Dhananjay Lal

A novel MAC layer protocol for space division multiple access in wireless ad hoc networks

Recently, MAC protocols using directional antennas for wireless ad hoc networks that are based on and similar to IEEE 802.11 type WLAN have been proposed. These protocols, however, are unable to attain substantial performance improvements because they do not enable the nodes to perform multiple simultaneous transmissions/receptions. In this paper, we propose a MAC layer protocol that exploits space division multiple access, thus using the property of directional reception to receive more than one packet from spatially separated transmitter nodes (equipped with smart antenna systems). Our simulation results show that drastic throughput improvements may be achieved through this scheme.

A Kalman Filter Based Link Quality Estimation Scheme for Wireless Sensor Networks

Communication among wireless sensor nodes that employ cheap low-power transceivers is often very sensitive to the variations of the wireless channel. Sensor network routing protocols thus strive to continually adapt to temporal variations in wireless links in order to avoid wasteful transmissions over low-quality links. Such adaptive routing protocols must rely on a scheme that can not only accurately estimate the quality of wireless links in terms of a quantitative measure, such as the packet success rate (PSR), but also quickly adapt to temporal dynamics of the links. Traditionally, the PSR is estimated from the fraction of successful transmissions over a window of test- packets. However, we demonstrate that counting based methods do not react to changes in the wireless channel fast enough and that the only way to address this problem is to estimate the PSR based on the receivers characteristics and on the signal to noise ratio (SNR) at the receiver. We thus propose a scheme that uses a pre-calibrated SNR-PSR relationship and instantaneous SNR estimates to calculate the PSR of the link. In our scheme, each receiver continuously tracks the SNR using a Kalman Filter to minimize the estimation error and uses a locally available SNR- PSR curve to estimate the PSR. Through extensive experiments we demonstrate that our scheme adapts to variations in the channel faster than counting-based PSR estimators and that it also provides better PSR estimates than these counting-based approaches.

Towards automatic detection of falls using wireless sensors

Accurate detection of falls leading to injury is essential for providing timely medical assistance. In this paper, we describe a wireless sensor network system for automatic fall detection. To detect falls, we use a combination of a body- worn triaxial accelerometer with motion detectors placed in the monitored area. While accelerometer provides information about the body motion during a fall, motion detectors monitor general presence or absence of motion. From all sensors, the data is transmitted wirelessly using the IEEE 802.15.4 protocol to a central node for processing. We use an implementation of carrier sense multiple access - collision avoidance scheme for channel reuse. A simple forwarding scheme is used to provide an extended coverage for a home environment. Fall detection is accomplished by a two-stage algorithm that utilizes the triaxial acceleration and the motion data sequentially. In the first stage, the algorithm detects plausible falls using a measure of normalized energy expenditure computed from the dynamic acceleration values. In the second stage, falls are confirmed based on the absence of motion. Systematic evaluation on simulated falls using 15 adult subjects shows that the proposed system provides a highly promising solution for real-time fall detection.

Performance evaluation of medium access control for multiple-beam antenna nodes in a wireless LAN

We consider the medium access control (MAC) protocol design for nodes in a wireless LAN that use a wide-azimuth switched beam smart antenna system comprised of a multiple beam antenna array. The one-hop performance of carrier sense multiple access (CSMA) as well as slotted aloha for such a system is presented analytically and through simulation. The problem of synchronization of multiple beams in CSMA is investigated in our analysis. Our results show that, under heavy offered load conditions, CSMA is a good choice with nodes that have multiple-beam smart antennas, despite the performance loss due to the beam synchronization, providing a stable throughput that approaches unity and is invariant to fluctuations in the offered load. Slotted Aloha, on the other hand, is capable of higher peak throughput in a narrow range of offered loads as more switched beams are employed, but performance drastically reduces beyond optimum offered loads. We also introduce a method, expanded receive rule (ERR), whereby the tight synchronization among different beams of a receiver node in CSMA is relaxed, which is observed to provide better throughput. Finally, we also present performance results for a 4-way-handshake-type carrier sense mechanism using multiple beam antennas.

A cross layer MAC with explicit synchronization through intelligent feedback for multiple beam antennas

This paper introduces a novel cross layer medium access control protocol for multiple beam antennas to fully utilize their multiple-beam-forming capability. Our protocol, explicit synchronization via intelligent feedback (ESIF), uses feedback from neighboring nodes to synchronize data communication at multiple beams. ESIF exploits routing information to guarantee long-term fairness and minimize the energy and latency overheads. Unlike previous works on directional antennas that discuss range extension, we focus on optimal spatial reuse. Simulation results demonstrate that ESIF outperforms other on-demand access schemes based on IEEE 802.11 DCF for multiple beam antennas

IEEE 802.11 DCF based MAC protocols for multiple beam antennas and their limitations

Multiple beam antennas can receive (or transmit) data on multiple beams simultaneously. The aim of this paper is to show the limitations of using IEEE 802.11 distributed coordination function (DCF) based schemes for medium access control (MAC) in such antennas. We provide four different variants of IEEE 802.11 DCF based on-demand protocols to study this phenomenon. Our simulation results and analyses emphasize the need to develop a novel MAC protocol to fully exploit the capabilities of multiple beam antennas. We further provide some embryonic guidelines for the development of such a protocol

From Experience with Indoor Wireless Networks: A Link Quality Metric that Captures Channel Memory

With ARQ based link-layer schemes, energy consumption on a link increases with packet retransmissions. We define the inefficiency of a link as the expected number of transmissions before a packet is successfully received on that link. Memory in the packet success process, caused by long indoor coherence times, strongly influences inefficiency. Based on measurements, we build a simple model for the packet success process that incorporates memory and predicts our metric practically and accurately. In particular, inefficiency is asymptotically linear in the memory duration when there is a nonzero probability of a deep fade, and approximately logarithmic otherwise.

A multiple-beam antenna protocol at a wireless access point for exploiting spatial parallelism

We propose a protocol for use at a wireless access point (AP) that uses multiple beam smart antennas. Such APs can be deployed in traffic hotspots to overcome bottlenecks by enhancing capacity significantly. Our protocol is fully compatible with IEEE 802.11 in its current form, and can function with omnidirectional devices. The protocol seeks to exploit parallel packet transmission of downstream data (from the AP to the nodes), while reverting to normal 802.11 for upstream data (from a node to the AP). The performance gain expected from our protocol is proportional to the spatial resolution (number of beams) and the amount of downstream data. Typically, mobile terminals running applications download data from other servers, and hence the volume of downstream data far exceeds the upstream data. In such conditions, our protocol provides substantial gain.

Localized transmission scheduling for spatial multiplexing using smart antennas in wireless adhoc networks

In our paper, we investigate some MAC layer issues associated with the use of multiple-beam smart antennas at nodes in a distributed wireless ad-hoc network. We propose a transmitter-initiated on-demand MAC protocol that exploits the spatial parallelism in the transmission of packets at a node. We devise two MAC scheduling strategies in the one-hop neighborhood of a node, largest edge first (LEF) and largest degree node first (LDNF), that allow spatial parallelism to be exploited in the transmission as well as reception of packets. Our simulation results indicate that significant gain may be achieved through our scheduling strategies in a one-hop traffic bottleneck.