An Efficient Rate Compatible Modulation With Variable Weight Sets
Rate Compatible Modulation (RCM) has been shown to be capable of achieving smooth rate adaptive transmission in highly dynamic channel scenarios. In this paper, the capacity of RCM is analyzed for different channel conditions, and an important result is obtained, i.e., in low-to-middle signal to noise ratio (SNR), the same achievable capacity in RCMs may be obtained for different weight sets being used. Based on this result, instead of employing fixed-weight sets with large values or big cardinalities for all channel conditions as in conventional RCM, a novel RCM scheme with variable weight sets (RCM-VWS) is proposed by employing different weight sets for different SNR ranges such that the demodulation complexity can be significantly reduced. In RCM-VWS, the SNR value of the received signal is divided into five unoverlapped ranges, which are estimated and encoded into 3-bit information at the receiver. This 3-bit information is delivered back to the transmitter for weight set selection when the SNR value of the received signal changes from one range to another or for some transmission interval. Thus, the cost of the feedback load is trivial in practice. Theoretical analysis and simulations show that compared with conventional RCM with fixed weight sets, the computational complexity of RCM-VWS can be reduced by as much as 97% at SNR < 6 dB and 90% at 6 dB < SNR < 9 dB while maintaining the same transmission rate. In addition, the proposed scheme achieves a better BER performance than the conventional RCM.