Comparison of hybrid ARQ schemes and optimization key parameters for high-speed packet transmission in W-CDMA forward link

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 ill the W-CDMA forward link by comparing the throughput performance of three types of hybrid ARQ schemes: type-I hy brid ARQ with packet combining (PC), type-II hybrid ARQ, and basic type-I hybrid ARQ as a reference. Moreover, from the view point 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 (E-c/N-0) at the throughput efficiency of 0.2/0.4/0.6 is improved by 0.7/0.3/0.1dB and 3.9/1.8/0.5SdB. 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 x 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 E-c/N-0 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 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-II with PC schemes it is around between 3/4 and 8/9 in type-II. resulting in a higher throughput efficiency. In addition, for highspeed packer 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 tory maximum Doppler frequency and a high transmit E-c/N-0 region.