Cycle slips -What are these in Coherent receivers and their mitigation

In this article we will have a overview of cycle slips which are encountered in coherent receivers .So the constellation diagram when plotted if we rotate it to 90 degrees they will become exactly similar , isn't it ?

So in turn which means that we may estimate the phase incorrectly .This is known as cycle slip .This occurs primarily because we use M-ary encoding .The are also known as Phase errors and typically occur in the multiples of 2π/M .

Again what could be the cause of cycle clips ?- There could be many but to name a few are laser phase noise, order and shape of constellation, nonlinear??cross-phase modulation (XPM) etc ., all of these may?rotate the quadrant axis inducing cycle slips or errors .

There are various methods to counter the cycle slips .One way is that the symbols can be encoded differentially which means that the information is carried by the phase difference between two adjacent symbols, rather than by the symbol phase itself .Another way is to induce pilot symbols which are ?inserted periodically into the data sequence in order to limit the length of an error burst. This is done using error correcting code for example LDPC .