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In this paper, we investigate the strategy of transmission mode switching for device-to-device (D2D) communication in both single-cell scenario and multi-cell scenarios, which selects the transmission mode to guarantee the maximum ergodic achievable sum-rate among three transmission modes. We first introduce the basic operation principles of three communication transmission modes which are named as traditional cellular communication mode, direct D2D communication mode and two-way decode-and-forward (DF)-relayed D2D communication mode. Then we derive the corresponding expressions for the ergodic achievable sum-rates of each transmission mode, and get the crossing points of different transmission modes to attain maximum ergodic achievable sum-rate of the system. From the analytical results, we can see that the proper operating region of each transmission mode is related to different interference level and distance of the D2D users. Based on the analytical results, we obtain a reliable communication transmission mode switching strategy which guarantees the system to choose the mode with the maximum ergodic achievable sum-rate so as to improve the performance of D2D communication. Numerical results demonstrate that by applying mode switching, the ergodic achievable sum-rate of the system achieves a remarkable enhancement.

This paper investigates the antenna selection scheme choosing the antenna with the largest channel gain to the the cellular user for device-to-device (D2D) communication underlaying cellular networks. We derive an exact closed-form expression of the ergodic achievable rate and examine its asymptotic behavior in the high signal-to-noise ratio (SNR) regime. It is demonstrated that the high SNR approximation can be much improved by a higher transmit power ratio between the base station (BS) and the D2D transmitter in the small cell setting where all users are closely located. However, in the macro cell setting in which only the D2D terminals are fairly close, the influence of the transmit power ratio becomes insignificant. In addition, we present upper and lower bounds of the ergodic achievable rate. Based on these results, we illustrate that the D2D communication cannot help in the cellular network for elevating the rate when the transmit SNR at the BS grows high. If the BS SNR is lower, then the D2D communication can effectively increase the ergodic achievable rate. Numerical results are provided to justify the correctness of the expressions and the relevant performance analysis.