Frederick Vook

Kronecker product correlation model and limited feedback codebook design in a 3D channel model

A 2D antenna array introduces a new level of control and additional degrees of freedom in multiple-input-multiple-output (MIMO) systems particularly for the so-called “massive MIMO” systems. To accurately assess the performance gains of these large arrays, existing azimuth-only channel models have been extended to handle 3D channels by modeling both the elevation and azimuth dimensions. In this paper, we study the channel correlation matrix of a generic ray-based 3D channel model, and our analysis and simulation results demonstrate that the 3D correlation matrix can be well approximated by a Kronecker production of azimuth and elevation correlations. This finding lays the theoretical support for the usage of a product codebook for reduced complexity feedback from the receiver to the transmitter. We also present the design of a product codebook based on Grassmannian line packing.

Moving Towards Mmwave-Based Beyond-4G (B-4G) Technology

Availability of large untapped spectrum resources in the millimeter wave (Mmwave) band is suitable for providing a gigabit experience with true local feel using high capacity small cells. Unlike traditional cellular systems, millimeter wave transmissions do not benefit from diffraction and dispersion making it difficult for them to propagate around obstacles thus resulting in higher shadowing loss. They also have less favorable link budgets due to lower power amplifier (PA) output powers and greater pathloss at these higher frequencies. Also, current costs of the Mmwave circuits are higher, but the costs will become much lower when the technology becomes mainstream. One advantage of millimeter wave, however, is that the smaller wavelengths allow for the fabrication of antenna arrays having a much higher number of antenna elements in a much smaller area than is typical at microwave bands. In this article, we outline a framework for Beyond-4G (B-4G) local area network in the millimeter wave band for both access and backhaul including air-interface, antenna-arrays and IC technology. It is shown that Mmwave B-4G small cell technology can provide peak and cell edge rates greater than 10 Gbps and 100 Mbps respectively with latency less than 1msec for local area network.

MIMO and beamforming solutions for 5G technology

Multi-antenna technologies such as beamforming and Multiple-Input, Multiple-Output (MIMO) are anticipated to play a key role in “5G” systems, which are expected to be deployed in the year 2020 and beyond. With a class of 5G systems expected to be deployed in both cm-wave (3-30 GHz) and mm-wave (30-300 GHz) bands, the unique characteristics and challenges of those bands have prompted a revisiting of the design and performance tradeoffs associated with existing multi-antenna techniques in order to determine the preferred framework for deploying MIMO technology in 5G systems. In this paper, we discuss key implementation issues surrounding the deployment of transmit MIMO processing for 5G systems. We describe MIMO architectures where the transmit MIMO processing is implemented at baseband, RF, and a combination of RF and baseband (a hybrid approach). We focus on the performance and implementation issues surrounding several candidate techniques for multi-user-MIMO (MU-MIMO) transmission in the mm-wave bands.

3D channel model in 3GPP

Multi-antenna techniques capable of exploiting the elevation dimension are anticipated to be an important air-interface enhancement targeted to handle the expected growth in mobile traffic. In order to enable the development and evaluation of such multi-antenna techniques, the 3rd Generation Partnership Project (3GPP) has recently developed a three-dimensional (3D) channel model. The existing two-dimensional (2D) channel models do not capture the elevation channel characteristics, making them insufficient for such studies. This article describes the main components of the newly developed 3D channel model and the motivations behind introducing them. One key factor is the ability to model channels for users located on different floors of a building (at different heights). This is achieved by capturing a user height dependency in modelling some channel characteristics including pathloss, lineof- sight (LOS) probability, etc. In general, this 3D channel model follows the framework of WINNERII/WINNER+ while also extending the applicability and the accuracy of the model by introducing some height dependent and distance dependent elevation related parameters.

Transparent user-specific 3D MIMO in FDD using beamspace methods

In typical macro-cell deployments for 3GPP LTE the base station employs an array of cross-polarized or co-polarized antennas that are linearly spaced in azimuth. In many panel array designs used in cellular deployments, each antenna port is actually formed by co-phasing some number of vertically-arranged physical sub-elements to achieve a desired elevation pattern and overall gain. If these vertically-arranged sub-elements could be individually and adaptively controlled then the antenna array could adapt its transmission in both the elevation and azimuth dimensions on a per-user basis (aka 3D MIMO) while maintaining the same form factor as the original array. In this paper we consider the problem of incorporating user-specific 3D-MIMO into the LTE FDD OFDMA downlink in a way that is transparent to the existing mobile devices. We propose using the beamspace concept along with uplink transmissions to control the elevation dimension while using the existing LTE codebook feedback methods to control the azimuth dimension. We show that the ability to adapt in both the elevation and azimuth dimensions on a per-user basis can provide significant gains in performance.

Hybrid structure in massive MIMO: Achieving large sum rate with fewer RF chains

Massive multiple-input multiple-output (MIMO) systems can attain a high channel capacity and spectral efficiency by using a very large antenna array at the base station (BS). However, the cost of having one radio frequency (RF) chain behind every antenna element can be prohibitive. In addition, the overall power consumption of the RF hardware can be excessively high. A hybrid analog-digital structure can be utilized to reduce the required number of RF chains at the BS. In this paper, we present achievable rates of hybrid beamforming in multi-user MIMO (MU-MIMO) when employing only one RF chain per user. The analysis and simulation results show that the asymptotic signal-plus-interference-to-noise ratio (SINR) of hybrid beamforming is reduced by a factor of π/4 compared to conventional beamforming methods, and the resulting achievable sum-rate degradation can be compensated by simply employing 27% more transmit antennas.

Rate analysis and feasibility of dynamic TDD in 5G cellular systems

In conventional applications of time division duplex (TDD) in cellular systems, the time resource split between uplink (UL) and downlink (DL) is fixed across all base stations (BSs) in the network. This leads to under utilization of BS resources when there is a mismatch between the expected and experienced UL/DL traffic in a given cell. A dynamic split that varies in each cell is desirable, but is challenging due to the high interference experienced by UL receivers in one cell from DL transmissions in adjacent cells. This paper analyzes the performance of UL users in dynamic TDD enabled next generation cellular networks using a stochastic geometry framework. The analysis highlights the trade-off between spectral efficiency and resource utilization for dynamic TDD. With appropriate interference mitigation, dynamic TDD offers a significant gain in data rates as compared to static TDD, which is higher when the BSs are lightly loaded and/or the fraction of UL users is low.

Sub-sector-based codebook feedback for massive MIMO with 2D antenna arrays

Massive MIMO is a promising technology for next generation cellular networks. It differentiates from conventional MIMO systems because of the excessive number of transmit antennas at the base stations. To implement a massive MIMO system with FDD, a practical low-overhead downlink channel training and estimation method is required. We propose a codebook based feedback framework for FDD massive MIMO systems that divides the coverage area into sub-sectors, where each sub-sector is formed by a set of narrow beams that cover a pre-assigned area in azimuth and elevation. A feedback process is then used where the codebook based feedback is limited to the beams covering the sub-sector to which the user device belongs. Our simulation results show that the proposed feedback framework with a large 2D antenna array provides substantial performance improvement compared to existing LTE/LTE-Advanced systems that currently support no more than eight antenna ports.

Elevation beamforming with beamspace methods for LTE

Recently, there has been interest in extending MIMO processing techniques to exploit the elevation dimension of the multipath channel in addition to the azimuth dimension. In this paper, we explore the use of beamspace methods for creating virtual antenna ports in the vertical direction to enable a base station antenna array to exploit the elevation dimension of the multipath channel. We consider two ways of configuring the vertical antenna array: vertical sectorization, which creates additional sectors in the vertical domain, and vertical beamforming, which simply leverages the additional vertical antenna ports with existing MIMO processing techniques. For Rel-8/10 LTE, we show that both vertical sectorization and vertical beamforming can provide significant gains in the average and cell-edge throughputs, both in full buffer traffic as well as bursty traffic. We also highlight the sensitivity of the results to the specific assumptions of the underlying multipath channel characteristics and other system parameters.

Massive MIMO for mmWave systems

The large bandwidths available at the millimeter wave (mmWave) carrier frequencies (e.g., 30-100 GHz) have sparked significant interest in developing cellular systems in those bands to meet the ever-increasing demand for high data rates. Large-scale antenna arrays with tens or hundreds of antennas are envisioned to be a prerequisite for operating in the mmWave bands due to the poor path loss conditions in those bands. Previous studies for 72GHz carrier frequencies have shown how extremely high data rates can be achieved in ultra-dense small cell deployments through simple single-user MIMO techniques mainly by virtue of the high system bandwidth (on the order of 1-2GHz). In this paper, we extend the prior work on single-user MIMO (SU-MIMO) for mmWave bands and examine the question of whether Multi-User MIMO (MU-MIMO) is a useful approach for mmWave bands. We show that there are definite cases where MU-MIMO can provide significant system capacity gains over SU-MIMO in the mmWave bands, which is in contrast to the expectation that the poor path loss conditions necessitate simple high gain beamforming techniques. We show that in many cases, a large-scale array provides sufficient SINR gain that can enable further gains from multi-user spatial multiplexing. We show how those gains depend on a variety of factors such as the user density and the transmission strategy.