Code-division multiple access | EPFL Graph Search

Code-division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communication channel. This allows several users to share a band of frequencies (see bandwidth). To permit this without undue interference between the users, CDMA employs spread spectrum technology and a special coding scheme (where each transmitter is assigned a code). CDMA optimizes the use of available bandwidth as it transmits over the entire frequency range and does not limit the user's frequency range. It is used as the access method in many mobile phone standards. IS-95, also called "cdmaOne", and its 3G evolution CDMA2000, are often simply referred to as "CDMA", but UMTS, the 3G standard used by GSM carriers, also uses "wideband CDMA", or W-CDMA, as well as TD-CDMA and TD-SCDMA, as its radio technologies. Many carriers (such as AT&T and Verizon) sh

Schemes and architectures for wireless ad hoc networks and cooperative communication

Tarik Tabet

The focus of this thesis is on the study of decentralized wireless multi-hop networks. We are particularly interested in establishing bounds on the traffic-carrying capabilities of wireless ad hoc networks and conditions on the scalability of such networks with node mobility. This theoretical investigation brings forward challenges on the design of such networks. This leads to a second part of this thesis that considers the feasibility and the design of physical layer architectures and schemes for decentralized wireless multi-hop networks. In the first part of this thesis, bounds on the capacity of wireless ad hoc networks with two types of non-uniform traffic patterns are established. We focus on the impact of traffic patterns where local communications predominate and show the improvement in terms of per user-capacity over ad hoc networks with unbounded average communication distances. We then study the capacity of hybrid wireless networks, where long-distance relaying is performed by a fixed overlay network of base-stations. We investigate the scaling of capacity versus the number of nodes and the density of base-stations in the area of the network. It is shown that the gain in performance is mainly due to the reduction in the mean number of hops from source to destination. Then, we investigate the impact of mobility on the ad hoc network capacity. We propose a set of necessary and sufficient conditions under which the long-term averaged throughput in an ad hoc network can remain constant as the number of nodes increases. The main idea is to use a connectivity graph that does not represent the actual physical network, but rather the available communication resources. This graph also allows to translate the problem of maximizing the throughput in ad hoc networks to the multi-commodity flow problem and directly apply related results. In contrast to these macroscopic studies, in the second part we focus on a microscopic analysis of ad hoc wireless networks. We are interested in characterizing the performance of decentralized multiple-access and retransmission schemes for multi-hop wireless networks with the goal of drawing conclusions on cross-layer design. We investigate different transmission strategies in order to assess the tradeoff between spatial density of communications and the range of each transmission. We present tools for characterizing the spatial throughput as a function of topological parameters (e.g node population density) and system parameters (propagation, bandwith etc). The results of this work also show that coding and retransmissions provide means of reliable communication coupled with a completely decentralized multiple-access strategy. Finally, an efficient protocol for the delay-limited fading Automatic Retransmission reQuest (ARQ) single relay channel is considered for cooperative communications. The proposed protocol exploits two kinds of diversity: (i) space diversity available through the cooperative (relay) terminal, which retransmits the source's signals, (ii) ARQ diversity obtained by leveraging the retransmission delay to enhance the reliability. The performance characterization is in terms of the achievable diversity, multiplexing gain and delay tradeoff for a high signal-to-noise ratio (SNR) regime. Then, by letting the source's power level vary over the retransmission rounds, we show the benefits of power control on the diversity.

EPFL2007

Game Theory in Communications

Vojislav Gajic

Multi-user communication theory typically studies the fundamental limits of communication systems, and considers communication schemes that approach or even achieve these limits. The functioning of many such schemes assumes that users always cooperate, even when it is not in their own best interest. In practice, this assumption need not be fulfilled, as rational communication participants are often only interested in maximizing their own communication experience, and may behave in an undesirable manner from the system's point of view. Thus, communication systems may operate differently than intended if the behavior of individual participants is not taken into account. In this thesis, we study how users make decisions in wireless settings, by considering their preferences and how they interact with each other. We investigate whether the outcomes of their decisions are desirable, and, if not, what can be done to improve them. In particular, we focus on two related issues. The first is the decision-making of communication users in the absence of any central authority, which we consider in the context of the Gaussian multiple access channel. The second is the pricing of wireless resources, which we consider in the context of the competition of wireless service providers for users who are not contractually tied to any provider, but free to choose the one offering the best tradeoff of parameters. In the first part of the thesis, we model the interaction of self-interested users in a Gaussian multiple access channel using non-cooperative game theory. We demonstrate that the lack of infrastructure leads to an inefficient outcome for users who interact only once, specifically due to the lack of coordination between users. Using evolutionary game theory, we show that this inefficient outcome would also arise as a result of repeated interaction of many individuals over time. On the other hand, if the users correlate their decoding schedule with the outcome of some publicly observed (pseudo) random variable, the resulting outcome is efficient. This shows that sometimes it takes very little intervention on the part of the system planner to make sure that users choose a desirable operating point. In the second part of the thesis, we consider the competition of wireless service providers for users who are free to choose their service provider based on their channel parameters and the resource price. We model this situation as a two-stage game where the providers announce unit resource prices in the first stage and the users choose how much resource they want to purchase from each provider in the second stage. Under fairly general conditions, we show that the competitive interaction of users and providers results in socially optimal resource allocation. We also provide a decentralized primal-dual algorithm and prove its convergence to the socially optimal outcome.

EPFL2010

Game Theoretic Considerations for the Gaussian Multiple Access Channel

Vojislav Gajic, Bixio Rimoldi

We study the behavior of users in a classical Additive White Gaussian Noise Multiple Access Channel. We model users as rational entities whose only interest is to maximize their own communication rate, and we model their interaction as a noncooperative one-shot game. The Nash equilibria of the two-user game are found, and the relation between the pure-strategy and mixed-strategy Nash equilibria is discussed. As in most games, the absence of cooperation and coordination leads to inefficiencies. We then extend our setting using evolutionary game theory, which we use to model a large population of users playing the MAC game over time. A unique evolutionary stable strategy is found for this case, corresponding to the strategy achieving the Nash equilibrium in a simplified one-shot game. Finally, we investigate what happens to the distribution of strategies in a population when we assume that the number of offsprings of a user is equal to the payoff of this user in a one-shot game. We find that the system converges to a state in which the average strategy of the population is the evolutionary stable strategy.