Diversity

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Diversity

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It is a powerful receiver technique that provides link improvement at relatively low cost. It exploits a random nature of the radio propagation by finding the independent signal paths for communication. 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 SNR at the receiver can be improved by around 20 to 30dB.

The two categories
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    Depending upon the type of fading the diversity techniques have been categorized as
1. Microscopic diversity techniques
2. Macroscopic diversity techniques

Macroscopic Diversity techniques:

    These techniques are used in small scale fading environment. They exploit the rapidly changing signal to avoid deep fades. For example: Incase of small scale fading it is known that if the if the 2 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 signal at all times, a receiver can mitigate small-scale fading effects. Space diversity is a macroscopic diversity technique.

Microscopic diversity techniques:

    These techniques are used in large Scale fading techniques. Here we generally select the base-stationwhich not shadowed when others are, the mobile can improve substantially the average signal-to-noise ratio on the forward link.

Types of diversity
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    The basic types of diversity techniques are:

1. Space diversity
2. Time diversity
3. Frequency diversity

Space diversity

This is used at the transmitter or the receiver. Space diversity reception methods can be classified into four types:
1. Selection diversity
2. Feedback diversity
3. Maximal Ratio combining diversity
4. Equal gain diversity

a.Selection Diversity

M Modulators are used to provide m diversity branches whose gains are adjusted to providethe sasme SNR for each branch. The receiver branch having the highest instantaneous SNR is then connected to the demodulator.
Pros: simple to implement
Cons: this system cannot function on a truly instantaneous basis

b.Feedback or Scanning diversity

This is very similar to Selection diversity except that instead of always using he best of M signals , the M signals are scanned in a fixed sequence until one is found to be above the predetermined threshold.this signal is then received until it falls below the threshold and the scanning process is again initiated. Pros: Simple t implement Cons:Statistics obyained by this method is inferior to those obtained from the other methods

c.Maximal Ratio combining

Here all the incoming signals from all the M branches are weighted according to their individual signal voltage to noise power ratios and then summed. All the individual signals must be co-phased before being summed. This requires sn individual receiver circuitry and phasing circuit for each antennas element. Pros: It produces an acceptable SNR compared to othe rtechniques Cons: It has a complicated circuitry

d.Equal gain combining

    Here the branch weights are all set to unity, but the signals from each branch arae co-phased to provide equal gain combining.this allows the receiver to exploit signals that are simultaneously received on each branch.
Pros: Simpler circuitry compred to Maximal Ratio Combining
Cons: Provides inferior results compared to Maximal Ratio Combining.

Time diversity
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    Here we repeatedly transmit information at time spacings that exceed the coherence time of the channel, so that multiple repetetions of the signal will be received with independent fading conditions, thereby providing for diversity.

Frequency diversity
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    Here information is transmitted on more than one carrier frequency.The reson being that the frequencies separated by more than the coherence bandwidth of the channel will not experience the same fading . This is often employed in microwave line of sight links which carry sevral channels in a FDM mode. In practice a 1:N protection switching is provided by a radio liensee, wherein one frequency is is nominally idle but is available on a stand by basis to provide the frequency diversity switching for any one of the N other carriers. When diversity is needed the appropriate traffic is simply switched to the backup frequency.

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