Channel measurements and modelling have been long considered as the foundation for effective and efficient wireless communication system designs. Recently, there has been an explosive growth of research work dedicated to the so-called device-to-device (D2D) communications. In the mean time, however, measurements and modelling of D2D channels seem to somewhat fall behind. To promote research on these aspects, in this study, the authors provide a critical overview of the current state of research on D2D channels, and comprehensively discuss future trends and research directions.

This study presents a ‘subblock tracking’ based equalisation technique for orthogonal frequency division multiplexing (OFDM) based device-to-device communications over high mobility environment. This technique is to use a rectangular-shaped receive window to partition the OFDM block into subblocks rendering the channel response of each subblock to be time-invariant, thereby allowing one to equalise the channel frequency response on each subcarrier of the subblocks simply by a single tap. The equalised signals are then combined to give the final output, where the combination weights are designed to minimise detection errors. The authors mathematically characterise the detection performance by deriving the signal-to-interference ratio as a function of channel-time-variation and the numbers of subblocks. To further enhance the proposed design without imposing computational consumption, a subblock tracking scheme with partially overlapped partition is presented and theoretically analysed. Simulation results demonstrate the advantages this method over the conventional schemes in high mobility environment.

This study considers spectrum sharing problems in the heterogeneous wireless networks where different device-to-device (D2D) users coexist with the cellular users. The authors propose a novel scheme, called ‘spectrum partition-based D2D transmission’ (SPDT), to improve spectrum efficiency of the D2D and cellular networks. In SPDT, the D2D users assist the cellular transmissions to gain some spectrum released from the cellular system. Then, the obtained spectrum is divided into several frequency bands and each band is assigned to a different D2D pair for its data transmission. Under the quality-of-service (QoS) constraints of both the D2D and cellular users, the authors exploit the tradeoff in the power allocation of the D2D transmitters and show that the number of the allowed accessing D2D pairs can be maximised by optimising the D2D transmitters power for SPDT. For comparison the power optimisation problem is also investigated with the objective of maximising the number of allowed users accessing D2D pairs in the power control scheme, called ‘underlay D2D transmission’ (UDT). This is where the D2D users access the spectrum being used by the cellular users with power control while ensuring that the QoS of the cellular transmissions is satisfied. Finally, the simulation results show evident performance gains of the proposed SPDT scheme over the UDT scheme.

Exploiting direct transmissions between geographically close mobile users without passing through the base stations, device-to-device (D2D) communications contribute significant improvement to the spectral efficiency of cellular networks. In D2D-assisted cellular networks, the social interaction of mobile users is an important property that will affect the practical performance and should be seriously accounted in the network resource allocation, which is yet to be fully explored. In this study, the authors investigate the social interactions for D2D transmissions and develop a contact time model to characterise the D2D links. A D2D link can be considered for resource allocation only when the two users encounter and their contact time is enough long to complete a meaningful transmission. They formulate and compare both sociality-blind and sociality-aware optimisation problems for resource allocation in D2D-assisted cellular networks. Extensive numerical results are presented, validating that the sociality-aware resource allocation can achieve higher performance than that of the sociality-blind approach.

This work studies the dynamic resource allocation for device-to-device communication and proposes a service time prediction-based dynamic resource allocation mechanism for a cellular system. The proposed allocation mechanism introduces the concept of ‘utility’ as a metric to reflect the quality of experience of users with different service requirements, and takes into account service time prediction in spectral resource allocation. Specifically, it enables a base station to predict the service time of a new user and the remaining service time of existing users in the system, and takes into account the predicted service time and remaining service time in calculating the effective utility gain caused by the arrival of a new user. Based on the calculated effective utility gain, the base station will select the physical resource block (PRB) with the largest effective utility gain every time it allocates a PRB to the new user. Simulation results show that the proposed dynamic allocation mechanism can significantly improve the system performance in terms of the overall average utility and the number of users successfully admitted to the system.

New distributed link scheduling schemes based on a recently proposed device-to-device (D2D) communication technology, FlashLinQ are studied. FlashLinQ is an orthogonal frequency division multiplexing-based synchronous D2D communication system, which achieves a high data rate at longer communication ranges over a licensed spectrum. In this method, some links are selected for data traffic for efficient frequency reuse to avoid interference in D2D links. Where every link is assigned a scheduling priority, so in TX (transmitter) and RX (receiver) yielding, the links which are highly interfered by higher priority links give up communication. However, this technique causes inefficiency in the resource reuse from the cascade yielding problem because link scheduling is performed individually by its RX and TX nodes. To deal with this problem, the authors propose two new link scheduling schemes based on a binary matrix called on-off interference map (I-Map), which indicates strong interference between links. In these schemes, each TX generates a binary I-Map matrix with the collective information from TX and RX blocks. With the common I-Map matrix that displays inter-link interference between D2D links, the proposed schemes achieve better performance as compared to the conventional distributed FlashLinQ scheduling scheme.

User-specific network controlled device-to-device (D2D) communication is a promising method to improve the system performance of the future cellular networks. It is a challenging task to realise the optimal link adaptation for D2D reliable multicast because of the nature of wireless radio channels. In this study, network coding is employed to enhance D2D multicast by introducing the characteristic that different information network-coded into a single multicast packet is destined to different receivers. A user-specific bit mapping algorithm is then proposed to make different information having a different equivalent coding rate before network-coded. A corresponding user-specific link adaptation scheme is proposed to adaptively choose an optimal modulation and coding scheme (MCS) for D2D multicast. Furthermore, the receiver detection procedure is then derived to recover the expected information for different users. As a result, different users obtain their respective information from a single network-coded multicast packet with different equivalent MCS according to the respective channel quality from the transmitter to them. Numerical results show that the user-specific link adaptation scheme can improve the capacity performance of network controlled D2D multicast. The throughput gain varies from 13 to 45% in different channel quality scenarios.

Despite the numerous benefits brought by device-to-device (D2D) communications, the introduction of D2D into cellular networks poses many new challenges in the resource allocation design because of the co-channel interference caused by spectrum reuse and limited battery life of user equipment's (UEs). Most of the previous studies mainly focus on how to maximise the spectral efficiency and ignore the energy consumption of UEs. In this study, the authors study how to maximise each UE's Energy Efficiency (EE) in an interference-limited environment subject to its specific quality of service and maximum transmission power constraints. The authors model the resource allocation problem as a non-cooperative game, in which each player is self-interested and wants to maximise its own EE. A distributed interference-aware energy-efficient resource allocation algorithm is proposed by exploiting the properties of the nonlinear fractional programming. The authors prove that the optimal solution obtained by the proposed algorithm is the Nash equilibrium of the non-cooperative game. The authors also analyse the tradeoff between EE and SE and derive closed-form expressions for EE and SE gaps.

Device-to-device (D2D) communication has recently attracted much research attention because of its potential to increase the capacity of cellular networks. Most existing works aim to maximise the overall system throughput (system-centric), which ignores the actual traffic demands of D2D users. In this study, the authors consider user-centric relay assisted D2D communications where D2D users have different evaluations for the significance of every unit of increased data rate. By considering the traffic demands of D2D users, the authors propose a Vickrey–Clarke–Groves auction based relay allocation mechanism (ARM) in which every D2D user submits a bid to the basestation (BS). The submitted bids indicate D2D users’ valuation on every unit of the increased data rate. The BS then allocates relays to D2D users by maximising the social welfare of D2D users while maintaining a predefined data rate requirement for cellular users. A payment scheme to charge D2D users for using relays is designed, and the authors show that the auction is truthful. The authors also extend the results to a general case and provide a general ARM accordingly. Extensive simulation results are provided to demonstrate the performance of the proposed mechanisms.

In the present study, the authors investigate robust secrecy rate optimisation problems for a multiple-input-single-output secrecy channel with multiple device-to-device (D2D) communications. The D2D communication nodes access this secrecy channel by sharing the same spectrum, and help to improve the secrecy communications by confusing the eavesdroppers. In return, the legitimate transmitter ensures that the D2D communication nodes achieve their required rates. In addition, it is assumed that the legitimate transmitter has imperfect channel state information of different nodes. For this secrecy network, the authors solve two robust secrecy rate optimisation problems: (a) robust power minimisation problem, subject to the probability based secrecy rate and the D2D transmission rate constraints; (b) robust secrecy rate maximisation problem with the transmit power, the probabilistic based secrecy rate and the D2D transmission rate constraints. Owing to the non-convexity of robust beamforming design based on two statistical channel uncertainty models, the authors present two conservative approximation approaches based on ‘Bernstein-type’ inequality and ‘S-Procedure’ to solve these robust optimisation problems. Simulation results are provided to validate the performance of these two conservative approximation methods, where it is shown that ‘Bernstein-type’ inequality based approach outperforms the ‘S-Procedure’ approach in terms of achievable secrecy rates.

In vehicle-to-vehicle (V2V) communications, low delay and long propagation distance are very important for multi-hop safety-related message broadcasting. Most earlier studies focused on one-hop broadcasting while little attention has been paid to multi-hop delay and propagation distance. In this study, a new model for analysing the connectivity probability, average hop count and one-hop delay of multi-hop safety-related message broadcasting in V2V communications is built, taking into account the following factors: propagation distance, one-hop transmission range, distribution of vehicles, vehicle density, average length of vehicles and minimum safe distance between vehicles. Simulation results demonstrate that the proposed model can provide better performance in terms of multi-hop delay and there exists an optimal one-hop transmission range to minimise the multi-hop delay. After that, A new scheme is proposed to track the optimal one-hop transmission range by using a Genetic Algorithm. With this scheme, vehicles are allowed to adjust the one-hop transmission range based on vehicle density to reduce the multi-hop delay. The proposed scheme is validated by simulations using realistic vehicular traces.

Device-to-device (D2D) is considered as a promising technique to increase resource utilisation and to provide new application in future wireless networks. This study reviews the standardisation progress of D2D in third-generation partnership project. Three scenarios, in-coverage, partial-coverage and out-of-coverage, are defined. For these scenarios, channel model is obtained by the amendment of existing channel model. The function of D2D operation is divided into three building blocks: synchronisation, discovery and communication. The synchronisation procedure and synchronisation signal/channel are discussed first. Then the design aspects of D2D discovery and communication are discussed in detail, including channel structure design, resource allocation mechanisms, interference management and so on. Future works towards post Rel-12 releases are also briefly outlined.

Machine-to-machine (M2M) communication plays an important role in various kinds of intelligent networks. In this study, a hybrid cooperation scheme for data collection in hierarchical smart building networks (SBN) is proposed under the framework of M2M communications. The hierarchical network structure means that the data collection process is carried out via multi-layer communications. In the first layer, smart metres organise themselves into clusters and send information to the cluster-heads. Then all cluster-heads forward the received information to the base station automatically in the second layer. In particular, the roles of cluster-head can be acted by either fixed nodes or user terminals in the building, and this endow a hybrid cooperation mode to the data collection process. To construct the network structure and utilise the resources efficiently, the authors first provide some theoretical analysis on the influence of network structure and bandwidth constraints. Then a distributed scheme for joint structure formation and subband allocation is proposed based on coalitional game theory. Furthermore, for the feasibility of this scheme in practical applications, some improvements of the proposed scheme have also been made at last. The advantages of the proposed scheme are verified by simulation results.

This study presents the design of a service-oriented networking platform to offer dynamic networking services in support of mobile group communications among connected smart devices. This platform enables users to access smart services anywhere and anytime with embedded devices while satisfying their requirements of various smart services. The proposed platform adopts a session initiation protocol (SIP) protocol as service signalling protocol that provides high extensibility and compatibility with an existing service system. The overlay network structure suggested in our platform provides an advantage of building a scalable and dynamic service network based on multiple smart zones. Furthermore, the quality of end-to-end service in dynamic network environments is guaranteed by virtue of the smart delivery scheme in the platform.