good conduction a t the mater ia l joints and ensures a good interface to the cavity interior. A fan cools the combiner, aids its t empera tu re stability, and stabilizes the characteris t ics of the combiner stripline.

7.2.4 Power units T h e pr imary power for the cell-site equipment is a +24-volt dc

reserve power system, described in more detail in Section VI. T h e 24-volt direct cur ren t is distr ibuted to each frame and e i ther is used directly, in the case of R F power amplifiers, or is converted to the correct, regulated power needed by each frame. Th i s conversion re-


quires the use of various quant i t ies and combinat ions of four plug-in codes of dc-to-dc converters . One unit , coded 121A, is of t h e nominal 50-watt type which occupies only one-half of an appa ra tus housing, and has an ou tpu t capacity of 2.3 amperes a t —14.7 volts. T h e o ther th ree units—coded 122D, E, and F—are of the nominal 150-watt type, occupy a full appara tus housing, and have ou tpu t s of 7 amperes a t +14.7 volts, 7 amperes a t —14.7 volts, and 17.5 amperes a t +5.3 volts, respectively. Al though each of these codes was specified to unique A M P S requirements , they are par t of a larger s tandard family of Bell Sys tem power converters . T h e converters of each code within a frame are connected to a common bus. Moreover, a t least one extra power uni t is provided for r edundancy on each bus. In general, t h e loss of any single power uni t will resul t in an a la rm bu t no loss of service to customers.

7.2.5 Antennas and mast

T r a n s m i t and receive an tennas for the trial system are high-gain, omnidirectional, and vertically polarized. T h e y are end-supported but are electrically center-fed to minimize an tenna-pa t t e rn squint-angle change over the frequency band. Two receive an tennas per cell site provide diversity and there is one t ransmi t an tenna per radio frame (16 radio channels) . T h e an tennas are approximately 13 feet long, including the mounting, and are placed in a 2-Vfe-in. d iameter fiberglass housing. When the system requires directional capability, t he omnidirectional an tennas will be augmented by directional t ransmi t and receive an tennas .

T h e omnidirect ional an t ennas are typically moun ted a t the corners of a t r iangular platform (about 10 feet on a side) a t t he top of a 150-foot free-standing steel mast , as shown in Fig. 25. Later , the directional an t ennas will be moun ted behind the contoured dielectric covers. T h e r e will be two receive an tennas (for diversity) and one or two t ransmi t an t ennas per face. Where an existing s t ructure such as a microwave tower or downtown central office building is used, special mount ing a r rangements must be engineered for each site. As the sys tem grows and cells are subdivided, the an tenna height may be reduced to about 100 feet.

Since the vertical pa t t e rn has a half-power beamwidth of abou t 7 degrees, t h e omnidirect ional an tenna has the disadvantage of being susceptible to relatively small angular deflections from vertical. T h e an tenna mas t and platform were designed to minimize deflection and cost. T h e two major sources of deflections are the wind and uneven solar heat ing of the steel mast . In general, the an tenna ha rdware was designed to mee t two wind criteria; (i) sys tem operat ion within specifications for normally encountered wind conditions, and (ii) survival unde r ext reme b u t rare conditions such as winds u p to 100 m p h . 1 2

T h e R F t ransmission line to t h e an tennas uses semirigid air-filled


κ Ι ~- RECEIVE

A N T E N N A S

Fig. 25—Mast-mounted antennas.

coaxial cables to keep the R F losses low. T h e cables are routed inside the mas t th rough a suspended conduit cluster. T h e conduit allows for cable system growth and ease of rep lacement if required. T h e interior mount ing provides protect ion to the cables and significantly improves the visual appearance . An air pressurization system keeps the cables pressurized and dry internally.

APPENDIX

Operating Theory of the Two Branch, Equal Gain, Diversity Combiner

In Figs. 19 and 26, consider the two 1.8-MHz input signals, A and B, which arrive from the two 1.8-mHz I F amplifiers.


^ A = a(t) C O S I i ^ t + i f r J t l + a l

1.8 ΜΗ;

Ο = a It) COSI(u) , + ujq) X * a\

I

C - C O S ( u 0 ' - * m ( I I I

TO DISC 400 kHz

2.2 MHz

BPF BW= 1.0 kHz LIMITER

0 MIX LIMITER MIX

Ε = a (I) C D S ( u o t - 0 m (t))

30 kHz 2 F » b (II COS + ω 0 ) t + 0 | G = b (t) C0S<u)„t - tJ„ , (II)

1.8 MHz

2.2 MHZ

BPF BW= 1.0 kHz

B = b (t) C O S ( u , l + t>m ( ι ) 4 β )

Fig. 26—Two-branch equal-gain diversity combiner.

A = a (t) cos (ωι t + <t>m (t) + a), B = b(t) cos (ωι t + φη, (t) + β),

where

a (t), b (t) are slowly varying u n c o r r e c t e d Rayleigh ampl i tude functions,

ωι = 2w (1.8 MHz) = carrier frequency,

φη, (t) " t he voice phase modulat ion, and

α, β = slowly varying, random, u n c o r r e c t e d , carrier phases of channels A and B, respectively.

I t is t he function of t h e combiner to co-phase A and Β (set a = β), so t h a t A and Β can t hen be coherent ly added together. Assume t h a t this regenerat ive loop is already in operat ion so t h a t there exists the cons tan t ampl i tude ou tpu t signal

C = cos (ω 0 t - φ„ ( r ) ) ,

where

ω 0 = 2w (400 kHz) = ou tpu t signal.

No te t h a t a and β a re absent . T h e upper left mixer (modulator) takes the product A and C whose

upper s ideband is D = a (t) cos [(ωι + ωο) t + a].


Note t ha t the voice modulat ion t e rm φ™ (t) has been removed, but the r andom modulat ion α remains.

T h e signal D, which is a 2.2-MHz carrier with slowly varying phase modulat ion, passes through the nar rowband filter and limiter, which removes t h e ampl i tude t e rm a (t). T h e subtlet ies associated with the choice of the filter bandwidth are thoroughly discussed by Halpern 9 . T h e result ing constant-ampli tude signal,

U — cos [ ( ω ι + too) t + o] ,

is product -modula ted with A in the upper right-hand "mixer." T h e lower s ideband is given by

Ε = a (t) cos [ωο t - φ„ U ) ] .

N o t e t ha t this second modulat ion process has removed the random phase t e rm a.

By similar reasoning, the signal

G = b (t) cos [ω 0 ί - <t>m (t)]

is generated by t h e lower regenerative loop. Ε and G, being phase-coherent , can be summed in a hybrid. Th i s sum passes through a third limiter where the ampl i tude function is removed, giving the constant ampl i tude ou tpu t signal

C = cos [ω 0 t - φ„ (<)]·

REFERENCES

1. Z. C. Fluhr and P. T. Porter, "AMPS: Control Architecture," B.S.T.J.. this issue, pp. 43-69.

2. G. A. Arredondo, J. C. Feggeler, and J. I. Smith, "AMPS: Voice and Data Transmission," B.S.T.J., this issue, pp. 97-122.

3. R. E. Fisher, "AMPS: A Subscriber Set for the Equipment Test," B.S.T.J., this issue, pp. 123-143.

4. A. K. Johnson and J. I. Smith, "Radio Frequency Design for a High-Capacity Mobile Telephone System," Nat. Telecommun. Conf. Record (Los Angeles, Cel., December 1977), / , pp. 16:4-1,2.

5. A. K. Johnson, "Determination of Filter Requirements for a High Capacity Mobile Telephone System Base Station," Svmp. Microwave MobUe Commun. Conf. Record (Boulder, Col., September 1974), paper V-4, p. 32.

6. A. K. Johnson, "Computer Simulation of High Capacity Mobile Telephone System 16 Channel Multiplexer," IEEE Veh. Tech. Conf. Record (Orlando, Fla.. March 1, 1977), p. 109.

7. K. Kurokawa, "Design Theory of Balanced Transistor Amplifier," B.S.T.J., 44, No. 8 (October 1965), pp. 1675-1698.

8. J. Granlund, "Topic in T h e Design of Antennas for Scatter," Lincoln Laboratories, Massachusetts Institute of Technology, Technical Report 135, November 1956.

9. S. W. Halpern, "The Theory of Operation of an Equal-Gain Predetection Combiner with Rayleigh Fading Channels," IEEE Trans. Commun., COM-22, No. 8 (August 1974), pp. 1099-1106.

10. W. C. Jakes, Jr., ed., Microwave Mobile Communications, New York: John Wiley, 1974, Ch. 5.

11. D L . Huff, "AMPS: T h e Developmental System," B.S.T.J., this issue, pp. 249-269. 12. D. L. Chandler, "Deflection Analysis of Supporting Structures for Antennas to be

Used in a Mobile Telephone Svstem", IEEE Trans. Veh. Tech. , VT-26. No. 2 (May 1977).
