Device to Device (D2D) Communication

Cognitive radio [96] has equally gained much attention in recent years primarily due to the offered potential to reusing the assigned spectrum. Cognitive radio networks have therefore been developed in order to enhance throughput and coverage. In this direction D2D communication utilises the cellular spectrum: with potential of better QoS as it occurs in the controlled cellular spectrum.

As a successor and inspired by cognitive radio and relay networks, D2D have recently been investigated as a potential way to improve secondary user throughput [40, 88, 97]. As a type of cognitive technology, D2D communication could optimize the system capacity over the shared uplink (UL) or downlink (DL) resources while fulfilling prioritized cellular service constrains [96, 98]. D2D communications underlaying a cellular infrastructure has been

36 proposed as a means of taking advantage of the physical proximity of communicating devices, increasing resource utilization, and improving cellular coverage [99].

In D2D-enabled cellular networks, data communications between user equipment can be completed by two modes: the cellular mode and the D2D mode where UEs bypass the base station and directly communicate with each other. Usually the transmission mode is selected based on the distance between UEs [100]. Figure 2-13 [101] represents a simple D2D communication scenario. The eNodeB transports the cellular traffic, while the D2D transmission is carried out using the D2D link with partial involvement of the eNodeB (network assisted) for handling control information and session setup.

There are two different types of D2D, with network assist and without network assist as

shown in Figure 2-14 [101]. A major difference between D2D communications over a

cellular network (network assisted) and a traditional D2D system is that D2D communications in a cellular network assume the underlay of a cellular network. This

provides significant potential advantages to device discovery such that resource allocation, interference coordination, and collision avoidance can be carried out more efficiently.

However, it must also support the cases where the network coverage is not available. As an example, coverage holes due to imperfect cell coverage or temporary failure of network access points, implying that D2D communications must be sustained when devices are partially or completely lost or outside the network coverage [102].

D2D pairs should keep a certain distance away from base stations and primary cellular UEs to avoid generating or receiving heavy interference. As D2D communications underlaying a cellular infrastructure introduce many new interference scenarios such as resources sharing between D2D communications and cellular networks, also additional interference introduced by new D2D transmissions, interference management becomes more critical. Some of the key design questions raised in the implementation of the D2D technology include interference mitigation and resource allocation in the integrated networks (network with cellular and D2D UEs) amongst others. Current research topics of interference management include mode selection, resource allocation and power control. Power control is a common method studied

38 for performance improvement of networks with different entities (both D2D and cellular) [103]. It involves the adjustment of the D2D transmitter power and/or the eNodeB transmitter power, or adopting some CoMP functionalities such as beamforming using smart antennas to achieve it. Several contributions have been provided in literature to identify efficient techniques to achieve the above in an integrated environment.

In [104], a novel scenario which is based on the coexistence of coordinated system (CoMP) and D2D for Interference management is presented, enhancing system throughput and energy efficiency. It uses coordination and zero forcing algorithm to mitigate inter and intra cell interference. In [105], an antenna based solution where transmit and receive beams are formed based on the channel state information (CSI) through traditional D2D links was presented.

5G is expected to also support heterogeneous networks, with macro-cells, micro-cells, small- cells, and relays [106]. One way to expand an existing macro-network, while maintaining it as a homogeneous network, is to “densify” it by adding more sectors per eNodeB or deploying more macro-eNodeBs. However, reducing the site-to-site distance in the macro- network can only be pursued to a certain extent because finding new macro-sites becomes increasingly difficult and can be expensive, especially in city centres. An alternative is to introduce small cells through the addition of low-power base stations Home eNodeBs (HeNodeBs) or Relay Nodes (RNs) or Remote Radio Heads (RRH) to existing macro- eNodeBs. Site acquisition is easier and cheaper with this equipment which is also correspondingly smaller [107, 108]. Nokia Siemens Networks [24, 55] has shown that a thousand fold increase in network capacity with a 10 Mbit/s minimum downlink user data rate can be achieved by a HetNet configuration that reuses all the existing macro sites and deploys ten times as many outdoor micro sites [55]. Femto-cell networks are currently seen as a new communication paradigm for the ever increasing ubiquitous wireless traffic

demands. Being pervasive by nature, its proximity to the subscriber opens a new world of possibilities for the development of applications.

Small cells are primarily added to increase capacity in hot spots with high user demand and to fill in areas not covered by the macro network both outdoors and indoors. They also improve network performance and service quality by offloading from the large macro-cells. The result is a heterogeneous network with large macro-cells in combination with small-cells providing increased bitrates per unit area [107]. Among them, D2D communication and cloud computing services demanded by smartphones could be moved from large server farms to Home eNodeBs, provided that these are equipped with computational and storage resources, thus improving user experience on latency and download/upload speed.

In view of the general overview of heterogeneous networks and D2D technology in the above subsection, the issue of interference between the simultaneous transmission of cellular and D2D traffic in a small-cell environment has been identified as a key issue in realising the hybrid network. Thus, specific algorithms proposed for interference mitigation in the network will be further presented in Chapter 6 to compare and contrast implementation methods and performance results with the algorithm proposed in this research work.