Saifur Rahman

Rate Region of the Gaussian Scalar-Help-Vector Source-Coding Problem

We determine the rate region of the Gaussian one-helper source-coding problem in which the helper observes a scalar, the main encoder observes a vector, and the distortion constraint is a positive semidefinite upper bound on the error covariance matrix of the main source. The rate region is achieved by a Gaussian achievable scheme. We introduce a novel outer bounding technique to establish the converse. Our approach is to create a reduced dimensional problem by projecting the main source and the distortion constraint in certain directions determined by the optimal Gaussian scheme. We also provide an outer bound to the rate region of the more general problem in which there are distortion constraints on both sources. This outer bound is partially tight in general and completely tight in some nontrivial cases.

Full dimension MIMO for LTE-Advanced and 5G

Elevation beamforming and Full Dimension MIMO (FD-MIMO) has been an active area of research and standardization in 3GPP LTE-Advanced. In an FD-MIMO system, a base station with 2-dimensional (2D) active array supports multi-user joint elevation and azimuth beamforming (a.k.a. 3D beamfoming), which results in much higher cell capacity compared to conventional systems. Recent study has shown that with these new FD-MIMO technologies, we can achieve promising 3-5× gain in both cell capacity as well as cell-edge throughput. In this paper, we will provide a brief summary of recent 3GPP activities, including the recently completed 3D channel model, ongoing study on FD-MIMO scenarios, antenna/RF (radio frequency) transceiver architectures, as well as potential network performance benefits. In addition, we also discuss some methods for reducing CSI (channel state information) feedback overhead and ensuring efficient operation of large size FD-MIMO for both TDD and FDD systems.

Vector Gaussian hypothesis testing and lossy one-helper problem

We study the vector Gaussian versions of two problems: hypothesis testing under a communication constraint and the lossy one-helper problem. In the hypothesis testing problem, a test against independence is considered when a vector Gaussian source is available at the detector which receives a message about another vector Gaussian source at a specified rate. Two equivalent characterizations of the optimal type 2 error exponent are given when the type 1 error is at most a fixed constant. The first characterization is based on enhancement technique introduced by Weingarten et. al. and the other is transform-based. The transform-based characterization directly yields a water pouring interpretation, and establishes successive refinability. For the lossy one-helper problem, we determine a portion of the boundary of the rate region.

Linear MU-MIMO pre-coding algorithms for a millimeter wave communication system using hybrid beam-forming

This paper presents the theoretical capacity performance of different linear multi-user MIMO (MU-MIMO) pre-coding algorithms for a millimeter wave communication system using hybrid beam-forming. Hybrid beam-forming uses both RF and digital beam-forming and is a practical way to achieve the required array gains when the high bandwidths available at millimeter wave frequencies are utilized. The performances of the different linear pre-coding schemes are simulated using a millimeter wave channel model in different interference scenarios under the assumption of perfect channel knowledge at the transmitter and at the receiver. Furthermore, enhancements to the linear pre-coding algorithms are described and the resulting performance is presented.

Reduced space channel feedback for FD-MIMO

To support downlink full-dimension (FD-MIMO) in FDD scenarios, an accurate yet reasonably efficient channel-state-information (CSI) feedback scheme is required to harness the potential beamforming gain offered by the use of a large number of antenna elements in a 2D active antenna array. In this paper, a reduced-spaced channel quantization and feedback scheme which exploits slow variation of the angle-of-departure (AoD) distribution is proposed. By perceiving the instantaneous spatial channel to be a linear combination of slowly varying subset of matrices, it is demonstrated that the proposed scheme offers up to 62.5% dimensionality reduction with negligible throughput loss in 16-antenna FD-MIMO system. When used together with vector quantization, the proposed scheme offers substantial gain over that supported by 3GPP LTE.

Modular and High-Resolution Channel State Information and Beam Management for 5G New Radio

This article provides an overview of key features pertaining to CSI reporting and beam management for the 5G New Radio (NR) currently being standardized in 3GPP. For CSI reporting, the modular design framework and high-resolution spatial information feedback offer not only flexibility in a host of use cases and deployment scenarios, but also improved average user throughput over state-of-the-art 4G LTE. To accommodate cellular communications in the milimeter-wave regime where a combination of analog and digital beamforming is typically used at both a base station and user equipment, beam management procedures such as measurement, reporting, and recovery are introduced. The utility and joint usage of these two features are demonstrated along with some potential upgrades for the next phase of 5G NR. Introduction

A low overhead synchronization mechanism for beamformed systems

Small cells deployments, at carrier frequencies much higher than the current cellular deployments, featuring electronically steerable transmit and receive antenna arrays offering high-gain beam-forming, are likely to form the basis of next generation cellular networks (sometimes referred to as 5G). This shift to smaller cells and high-gain directional transmit/receive technologies will be accompanied with challenges of handling increased number of cell edges and “more fragile” communication links, compared to the traditional macro cell operation in the cellular bands with omni-directional receivers. Cloud Cell - a system framework for virtually eliminating the edges and handling fragility, by ensuring that the Mobile Station (MS) always has multiple “good” links to choose from, was introduced in an earlier work. A fundamental challenge in enabling Cloud Cell, or another system where an MS is served by multiple base stations (BSs), is to come up with schemes where the synchronization overhead can be kept to a minimum without incurring excessive losses in performance. In this paper, we propose and evaluate a low-pilot overhead scheme for uplink synchronization in a Cloud Cell based beam-formed system, suitable for use in mm-Wave bands.

Multicasting for wireless video-on-demand

We consider a video delivery problem in which a server multicasts a single video to several users over a wireless broadcast channel. Users arrive and depart asynchronously, and when a user arrives, he or she wishes to watch the video from its beginning. Under a natural stochastic model for this problem, the optimal scheduling algorithm can be described by dynamic programming, but it is too complex to be practical. We describe a suite of approximations to the optimal scheme that lead to practical scheduling algorithms. The algorithms work with partially observed state, multirate multicast, and a variety of cost functions. Numerical results show significant performance gains over previously known schemes.