Marco Belleschi

Performance analysis of a distributed resource allocation scheme for D2D communications

Device-to-device (D2D) communications underlaying a cellular infrastructure has recently been proposed as a means of increasing the cellular capacity, improving the user throughput and extending the battery lifetime of user equipments by facilitating the reuse of spectrum resources between D2D and cellular links. In network assisted D2D communications, when two devices are in the proximity of each other, the network can not only help the devices to set the appropriate transmit power and schedule time and frequency resources but also to determine whether communication should take place via the direct D2D link (D2D mode) or via the cellular base station (cellular mode). In this paper we formulate the joint mode selection, scheduling and power control task as an optimization problem that we first solve assuming the availability of a central entity. We also propose a distributed suboptimal joint mode selection and resource allocation scheme that we benchmark with respect to the centralized optimal solution. We find that the distributed scheme performs close to the optimal scheme both in terms of resource efficiency and user fairness.

A comparative study of power control approaches for device-to-device communications

Device-to-device (D2D) communications integrated into cellular networks is a means to take advantage of the proximity of devices and thereby to increase the user bitrates and system capacity. D2D communications has recently been proposed for the 3GPP Long Term Evolution (LTE) system as a method to increase the spectrum- and energy-efficiency. Such systems support a wide range of power control schemes based on a combination of open-loop and closed-loop components and there is a need to set the associated control parameters such that spectrum- and energy-efficiency targets are met. In this paper we study the performance of various power control strategies applicable to D2D communications in LTE networks and compare them with a utility function maximization approach that balances spectrum efficiency and the total transmission power. Our reference scheme is based on a fully distributed algorithm that iteratively sets the signal-to-interference-plus-noise (SINR) targets and corresponding transmit power levels. We find that the LTE-based power control approach performs close to the optimal scheme provided that the associated parameters are properly set1.

Benchmarking Practical RRM Algorithms for D2D Communications in LTE Advanced

Device-to-device (D2D) communication integrated into cellular networks is an advanced tool to take advantage of the proximity of devices and allow for reusing cellular resources and thereby to increase the user bitrates and the system capacity. However, the introduction of D2D in legacy long term evolution (LTE) cellular spectrum requires to revisit and modify the existing radio resource management and power control (PC) techniques in order to fully realize the potential of the proximity and reuse gains and to limit the interference to the cellular layer. In this paper, we examine the performance of the legacy LTE PC tool box and benchmark it against an utility optimal iterative scheme. We find that the open loop PC scheme of LTE performs well for cellular users both in terms of the used transmit power levels and the achieved signal-to-interference-and-noise-ratio distribution. However, the performance of the D2D users as well as the overall system throughput can be boosted by the utility optimal scheme, by taking better advantage of both the proximity and the reuse gains. Therefore, in this paper we propose a hybrid PC scheme, in which cellular users employ the legacy LTE open loop PC, while D2D users exploits the utility optimizing distributed PC scheme. We also recognize that the hybrid scheme is not only nearly optimal, and can balance between spectral and energy efficiency, but it also allows for a distributed implementation at the D2D users, while preserving the LTE PC scheme for the cellular users.

Message Passing Resource Allocation for the Uplink of Multi-Carrier Multi-Format Systems

We propose two novel distributed resource allocation (RA) schemes for the uplink of a cellular multi-carrier multi-format system based on the message passing (MP) technique. In the proposed approaches each transmitter iteratively sends and receives information messages to/from the base station with the goal of achieving an optimal RA strategy. The exchanged messages are the solution of small distributed allocation problems. Hence, despite the NP-hardness of the original RA problem, they distribute the computational effort in the cell among all the transmitters and the base station. Specifically, the first algorithm combines MP with a dynamic programming formula solved at each step, while the second method initially solves to optimality a simplified single-format RA via MP, and eventually performs format allocation to satisfy the rate constraints. Compared to alternatives, numerical results assess the validity of MP-based schemes both in terms of efficiency and complexity.

On the Convergence and Optimality of Reweighted Message Passing for Channel Assignment Problems

Many assignment problems, and channel allocation in OFDMA networks is a typical example, can be formulated as bipartite weighted b-matching (BWBM) problems. In this letter we provide a proof of the convergence and the optimality of the reweighted message passing (ReMP) algorithm when applied to solve BWBM problems in a distributed fashion. To this aim, we first show that the ReMP rule is a contraction mapping under a maximum mapping norm. Then, we show that the fixed convergence point is an optimal solution for the original assignment problem.

Fast Power Control for Cross-Layer Optimal Resource Allocation in DS-CDMA Wireless Networks

This paper presents a novel cross-layer design for joint power and end-to-end rate control optimization in DS-CDMA wireless networks, along with a detailed implementation and evaluation in the network simulator ns-2. Starting with a network utility maximization formulation of the problem, we derive distributed power control, transport rate and queue management schemes that jointly achieve the optimal network operation. Our solution has several attractive features compared to alternatives: it adheres to the natural time-scale separation between rapid power control updates and slower end-to-end rate adjustments, and uses simplified power control mechanisms with reduced signalling requirements. We argue that these features are critical for a successful real-world implementation. To validate these claims, we present a detailed implementation of a cross-layer adapted networking stack for DS-CSMA ad-hoc networks in ns-2. We describe several critical issues that arise in the implementation, but are typically neglected in the theoretical protocol design, and evaluate the alternatives in extensive simulations.

Near-optimal practical power control schemes for D2D communications in cellular networks

Device-to-device (D2D) communication has the potential of increasing the system capacity, energy efficiency and achievable peak rates while reducing the end-to-end latency. To realize these gains, recent works have proposed resource allocation (RA) and power control (PC) approaches that show near optimal performance in terms of spectral or energy efficiency. However, the proposed schemes either consider a single cell environment or assume instantaneous channel state information (CSI) and/or rely on iterative algorithms that require excessive inter-node message exchange and suffer from slow convergence time. For D2D user equipment (UE), we propose a distributed utility optimization based PC scheme that relies on locally available measurement data and is made practical by constraining the number of iterations and the interference caused to the cellular receiver, while legacy UEs employ the standard Long Term Evolution (LTE) PC. We investigate the performance of the proposed PC scheme when combined with two RA schemes that differ in terms of the required channel state information. We find that when properly tuned, this practical PC scheme combined with a RA algorithm that requires limited channel knowledge, not only shows near optimal performance, but it also constraints the impact of D2D communications on the cellular layer.

Complexity analysis and heuristic algorithms for radio resource allocation in OFDMA networks

In this paper, we address the problem of allocating users to radio resources (i.e. subcarriers) in the downlink of an OFDMA system. In particular, we consider a multi-format resource allocation problem (MF-RAP) in which the link adaptation adjusts the spectral efficiency for each user-subcarrier pair, i.e. for each radio link, in order to minimize the total transmission power while fulfilling a rate request for each user. We propose an integer linear programming (ILP) formulation of the problem and exhaustively discuss the computational complexity. Specifically, we prove that the problem is NP-hard in the strong sense and demonstrate that it is hard to be approximated in polynomial time within a constant factor. Hence, we present heuristic approaches that achieve “reasonably good” solutions in the general case. Computational experiences show that, in comparison with a commercial state-of-the-art ILP optimization solver, the proposed algorithms are effective in terms of solution quality and CPU times.

A Min-Sum Approach for Resource Allocation in Communication Systems

This paper considers distributed protocol design for resource allocation (RA) problems. We propose a fully decentralized RA scheme based on the min-sum message passing (MP) approach in which each message is the solution of small distributed allocation problems. Due to the presence of cycles in the network graph, the MP routine may not converge to a fixed point. To this end, we introduce a reweighted MP (ReMP) algorithm that perturbs the ordinary min-sum algorithm by suitably re-weighting messages. ReMP distributes the computational effort of achieving an optimal RA among nodes. Such feature makes ReMP particularly attractive in wireless networks allowing the convergence to a fixed and provably optimum point without employing any central controller. Numerical results show that ReMP outperforms conventional MP-based algorithms for RA problems in terms of computation time.

A message passing approach for resource allocation in cellular OFDMA communications

This paper proposes a distributed and low-complexity resource allocation scheme for cellular OFDMA networks. In particular, we consider ReMP, a reweighted message passing algorithm that perturbs the standard max-sum algorithm by suitably reweighting messages. In a single-cell scenario, such a scheme allows to achieve convergence to a fixed and provably optimum point without employing any central controller. The ReMP algorithm is then adapted to a multi-cell environment. To this aim, we devise X-ReMP, a ReMP-based algorithm that combines cross-cell signaling and the regular ReMP routine that still runs within each cell. The cross-signaling among cells aids ReMP to deal with the inter-cell multiple-access interference, so that X-ReMP allows convergence to a good working point in terms of system throughput even in presence of strong inter-cell interference.