Experimental evaluation on effect of hybrid ARQ with packet combining in forward link for VSF-OFCDM broadband wireless access

This paper experimentally investigates the throughput performance employing hybrid automatic repeat request (ARQ) with a packet combining algorithm, i.e.. incremental redundancy and Chase combining, from the viewpoints of the achievable throughput satisfying the required residual packet error rate (PKR), coupled with real-time adaptive modulation and channel coding (AMC) operation, for variable spreading factor-orthogonal frequency and code division multiplexing (VSF-OFCDM) broadband wireless access in a forward-link multipath fading channel. Since the implemented base station (BS) and mobile station (MS) transceivers can achieve a high peak throughput of greater than 100 Mbps, the time diversity effect by packet combining or code combining in hybrid ARQ is thoroughly examined associated with the frequency diversity effect over a broadband channel and antenna diversity reception. The experimental results demonstrate that even in a six-path Rayleigh fading channel, incremental redundancy is superior to Chase combining under high Doppler frequency conditions such as f_D = 140 Hz, especially when the number of maximum retransmissions is small, because the larger coding gain after code combining is effective in compensating for packet errors due to inappropriate modulation and coding scheme (MCS) selection caused by the tracking error of the AMC loop for the fast fading variations. Furthermore, it is elucidated that the throughput values of 7 Mbps and 10 Mbps (achieved at the average received signal energy per symbol-to-background noise power spectrum density ratio (E_sM₀) of approximately -5 dB) satisfying the required PER of below 10^-4 are achieved when the maximum number of retransmissions is approximately 3 and 7, respectively, in the six-path fading channel with f_D = 20 Hz.