If an echo is generated during transmission, then along with the principal wave (directly transmitted wave), an echo with a certain time delay arrives at the receiver, which causes degradation of receiver performance.
In this paper, the bit error rate for optical MSK communication systems with a limiterdiscriminatorintegrator-type receiver is analyzed and the influence of echo is evaluated qualitatively as well as quantitatively.
In general, there exists a floor value on the error rate curve due to the phase fluctuation of laser light in the coherent optical communication systems, which is not reduced upon increasing the received optical power. It is shown that this floor value increases due to the presence of echo and its value is a maximum when the delay time of echo with respect to the principal wave is an integral multiple of the bit duration. Moreover, when the spectral linewidth of the light with high bit rate is narrow, then the effect of echo becomes even stronger.
A relationship between the received optical power and bit error rate is derived, and the power penalty is determined for given echo amplitude, delay time, and linewidth-bit duration product. The conditions under which the effect of echo cannot be neglected are clarified. Moreover, the optical MSK system is compared with the optical DPSK system and it is shown that the MSK signal is degraded by echo greater than the DPSKsignal.
The special feature of this paper is an exact analysis of the case when some phase correlation exists between the principal wave and echo due to coding considerations.