A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering, School of Advanced Technologies

Thesis (M.Eng.) - Asian Institute of Technology, 1998

The delay spread, fading and the Doppler spread in a mobile-radio channel cause a significant degradation of the signal to noise ratio of a wireless communication system. The degradation is prominent when the data rate associated is high as in the case of wireless ATM (W ATM). Proposing a suitable coding and modulation scheme for W ATM to combat the channel distortions is the prime concern of this study. Orthogonal frequency division multiplexing (OFDM) is one of the most promising solutions for high data rate transmissions in a frequency selective radio channels. ATM is designed for transmission links having very low bit error rate. OFDM incorporated with a suitable channel coding scheme can be used to transmit ATM cells, in frequency selective fading environment. The concatenated coding schemes, Reed Solomon (RS) code with trellis coded modulation (TCM) and Reed Solomon code with turbo trellis coded modulation (TTCM) are investigated for fading channels. Signal to noise ratio required to achieve an average bit error rate (BER) of 10-5 was found by simulation. This particular BER was selected as ATM transmission needs high quality transmission links, in which a BER less than 10- 5 can be achieved. Neither TTCM nor TCM can be used for WATM applications alone as their BER performances give rise to a higher error floors. The concatenated schemes investigated here do not have an error floor in fading channels. Both concatenated schemes (TCMIRS & TTCM/RS) investigated have a very high coding gain compared to uncoded case (TCM/RS-8 dB- and TTCMIRS-9 dB at 10-2 BER). TTCM/RS scheme outperforms TCM/RS scheme in frequency selective fading channel, having a 3 dB gain at 10-5 BER. CC(16-QAM, 64 states)/RS scheme showed a performance very close to the TCM/RS performance, having slightly better performance after 15 dB. High peak to average power ratio and the higher sensitivity to frequency offset are the major draw-backs of OFDM. Peak windowing to reduce P APR is studied here. Peak windowing can be used to reduce the P APR by 40% with only a small degradation of BER performance. The maximum PAPR observed was 13.6 dB and it was reduced up to 8 dB with peak windowing giving only about 1.25 dB degradation of BER at 10-5 {u2022} It was also found that the proposed system is highly sensitive to frequency offset. Therefore, it is very important to use a reliable frequency offset estimation and correction scheme with the proposed system.