This letter describes how to improve performance of cellular systems by combining non-equiprobable signaling with low-density parity check (LDPC) coding for an orthogonal frequency division multiplexing system. We focus on improving performance of wireless transmission with rate $3/4$ LDPC codes employing $4$-QAM at the cell edge. We double the size of the $4$-QAM constellation by introducing a second shell of signal points, and we implement non-equiprobable signaling through a shaping code which selects the high energy shell less frequently than the low energy shell. We emphasize that the shaping code is also used to transmit information. We employ rate $1/2$ LDPC coding and select the rate of the shaping code to match that of rate $3/4$ LDPC coding using $4$-QAM. We describe how to combine coding and shaping by integrating shaping into the calculation of log-likelihood ratios (LLRs) necessary for decoding LDPC codes. We present simulation results for a representative Veh-A channel showing gains of $4$ dB at a bit error rate (BER) of $10^{-3}$. Our results suggest that it is better to increase rate at the cell edge by introducing shaping, rather than by increasing the rate of the LDPC code.

翻译：本通信描述如何通过将非等概率信号与低密度奇偶校验编码相结合，以提升正交频分复用系统在蜂窝网络中的性能。我们专注于改善采用4-QAM调制与码率为3/4的LDPC编码在小区边缘的无线传输性能。通过引入第二层信号点，我们将4-QAM星座的规模扩大一倍，并借助成形码实现非等概率信号传输，使得高能量层的选择频率低于低能量层。我们强调该成形码亦用于传输信息。我们采用码率为1/2的LDPC编码，并选择成形码的码率以匹配采用4-QAM的码率3/4 LDPC编码。我们阐述了如何通过将成形整合到解码LDPC码所需的对数似然比计算中，实现编码与成形的结合。针对典型的Veh-A信道，我们给出了仿真结果，显示在误比特率为10^{-3}时可获得4 dB的增益。我们的结果表明，在小区边缘通过引入成形技术来提升速率，优于单纯提高LDPC编码的码率。