Error Compensation MechanislTIS

Error Compensation MechanislTIS

The efforts to compensate for the errors and distortions introduced by multipath fading
fall into three general categories: forward error correction, adaptive equalization,
and diversity techniques. In the typical mobile wireless environment, techniques
from all three categories are combined to combat the error rates encountered.
Forward Error Correction Forward error correction is applicable in digital
transmission applications: those in which the transmitted signal carries digital data
or digitized voice or video data. The term forward refers to procedures whereby a
receiver, using only information contained in the incoming digital transmission, corrects
bit errors in the data. This is in contrast to backward error correction, in which
the receiver merely detects the presence of errors and then sends a request back to
the transmitter to retransmit the data in error. Backward error correction is not
practical in many wireless applications. For example, in satellite communications, the

amount of delay involved makes retransmission undesirable. In II obile communications,
the error rates are often so high that there is a high probabib y that the retransmitted
block of bits will also contain errors. In these applicati ns, forward error
correction is required. In essence, forward error correction is ach ved as follows:
1. Using a coding algorithm, the transmitter adds a number 0 additional, redundant
bits to each transmitted block of data. These bits form n error-correcting
code and are calculated as a function of the data bits.
2. For each incoming block of bits (data plus error-correcting code), the receiver
calculates a new error-correcting code from the incoming d ta bits. If the calculated
code matches the incoming code, then the receiver a~ 'umes that no error
has occurred in this block of bits.
3. If the incoming and calculated codes do not match, then OIl or more bits are in
error. If the number of bit errors is below a threshold t at depends on the
length of the code and the nature of the algorithm, it is pos 'ible for the receiver
to determine the bit positions in error and correct all errOl .
Typically in mobile wireless applications, the ratio of total bits sent to data bits
sent is between 2 and 3. This may seem an extravagant amount 01 overhead, in that the
capacity of the system is cut to one-half or one-third of its poVntial, but the mobile
wireless environment is so difficult that such levels of redundan :y are necessary.
Chapter 8 examines forward error correction techniques in detail.