In [10], a variable rate adaptive trellis-coded QAM is
discussed, offering lower average BER and higher average
throughput as compared to fixed rate schemes.
In [12], another AMC technique is proposed using M-ary
Phase-Shift Keying (MPSK) modulation, which offers 3–20 dB
gain in BER performance.
Turbo codes [13], which can achieve near-capacity performance
on Additive White Gaussian Noise (AWGN) channels,
have also been proposed in adaptive transmission systems to
further improve performance [14]. Results show that a gain of
about 3 dB can be obtained over an AMC scheme using trellis
coded modulation.
The performance of turbo code in AMC systems depends
heavily on the accurate prediction of the channel condition,
which may be a difficult task given the time-varying nature of
the mobile environment. This is due to the fact that turbo codes
operate close to the channel capacity and thus have steep performance
curves. The sensitivity of turbo codes to prediction
errors may cause the system to produce much less favorable
results than theoretically predicted. With much of the industry
interest in 3G development, it is essential to overcome this
shortcoming and find methods for using turbo codes in AMC
systems under a more realistic environment, where prediction
errors can often occur.