However, in the upstream channel, ONUs can not transmit optical data signal in CM.
To solve this problem, burst mode (BM) transmission is adopted for upstream channel.
Such circuit can generate local clock with the frequency and phase same as the individual received optical packet in a short locking time, for example within 40 ns.
[1] Such generated local clock can in turn perform correct data decision.
The conventionally used PLL based clock recovery schemes can not meet such strict requirement on locking time.