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With increasing order of quadrature amplitude modulation (QAM), the bandwidth efficiency is improved in digital communication. However, in practice, the modulation order is limited, since conventional digital carrier recovery (CR) algorithms give rise to unacceptable phase error. The authors present an efficient software-aided technique for phase error reduction in CR for high-order QAM, based on the simple and well-known fourth power CR loop. Analytical and simulation results indicate that the new technique has several attractive features such as approximate of invariance of phase error improvement over modulation order and low hardware complexity for modulation orders as high as 256-QAM. Experimental results for 64 and 256-QAM illustrate phase error variance of less than -110-dBc/Hz at the frequency offset of 10-kHz, that is, 30-dB reduction of phase error variance or 3-dB increase in system processing gain compared to the conventional fourth power CR loop. This allows a significant improvement of bandwidth efficiency by increasing the modulation order, at the cost of slight complexity overhead.
Date of Publication: December 17 2010