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`SAMSUNG 1021
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`Principles @Prac en.
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`UCR ee
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`1
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`SAMSUNG 1021
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`© 1996 by Prentice Hall PTR
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`A Simon & Schuster Company
`Upper Saddle River, New Jersey 07458
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`ISBN 0-13-375536-3
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`2
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`———ee
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`poor
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`al com-
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`ming,
`use
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`as good
`plex
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`uming
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`iming,
`low,
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`6.10 Diversity Techniques
`Diversity is a powerful communication receiver technique that provides
`wireless link improvementat relatively low cost. Unlike equalization, diversity
`requires no training overheadsince a training sequence is not required by the
`transmitter. Furthermore, there are a wide range of diversity implementations,
`many whichare very practical and provide significant link improvement withlit-
`tle added cost.
`Diversity exploits the random natureof radio propagation by finding inde-
`pendent(or at least highly uncorrelated) signal paths for communication. In vir-
`tually all applications, diversity decisions are made by the receiver, and are
`unknownto the transmitter.
`The diversity concept can be explained simply. If one radio path undergoes
`a deep fade, another independent path may have a strong signal. By having
`more than one path to select from, both the instantaneous and average SNRsat
`the receiver may be improved,often by as much as 20 dB to 30 dB.
`As shown in Chapters 3 and 4, there are two types of fading — small-scale
`and large-scale fading. Small-scale fades are characterized by deep and rapid
`amplitude fluctuations which occur as the mobile movesover distances ofjust'a
`few wavelengths. These fades are caused by multiple reflections from the sur-
`roundings in the vicinity of the mobile. Small-scale fading typically results in a
`Rayleigh fading distribution of signal strength over small distances. In order to
`prevent deep fades from occurring, microscopic diversity techniques can exploit
`the rapidly changing signal. For example, the small-scale fading shown in Figure
`8.1 reveals that if two antennas are separated by a fraction of a meter, one may
`receive a null while the other receives a strong signal. By selecting the bestsig-
`nal at all times, a receiver can mitigate small-scale fading effects (this is called
`antenna diversity or space diversity).
`Large-scale fading is caused by shadowingdueto variationsin both theter-
`rain profile and the nature of the surroundings. In deeply shadowedconditions,
`the received signal strength at a mobile can drop well below that of free space. In
`Chapter 3, large-scale fading was shown to be log-normally distributed with a
`standard deviation of about 10 dB in urban environments. Byselecting a base
`station which is not shadowed whenothers are, the mobile can improve substan-
`tially the average signal-to-noise ratio on the forward link. This is called macro-
`scopic diversity, since the mobile is taking advantage of large separations
`between the serving basestations.
`Macroscopic diversity is also useful at the base station receiver. By using
`base station antennasthat are sufficiently separated in space, the basestationis
`able to improve the reverse link by selecting the antenna with the strongestsig-
`nal from the mobile.
`
`| a
`
`al com-
`
`matched
`matched
`ignificant
`xxtremely
`ictionally
`ist as fast
`distortion
`ition, the
`phase. In
`izer into a
`ess of the
`weshi and
`
`ining pop-
`3.7.2). The
`rk in this
`
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`3
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`virtually any diversity application, although often at much greater cost and com-
`plexity than other diversity techniques.
`
`
`
`Switching
`Logic
`or
`Demodulators
`
`6.10.3 Practical Space Diversity Considerations
`Space diversity, also known as antennadiversity, is one of the most popular
`forms of diversity used in wireless systems. Conventional cellular radio systems
`consist of an elevated base station antenna and a mobile antennaclose to the
`ground. Theexistence of a direct path between the transmitter and the receiver
`is not guaranteed and the possibility of a numberof scatterers in the vicinity of
`the mobile suggests a Rayleigh fading signal. From this model [Jak70], Jakes
`deduced that the signals received from spatially separated antennas on the
`mobile would have essentially uncorrelated envelopes for antenna separationsof
`one half wavelength or more.
`The concept of antenna space diversity is also used in base station design.
`At each cell site, multiple base station receiving antennas are used to provide
`diversity reception. However, since the important scatterers are generally on the
`groundin the vicinity of the mobile, the base station antennas must be spaced
`considerably far apart to achieve decorrelation. Separations on the order of sev-
`eral tens of wavelengths are required at the base station. Space diversity can
`thus be used at either the mobile or base station, or both. Figure 6.12 shows a
`general block diagram of a space diversity scheme [Cox83a].
`
`4. Equal gain diversity
`
`Variable Gain
`
`Figure 6.12
`Generalized block diagram for space diversity.
`
`Space diversity reception methods can beclassified into four categories
`[Jak71]:
`1. Selection diversity
`2. Feedback diversity
`3. Maximal ratio combining
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`4
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`ater cost and com-
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`*the most popular
`ular radio systems
`senna close to the
`r andthe receiver
`3 in the vicinity of
`lel [Jak70], Jakes
`antennas on the
`ona separationsof
`
`ise station design.
`re used to provide
`-e generally on the
`as must be spaced
`n the order of sev-
`pace diversity can
`gure 6.12 shows a
`
`Output
`
`6.10.3.1 Selection Diversity
`Selection diversity is the simplest diversity technique analyzed in section
`6.10.1. A block diagram of this method is similar to that shown in Figure 6.12,
`where m demodulators are usedto provide m diversity branches whosegains are
`adjusted to provide the same average SNRfor each branch. As derived in section
`6.10.1, the receiver branch having the highest instantaneous SNR is connected
`to the demodulator. The antenna signals themselves could be sampled and the
`best one sent to a single demodulator. In practice, the branch with the largest
`(S+N)/N is used,sinceit is difficult to measure SNR. A practical selection
`diversity system cannot function on a truly instantaneous basis, but must be
`designedso that the internal time constants of the selection circuitry are shorter
`than the reciprocalof the signal fadingrate.
`
`6.10.3.2 Feedback or Scanning Diversity
`Scanning diversity is very’ similar to selection diversity except that instead
`of always using the best of M signals, the M signals are scanned in a fixed
`sequence until oneis found to be above a predetermined threshold. This signalis
`then received until it falls below threshold and the scanning processis again ini-
`tiated. The resulting fading statistics are somewhatinferior to those obtained by
`the other methodsbut the advantage with this methodis thatit is very simple to
`implement — only one receiver is required. A block diagram of this methodis
`shownin Figure 6.13.
`
`senna y
`
`Control Comparator
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`
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`
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`
`
`Short-Term
`Average
`
`Preset Threshold
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`ito four categories
`
`Figure 6.13
`Basic form of scanning diversity.
`
`6.10.3.3 Maximal Ratio Combining
`In this methodfirst proposed by Kahn [Kah54], the signals from all of the
`M branches are weighted according to their individual signal voltage to noise
`powerratios and then summed.Figure 6.14 shows a block diagram of the tech-
`nique. Here, the individual signals must be co-phased before being summed
`
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`5
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`Ch. 6 « Equalization, Diversity, and Channel Coding
`332
`(unlike selection diversity) which generally requires an individual receiver and
`phasingcireuit for each antenna element. Maximal ratio combining produces an
`output SNR equal to the sum of the individual SNRs, as explained in section
`6.10.2. Thus, it has the advantage of producing an output with an acceptable
`SNR even when noneof the individual signals are themselves acceptable. This
`technique gives the best statistical reduction offading of any knownlinear diver-
`sity combiner. Modern DSP techniques anddigital receivers are now making this
`optimal form of diversity practical.
`
`Antenna
`
`Figure 6.14
`Maximal ratio combiner.
`
`Adaptive control
`
` SeennpamenelaeereTaree
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`vertical polarization due to hand-tilting when the portable cellular phone is
`
`6.10.3.4 Equal Gain Combining
`In certain cases, it is not convenient to provide for the variable weighting
`capability required for true maximal ratio combining. In such cases, the branch
`weights are all set to unity but the signals from each branch are co-phased to
`provide equal gain combining diversity. This allows the receiver to exploit sig-
`nals that are simultaneously received on each branch. The possibility of produc-
`ing an acceptable signal from a number of unacceptable inputsis still retained,
`and performance is only marginally inferior to maximal ratio combining and
`superiorto selection diversity.
`
`6.10.4 Polarization Diversity
`At the base station, space diversity is considerably less practical than at the
`mobile because the narrow angle of incidentfields requires large antenna spac-
`ings [Vau90]. The comparatively high cost of using space diversity at the base
`station prompts the consideration of using orthogonal polarization to exploit
`polarization diversity. While this only provides two diversity branches it does
`allow the antenna elementsto be co-locate™.
`In the early days of cellular radio, all subscriber units were mounted in
`vehicles and used vertical whip antennas. Today, however, over half of the sub-
`scriber units are portable. This means that most subscribers are no longer using
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`6
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