Bit Interleaving And Bit Allocation In BICM-OFDM Systems

In worldwide telecommunication industry, orthogonal frequency division multiplexing (OFDM) has been a promising solution and applied in many wire-less systems. One of the main advantage is that OFDM offers more efficient spectrum utilization. It divides available bandwidth into multiple parallel nar-rower orthogonal sub-carriers. By introducing a guard interval between consec-utive OFDM symbols, inter-symbol interference can be practically eliminated. Also, it turns a frequency selective channel into a near flat fading channel be-cause groups of sub-carriers lie within the same coherent bandwidth. For future broadband mobile communication it has been intensively studied. On the other hand, bit interleaved coded modulation (BICM) is proved to offer higher di-versity gains for the same coding complexity. It is the key technology in the so-called BICM-OFDM system.In convention, same signal constellation size is applied on all sub-carriers. However, if the transmitter has channel state information (CSI) obtained via a feedback channel, the negative effects of the fading can be further alleviated by an adjustable transmission adapted to the varying channel gain. The main contributions of the dissertation are as follows:1. A novel adaptive interleaving scheme is proposed addressed to the opti-mization of decoding performance in BICM-OFDM. The new scheme has improvement in the following two aspects:(1) It increases the efficiency of Viterbi decoding for BICM application; (2) A more reliable mechanism for interleaving pattern synchronization is included. It avoids the decod-ing floor caused by feedback error propagation. The simulation results show that the proposed scheme has better performance, less overhead and is more robust to the feedback errors.2. Based on CSI (Channel State Information), the approach selects one in- terleaver from a pool of candidate interleavers. As the selected one is ex-pected to have larger MED (Minimum Euclidean Distance) at receiver, the technique can significantly improve the decoding error rate performance. Compared with the adaptive interleaving schemes proposed in literatures, the proposed scheme has no interleaving pattern synchronization problem and requires only a few bits for the signaling.3. We approximate the M-ary-input soft-output channel on each sub-carrier of OFDM as log2 (M) equivalent parallel channels with binary-input soft-output, and two sets of efficient metrics in closed form are defined and derived to indicate the quality of these equivalent channels.4. A new ABL (Adaptive Bit Loading) algorithm is presented by simply selecting the best equivalent channels to load code bits. Simulation results have verified the superior performance to the classical algorithm.5. Regarding to throughput maximization, we define the metrics as effective bit level SNR which provides more accurate indication and direct relation to BER performance. One more ABL proposed hereafter offers as high as 56% more throughput than the conventional in low SNR region.