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Comparison of Hybrid ARQ Schemes and Optimization of Key Parameters for High-Speed Packet Transmission in W-CDMA Forward Link
IEICE TRANSACTIONS on Fundamentals of Electronics, Communications and Computer Sciences
Publication Date: 2001/07/01
Print ISSN: 0916-8508
Type of Manuscript: Special Section PAPER (Special Section on Multi-dimensional Mobile Information Networks)
mobile radio, W-CDMA, high-speed downlink packet access, hybrid ARQ,
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This paper elucidates the most appropriate hybrid automatic-repeat-request (ARQ) scheme, i.e., which can achieve the highest throughput, for high-speed packet transmission in the W-CDMA forward link by comparing the throughput performance of three types of hybrid ARQ schemes: type-I hybrid ARQ with packet combining (PC), type-II hybrid ARQ, and basic type-I hybrid ARQ as a reference. Moreover, from the viewpoint of maximum throughput, the respective optimum roles of ARQ and channel coding in hybrid ARQ are also clarified, such as the optimum coding rate and the packet length related to the interleaving effect. The simulation results reveal that the type-II scheme exhibits the best throughput performance, and the required received signal energy per chip-to-background noise spectral density ratio (Ec/N0) at the throughput efficiency of 0.2/0.4/0.6 is improved by 0.7/0.3/0.1 dB and 3.9/1.8/0.5 dB, respectively, compared to the type-I scheme with and without PC in a 2-path Rayleigh fading channel with the average equal power at the maximum Doppler frequency of 5 Hz and the packet length of 4 slots (= 0.667 4 = 2.667 msec). However, the improvement of the type-II scheme compared to the type-I scheme with PC is small or the achievable throughput is almost identical in the high-received Ec/N0 region. On the other hand, the type-I scheme with PC is much less complex and thus preferable, while maintaining almost the same throughput performance or allowing very minor degradation compared to that with type-II. The results also elucidate that, while the optimum coding rate depends on the required throughput in the basic type-I and type-I with PC schemes, it is around between 3/4 and 8/9 in type-II, resulting in a higher throughput efficiency. In addition, for high-speed packet transmission employing a hybrid ARQ scheme, a shorter retransmission unit size is preferable such as 1 slot, and the fast transmit power control is effective only under conditions such as a low maximum Doppler frequency and a high transmit Ec/N0 region.