Irregularly distributed triangular quadrature amplitude modulation

The recently suggested triangular quadrature amplitude modulation (TQAM) provides considerable power gain over square quadrature amplitude modulation (SQAM) at the expense of slight increase in detection complexity. However, power gain of the TQAM is limited on account of the constraint that signal points are regularly distributed at the vertexes of contiguous equilateral triangles. In this paper, we investigate a modified signal constellation of the TQAM where signal points are irregularly distributed while preserving triangular structure, and calculate achievable power gain of the proposed scheme. We also address optimum and suboptimum bit stream mapping methods and suggest a simple and optimum detection method for the constellation to be meaningful in practical implementation. Analysis and simulation show that the proposed constellation provides power gains of 0.62 dB and 0.20 dB over the SQAM and the TQAM at a target symbol error rate of 10^-6 for 64-ary case.