| Orthogonal frequency division multiplexing (OFDM) is a transmission technique which divides the data into a number of independent frequency subchannels whose bandwidth is much less than the total data rate. The OFDM technique has many advantages in wireline and wireless communications and is used in many practical systems. One of the advantages of the OFDM technique is robustness against the inter-symbol interference caused by multipath channels.; If the channel is frequency selective, the error rate of each subchannel can be different. One of the transmission schemes to overcome the frequency selectivity is coded OFDM. In a coded OFDM system, data are encoded by a channel coder, interleaved and divided into several frequency subchannels. Due to the channel coding and interleaving, frequency diversity can be archived. This coded OFDM is used in practical communication systems including Wireless LAN. However, the power allocated to the nulls of the frequency response is likely to be wasted in a coded OFDM system. Therefore, if the power is allocated to the best subchannels, the performance of a coded OFDM can be improved.; The FER and BER of a coded OFDM system are bounded by sums of exponential functions or by sums of Q-functions. In this paper, these bounds are shown to be convex functions. Therefore, power allocation to minimize the target BER or FER is shown to be a convex optimization problem with a total transmission power constraint. With the power allocation obtained from the optimization, the performance of a coded OFDM system is improved. The proposed scheme provides a near-optimum power allocation for a given channel coding, interleaver and modulation. In this dissertation, we also present a joint coded bit and power allocation for a coded OFDM system with bit-interleaved coded modulation and a soft-decision decoder.; The performance of the proposed allocation schemes is simulated with a simple OFDM system in static and dynamic channel environments. From the simulation and analytical results, we show that the proposed allocation schemes provide performance gain over conventional coded OFDM systems. |
