The high demand for multimedia applications in environmental monitoring, invasion detection, and disaster aid has led to the rise of wireless sensor network (WSN). With the increase of reliability and diversity of information streams, the higher requirements on throughput and quality of service (QoS) have been put forward in data transmission between two sensor nodes. However, lower spectral efficiency becomes a bottleneck in non-line-of-sight (NLOS) transmission of WSN. This paper proposes a novel nondata-aided error vector magnitude based adaptive modulation (NDA-EVM-AM) to solve the problem. NDA-EVM is considered as a new metric to evaluate the quality of NLOS link for adaptive modulation in WSN. By modeling the NLOS scenario as the η−μ fading channel, a closed-form expression for the NDA-EVM of multilevel quadrature amplitude modulation (MQAM) signals over the η−μ fading channel is derived, and the relationship between SER and NDA-EVM is also formulated. Based on these results, NDA-EVM state machine is designed for adaptation strategy. The algorithmic complexity of NDA-EVM-AM is analyzed and the outage capacity of NDA-EVM-AM in an NLOS scenario is also given. The performances of NDA-EVM-AM are compared by simulation, and the results show that NDA-EVM-AM is an effective technique to be used in the NLOS scenarios of WSN. This technique can accurately reflect the channel variations and efficiently adjust modulation order to better match the channel conditions, hence, obtaining better performance in average spectral efficiency.
Transmission in non-line-of-sight (NLOS) conditions has a poor throughput in device-to-device (D2D) communications. In order to achieve high throughput, adaptive modulation has been selected as a spectrally-efficient transmission technology. Moreover, quantifying the performance of a fading channel in NLOS conditions becomes a core issue. In general, the η − µ distribution can be employed to characterize and model the NLOS fading channel. Effective evaluating the quality of η − µ channels can provide an efficient theoretical reference for determining the optimum switching thresholds in adaptive modulation. However, due to the sensitivity of D2D transmission to fading channels, providing an efficient theoretical benchmark for evaluating, the quality of η − µ channels is still a challenge in transmission design. In this paper, the nondata-aided error vector magnitude (NDA-EVM) is considered as a novel metric to evaluate the quality of wireless fading channels. Specifically, the NDA-EVM of the multilevel quadrature amplitude modulation (MQAM) signals over η−µ fading channels and its lower bound is analytically derived. This can be used to further determine the optimum switching thresholds in adaptive modulation in NLOS conditions. The numerical results validate the effectiveness of the proposed formulation and also reveal the influence of the channel parameters on the lower bound of the NDA-EVM. INDEX TERMS η − µ fading channels, D2D, NLOS, nondata-aided error vector magnitude, lower bound. I. INTRODUCTION Device-to-device (D2D) communication is one of the key techniques in the 5th generation (5G) wireless communication systems that allows direct communication between mobile nodes [1]. However, D2D transmission schemes are sensitive to channel conditions and provide poor throughput in Non-line-of sight (NLOS) scenarios [2]. Adaptive modulation can select the optimal modulation parameters relative to the channel characteristics, which can provide an effective leverage to achieve high throughput [3]. Hence, quantifying the performance of a fading channel in NLOS conditions is a core issue of adaptive modulation in D2D communications. The associate editor coordinating the review of this manuscript and approving it for publication was Rui Wang.
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