Theodoros Salonidis

Design and experimental evaluation of multi-user beamforming in wireless LANs

Multi-User MIMO promises to increase the spectral efficiency of next generation wireless systems and is currently being incorporated in future industry standards. Although a significant amount of research has focused on theoretical capacity analysis, little is known about the performance of such systems in practice. In this paper, we present the design and implementation of the first multi-user beamforming system and experimental framework for wireless LANs. Using extensive measurements in an indoor environment, we evaluate the impact of receiver separation distance, outdated channel information due to mobility and environmental variation, and the potential for increasing spatial reuse. For the measured indoor environment, our results reveal that two receivers achieve close to maximum performance with a minimum separation distance of a quarter of a wavelength. We also show that the required channel information update rate is dependent on environmental variation and user mobility as well as a per-link SNR requirement. Assuming that a link can tolerate an SNR decrease of 3 dB, the required channel update rate is equal to 100 and 10 ms for non-mobile receivers and mobile receivers with a pedestrian speed of 3 mph respectively. Our results also show that spatial reuse can be increased by efficiently eliminating interference at any desired location; however, this may come at the expense of a significant drop in the quality of the served users.

Experimental characterization of 802.11n link quality at high rates

802.11n has made a quantum leap over legacy 802.11 systems by supporting extremely higher transmission rates at the physical layer. In this paper, we ask whether such high rates translate to high quality links in a real deployment. Our experimental investigation in an indoor wireless testbed reveals that the highest transmission rates advertised by the 802.11n standard typically produce losses (or even outages) even in interference-free environments. Such losses become more acute and persist at high SNR values, even at low interference intensity. We find that these problems are partly due to bad configurations that do not allow exploitation of spatial diversity, partly due to the wider 802.11n channels that expose these sensitive high rates to more interference. We show that these problems can be alleviated using the 802.11n MAC layer enhancements jointly with packet size adaptation.

XPRESS: a cross-layer backpressure architecture for wireless multi-hop networks

Contemporary wireless multi-hop networks operate much below their capacity due to the poor coordination among transmitting nodes. In this paper we present XPRESS, a cross-layer backpressure architecture designed to reach the full capacity of wireless multi-hop networks. Instead of a collection of poorly coordinated wireless routers, XPRESS turns a mesh network into a wireless switch. Transmissions over the network are scheduled using a throughput-optimal backpressure algorithm. Realizing this theoretical concept entails several challenges, which we identify and address with a cross-layer design and implementation on top of our wireless hardware platform. In contrast to previous work, we implement and evaluate backpressure scheduling over a TDMA MAC protocol, as it was originally proposed in theory. Our experiments in an indoor testbed show that XPRESS can yield up to 128% throughput gains over 802.11.

Design and deployment considerations for high performance MIMO testbeds

MIMO (Multiple Input Multiple Output) enabled systems are characterized by higher reliability and transmission rates, as compared to conventional SISO (Single Input Single Output) systems. However, unless administered properly, the MIMO technology may not facilitate very high throughputs on point-to-point wireless links. Therefore, it becomes imperative for the network architect to design such networks in ways that fully exploit the inherent properties of MIMO. In this paper, we first conduct an extensive experimental study, using a powerful hardware platform, in order to understand the behavior of MIMO links in different topological scenarios. Our experiments involve scenarios with MIMO links in isolation, as well as in competition with other MIMO and SISO links. Second, we perform measurements with different commercial platforms towards assessing the ability of each platform to efficiently support the MIMO technology. Based on our experimental observations we deduce that the CPU processing speed of the underlying hardware platform is an important factor that can hide the performance benefits of a MIMO enabled tranceiver. We comment on the applicability of the different hardware choices that we test; furthermore, we suggest the most appropriate choice for building a MIMO testbed, taking into account the cost, the extend-ability and the re-usability of the selected platform. Finally, having adopted this choice in our testbed design, we provide a description of our testbed architecture.

Video-Aware Rate Adaptation for MIMO WLANs

The IEEE 802.11n standard supports very high physical layer data rates using Multiple Input Multiple Output (MIMO) antenna technologies. Despite such high rates, High Definition (HD) video streaming is still challenging in WLAN deployments. In this paper, we show that the wireless channel probing overhead of existing 802.11n data rate adaptation mechanisms can be detrimental to HD video performance. We propose VARA, a Video-Aware Rate Adaptation protocol that addresses this problem by adapting the frequency and timing of wireless probing to both video encoding rate variations and wireless channel variations. In addition, VARA employs novel strategies that multiplex several Variable Bit Rate (VBR) HD video streams by minimizing their aggregate peak rate requirement. Our experimental evaluations for static and mobile scenarios in a MIMO 802.11n wireless testbed demonstrate the practical benefits of VARA over state-of-the-art 802.11n rate adaptation protocols.

MIMO wireless networks with directional antennas in indoor environments

We perform an experimental characterization of an indoor MIMO system with directional antennas (our prototype multi-sector antennas). The study reveals that, even without antenna directivity gain, the directionality of signals changes the MIMO channel structure and provides a way to improve MIMO throughput performance. It also shows that it is sufficient for the improvement to consider only a small subset out of all possible antenna direction combinations. Finally, our study shows that it is possible to achieve both throughput gains and interference reduction, thus increasing network spatial reuse.

Optimization driven multi-hop network design and experimentation: the approach of the FP7 project OPNEX

The OPNEX project exemplifies system and optimization theory as the foundations for algorithms that provably maximize capacity of wireless networks. The algorithms termed in abstract network models have been converted to protocols and architectures practically applicable to wireless systems. A validation methodology through experimental protocol evaluation in real network testbeds has been proposed and used. OPNEX uses recent advances in system theoretic network control, including the Back-Pressure principle, max-weight scheduling, utility optimization, congestion control, and the primaldual method for extracting network algorithms. These approaches exhibited vast potential for achieving high capacity and full exploitation of resources in abstract network models and found their way to reality in high performance architectures developed as a result of the research conducted within OPNEX.

Design and implementation of backpressure scheduling in wireless multi-hop networks: from theory to practice

• Multi-hop TDMA MAC and slot synchronization • Non-interfering link sets must be known • Computation and protocol overhead ‒ Optimal schedule computation is NP-hard ‒ Flow queues lengths and schedule known at each slot • Hardware constraints ‒ Memory constrained and unaware of network flows ‒ Backpressure requires link scheduling • Wireless multi-hop network operate below their capacity ‒ Poor coordination among transmitting nodes ‒ Protocols operate independently at each layer • Backpressure scheduling ‒ A cross-layer approach to achieve full network capacity ‒ Performs routing and scheduling at each time slot ‒ Requires time-slotted MAC protocol and central controller ‒ So far a theoretical concept: no real system exists to date

Interference mitigation in WiFi networks using multi-sector antennas

Sectorized antennas provide an attractive solution to increase wireless network capacity through interference mitigation. Despite their increasing popularity, the real-world performance characteristics of such antennas in dense wireless mesh networks are not well understood. We demonstrate our multi-sector antenna prototypes and their performance through video streaming over an indoor wireless network in the presence of interfering nodes. We use our graphical tool to vary the sender, receiver, and interferer antenna configurations and the resulting performance is directly visible in the video quality displayed at the receiver.