Performance of TCM with concatenated codes over fading channels

Abstract

The performance of digital transmission sysems is highly affected by the phenomenon of fading. Concatenated codes are used to enhance the system performance. However, this technique suffers from the excessive bandwidth expansion. One way to mitigate this effect is to use a trellis coded modulation (TCM) technique as an inner code. The use of TCM reduces the bandwidth requirement and yet gives a very good performance. The performance of outer codes with a TCM encoder/modulator over a slow Rayleigh amplitude distributed channel is evaluated analytically. It is shown that if the Golay code is used as the outer code, coding gain around 30 dB can be achieved. In the case of frequency selective fading, a digital communication system having an equalizer, a trellis-coded modulator and an outer code is simulated. The effect of frequency selective fading is simulated using the simplified three path model and performance curves are obtained. It is shown that an outer code with a TCM encoder/modulator can not be considered alone as a fading countermeasure and adaptive equalization has to be used in the system.