Hamid R. Sadjadpour

Challenges: towards truly scalable ad hoc networks

The protocols used in ad hoc networks today are based on the assumption that the best way to approach multiple access interference (MAI) is to avoid it. Unfortunately, as the seminal work by Gupta and Kumar has shown, this approach does not scale. Recently, Ahlswede, Ning, Li, and Yeung showed that network coding (NC) can attain the max-flow min-cut throughput for multicast applications in directed graphs with point-to-point links. Motivated by this result, many researchers have attempted to make ad hoc networks scale using NC. However, the work by Liu, Goeckel, and Towsley has shown that NC does not increase the order capacity of wireless ad hoc networks for multi-pair unicast applications. We demonstrate that protocol architectures that exploit multi-packet reception (MPR) do increase the order capacity of random wireless ad hoc networks by a factor Θ(log n) under the protocol model. We also show that MPR provides a better capacity improvement for ad hoc networks than NC when the network experiences a single-source multicast and multi-pair unicasts. Based on these results, we introduce design problems for channel access and routing based on MPR, such that nodes communicate with one another on a many-to-many basis, rather than one-to-one as it is done today, in order to make ad hoc networks truly scalable.

Interleaver design for turbo codes

The performance of a turbo code with short block length depends critically on the interleaver design. There are two major criteria in the design of an interleaver: the distance spectrum of the code and the correlation between the information input data and the soft output of each decoder corresponding to its parity bits. This paper describes a new interleaver design for turbo codes with short block length based on these two criteria. A deterministic interleaver suitable for turbo codes is also described. Simulation results compare the new interleaver design to different existing interleavers.

A Unifying Perspective on the Capacity of Wireless Ad Hoc Networks

We present the first unified modeling framework for the computation of the throughput capacity of random wireless ad hoc networks in which information is disseminated by means of unicast routing, multicast routing, broadcasting, or different forms of anycasting. We introduce (n,m, k)-casting as a generalization of all forms of one-to-one, one-to-many and many-to-many information dissemination in wireless networks. In this context, n, m, and k denote the total number of nodes in the network, the number of destinations for each communication group, and the actual number of communication-group members that receive information (i.e., k lesm), respectively. We compute upper and lower bounds for the (n, m, k)- cast throughput capacity in random wireless networks. When m = k = ominus(1), the resulting capacity equals the well-known capacity result for multi-pair unicasting by Gupta and Kumar. We demonstrate that ominus(1/radic(mnlogn)) bits per second constitutes a tight bound for the capacity of multicasting (i.e., m = k < n) when m les ominus (n/(log n)). We show that the multicast capacity of a wireless network equals its capacity for multi-pair unicasting when the number of destinations per multicast source is not a function of n. We also show that the multicast capacity of a random wireless ad hoc network is ominus (1/n), which is the broadcast capacity of the network, when m ges ominus(n/ log n). Furthermore, we show that ominus (radicm/(kradic(n log n))),ominus(1/(k log n)) and ominus(1/n) bits per second constitutes a tight bound for the throughput capacity of multicasting (i.e., k < m < n) when ominus(1) les m les ominus (n/ log n), k les ominus(n / log n) les m les n and ominus (n/ log n) les k les m les n respectively.

Construction of OFDM M-QAM sequences with low peak-to-average power ratio

We present a technique to derive M-quadrature amplitude modulation (QAM) signals from quaternary phase-shift keying (QPSK) constellations when M=2/sup n/ and n is an even number. By utilizing QPSK Golay sequences, we have constructed M-QAM sequences with low peak-to-mean envelope power ratios. Several upper bounds for these M-QAM sequences were derived.

Amplify-forward and decode-forward: the impact of location and capacity contour

Successful message relay, or the quality of the inter-user channel, is critical to fully realize the cooperative benefits promised by the theory. This in turn points out the importance of the relative location of the users. This paper investigates the impact of the location on the system capacity and outage probability for both amplify-forward (AF) and decode-forward (DF) schemes. Signal attenuation is modeled using power laws and capacity is evaluated using the max-flow min-cut theory. The resemblance and difference between cooperative systems and multi-input multi-output (MIMO) systems are also discussed. Finally a capacity contour for DF, the more popular mode of the two, is provided to facilitate the derivation of engineering rules

A unified analysis of routing protocols in MANETs

This paper presents a mathematical framework for the evaluation of the performance of proactive and reactive routing protocols in mobile ad hoc networks (MANETs). This unified framework provides a parametric view of protocol performance, which in turn provides a deeper insight into protocol operations and reveals the compounding and interacting effects of protocol logic and network parameters. The parametric model comes from a combinatorial model, where the routing logic is synthesized along with the characterization of MAC performance. Each wireless node is seen independently as a two-customer queue without priority, where the two types of customers are unicast and broadcast packets. The model captures the essential behavior and scalability limits in network size of both classes of routing protocols, and provides valuable guidance on the performance of reactive or proactive routing protocols under various network configurations and mobility conditions. The analytical results obtained with the proposed model are in close agreement with simulation results obtained from discrete-event Qualnet simulations.

Routing Overhead as A Function of Node Mobility: Modeling Framework and Implications on Proactive Routing

rdquoThe paper presents a mathematical framework for quantifying the overhead of proactive routing protocols in mobile ad hoc networks (MANETs). We focus on situations where the nodes are randomly moving around but the wireless transmissions can be decoded reliablely, when nodes are within communication range of each other. We explicitly present a framework to model the overhead as a function of stability of topology and analytically characterize the statistical distribution of topology evolutions. The OLSR protocol is further singled out for a detailed analysis, incorporating the proposed analytical model. Results are compared against Qualnet simulations for random movements, which corroborate the essential characteristics of the analytical results. The key insight that can be drawn from the analytical results of this paper is that nodal movements will drive up the overhead by a penalty factor, which is a function of the overall stability of the network.

Throughput-delay analysis of mobile ad-hoc networks with a multi-copy relaying strategy

Multiuser diversity has been shown to increase the throughput of mobile ad-hoc wireless networks (MANET) when compared to fixed wireless networks. This paper addresses a multiuser diversity strategy that permits one of multiple one-time relays to deliver a packet to its destination. We show that the /spl theta/(1) throughput of the original single one-time relay strategy is preserved by our multi-copy technique. The reason behind achieving the same asymptotic throughput is the fact that, as we demonstrate in this paper, interference for communicating among closest neighbors is hounded for different channel path losses, even when n goes to infinity. We find that the average delay and its variance scale like /spl theta/(n) and /spl theta/(n/sup 2/), respectively, for both the one and multi-copy relay strategies. Furthermore, while for finite n the delay values in the single-copy relaying strategy are not bounded, our multi-copy relay scheme attains bounded delay.

Many-to-Many Communication: A New Approach for Collaboration in MANETs

We introduce a collaboration-driven approach to the sharing of the available bandwidth in wireless ad hoc networks, which we call many-to-many cooperation, that allows concurrent many-to-many communication. This scheme is based on the integration of multi-user detection and position-location information with frequency and code division in mobile ad hoc networks (MANETs). Transmissions are divided in frequency and codes according to nodal locations, and successive interference cancellation (SIC) is used at receivers to allow them to decode and use all transmissions from strong interfering sources. Consequently, the interference is divided into constructive interference (COI) and destructive interference (DEI). We show that, if each node is allowed to expand its bandwidth, both the links Shannon capacity and the per source-destination throughput scale like O(nalpha/2 ) (upper-bound) and Omega[f(n)] (lower-bound), for n nodes in the network, a path loss parameter alpha > 2, and 1 les f(n) < nalpha/2. Many-to-many cooperation allows multi-copy relaying of the same packet, which reduces the packet delivery delay compared to single-copy relaying without any penalty in capacity.

The capacity and energy efficiency of wireless ad hoc networks with multi-packet reception

We address the cost incurred in increasing the transport capacity of wireless ad hoc networks over what can be attained when sources and destinations communicate over multi-hop paths and nodes can transmit or receive at most one packet at a time. We define the energy efficiency ·(n) as the bit-meters of information transferred in the network for each unit energy. We compute the energy efficiency of many different techniques aimed at increasing the capacity of wireless networks and show that, in order to achieve higher transport capacity, a lower energy efficiency must be attained. Using the physical model, we compute the throughput capacity of random wireless ad hoc networks in which nodes are endowed with multi-packet reception (MPR) capabilities. We show that λ(n)= Θ (<i>R</i>(<i>n</i>))^(1-2/α) / <i>n</i>^1/α) bits per second constitutes a tight upper and lower bound for the throughput of random wireless ad hoc networks, where α>2 is the path loss parameter in the physical model, <i>n</i> is the total number of nodes in the network, and <i>R</i>(<i>n</i>) is the MPR receiver range. In doing so, we close the gap between the lower and upper bounds for the throughput capacity of wireless networks in the physical model. Compared to the original result derived for plain routing by Gupta and Kumar, MPR achieves a capacity gain of at least Θ((log <i>n</i>)α-2/2α) when R<i>R</i>(<i>n</i>)= Θ(√log <i>n</i>/<i>n</i>).