In optical communication

high modulation level（M） of a transceiver results in its high power efficiency

yet leading to the increase of time slot rate and affecting its error performance.In this paper

the effect of the time slot rate on the error performance of the APD-based direct detection receiver is studied.In the case of pulse position modulation

the code error rate versus time slot rate is derived and the root cause of effect production analyzed.Numerical results show that the increase of time slot rate weakens the system noise rejection performance and

for example

approximately 1 dBm more average signal power is required from M to M+1 with code error rate 10-6.