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Digital Bridge Institute Page 1

DAY 5

Microwave Diversity Techniques

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Introduction to DiversityTypes of Diversity

Frequency DiversityTime DiversitySpace Diversity

Polarization DiversityConvergence and Fixed ServicesEconomics of Wired versus Wireless Systems

Day 8: Microwave Diversity Techniques

RF/Microwave Systems : PDD

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Fading in Wireless CommunicationsTypes of fading depend on

Symbol period vs. Coherent delay spread Coherent time

Transmission BW vs. Doppler spread Coherent BW

Spatial location of antenna vs. angle spread Coherent distance

Fading effects

BER and capacity

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Fade Mitigation Techniques

EqualizationDigital Signal Processing Techniques (DSP)technique used to compensate for IntersymbolInterference (ISI) created by multipath fadingwithin time dispersive channels

Channel Coding To improve mobile communication linkperformance by including redundant bits in thetransmitted message

Diversity To reduce the depth and duration of fadesexperienced by a receiver in a frequency or time

selective channel

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Diversity TechniquesDiversity is the primary technique used to improveperformance on a fading channel.Diversity is a process of obtaining multiple independent signalthrough many dimensions

space, frequency, time, polarization, etc.)

Main IdeaProvide the Rx with multiple versions of the same transmittedsignal over independent channels or pathsSend the same information over independent fading pathsThe probability of all signals being faded will be less than theprobability that just one is faded!

i.e., multiple paths unlikely to fade simultaneouslyDiversity overcomes effects of fading by combining path signals

At Rx, combine some or all the paths to mitigate fading effects

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Why Diversity?

Diversity works on the notion that two heads is better than one Diversity schemes provide 2 or more inputs at the Rxsuch that fading among these inputs are uncorrelated

If one radio path undergoes a deep fade at a particular point in time, another independent (or at least highlyuncorrelated) path may have a strong signal at thatpoint

By having more than one path to select from, both theinstantaneous and average SNR at the Rx may beimproved often as much as by 20 to 30 dB

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Types of Diversity TechniquesSpace diversity (also called Antenna Diversity) :

Multiple antennas are used to transmit/receive the signal Antenna spacing must be such that fading at each antenna isindependent (coherence distance), i.e., multiple of

There are several types of space diversities

1) Receive Diversity (SIMO):Multiple antennasdeployed at the receiveside

Rx Tx

Tx Rx

Base Station Mobile station

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Why Multiple AntennasFrequency and time processing are at limitsSpace processing is interesting because it does not increasebandwidth

Phased arrayrange extension,

interference reduction

Adaptive Antennasinterference cancellation

MIMO Systems(diversity)

Specular channels Scattering channels

IndoorOutdoor

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In downlink,Receive diversity isdifficult to implementRequires multipleantennas andadditional processingat the mobile stationNot suitable due tosize and batterypower limitation atmobile

Put additionalprocessing andcomplexity at the basestation => TX-Diversity

Not suitable due to size andbattery power limitation atmobilePut additional processingand complexity at the basestation => TX-Diversity

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2) Transmitter Diversity (MISO):Multiple antennas deployed at the transmitter side

TxRx

Tx Rx


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Tx Rx

Tx Rx


Convert flat fading channel intentionally into a frequency selective channelTransmission of same information from M antennas simultaneously with differentdelays

3) Time diversity (Delay diversity)Main idea:

Provide diversity benefit byintroducing intentional multipath

Transmit the original signalfrom the first antenna anduse delayed signals in thesubsequent antennasReceiver uses an equalizer or MLSE equalizer or MLSEfor detectionNo loss of BW efficiency

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Diversity order:Obtains an M- th order diversity with the optimum decoder

Coding point of view:Delay diversity corresponds to repetition code of length M

Constraint : Inserted delay < smaller than RMS delay spread Not a good scheme, especially when channel has higher

delay spread

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4) Space Time (S-T) Diversity:Utilize spatial and temporal domain for obtaining diversity

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Space Diversity Antenna Configurations

Tx

Tx

Rx

Rx

Rx Tx

Tx Rx


Single-Input-Single Output

Single-Input-Multiple Output

Multiple-Input-Single Output

Multiple-Input-Multiple Output(MIMO)

(MISO)

(SIMO)

(SISO)

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Advantages of Multi-AntennaPerformance improvements achieved by multi-antennasystems are mainly due to:

Array gainIncrease power Beamforming

Diversity gainMitigate fadingSpace-time coding

Spatial multiplexing gain

Multiply data ratesSpatially orthogonal channels

Interference reduction Aggressive frequency reuse

Users spatial signatures

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5) Space-Frequency (S-F) Diversity:Orthogonal Frequency Division Multiplexing (OFDM) systemspecific diversity technique by exploiting sub-carriers andspatial domain.

6) Space-Time-Frequency (S-T-F) Diversity: Another OFDM system specific diversity technique

7) Transmit-Receive Diversity (MIMO-Diversity):S-T diversity at the Tx and space diversity at the receiver

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Frequency DiversityTransmission of same signal at different frequencies

Use different frequency carriers separated by adistance larger than the coherence bandwidth of thechannel.Not bandwidth-efficient (Bw Expansion).Not effective on flat (non-frequency selective)channelsTechniques that exploit frequency diversity include:

RAKE Receivers, OFDM, equalization

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Time DiversityTransmission of same signal sequence at different

timeUse different time slots separated by interval longer than coherence time of the channelChannel coding plus interleaving is used to providetime diversityNot effective over slow fading channels

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Multipath DiversityDelay and Doppler discrimination using RAKE

receivers

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Polarization DiversityUses two antennas with different polarization for

reception and/or transmission

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Field Component Diversity Antenna pattern diversity

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Angle DiversityDirection diversity (Angle discrimination)

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Diversity Combining Techniques A diversity system combines the independent fading paths toobtain a resultant signal which is then passed through astandard demodulator

The combining can be done in several ways which vary incomplexity and overall performanceWe will use space diversity as a reference to describe thedifferent combining techniques, although the techniques canbe applied to any type of diversity

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Thus, the combining techniques will be defined as operationson an antenna array.

Most combining techniques are linear:the output of the combiner is just a weighted sum of thedifferent fading paths or branches, as shown below

For ideal coherent detection, the weighting can be performed

either before detection (pre-detection) or after detection (post-detection) with essentially no difference in performance.There is a slight performance degradation in using pre-detection combining for differentially coherent detection.Usually combining is performed post-detection, since thebranch signal power and/or phase is required to determine theappropriate weights.Post-detection combining requires a dedicated receiver for each branch, which increases the hardware complexity and

cost, particular for a large number of branches.

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In post-detection combining is also perform co-phasing, so that eachsignal path entering the summer is just a real number

The main purpose of diversity is to combine the independent fadingpaths to mitigate the effects of fadingThe signal output from the combiner equals the original transmittedsignal s(t) multiplied by a random complex amplitude term that

results from the path combiningThis complex amplitude term results in a random SNR at thecombiner output, where the distribution of is a function of thenumber of diversity paths, the fading distribution on each path, andthe combining technique

Since the combiner output is fed into a standard demodulator for thetransmitted signal s(t), the performance of the diversity system interms of P b and P out is given as

eP Q f d

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Types of Diversity CombiningThe 3 most prevalent space diversity combining techniquesare selection diversity (SD), equal gain combining (EGC), and

maximum ratio combining (MRC)Selection Diversity (SD)

In SD, the strategy is to choose the branch with the highestSNR or more practically, the highest S + N, since the noiseN is assumed to be the same on all branches

Fading path with highest gain selectedCombiner output SNR is the maximum of the branchSNRs

the path output from the combiner has an SNRequal to the maximum SNR of all the branches

CDF easy to obtain, pdf found by differentiatingCan get up to about 20 dB of gain

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Equal Gain Combining (EGC) All paths cophased and summed with equal weighting A simpler technique is equal-gain combining, which isessentially a maximal-ratio combiner with all of the gains = 1.The SNR of the combiner output, assuming equal noisepower in each branch, is then given by

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Maximal Ratio Combining (MRC)In MRC the output is a weighted sum of all branches

All paths are cophased and summed with optimal weightingto maximize combiner output SNROptimal technique (maximizes output SNR)

Combiner SNR is the sum of the branch SNRs.Distribution of SNR hard to obtain.Exhibits 10-40 dB gains in Rayleigh fading.

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Orthogonal Frequency DivisionMultiplexing (OFDM)

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Basic Definitions A block modulation scheme where data symbols aretransmitted in parallel using large number of orthogonal sub-carries

A block of N serial data symbols (each w/duration T s ) isconverted to N parallel data block (each with duration T=NT s )Choose NT s >> N parallel data symbols modulate the N sub-carriers

Increase symbol duration leads to reduction in distortion (ISI)

Serialto

ParallelConverter

1

N

N-1

2

N R

T

12cos f t p

1 N

22 f t cos p

12 N f t cos p

2 N f t cos p

1 R

T

1 R

T

1 R

T

1 R

T

x(t)

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OFDM increase spectral efficiency by allowing overlapping

The sub-carriers are spaced apart by 1/T Hz

Each block of data is modulated by a separate carrier signalw/freq.

f f

0 f

1f

2 f

3 f

4 f

5 f

6

f D

0n

n f f

T

1

1n n f f f

T

D

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How do we separate the sun-channels at the receiver?

1

N

N-1

2

12cos f t p

22 f t cos p

12 N f t cos p

2 N f t cos p

s t S P SM

1 x

2 x

1 x N x N

x n RF x t

1

N

nn

x Re x exp j t t n

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OFDM TransmitterUsually QAM signals are used

S/P

d(1)

d(N-1)

12cos f t p

22 f t cos p

12 N f t cos p

2 N f t cos p

S x( t )M

QAM

Encoder

d( n )

d(2)

RFD(t)

d(N)

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Advantages of OFDMIncreased efficiency because carrier spacing is reduced (orthogonalcarriers overlap)Equalization simplified, or eliminatedMore resistant to fadingData transfer rate can be scaled to conditions

Single Frequency Networks are possible (broadcast application)Now possible because of advances in signal processing horsepower

Disadvantages of OFDMHigher Peak-to-averageMore sensitive to phase noise, timing and frequency offsetsGreater complexityMore expensive transmitters and receiversEfficiency gains reduced by requirement for guard interval

h d

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Diversity Techniques Based on LocationsReceiver Diversity (SIMO):

Usually space diversityTransmitter Diversity (MISO) :

Time diversity (Delay diversity)S-T Diversity

S-F DiversityS-T-F Diversity

Transmit-Receive Diversity (MIMO-Diversity):S-T diversity at the transmitter and space diversity at the

receiver

Ti Di i i OFDM S

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Time Diversity in OFDM SystemsDelay Diversity (DD):

Transmit the original signal from the first antenna and usedelayed signals in the subsequent antennasConstraint:

Inserted delay must be smaller than RMS delay spread

Not a good scheme, especially when channel has higher delay spread. In those cases, the affectivity of CP isreduced and the performance may be worse in certainscenario.

MIMO S

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MIMO SystemWhat is a MIMO System?

A MIMO system consists of several antenna elements, plusadaptive signal processing, at both transmitter andreceiver, the combination of which exploits the spatialdimension of the mobile radio channel.

Benefitshigher capacity (bits/s/Hz):

spectrum is expensive; number of BS is limitedbetter transmission quality

Increased coverageImproved user position estimation

N b d MIMO Ch l M d l

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Narrowband MIMO Channel Model

Frequency flat channelmodel

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