Crosstalk penalty in multichannel ASK heterodyne lightwave systems

The author addresses the issue of optimum channel spacing in multichannel amplitude-shift-keying (ASK) heterodyne lightwave systems by considering the bit-error-rate (BER) degradation resulting from interchannel crosstalk. An expression for the BER of a specific channel is obtained by considering the crosstalk from both of its nearest neighbors. The general result is used to calculate the power penalty as a function of the crosstalk level. The crosstalk is shown to depend on a large number of system parameters, such as the bit rates, the received powers, the channel spacing, and the response of the bandpass filter. For the case of equal bit rates and equal received powers in all channels, the crosstalk from each channel should be below -12 dB to keep the power penalty below 1 dB, which means a minimum channel spacing of about four or five times the bit rate. If the objective is to reduce the power penalty below 0.1 dB, the interchannel crosstalk should be less than -18 dB, which would require a minimum channel spacing of about ten times the bit rate