The Cellular Radio Handbook

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Chapter 3: Basic Radio

OVERVIEW

Radio was first postulated in a roundabout way in 1873 by Maxwell (who foundmathematically that a wave which was both electric and magnetic, and traveled at the speedof light should exist), demonstrated in 1888 by Hertz (who commented that it was aninteresting phenomenon, but of no practical consequence), and used for practicalcommunications in 1895 by Marconi. Radio is an electromagnetic phenomenon andradiates as photons. It belongs to the family of radiation that includes X-rays, light, andinfrared (heat) waves. The different categories of radiation differ in frequency, as shown inFigure 3.1. They also differ in energy and ability to propagate through different media.

Figure 3.1: Electromagnetic spectrum.

BASIC ELEMENTS

All practical radio systems can be reduced to the basic scheme shown in Figure 3.2. For thepurposes of telecommunications the radio spectrum is divided into frequency bands,virtually all of which have some application for communications. The bands are defined in

Table 3.1.

Figure 3.2: A simplified analog transmitter/receiver with an audio AM modulation.

Table 3.1: The RF Communications Bands

Frequency Range Name Abbreviation Main Uses

Less than 300 Hz Extremely low ELF Submarine

300 Hz3 kHz Infra low ILF Voice band

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Table 3.1: The RF Communications Bands

Frequency Range Name Abbreviation Main Uses

3 kHz30 kHz Very low VLF Audio band

30 kHz300 kHz Low LF Broadcast/long range

300 kHz3 MHz Medium MF Broadcast/long range/shortwave3 MHz30 MHz High HF Shortwave band

30 MHz300 MHz Very high VHF Mobile radio/paging/VHF TV

300 MHz3 GHz Ultra high UHF Mobile/microwave/TV

3 GHz30 GHz Super high SHF Microwave/satellite

30 GHz300 GHz Extremely high EHF Microwave

300 GHz3000 GHz Tremendously high THF Experimental

The names reflect the development of technology, with relatively low frequencies, such as

shortwave, being called "High Frequency" (HF). In the days when this band was named,most practical transmitters where mechanical and these frequencies were obtained onlywith great difficulty. (Although some spark transmitters were capable of generating GHzfrequencies, because the detectors were so crude, the range was limited to meters, and thesefrequencies were not regarded as being of practical use.) Consequently, as the upper limitswere pushed away, new superlatives for "Very High" (VH) needed to be used.

Virtually all cellular applications are in the ultra HF (UHF) band, although the microwavelinks can be in the extremely HF (EHF) band. At the lower end there is the extremely lowband (ELF) which includes the power supply frequency (50 or 60 Hz).

Transmitter

A transmitter consists of two basic parts: a modulator and a carrier. Figure 3.2shows ananalog transmitter with an audio input that is converted into the form to be transmitted (inthis case, via a microphone to the modulator). A radio frequency generator generates theradio energy that will carry the signal. This generally consists of an oscillator (whichproduces the initial signal) and a number of amplifier stages (which amplify the level tothat required at the antenna). A modulator mixes the signal to be transmitted with the radiofrequency signal (called the carrier) in such a way that the signal can be decoded at adistant receiver.

Receiver

The receiver in Figure 3.2gets a signal from its antenna, which also receives a number ofunwanted signals. The tuned circuit tunes out all but the wanted signal, which is thendemodulated (decoded) by the demodulator.

The very simple receiver illustrated in Figure 3.2, consisting only of a tuned section and ademodulator, is known as a tuned radio frequency (TRF) receiver. Until about 1930 most

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receivers were of this type. Such a receiver is seen in Figure 3.3. This is a 5-valve set with3 tuned RF stages followed by two transformer coupled

Figure 3.3: The Hazeltine TRF receiver, designed in 1927 in the United States. This

receiver uses 3 independent dials for tuning.

audio stages. The 1927 U.S. designed Hazeltine TRF set is still used daily in my home. Itworks fine in the day-time but lacks selectivity for night time reception. The downside isthe tuning, which uses three separate dials that make the task a bit like finding a safecombination.

The TRF was replaced as the main commercial receiver by the superheterodyne,commonly known today as thesuperhet, shown as Figure 3.4. The name was derived fromthe word heterodyne, which means the beating of two signals together to produce the sumand difference frequencies; (CW orcontinuous wave Morse code signals were decoded by

beating the incoming signal with one of almost the same frequency so that an audio toneequal to the difference frequency was heard). The tuning and signal amplification in theearly superheterodynes was done at supersonic frequencies of around 50 to 60 kHz (theheterodyne frequency), and hence the word "super." These frequencies were chosenbecause the early valves worked better at these lower frequencies than at RF(approximately 1 MHz). The

Figure 3.4: A basic superhetrodyne receiver.

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heterodyne frequency is now known as the intermediate frequency or IF, and in modernreceivers is well above the supersonic rangetypically 455 kHz or 10.7 MHz.

Even today, considerable advantages are had by doing a lot of the amplification and tuningat relatively low frequencies where technology is well-developed and components are

inexpensive.

The superhetrodyne uses a mixer to down convert the carrier. In the process of mixing, themixer will present not one but two carrier frequencies to the IF stage. Let's assume that IFfrequency is 500 kHz and the mixer oscillator frequency is 800 Mhz. As a result of mixing,the 800.5-MHz carrier and the one that is 799.5 MHz, both will be presented to the IFamplifier as the "wanted" 500-kHz signal (i.e., there are two frequencies 500 kHz awayfrom the IF frequency). The second, unwanted frequency is known as the image frequency.To prevent this from being a problem, it will be necessary to pre tune the carrier toeliminate the unwanted signal. Let's assume the wanted signal was the 800.5 MHz and arejection of 60 dB was required. This would require that that the preselector filter had a Q

of around 400,000; a figure that even a superconducting filter would not (readily) be able toachieve! To make it easier, we could instead of a 500-kHz IF, start with a 10.7-MHz IF (acommon commercial IF frequency) so now the image frequency is 21.4 MHz away. Thisimage can be rejected to the extent of 60 dB with a Q of only ~19,000; still asking a bitmuch. However if the IF frequency is further increased to 70 MHz, a more manageable Qof just more than 3000 is required. However for 800-MHz systems, the image frequency(which is now 140 MHz away) will be outside the passband of the RX preselector filter, somuch of the rejection will occur before the RF stage. This approach is what is in fact donein cellular RX design.

Although a wideband RF stage like the 70 MHz just discussed will have good image

rejection, it will not have good selectivity and so often a second stage of IF at a lowerfrequency again will be used to attend to this problem. In this case, you have a doubleconversion superhetrodyne; this technique is commonly used in communications receivers,some of which are even triple conversion.

A final improvement that is characteristic of modern receivers is the addition of an RFstage, which consists usually of a single stage, low-noise amplifier to improve the signal-to-noise (S/N) performance of the set. In fact, it will be seen in the chapter on noiseperformance, that in a normally operating superhet, it is the noise figure of that RF stagethat will virtually determine the overall receiver performance.

A receiver incorporating all these elements will be found in the typical cellular receiver inboth base stations and in mobile phonesshown inFigure 3.5.

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Figure 3.5: A typical superhet configuration of the type used in cellular radios.

Portable FM radio was first developed by Motorola for the US Army Signal Corps duringWorld War II. The first unit was a backpack weighing around 16 kilograms and having arange of about 30 kilometers.

Modulator

The modulation system used in most analog cellular radio systems is known as frequency

modulation (FM). Digital modulation is usually phase modulation. In FM modulation thefrequency of the carrier is varied proportionally to the signal to be transmitted. A typicalFM modulator is shown in Figure 3.6.

Figure 3.6: Typical FM modulator.

The audio input varies the bias on the varactor (a solid-state variable capacitor, illustratedin Figure 3.6), which in turn changes the frequency of the tuned circuit. The maximumamount that the frequency can deviate from its central carrier frequency is called the peakdeviation.

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The S/N performance of FM systems is very high, provided the noise level is reasonablylow. FM systems with wide deviation have better S/N performance than those with narrowdeviation.

Some analog systems use phase modulation, particularly for data transmission. Phase

modulation is closely related to frequency modulation and can be derived from it bypassing the signal through a simple differential circuit before frequency modulation (seeFigure 3.7).

Figure 3.7: Phase modulation from frequency modulation.

Frequency shift keying (FSK), at relatively low speeds (for example, the Manchester codeat 6.4 kbits is for TACS and 8 kbits for AMPS on the control channel), is often used

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for data, because it has better S/N performance than FM at low signal levels. Thisenhances signaling in areas of poor reception.

DYNAMIC CHANNEL ALLOCATION

Because cellular systems use many channels, it is necessary for the mobile to automaticallyswitch to the correct channel. This is done by sending an instruction (data) that indicatesthe channel number required. The mobile system then switches to the channel indicated byusing synthesized tuning, a system where the frequency of the oscillator is numericallycompared to the required frequency and adjusted by a "phase-locked loop" until the twofrequencies match.

NOISE AND SIGNAL-TO-NOISE PERFORMANCE

All radio systems are ultimately limited in range by noise. When the intrusion of noise issuch that an acceptable signal can no longer be obtained, then the system is said to benoise-limited.

Medium-wave (broadcast band) and short-wave broadcasts operate in a very noisyenvironment; background noise limits the performance in the broadcast/shortwave bands.The very high frequency (VHF) and ultra high frequency (UHF) bands where cellular radiooperates are relatively quieter and most of the noise is generated in the radio frequencypreamplifier of the receiver itself. Regardless of how well designed the receiver is, there isa theoretical noise power level that, at a given temperature, cannot be improved upon. Thisis because of the thermal noise, generated by the movement of atomic particles (in the

receiver and most particularly in the first radio frequency amplifier). This noise isproportional to the operating temperature. Hence, the antenna and RF amplifier stages willgenerate thermal noise continuously. For this reason high-quality receivers, such as radiotelescopes, operate their input stage RF amplifiers at liquid-nitrogen temperatures or lower.

In order to perceive a relatively noise-free signal, the incoming signal must exceed thenoise level by a respectable margin, known as the signal-to-noise ratio (SNR). For cellularradio systems, this level is usually regarded as about 12 dB for marginal reception and 30dB for good-quality conversations.

Signal-to-noise ratio is usually expressed as:

Because modern receivers closely approach the theoretical noise limits for their operatingtemperatures, it can easily be deduced what minimum received signal level is required toachieve satisfactory signal-to-noise ratio.

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Thus when we speak of a 39-dBV/m boundary level for an Advanced Mobile PhoneService (AMPS) system, this is equivalent to specifying the point at which the signal-to-noise ratio is regarded as satisfactory.

A "satisfactory" level of signal-to-noise for cellular systems is usually regarded as one

where the noise is just noticeable. For public mobile radio (PMR), a usable but noisy levelis often regarded as satisfactory. In FM systems, the noise usually occurs as sharp clicks(sometimes known as "picket fence" noise because it is similar to the sound produced bydragging a stick along a picket fence).

In digital systems the RF signal level still determines the signal quality, but the qualityitself is measured in bit error rates (BER). It is standard practice to take the standard as araw error rate of 2 parts in 100 as the minimum acceptable level [this is the digitalequivalent of a Signal to Noise and Distortion (SINAD) level of 12 dB]. However, forgood-quality digital signals the BER is very low indeed, so much so as to be almostmeaningless for speech. What is more important is the margin above the minimum BER.

This is usually taken to be something of the order of 1520 dB.

Digital systems degrade rapidly when the minimum BER is approached, and a muchsharper boundary is established than is the case with analog.

dBs

Human senses perceive power approximately logarithmically. For example, doubling theenergy level of a sound pulse produces only a 3-dB increase in the perceived levelandthat increase is only just noticeable. The term dB was introduced to define relative powerlevels logarithmically.

The term dB is used often in radio systems and can be a major source of confusion to theuninitiated because of the large number of different units of dBs. Essentially, the dB levelis the log of a power ratio: dBm, the most common form of dB, is the power of thesystem measured compared to 1 milliwatt. Mathematically, this can be expressed as:

Thus

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A third property, reflection, is significant at all cellular radio frequencies. Coverage inhigh-density city areas is significantly enhanced by the ability of the radio system to reflectinto most areas that are inaccessible via a direct path.

These three modes of propagation are summarized in Figure 3.9.

Figure 3.9: Modes of propagation at cellular frequencies. Super-refraction, or ducting, is asporadic phenomenon responsible for propagation over very large distances under certainatmospheric conditions.

Radio base stations are effectively radio concentrators that serve a large number of

customers using a few channels. Typically, 2030 customers can be handled on eachchannel (depending on the calling rate). The base station is always modular in construction.Figure 3.10 shows the basic components of a base station. These elements of the basestation shown in Figure 3.10 are discussed in the following sections. Figure 3.11 shows arural base station with 120-degree sector antennas.

Figure 3.10: Diagram of a typical base station.

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Figure 3.11: A base station with a purpose-built tower and transportable hut.

RADIO CONTROLLER

The radio controller is the interface between the mobile switch and the base station, andoperates similarly to a remote-subscribers switch. Under the control of the mobile switch,the radio controller selects (switches) the radio channels as required. It also supervisesvarious system parameters, including alarm conditions and radio field strength.

ANTENNASAntennas used in cellular radio are usually gain antennas, meaning that they have gain,compared to the simplest form of antenna, the dipole. A dipole is shown in Figure 3.12.The simplest vehicle-mounted antenna with

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Figure 3.12: Dipole antenna.

the same gain as a dipole antenna is the quarter-wave antenna, illustrated in Figure 3.13. A

mobile gain antenna is usually easily recognized by its loading coil, as shown in Figure3.14. The loading coil is usually visible as a "bump" in the antenna.

Figure 3.13: Quarter-wave antenna.

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Figure 3.14: Three-dB gain antenna for roof mounting.

Mobile (vehicular) cellular antennas are usually between 3 and 4.5 dB gain whilehandhelds have gains around 0 dB. Base-station omnidirectional antennas, which stackmany radiating elements in series, are often 9 dB in gain; unidirectional base-stationantennas can have gains as high as 17 dB. Very high gain antennas are only practical infixed locations, because they are also physically large and must be exactly vertical tooperate satisfactorily. Sometimes, however, they are deliberately tilted down to limit thebase-station range.

MOBILE TRANSMIT POWER AND HEALTH

The radiation from a radio system is non-ionizing (distinguishing it from radioactive decay

products); the main effect on the human body is a rise in temperature.

In cellular systems, the mobile's power is usually limited to a few watts. It is generallybelieved that these powers and the radio frequency radiation levels do not pose any healthhazard. However, it is not recommended to hold or touch the antenna of a mobile in use.Some authorities have expressed doubts about the wisdom of long-term use of handheldunits.

Contrary to popular belief, it is not the high frequencies used by cellular radio (800900MHz) that are inherently most harmful to humans. At the lower frequenciesaround 100MHzthe body can become resonant and, therefore, very absorbent. Hence, these lower

frequencies are potentially more hazardous. Early experiments using small laboratoryanimals pointed to relatively more harmful effects at higher frequencies. However, thesmall size of the animals, which gave the animals a high resonant frequency, probablyaccounted for these results.

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Chapter 4: PlanningAn Essential

Network Function

OVERVIEW

Very few cellular networks are well planned. Most are installed with almost indecent haste,with a lot of corners cut and too many compromises made. The situation in cellular hasbeen exacerbated by consistent underestimation of demand and the high capital investmentrequired, which causes management to be very impatient with system engineers who wouldprefer to do the job properly rather than just quickly. As a result badly planned cellularnetworks are the rule not the exception. They are characterized by

Almost exclusive use of pre-existing sites (for wire-line operators) Lack of in-house survey facilities Poor performance in a frequency reuse environment, particularly with regard to

dropped calls and interference (which has the characteristics of cross talk) Poor coverage Inefficient use of resources Dropped calls Excessive blocking

FLEXIBILITY

Flexibility is the key to cellular system planning. If you rely on marketing forecasts, the

business plan or other sources of soothsaying, you are bound to have problems. Thecellular industry is growing too fast and too uncertainly for anyone to be able to forecastthe future with even reasonable accuracy. The system must be planned and designed toallow for any and all reasonable contingencies. It mustabove allbe flexible.

TIME FRAME

The time frame for planning of a properly planned system is such that the planning shouldbe at least a year ahead of the implementation. This time frame is dictated mainly by leadtimes on hardware and sites. Some particular lead times are

Site acquisition and town planning approvals, 612 months Construction of a major switch building, 1218 months Microwave and switching equipment deliveries, 6 months Antennas, rectifiers, and batteries, 4 months Base station equipment [radio frequency (RF) channels, etc.], 3 months

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These are the lead times that are considered "normal." Under "exceptional" circumstances,which seem to occur so often that they seem to be the rule rather than the exception, longerdelays can be experienced.

One of the problems engineers have with managers who refuse to plan, is that when the

crunch is really on they can always get "something" up and running in much shorter timeframes than those listed above. When this is done it will result in some built-incompromises. Some of these compromises include

Less than optimal base-station location The site may not be suitable for future expansions Future frequency reuse may be limited The desired level of redundancy may not be built in Equipment purchased because of its availability may not be compatible with the

rest of the network, and may require a stock of dedicated spares and test equipment.Also, staff training and future support problems may not have been considered

The cost of the badly planned network will be higher The future cost to tidy up the compromises may be overlooked

With continued crisis management, the network becomes a patchwork that is expensive,inefficient, and difficult to maintain. However, management often doesn't bother too muchabout these "technical" details and the next expansion is handled the same way, and so thecycle continues. Management comes to believe that planning is not necessary and that shorttime-frames can be delivered by "good" management.

RADIO SURVEYS

Knowing the coverage and potential interference problems is the key to a good cellulardesign. Very few cellular managers/engineers understand thisconsequently there are veryfew well-designed cellular systems to be found. Less than one-half of all cellular systemscould be regarded as well designed! (If you think that statement is a little harsh, thenobviously you haven't been using your cellular phone very much.) Some are very poor.

A good network can only be designed by good engineers who know its coverage andinterference habits intimately. For that reason it is essential that the designerbe a userofthe system, since this is an excellent way to learn the performance.

Radio surveys should be done in-house and be at least one year (but preferably two) ahead

of the implementation timetable. This will allow for the acquisition time, site preparation,and a safety factor so that alternatives can be found if necessary. If computerized radiocoverage prediction techniques are used, it is possible that this time frame could be reducedby a few months.

SITE ACQUISITIONS

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Site acquisitions are very time-consuming, considering that after obtaining either a contractto buy a site or a long-term rental contract, it will then be necessary to obtain one or moreof the following:

Tower construction permits

Aviation department clearances Local government building approvals Frequency permits for microwave links Road access permits (if transportable huts are used) Connection of electric power

Although construction personnel can sometimes do wonders, at least six months should beallowed for completion. Most operators of large systems have found that it is cost-effectiveto have dedicated personnel for property acquisition. These people can negotiate with theproperty owners and follow through on approvals and permits.

Opposition from local authorities that the installation will be an eyesore can be expected.However, with some attention to design, a base stationeven though still a little prominentneed not be altogether out-of-character with its surroundings, as can be seen in Figure4.1. Here

Figure 4.1: Base stations need not clash with the surroundings. This base station seems toblend with the surroundings.

even the 30-meter monopole seems to harmonize with the surroundings. (See also disguiseantennas in Chapter 12.)

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Sometimes a quick fix can be a future liability on a grand scale. Inaccessible sites thatrequire helicopter access or four-wheel drives can be ideal microwave repeater and relaysites, but the cost of maintaining them can be prohibitive. Don't be lured into thinking that amicrowave system that has a mean time between failures (MTBF) of 10 years can be leftunattended. Don't forget that the batteries, solar cells, buildings, towers, and even the site

vegetation require regular attention. I know of such sites that cost over $1 million per yearto maintain.

FREQUENCY PLANNING

Frequency reuse is the essence of cellular radio [except for code division multiple access(CDMA)]. Good frequency planning starts with well-chosen sites. Although somedesigners make a lot of fuss about site location relative to a hypothetical hexagonal grid, amuch more critical consideration is coverage and reuse potential. Fortunately a systemfrequency retune can be done relatively easily, and so bad designs can often be corrected ata reasonable cost.

The frequency plan should not be done with the sole consideration being the "as-builtconfiguration." Planned future cells should be allowed for and their future allocationsreserved. This permits an orderly expansion and will minimize retunes.

TRUNK NETWORK PLANNING

The general rule for trunk capacity is that there is never enough. No matter how generousyou thought the initial installation was in regard to future trunk provisioning, it is alwaystoo little. Fiber optics and relatively low-cost, high-capacity digital microwave have made

this problem a little easier, but it still has not gone away.

While some companies still use 2- and 8-Mbit (or T1), more forward-thinking ones aredoubting if 34-Mbit is adequate and have standardized on 140 Mbit for main trunk routes.The extra cost is minimal for fiber optic, but the shorter hops (and hence additionalrepeaters and towers) for microwave have to be justified.

It is necessary to always consider future alternative trunk routes as well as route diversity.The route diversity should include the provision for future nodes. All sites, no matter howremote, may one day be needed as drop and insert points. Don't design this capability out ofthe system.

Where applicable, alternate routing can be used to reduce the cost of the trunk networkandimprove its survivability.

ROOM TO GROW

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Poorly planned systems rarely have built-in capacity for future growth. This can be aparticularly expensive problem if building and site plans do not allow for expansion. Howoften do we encounter a site that is difficult to expand, just because the initial building waslocated so as to prohibit growth? How often do site managers lament that the site next doorwas not purchased when it became available? Or even that the tower was not designed to

carry the number of dishes that are now needed? As seen inFigure 4.2 towers can easilybecome fully loaded, and the one illustrated really has little room for any expansion. Agood rule of tower design is to estimate the future demand as accurately as possible, andthen double the estimate.

Figure 4.2: This tower is fully loaded with an array of microwave dishes and Yagiantennas. Towers become full all too easily.

Towers can present difficult problems for the installer if the site and site access have notbeen well planned. Not only does a tower require a good deal of space to build and erectbut the foundations for the footings require extensive excavations, which can be potentiallyhazardous to the pre-existing structures.

All of these problems result from shortsighted thinking and lack of planning.

OTHER FACILITIES

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Some cellular operators will intend to expand into related areas such as paging, trunkmobile, backbone provisioning, and voice messaging. If your operation does intend toexpand, leave room for the equipment. This can be done either by dimensioning the roomsa little larger or by designing equipment rooms that can be easily expanded. Non-structuralwalls improve flexibility.

Often opportunities will arise to rent tower and equipment space to other users. This can bea good additional source of income, but if you intend to do this, you must plan the accessfor the "other users" so that they can install and maintain their equipment withoutdisruption to your service, and preferably without access to sensitive areas like the switch.Some operators have a self-contained "other users" room that has no access to the mainequipment rooms. Increasingly, shared sites are being mandated by governments.

Chapter 5: Cell Site Selection and System

DesignOVERVIEW

Cell site selection is the process of selecting good base-station sites. The process is half art,half science. To the customer, the most vital feature of a cellular system is its coverage.Therefore, it is important for the system to deliver what the customer wantsgoodcoverage within a logically defined service area. The service provider is usually interestedin both extensive frequency reuse (except in small towns) and good coverage. The selectionof the best cell sites is essential. Often this part of cellular-system design is poorlyconsidered and results in poor system performance.

DESIGN OBJECTIVES

The design objective should be to cover the service area without any serious discontinuitiesas economically as possible; and, where applicable, to allow for future frequency reuse. Inmost technical journals, coverage is defined as 90 percent of the area covered for 90percent of the time. Were this "objective" to be achieved literally, the coverage wouldindeed be poor (that is, 81 percent of the service area would provide adequate field strengthfor successful calls; 19 percent of the target area would be substandard). In fact, the 90percent/90 percent coverage standard means something quite different. The 90 percent/90

percent definition means 90 percent of the area should be covered at any time or,alternatively, that 90 percent of the regions achieve a defined standard at any point in spaceand time.

The FCC has specified a field strength of 39 dBV/m average [for Advanced MobilePhone Service (AMPS); code division multiple access (CDMA) coverage is similar] as theboundary of a cell. Although this figure is realistic, it is a compromise. In practice, theboundary field strength for acceptable service is a function of the terrain. The real objective

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is to obtain a signal-to-noise (S/N) ratio comparable, but not necessarily equal, to a land-line telephone, which is usually accepted as 30 dB (some European authorities use 20 dB,which is below the quality of a very poor telephone line). Land-line systems, however,generally achieve 40 dB or more.

As a guide, Table 5.1 lists more realistic boundaries. No firm rule for urban centralbusiness district (CBD) can be formulated, but it is generally accepted that a 2030-dBmargin over mobile levels is required for handheld use in multistory buildings. Goodhandheld coverage is defined as a signal level yielding a voice quality that is usablewithout discomfort in buildings from the ground floor up, excluding elevators and theirimmediate vicinity (2 meters).

Table 5.1: Boundary of Cell Sites for AMPS Systems

Area Recommended Field Strength AMPS/NMT450/CDMA NMT900 GSM[a]

Urban CBD 60 dBV/m 68 72

Suburban 39 dBV/m 47 51Rural 34 dBV/m 41 46[a]Still not widely agreed upon.

ASSUMPTIONS AND LIMITATIONS

All new systems need good handheld coverage, a fact that provides a good starting point.Individual cellular operators will probably have their own preferences for buildings tohouse cellular bases, such as telephone switching centers, buildings designated as radiotelephone sites, or maybe just a few particular buildings to which access can easily be

arranged. The cellular-system designer should accommodate these preferences, but not atthe expense of sound design.

A cell plan (that is, N = 4, 7, or 12) should be chosen on the basis of the long-termcustomer density, noting that high density favors the 4-cell system. Initially, however, the4-cell system is both more costly and less tolerant of site selections that do not approximatethe theoretical cell plan.

Defined Coverage

Before a design is implemented, the number of bases to be used should be defined based on

the techniques outlined in Chapter 25, "Budgets."

Defining Boundaries

Radio waves propagate according to natural laws that have little to do with city boundariesand customer service areas. Therefore, there is little value in defining a precise area to becovered until the "natural" boundaries defined by the propagation characteristics are

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known. Of course, it is still practical to define certain areas as essential for coverage,leaving the "fine-tuning" of the actual boundaries as flexible as possible.

When boundaries must be decided, the decision should be made in consultation withengineering and marketing staff once alternative boundaries (those possible, given the

resource constraints) are known. At coverage boundaries, high sites become increasinglyattractive in gaining an economic population of customers in a given service area. Thesesites are usually in low-density suburban or rural areas.

Usually, the physical boundaries of a city are poorly defined (in terms of customer density).At the "edges," a number of options are available. For example, one base to the north of acity may provide a potential 1000 km2 coverage along a sparsely populated highway, whilethe same base to the south may cover only 300 km2 but includes two important towns. Forthis reason it is important that the system designer have good communication with themarketing staff.

While the designer should be free to select sites that provide continuous coverage in high-density regions, the engineering staff must recognize that there are no clear-cut "best"designs and that many alternate solutions, with different coverage, could be equally viable.Indeed, detailed marketing studies can be undertaken to "resolve" the problem.

Note It is important to provide your marketing staff with a clearly marked map showingthe expected coverage of the alternatives. Without this information, they have nofoundation on which to draw their conclusions.

SUITABLE SITES

All selected sites must have reasonable access for installation and maintenance, andsuitable accommodation (usually a minimum of 18 m2) with a ceiling of 0.6 meter abovethe equipment rack. Equipment racks are usually 2.2, 2.7, or 2.9 meters high, depending onthe manufacturer. It is important that equipment access is available and that provision ismade to accommodate heavy crates of equipment. Access for maintenance must also beprovided. If the site is located in a rural area, power and reasonable road access should beavailable. All of these improvements can be expensive if they have to be provided afterinitial installation.

Links to the base stations (either radio or physical) must be provided, and power is alwaysessential. Consequently, the designer should undertake a site visit to ensure suitability

before pursuing the design too far. If microwave links are used, the path back to the switch(or nearest hub) must be considered. Remember that base-site antennas can be large and 12(or more) sector antennas with dimensions of 0.5 m 3 m (approximately) may need to befitted. Although it is usually impossible to guarantee the life of a site, some sites are morevulnerable than others and it is best to avoid those where new building that will obscure theradio path may occur.

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The site needs access to commercial power (about 400 watts/carrier, including air-conditioning), and usually some provision for an emergency power plant must be made.Hospitals are often good sites because they are usually relatively low buildings with goodclearance around them (parking lots and gardens). They also have emergency power. Onsome tall buildings where emergency power is not available, it may be feasible to run a

cable down the building to an external socket that can be powered by a portable generator.This saves cost, space, and possible objections to a permanent installation.

Because a cell site is a substantial building and, usually has, a prominent antenna-supportstructure, the zoning of the site must be appropriate. Although it is often possible to get cityplanning approval to build base sites in residential areas, the process is uncertain and slow.Areas zoned for industrial or commercial use are usually easier to acquire for cellularpurposes.

The availability of a site will be determined by local building codes, neighborhoodenvironmental attitudes (particularly if a substantial antenna structure is involved), as well

as any limitations imposed by the site owner.

If rental property is being considered, leases should be for at least five years, preferablywith an option to extend the lease. If possible, it is advisable to house on-site equipment intransportable buildings, which will minimize costs in the event of lease termination. Also,an installation should be done with the expectation that at some future date you will need tomove the equipment.

Where an antenna support structure must be built, especially in a residential area, thefollowing questions will be posed by the local residents:

Will it interfere with the TV reception in the area? Is the tower safe? Will the microwave make me sterile? How big is the tower? Will it cause cancer?

Of course, there are usually no problems, but the questions will be asked, and the designengineer should be ready with the answers.

JOINT USER SITES

For most cellular operators, the days of intense competition to get a little more coveragethan the competitor are over. During start-up coverage was everything, but mostestablished systems have adequate coverage, and new cell sites are needed mainly forcapacity. There could be significant savings if the operators were able to get together andshare the costs of sites, towers, and infrastructure. Separate lockable buildings can beprovided, but all else could be shared.

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Existing sites could either be excluded, cross-shared, or sold to a third company, of whichboth (or all) carriers are shareholders. Cost savings could easily amount to 3040 percentof the total cost of a base station.

While sharing is desirable for cellular applications, it is probably imperative for PCS/PCN.

GETTING A STARTING POINT FOR THE DESIGN

Inasmuch as the choice of suitable sites is an interactive one, the sooner a desirable centralsite can be identified, the better. A starting point for the design must be established.Usually, that point will be the primary site selected for the central business district (CBD)coverage. The primary CBD site is a good starting point because it can be selected to coverthe CBD regardless of other coverage requirements. Also, CBD sites are difficult to acquireand once one has been found, it is a good idea to make it fit into the final pattern.

In small towns, frequency reuse may not be necessary, so high "broadcast" sites can be

chosen. Therefore, height can be used to select a good first site. This type of design is verydifferent from high-density designs and is a good deal easier. As handheld use iscontemplated, it is often necessary to find a prominent central urban site to providehandheld coverage in CBD buildings. This is particularly true if all other sites are morethan 4 km from the CBD.

In big cities the opposite is true: a selected site(s) should offer good handheld penetrationwhile at the same time containing the coverage. It may be possible to use surroundingbuildings as radio-path shields. If frequency reuse is a consideration, the ability to provideadequate frequency reuse should be a major consideration. It should be noted that althoughplacing the antenna in a situation where it is surrounded by tall buildings, as shown in

Figure 5.1, is a good way of confining the radiation for that antenna, the surroundingbuildings will cause reflections that will seriously degrade the back-to-front isolation of thesector antennas used. This can itself be a problem if there is to be frequency reuse withinthat confined area.

Figure 5.1: Preferred frequency reuse.

When tall buildings are used, as in Figure 5.2, virtually all mobiles in the area will interferein this antenna. As the front-to-back ratio in real life (as distinct from free space values) isusually only about 10 dB, even interference from the back of the antenna will be a

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Figure 5.2: Poor frequency reuse.

problem. This configuration is good for coverage but bad for frequency reuse.

Frequency reuse would eliminate the tallest buildings (even considerable antenna downtiltwill not help much) as CBD sites. A good choice might be to use a smaller building with aclear area of about 500 meters from the next obstructing building.

Site availability is a major limitation. Many ideal sites have uncooperative owners. Thedesigner often has little choice, particularly with inner-city sites.

For some rooftop installations, weight may also be a problem.

SPECIAL CONSIDERATIONS

Significant reflections occur from large buildings. As a result, some of the front-to-backratio immunity of sectored antennas will be lost. For this reason it is essential to surveysectored sites facing tall buildings, paying particular attention to the area outside thenominal cell area. The front-to-back ratio of an antenna that may nominally be 22 dB canbe reduced to 615 dB because of reflections (see Figure 5.3). This reduction can result in

interference problems from behind the cell.

Figure 5.3: Using buildings and natural boundaries to confine coverage.

Figure 5.3 shows one way of exploiting natural or man-made boundaries to improvefrequency reuse. Assuming that the buildings form long rows following the coastline, asshown inFigure 5.3, then the buildings can be effectively serviced from a seashore site. Atthe same time, the buildings shield the site to permit effective frequency reuse behind them.Experience shows that ship-to-land cellular communications are usually made via thehighest base station rather than the one nearest the sea. This fact should be consideredwhen deciding if reflected signals are likely to be a source of interference. Use by sea

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vessels is generally limited, and reflections are not likely to cause problems with seacoverage. Where significant sea traffic is anticipated, a high base should be dimensionedaccordingly. Line-of-sight propagation losses (rather than mobile environment losses)should be assumed. Because of reduced multipath, a lower field strength of 32 dBV/m forAMPS/CDMA & 44 V/m for GSM 900 will suffice. This lower figure results in

significantly greater coverage. Although with digital systems over-water range is likely tobe propagation-time limited rather than field-strength limited. For the path losscalculations, the boat antenna can be considered to be at 3 meters.

These same reflections can, however, be a serious source of interference in high-densityareas when the reflected signal reappears behind the cell site. Downtilt minimizes thisproblem.

Special problems occur when buildings are not built on flat ground and excavation isnecessary. Figure 5.4 shows such a building. The building in Figure 5.4 is, in effect,underground from some directions. For example, the building is clearly underground with

respect to base A. The first two levels probably will not have coverage from that base, butthey will achieve coverage from base B. Many buildings are constructed this way,particularly in hilly cities, and they can make good handheld coverage extremely difficult.

Figure 5.4: Land topography can cause dead spots. This building is partially undergroundfrom some directions. Note that the building may not be underground with respect to baseB, but the first two levels of the building are underground with respect to base A.

Using these guidelines, a central site can be selected according to availability, cost, access,and the site's building potential. Having selected the pivotal site, the next step is to surveythe site to determine its actual coverage. (For more information, see Chapter 6, "RadioSurvey.")

In Figure 5.5, the poor city penetration in the north-easterly and northwesterly directionsimplies significant obstructions in these directions. Additional city sites may be necessaryto remedy this situation. Sites should be selected from a combination of visual siteinspections and map studies to provide continuous coverage.

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Figure 5.5: The surveyed field strength of the first selected site.

WHAT THE CUSTOMER WILL ACCEPT

There have been experiments conducted in the past on conventional wireline systems todetermine just how good the customer's subjective evaluation of the quality of service is. If

a group of subscribers are connected to a switch with access grade of service (GOS) of 0.01(meaning that one call in one hundred will fail to complete because of lack of access toswitching equipment0.01 is a typical value for a suburban subscriber) and that GOS issuddenly reduced to 0.1 (one in ten calls fail), there will be a flood of complaints.However, should the GOS be reduced gradually over a period of months, the subscriberbecomes acclimatized, and the GOS may get as bad as 0.2 (one in five calls fail) or evenworse, before a significant number of customers will complain.

Major system reconfigurations always bring in subscriber complaints. It doesn't reallymatter too much what the change is, it seems as if once the subscribers are aware thatsomething has changed there is an increase in awareness of system performance and the

complaints will come in.

In the experience of the author some small systems that perform dismally (at least one callin five failing to mature for system reasons) have acceptably low levels of customercomplaint. These systems have something in common; they are either single monopolysystems or they are working alongside an equally dismal competitor, so the subscriber hasnothing to compare the performance against. They think that "radio" systems are alwaysthis bad! Poor-performance systems are usually managed by "managers" who don'tunderstand the engineering and assume everything is fine as long as the customers are notcomplaining.

There is just one other factor that needs to be considered when trying to understand thelimits of customers tolerance, and that is that about 50 percent of all calls will fail to matureeven in a good system. This percentage might seem very high, and it is, but consider thenature of the calls:

Calls to mobiles frequently receive the message "that the mobile is unattended orturned off" and in fact it probably is!

Calls to land-line parties are often unanswered.

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Calls made by dialing while driving often result in wrong numbers. Correctly dialed calls (particularly in big cities) often are routed to "This number

has been changed." Overseas calls seem to fail as a matter of course, especially if they are not directed

to the major industrial countries.

Calls are often terminated by the dialer before completion. The called party is busy.

In this environment a badly designed network can often operate without excessivecomplaints for a long time. Subscribers' complaints are no judge of the performance of thenetwork, and good engineering standards should be kept at all times. When a poorlydesigned network finally reaches the stage where the complaints begin to rise significantly,it will be very expensive to reconfigure. Undoubtedly many base stations will requirerelocation, new bases may be needed, and some may become redundant. The single mostcommon problem in a big system will be sites that are too high and so radiate too far. In asmall system the biggest problems may be patchy coverage due to inappropriately located

sites and to overload at some of the sites. New sites are expensive and time-consuming toacquire. If the old sites cannot be reused, they may be difficult to dispose of or may havelong leases that have to be honored. While all this expensive relocation is going on theGOS will drop even further, and the complaints will rise again.

A well-engineered system will be designed to quantifiable performance standards and thesestandards will be met. It is not unusual that very large systems that are well engineeredperform better than much more modest ones. The reason is that the big systems mustbewell engineered to work at all. The operators usually know the importance of goodengineering, and if they don't, they will soon be left behind by their competitors.

Doing It the Hard Way

Today there are many engineers who are "designing" cellular networks with planning toolswho simply don't understand the underlying algorithms used by those tools. As a result, alot of very poor designs are produced. Every design engineer should get some experienceof doing it manually; then they can understand what the design tool is telling them.

MAP STUDIES

The first approximation of coverage is made from a map study or computer prediction.This process is only the first step in a design and should not be confused with the design

itself. Cellular design is highly iterative.

The following approximation methods are recommended:

CCIR. "Recommendations and Reports of the CCIR," 1982, Volume V,Propagation in Nonlonized Media, Report 567-2

Yoshihisa Okumura et al., Review of the Electrical Communication Laboratory,Volume 16, Nos. 910 SeptemberOctober 1968, NTT, Japan

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Computer techniques

The first two methods are essentially based on similar empirical techniques and yieldalmost equivalent (but not identical) results. These methods use a series of curves that werederived from field studies of ranges obtained from transmitters on various frequencies and

from various elevations. They have become classic studies that are widely used. In fact,algorithms derived from these and other similar studies are often used in the computermodels. Their main attractions are simplicity and proven utility. These methods will beused later in the chapter.

The Hata Model

The Hata model is a computerized implementation of the Okumura studies (as detailedabove) in Japan.

COMPUTERIZED TECHNIQUES

Computerized methods using digitized maps can produce a higher degree of accuracy, butare very costly. The accuracy of good computer techniques is still only 6 dB (compared to10 dB for manual methods).

Computer prediction methods normally assume a two-dimensional path between thetransmitter and receiver. In real-life propagation, however, contributions to the far-fieldpattern are made by reflected, scattered, and refracted paths that are in other planes. Wavescattering produces a spatial spectrum that is a complex holographic image of the surfacecausing the scattering. It is generally not practical to take these effects into account.Therefore, there is a limit to the degree to which any calculations can reflect reality.

The detail needed to accurately determine the path profiles is also very large. If a city of2000 km2 is characterized by 100-m 100-m areas, then there are 2000 10 10, or200,000 such areas. A minimum representation would contain two pieces of informationabout each area, namely height above mean sea level and type of terrain (that is, urban,rural, open, or water), and thus would contain a total of 400,000 pieces of information. It isalso desirable to store information about surface clutter (whether man-made, like buildings,or natural, like trees) and its height and distribution. This information can be the mostdifficult to obtain and to update. Moreover, for computer systems to be useful, they mustnot be limited to one city.

Thus, a large digital database and data storage in the high-megabyte range is necessary.The acquisition of these data is costly and is, ultimately, the limiting factor. Unless adatabase is available from other sources (for example, mapping authorities), the cost ofproducing one may be too high to be undertaken by a cellular operator. Most computer-prediction databases lack surface-clutter information and so are of limited use.

Despite the apparent limitations, computerized techniques have found favor among manydesigners, and most major operators and suppliers have an in-house system (which has

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generally been customized). These systems can generally do more than just forecastpropagation. Most can graphically depict composite coverage, which has been derivedfrom the files of a number of individual site predictions. As an extension of this, compositeinterference maps can be produced. These can be particularly useful for gaining anoverview of a system's performance. A number of these forecasting systems are available

for sale. Typically, the cost will be about $100,000 to $200,000. Most will run adequatelyon a high-end PC.

HANDHELDS

The measurement of field strength inside buildings reveals high standard deviations(meaning a large variability in the readings) and is not recommended for practical surveybecause it is difficult to do and does not yield useful results. However, on-site tests withhand-helds, after the installation of bases, can be informative.

Street-level field strengths can be used to characterize levels inside buildings as an

"average" building loss. Losses vary from 10 to 35 dB at ground level and are a function offloor level (decreasing by about 0.5 to 2 dB per floor). The loss measured from window ordoor increases as the building is entered, at a rate of about 0.6 dB per meter. The actual lossdepends on the type of building, its size, and the amount of steel reinforcement used in itsconstruction. Thus, in earthquake-, hurricane-, and tornado-prone areas, losses are likely tobe greater. Tokyo has average building losses of 28 dB, while US cities generally average20 dB.

Some cities were planned with wide streets, most of which run at right angles to each other,with many open areas such as parks and squares. Other cities just grew with roadsfollowing cart tracks. Planned cities offer less impediment to cellular propagation, and

spaces between buildings act as large and fairly efficient waveguides. In these cities,handheld penetration at considerable distance (up to 4 km at 800 or 900 MHz) can beachieved, particularly if the terrain is relatively flat. Propagation along the direction of amajor road can be 6 dB better (at the same distance) than at right angles to that direction. Inunplanned cities, a significant reduction in range can be expected.

For these reasons, it is not possible to have a universally applicable rule. It is advisable tosurvey each new city or region in order to get a clear understanding of the possible rangebefore drawing conclusions from results obtained in other, different regions. Handheldcoverage from 0.4 to 6 km (at 800 or 900 MHz) has been reported for cities of similar sizewith different construction and features. A street-level field strength of 60 dBV/m (70

dBV for GSM 800) will generally be sufficient to ensure adequate handheld coverageinside normal office buildings at ground level.

With their low power outputs, handhelds represent the real challenge of cellular radiofrequency (RF) design and also represent a new environment. Design rules need to accountfor the fact that handhelds are not usually used in high, multipath environments. Becausehandhelds are also the "weakest link" in the cellular path, a system designed for handheldsis generally adequate for vehicle-mounted units.

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Base-Station Sensitivity and Improved Handheld Performance

In many instances it is worthwhile to actively improve the uplink budget by improving thebase-station sensitivity. There are a number of ways to achieve this. First, diversity can beused. Diversity takes advantage of the fact that two different radio paths are likely to have a

fading pattern that is mostly independent. So two antennas separated in space by sufficientdistance (for cellular purposes this usually means a few meters) will receive signals thathave traveled along different paths, and therefore the received signals will have a lowcorrelation. By combining these signals appropriately (using combining diversity, a processwhich sums the two signals in phase), a gain of around 6 dB can be achieved relative to thegain of a single antenna. A simpler form of diversity (switched diversity) simply choosesthe "best" antenna at any given time and switches to it, which will result in anapproximately 3 dB gain.

Another form of diversity that is currently popular is polarization diversity. Because thiscan be done without spatial separation, and two diversity antennas can be built into one

antenna radome, it is very attractive from a clutter point of view. It works on the principlethat a signal that has been refracted or reflected will undergo a polarization change as aresult. Hence signals that arrive in different polarizations have undergone different pathsalong the way, and so will be uncorrelated. The cross-polarized antennas are usuallypolarized at two angles each 45 degrees off the vertical. Gains of about 4.5 dB areobtainable from this technique.

Low-noise amplifiers (LNA), mounted as close as possible to the antenna, will improve thelink budget, typically by about 35 dB, but this may be offset somewhat by higherintermodulation levels. Nevertheless, with good filtering and good (low) noise figureamplifiers significant sensitivity gains can be achieved.

Good quality high-Q pre-selector filters can significantly enhance base-station sensitivity.By reducing adjacent channel interference (and hence reducing the desensitizing effect),quality filtering can enhance the receiver (RX) sensitivity by several dB (the amountdepending greatly on the type and magnitude of the adjacent channel signals).

Smart antennas can significantly improve the received signal level, while at the same timereducing unwanted signals. The net result can be a very significant improvement in theoverall link budget.

A PROGRAM FOR CALCULATING RANGE OR

PATH LOSS

A useful BASIC program to enable the quick calculation of range and path loss can befound in Appendix A. Based on CCIR Report 567-2, the program was written to enabledetermination of coverage from most cellular sites. The range of validity of the algorithm islimited to

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Frequency: 4502000 MHz Base-station effective height: 30200 m Vehicle antenna height: 110 m Range: 120 km Buildings/land: 350 percent

These limits should not cause any problems in the majority of cellular applications. Notethat use beyond these limits (particularly for range) is not advised, as the results will not beaccurate.

GETTING TO KNOW THE TERRAIN

Most problems with cellular systems begin with poor design. Poor design begins withmisuse of computer models. Even the best models need calibration for the actual terrain.Uncalibrated computer models will work best in rugged terrain with plenty of hills andprominent obstructions. They are at their worst when the terrain is flat with a lot of low-

level clutter, such as trees and houses. Frequently problems arise because the designerapplies a model from a previous design to a new area

Figure 5.6: Field strength (dBV/m for 100-W ERP) in an urban area. (Source: CCIRReport 567-2.)

without accounting for the different terrain characteristics. With many years of experiencethe designer can get a feel for the terrain, and should recognize right away if it is unusual ordifferent from what has been encountered before.

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MANUAL PROPAGATION PREDICTION

A number of empirical studies have produced algorithms that can be used to determine thefar-field strength as a function of effective radiated power (ERP) and range.

Figure 5.7: Okumura relationships between field strength and distance for a 150-MHzband.

Generally, these results were produced after an extensive series of propagation tests in oneor, at most, a few countries.

Studies done in different countries (with different terrains) revealed substantial variationsin path attenuation over similar terrain. One of the main sources of discrepancy is in thedescription of topography. A hill in Venice may be compared to a molehill in SanFrancisco!

Notice that Figure 5.6is calibrated forurban areas. For typical suburban areas, the rangewill be improved by a factor of about 7 dB, and for rural areas at least 12 dB. To calibratethe curve, take a few measurements in the area being studied and find a correction factor(in dB) to get the curve to correspond to the measured range. A correction of as much as 28dB can be obtained in very flat rural areas. It should be noted, however, that once the factorhas been applied the curves will yield good values for future studies at different antennaheights and for different ranges without further recalibration.

THE OKUMURA STUDIES

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A landmark study by Yoshihisa Okumura with Ohmori, Kawano, and Fukuda, and printedin the Review of the Electrical Communications Laboratory (Volume 16, Numbers 910,September 1968), was one of the first attempts to quantify, in a statistical way, propagationin the mobile environment for frequencies from 150 MHz to 2000 MHz. The study wasdone in Japan and was conducted from a van, moving at 30 km/h, the results being

measured on recording paper. The equipment had a high sensitivity of 125 dBm, and adynamic range of 50 dB.

Referring to some early attempts to get something definitive from their studies, they reporton page 831: "Some propagation curves hitherto published were not definitive, becausethey took in all sorts of terrain irregularities and environmental clutter haphazardly so werenot very useful for estimating the field strength or service

Figure 5.8: Okumura relationship between field strength and distance for a 450-MHz band.

area adapted to the real situation." What this is saying is that the real world is a mess, andtoo hard to generalize. To get around this they went on to define types of terrain and tocompare like with like. While this is consistent with the reductionist scientific method, the

engineers and computer modelers who came along afterwards to use these results generallyignored the context in which they were derived. It would be fairly rare that the service areaof a cellular radio site would be entirely one terrain type; more likely it would be a mixtureof types, like rolling hills, flat terrain, urban, and plains, with the population density andfoliage varying considerably. Despite the shape of the real world, many computeralgorithms accept only one terrain description for the service area of a site. Theconsequence of using the information out of context is wrong answers!

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Used intelligently (more of this later), and not blindly in a computer model, the results ofthe Okumura study can give some good insights into RF propagation. Figures 5.7 to 5.10 togive the Okumura relationships between field strength (referenced to a transmitter with a 1-kW ERP) and distance in kilometers.

Correction Factors

The Okumura curves refer to the signal level in 50 percent of locations, and is thereforereferring to the median signal level. The field strength at any range is near to being lognormally distributed, and so is virtually symmetrically distributed about the median. In thatcase, the mean is equal to the median. It sho

Documents

The Cellular Radio Handbook