Sriram Lakshmanan

Component based channel assignment in single radio, multi-channel ad hoc networks

In this paper, we consider the channel assignment problem in single radio multi-channel mobile ad-hoc networks. Specifically, we investigate the granularity of channel assignment decisions that gives the best trade-off in terms of performance and complexity. We present a new granularity for channel assignment that we refer to as component level channel assignment. The strategy is relatively simple, and is characterized by several impressive practical advantages. We also show that the theoretical performance of the component based channel assignment strategy does not lag significantly behind the optimal possible performance, and perhaps more importantly we show that when coupled with its several practical advantages, it significantly outperforms other strategies under most network conditions.

Diversity Routing for Multi-hop Wireless Networks with Cooperative Transmissions

In this paper, we consider the use of cooperative transmissions in multi-hop wireless networks to achieve virtual MISO (Multiple Input Single Output) links. Specifically, we investigate how the physical layer VMISO benefits translate into network level performance improvements. We show that the improvements are non-trivial (15% to 300% depending on the node density) but rely on two crucial algorithmic decisions: the number of co-operating transmitters for each link; and the cooperation strategy used by the transmitters. Finally, we present Proteus, an adaptive diversity routing protocol that includes algorithmic solutions to the above two decision problems and leverages VMISO links in multi-hop wireless network to achieve performance improvements. We evaluate Proteus using NS2 based simulations with an enhanced physical layer model that accurately captures the effect of VMISO transmissions.

Asymmetric caching: improved network deduplication for mobile devices

Network deduplication (dedup) is an attractive approach to improve network performance for mobile devices. With traditional deduplication, the dedup~source uses only the portion of the cache at the dedup~destination that it is aware of. We argue in this work that in a mobile environment, the dedup~destination (say the mobile) could have accumulated a much larger cache than what the current dedup~source is aware of. This can occur because of several reasons ranging from the mobile consuming content through heterogeneous wireless technologies, to the mobile moving across different wireless networks. In this context, we propose asymmetric caching, a solution that is overlaid on baseline network deduplication, but which allows the dedup~destination to selectively feedback appropriate portions of its cache to the dedup~source with the intent of improving the redundancy elimination efficiency. We show using traffic traces collected from 30 mobile users, that with asymmetric caching, over 89% of the achievable redundancy can be identified and eliminated even when the dedup~source has less than one hundredth of the cache size as the dedup~destination. Further, we show that the ratio of bytes saved from transmission at the dedup~source because of asymmetric caching is over 6x that of the number of bytes sent as feedback. Finally, with a prototype implementation of asymmetric caching on both a Linux laptop and an Android smartphone, we demonstrate that the solution is deployable with reasonable CPU and memory overheads.

Multi-gateway association in wireless mesh networks

Most traditional models of wireless mesh networks involve a mobile device connecting to the backbone through one of the available gateways in a wireless mesh network. In this paper, we present an alternate model, in which mobile devices are allowed to connect through more than one of the available gateways. We call the model multi-gateway association (MGA). We present arguments for why such a model can result in better capacity, fairness, diversity and security when compared to the default single-association model. We also identify the primary challenges that need to be addressed when using multiple-gateway associations, and propose solutions to handle these challenges.

Securing Wireless Data Networks against Eavesdropping using Smart Antennas

In this paper, we focus on securing communication over wireless data networks from malicious eavesdroppers, using smart antennas. While conventional cryptography based approaches focus on hiding the meaning of the information being communicated from the eavesdropper, we consider a complimentary class of strategies that limit knowledge of the existence of the information from the eavesdropper. We profile the performance achievable using simple beamforming strategies using a newly defined metric called exposure region. We then present three strategies within the context of an approach called virtual arrays of physical arrays to significantly improve the exposure region performance of a wireless LAN environment. Using simulations and analysis, we validate and evaluate the proposed strategies.

On multi-gateway association in wireless mesh networks

Most traditional models of wireless mesh networks involve a mobile device connecting to the backbone through one of the available gateways in a wireless mesh network. In this paper, we present an alternate model, in which mobile devices are allowed to connect through more than one of the available gateways. We call the model multi-gateway association (MGA). We present arguments for why such a model can result in better capacity, fairness, diversity and security when compared to the default single-association model. We also identify the primary challenges that need to be addressed when using multiple-gateway associations, and propose solutions to handle these challenges.

Towards Adaptive Beamforming in Indoor Wireless Networks: An Experimental Approach

Several research works have argued that adaptive beamforming has the potential to realize the high spectral efficiency requirements of next-generation wireless standards, and is especially well-suited for multipath-rich environments such as indoors. Most works have been limited to theory; few works in literature address the practical benefits and realizability of adaptive beamforming. In this paper, we design and implement the first indoor WLAN beamforming system with multi-element array antennas and software radio platforms, that forms a testbed for exploration of practical benefits of beamforming, and evaluation of algorithms for efficient beamforming in diverse environments. In the process of building the system, we identify and address several challenges with practical beamforming that are often ignored in theoretical works. Most importantly, channel estimation for forming the best beam to a user is hindered by oscillator drifts on the transmitter and receiver side that introduce hard-to-isolate phase and frequency offsets from the estimated channel coefficients. We describe these issues and incorporate novel solutions in our system to address them without requiring hardware modifications. We use the system to demonstrate the realizable benefits of adaptive beamforming in a typical indoor office environment.

Realizing high performance multi-radio 802.11n wireless networks

We explore the design of a high capacity multi-radio wireless network using commercial 802.11n hardware. We first use extensive real-life experiments to evaluate the performance of closely located 802.11n radios. We discover that even when tuned to orthogonal channels, co-located 802.11n radios interfere with each other and achieve significantly less throughput than expected. Our analysis reveals that the throughput degradation is caused by three link-layer effects: (i) triggering of carrier sensing, (ii) out of band collisions and (iii) unintended frequency adaptation. Using physical layer statistics, we observe that these effects are caused by fundamental limitations of co-located radios in achieving signal isolation. We then consider the use of beamforming antennas, shielding and antenna separation distance to achieve better signal isolation and to mitigate these problems. Our work profiles the gains of different physical isolation approaches and provides insights to network designers to realize high-performance wireless networks without requiring synchronization or protocol modifications.

On link rate adaptation in 802.11n WLANs

The IEEE 802.11n standard is gaining popularity to achieve high throughput in Wireless LANs. In this paper, we explore link adaptation in practical 802.11n systems using experiments with off-the-shelf hardware. Our experiments reveal several non-trivial insights. Specifically, (1) trivial extensions of algorithms developed for 802.11g provide minimal benefits in 802.11n systems; (2) in contrast to theoretical expectation, multiple antenna transmission does not always lead to higher throughput in practice; (3) both stream and antenna selection are essential to reap the full benefits of MIMO technologies. We use insights developed from experiments to develop a new metric for stream selection called the Median Multiplexing Factor (MMF). The proposed metric can be used to develop intelligent rate selection algorithms that can achieve high throughput with purely software changes.

Practical beamforming based on RSSI measurements using off-the-shelf wireless clients

WLANs have become an important last-mile technology for providing internet access within homes and enterprises. In such indoor deployments, the wireless channel suffers from significant multipath scattering and fading that degrades performance. Beamforming is a smart antenna technology that adjusts the transmissions at the transmitter to reenforce the signals received through multiple paths at the receiver. However, doing this requires the accurate estimation of the channel coefficients at the receiver and its knowledge at the transmitter which off-the-shelf WiFi clients are incapable of doing. In this work, we develop a novel procedure that uses Received Signal Strength Indicator (RSSI) measurements at the receiver along with an intelligent estimation methodology at the transmitter to achieve beamforming benefits. Using experiments in an indoor office scenario with commercial WiFi clients, we show that the scheme achieves significant performance improvements across diverse scenarios.