VLSI Implementation of High Throughput Turbo Decoder for Mobile WiMAX Modem

Abstract

In this paper, we propose the architecture for the high-throughput turbo (CTC) decoder used in the Mobile WiMAX modem. The proposed CTC decoder uses Min-Log-MAP algorithm, unsigned bits for quantization of metrics, and efficient architecture for finding a minimum value and preventing overflows in the computation of state metrics. Instead of using two stages of comparators to find a minimum value, which is conventional in the state metric computation unit, the proposed CTC decoder replaces the second stage of comparators by AND gates for the Most Significant Bits (MSB) of the first stage outputs. Thanks to the reduction of the complexity resulting from the efficient architecture, the CTC decoder achieves high operating frequency. The proposed CTC decoder architecture allows parallel computation of forward state metrics and backward state metrics. As a result, the number of cycles required to complete one iteration of decoding are reduced by half compared with those of conventional CTC decoders. A permuter, used as interleaver and deinterleaver alternatively, is designed to reduce by half the cycle required for data writing and thus to be compatible with the parallel computation of state metrics. The proposed CTC decoder is implemented by Samsung 0.18um CMOS process. A single CTC decoder designed by proposed architecture supports the operating frequency of 275 MHz and the throughput of 68.3 Mbps. It is verified that the throughput requirement of Mobile WiMAX standard is satisfied by using two CTC decoders in a parallel manner.