`
`Principles @Prac en.
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`CaaS
`CoDSeeeee
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`APPLEINC./ Page 1 of 6
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`UCR ee
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`Ex.1019
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`Ex.1019
`APPLE INC. / Page 1 of 6
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`

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`Ex.1019
`APPLE INC. / Page 2 of 6
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`

`

`1el Coding
`
`Diversity Techn iques
`
`325
`
`6.1 O Diversity Techniques
`Diversity is a powerful communication receiver technique that provides
`wireless link improvement at relatively low cost. Unlike equalization, diversity
`requires no training overhead since a training sequence is not required by the
`transmitter. Furthermore, there are a wide range of diversity implementations,
`many which are very practical and provide significant link improvement with lit(cid:173)
`tle added cost.
`Diversity exploits the random nature of radio propagation by finding inde(cid:173)
`pendent (or at least highly uncorrelated) signal paths fo~ communication. In vir(cid:173)
`tually all applications, diversity decisions are made by the receiver, and are
`unknown to 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 t1'e instantaneous and average SNRs at
`the receiver may be improved, often by as ~uch 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 moves over distances of just a
`few wavelengths. These fades are caused by multiple reflections from the sur(cid:173)
`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
`3.1 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 best sig(cid:173)
`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 shadowing due to variations in both the ter(cid:173)
`rain profile and the nature of the surroundings. In deeply shadowed conditions,
`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. By selecting a base
`station which is not shadowed when others are, the mobile can improve substan(cid:173)
`tially the average signal-to-noise ratio on the forward link. This is called macro(cid:173)
`scopic diversity, since the mobile is taking advantage of large separations
`between the serving base stations.
`Macroscopic diversity is also useful at the base station receiver. By using
`base station antennas that are sufficiently separated in space, the base station is
`able to improve the reverse link by selecting the antenna with the strongest sig(cid:173)
`nal from the mobile.
`
`poor
`
`al com-
`
`m in g,
`use
`
`:1.s good
`1plex
`
`Lming
`
`1ming,
`low,
`
`1al com-
`
`matched
`matched
`ignificant
`)Xtremely
`zctionally
`LSt as fast
`distortion
`1ition, the
`phase. In
`lzer into a
`.ess of the
`1reshi and
`
`ining pop(cid:173)
`i. 7.2). The
`,rk in this
`
`Ex.1019
`APPLE INC. / Page 3 of 6
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`

`330
`
`Ch. 6 • Equalization, Diversity, and Channel Coding
`
`virtually any diversity application, although often at much greater cost and com(cid:173)
`plexity than other diversity techniques.
`
`6.10.3 Practical Space Diversity Considerations
`Space diversity, also known as antenna diversity, 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 antenna close to the
`ground. The existence of a direct path between the transmitter and the receiver
`is not guaranteed and the possibility of a number of 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 separations of
`one half wavelength or more.
`The concept of antenn~ 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
`ground in the vicinity of the mobile, the base station antennas must be spaced
`considerably far apart to achieve decorrelation. Separations on the order of sev(cid:173)
`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].
`
`Switching
`Logic
`or
`Demodulators
`
`Output
`
`Antenna
`
`Variable Gain~----~
`
`Figure 6.12
`Generalized block diagram for space diversity.
`
`Space diversity reception methods can be classified into four categories
`[Jak71]:
`1. Selection diversity
`2. Feedback diversity
`3. Maximal ratio combining
`4. Equal gain diversity
`
`Ex.1019
`APPLE INC. / Page 4 of 6
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`

`

`y, and Channel Coding
`
`1ter cost and com-
`
`'the most popular
`1.lar radio systems
`;enna close to the
`r and the receiver
`3 in the vicinity of
`lel [Jak70], Jakes
`antennas on the
`rma separations of
`
`tse station design.
`,e used to provide
`:e generally on the
`1s must be spaced
`n the order of sev(cid:173)
`pace diversity can
`gure 6.12 shows a
`
`Diversity Techniques
`
`331
`
`6.10.3.1 Selec.tion 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 used to provide m diversity branches whose gains are
`adjusted to provide the same average SNR for 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) I N is used, since it is difficult to measure SNR. A practical selection
`diversity system cannot function on a truly instantaneous basis, but must be
`designed so that the internal time constants of the selection circuitry are shorter
`than the reciprocal of the signal fading rate.
`
`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 one is found to be above a predetermined threshold. This signal is
`then received until it falls below threshold and the scanning process is again ini(cid:173)
`tiated. The resulting fading statistics are somewhat inferior to those obtained by
`the other methods but the advantage with this method is that it is very simple to
`implement -
`only one receiver is required. A block diagram of this method is
`shown in Figure 6.13.
`
`Output
`
`- - - - - - - - - - - - - 9gJ!t.r<.~1- - Comparator
`
`Preset Threshold
`
`1to four categories
`
`Receiver
`
`Figure 6.13
`Basic form of scanning diversity.
`
`Short-Term
`Average
`
`6.10.3.3 Maximal Ratio Combining
`In this method first proposed by Kahn [Kah54], the signals from all of the
`M branches are weighted according to their individual signal voltage to noise
`power ratios and then summed. Figure 6.14 shows a block diagram of the tech(cid:173)
`nique. Here, the individual signals must be co-phased before being summed
`
`Ex.1019
`APPLE INC. / Page 5 of 6
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`

`332
`
`Ch . 6 • Equalization, Diversity, and Channel Coding
`(unlike selection diversity) which generally requires an individual receiver and
`phasing circuit 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 none of the individual signals are themselves acceptable. This
`technique gives the best statistical reduction of fading of any known linear diver(cid:173)
`sity combiner. Modern DSP techniques and digital receivers are now making this
`optimal form of diversity practical.
`
`Cophase
`and
`
`Sum
`
`Detector
`
`Output
`
`Adaptive control
`
`Figure 6.14
`Maximal ratio combiner.
`
`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(cid:173)
`nals that are simultaneously received on each branch. The possibility of produc(cid:173)
`ing an acceptable signal from a number of unacceptable inputs is still retained,
`and performance is only marginally inferior to maximal ratio combining and
`superior to 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 incident fields requires large antenna spac(cid:173)
`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 elements to be co-locate..J.
`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(cid:173)
`scriber units are portable. This means that most subscribers are no longer using
`vertical polarization due to hand-tilting when the portable cellular phone is
`
`Ex.1019
`APPLE INC. / Page 6 of 6
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