Adaptive Equalization

Adaptive Equalization Adaptive equalization can be apJ lied to transmissions
that carry analog information (e.g., analog voice or video) or di ital information (e.g.,
digital data, digitized voice or video) and is used to combat intI rsymbol interference.
The process of equalization involves some method of gatherinl. the dispersed symbol
energy back together into its original time interval. Equaliz, tion is a broad topic;
techniques include the use of so-called lumped analog circu ts as well as sophisticated
digital signal processing algorithms. Here we give a fla\ r of the digital signal
processing approach.
a common approach using a linear 'qualizer circuit. In this
specific example, for each output symbol, the input signal is sa: pled at five uniformly
spaced intervals of time, separated by a delay T. These sa pIes are individually
weighted by the coefficients Ci and then summed to produce t e output. The circuit is
referred to as adaptive because the coefficients are dynamic lly adjusted. Typically,
the coefficients are set using a training sequence, which is a I nown sequence of bits.
The training sequence is transmitted. The receiver compan s the received training
sequence with the expected training sequence and on the bas of the comparison calculates
suitable values for the coefficients. Periodically, a new raining sequence is sent
to account for changes in the transmission environment.
For Rayleigh channels, or worse, it may be necessary t) include a new training
sequence with every single block of data. Again, this repre ·ents considerable overhead
but is justified by the error rates encountered in a mob e wireless environment.