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Performance Comparison of Channel Interleaving Methods in Frequency Domain for VSF-OFCDM Broadband Wireless Access in Forward Link
Noriyuki MAEDA Hiroyuki ATARASHI Mamoru SAWAHASHI
IEICE TRANSACTIONS on Communications
Publication Date: 2003/01/01
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Issue on Multiple Access and Signal Transmission Techniques for Future Mobile Communications)
channel interleaving, bit interleaving, symbol interleaving, chip interleaving, VSF-OFCDM,
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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.