Sudhir Ramakrishna

On the Feasibility of Codebook-Based Beamforming in Millimeter Wave Systems With Multiple Antenna Arrays

The use of the millimeter (mm) wave spectrum for next generation (5G) mobile communication has gained significant attention recently. The small carrier wavelengths at mmwave frequencies enable synthesis of compact antenna arrays, providing beamforming gains that compensate the increased propagation losses. In this work, we investigate the feasibility of employing multiple antenna arrays (at the transmitter and/or receiver) to obtain diversity/multiplexing gains in mmwave systems, where each of the arrays is capable of beamforming independently. Considering a codebook-based beamforming system (the set of possible beamforming directions is fixed a priori, e.g., to facilitate limited feedback), we observe that the complexity of jointly optimizing the beamforming directions across the multiple arrays is highly prohibitive, even for very reasonable system parameters. To overcome this bottleneck, we develop reduced complexity algorithms for optimizing the choice of beamforming directions, premised on the sparse multipath structure of the mmwave channel. Specifically, we reduce the cardinality of the joint beamforming search space, by restricting attention to a small set of dominant candidate directions. To obtain the set of dominant directions, we develop two complementary approaches: 1) based on computation of a novel spatial power metric; a detailed analysis of this metric shows that, in the limit of large antenna arrays, the selected candidate directions approach the channels dominant angles of arrival and departure, and 2) precise estimation of the channels (long-term) dominant angles of arrival, exploiting the correlations of the signals received across the different receiver subarrays. Our methods enable a drastic reduction of the optimization search space (a factor of 100 reduction), while delivering close to optimal performance, thereby indicating the potential feasibility of achieving diversity and multiplexing gains in mmwave systems.

On the feasibility of beamforming in millimeter wave communication systems with multiple antenna arrays

The use of the millimeter (mm) wave spectrum for next generation (5G) mobile communication has gained significant attention recently. The small carrier wavelengths at mmwave frequencies enable synthesis of compact antenna arrays, providing beamforming gains that compensate the increased propagation losses. In this work, we investigate the feasibility of employing multiple antenna arrays (at the transmitter and/or receiver) to obtain diversity/multiplexing gains, where each of the arrays is capable of beamforming independently. Considering a codebook based beamforming system (the set of possible beamforming directions is fixed a priori, e.g., to facilitate limited feedback), we observe that the complexity of jointly optimizing the beamforming directions across the multiple arrays is highly prohibitive, even for very reasonable system parameters. To overcome this bottleneck, we develop two complementary approaches, premised on the sparse multipath structure of the mmwave channel. Specifically, we reduce the complexity of the joint beamforming optimization problem by restricting attention to a small set of candidate directions, based on: (a) computation of a novel spatial effective power metric, and (b) estimation of the channels angles of arrival. Our methods enable a drastic reduction of the optimization search space (a factor of 100 reduction), while delivering close to optimal performance, thereby indicating the potential feasibility of achieving diversity and multiplexing gains in mmwave systems.

Full Dimension MIMO (FD-MIMO): Demonstrating Commercial Feasibility

Massive multi-input multi-output (MIMO) is shown to significantly increase spectral efficiency by exploiting a large number of antennas to support high-order multiuser MIMO. In 3GPP Release-13, a full-dimension MIMO (FD-MIMO) technology was introduced to address practical aspects for massive MIMO in cellular systems; extensive simulations show 2–4 times capacity gain compared with current LTE systems. FD-MIMO has been identified as one of the key 5G technologies and is being continuously improved in 3GPP new radio standards. However, several practical challenges such as interference mitigation among MIMO streams for a large number of users with limited channel feedback, hardware limitations, such as calibration errors that limit the precoding capabilities need to be addressed carefully. A proof of concept (PoC) base-station and user equipment (UE) prototype has been designed to validate the potential of FD-MIMO technology and demonstrate commercial implementation feasibility. In this paper, we present theory and architecture behind an FD-MIMO prototype and share the field test results with LTE-based UEs in a multi-user MIMO setup. We also analyze the impact of transmitter and receiver calibration errors on the performance of the FD-MIMO system.

Full Dimension MIMO (FD-MIMO) - Reduced Complexity System Design and Real-Time Implementation

Full-Dimension MIMO (FD-MIMO) technology has been shown to increase spectral efficiency 2-4X compared to current LTE systems by exploiting a large number of antennas to support high order multiuser MIMO. High order multiuser MIMO with large number of antennas increase design and implementation complexity significantly. Furthermore, practical challenges such as antenna calibration for RF mismatches and failure need to be considered. In this paper, reduced-complexity precoding algorithm is introduced and optimized for real-time FPGA based implementation. Novel antenna calibration architecture is designed for FD-MIMO large 2-dimension array with the consideration of RF failure in practice. Field experimental results are also presented based on a proof of concept (PoC) FD-MIMO base-station (BS) with 32 antennas that supports up to 12 users and achieves spectral efficiency of ~21 bits/sec/Hz.

MIMO designs for mmWave wireless LAN systems

In this paper, we explore unique aspects of MIMO designs for mmWave wireless LAN systems, focusing on SU-MIMO feasibility at 60 GHz and implications on the MAC protocol design. We study the spectral efficiency gains achievable by using and MIMO with dual polarization in an indoor environment. We show that beyond 2 streams, MIMO gains (for SU-case) are rather limited and that fine beamforming with narrow beams is important for MIMO capacity gains. In this paper, we also provide efficient association and beamforming techniques for SU/MU-MIMO, enabling greater number of STAs to associate and refine beams in a given amount of time compared to the IEEE 802.11ad protocol.