diversity techniques in mobile communications

PRESENTED BY:- Diwaker Pant me (ece regular)

May 3, 2023 Diversity Techniques By Diwaker Pant 1

Introduction. Need of Diversity. Classification of Diversity. Diversity Techniques. Frequency diversity. Time Diversity. Related Paper. Conclusion. References.


Diversity- is a powerful communication technique that provides wireless link improvements at relatively low cost.

Diversity exploits the random nature of radio propagation by finding independent signal path for communication.

These independent paths are highly uncorrelated.


If one radio path undergoes a deep fade another If one radio path undergoes a deep fade another independent path may have a strong signal.independent path may have a strong signal.


Two criteria are necessary to obtain a high degree of improvement from a diversity system are :

First, the fading in individual branches should have low cross-correlation or highly uncorrelated.

Second, the mean power available from each branch should be almost equal.


Macro diversity: provides a method to mitigate the effects of shadowing , as in case of Large scale fading.

Micro diversity: provides a method to mitigate the effects of multi-path fading as in case of small scale fading.


Large scale fading is caused by shadowing due to the presence of fixed obstacles in the radio path.

Long term fading can be mitigated by macroscopic diversity (apply on separated antenna sites) like the diversity using two base stations


Small scale fades are characterized by deep and rapid amplitude fluctuations which occur as the mobiles moves over distances of just a few wavelengths.

These fades are caused by multiple reflections from surroundings in the vicinity of the mobile.

Short term fading can be mitigated by the diversity using multiple antennas on the base station or mobile unit.


Space Diversity: Using antennas spaced enough (at Tx or Rx).

Polarization Diversity:Using antennas with different polarizations.

Frequency Diversity:Using frequency channels separated in frequency more than the channel coherence bandwidth.

Time Diversity:Using time slots separated in time more than the channel coherence time.


Use more than one antenna to receive the signal.


Polarization diversity uses antennas of different polarizations i.e. horizontal and vertical.

The antennas take advantage of the multipath propagation characteristics to receive separate uncorrelated signals

SNR is improved by up to 12 dB even in line-of-sight channels.


Frequency diversity is implemented by transmitting same information on more than one carrier frequency.

Our aim is to make these carrier frequency uncorrelated to each other, so that they will not experience the same fades.

To make them least correlated, these carrier frequencies are separated by more than the coherence bandwidth of the channel.


Theoretically if the channels are uncorrelated , then the probability of simultaneous fading will be the product of the individual fading probabilities.

Frequency diversity is often employed in microwave line –of-sight links.

These links uses Frequency division multiplexing mode(FDM).


In Practice 1:N protection switching is used as shown below.


Transmitter

Receiver

1

N

2....

Idle(Backup Frequency)

Normal Frequency

Normal Frequency

Protection switching is provided by a radio licensee.

In this case one frequency is nominally idle but is available on a stand by basis to provide frequency diversity switching for any one of the other N carrier.

When diversity is needed , the appropriate traffic is simply switched to backup frequency.


FH-SS is a special case of frequency diversity.

New OFDM modulation and access techniques exploit frequency diversity.

This can be achieved by providing simultaneous modulation signals with error control coding across a large bandwidth.

If a particular frequency undergoes a fade , the composite signal will still be demodulated.


Time diversity repeatedly transmits information at the time spacing that exceeds the coherence time of the channel.

Multiple repetitions of the signals will be received with independent fading conditions, thereby providing diversity.

Our modern implementation of time diversity involves the use of RAKE receiver for spread spectrum CDMA.



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

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" called fingers.

The rake receiver was patented in the US in 1956 by “Price and Green”.

Each correlator detects a time-shifted version of the original transmission, and each finger correlates to a portion of the signal, which is delayed by at least one chip in time(1/Rc) from the other fingers.

This will result in higher SNR (Eb/N0) in a multipath environment than in a "clean" environment.


http://en.wikipedia.org/wiki/Eb/N0




Multipath component appears like uncorrelated noise at a CDMA receiver and equalization is not required.

The outputs of each correlator are weighted to provide better estimate of the transmitted signal than is provided by a single component.

The weighting coefficients are based on the power or the SNR from each correlator output.

o If the power or SNR is small out of a particular correlator, it will be assigned a small weighting factor or vice versa.



Interleaving is used to obtain time diversity in the digital communication system.

Interleaving is a way to arrange data in a non-contiguous way to increase performance.

It is typical for many speech coders to produce several important bits in succession.

The function of interleaver is to spread these bits out in time so that if there is a deep fade or error burst, the important bits from a block of source data are not corrupted at the same time.


At the receiver , de-interleaver is used.

Convolutional interleaver can be used in place of block interleaver and ideally suited for use with convolutional codes.

Jin-Tao Wang, Zhi-Xing Yang, Chang-Yong Pan, Jian Song and Lin Yang, “Design of Space-Time-Frequency Transmitter Diversity Scheme for TDS-OFDM System”, IEEE Transactions on Consumer Electronics, Vol. 51, No. 3, AUGUST 2005 .


Abstracto To improve the performance of the Time Domain Synchronous - Orthogonal

Frequency Division Multiplexing (TDS-OFDM) system, an efficient space-time-frequency coding scheme is proposed. The block coding is employed in both time and frequency dimensions simultaneously and proved to be a quasi-orthogonal design. The performance improvement of TDS-OFDM system with this method is theoretically analyzed and computer simulated, showing its superior performance in the presence of both time and frequency selective environments. Comparing to the pure space-time or space-frequency coding scheme with straightforward implementations, the proposed implementation scheme reduces the computational load by about 70% in TDS-OFDM system.


Recently, transmitter diversity has been extensively studied to combat the detrimental effects in wireless fading channels because of its relative simplicity in implementation.

One attractive approach to the transmitter diversity is the space-time block coding scheme in which a very simple maximum-likelihood (ML) decoding algorithm is used at the receiver.

Space-time block codes (STBCs) from orthogonal designs can achieve full diversity.

The main characteristic of these codes is the orthogonality.


A design of space-time-frequency transmitter diversity scheme is proposed in this paper and its effectiveness is demonstrated using the Time Domain Synchronous - OFDM (TDS-OFDM) system as an example.


Fig. (A) Diagram of the space-time-frequency diversity method for TDS-OFDM system

The BER performance of the introduced STFC-OFDM method for TDS-OFDM system was further verified by computer simulations.

The guard interval was set to 1/9 of the length of the frame body.

The simulation system used rate 2/3 Convolutional code and QPSK modulation on each subcarrier.



channel A is a six-ray model from the Brazil digital television test report, channel model B is constructed by replacing the sixth ray with a 0dB echo, representing a channel with severe frequency selective fading.


The BER performance comparison among the STC-OFDM, SFC-OFDM and STFC-OFDM systems under channel A without Doppler shift.


The BER performance comparison among the STC-OFDM, SFC-OFDM and STFC-OFDM systems under channel B without Doppler shift.


To improve the transmission performance of TDS-OFDM system developed for digital television broadcasting, an efficient STFC-OFDM transmitter diversity scheme is introduced and the performance improvement is both theoretically analyzed and computer simulated.

STFC-OFDM scheme is proved to be essentially a quasi-orthogonal design. The block coding is employed in both time and frequency dimensions simultaneously. Compared with the conventional STC-OFDM and SFC-OFDM systems, this scheme performs better in both time and frequency selective channels because the channel condition assumption is much more close to the reality.


Diversity is an extremely powerful technique for improving the quality of communication systems and it is easy to achieve gains equivalent to power savings in excess of 10 dB.

These gains are achieved at the expense of extra hardware, particularly in terms of extra antennas, bandwidth and receivers, which must be balanced against the benefits.

The key requirements for achieving the maximum benefit are that the multiple branches of the system should encounter substantially equal mean powers and near-zero cross-correlation of the fading signals.

Wireless Communications “Principles and Practice” by Theodore S. Rappaport. (2nd edition).

http://en.wikipedia.org/wiki/Rakereceiver.

J.T. Wang et al, “Design of Space-Time-Frequency Transmitter Diversity Scheme for TDS-OFDM System”. IEEE Transactions on Consumer Electronics, Vol. 51, No. 3, AUGUST 2005 .


http://en.wikipedia.org/wiki/Rakereceiver

May 3, 2023Diversity Techniques By Diwaker

Pant 36

diversity techniques in mobile communications

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