Junyu Liu

On-demand scheduling: achieving QoS differentiation for D2D communications

As a major supplement to LTE-Advanced, D2D communications underlaying cellular networks have proven efficient in offloading network infrastructures and improving network performance. The scheduling mechanism plays a key role in providing better user experience in D2D communications. However, controlled by operators, D2D communications pose specific problems that do not exist in available wireless networks. Therefore, mature scheduling mechanisms devised for cellular networks or ad hoc networks are not directly applicable to D2D communications. In this article, we first review recent research on scheduling mechanisms for D2D communications, and discuss the design considerations and implementation challenges. Then we propose an on-demand scheduling mechanism, DO-Fast, which can provide QoS differentiation capabilities. Next, we provide performance evaluations based on simulations and an experimental testbed. Finally, we conclude this article and point out possible directions for future research.

Enhancement for content delivery with proximity communications in caching enabled wireless networks: architecture and challenges

To cater for the exploding growth of video traffic, small cell base stations (SBSs) and device-to-device-enabled caching and delivery have been regarded as promising techniques for future wireless networks. In this article, we design a proximity communications enhanced multilayer caching and delivery architecture. Then merits possessed by the proposed architecture are highlighted, and challenges and open issues are comprehensively presented. Specifically, we shed light on the trade-offs between key performance indicators (e.g., hit ratio, latency, and coverage) and operation costs (e.g., device storage space, wireless bandwidth, and device battery life), and then clarify fundamental coupling between content caching and delivery. To further verify the effectiveness of the cooperation among SBSs and user equipments, we propose a distributed content caching and delivery strategy, jointly considering popularity distribution, diverse storage capability, and user mobility. Simulation results demonstrate that the proposed strategy can significantly lower the content retrieval latency and reduce the traffic flowing to core networks. Furthermore, design details of the experimental testbed are presented, and the validity of our developed strategy is verified.

On Transmission Capacity Region of D2D Integrated Cellular Networks With Interference Management

In this paper, we characterize the transmission capacity region (TCR) in D2D integrated cellular networks when two prevalent interference management techniques, power control and Successive Interference Cancellation (SIC) are utilized. The TCR is defined as the enclosure of all feasible sets of active transmitter intensities in cellular and D2D systems. Closed-form approximate expressions of TCR are derived for two spectrum sharing modes, i.e., reuse mode and dedicated mode. The analysis provides insights into the impact of network parameters, interference management methods, as well as bandwidth allocation policy on the TCR. Moreover, we compare the reuse mode and dedicated mode in terms of TCR. Specifically, with power control, given the same target rate for cellular users and D2D users, the TCR of the dedicated mode is shown to be entirely enclosed by that of the reuse mode when 2α/2 ≤ θ+2, where α and θ are, respectively, the path loss exponent and decoding threshold. However, with SIC utilized, numerical results show that when θ > 1, better performance can always be achieved by the reuse mode in terms of TCR. The results can serve as a guideline for the design of efficient interference management techniques and spectrum regulation in D2D integrated cellular networks.

A Distributed Opportunistic scheduling protocol for device-to-device communications

In this paper, we consider the distributed scheduling problem for the OFDM based device-to-device (D2D) communications. In order to fully exploit the spatial diversity of the channel variation as well as provide access fairness for all D2D links, we propose a synchronous Distributed Opportunistic scheduling protocol under Fairness constraints (DO-Fast). DO-Fast incorporates the opportunistic scheduling with a round-robin strategy. By exchanging local Channel State Information (CSI) in a distributed way, the opportunistic scheduling strategy enables the links with better channel conditions to take precedence for higher access priorities. It leads to more concurrent transmissions and higher system throughput than the random scheduling strategy, where links are allocated with priorities in a random manner regardless of channel conditions. Meanwhile, we prompt a round-robin strategy so that the D2D links would take high priorities alternately, which guarantees the short-term fairness requirements of the links with poor channel conditions. We show via simulations that DO-Fast achieves throughput improvement over the existing scheduling protocol from the network perspective with acceptable delay performance.

D2D Enhanced Co-Ordinated Multipoint in Cloud Radio Access Networks

Coordinated multipoint (CoMP) is an efficient technique to increase cell-edge coverage probability and throughput in cloud radio access network (C-RAN). In this paper, we integrate device-to-device (D2D) communications with CoMP by applying a distance based mode selection rule for downlink users in C-RAN, exploiting the proximity of D2D communications to improve system spectral efficiency. Using stochastic geometry, we first derive the signal-to-interference ratio distribution at a typical downlink user and a typical D2D receiver when two types of CoMP schemes, namely, zero-forcing beamforming (ZFBF) and noncoherent joint transmission (NC-JT), are applied in C-RAN. In addition, we analytically compare ZFBF and NC-JT using rate coverage probability. Meanwhile, we analyze the effect of D2D communications on enhancing the area spectral efficiency (ASE) of CoMP enabled C-RAN system. Numerical results show that increasing the co-operative cluster size would potentially degrade the system ASE when the network is heavily loaded. Furthermore, it is observed that enabling D2D mode in C-RAN can effectively offload the traffic of C-RAN and provide significant ASE gains if mode selection threshold is properly designed. Lastly, it is analytically demonstrated that spectrum resources can be better exploited by D2D users if they coexist with ZFBF enabled radio units (RUs) rather than NC-JT enabled RUs.

Interference-aware resource allocation for D2D underlaid cellular network using SCMA: A hypergraph approach

Device-to-Device (D2D) communication underlaid cellular networks has been regarded as a technology with great promise to provide higher transmission rate, lower latency and better energy efficiency in services between user terminals in the future fifth generation (5G) wireless network. In this paper, we consider the resource allocation problem to enhance the system performance. Specifically, we use hypergraph to characterize the interference among cellular uplinks and D2D links when sparse code multiple access (SCMA) is applied as the multiple access strategy. Targeting at maximizing system sum rate, we propose an Interference-Aware Hypergraph based Codebook Allocation (IAHCA) algorithm. Using IAHCA, each orthogonal SCMA resource, i.e., SCMA codebook, is allowed to be shared by one cellular uplink and more than one D2D links. As a consequence, available SCMA resources can be fully exploited, thereby effectively achieving higher system throughput and activating more D2D links. Simulation results confirm that IAHCA outperforms conventional graph based algorithm and other hypergraph based algorithms.

Resource allocation in device-to-device communication underlaid cellular network using SCMA: An opportunistic approach

Device-to-Device (D2D) communication underlaid cellular network has been proven to enable a significant performance improvement in future fifth generation (5G) wireless networks. Recently, sparse code multiple access (SCMA) has been proposed as an efficient multiple access method to support massive connectivity and diverse applications. In this paper, we incorporate SCMA into D2D communication underlaid cellular network, targeting at enhancing overall network performance. In order to fully exploit the potential of SCMA, we consider that SCMA codebooks can be reused by cellular uplinks and D2D links. However, if not properly handled, the resulting mutual interference may greatly degrade the performance of the hybrid system. To tackle the problem, we have proposed an Opportunistic SCMA Codebook Allocation (OSCA) strategy, aiming to maximize the sum rate of the hybrid system. Specifically, the idea of opportunistic scheduling has been applied in OSCA to implement codebook allocation for cellular user and D2D user. We show via simulation results that the proposed strategy could yield performance enhancement over the conventional strategies in terms of system spectral efficiency.

Analysis of transmission capacity region in D2D integrated cellular networks with power control

The integration of Device-to-Device (D2D) communications into cellular networks, albeit improving spectrum efficiency, may inevitably lead to cross-tier interference between cellular users and D2D users. In this paper, we endow D2D users with the capability of power control to address the cross-tier interference and theoretically analyze the benefits of power control in enhancing the transmission capacity region (TCR). In particular, based on transmission capacity, the TCR is defined as the enclosure of all feasible combinations of transmitter intensities in cellular and D2D networks. We first employ the stochastic geometry framework to derive closed-form expressions of the TCR for two prevalent spectrum sharing modes, i.e., reuse mode and dedicated mode. As for the reuse mode, we then study how to enlarge the TCR through initializing the power levels of cellular users and D2D users. Finally, the dedicated mode is compared with the reuse mode through TCR. Specifically, given the same target rate for cellular users and D2D users, the reuse mode is shown to outperform the dedicated mode in terms of the TCR when 2α/2 ≤ θ + 2, where α and θ are, respectively, the path loss exponent and decoding threshold. The analysis provides useful guidance for spectrum regulation and design of efficient power control techniques in D2D integrated cellular networks.

Modeling and Analysis of SCMA Enhanced D2D and Cellular Hybrid Network

Sparse code multiple access (SCMA) has been recently proposed for the future wireless networks, which allows nonorthogonal spectrum resource sharing and enables system overloading. In this paper, we apply SCMA into device-to-device (D2D) communication and cellular hybrid network, targeted at using the overload feature of SCMA to support massive device connectivity and expand network capacity. Particularly, we develop a stochastic geometry-based framework to model and analyze SCMA, considering underlaid and overlaid modes. Based on the results, we analytically compare SCMA with orthogonal frequency-division multiple access (OFDMA) using area spectral efficiency (ASE) and quantify closed-form ASE gain of SCMA over OFDMA. Notably, it is shown that system ASE can be significantly improved using SCMA and the ASE gain scales linearly with the SCMA codeword dimension. Besides, we endow D2D users with an activated probability to balance cross-tier interference in the underlaid mode and derive the optimal activated probability. Meanwhile, we study resource allocation in the overlaid mode and obtain the optimal codebook allocation rule. It is interestingly found that the optimal SCMA codebook allocation rule is independent of cellular network parameters when cellular users are densely deployed. The results are helpful in the implementation of SCMA in the hybrid system.

Comp Transmission in Cloud Radio Access Networks

Coordinated multi-point (CoMP) is an efficient technique to increase cell-edge coverage probability and throughput in cloud radio access network (C- RAN). In this paper, we develop a tractable framework to analytically evaluate the performance of two typical CoMP schemes, namely, zero-forcing beamforming(ZFBF) and non-coherent joint transmission (NC-JT). Using the stochastic geometry based approach, we characterize the signal-to- interference ratio (SIR) distribution at a typical downlink user. Furthermore, we consider the effect of imperfect channel state information (CSI) on coverage probability of the typical user from the practical design perspective. It is observed that ZFBF can outperform NC-JT only when there is sufficient overhead to convey the quantized CSI. In summary, this work serves as a good rule of thumb to compare the effect of CoMP techniques in C-RAN.