Mahmudur Rahman

Enhancing cell-edge performance: a downlink dynamic interference avoidance scheme with inter-cell coordination

Interference management has been a key concept for designing future high data-rate wireless systems that are required to employ dense reuse of spectrum. Static or semi-static interference coordination based schemes provide enhanced cell-edge performance but with severe penalty to the overall cell throughput. Furthermore, static resource planning makes these schemes unsuitable for applications in which frequency planning is difficult, such as femtocell networks. In this paper, we present a novel dynamic interference avoidance scheme that makes use of inter-cell coordination in order to prevent excessive inter-cell interference, especially for cell or sector edge users that are most affected by inter-cell interference, with minimal or no impact on the network throughput. The proposed scheme is comprised of a two-level algorithm - one at the base station level and the other at a central controller to which a group of neighboring base stations are connected. Simulation results show that the proposed scheme outperforms the reference schemes, in which either coordination is not employed (reuse of 1) or employed in a static manner (reuse of 3 and fractional frequency reuse), in terms of cell edge throughput with a minimal impact on the network throughput and with some increase in complexity.

An Overview of Radio Resource Management in Relay-Enhanced OFDMA-Based Networks

Researchers in both academia and industry have accepted OFDMA as the most appropriate air-interface for the emerging broadband wireless access networks and standards. A number of IEEE working groups and various research forums are focusing on developing relay and mesh-enabled networks with cooperative communication features. Among these research efforts are IEEE 802.11s, IEEE 802.16j/m, and 3GPPs advanced long term evolution (LTE-advanced). The combination of OFDMA with relaying techniques provides rich opportunities for cost-effective and high-performance networks. To exploit such opportunities requires intelligent radio resource management (RRM) algorithms. Although a number of publications have highlighted the important and challenging issues involved in designing RRM algorithms for OFDMA networks, only recently a number of papers have investigated relay-enhanced OFDMA-based multicellular networks. By and large, the literature indicates that these issues constitute a hot research topic that will continue to attract interest. This paper provides a survey of the current literature on OFDMA networks enhanced with decode-and-forward relaying and provides their link to earlier literature in non-OFDMA networks. In addition, a rich list of references is provided to direct the readers toward some of the emerging techniques.

Fairness-aware radio resource management in downlink OFDMA cellular relay networks

Relaying and orthogonal frequency division multiple access (OFDMA) are the accepted technologies for emerging wireless communications standards. The activities in many wireless standardization bodies and forums, for example IEEE 802.16 j/m and LTE-Advanced, attest to this fact. The availability or lack thereof of efficient radio resource management (RRM) could make or mar the opportunities in these networks. Although distributed schemes are more attractive, it is essential to seek outstanding performance benchmarks to which various decentralized schemes can be compared. Therefore, this paper provides a comprehensive centralized RRM algorithm for downlink OFDMA cellular fixed relay networks in a way to ensure user fairness with minimal impact on network throughput. In contrast, it has been observed that pure opportunistic schemes and fairness-aware schemes relying solely on achievable and allocated capacities may not attain the desired fairness, e.g., proportional fair scheduling. The proposed scheme is queue-aware and performs three functions jointly; dynamic routing, fair scheduling, and load balancing among cell nodes. We show that the proposed centralized scheme is different from the traditional centralized schemes in terms of the substantial savings in complexity and feedback overhead.

Interference Avoidance with Dynamic Inter-Cell Coordination for Downlink LTE System

The investigation of co-channel interference mitigation techniques (such as, interference cancellation through receiver processing, interference randomization by frequency hopping, and interference avoidance through resource usage restrictions imposed by frequency and power planning) has become a key focus area in achieving dense spectrum reuse in next generation cellular systems such as 3GPP LTE, LTEadvanced, and WiMAX. In this paper, we propose an interference avoidance scheme for LTE downlink that uses dynamic inter-cell coordination facilitated through X2 interface among neighbouring evolved UTRAN nodeBs (eNBs, i.e., LTE base stations). Proposed scheme is evaluated by extensive simulations and compared with a number of reference schemes available in the literature. It has been observed that the proposed scheme attains superior performance in terms of cell-edge and sector throughput compared to those in the reference schemes.

Interference Avoidance through Dynamic Downlink OFDMA Subchannel Allocation using Intercell Coordination

OFDMA subchannel allocation has been well-studied. However, most of the available literature considers a single cell system without cochannel interference. We present a novel intercell interference avoidance scheme, in which the downlink OFDMA subchannels are allocated through intercell coordination in a multicell environment. The proposed scheme not only aims to achieve maximized network throughput but also focuses on providing improved throughput for cell or sector edge users that are most affected by intercell interference. Enhanced cell edge performance may result in fewer base stations needed to cover a region; this, in turn, may yield substantial savings in deployment cost. The scheme is comprised of two different algorithms; one at the base station level, and the other at a central controller to which a group of neighboring base stations are connected. The performance of the proposed scheme is compared with that of a reference scheme in which coordination is not employed. It is observed from simulation results that the proposed scheme outperforms the reference scheme in terms of network and cell edge throughput.

Fairness-Aware Joint Routing and Scheduling in OFDMA-Based Cellular Fixed Relay Networks

Relaying and orthogonal frequency division multiple access (OFDMA) are the accepted technologies for emerging wireless communications standards. The activities in many wireless standardization bodies and forums, for example IEEE 802.16 j/m and LTE-Advanced, attest to this fact. The availability or lack thereof of efficient radio resource management (RRM) could make or mar the opportunities in these networks. This paper therefore provides a comprehensive RRM algorithm for OFDMA-based multi-cellular fixed relay networks in a way to ensure fairness among users with minimal impact on the network throughput (in contrast, pure opportunistic RRM techniques always favor users with good channel conditions). Unlike the majority of works in the literature, our proposed scheme is queue-aware and jointly performs routing, fair scheduling, and load balancing among cell nodes. The routing strategy has inherent learning ability and it dynamically converges to better routes.

Cross-layer resource scheduling for video traffic in the downlink of OFDMA-based wireless 4G networks

Designing scheduling algorithms at the medium access control (MAC) layer relies on a variety of parameters including quality of service (QoS) requirements, resource allocation mechanisms, and link qualities from the corresponding layers. In this paper, we present an efficient cross-layer scheduling scheme, namely, Adaptive Token Bank Fair Queuing (ATBFQ) algorithm, which is designed for packet scheduling and resource allocation in the downlink of OFDMA-based wireless 4G networks. This algorithm focuses on the mechanisms of efficiency and fairness in multiuser frequency-selective fading environments. We propose an adaptive method for ATBFQ parameter selection which integrates packet scheduling with resource mapping. The performance of the proposed scheme is compared to that of the round-robin (RR) and the score-based (SB) schedulers. It is observed from simulation results that the proposed scheme with adaptive parameter selection provides enhanced performance in terms of queuing delay, packet dropping rate, and cell-edge user performance, while the total sector throughput remains comparable. We further analyze and compare achieved fairness of the schemes in terms of different fairness indices available in literature.

Inter-Cell Interference Coordination in OFDMA Networks: A Novel Approach Based on Integer Programming

We study a dynamic inter-cell interference coordination scheme formulated using a novel binary integer linear programming optimization technique. The scheme aims to maximize utility, where different utility measures have been defined to model varying levels of user fairness. The formulated problem is decomposed into a number of smaller sub-problems and solved iteratively to ease the computational complexity. We observe the cell-edge and sector throughput as well as user fairness and compare with a reference scheme, where interference coordination is not used. It is observed that the proposed scheme always outperforms the reference scheme in terms of cell-edge and sector throughput as well as in fairness.

Opportunistic nonorthogonal packet scheduling in fixed broadband wireless access networks

In order to mitigate high cochannel interference resulting from dense channel reuse, the interference management issues are often considered as essential part of scheduling schemes in fixed broadband wireless access (FBWA) networks. To that end, a series of literature has been published recently, in which a group of base stations forms an interferer group (downlink transmissions from each base station become dominant interference for the users in other in-group base stations), and the scheduling scheme deployed in the group allows only one base station to transmit at a time. As a result of time orthogonality in transmissions, the dominant cochannel interferers are prevented, and hence the packet error rate can be improved. However, prohibiting concurrent transmissions in these orthogonal schemes introduces throughput penalty as well as higher end-to-end packet delay which might not be desirable for real-time services. In this paper, we utilize opportunistic nonorthogonality among the in-group transmissions whenever possible and propose a novel transmission scheduling scheme for FBWA networks. The proposed scheme, in contrast to the proactive interference avoidance techniques, strives for the improvements in delay and throughput efficiency. To facilitate opportunistic nonorthogonal transmissions in the interferer group, estimation of signal-to-interference-plus-noise ratio (SINR) is required at the scheduler. We have observed from simulations that the proposed scheme outperforms the reference orthogonal scheme in terms of spectral efficiency, mean packet delay, and packet dropping rate.

Fairness Assessment of the Adaptive Token Bank Fair Queuing Scheduling Algorithm

Adaptive token bank fair queuing (ATBFQ) algorithm has been proposed as a cross-layer scheduling technique for 4G wireless systems recently. This algorithm takes higher layer quality of service (QoS) attributes such as priorities, interflow fairness, and delay constraints into account. By selecting the user terminals (UTs) in a certain prioritized manner derived from QoS attributes, the performance of the UTs, suffering from high interference and/or shadowing in particular, can be improved. The ATBFQ algorithm has been tested in a multicell environment in the presence of intercell interference by comparing with reference Score Based (SB) and Round Robin (RR) algorithms. In this paper, we further analyze ATBFQ primarily with regard to fairness along with other performance metrics accessed in a more elaborate system considering varying interference and loading conditions. Furthermore, an adaptive method for the allocation of resources is proposed for ATBFQ parameter selection, and is shown to have better performance in various loading conditions. It is observed from simulation results that ATBFQ with adaptive parameter selection outperforms the reference schemes in terms of queuing delay and UT throughput for different network loading cases.