In today's DWDM systems 40-160 10 Gbps channels can be transmitted over a single fiber. This huge bandwidth was enough till now, but in a little while there will not be free wavelengths in some part of the network. So the goal is to increase the number of free channels. It can be accomplished by increasing the data rate of one channel. We would like to solve this problem cost-effectively, so we do not want to change many things in our network.
If we increase data rate, the distorting effect of chromatic dispersion will increase as well. So we have to change the modulation format to phase modulation instead of the widely used intense modulation.
My thesis is about the investement of this problem and a real system. The real system is the 40 Gbps solution of Cisco. The used modulation in this realization is CP-DQPSK. There are three basic technologies behind CP-QPSK: dual polarization, DQPSK modulation, and coherent detection with DSP.
I had chance to measure the properties of this system in Magyar Telekom's network and I was able to compare the result of measure to my theoretical considerations.