THESIS FOR THE DEGREE OF MASTER OF SCIENCE Performance Analysis and Comparison of OFDM Based Packet Transmission System with QAM Modulation

Nowadays, mobile radio communication systems are mainly affected by frequency and time selective radio channel due to multipath propagation and Doppler shifts. For combating these two effects of radio channels, an OFDM based system that is with fixed and adaptive order QAM schemes and TFL-CDMA technique will be described and studied in this thesis. OFDM technique can remove the inter-symbol-interference that comes from delay difference of multipath channels by inserting a guard interval into each OFDM symbol. TFL-CDMA, named Time-Frequency Localized CDMA, can overcome the multipath effect due to diversity and user separation from the placing codeword-chips in a rectangular block over a time-frequency-bin. QAM scheme can effectively improve the performance when transmission over a flat fading channel, especially an adaptive QAM scheme can gain the best orthogonal time-frequency resource utilization by using channel prediction and feedback of this information to the scheduler. Therefore, an OFDM based system combined with the fixed or adaptive QAM scheme and TFL-CDMA technique is proposed. From simulation results, it is found that the proposed OFDM based adaptive QAM system outperforms other competing system on spectral efficiency, such as OFDM based fixed QAM system without or with TFL-CDMA chip mapping technique.