A study of bandwidth constraints in bi-phase modulated PCM/NRZ telemetry channels

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1970

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The performance degradation of bi-phase modulated NRZ/ PCM telemetry channels, resulting from constraining the biphase modulated signal bandwidth, is investigated. Emphasis is toward development of the analytical techniques necessary to evaluate the effects of bandwidth limiting on the channel's bit error probability. Analysis of the effects of bandwidth limiting require utilization of deterministic PCM signals since the amplitude and phase spectrum of the bi-phase modulated signal must be defined in order to evaluate the change in the telemetry signal waveform due to the bandpass filter's amplitude and phase characteristics. Due to the nature of the signals involved, series expansions are required to define many of the functions. Evaluation of the resulting analytical expressions thus required utilization of computer techniques. A computer program was utilized for evaluation of the demodulated signal waveform after filtering of the bi-phase modulated signal occurred. An additional computer program is utilized to evaluate the expression developed for predicting bit error probabilities when bandwidth limiting effects are considered. A comparison is made between the bit error probability performance of an ideal non-restricted bandwidth channel and channels of varying degrees of bandwidth limiting. It is shown that when the ratio of channel bandwidth to bit rate is four or less, a significant loss in channel bit error probability performance will result. An experimental verification of the analytical results is provided. The experimental data agrees very well with that obtained from the computer evaluation. The addition of a predetection amplitude limiter to the telemetry demodulator is shown to significantly alter the results of both the analytical and experimental data. It is seen that the amplitude-limiter provides an optimized detection capability to the bandwidth limited channel, and thus significantly reduces the loss in bit error probability performance resulting from the bandwidth constraints. The analytical and experimental data presented can be utilized to effectively tradeoff bi-phase modulated PCM telemetry channel bandwidth requirements with channel crosstalk requirements in frequency division multiplexed systems, such as the Phase II Apollo Lunar Scientific Data Communication System.

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