Breadth-first trellis decoding with adaptive effort - Department of ...

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8 IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 38, NO. 1, JANUARY 19908-PSK__ I1 +D5+D1', M=21d& = 8.0. Stales = 1024le'+ 1 I I I I I I ,4.5 5 5.5 8 6.5 7 7.5 B 8.5 9 9.5SNR (DB)(a)4.5 5 5.5 8 6.5 7 7.5 B 8.5 9 9.5SNR
SIMMONS: BREADTH-FIRST TRELLIS DECODING 9,,0 1000 2000 3000 4000 5000 e000 7000 8888 90w 100008%:-;EBFig. 8.0 100 200 300 400 500 600 700 E00 900 1000DECODING INTERVALS@)Typical decoder dynamics (a) and time-scale expansion (b) [actualcode used is 4-AM, U = 10 at SNR=8.O dB, TE = 801.100 200 300 400 588 e000 100 200 300 400 500 e00DECODING INTERVALSFig. 9. Decoder dynamics with loss of transmitted path (code of Fig. 8 withSNR = 1.5 dB, SL = 64).sample buffer cleared. When one or several larger noise samplesoccur, all path metrics tend to climb and the decoding algorithmresponds with an increase in the number of survivor paths, whichtends to be sustained over several symbol intervals. The decodingalgorithm, in effect, uses this extra observation time to establishwhich paths may then be safely discarded. Computational demandsnow outpace decoder capability, and received samples accumulate inthe buffer. Once the noise event has passed, it takes some time toclear the computational backlog. Also note that the periodic stateforcingemployed in the simulations drives the number of survivorsback to just one at the end of every 200-symbol block.What is being shown here on the horizontal axis of Fig. 8 isthe symbol interval that the decoder is processing. These decodingintervals take varying amounts of time to complete, and ther durationonly corresponds on average to the fixed duration of a transmissionbaud. The backlog simply shows how many symbols the decoder isbehind the transmitter. This explains why the figures do not showthe buffer backlogs immediately dropping to zero when state-forcingoccurs at the end of each block. The backlog represents unprocessedsymbols that we simply cannot discard.Fig. 9 shows the behavior when the transmitted path is lost and anerror event begins. Note that the number of survivors gets very large,and the buffer backlog grows steadily. Soft-limiting to 64 survivorsis used to show behaviour under realistic conditions. A practicalimplementation may require aborting the decoding of such a blockand flushing the corresponding portion of the received-sample buffer(the whole block is discarded but it contains errors anyway).Finally, simulations with the SL soft limit on the number of sur-
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