This paper presents a performance comparison of the channel-interleaving method in the frequency domain, i.e., bit interleaving after channel encoding, symbol interleaving after data modulation, and chip interleaving after spreading, for Variable Spreading Factor-Orthogonal Frequency and Code Division Multiplexing (VSF-OFCDM) wireless access with frequency domain spreading, in order to reduce the required average received signal energy per symbol-to-background noise power spectrum density ratio (Es/N0) and achieve the maximum radio link capacity. Simulation results show that, for QPSK data modulation employing turbo coding with the channel coding rate R=3/4, the chip-interleaving method decreases the required average received Es/N0 the most for various radio parameters and propagation model conditions, where the number of code-multiplexing, Cmux, the spreading factor, SF, the r.m.s. delay spread, σ, the number of multipaths, L, and the maximum Doppler frequency, fD, are varied as parameters. For example, when Cmux=12 of SF=16, the improvement in the required average received Es/N0 from the case without interleaving at the average packet error rate (PER) of 10-2, is approximately 0.3, 0.3, and 1.4 dB for the bit, symbol, and chip interleaving, respectively, in a L=12-path exponential decayed Rayleigh fading channel with σ of 0.043 µsec and fD of 20 Hz. This is because the chip interleaving obtains a higher diversity gain by replacing the chip assignment over the entire bandwidth. Meanwhile, in 16QAM data modulation with R=1/2, the performance of the chip interleaving is deteriorated, when Cmux/SF>0.25, due to the inter-code interference caused by different fading variations over the spreading duration since the successive chips during the spreading duration are interleaved to the separated sub-carriers. Thus, bit interleaving exhibits the best performance although the difference between bit interleaving and symbol interleaving is slight. Consequently, we conclude that the bit-interleaving method is the best among the three interleaving methods for reducing the required received Es/N0 considering the tradeoff between the randomization effect of burst errors and the mitigation of inter-code interference assuming the application of adaptive modulation and channel coding scheme in OFCDM employing frequency domain spreading.

In wireless ATM networks, the condition of the transmission channel fluctuates significantly as a result of many causes such as multipath, shadow phasing, and so forth. Several schemes have been proposed to take care of transmission errors. Some schemes are suitable for favorable channel conditions, while others are more suitable for unfavorable channel conditions. Thus, using a fixed error control scheme regardless of channel conditions is destined to be inefficient. This paper presents a dynamic error control scheme that dynamically selects an appropriate error control scheme according to the conditions of the transmission channel. Using this method, we can decrease both errors and the overhead of redundancy.

In this work, we present an 8-state trellis code for bit interleaved 16-QAM and the BER performance on Rayleigh fading channel is evaluated. We analyze the BER and show that the effective code length and minimum productive distance are also important criterion for code design on bit interleaved system. We design the code by considering not only minimum Hamming distance but also the effective code length and minimum productive distance. As a result, we found that the scheme employing the code achieves good BER performance on Rayleigh fading channel even with the finite interleaving size.