Dola Saha

A network-aware MAC and routing protocol for effective load balancing in ad hoc wireless networks with directional antenna

Use of directional antenna in the context of ad hoc wireless networks can largely reduce radio interference, thereby improving the utilization of wireless medium. Our major contribution in this paper is to devise a routing strategy, along with a MAC protocol, that exploits the advantages of directional antenna in ad hoc networks for improved system performance. In this paper, we have illustrated a MAC and routing protocol for ad hoc networks using directional antenna with the objective of effective load balancing through the selection of maximally zone disjoint routes. Zone-disjoint routes would minimize the effect of route coupling by selecting routes in such a manner that data communication over one route will minimally interfere with data communication over the others. In our MAC protocol, each node keeps certain neighborhood status information dynamically in order that each node is aware of its neighborhood and communications going on in its neighborhood at that instant of time. This status information from each node is propagated periodically throughout the network. This would help each node to capture the approximate network status periodically that helps each node to become topology-aware and aware of communications going on in the network, although in an approximate manner. With this status information, each intermediate node adaptively computes routes towards destination. The performance of the proposed framework has been evaluated on QualNet Network Simulator with DSR (as in QualNet) as a benchmark. Our proposed mechanism shows four to five times performance improvement over DSR, thus demonstrating the effectiveness of this proposal.

SMACK: a SMart ACKnowledgment scheme for broadcast messages in wireless networks

Network protocol designers, both at the physical and network level, have long considered interference and simultaneous transmission in wireless protocols as a problem to be avoided. This, coupled with a tendency to emulate wired network protocols in the wireless domain, has led to artificial limitations in wireless networks. In this paper, we argue that wireless protocols can exploit simultaneous transmission to reduce the cost of reliable multicast by orders of magnitude. With an appropriate application interface, simultaneous transmission can also greatly speed up common group communication primitives, such as anycast, broadcast, leader election and others. The proposed method precisely fits into the domain of directly reachable nodes where many group communication mechanisms are commonly used in routing protocols and other physical-layer mechanisms. We demonstrate how simultaneous transmission can be used to implement a reliable broadcast for an infrastructure and peer-to-peer network using a prototype reconfigurable hardware. We also validate the notion of using simple spectrum sensing techniques to distinguish multiple transmissions. We then describe how the mechanism can be extended to solve group communication problems and the challenges inherent to build innovative protocols which are faster and reliable at the same time.

Coordinated dynamic spectrum management of LTE-U and Wi-Fi networks

This paper investigates the co-existence of Wi-Fi and LTE networks in emerging unlicensed frequency bands which are intended to accommodate multiple radio access technologies. Wi-Fi and LTE are the two most prominent wireless access technologies being deployed today, motivating further study of the inter-system interference arising in such shared spectrum scenarios as well as possible techniques for enabling improved co-existence. An analytical model for evaluating the baseline performance of co-existing Wi-Fi and LTE networks is developed and used to obtain baseline performance measures. The results show that both Wi-Fi and LTE networks cause significant interference to each other and that the degradation is dependent on a number of factors such as power levels and physical topology. The model-based results are partially validated via experimental evaluations using USRP-based SDR platforms on the ORBIT testbed. Further, inter-network coordination with logically centralized radio resource management across Wi-Fi and LTE systems is proposed as a possible solution for improved co-existence. Numerical results are presented showing significant gains in both Wi-Fi and LTE performance with the proposed inter-network coordination approach.

An adaptive framework for multipath routing via maximally zone-disjoint shortest paths in ad hoc wireless networks with directional antenna

Application of multipath routing techniques in mobile ad hoc networks has been explored earlier, as multipath routing may help to reduce end-to-end delay, perform load balancing and consequently improve throughput. However, it has also been shown that the success of multipath routing in ad hoc wireless network depends on network topology and channel characteristics can severely limit the gain offered by multipath routing strategies. The most significant challenge to making the use of multipath routing protocols effective in this environment involves considering the effects of route coupling. Route coupling in wireless medium occurs when two routes are located physically close enough to interfere with each other during data communication. As a result, the nodes in multiple routes are constantly contending for access to the medium they share and can end up performing worse than a single path protocol. In this paper, we propose a notion of zone-disjoint routes in wireless medium where paths are said to be zone-disjoint when data communication over one path will not interfere with data communication in other path. The notion of zone-disjointness is used as route selection criteria. However, zone-disjointness alone is not sufficient for performance improvement. If the path-length (number of hops) were large, that would increase the end-to-end delay even in the context of zone-disjointness. So, it is imperative to select maximally zone-disjoint shortest paths. However, getting zone-disjoint or even partially zone-disjoint routes in ad hoc network with omni-directional antenna is difficult, since the transmission zone of each node is larger compared to that with directional antenna. Hence, one way to reduce this transmission zone of a node is to use directional antenna. In this paper, we investigate the effect of directional antenna on zone-disjoint multipath routing and evaluated its effectiveness in QualNet network simulator.

Improving End-to-End Delay through Load Balancing with Multipath Routing in Ad Hoc Wireless Networks Using Directional Antenna

Multipath routing protocols are distinguished from single-path protocol by the fact that they use several paths to distribute traffic from a source to a destination instead of a single path. Multipath routing may improve system performance through load balancing and reduced end-to-end delay. However, two major issues that dictate the performance of multipath routing – how many paths are needed and how to select these paths. In this paper, we have addressed these two issues in the context of ad hoc wireless networks and shown that the success of multipath routing depends on the effects of route coupling during path selection. Route coupling, in wireless medium, occurs when two routes are located physically close enough to interfere with each other during data communication. Here, we have used a notion of zone-disjoint routes to minimize the effect of interference among routes in wireless medium. Moreover, the use of directional antenna in this context helps to decouple interfering routes easily compared to omni-directional antenna.

A phased array antenna testbed for evaluating directionality in wireless networks

One of the most important components of any mobile system is the antenna; antenna design can overcome or cause a number of problems that then must be addressed at other technology layers. Modern mobile platforms are beginning to include novel antenna technology such as MIMO and beam steering; these technologies increase the complexity of evaluating the effectiveness of topology formation algorithms, routing and overall performance due to the large number of configuration states the system can contain. Directional antennas allow for significant improvements in link quality and spatial reuse in wireless communication. Traditional antennas with fixed direction are effective but unable to respond to station mobility or a dynamic environment including such factors as wind and foliage growth. There is a growing body of work on using steerable and sectored antenna systems to harness the efficiency of directional antennas while retaining the flexibility of ad-hoc networks; however, there has been very little work on implementation and measurement of such networks. We examined the physical-layer properties of directional links in two real RF environments, and have evaluated higher-layer strategies for utilizing these antennas. Our results indicate the topology formation process must be a network operation, and that simple link-by-link topology optimization is likely to lead to poor overall performance. These observations drive the formation of the testing and evaluation tools we have developed. This paper describes the tools, methodology and metrics we are using in the evaluation of topology formation algorithms using a dynamically steerable phase array system.

A Rotational Sector-Based, Receiver-Oriented Mechanism for Location Tracking and Medium Access Control in Ad Hoc Networks Using Directional Antenna

The use of directional antenna in wireless ad hoc networks potentially increases simultaneous communication by directing the transmitting and receiving beams towards the receiver and transmitter node as compared to omni-directional antenna, where nodes in the vicinity of a communication are kept silent. However, in order to implement effective directional MAC protocol using directional antenna, a node should know how to set its transmission direction to transmit a packet to its neighbors and to avoid transmission in other directions where data communications are already in progress. So, it becomes imperative to have a mechanism at each node to track the locations of its neighbors and to know the communication status of neighboring nodes. In this paper, we propose a receiver-centric approach for location tracking and MAC protocol. The performance evaluation on QualNet network simulator indicates that our protocol is highly efficient with increasing number of communications and increasing data rate.

Practical implementation of blind synchronization in NC-OFDM based cognitive radio networks

Spectrum Pooling has lead the cognitive radio research community to a new frontier where it is imperative to examine the architectures of wireless network protocols as well as the underlying hardware. Challenges at the physical layer (PHY) have increased the complexity of the algorithms used in processing a signal with multifarious properties. Synchronization of Non-Contiguous Orthogonal Frequency Division Multiplexing (NC-OFDM) is one such example where a clean slate algorithm and implementation is of utmost need. In this paper, we present a novel algorithm to perform NC-OFDM synchronization in the presence of an incumbent and also provide an outline for a FPGA based implementation of the proposed synchronizer. Extensive simulations under varying signal-to-noise ratio (SNR) and bandwidth reveal significant improvement over existing algorithms employed for NC-OFDM synchronization in cognitive radios.

PHY Aided MAC - A New Paradigm

Network protocols have traditionally been designed using a layered method in part because it is easier to implement some portions of network protocols in software and other portions must be implemented in hardware for performance reasons. These different implementation techniques enforce layer boundaries. In this paper, we show that with the advent of software defined radios, it becomes possible to blur those layer boundaries and produce higher performance network protocols as a result. In this paper we exploit a programmable physical layer and simultaneous transmission to have clients signal whether they have packets to send. By detecting the high energy at the simultaneous transmission, the AP gets the following information: a) which stations have packets to send and b) whether the traffic load is high, medium or low. Again, using the programmable physical layer, the AP schedules clients efficiently while wasting little of the spectrum on signaling overhead. The proposed protocol is a) fast, since no packet transmission is required for polling responses and all clients respond concurrently; b) reliable, as the poll response is contention free and c) scalable. We demonstrate the feasibility of implementing such a system using a FPGA based prototype software defined radio platform. We then show how the MAC protocol can scale using the QualNet network simulator and compare the performance to a contention based protocol.

An architecture for software defined cognitive radio

As we move forward towards the next generation of wireless protocols, the push for a better radio physical layer is ever increasing. Conventional radio architectures are limited to narrow operating regions and fails to adapt with changing technology. This is further strengthened with the advent of cognitive radio, which needs a more versatile and flexible framework that is programmable within the timing constraints of a protocol. In this paper we present an architecture for Software Defined Cognitive Radio that caters to the specific baseband processing requirements in a changing environment. We aim to provide more flexibility by de-constructing the radio pipeline into a framework of user controlled kernels that can be reconfigured at run-time. This architecture provides the barebones of a OFDM based radio physical layer that can adapt to perform a varied number of tasks in different radio networks. We also present a novel message based real-time reconfiguration method to transmit and receive a wide range of waveforms used in concurrent wireless protocols.