Mehrdad Shariat

On the Evolution of Multi-Cell Scheduling in 3GPP LTE / LTE-A

This paper provides a holistic overview of multi-cell scheduling strategies in emerging wireless systems. Towards this objective, the evolution of interference management techniques is thoroughly investigated from simple inter-cell interference coordination (ICIC) techniques towards more advanced coordinated multipoint transmissions (CoMP), while comparing and contrasting their common features and differences. Finally CoMP is explored in detail as an advanced and challenging mechanism to fully cooperate between adjacent cells in order to have an efficient resource allocation and inter-cell interference mitigation in multi-cell environments.

Dynamic Clustering Framework for Multi-Cell Scheduling in Dense Small Cell Networks

This letter proposes a novel graph-based multi-cell scheduling framework to efficiently mitigate downlink inter-cell interference in small cell OFDMA networks. This framework incorporates dynamic clustering combined with channel-aware resource allocation to provide tunable quality of service measures at different levels. Our extensive evaluation study shows that a significant improvement in users spectral efficiency is achievable, while also maintaining relatively high cell spectral efficiency via empirical tuning of re-use factor across the cells according to the required QoS constraints.

Scheduling as an important cross-layer operation for emerging broadband wireless systems

Cross-layer scheduling is a promising solution for improving the efficiency of emerging broadband wireless systems. In this tutorial, various cross-layer design approaches are organized into three main categories namely air interface-centric, user-centric and route-centric and the general characteristics of each are discussed. Thereafter, by focusing on the air interface-centric approach, it is shown that the resource allocation problem can be formulated as an optimization problem with a certain objective function and some particular constraints. This is illustrated with the aid of a customer-provider model from the field of economics. Furthermore, the possible future evolution of scheduling techniques is described based on the characteristics of traffic and air interface in emerging broadband wireless systems. Finally, some further challenges are identified.

Joint TDD Backhaul and Access Optimization in Dense Small-Cell Networks

This paper addresses the problem of joint backhaul (BH) and access link optimization in dense small-cell networks with a special focus on time-division duplexing (TDD) mode of operation in BH and access link transmission. Here, we propose a framework for joint radio resource management, where we systematically decompose the problem in BH and access links. To simplify the analysis, the procedure is tackled in two stages. At the first stage, the joint optimization problem is formulated for a point-to-point scenario where each small cell is simply associated with a single user. It is shown that the optimization can be decomposed into separate power and subchannel allocation in both BH and access links, where a set of rate-balancing parameters in conjunction with duration of transmission governs the coupling across both links. Moreover, a novel algorithm is proposed based on grouping the cells to achieve rate balancing in different small cells. Next, in the second stage, the problem is generalized for multiaccess small cells. Here, each small cell is associated with multiple users to provide the service. The optimization is similarly decomposed into separate subchannel and power allocation by employing auxiliary slicing variables. It is shown that similar algorithms, as in the previous stage, are applicable by a slight change with the aid of slicing variables. Additionally, for the special case of line-of-sight BH links, simplified expressions for subchannel and power allocation are presented. The developed concepts are evaluated by extensive simulations in different case studies from full orthogonalization to dynamic clustering and full reuse in the downlink, and it is shown that the proposed framework provides significant improvement over the benchmark cases.

Effective RF codebook design and channel estimation for millimeter wave communication systems

Millimeter wave (mmWave) technologies can enable current mobile communication systems to achieve higher data rates. However, wireless channels at mmWave frequencies experience higher isotropic path loss. Therefore, employing a suitable beamforming algorithm is an indispensable element of any mmWave system. Traditional multiple-input multiple-output (MIMO) systems employ digital beamforming where each antenna element is equipped with one RF chain. In case of mmWave systems, however, the power consumption, signalling and hardware cost impose the designers to deploy analog or hybrid beamforming strategies. This paper addresses two key problems in beamforming for millimeter wave communication systems. First, an effective codebook is designed, using a genetic algorithm that achieves a near-optimal array gain in all directions. This RF codebook is shown to perform better compared to the state-of-the-art RF codebooks with fewer RF chains and lower resolution phase shifters. Secondly, a low complexity channel estimation scheme is proposed that requires less signalling overhead and is effective with low-resolution phase shifters. Finally, the performance of the proposed RF codebook and channel estimation scheme is thoroughly investigated in terms of spectral efficiency.

Enabling wireless backhauling for next generation mmWave networks

This paper presents some key findings w.r.t. the Radio Resource Management (RRM) in wireless Backhaul (BH) of mmWave networks. First, the envisioned design of mmWave backhaul architecture is outlined highlighting the most important newly needed functional blocks and in what they differ from a non-mmWave architectures. Next, the challenges and functionality of RRM techniques are discussed, focusing on Routing and Link scheduling algorithms in such BH architecture. Furthermore different possible interactions between RRM functions are explored. Finally, preliminary analytical and experimental study on the performance of different link scheduling and routing functions for mmWave backhauling are provided, highlighting in particular the impact of traffic load and dynamic route selection on BH End to End delay.

On the analysis of co-tier interference in femtocells

This paper quantifies the impact of co-tier interference in femtocells (i.e. inter-cell interference arising from other neighbouring femtocells) through a semi-analytical approach concluding that without interference mitigation techniques, QoS in the form of acceptable outage probability is not achievable in scenarios representing medium to worst cases of interference. Results obtained from semi-analytical approach are also compared against Monte Carlo simulations for evaluation purposes. Subsequently, some important radio access parameters and deployment configurations, such as path loss model, shadowing, wall penetration loss, location of femtocells and user distribution are further examined as key elements that can potentially affect, positively or negatively, the femtocell-to-femtocell interference in a multi-femtocell deployment. The results can be used as guidelines in the deployments of femtocells.

Graph-Based Multicell Scheduling in OFDMA-Based Small Cell Networks

This paper proposes a novel graph-based multicell scheduling framework to efficiently mitigate downlink intercell interference in OFDMA-based small cell networks. We define a graph-based optimization framework based on interference condition between any two users in the network assuming they are served on similar resources. Furthermore, we prove that the proposed framework obtains a tight lower bound for conventional weighted sum-rate maximization problem in practical scenarios. Thereafter, we decompose the optimization problem into dynamic graph-partitioning-based subproblems across different subchannels and provide an optimal solution using branch-and-cut approach. Subsequently, due to high complexity of the solution, we propose heuristic algorithms that display near optimal performance. At the final stage, we apply cluster-based resource allocation per subchannel to find candidate users with maximum total weighted sum-rate. A case study on networked small cells is also presented with simulation results showing a significant improvement over the state-of-the-art multicell scheduling benchmarks in terms of outage probability as well as average cell throughput.

On the efficiency of interference coordination schemes in emerging cellular wireless networks

In this paper, the efficiencies of different interference coordination schemes are evaluated for emerging wireless networks and the possible impact on intra-cell scheduling is studied through extensive simulations. The results show that pure fractional frequency reuse can provide similar improvement in the cell-edge throughput compared to power coordinated counterpart at a less cost in terms of overall throughput. Moreover, it can provide fairer distribution of throughput in both central as well as cell-edge areas. However, this scheme can not mange asymmetrical changes in the distribution of users across different cells in the entire system. As a result, a power coordination mechanism would be still necessary on top of such flexible frequency reuse schemes.

Dynamic graph-based multi-cell scheduling for femtocell networks

This paper proposes a dynamic semi-centralized resource partitioning algorithm to mitigate the problem of co-tier interference in dense femtocell deployments. This algorithm incorporates graph-colouring and network utility concepts to address inter-cell interference in femtocell networks. The aforementioned scheme acts as a multi-cell coordination mechanism on top of intra-cell scheduling by applying a low complexity graph-based algorithm. The objective of the coordination mechanism is the efficient management of resource conflicts due to interference in a multi-cell environment consisting of femtocells. This coordination mechanism defines a novel category of graph-based ICIC algorithms that uses bipartite graph colouring to avoid resource conflicts by randomizing them in time domain.