2001/12/14Prof. Huei-Wen Ferng 1 Chapter 6 Advanced Mobile Phone System (AMPS)

2001/12/14 Prof. Huei-Wen Ferng 1

Chapter 6Chapter 6Advanced Mobile Phone Advanced Mobile Phone

System (AMPS)System (AMPS)

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PreliminaryPreliminaryTechnology TutorialsTechnology Tutorials

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Multiple AccessMultiple Access• Frequency Division Multiple Access

(FDMA)– AMPS and CT2

• Time Division Multiple Access (TDMA)• Hybrid FDMA/TDMA• Code Division Multiple Access

– a physical channel corresponds to a binary code

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CDMACDMA

• Each station has its own unique chip sequence (CS)

• All CS are pair-wise orthogonal• For example :(codes A, B, C and D are

pair-wise orthogonal)– A: 00011011 => (-1-1-1+1+1-1+1+1)– B: 00101110 => (-1-1+1-1+1+1+1-1)– C: 01011100 => (-1+1-1+1+1+1-1-1)– D: 01000010 => (-1+1-1-1-1-1+1-1)

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CDMACDMA• A·B = (1+1-1-1+1-1+1-1) = 0• B·C = (1-1-1-1+1+1-1+1) = 0• Example: if station C transmits 1 to station

E, but station B transmits 0 and station A transmits 1 simultaneously then the signal received by station E will become S = (-1+1-3+3-1-1-1+1). E can convert the signal S to S·C = (1+1+3+3+1-1+1-1)/8 = 1

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Mobile Radio SignalsMobile Radio Signals• Four main effects produced by

physical conditions:– Attenuation that increases with distance– Random variation due to environmental

features, i.e., shadow fading.– Signal fluctuations due to the motion of

a terminal, i.e., Rayleigh fading.– Distortion due to that the signal travels

along different paths, i.e., multi-path fading.

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Attenuation Due to DistanceAttenuation Due to Distance

• The signal strength decreases with distance according to the relationship:

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Slow/Shadow FadingSlow/Shadow Fading• Random Environmental Effects

– As a terminal moves, the signal strength gradually rises and falls with significant changes occurring over tens of meters.

– Let P (received power) be a log-normal distributed random variable with mean Preceive and S (signal strength in dBm), i.e., S=10log10(1000P) dBm.

– The log-normal of P implies that S is normal distributed.

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Fast/Rayleigh FadingFast/Rayleigh Fading• Fast (Rayleigh) Fading Due to Motion of

Terminals– As the terminal moves, each ray undergoes

a Doppler shift, causing the wavelength of the signal to either increase or decrease

– Doppler shifts in many rays arriving at the receiver cause the rays to arrive with different relative phase shifts

– At some locations, the rays reinforce each other. At other locations, the ray cancel each other

– These fluctuations occur much faster than the changes due to environmental effects

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Multi-path PropagationMulti-path Propagation• There are many ways for a signal to

travel from a transmitter to a receiver (see Fig 9.5)

• Multiple-path propagation is referred to as inter-symbol interference (see Fig. 9.6)

• Path delay = the maximum delay difference between all the paths

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Technology ImplicationsTechnology Implications• Systems employ power control to

overcome the effects of slow fading• Systems use a large array of techniques

to overcome the effects of fast fading and multi-path propagation– Channel coding– Interleaving– Equalization– PAKE receivers– Slow frequency hopping– Antenna diversity

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Spectrum EfficiencySpectrum Efficiency

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Spectrum Efficiency (Cont’d)Spectrum Efficiency (Cont’d)

• Compression Efficiency and Reuse Factor– Compression Efficiency = C conversations/per

MHz (one-cell system)– If N is the number of reuse factor, spectrum

efficiency E = C/N conversations per base station per MHz

– A measure of this tolerance is the signal-to-interference ratio S/I

– A high tolerance to interference promotes cellular efficiency

– S/I is an increasing function of the reuse factor N

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Spectrum Efficiency (Cont’d)Spectrum Efficiency (Cont’d)• Channel Reuse Planning

– A channel plan is a method of assigning channels to cells in a way that guarantees a minimum reuse distance between cells using the same channel.

– N ≥ 1/3(D/R)^2 where D is the distance between a BS and the nearest BS that use the same channel and R is radius of a cell.

– Practical value of N range from 3 to 21.

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Slow Frequency HoppingSlow Frequency Hopping• The signal moves from one

frequency to another in every frame• The purpose of FH is to reduce the

transmission impairments • Without FH, the entire signal is

subject to distortion whenever the assigned carrier is impaired

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RAKE ReceiverRAKE Receiver• Synchronization is a major task of a

SS receiver– Difficulty: multi-path propagation

• Solution: Multiple correlator (demodulator) in each receiver– Each correlator operates with a digital

carrier synchronized to one propagation path

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Channel CodingChannel Coding• Channel codes protect information signals

against the effects of interference and fading• Channel coding decrease the required signal-to-

interference ratio (S/I)req and the reuse factor N

• Channel coding will decrease the compression efficiency C

• The net effect is to increase the overall spectrum efficiency

• Channel codes can serve two purposes:– error detection and forward error correction (FEC)

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Block CodesBlock Codes• Block code (n, k, dmin)

– Used to Protect The Control Information– n is the total number of transmitted bits per

code word– k is the number of information bits carried

by each code word– dmin the minimum distance between all

pairs of code word• Ex: n = 3, k = 2, dmin = 2 (000, 011,

101, 110)– Code rate r=k/n.

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Block CodesBlock Codes• When dmin = 5, there are three

possible decoder actions– The decoder can correct no errors and

detect up to four errors– It can correct one error and detect two

or three errors– It can correct two errors, three or more

bit errors in a block produce a code word error

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Convolutional CodesConvolutional Codes• Each time a new input bit arrives at

the encoder, the encoder produces m new output bits– the encoder obtains m output bits by

performing m binary logic operations on the k bits in the shift register

– The code rate is r = 1/m

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V1 = R1

V2 = R1 R2 R3

V3 = R1 R3

Example:

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InterleavingInterleaving• Most error-correcting codes are

effective only when transmission error occurs randomly in time.

• To prevent errors from clustering, cellular systems permute the order of bits generated by a channel coder.

• Receivers perform the inverse permutation.

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InterleavingInterleaving• Example:

– WHAT I TELL YOU THREE TIMES IS TRUE– If there are four consecutive errors in the

middle, the result is WHAT I TELL YBVOXHREE TIMES IS TRUE– Alternatively, it is possible to interleave the

symbol using a 5 x 7 interleaving matrix (See pp. 364-365)

– WHOT I XELL YOU THREE TIMEB IS VRUE

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Adaptive EqualizationAdaptive Equalization• An adaptive equalizer operates in two

modes– Training mode: Modem transmits a signal,

referred to as a training sequence, that is known to receiver. The receiving modem process the distorted version of training sequence to obtain a channel estimate

– Tracking mode: The equalizer uses the channel estimate to compensate for distortions in the unknown information sequence

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Walsh Hadamard MatrixWalsh Hadamard Matrix• The CDMA system uses a 64 x 64

WHM in two ways:– In down-link transmissions, it used as an

orthogonal code, which is equivalent to an error-correcting block code with (n, k; dmin) = (64, 6; 32)

– In up-link transmissions, the matrix serve as a digital carrier due to its orthogonal property

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Walsh Hadamard MatrixWalsh Hadamard Matrix W1 = | 0 |

0 00 1W2 =

0 00 1

0 00 1

0 00 1

1 11 0

W4 =

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AMPS SystemAMPS SystemThe first generation The first generation

cellular phone systemcellular phone system

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Network ElementsNetwork Elements

• The AMPS specification refers to terminals as mobile stations and to base station as land stations.

• The common terminology for an AMPS switch is mobile telephone switching office (small and large MTSO).

• The communication links between the base stations and switch are labeled land lines (copper wires, optical fibers or microwave systems)

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AMPS Identification CodesAMPS Identification Codes• Mobile Identification Number (MIN)

– Area code (3 digits), Exchange number (3 digits) and subscriber number (4 digits)

• Electronic Serial Number (ESN)• System Identifier (SID)• Station Class Mark (SCM)

– Indicates capabilities of a mobile station

• Supervisory Audio Tone (SAT)• Digital Color Code (DCC)

– Help mobile stations distinguish neighboring base stations from one another

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Frequency Bands and Frequency Bands and Physical ChannelsPhysical Channels

• The band for forward transmissions, from cell site to mobile station, is 870-890 MHz.

• The reverse band, for transmissions by mobiles, is 45 MHz lower.

• An AMPS physical channel occupies two 30 KHz frequency bands, one for each direction.

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Radiated PowerRadiated Power• An AMPS terminal is capable of radiating

signals at 6 or 8 different power levels (6 mW to 4W).– 10 log 4000 = 36 dBm

• The radiated power at a a base station is typically 25 W.

• Discontinuous transmission (DTX)– Speech activity detector– ON-OFF state– Power saving and Interference reducing

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Analog Signal ProcessingAnalog Signal Processing• Compression and pre-emphasis are established

techniques for audio signal transmission.• An amplitude limiter confines the maximum

excursions of the frequency modulated signal to 12 KHz.

• Low pass filter Attenuates signal components at frequencies above 3 KHz, refer to Fig. 3.5.

• The notch (at 6KHz) removes signal energy at the frequencies associated with the 3 SAT of the AMPS system.

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SAT and STSAT and ST• The SAT (Supervisory Audio Tone) transmitted

with user information serves to identify the base station assigned to a call.

• Each base station has its own SAT- at 5970 Hz, 6000 Hz, or 6030 Hz.

• An analog signals from AMPS terminals can also contain a 10 KHz sine wave referred to as a ST (Supervisory Tone).– On-hook and Off-hook indications signaling

• The channel reuse principles (Section 9.3.2)

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Digital SignalsDigital Signals• AMPS also transmits important network

control information in digital form.• AMPS digital signal are sine waves either 8

KHz above or 8 KHz below the carrier.• The signal format is Manchester coded

binary frequency shift keying at a rate of 10 Kbps

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Spectrum EfficiencySpectrum Efficiency

• Frequency modulation in 30 KHz physical channels

• Signal-to-Interference ratio (SIR)– SIR >= (SIR)req = 18 dB– Reuse factor N = 7 (Figure 9.9)

• Spectrum efficiency– E=395 /7*25 = 2.26 conversations/cell/MHz– 395 traffic channels, 25 MHz/system, 7 cells

in a cluster

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Logical Channel CategoriesLogical Channel Categories• FOCC: Forward (Downlink) Control Channel

– Carries the same information from one base station to all of the mobile terminals (Broadcast)

• RECC: Reverse (Uplink) Control Channel– Carries information from many mobile

terminals that do not have voice channel (Random access)

• FVC: Forward Voice Channel (Dedicated)• RVC: Reverse Voice channel (Dedicated)• Forward and reverse traffic channel

– User information (Dedicated)

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Tasks Performed by Tasks Performed by TerminalsTerminals

• Initialization mode– The terminal turns the power on– A conversation ends– Loses contact with the current base station

• Idle mode• Access mode (from Idle mode)

– The terminal presses the SEND button– An incoming call request detected (MIN)– A registration event stimulated

• Conversation mode

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CapacityCapacity

• There are 3 ways to increase the capacity– Operate with smaller cells– Obtain additional spectrum allocations– Improve spectrum efficiency

• NAMPS (Narrowband-AMPS)– Messages similar to AMPS– Synchronization sequences– Digital versions of the SAT and ST

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Review ExercisesReview Exercises• What is the purpose of the busy/ idle bits in

the FOCC? Why are they not used in the other control channel formats?

• Explain how the AMPS system users supervisory audio tones (SAT) and a digital color code (DCC). Why are both required?

• Explain why it is sometimes desirable for the AMPS system to set up a call through a base station that is not the nearest base station to the terminal. How does the AMPS system achieve this effect?

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ReferencesReferences• D.J. Goodman, “Wireless Personal

Communications Systems”, Ch9 and Ch3.

• Ch9: Preliminary• Ch3: AMPS system