In-band Wireless Backhaul Research Articles

Backhaul-Aware Resource Allocation and Optimum Placement for UAV-Assisted Wireless Communication Network

Driven by its agile maneuverability and deployment, the unmanned aerial vehicle (UAV) becomes a potential enabler of the terrestrial networks. In this paper, we consider downlink communications in a UAV-assisted wireless communication network, where a multi-antenna UAV assists the ground base station (GBS) to forward signals to multiple user equipments (UEs). The UAV is associated with the GBS through in-band wireless backhaul, which shares the spectrum resource with the access links between UEs and the UAV. The optimization problem is formulated to maximize the downlink ergodic sum-rate by jointly optimizing UAV placement, spectrum resource allocation and transmit power matrix of the UAV. The deterministic equivalents of UE’s achievable rate and backhaul capacity are first derived by utilizing large-dimensional random matrix theory, in which, only the slowly varying large-scale channel state information is required. An approximation problem of the joint optimization problem is then introduced based on the deterministic equivalents. Finally, an algorithm is proposed to obtain the optimal solution of the approximate problem. Simulation results are provided to validate the accuracy of the deterministic equivalents, and the effectiveness of the proposed method.

Backhaul-Aware Trajectory Optimization of Fixed-Wing UAV-Mounted Base Station for Continuous Available Wireless Service

Due to high mobility and flexibility, unmanned aerial vehicles (UAVs) have attracted significant attention from both academia and industry in wireless communications. Different types of UAVs are used for different wireless applications. However, there is a dilemma that no suitable approach is available to provide continuous available long-time wireless coverage from UAV-mounted base stations (UBSs) since rotary-wing UAVs require frequent energy replenishment while fixed-wing UAVs cannot hover at fixed locations. In this paper, the fixed-wing UBS is considered for providing continuous available wireless services to ground users with a novel dynamic resource allocation framework. In particular, the optimal resource allocations for both out-band and in-band wireless backhaul schemes are derived using convex optimization theory. Moreover, the sequential convex optimization method is used to obtain the energy-efficient trajectories. Finally, extensive simulation results are provided to verify the effectiveness of our proposed method. A comparison of out-band and in-band backhaul schemes is also provided.

Joint Resource Allocation, Placement and User Association of Multiple UAV-Mounted Base Stations With In-Band Wireless Backhaul

The in-band wireless backhaul that shares spectrum resources between fronthaul and backhaul is a promising method for unmanned aerial vehicle-mounted base stations (UBSs) to provide wireless services with limited resources. However, the existing works mainly considered the out-band wireless backhaul, where the resources of fronthaul and backhaul are allocated separately. Moreover, the UBS placement methods usually focused on the optimization of fronthaul while neglecting the backhaul. In this letter, a placement algorithm that jointly optimizes the resource allocation and user association for multiple UBSs with in-band wireless backhaul is proposed. In particular, the joint optimal resource allocation for both fronthaul and backhaul is derived using convex optimization theory. Given the optimal resource allocation, an equivalent spectrum efficiency is defined to simplify the optimization problem. As a result, an effective algorithm is proposed by solving the user association problem and UBS placement problem iteratively. In addition, simulation results are provided to verify the efficiency and effectiveness of our proposed method.

Beamforming Design for Low-Power In-Band Wireless Backhaul Systems: Centralized and Distributed Approaches

In this paper, we develop beamforming (BF) schemes to minimize the total transmitted power of both uplink (UL) and downlink (DL) in two-tier network with in-band wireless backhaul (WB) while the target rates of user equipment (UE) are satisfied under the backhaul constraints. We propose a centralized BF scheme with an alternate update algorithm optimizing the BF matrices and power allocation vectors. Then the UL-DL feasible target rate region of the proposed scheme is analyzed, which can be enlarged in both UL and DL by increasing the number of antennas of small cell base stations (SBSs). Simulation results show that the proposed in-band WB system for two-tier network can save the power consumption by 33.5% and 82.1% compared with the out-of-band WB system and conventional one-tier network, respectively, at the UE target rates of 80 Mb/s in DL and 40 Mb/s in UL. In addition, we propose a distributed BF scheme to locally optimize the variables at each SBS, which can achieve above 90% of the feasible target rate region of the centralized scheme without the instantaneous information exchange.

Joint Downlink Cell Association and Bandwidth Allocation for Wireless Backhauling in Two-Tier HetNets With Large-Scale Antenna Arrays

The problem of joint downlink cell association (CA) and wireless backhaul bandwidth allocation (WBBA) in two-tier cellular heterogeneous networks (HetNets) is considered. Large-scale antenna array is implemented at the macro base station (BS), while the small cells within the macro cell range are single-antenna BSs and they rely on over-the-air links to the macro BS for backhauling. A sum logarithmic user rate maximization problem is investigated considering wireless backhauling constraints. A duplex and spectrum sharing scheme based on co-channel reverse time-division duplex (TDD) and dynamic soft frequency reuse (SFR) is proposed for interference management in two-tier HetNets with large-scale antenna arrays at the macro BS and wireless backhauling for small cells. Two in-band WBBA scenarios, namely, unified bandwidth allocation and per-small-cell bandwidth allocation scenarios, are investigated for joint CA-WBBA in the HetNet. A two-level hierarchical decomposition method for relaxed optimization is employed to solve the mixed-integer nonlinear program (MINLP). Solutions based on the General Algorithm Modeling System (GAMS) optimization solver and fast heuristics are also proposed for cell association in the per-small-cell WBBA scenario. It is shown that when all small cells have to use in-band wireless backhaul, the system load has more impact on both the sum log-rate and per-user rate performance than the number of small cells deployed within the macro cell range. The proposed joint CA-WBBA algorithms have an optimal load approximately equal to the size of the large-scale antenna array at the macro BS. The cell range expansion (CRE) strategy, which is an efficient cell association scheme for HetNets with perfect backhauling, is shown to be inefficient when in-band wireless backhauling for small cells comes into play.

Small Cell In-Band Wireless Backhaul in Massive MIMO Systems: A Cooperation of Next-Generation Techniques

Massive multiple-inputmultiple-output (MIMO) systems, dense small-cells (SCs), and full duplex are three candidate techniques for next-generation communication systems. The cooperation of next-generation techniques could offer more benefits, e.g., SC in-band wireless backhaul in massive MIMO systems. In this paper, three strategies of SC in-band wireless backhaul in massive MIMO systems are introduced and compared, i.e., complete time-division duplex (CTDD), zero-division duplex (ZDD), and ZDD with interference rejection (ZDD-IR). Simulation results demonstrate that SC in-band wireless backhaul has the potential to improve the throughput for massive MIMO systems. Specifically, among the three strategies, CTDD is the simplest one and could achieve decent throughput improvement. Depending on conditions, with the self-interference cancellation capability at SCs, ZDD could achieve better throughput than CTDD, even with residual self-interference. Moreover, ZDD-IR requires the additional interference rejection process at the BS compared to ZDD, but it could generally achieve better throughput than CTDD and ZDD.

Mobile association for wireless heterogeneous networks with cooperative relays: optimal framework and implementation schemes

Heterogeneous networks improve the spectrum efficiency and coverage of wireless communication networks by deploying low-power nodes on top of the conventional network nodes. In this paper, we consider a heterogeneous cellular network with multiple low-power relay nodes (RN) deployed in each cell using in-band wireless backhaul link between the RN and the base station (BS). The RNs work cooperatively with the BSs on the downlink communication towards the user equipments. Due to the disparity between the transmit powers of the BSs and the RNs, mobile association schemes developed for the conventional homogeneous networks may lead to a highly unbalanced traffic load distribution with most of the traffic being concentrated on the BSs. In this paper, we propose a new mobile association framework for the heterogeneous wireless networks with node cooperation. The proposed framework aims to maximize the network capacity and balance the traffic load among the network nodes. A pseudo-optimal solution is obtained for the proposed mobile association framework using a centralized gradient-descent algorithm. To enable online implementation, we further introduce a distributed algorithm using a pricing mechanism. Numerical results show that a prominent improvement in network capacity can be achieved by the proposed load balancing-based mobile association scheme compared with the conventional counterpart. Moreover, compared with the pseudo-optimal solution, the proposed pricing-based distributed algorithm only suffers a small loss in network capacity and thus can be considered as an efficient real-time implementation alternative.

Point-to-multipoint in-band mmwave backhaul for 5G networks

Cost-effective and scalable wireless backhaul solutions are essential for realizing the 5G vision of providing gigabits per second anywhere. Not only is wireless backhaul essential to support network densification based on small cell deployments, but also for supporting very low latency inter-BS communication to deal with intercell interference. Multiplexing backhaul and access on the same frequency band (in-band wireless backhaul) has obvious cost benefits from the hardware and frequency reuse perspective, but poses significant technology challenges. We consider an in-band solution to meet the backhaul and inter-BS coordination challenges that accompany network densification. Here, we present an analysis to persuade the readers of the feasibility of in-band wireless backhaul, discuss realistic deployment and system assumptions, and present a scheduling scheme for inter- BS communications that can be used as a baseline for further improvement. We show that an inband wireless backhaul for data backhauling and inter-BS coordination is feasible without significantly hurting the cell access capacities.