Massive multiple-input multiple-output (mMIMO) technology uses a very large number of antennas at base stations to significantly increase efficient use of the wireless spectrum. Thus, mMIMO is considered an essential part of 5G and beyond. However, developing a scalable and reliable mMIMO system is an extremely challenging task, significantly hampering the ability of the research community to research nextgeneration networks. This "research bottleneck" motivated us to develop a deployable experimental mMIMO platform to enable research across many areas. We also envision that this platform could unleash novel collaborations between communications, computing, and machine learning researchers to completely rethink next-generation networks.

Renew

Hybrid analog/digital precoding offers a compromise between hardware complexity and system performance in millimeter wave (mmWave) systems. This type of precoding allows mmWave systems to leverage large antenna array gains that are necessary for sufficient link margin, while permitting low cost and power consumption hardware. Most prior work has focused on hybrid precoding for narrow-band mmWave systems, with perfect or estimated channel knowledge at the transmitter. MmWave systems, however, will likely operate on wideband channels with frequency selectivity. Therefore, this paper considers wideband mmWave systems with a limited feedback channel between the transmitter and receiver. First, the optimal hybrid precoding design for a given RF codebook is derived. This provides a benchmark for any other heuristic algorithm and gives useful insights into codebook designs. Second, efficient hybrid analog/digital codebooks are developed for spatial multiplexing in wideband mmWave systems. Finally, a low-complexity yet near-optimal greedy frequency selective hybrid precoding algorithm is proposed based on Gram–Schmidt orthogonalization. Simulation results show that the developed hybrid codebooks and precoder designs achieve very-good performance compared with the unconstrained solutions while requiring much less complexity.

Frequency Selective Hybrid Precoding for Limited Feedback Millimeter Wave Systems

This paper considers the design of optimal resource allocation policies in wireless communication systems, which are generically modeled as a functional optimization problem with stochastic constraints. These optimization problems have the structure of a learning problem in which the statistical loss appears as a constraint, motivating the development of learning methodologies to attempt their solution. To handle stochastic constraints, training is undertaken in the dual domain. It is shown that this can be done with small loss of optimality when using near-universal learning parameterizations. In particular, since deep neural networks (DNNs) are near universal, their use is advocated and explored. DNNs are trained here with a model-free primal-dual method that simultaneously learns a DNN parameterization of the resource allocation policy and optimizes the primal and dual variables. Numerical simulations demonstrate the strong performance of the proposed approach on a number of common wireless resource allocation problems.

/pdf/learning-optimal-resource-allocations-in-wireless-systems-18s43x1yn0.pdf

Learning Optimal Resource Allocations in Wireless Systems

High directivity of 60 GHz links introduces new link training and adaptation challenges due to both client and environmental mobility. In this paper, we design, implement and evaluate MOCA, a protocol for Mobility resilience and Overhead Constrained Adaptation for directional 60 GHz links. Since mobility-induced link blockage and misalignment cannot be countered with data rate adaptation alone, we introduce Beam Sounding as a mechanism invoked before each data transmission to estimate the link quality for selected beams, and identify and adapt to link impairments. We devise proactive techniques to restore broken directional links with low overhead and design a mechanism to jointly adapt beamwidth and data rate, targeting throughput maximization that incorporates data rate, overhead for beam alignment, and mobility resilience. We implement a programmable node and testbed using software defined radios with commercial 60 GHz transceivers, and conduct an extensive over-the-air measurement study to collect channel traces for various environments. Based on trace based emulations and the IEEE 802.11ad channel model, we evaluate MOCA under a variety of propagation environments and mobility scenarios. Our experiments show that MOCA achieves up to 2x throughput gains compared to a baseline WLAN scheme in a diverse set of operational conditions.

https://dl.acm.org/doi/pdf/10.1145/2942358.2942380

Mobility resilience and overhead constrained adaptation in directional 60 GHz WLANs: protocol design and system implementation

Wi-Fi localization and tracking face accuracy limitations dictated by antenna count (for angle-of-arrival methods) and frequency bandwidth (for time-of-arrival methods). This paper presents mD-Track, a device-free Wi-Fi tracking system capable of jointly fusing information from as many dimensions as possible to overcome the resolution limit of each individual dimension. Through a novel path separation algorithm, mD-Track can resolve multipath at a much finer-grained resolution, isolating signals reflected off targets of interest. mD-Track can localize human passively at a high accuracy with just a single Wi-Fi transceiver pair. mD-Track also introduces novel methods to greatly streamline its estimation algorithms, achieving real-time operation. We implement mD-Track on both WARP and cheap off-the-shelf commodity Wi-Fi hardware, and evaluate its performance in different indoor environments.

https://dl.acm.org/doi/pdf/10.1145/3300061.3300133

mD-Track: Leveraging Multi-Dimensionality for Passive Indoor Wi-Fi Tracking

The IEEE 802.11ac amendment has been proposed to enhance the throughput of IEEE 802.11n beyond gigabit-per-second rates. In this article we present an overview of the most important features proposed in the 802.11ac amendment, including channel bonding mechanisms and multi-user MIMO.

IEEE 802.11ac: from channelization to multi-user MIMO

In an 802.11ac-based MU-MIMO network comprised of multiple cells1, inter-cell interference allows only a single AP to serve its clients at the same time, significantly limiting the network capacity. In this work, we overcome this limitation by letting the APs and clients in interfering cells coordinately cancel the inter-cell interference using their antennas for beamforming. To achieve such coordinated interference cancellation in a practical way, we propose a novel two-step optimization. First, without requiring any channel knowledge, each AP and client optimizes the use of its antennas for either data communication or inter-cell interference cancellation, in order to maximize the total number of deliverable streams in the MU-MIMO network. Second, with only partial channel knowledge, each AP and client optimizes their beamforming weights after the optimal antenna usage has been identified in the first step. Our solution, CoaCa, integrates this two-step optimization into 802.11ac with small modifications and negligible overhead, allowing each AP and client to locally perform the two-step optimization. Our experimental evaluation indicates that for a MU-MIMO network with two cells, by cancelling the inter-cell interference CoaCa can convert the majority of the expected number of streams increase (50%-67%) into network capacity improvement (41%-52%).

/pdf/combating-inter-cell-interference-in-802-11ac-based-multi-1iyfltwq7w.pdf

Combating inter-cell interference in 802.11ac-based multi-user MIMO networks

In this paper, we present the design, implementa- tion, and evaluation of the novel downlink Multi-User MIMO sounding protocol called MUTE. Our protocol decouples the sounding set selection used to collect Channel State Information (CSI), from the transmission set selection in order to minimize or even eliminate the overhead associated with sounding, while maximizing user selection performance. To this end, MUTE exploits channel statistics to all the different users to predict whether a particular user's channel will remain sufficiently stable, thereby allowing the access point to preclude channel sounding before a MU-MIMO transmission. We show that in indoor WLANs, MUTE can reduce sounding overhead by close to 73% under certain conditions while minimizing rate performance losses due to inaccurate channel estimation. Zero-Forcing Multi-User-MIMO beamforming systems (ZFBF MU-MIMO) rely on channel sounding to provide the beamformer or Access Point (AP) with Channel State Information (CSI) about each beamformee or user. This is necessary to generate the steering beam weights required to perform the zero-forcing precoding prior to a beamformed transmission (13). Additionally, it is advantageous to acquire CSI from all associated users in order to maximize user diversity, 1 thereby improving the user selection process by increasing the likelihood of finding beamformees with orthog- onal or semi-orthogonal channel vectors. This can lead to complete suppression of interference between the different data streams serving the different beamformees and therefore to rate maximization at every transmission. To this end, the beamformer can acquire channel estimates from all potential beamformees before every packet transmis- sion. This provides the AP with accurate, up-to-date CSI about all users to be served, hence improving the performance of the precoding scheme. That is, having the most updated CSI for all users allows the AP to find the optimal user grouping strategy at every transmission. Unfortunately, the overhead required for CSI acquisition is directly proportional to the number of users to be sounded as well as the frequency with which this process takes place. Therefore, in a practical system, the beamformer should find a balance between sounding frequency and CSI accuracy, in the interest of minimizing sounding overhead. In this paper we propose a multi-user zero-forcing beam- forming sounding protocol that addresses the issue of overhead associated with channel sounding, with the goal of eliminating 1 We define user diversity as the accommodation of a finite set of users with distinct channel characteristics. it temporarily based on channel stability. We name our protocol MUTE which stands for Multi-User Transmission Enhancer. In the best case, in the presence of users with stable channels, MUTE will invoke a MU-MIMO transmission without any im- mediately preceding channel sounding, thereby vastly reducing overhead and correspondingly increasing transmission air time and throughput. Nonetheless, MU-MIMO is very sensitive to the accuracy of CSI, specially as the number of concurrent streams increases. Therefore, MUTE strives to find a balance between CSI degradation and sounding suppression. We argue that the decoupling of the sounding selection pro- cedure from the transmission user selection procedure provides the flexibility to choose whether to sound a particular user or not, independently from the set of them to be served in the next ZFBF transmission. This in turn decreases overhead associated with sounding by exploiting the presence of users with stable channels, while independently providing sufficiently accurate information to the AP about channel statistics of associated users. Then, based on this information, the AP can select the combination of users that maximizes an objective function such as achievable rate or a fairness criteria, for example. This is in contrast to existing MU-MIMO implementations where the set of sounded users is the same as the set of users to be served next (4), (7), (12). Furthermore, two of the major strengths of MUTE are interoperability with IEEE 802.11ac (3) devices as well as the fact that it can operate independently of the scheduler implemented.

/pdf/mute-sounding-inhibition-for-mu-mimo-wlans-3qx62geuax.pdf

MUTE: Sounding Inhibition for MU-MIMO WLANs

This paper investigates the application in IEEE 802.11ac/ax systems of received bit information rate (RBIR) technique in order to estimate the effective signal-to-interference-plus-noise ratio used to abstract the physical layer (PHY) performance in system level simulations. The RBIR technique is evaluated based on simulation results of 802.11ac PHY operating over both canonical flat fading and independent and identical distributed and spatial-correlated frequency selective single-user and multi-user multiple input multiple output channels. We have concluded that RBIR PHY abstraction methodology is accurate enough to provide first order insights on system level performance and design options with reduced computational complexity. Finally, it is also described the application of RBIR PHY abstraction scheme to schedule on the flight the modulation code scheme that achieve a target quality-of-service.

/pdf/on-application-of-phy-layer-abstraction-techniques-for-40gcgiansl.pdf

On Application of PHY Layer Abstraction Techniques for System Level Simulation and Adaptive Modulation in IEEE 802.11ac/ax Systems

Downlink Multi-User MIMO (MU-MIMO) enables the simultaneous spatial sharing of the channel by multiple users to achieve a capacity gain over Single-Input Single-Output (SISO) systems. Unfortunately, the overhead required to enable multi-user MIMO is much higher than the overhead required for single-stream systems. Namely, for K users, collection of channel state information requires K transmission exchanges (i.e., O(K)) between the AP and users. Likewise, the MU-MIMO acknowledgement process also requires the same amount of exchanges, thus reducing the performance gains attained via simultaneous downlink transmission. In this paper, we design, implement, and experimentally evaluate Concurrent Uplink Control Messages (CUiC) to scale the MU-MIMO control information exchange process and improve the efficiency of 802.11ac-based MU-MIMO networks. Our key technique is the design of new channel sounding and acknowledgement mechanisms that enable multiple users to transmit their reverse-direction control messages (i.e., beamforming reports and acknowledgments) concurrently to the AP, in O(1) transmission slots. We implement CUiC and perform an extensive set of experiments and demonstrate throughput gains of more than 100% compared to 802.11ac.

Oscar Bejarano

Papers

IEEE 802.11ac: from channelization to multi-user MIMO

Combating inter-cell interference in 802.11ac-based multi-user MIMO networks

MUTE: Sounding Inhibition for MU-MIMO WLANs

On Application of PHY Layer Abstraction Techniques for System Level Simulation and Adaptive Modulation in IEEE 802.11ac/ax Systems

Scaling multi-user MIMO WLANs: The case for concurrent uplink control messages