Randeep Bhatia

Joint channel assignment and routing for throughput optimization in multi-radio wireless mesh networks

Multihop infrastructure wireless mesh networks offer increased reliability, coverage, and reduced equipment costs over their single-hop counterpart, wireless local area networks. Equipping wireless routers with multiple radios further improves the capacity by transmitting over multiple radios simultaneously using orthogonal channels. Efficient channel assignment and routing is essential for throughput optimization of mesh clients. Efficient channel assignment schemes can greatly relieve the interference effect of close-by transmissions; effective routing schemes can alleviate potential congestion on any gateways to the Internet, thereby improving per-client throughput. Unlike previous heuristic approaches, we mathematically formulate the joint channel assignment and routing problem, taking into account the interference constraints, the number of channels in the network, and the number of radios available at each mesh router. We then use this formulation to develop a solution for our problem that optimizes the overall network throughput subject to fairness constraints on allocation of scarce wireless capacity among mobile clients. We show that the performance of our algorithms is within a constant factor of that of any optimal algorithm for the joint channel assignment and routing problem. Our evaluation demonstrates that our algorithm can effectively exploit the increased number of channels and radios, and it performs much better than the theoretical worst case bounds

On power efficient communication over multi-hop wireless networks: joint routing, scheduling and power control

With increasing interest in energy constrained multi-hop wireless networks (Bambos, N. et al., 1991), a fundamental problem is one of determining energy efficient communication strategies over these multi-hop networks. The simplest problem is one where a given source node wants to communicate with a given destination, with a given rate over a multi-hop wireless network, using minimum power. Here the power refers to the total amount of power consumed over the entire network in order to achieve this rate between the source and the destination. There are three decisions that have to be made (jointly) in order to minimize the power requirement. (1) The path(s) that the data has to take between the source and the destination. (Routing). (2) The power with each link transmission is done. (Power Control). (3) Depending on the interference or the MAC characteristics, the time slots in which specific link transmissions have to take place. (Scheduling). (4) To the best of our knowledge, ours is the first attempt to derive a performance guaranteed polynomial time approximation algorithm for jointly solving these three problems. We formulate the overall problem as an optimization problem with non-linear objective function and non-linear constraints. We then derive a polynomial time 3-approximation algorithm to solve this problem. We also present a simple version of the algorithm, with the same performance bound, which involves solving only shortest path problems and which is quite efficient in practice. Our approach readily extends to the case where there are multiple source-destination pairs that have to communicate simultaneously over the multi-hop network.

Minimizing Service and Operation Costs of Periodic Scheduling

We study the problem of scheduling activities of several types under the constraint that, at most, a fixed number of activities can be scheduled in any single time slot. Any given activity type is associated with a service cost and an operating cost that increases linearly with the number of time slots since the last service of this type. The problem is to find an optimal schedule that minimizes the long-run average cost per time slot. Applications of such a model are the scheduling of maintenance service to machines, multi-item replenishment of stock, and minimizing the mean response time in Broadcast Disks. Broadcast Disks recently gained a lot of attention because they were used to model backbone communications in wireless systems, Teletext systems, and Web caching in satellite systems. The first contribution of this paper is the definition of a general model that combines into one several important previous models. We prove that an optimal cyclic schedule for the general problem exists, and we establish the NP-hardness of the problem. Next, we formulate a nonlinear program that relaxes the optimal schedule and serves as a lower bound on the cost of an optimal schedule. We present an efficient algorithm for finding a near-optimal solution to the nonlinear program. We use this solution to obtain several approximation algorithms. 1 A 9/8 approximation for a variant of the problem that models the Broadcast Disks application. The algorithm uses some properties of “Fibonacci sequences.” Using this sequence, we present a 1.57-approximation algorithm for the general problem. 2 A simple randomized algorithm and a simple deterministic greedy algorithm for the problem. We prove that both achieve approximation factor of 2. To the best of our knowledge this is the first worst-case analysis of a widely used greedy heuristic for this problem.

Throughput Optimization of Wireless Mesh Networks with MIMO Links

Multiple input multiple output (MIMO) antennas use sophisticated physical layer techniques to provide significant benefits over conventional antenna technology. Multiple independent data streams can be sent over the MIMO antenna elements. MIMO link can also suppress interference from neighboring links as long as the total useful streams and interfering streams are no greater than the number of receiving antenna elements. For these reasons MIMO antennas are increasingly being considered for use in interference limited wireless mesh networks and have been adopted by WLAN and WIMAX standards. However, the benefits of the MIMO technology in improving network performance are limited unless the higher layer protocols also exploit these capabilities. In this paper we are interested in characterizing the benefits of cross-layer optimizations in interference limited wireless mesh networks with MIMO links. We formulate a framework where data routing at the protocol layer, link scheduling at the MAC layer and stream control at the physical layer can be jointly optimized for throughput maximization in the presence of interference. We then develop an efficient algorithm to solve the resulting throughput optimization problem subject to fairness constraints.

ICAM: integrated cellular and ad hoc multicast

In third generation (3G) wireless data networks, multicast throughput decreases with the increase in multicast group size, since a conservative strategy for the base station is to use the lowest data rate of all the receivers so that the receiver with the worst downlink channel condition can decode the transmission correctly. This paper proposes ICAM, integrated cellular and ad hoc multicast, to increase 3G multicast throughput through opportunistic use of ad hoc relays. In ICAM, a 3G base station delivers packets to proxy mobile devices with better 3G channel quality. The proxy then forwards the packets to the receivers through an IEEE 802.11-based ad hoc network. In this paper, we first propose a localized greedy algorithm that discovers for each multicast receiver the proxy with the highest 3G downlink channel rate. We discover that due to capacity limitations and interference of the ad hoc relay network, maximizing the 3G downlink data rate of each multicast receivers proxy does not lead to maximum throughput for the multicast group. We then show that the optimal ICAM problem is NP-hard, and derive a polynomial-time 4-approximation algorithm for the construction of the multicast forest. This bound holds when the underlying wireless MAC supports broadcast or unicast, single rate or multiple rates (4(1 + isin) approximation scheme for the latter), and even when there are multiple simultaneous multicast sessions. Through both analysis and simulations, we show that our algorithms achieve throughput gains up to 840 percent for 3G downlink multicast with modest overhead on the 3G uplink

Declustering using golden ratio sequences

We propose a new data declustering scheme for range queries. Our scheme is based on Golden Ratio Sequences (GRS), which have found applications in broadcast disks, hashing, packet routing, etc. We show by analysis and simulation that GRS is nearly the best possible scheme for 2-dimensional range queries. Specifically, it is the best possible scheme when the number of disks (M) is at most 22; has response time at most one more than that of the best possible scheme for M/spl les/94; and has response time at most three more than that of the best possible scheme for M/spl les/550. We also show that it outperforms the cyclic declustering scheme-a recently proposed scheme that was shown to have better performance than previously known schemes for this problem. We give some analytical results to suggest that the average performance of our scheme is within 14 percent of the optimal scheme. Our analytical results also suggest a worst case response time within a factor 3 of the optimal for any query, and within a factor 1.5 of the optimal for large queries. We also give a multidimensional extension of our scheme, which has better performance than the multidimensional generalization of the cyclic declustering scheme.

On Local Search and Placement of Meters in Networks

This work is motivated by the problem of placing pressure-meters in fluid networks. The problem is formally defined in graph-theoretic terms as follows. Given a graph, find a cotree (complement of a tree) incident upon the minimum number of vertices. We show that this problem is NP-hard and MAX SNP-hard. We design an algorithm with an approximation factor of

Hierarchical Declustering Schemes for Range Queries

2 + \epsilon

Asymptotically optimal declustering schemes for 2-dim range queries

for this problem for any fixed

Optimized network traffic engineering using segment routing

\epsilon >0