Software for Turbo Decoding on Digital Signal Processors

Software for decoding turbo codes that have been adopted as standard by the Consultative Committee for Space Data Systems is being developed along with hardware built around integrated-circuit digital signal processors (DSPs) that execute the software. The software enables reliable communication at data rates up to 700 kb/s at a signal-to-noise ratio (SNR) of –0.2 dB if a rate-1/6 code is used, or at an SNR of 0.8 dB if a rate-1/3 code is used. The software is written primarily in assembly language and runs on eight high-performance DSPs in parallel. Frames of data are distributed among six of the DSPs, which perform iterative decoding. A “stopping rule” is used to detect early convergence, thereby reducing the average number of iterations and, hence, increasing decoding speed. The remaining two DSPs perform ancillary functions, including frame synchronization, tracking of frame arrival times, de-randomization (sometimes used to ensure a bit-transition density sufficient for receiver tracking), and cyclic redundancy checking for verification of data.