Turbo equalization for an 8-PSK modulation scheme in a mobile TDMA communication system

An 8-PSK modulation scheme is proposed by ETSI to increase the data rates in a GSM system. The nonbinary symbol alphabet requires an excessively high number of states in a trellis based equalizer. Therefore, sub-optimum equalizers have to be considered. In this paper we investigated a block decision feedback equalizer derived from the minimum mean square error criterion. In order to enhance its performance we incorporate a priori information from the channel decoder. Turbo equalization iteratively detects and decodes coded data transmitted over a frequency selective channel. By feeding back a priori information from the channel decoder this technique enables the equalizer to make less erroneous decisions and provides more reliable soft values to the channel decoder. The simulation environment is based on the GSM system where the payload is modulated by 8-PSK. Simulation results show that the BER at the decoder output can be improved by about 2 to 3 dB using a couple of iterations in a typical urban channel environment.