A burst-mode receiver (BM-RX) is typically located in the Optical Line Termination (OLT) of a Passive Optical network (PON), as shown in FIG. 1. In general, the BM-RX comprises a photodiode, a burst-mode transimpedance amplifier (BM-TIA), a burst-mode limiting amplifier (BM-LA) and a burst-mode clock phase alignment (BM-CPA) block. In essence, the BM-RX converts the photodiode current into a voltage (BM-TIA), amplifies this voltage (amplitude/threshold recovery) and aligns this signal to the OLT clock (phase recovery). To achieve these functionalities the BM-RX requires time critical functions like activity detection, reset generation and clock phase alignment.
In conventional feedback receivers the threshold recovery is implemented as an offset compensation loop with fixed time constant. In burst-mode inaccurate peak-detectors are often used to realize a fast threshold setting. The clock and data recovery (CDR) that succeeds the amplitude recovery unit, generally uses a phase locked loop (PLL) or delay locked loop (DLL), again with fixed time constant.
Further, activity detection indicates a data burst being received. The activity detection signal is typically used to initiate the reset generation, the decision threshold extraction and the clock phase aligner (CPA) (so for both amplitude and phase recovery). In prior art burst mode receivers activity is detected by comparing the incoming signal with a reference voltage. This reference voltage depends on the combined DC offsets from the unipolar signal, the preceding transimpedance amplifier (TIA) and offsets from the activity detection circuitry itself. This system can only be used if this DC offset is the same for all bursts. In long-range optical networks with optical amplifiers or when using TIAs that already compensate part of the offset, this is no longer the case.
In prior art end-of-burst (EOB) detection solutions the number of consecutive zeros are counted. An EOB is detected when this number exceeds the maximum consecutive identical digits (CID) that can occur in the received bit stream. Although this technique works very well at low BER (e.g. EPON, GPON), it quickly starts to miss EOBs when the bit-error-rate (BER) rises. Extra functionality must then be added to make the EOB detection more reliable in these conditions. Consequently, there is a need for a method and circuit for end-of-burst detection that works well also in case of a high BER (or, equivalently, a low SNR).
USPTO Backgrounds
USPTO Backgrounds