Antonios Michaloliakos

AirSync: enabling distributed multiuser MIMO with full spatial multiplexing

The enormous success of advanced wireless devices is pushing the demand for higher wireless data rates. Denser spectrum reuse through the deployment of more access points (APs) per square mile has the potential to successfully meet such demand. In principle, distributed multiuser multiple-input-multiple-output (MU-MIMO) provides the best approach to infrastructure density increase since several access points are connected to a central server and operate as a large distributed multiantenna access point. This ensures that all transmitted signal power serves the purpose of data transmission, rather than creating interference. In practice, however, a number of implementation difficulties must be addressed, the most significant of which is aligning the phases of all jointly coordinated APs. In this paper, we propose AirSync, a novel scheme that provides timing and phase synchronization accurate enough to enable distributed MU-MIMO. AirSync detects the slot boundary such that all APs are time-synchronous within a cyclic prefix (CP) of the orthogonal frequency-division multiplexing (OFDM) modulation and predicts the instantaneous carrier phase correction along the transmit slot such that all transmitters maintain their coherence, which is necessary for multiuser beamforming. We have implemented AirSync as a digital circuit in the field programmable gate array (FPGA) of the WARP radio platform. Our experimental testbed, comprising four APs and four clients, shows that AirSync is able to achieve timing synchronization within the OFDM CP and carrier phase coherence within a few degrees. For the purpose of demonstration, we have implemented two MU-MIMO precoding schemes, Zero-Forcing Beamforming (ZFBF) and Tomlinson-Harashima Precoding (THP). In both cases, our system approaches the theoretical optimal multiplexing gains. We also discuss aspects related to the MAC and multiuser scheduling design, in relation to the distributed MU-MIMO architecture. To the best of our knowledge, AirSync offers the first realization of the full distributed MU-MIMO multiplexing gain, namely the ability to increase the number of active wireless clients per time-frequency slot linearly with the number of jointly coordinated APs, without reducing the per client rate.

Achieving high data rates in a distributed MIMO system

A distributed MIMO system consists of several access points connected to a central server and operating as a large distributed multi-antenna access point. In theory, such a system enjoys all the significant performance gains of a traditional MIMO system, and it may be deployed in an enterprise WiFi like setup. In this paper, we investigate the efficiency of such a system in practice. Specifically, we build upon our prior work on developing a distributed MIMO testbed, and study the performance of such a system when both full channel state information is available to the transmitters and when no channel state information is available. In the full channel state information scenario, we implement Zero-Forcing Beamforming (ZFBF) and Tomlinson-Harashima Precoding (THP) which is provably near-optimal in high SNR conditions. In the scenario where no channel information is available, we implement Blind Interference Alignment (BIA), which achieves a higher multiplexing gain (degrees of freedom) than conventional TDMA. Our experimental results show that the performance of our implementation is very close to the theoretically predicted performance and offers significant gains over optimal TDMA. We also discuss medium access layer issues in detail for both scenarios. To the best of our knowledge, this is the first time that the theoretical high data rates of multiuser MIMO systems have been showcased in a real world distributed MIMO testbed.

USC SDR, an easy-to-program, high data rate, real time software radio platform

We present USC SDR, a wireless platform designed for easy-to-program, high data rate, real time wireless experimentation. The design of our platform aims at removing most of the bottlenecks encountered in the design of current software radio architectures, e.g. the requirement to program new schemes in an FPGA, and the difficulty to run real-time experiments for a long time. The architecture combines generic PCI FPGA development boards with radio front-ends built as self-sufficient daughterboards. The daughterboards are connected to the FPGAs, which in turn are plugged into the PCIE slots of a general-purpose server. Interestingly, the connection of the daughterboards to the FPGA cards is implemented through a standard FMC (FPGA Mezzanine Card) interface, such that the same RF front-end can be reused with future FPGA generations. In this way, USC SDR is not limited to a specific FPGA choice and does not require a complete re-design in order to accommodate for future more powerful FPGAs. The hardware is supported by a real-time software architecture where signal processing tasks, PHY and MAC layer algorithms can be programmed as user-level applications. As an example, we will showcase a massive MIMO testbed built using a single server with multiple PCIE slots.

Efficient MAC for distributed multiuser MIMO systems

A distributed multiuser MIMO system consists of several access points which are connected to coordinating servers and operate as a large multi-antenna access point. Thanks to joint decoding and precoding, all transmitted signal power is useful, rather than “interference”. The system has the potential to support constant rates as the number of clients increases, thus offering a tremendous bandwidth boost. Despite the high gains and potential applicability to real world setups like enterprise networks, this approach is regarded today mostly as a theoretical solution because of some serious implementation difficulties. Motivated by our recent success in addressing synchronization issues in a real distributed multiuser MIMO testbed that we have developed, in this work we move one step further and design an efficient MAC scheme for such a system. First, we study and design optimal as well as practical user selection and scheduling schemes. Second, we investigate coding methods to realize the achievable rates at higher layers, and pay particular attention to recent rateless codes advancements. Last, we propose a specific MAC super-frame, and discuss how to make our system backward compatible with 802.11ac.

Power minimization with quality-of-information outages

In this paper, we consider Quality-of-Information (QoI) aware transmission policies for a dynamic environment. In particular, we focus on the time-varying nature of the observation quality of the environment in practical networks which leads to uncertainty in satisfying QoI requirements specified by end users. The goal of this paper is to meet QoI requests from end users with minimum resources. Specifically, power is allocated dynamically depending on observation accuracies and QoI requirements. We formulate a dynamic scheme for scheduling with the objective of minimizing the energy consumption at the network while satisfying constraints on outage probability for QoI. Lyapunov stability arguments are used to define a policy based on the instantaneous observation qualities and QoI requirement satisfaction levels. Numerical results demonstrate that significant improvements in delivered QoI are realized with identical power expenditure using our QoI-aware resource allocation algorithm compared with traditional maximum-rate schedulers.

Operational information content sum capacity

This paper considers Quality-of-Information (QoI) aware resource allocation policies for multiuser networks. QoI is a recently introduced composite metric which is impacted by a number of attributes of information communicated from the source(s) to the destination(s), and as such differs from traditional quality-of-service metrics considered to date. The focus of this work is defining the Operational Information Content Sum Capacity (OICC-S) of a network, achieved by the set of information attributes supported that maximize sum quality of the network. This quality is defined as a function of the information attributes provided by the source input, as well as the channel induced attributes that impact the QoI delivered to the destination(s). Optimum rate allocation to maximize the output sum quality of information and achieve OICC-S of the network for various settings is provided, and demonstrated to differ from the solution that provides maximum throughput, making QoI-awareness necessary in resource allocation. Insights arising from the analysis are provided, along with those from practical scenarios.

Asynchronously Coordinated Multi-Timescale Beamforming Architecture for Multi-Cell Networks

Modern wireless devices such as smartphones are pushing the demand for higher wireless data rates. The ensuing increase in wireless traffic demand can be met by a denser deployment of access points, coupled with a coordinated deployment of advanced physical layer techniques to reduce inter-cell interference. Unfortunately, advanced physical layer techniques, e.g., multi-user (MU) MIMO found in 802.11ac and LTE-advanced, are not designed to operate efficiently in a coordinated fashion across multiple densely deployed transmitters. In this paper, we introduce a new coordination architecture, which can achieve high performance gains without the high overhead and deployment cost that usually comes with coordination, thus making the vision of high capacity wireless access via densely deployed transmitters practical. The basic idea is to loosely coordinate nearby transmitters using slow varying channel statistics, while keeping all the functionality which depends on fast varying channel state information and has tight time deadlines locally. We achieve this via a smart combination of analog and digital beamforming using inexpensive front ends, a provably efficient algorithm to select compatible users and analog beams across all transmitters, and backward compatible protocol extensions. Our performance results, which include analysis, simulations, and experiments with software defined radios and directional antennas, show that our approach can achieve the

Scalable Synchronization and Reciprocity Calibration for Distributed Multiuser MIMO

10\times

Performance modeling of next-generation WiFi networks

gains of the theoretically optimal coordinated MU-MIMO approach, without the need to either tightly coordinate the clocks of the remote transmitters or meet tight delay constraints.