Question

Consider the noncoherent receiver consists of two paths; the band-pass filter in the top path is tuned to the carrier frequency representing symbol 1, and the band-pass filter in the lower path is tuned to the alternative carrier frequency representing symbol 0. These two filters are followed by a pair of energy-level detectors, whose outputs are applied to the comparator to recover the original binary data stream.

Similarly, the noncoherent BFSK receiver is twice as complex as the noncoherent BASK receiver. What would be the advantage of the BFSK system over the BASK system, given that they operate on the same binary data stream and the same communication channel? Justify your answer.

Answer 1

BASK and BFSK have the same performance, and BPSK has a 3-dB power advantage over both BASK and BFSK for both coherent and noncoherent detection schemes.

There is little difference in the complexity of the transmit equipment for all schemes. Receiver design simplification due to noncoherent detection is at the expense of performance degradation. Among the noncoherent schemes, DBPSK and noncoherent BFSK are more complex than noncoherent BASK.

Since the cost of transmitting and receiving equipment is generally a function of the peak power requirement rather than the average power requirement, BASK falls short. The power margin over noncoherent BFSK at low bit error rates is inconsequential. Because of the comparable performance and the added simplicity of noncoherent BFSK, it is almost exclusively employed instead of coherent FSK.

BASK is more sensitive to variations in received signal level due to variations in channel characteristics, such as amplitude nonlinearities and fading, as BFSK and BPSK are both constant amplitude signals.

Assuming a rectangular pulse, the power spectral density for each of these schemes decays as 1f2 for frequencies away from the carrier frequency. Except the discrete components, BASK and BPSK have the same power spectral density, and thus the same bandwidth requirements. BFSK has lower side-lobes than BPSK and BASK. In general, the bandwidth of BFSK signal is greater than the bandwidth of the BASK and BPSK signals.

With a smoother pulse than the rectangular pulse , the power spectral density for a modulation scheme can become more compact. This valuable reduction in bandwidth is of course at the expense of a modest increase in complexity of the pulse shaping.

Overall, BPSK scheme, in terms of both power and bandwidth utilization, is more efficient than BASK and BFSK schemes

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