Diversityis a technique used to compensate for fading channel impairments, and is usually implemented by using two or more receiving antennas.Diversity is usually employed to reduce the depth and duration of the fades experienced by a receiver in a flat fading (narrowband) channel.Diversity techniques can be employed at both base station and mobile receivers.
Diversity concept: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 SNRs improve.
Classification of Diversity technique:
Diversity techniques can be classified into four categories:
Space diversity:Space diversity also known as antenna diversity, is one of the most popularforms of diversity used in wireless systems.
- Antenna space diversity is used in base station design.At each cell site, multiple base station receiving antennas are used to provide diversity reception.
Space diversity is classified into four categories:
- Selection diversity: It is the simplest diversity technique
Block diagram is shown in side Figure where m de-modulators are used to provide m diversity branches
Gains of m diversity branches area adjusted to provide the same average SNR for each branch.
Receiver branch having the highest instantaneous SNR is connected to the demodulator. The antenna signals themselves could be sampled and thebest one sent to a single demodulator
- Feedback or Scanning Diversity:Here 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 initiated,block diagram of this method is shown in figure given below.
- Maximal ratio combining:the signals from all of the M branches are weighted according to their individual signal voltage to noise power ratios and then summed.
The individual signals are co-phased before being summed Maximal ratio combining produces anoutput SNR equal to the sum of the individual SNRs
- Equal gain diversity:In this the branch weights are all set to unity but the signals from each branch are co-phased toprovide equal gain combining diversity
- Frequency diversity transmits information on more than one carrier frequency
- Concept behind this technique is that frequencies separated by more than the coherence bandwidth of the channel does not experience the same fades
- Frequency diversity is employed in microwave line-of-sight links.
- 1:N protection switching is provided wherein one frequency is nominally idle but is available on a stand-by basis
- When diversity is needed, the appropriate traffic is simply switched to the backup frequency
Polarization diversity has been used for fixed radio links which vary slowly in time.
- Line-of-sight microwave links, for example, typically use polarization diversity to support two simultaneous users on the same radio channel.
- Since the channel does not change much in such a link, there is little likelihood of cross polarization interference