OUTBAND D2D
In this section, we review the papers in which D2D commu-nications occurs on a frequency band that is not overlappingwith the cellular spectrum. Outband D2D is advantageousbecause there is no interference issue between D2D andcellular communications. Outband D2D communication canbe managed by the cellular network (i.e., controlled) or it canoperate on its own (i.e., autonomous).A. ControlledIn works that fall under this category, the authors proposeto use the cellular network advanced management features tocontrol D2D communication to improve the efficiency andreliability of D2D communications. They aim to improvesystem performance in terms ofthroughput, power efficiency,multicast, and so on.The authors of [93] propose to use ISM band for D2Dcommunications in LTE. They state that simultaneous channelcontention from both D2D and WLAN users can dramaticallyreduce the network performance. Therefore, they propose togroup D2D users based on their QoS requirement and allowonly one user per group to contend for the WiFi channel. Thechannel sensing between groups is also managed in a way thatthe groups do not sense the same channel at the same time.They show via simulation that their approach increases theD2D throughput up to25%in comparison to the scenario inwhich users contend for the channel individually.The authors of [33] and [32] propose to use D2D communi-cations for increasing the throughput and energy efficiencyofcellular networks. The authors propose to form clusters amongcellular users who are in range for WiFi communication. Afterthe cluster is formed, only the cluster member with the highestcellular channel quality (i.e., cluster head) communicates withthe BS. The cluster head is also responsible to forward thecellular traffic of its clients (i.e., other users who belongtothe same cluster) to the BS. The authors provide an analyticalmodel to compute the throughput and power consumptionfor the proposed scheme. The advantages of this schemeare threefold:(i)the spectral efficiency increases becausethe cluster head has the highest channel quality among thecluster members which corresponds to transmissions with highModulation and Coding Schemes (MCS);(ii)the energyefficiency is increased because the cluster clients can go tocellular power saving mode;(iii)the fairness can be increasedbecause the cellular resources is distributed among clustermembers in a way that users with poor channel quality are notstarved. The authors show via numerical simulation that D2Dcommunications improve throughput and energy efficiencywith respect to classical Round Robin schedulers by50%and30%, respectively. The results also show that the proposedscheme can achieve almost perfect fairness