Author_Institution :
Titan Syst. Corp., Colorado Springs, CO, USA
Abstract :
We consider GMSK for use in frequency division multiple access (FDMA) satellite communication systems where only limited system bandwidth is available. The demodulator employs maximum likelihood sequence estimation (MLSE) of the GMSK signal using a simplified Viterbi processor. For the case of GMSK having a time-bandwidth product of BT=1/4, a two matched filter approximation gives very good performance, while, for BT=1/6, a two filter approximation is very similar to a four filter approximation. For a convolutional code with rate 1/2 and constraint length 7, simulation shows that soft output Viterbi algorithm (SOVA) decoding using a four-filter demodulator for BT=1/6 and BT=1/8, results in a performance improvement of 2.1 dB and 2.3 dB, respectively, in signal-to-noise ratio (SNR), compared to hard decision Viterbi algorithm (HOVA) decoding, at a bit error probability (BEP) of 10-5. When compared to a BPSK system that employs a similar soft decision decoder, the BEP power penalty for these schemes is 2.3 dB and 4.1 dB respectively. The spectral efficiency, relative the 95% RF power containment bandwidth, is 1.88 bits per sec per Hz (bps/Hz) and 2.1 bps/Hz of bandwidth for BT=1/6 and 1/8 GMSK respectively, and only 0.35 bps/Hz for BPSK. As ratios, the spectral efficiency improvement factor is 5.37 and 6.0 respectively.
Keywords :
Viterbi decoding; continuous phase modulation; convolutional codes; demodulation; demodulators; error statistics; frequency division multiple access; matched filters; maximum likelihood sequence estimation; minimum shift keying; modulation coding; satellite communication; BPSK; CPM; FDMA; MLSE; SOVA decoding; Viterbi processor; bit error probability; continuous phase modulation; convolutional code; frequency division multiple access; hard decision Viterbi algorithm decoding; matched filter; maximum likelihood sequence estimation; satellite communication; soft output GMSK demodulator; soft output Viterbi algorithm decoding; spectral efficiency; Binary phase shift keying; Bit rate; Demodulation; Frequency conversion; Frequency division multiaccess; Matched filters; Maximum likelihood decoding; Maximum likelihood estimation; Satellite communication; Viterbi algorithm;