text: Device-to-device (D2D) communications in cellular networks are promising technologies for improving network throughput, spectrum efficiency, and transmission delay. In this paper, we first introduce the concept of guard distance to explore a proper system model for enabling multiple concurrent D2D pairs in the same cell. Considering the Signal to Interference Ratio (SIR) requirements for both macro-cell and D2D communications, a geometrical method is proposed to obtain the guard distances from a D2D user equipment (DUE) to the base station (BS), to the transmitting cellular user equipment (CUE), and to other communicating D2D pairs, respectively, when the uplink resource is reused. By utilizing the guard distances, we then derive the bounds of the maximum throughput improvement provided by D2D communications in a cell. Extensive simulations are conducted to demonstrate the impact of different parameters on the optimal maximum throughput. We believe that the obtained results can provide useful guidelines for the deployment of future cellular networks with underlaying D2D communications.
  @cite_9: This paper analyzes the maximum achievable transmission capacity of the D2D communication system under heterogeneous networks. The heterogeneous networks contain two primary systems working on independent bands and D2D communication guarantees the target outage probabilities of both systems on each band. By utilizing stochastic geometry, the effects of the spatial densities and the transmission power allocation ratio on the achievable transmission capacity are presented. Moreover, the optimal transmission density of D2D pairs and the optimal power allocation ratio are derived. The maximum capacity of D2D communication is defined based on the former optimal value from theoretical results. It is shown that the optimal power allocation ratio is proportional to the product of bandwidth, node density and transmission power of two primary systems.
  @cite_25: It is quite important for Device-to-Device (D2D) communication to make decision in selecting which part of the celluar radio resource, namely uplink (UL) resource or downlink (DL) resource, to be reused for the purpose of higher system capacity. In this paper, we propose an interference control scheme in DL mode to limit the base station's (BS) interference to D2D user equipments (DUEs) by selecting cellular user equipments (CUEs) nearer to BS. By comparing the interference control scheme used in UL situation, we derived the two lower bounds of ergodic capacity of reusing UL and DL resource in D2D communication. Numeric results show that the system capacity can be optimized by utilizing effective reusing mode selection strategies.
  @cite_20: A new interference management strategy is proposed to enhance the overall capacity of cellular networks (CNs) and device-to-device (D2D) systems. We consider M out of K cellular user equipments (CUEs) and one D2D pair exploiting the same resources in the uplink (UL) period under the assumption of M multiple antennas at the base station (BS). First, we use the conventional mechanism which limits the maximum transmit power of the D2D transmitter so as not to generate harmful interference from D2D systems to CNs. Second, we propose a δD-interference limited area (ILA) control scheme to manage interference from CNs to D2D systems. The method does not allow the coexistence (i.e., use of the same resources) of CUEs and a D2D pair if the CUEs are located in the δD-ILA defined as the area in which the interference to signal ratio (ISR) at the D2D receiver is greater than the predetermined threshold, δD. Next, we analyze the coverage of the δD-ILA and derive the lower bound of the ergodic capacity as a closed form. Numerical results show that the δD-ILA based D2D gain is much greater than the conventional D2D gain, whereas the capacity loss to the CNs caused by using the δD-ILA is negligibly small.
  @cite_8: We study the uplink capacity gain when Device-to-Device (D2D) links are enabled in a FDD CDMA-based cellular system. In such a system, a direct link for data traffic is established if a communication pair is closely located, and better link quality is expected due to the short D2D transmission distance. We consider a single cell scenario, and assume uplink resource is utilized by D2D communications. We assume the D2D transmitter transmits at a power level no more than it transmits in a cellular mode; in this case no extra interference would be introduced into the CDMA system. Optimal transmit power for benefited D2D links is derived to improve system performance in terms of uplink capacity. Extensive simulations are conducted, which demonstrate that a significant capacity gain is obtained from D2D links. Subsequently, we discuss the performance gain and its affecting factors and try to derive some insights on practical implementation.
summary: For the theoretical analysis of the achievable performance bounds, available results are relatively fewer. In @cite_8 , the uplink capacity gain was derived when one D2D link was enabled in an FDD CDMA-based cellular cell. In @cite_20 , an interference-limited area (ILA) control scheme was proposed to manage the interference from CUEs to a D2D transaction when multiple antennas were used by the BS. By analyzing the coverage of ILA, a lower bound of the ergodic capacity was also obtained for DUEs using uplink cellular radio resources. After that, a similar approach was extended to the downlink resources sharing scenario in @cite_25 . In @cite_9 , the maximum achievable transmission capacity, which was defined as the spatial density of successful transmissions per unit area, was analyzed for the hybrid D2D and cellular network through stochastic geometry. However, due to the inevitable interference accumulated at the BS, most of the existing analytical results assuming a single D2D pair in a cellular network cannot be directly extended to a scenario with multiple coexisting D2D pairs. Therefore, the performance of D2D communications in the latter is still an under-developed issue, which could further improve the spectrum efficiency and increase the cellular network throughput.