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Diversity techniques for mobile wireless radio systems

Diversity @ Dictionary

“the presence of a wide range of variation , Including everyone”.

AJAL.A.J Assistant Professor , ECE DEPARTMENT

MET’S SCHOOL OF ENGINEERING - MALA

BY

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Communication - Mobile

TelephoneExchange

SubscriberLine

Inter-ExchangeJunction

Mobile SwitchingCentre

(MSC)

BSC BTS

MS

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HOW DO WE OVERCOME LIMITATIONS IMPOSED BY THE RADIO CHANNEL?

• Flat Fading Countermeasures

– Fade Margin

– Diversity

– Coding and Interleaving

– Adaptive Techniques• Delay Spread Countermeasures

– Equalization

– Multicarrier

– Spread Spectrum

– Antenna Solutions

8C32810.104-Cimini-7/98

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Cultural Dimensions

PERSONAL

CORPORATE

ETHNIC

GENERATIONAL

RELIGIOUS

CLASS

GENDER

REGIONAL

NATIONAL

PROFESSIONAL / EDUCATIONAL

UNITY IN DIVERSITY

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What is Diversity?

• Diversity may refer to: Multiculturalism, the ideology of including people of diverse

cultural and religious backgrounds Diversity (politics), the political and social policy of

encouraging tolerance for people of different backgrounds Diversity (business), the business tactic which encourages

diversity to better serve a heterogeneous customer base Diversity training, the process of educating employees,

students or volunteers to function in a diverse environment Biodiversity, the variation of life forms within a given

ecosystem Diversity scheme, a method for improving the reliability of a

message signal by using multiple communications channels

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DIVERSITY as far as mobile wireless

radio systems are

concerned !

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Common assumption:

• Common assumption:

signals that scatter off of different objects fade independently

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How / when diversity is created ?

• Diversity is created when these signals are separated in the receiver

Examples:

RAKE receiver - - separates paths by delay [PATH DIVERSITY]

Multi beam antenna - - separates paths by angle [ANGLE DIVERSITY]

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Why the name rake receiver ?

• The rake receiver is so named because it reminds the function of a garden rake, each finger collecting symbol energy similarly to how a rake collect leaves.

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Steel Rake

Leaf Rake

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SNOW RAKE

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GARDEN RAKE

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GARDEN RAKE

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Multi-path Energy Capture• In multi-path environments, the RMS delay spreads for a

given channel can be large (14 ns for CM3, 25 ns ).

• Un captured multi-path energy results in loss in performance of the communication device.

• One method for energy collection is to use a RAKE receiver.

SampledMatched-Filter

Output

w1 w2 w3 wN

RAKEoutput

t

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Propagation of Tx Signal

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Multipath

• Multipath occurs when RF signals arrive at a location via different transmission paths due to the reflection of the transmitted signal from fixed and moving objects.

• The combination of the direct and reflected signals most often leads to significant signal loss due to mutual cancellation.

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Rake Receiver – Multipath fading

• Rake receiver mitigates multipath fading effect• Multipath fading is a major cause of unreliable wireless channel

characteristic

x(t)

y(t) = a0x(t)y(t) = a0x(t)+a1x(t-d1)y(t) = a0x(t)+a1x(t-d1)+a2x(t-d2)

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RAKE Receiver: Basic Idea

• The RAKE receiver was designed to equalize the effects of multipath.

• It uses a combination of correlators, code generators, and delays, or “fingers”, to spread out the individual echo signals of the multipath.

• Each signal is then delayed according to peaks found in the received signal.

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Overview of Rake Receiver

• A rake receiver is a radio receiver designed to counter the effects of multipath fading. It does this by using several "sub-receivers" each delayed slightly in order to tune in to the individual multipath components.

• Each component is decoded independently, but at a later stage combined in order to make the most use of the different transmission characteristics of each transmission path.

• This could very well result in higher SNR (or Eb/No) in a multipath environment than in a "clean" environment

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RAKE Receiver Continued

• The same symbols obtained via different paths are then combined together using the corresponding channel information using a combining scheme like maximum ratio combining (MRC).

• The combined outputs are then sent to a simple decision device to decide on the transmitted bits.

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RAKE Receiver Block Diagram

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Maximum Ratio Combining of Symbols

• MRC corrects channel phase rotation and weighs components with channel amplitude estimate.

• The correlator outputs are weighted so that the correlators responding to strong paths in the multipath environment have their contributions accented, while the correlators not synchronizing with any significant path are suppressed.

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End Result of RAKE Receiver

• By simulating a multipath environment through a parallel combination of correlators and delays, the output behaves as if there existed a single propogation path between the transmitter and receiver.

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Another ways to create diversity:

?

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Another way to create diversity:

change the relative phases of the multi pathsignals

• Examples:• Identical antennas, slightly different locations

[SPACE DIVERSITY]• Same signal received on different RF carriers

[FREQUENCY DIVERSITY]. Required carrier separation depends inversely on delay spread

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Still other ways to generate diversity:

• Dual polarized antennas [POLARIZATION DIVERSITY]

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Fading VS Diversity

• Fading

Signal fluctuations caused by multipath propagation and shadowing effects

.• Diversity

Receiving the same information bearing signal over 2 or more fading channels.

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Introduction (cont’d)

• SpaceTransmission using multiple transmit/receive antennas.

• FrequencyTransmission using multiple frequency channels separated by at least the coherence bandwidth.

• Time Transmission using multiple time slots separated by at least the coherence time.

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Spatial diversity

• Single-input, single-output (SISO) channelNo spatial diversity

• Single-input, multiple-output (SIMO) channelReceive diversity

• Multiple-input, single-output (MISO) channel Transmit diversity

• Multiple-input, multiple-output (MIMO) channelCombined transmit and receive diversity

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The following classes of diversity schemes can be identified:

1. Space diversity 2. Polarization diversity 3. Frequency diversity 4. Time diversity 5. Angle or direction diversity

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1] Space diversity

• The signal is transferred over several different propagation paths.

• In the case of wired transmission, this can be achieved by transmitting via multiple wires.

• In the case of wireless transmission, it can be achieved by antenna diversity using multiple transmitter antennas (transmit diversity) and/or multiple receiving antennas (reception diversity).

• In the latter case, a diversity combining technique is applied before further signal processing takes place.

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2] Polarization diversity

• Polarization diversity: Multiple versions of a signal are transmitted and received via antennas with different polarization.

• A diversity combining technique is applied on the receiver side.

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POLARIZER

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3] Frequency diversity

• Frequency diversity: The signal is transferred using several frequency channels or spread over a wide spectrum that is affected by frequency-selective fading.

• Middle-late 20th century microwave radio relay lines often used several regular wideband radio channels, and one protection channel for automatic use by any faded channel. Later examples include: OFDM modulation in combination with subcarrier interleaving and

forward error correction Spread spectrum, for example frequency hopping or DS-CDMA.

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4] Time diversity

• Time diversity: Multiple versions of the same signal are transmitted at different time instants.

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Terrestrial microwave radio system with two antenna arrays configured for space-diversity

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Diversity Combining

• Once you have created two or morediversity channels, what do you do with them?

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Spatial diversity (cont’d)

•Maximum ratio combining (MRC)

h1

h2

h1*

h2*

yx

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Combining schemes:

•Selection combining (SC)

h1

h2

yx

MonitorSNR

Selectbranch

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• Switched diversity Switch-and-stay combining (SSC) Switch-and-examine combining (SEC)

h1

h2

x

ComparatorChannelestimator

switchingthreshold

Without excessively increasing

• 1. transmitting power• 2. Co-channel reuse distance

• One of the most effective technique to solve this issue is diversity reception technique

What are all the needs of diversity techniques??

• 1.No. of signal transmission paths• 2.A circuit to combine the received signal or

select one of them

CONCEPTS OF DIVERSITY BRANCH AND SIGNAL PATH

(a) Branch Construction method using frequency diversity

(b) Branch Construction method using Time diversity

COMBINING AND SWITCHNING METHODS

COMBINING AND SWITCHNINGMETHODS

FOR COHERENT DETECTION

FOR NON - COHERENT DETECTION

FOR COHERENT DETECTION

• There is no difference whether the combining is carried out in the pre detection or in the post detection stage

FOR NON-COHERENT DETECTION

• There is a difference in performance exists between whether pre detection & post detection combining methods

(a) Maximal ratio combining (MRC)

• MRC is the best performance improvement compared with the other methods

• However the trade off is :It requires 1. cophasing , 2. weighting 3.summing circuitsResulting in most complicated implementation

Complexity performance

(b) Equal Gain Combining (EGC)

• EGC is similar to MRC, except the weighting circuits are omitted

• The performance improvement obtained by equal gain combiner is slightly inferior to that of a MRC , since the interference and noise corrupted signal combined with high quality signals

For VHF, UHF and microwave wireless radio applications

• Both MRC and EGC methods are unsuitable• Compared with MRC, EGC methods, the

selection method is more suitable for mobile radio applications, because of its simple implementation

(c) Selection or Switching Combining

• The diversity branch having the highest signal level (or) lowest Pe is selected

• Also, in addition, stable operation is easily achieved , even in the fast multipath fading environments

• But the performance improvement obtained by the selection method is still only slightly inferior to that of MRC method

• Disadvantage of Selection or Switching Combining: The continuous monitoring of the signals requires the same no. of receivers as the no. of diversity branches

• This redundancy can be alleviated by the use of switching or scanning receiver

SWITCHNINGMETHODS

FIXED THERSHOLD VARIABLE THERSHOLD

(a) Switching Methods with Fixed threshold

• The switching from one branch to the other occurs when the signal level falls below a threshold value

• The threshold value is fixed• However this threshold value is not

necessarily the best over the entire service area

FIXED THERSHOLD

(b) Switching Methods with Variable threshold

• Here the threshold value is adjusted dynamically as the vehicle moves

• It includes the feedback using Estimation• But it may introduce the envelope and phase

transients that will reduce the performance improvement

VARIABLE THERSHOLD

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Conclusions

• The diversity is used to provide the receiver with several replicas of the same signal

• Diversity techniques are used to improve the performance of the radio channel without any increase in the transmitted power

• As higher as the received signal replicas are decorrelated, as much as the diversity gain

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Thank you for you patient attention

Mail: ec2reach@gmail.com