Performance evaluation of coherent high-speed TD-OFCDM broadband packet wireless access in forward link employing multi-level modulation and hybrid ARQ

This paper evaluates high-speed broadband packet wireless access in the for ward link using: coherent Time Division-Orthogonal Frequency and Code Divis ion Multiplexing (TD-OFCDM) by applying time-multiplexed pilot symbol assis ted channel estimation and integrating efficient multi-level modulation, hy brid automatic repeat request (ARQ), and code-multiplexing over a 50-100 MH z bandwidth. Computer simulation results first clarify that the common time -multiplexed pilot symbols with the transmit power of B dB higher than that of data symbols should be placed at both tile beginning and end of a packe t, and that the optimum averaging interval of channel estimates ill the fre quency domain is different according to the delay spread of a channel. Base d on these optimized parameters for packet transmission, we show that the o rthogonality among the code-multiplexed channels is destroyed due to severe frequency selective (multipath) fading and the accumulation of spread sign als using equal gain combining (EGC) in the frequency domain. This degrades the achievable throughput performance especially when improving multi-leve l modulation and a high coding rate. Consequently, coherent TD-OFCDM with 8 PSK data modulation and the convolutional coding of rate R = 2/3 employing sixteen-rods multiplexing (spreading factor (SF) is 16) achieves the highes t throughput of approximately 105 Mbps at the average received E-b/N-0 (sig nal energy per bit-to-noise power spectrum density ratio) of approximately 24 dB in a 3-path Rayleigh fading channel (rms delay spread, sigma = 0.1 mu sec). Furthermore, in coherent TD-OFCDM with QPSK and R = 1/5 or BPSK and R = 1/2, throughput performance greater than 80 Mbps is achieved at the ave rage received E-b/N-0 of approximately 20 dB even in a 24-path. Rayleigh fa ding channel (sigma = 0.2 mu sec).