Abstract
In this paper, we study the coded modulation schemes for pre-transformed (PT) Orthogonal Frequency Divi- sion Multiplexing (OFDM) systems, in short PT-OFDM systems. For quasi-static fading channels, we show that the signal to noise ratio (SNR) for different elements in a PT block is constant due to the spreading property of the PT matrix. Based on this observation, we propose trellis coded modulation (TCM) as the better coded modulation scheme for PT-OFDM system compared to bit interleaved coded modulation (BICM). This is further verified by a the BER comparison of coded PT-OFDM systems using different coded modulation schemes through computer simulations. one data packet, consisting of multiple OFDM symbols. While the channel is assumed independent for different data packets. In this paper, we study the performance of different coded modulation schemes for PT-OFDM systems. We show that under quasi-static channel condition, the instantaneous SNR's on all data symbols within the packet are equal. In such a system, TCM achieves better performance than BICM. This observation is further verified through computer simulations. We demonstrate that given FEC codes with the same code rate (2/3) and memory length (k =3 ), PT-OFDM system using TCM achieves better BER performance than BICM- PT-OFDM. We further show that TCM-PT-OFDM performs better than BICM-OFDM system as well for the given code. This paper is organized as follows. In section II, we describe the system structure of the PT-OFDM system with different coded modulation schemes. We also briefly discuss the MMSE detector for PT-OFDM systems. The analysis of instantaneous SNR for the PT-OFDM system and the choice of coded modulation schemes are presented in section III. The simulation results are given in section IV with the concluding remarks drawn in section V.
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