Pilot symbol aided carrier phase estimation

Abstract

Methods and systems for processing an optical signal in a communication system are disclosed. The disclosed methods yield benefits for estimation and tracking of carrier phase of received signals at a digital coherent receiver. Specifically, phase ambiguity is removed by the insertion of pilot symbols into a transmitted data stream. Pilot symbols are detected from a received signal, and carrier phase is estimated for the detected pilot symbols. If carrier phase track of received data symbols was lost, a correction is applied to recover the track. Coherent symbol decoding may be used which has not been possible with prior art techniques due to the possibility of phase tracking loss.

What is claimed is: 
     
         1 . A method, comprising:
 receiving an optical signal having a pilot symbol and multiple data symbols;   estimating a carrier phase of multiple data symbols using an M th  power scheme with joint polarization carrier phase estimation; and   detecting the carrier phase of at least one of said multiple data symbols adjacent said pilot symbol.   
     
     
         2 . The method of  claim 1 , further comprising determining a differential phase between the phase of at least one of said multiple data symbols adjacent said pilot symbol and a phase of said pilot symbol. 
     
     
         3 . The method of  claim 2 , further comprising:
 determining whether the differential phase is less than a phase tolerance; and   decoding the multiple data symbols based on the determination that the phase differential is less than the phase tolerance.   
     
     
         4 . The method of  claim 1 , wherein the phase tolerance is π/3 radians. 
     
     
         5 . The method of  claim 1 , wherein the pilot symbol is a set of pilot symbols comprising at least two adjacent pilot symbols. 
     
     
         6 . The method of  claim 1 , wherein the pilot symbol is a set of pilot symbols comprising a first pilot symbol and a second pilot symbol, the first pilot symbol corresponding to the first data symbol in the optical signal and the second pilot signal corresponding to the last data symbol in the optical signal. 
     
     
         7 . The method of  claim 1 , wherein said data symbols comprise 50 consecutive data symbols. 
     
     
         8 . The method of  claim 1 , wherein M equals 4. 
     
     
         9 . The method of  claim 1 , wherein estimating the carrier phase of the multiple data symbols comprises estimating the carrier phase of X and Y polarization tributaries associated with the multiple data symbols. 
     
     
         10 . The method of  claim 1 , wherein estimating the carrier phase of the multiple data symbols is based at least in part on a coupling coefficient, wherein the coupling coefficient has a value between 0 and 1. 
     
     
         11 . The method of  claim 9 , wherein the coupling coefficient is 0.7. 
     
     
         12 . An optical receiver comprising:
 a plurality of photodetectors each configured to generate an electrical signal proportional to a received optical signal, said optical signal having a plurality of channels each including a pilot symbol and a plurality of data symbols;   an analog to digital converter configured to convert each of said electrical signals to digital signals; and   a digital signal processor (DSP) communicating with said analog to digital converter, said DSP comprising a carrier phase estimation module configured to:   detect a carrier phase of one of the plurality of data symbol that is adjacent said pilot symbol;   determine a differential phase between the one of the plurality of data symbols adjacent said pilot symbol and said pilot symbol; and   estimate the phase of the data symbols using an M th  power scheme with joint polarization carrier phase estimation.   
     
     
         13 . The optical receiver of  claim 12  further comprising:
 a local oscillator circuit; and 
 a plurality of hybrid interferometers communicating with said local oscillator circuit, each of said hybrid interferometers configured to receive the plurality of optical signal channels and extract signals representing a phase and amplitude between the received optical channels and a signal received from said local oscillator and supply said phase and amplitude signals to a corresponding one of said plurality of photodetectors. 
 
     
     
         14 . The optical receiver of  claim 13  further comprising a polarization beam splitter configured to supply the plurality of optical signal channels to each of said plurality of hybrid interferometers.