A Rateless 16QAM Scheme for IoT Uplink Communications

How to achieve a high transmission rate at a low cost of energy consumption in a wide range of channel variations is a critical challenge in Internet of Things (IoT) applications. In this article, a rateless 16 quadrature amplitude modulation (R-16QAM) scheme for IoT uplink communications is proposed to deal with this problem. The R-16QAM transmitter consists of an encoder, a random bit selector, and a tailored modulation mapper. Different from the sequential bits selection and Gary mapping in conventional 16QAM modulation, bits are randomly selected and mapped into a 16QAM symbol by a weighted summation of selected bits in R-16QAM. The random bits selection introduces correlation among modulated symbols, enabling the transmitter to work in a rateless manner, i.e., the transmitter keeps generating and sending R-16QAM symbols until a feedback of successful decoding is received from the receiver. It means that the number of symbols transmitted varies with channel variations. In this way, the proposed R-16QAM can automatically adjust to channel conditions and work well in a wide range of channel conditions. The weighted-summation mapping can be illustrated by a “Tanner graph,” based on which we develop a low complexity iterative demodulation algorithm to improve receiver performance. The simulations show that compared with narrow band Internet of Things (NB-IoT) standard and Turbo coded light-weight rate compatible modulation (TLRCM) for IoT uplink communications, the proposed R-16QAM scheme can consistently achieve a lower energy consumption in a signal-to-noise-ratio (SNR) range of −10–15 dB. Moreover, it can achieve a maximum transmission throughput of 3.96 bits/s/Hz at SNR=15 dB while only 0.66 and 1.7 bits/s/Hz are achieved in NB-IoT and TLRCM, respectively. These advantages enable the proposed R-16QAM scheme to be particularly suitable to IoT application scenarios where nodes work with limit energy resources under a large range of channel variations.