Modulation format recognition scheme using orthogonal codewords for wavelength division multiplexing system

Abstract

A modulation format identification (MFI) scheme based on orthogonal coding scheme is proposed to solve the modulation format identification problem in wavelength division multiplexing (WDM) systems. The scheme uses sidebands to carry orthogonally encoded modulation format information. The basic orthogonal codeword is generated by the Hadamard matrix, and the codewords that are not equal to other codewords after the cyclic shift are selected. The receiving end decodes the modulation format information by using mutual information. We calculate the difference between the maximum and minimum values of the mutual information between the received symbols after cyclic shifting and the standard codewords to make the proposed scheme more robust. We numerically simulated the proposed scheme in a wavelength division multiplexing system, where its channels carry 12.5 Gbaud 4/16/64 QAM and 8/16 PSK. The simulation results show that the modulation format information in the sideband has little effect on the main band information. In the absence of symbol calibration, the receiver can still recover modulation format information from the sideband information with a high accuracy rate. Compared with the MFI algorithm based on Stokes space and the MFI based on signal amplitude performance, the proposed MFI still performs well at low OSNR, although it occupies some bandwidth resources. Moreover, this solution only requires a small number of symbols for decision, so it can achieve fast response requiring little extra storage space.