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Wireless Multiple Access Adaptive Communications Techniques

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1Wireless Multiple Access Adaptive Communications TechniquesGregory J. PottieElectrical Engineering DepartmentUniversity of California, Los AngelesAbstract: The extreme variability of wireless channels demands that adaptivity be the one con-stant feature of high-performance systems, whether for dedicated channels, cellular systems, ormulti-hop networks. We outline the fundamentals of the physical channel, and means of achievingreliable communications over highly variable links. Methods for managing multiple access inter-ference in cellular systems are described, as well as the additional challenges posed by multime-dia traffic and non-standard network topologies. At each step, further adaptive capability isdemanded of the system, and care must be taken to ensure that the adaptive algorithms interact ina complementary and stable fashion. Using such techniques, very large performance improve-ments over static designs may be obtained.I. IntroductionThe variability of wireless channels presents both challenges and opportunities in designingmultiple access communications systems. To maximize throughput for a given power budget, thelink must adapt to the actual channel conditions, changing the transmitter power level, antennabeam pattern, equalizer settings, and possibly the symbol rate and constellation size. On the otherhand, the attenuation and directionality of signals makes possible re-use of the time/frequencyresources in space, permitting a large number of users to access the shared medium. These usersare coupled by the mutual interference they cause one another. For an acceptable quality of ser-vice, each user will typically need a signal to interference ratio (SIR) above some target. The tech-niques which enhance reliability of a link also affect the interference seen by other users, as domultiple access techniques. Thus, it is counterproductive to separate consideration of the physicaland medium access layers in designing a wireless transmission system.A fundamental limit on the capacity of a multiple access wireless system is imposed by theinability of the various users to perfectly estimate and predict the time-varying channel and inter-ference. In the limit of large complexity and power levels, all other impairments can in principlebe overcome if the channel dynamics (including interference) are slow enough. In this article wewill describe practical techniques for ameliorating the impairments in point to point communica-tions (and their limitations), and then discuss the extension of these techniques to the multipleaccess setting. Throughout we pay particular attention to how the channel and interference couple2the adaptation of the different techniques, in both desired and undesired fashions.The remainder of the article is organized as follows. In section II we discuss radio links, pro-viding an introduction to radio propagation, and then focussing on means for dealing with multi-path interference. In section III we discuss multiple access techniques for cellular systems,assuming connection-oriented traffic, including TDMA, FDMA, and CDMA, along with meansto enhance the capacity of traditional approaches. These include dynamic resource management,smart antennas, and multi-user detection, as well as the interaction among the various methods. Insection IV we discuss alternative access scenarios, including network topologies such as peer-to-peer and multi-hop networks, and the impact of multimedia traffic. In section V we provide ourconclusions and suggestions for further research.II. Radio LinksWireless channels are distinguished from wireline channels by their large variability andinherent multiple access nature. Here we focus on the causes of the variability and the means fordealing with it, deferring discussion of multiple access until section III.II.1 Fundamentals of Radio Propagation ModelingRadio propagation can be understood at many levels: Maxwell’s equations, Huygen’s princi-ple and ray-tracing, and statistical models at various levels of abstraction. Which method to usedepends on the task at hand. Solution of Maxwell’s equations is mandated in design of high per-formance antennas, and to fully characterize the interaction of antenna array elements with eachother and their immediate environment [1,2]. However, the computations required to characterizepropagation within even a small structure are immense, and to be accurate must include precisemeasurements of the electromagnetic properties of the composite materials in the structure. Theeffort involved is comparable to directly taking measurements of the point to point link. A morefruitful approach in designing wireless links is to create a simplified model whose statistics matchwhat is found in some combination of measurements and sophisticated mathematical modeling.For these purposes, a model which starts from the theory typically found in first year physics text-books is reasonable [e.g. 3].Huygen’s principle for wave propagation states that we may regard each point on a wavefrontas the source for a new circular wavefront. The waves add by superposition. This principleaccounts for the circular expansion (and thus square law free space attenuation) of a wavefront3from a point source, the approximation of the wavefront as a plane wave at large distance from thesource, and the phenomenon of diffraction. The theory of optics abstracts plane waves using thenotion of ray tracing. In this point of view (which neglects diffraction), rays are emitted omnidi-rectionally by a point source. Reflection and refraction angles are computed through considerationof the differences in the index of refraction. Adding the attenuation of waves in various media tothe theory of ray tracing provides a reasonable first cut at understanding how electromagneticradiation intensity levels will vary. Multiple rays arriving at the same destination are added bysuperposition, with the phases of the rays determined by the lengths of the propagation paths andthe phase changes induced at reflective boundaries. This is the level of abstraction at which prop-agation models suitable for radio engineering are usually presented, since it captures most of theobserved dynamics in a model that is easy to visualize. It is also the basis for ray tracing and raylaunching simulation models. Moreover, these simple considerations provide guidance in design-ing large scale propagation studies to gather data which may later be abstracted into


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