Aaron Callard

Joint User Grouping and Transceiver Design in a MIMO Interfering Broadcast Channel

Consider a MIMO multi-cellular network (also known as an interfering broadcast channel) where each base station transmits signals to the users in its own cell. The basic problem is to design linear transmit/receive beamformers and schedule users across a fixed set of time slots so as to maximize the system throughput in the presence of both inter and intra cell interference. In this paper, we propose a joint linear transceiver design and user grouping scheme for sum utility maximization that is based on iterative minimization of weighted mean squared error (MSE). The proposed algorithm only needs local channel knowledge and its convergence to a stationary point is guaranteed for some well-known utility functions, while ensuring user fairness. The simulation results show that the proposed formulation/algorithm can offer significantly higher system throughput than the standard multi-user MIMO techniques such as the SVD-MMSE strategy, while maintaining user fairness. Furthermore, the proposed algorithm exhibits fast convergence and is amenable to distributed implementation with limited information exchange.

Joint transceiver design and user grouping in a MIMO interfering broadcast channel

Consider a MIMO interfering broadcast channel (multi-cellular network) where each base station transmits signals to the users in its own cell. The basic problem is to design linear transmit/receive beamformers that can maximize the system throughput in the presence of both inter and intra cell interference. To ensure user fairness in the system, we consider the joint user grouping, power allocation and beamformer design problem by maximizing a system utility which aims to strike a suitable trade-off between the user fairness and system throughput. We propose a simple algorithm to solve this nonconcave utility maximization problem and establish its convergence. The simulation results show that the proposed algorithm significantly outperforms the SVD-MMSE method and some other approaches in terms of system throughput while respecting user fairness. The proposed algorithm exhibits fast convergence and is amenable to distributed implementation with limited information exchange.

Step-Wise Optimal Low Power Node Deployment in LTE Heterogeneous Networks

In this paper, we propose a step-wise optimal low power node (LPN) deployment algorithm for LTE heterogeneous networks. Our proposed LPN deployment algorithm takes signal quality, relative loading among macro cells and LPN cells, and user density into account. Simulation results show that the proposed algorithm can achieve close to 100% coverage gain and more than 50% average cell throughput gain over a random LPN deployment scheme.

Handling real-time video traffic in software-defined radio access networks

In this paper, we introduce new solutions to handle real-time video traffic, such as video conferencing, in the future software-defined radio access networks. The real-time video is well-known for its high peak-to-mean rate ratio. This is a major challenge for traffic engineering and radio resource allocation, especially in small cell radio networks. We first propose an online method to dynamically estimate the effective rate of video flows, which is the rate the network should support in order to provide a satisfactory quality of experience. Second, traffic engineering methods taking into account characteristics of video flows are presented. Third, a radio coordination method to provide stable video rate across cells is discussed. Fourth, we give a fountain coding scheme to support mobile video users. The proposed solutions are investigated in an ultra-dense small cell network simulator. The simulation results show very significant gains over conventional technologies.

5G access-link provisioning and coordination: Tradeoff between proactive and reactive strategies

We explore the effect of imperfect knowledge on multipoint downlink traffic engineering (TE) (with dynamic point selection DPS) computed centrally and jointly with power control (PC) optimization versus distributed PC techniques. Instead of considering a single metric that shows qualities in a single dimension, we observe both performance and resilience by observing how many users get quality of experience (QoE) - or otherwise, how many users would complain that they do not receive the expected service. We identify that centralized traffic engineering provides the best performance (highest class of service) with distributed power control in a practical (imperfect) network. Although this has a clear drawback as it comes with a low resilience in face of bad service admission. We conclude that fifth generation networks will need to be able to dynamically change optimizing strategies to face different loading of the networks.

Downlink Transmission Optimization Framework

This paper presents an optimization framework for downlink transmission parameters of mobile cellular systems. A typical network optimization approach is to divide the network into disjoint clusters of base-stations (BS). Optimization is then performed within each cluster for important parameters such as transmit power, precoder, etc. This approach is widely adopted in academic research and industrial standard bodies, e.g. 3GPP LTE-Advanced. While reducing the optimization complexity, this strategy suffers from a performance limit due to interference from the nodes outside cluster. We thus propose a framework to overcome this limit by allowing clusters to exchange their parameters and optimization information via low-rate and non-zero delay backhauls. System-level simulations for LTE downlink transmit power optimization show that the proposed optimization model, while having the low-complexity of cluster-based approach, could nearly achieve the performance of network-wise optimization. Therefore, this model is particularly suitable for optimization of 4G and beyond-4G cellular networks.

LTE relay backhaul design for sparsely-populated environments

In the latest release of the Long Term Evolution (LTE) standards, which will be completed in September 2011, it was decided to standardize relay nodes (RN) for the purpose of improving coverage. This makes RNs very suitable tools for increasing the coverage of low population density areas. The RN is a new type of node that was not previously standardized in 3GPP before and, as such, created new challenges. In particular, for RN, the backhaul link (connecting the enhanced Node B (eNB) and the RN) needs to be defined, and both uplink and downlink grants need to be conveyed to the RNs, either using existing channels (e.g., the Packet Dedicated Control Channel (PDCCH)), or a new channel (commonly referred to as the Relay-PDCCH (R-PDCCH)). In this paper, we discuss both channels for the Frequency Division Duplex (FDD) mode and show that using a R-PDCCH is a more efficient channel. In addition, several features are provided to ensure good performance for the R-PDCCH, such as robust use of frequency diversity by transmitting the R-PDCCH using Distributed Virtual Resource Blocks (DVRBs), or use of rate-matching to ensure optimal usage of the allocated resources for R-PDCCH transmission. This robustness is considered necessary to ensuring good reliability for sparsely populated areas.

Multi-Cell Full-Duplex Wireless Communication for Dense Urban Deployment

Simultaneous bidirectional transmission and recep- tion of signals on a single channel in a full duplex (FD) system is gaining increased attention from both academia and industry. Recent advances in radio transceiver design have been able to minimize the self interference (SI) to a great extent making realization of a FD transceiver possible. For FD deployment in a multi-cell environment, minimizing SI is not sufficient due to the presence of a complex array of network-wide interference. This paper investigates the critical deployment challenges for a dense urban FD multi- cell network and provides solutions for successful deployment of FD cellular networks. Extensive simulation results show that the proposed user selection and power control algorithms are able to increase the FD performance gain by more than 80% when compared to the traditional half duplex (HD) counterpart.

Dynamic Allocation of Backhaul Resources for Distributed Interference Subtraction

This article explores a method to dynamically size the backhaul links (shared resources) used to support cooperative decoding and to forward decoded data, in the case of limited bandwidth of the backhaul. The practical approach provides a scheduler with a mean to know the backhaul limitations and to use it adequately while interacting with a traffic engineering optimizer which reacts to changes in demands of the radio access network balancing where to allocate cooperative backhaul resources to maximize fairness and throughput of users.