Equalization, Diversity and RAKE receivers
Equalization is a technique that compensates for ISI created by multipath in frequency selective, time dispersive channels.
If signal bandwidth exceeds coherence bandwidth of channel, ISI occurs and pulses are spread in time causing bit errors.
An equalizer compensates for amplitude and delay distortions introduced by the channel.
Equalizer is a digital filter used to remove the effects of ISI introduced by a time dispersive channel.
In frequency selective channels, an equalizer: enhance frequency components with small amplitude and attenuates frequency components with large amplitude.
In time varying channel, an adaptive equalizer is designed to track the channel variations so that the inverse channel equation is satisfied.
Two equalization methods:
– Linear equalization: output of the decision maker is not used in the feedback path to adapt the equalizer
– Nonlinear equalization: output of the decision maker is used in the feedback path to adapt the equalizer
Algorithm for equalization to determine filter coefficients:
1. Zero forcing:
– force equalizer output to go to zero at all sample points to either side of the desired peak output.
– it is designed to remove ISI completely
– enhances noise after the equalizer at frequencies where the channel has high attenuation.
2. Minimizing of Mean Square Error:
– calculate filter tap weights that will minimize the mean square error between the output of the decision circuit and output of the equalizer.
– MSE equalizer minimizes noise enhancement as compared to ZF equalizer, but at the cost of not totally removing the ISI
– it is a technique to combine several copies of the same message received over different channels
– main purpose is to extract and merge signals from the independent fading paths so that the effects of fading are mitigated.
– multiple signal copies must be uncorrelated or weakly correlated, i.e. < 0.5
– at any given moment of time, pick up the branch with the largest SNR.
Techniques of diversity combining:
– selection combining: select with highest SNR
– maximal ratio combining: weighted sum of signals of all branches
– equal gain combining: all branch weight set to unity but co-phased
Types of diversity:
1. space diversity: fading is minimized by simultaneous use of two or more physically separated antennas.
2. Time diversity: transmitter repeatedly transmits information at a time spacing that exceeds the coherence time of the channel
3. Frequency diversity: messages are transmitted simultaneously at two or more different frequencies.
4. Polarization diversity: pairing two complementary polarization to improve performance
5. Directional diversity: messages are captured from various directions and angles.
– an individual signal received by a RAKE finger may be too weak to produce a correct result