This paper is motivated by the need for fundamental understanding of ultimate limits of bandwidth efficient delivery of higher bit-rates in digital wireless communications and to also begin to look into how these limits might be approached. We examine exploitation of multi-element array (MEA) technology, that is processing the spatial dimension (not just the time dimension) to improve wireless capacities in certain applications. Specifically, we present some basic information theory results that promise great advantages of using MEAs in wireless LANs and building to building wireless communication links. We explore the important case when the channel characteristic is not available at the transmitter but the receiver knows (tracks) the characteristic which is subject to Rayleigh fading. Fixing the overall transmitted power, we express the capacity offered by MEA technology and we see how the capacity scales with increasing SNR for a large but practical number, n, of antenna elements at both transmitter and receiver.

We investigate the case of independent Rayleigh faded paths between antenna elements and find that with high probability extraordinary capacity is available. Compared to the baseline n = 1 case, which by Shannon‘s classical formula scales as one more bit/cycle for every 3 dB of signal-to-noise ratio (SNR) increase, remarkably with MEAs, the scaling is almost like n more bits/cycle for each 3 dB increase in SNR. To illustrate how great this capacity is, even for small n, take the cases n = 2, 4 and 16 at an average received SNR of 21 dB. For over 99% of the channels the capacity is about 7, 19 and 88 bits/cycle respectively, while if n = 1 there is only about 1.2 bit/cycle at the 99% level. For say a symbol rate equal to the channel bandwith, since it is the bits/symbol/dimension that is relevant for signal constellations, these higher capacities are not unreasonable. The 19 bits/cycle for n = 4 amounts to 4.75 bits/symbol/dimension while 88 bits/cycle for n = 16 amounts to 5.5 bits/symbol/dimension. Standard approaches such as selection and optimum combining are seen to be deficient when compared to what will ultimately be possible. New codecs need to be invented to realize a hefty portion of the great capacity promised.

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On Limits of Wireless Communications in a Fading Environment when UsingMultiple Antennas

The general technique of parallel transmission on many carriers, called multicarrier modulation (MCM), is explained. The performance that can be achieved on an undistorted channel and algorithms for achieving that performance are discussed. Ways of dealing with channel impairments and of improving the performance through coding are described, and implementation methods are considered. Duplex operation of MCM and the possible use of this on the general switched telephone network are examined. >

Multicarrier modulation for data transmission: an idea whose time has come

The use of infrared radiation as a medium for high-speed short-range wireless digital communication is discussed. Available infrared links and local-area networks are described. Advantages and drawbacks of the infrared medium are compared to those of radio and microwave media. The physical characteristics of infrared channels using intensity modulation with direct detection (IM/DD) are presented including path losses and multipath responses. Natural and artificial ambient infrared noise sources are characterized. Strategies for designs of transmitter and receivers that maximize link signal-to-noise ratio (SNR) are described. Several modification formats are discussed in detail, including on-off keying (OOK) pulse-position modulation (PPM), and subcarrier modulation. The performance of these techniques in the presence of multipath distortion is quantified. Techniques for multiplexing the transmissions of different users are reviewed. The performance of an experimental 50-Mb/s on-off-keyed diffuse infrared link is described.

Wireless Infrared Communications

Conventional equalization and carrier recovery algorithms for minimizing mean-square error in digital communication systems generally require an initial training period during which a known data sequence is transmitted and properly synchronized at the receiver. This paper solves the general problem of adaptive channel equalization without resorting to a known training sequence or to conditions of limited distortion. The criterion for equalizer adaptation is the minimization of a new class of nonconvex cost functions which are shown to characterize intersymbol interference independently of carrier phase and of the data symbol constellation used in the transmission system. Equalizer convergence does not require carrier recovery, so that carrier phase tracking can be carried out at the equalizer output in a decision-directed mode. The convergence properties of the self-recovering algorithms are analyzed mathematically and confirmed by computer simulation.

Self-Recovering Equalization and Carrier Tracking in Two-Dimensional Data Communication Systems

This paper discusses the analysis and simulation of a technique for combating the effects of multipath propagation and cochannel interference on a narrow-band digital mobile channel. This system uses the discrete Fourier transform to orthogonally frequency multiplex many narrow subchannels, each signaling at a very low rate, into one high-rate channel. When this technique is used with pilot-based correction, the effects of flat Rayleigh fading can be reduced significantly. An improvement in signal-to-interference ratio of 6 dB can be obtained over the bursty Rayleigh channel. In addition, with each subchannel signaling at a low rate, this technique can provide added protection against delay spread. To enhance the behavior of the technique in a heavily frequency-selective environment, interpolated pilots are used. A frequency offset reference scheme is employed for the pilots to improve protection against cochannel interference.

Analysis and Simulation of a Digital Mobile Channel Using Orthogonal Frequency Division Multiplexing

Une architecture de memorisation et de distribution qui memorise et distribue des programmes d'informations a des souscripteurs comprend: des depots d'informations qui archivent des programmes d'informations et fournissent par rafales des segments de programmes d'informations a des centraux; des centraux qui gerent les demandes de services faites par les souscripteurs et les segments de memoire intermediaire de programmes d'informations, pour les fournir aux souscripteurs en temps reel sous le controle interactif du souscripteur; et un equipement base chez les clients. Les centraux ont des memoires intermediaires centrales et chaque memoire intermediaire centrale comporte: des processeurs (402, 405) pour administrer les operations internes des memoires intermediaires et pour traiter les demandes des souscripteurs basees sur des macroinstructions de presentation de services et un plan de presentation de programmes; des interfaces (43, 46) permettant un acces externe; des bus (411, 412) pour le transport interne; des memoires intermediaires (403, 404) pour enregistrer des segments de programmes d'informations; et une memoire de stockage (407, 413) pour conserver la macroinstruction et le plan.

Architecture de communication et memoire intermediaire permettant la distribution de services informatiques

In Chapter 4, we examined signal design for baseband pulse transmission, and described the compromises among the bandwidth of the transmitted signal, noise immunity, and mitigation of intersymbol interference inherent in any design. Peak and mean-square intersymbol interference were defined, and it was shown how linear channel distortion can degrade performance. This chapter describes system structures that are optimum, either with respect to maximizing the probability that a sequence of symbols is correctly received or minimizing the output mean-square error; these receivers are primarily designed to correct the degradation caused by noisy channels and linear distortion. It is not possible, except in certain singular cases, to achieve the performance of an impairment-free system, or that of a system which attains the matched filter bound (which is equivalent to the transmission of isolated pulses). A further limitation discussed in Chapter 2 was the assumption of pulse-by-pulse (i.e. symbol-by-symbol) detection, which is optimum only in the absence of intersymbol interference. In the case of partial response signaling, we described precoding operations that eliminated the intentional intersymbol interference, but with a penalty in noise immunity. This penalty can be avoided if pulse-by-pulse detection is replaced by a process (the maximum likelihood receiver) that uses the entire received sequence for detection. This technique, the Viterbi algorithm, was applied to the decoding of convolutional codes in Chapter 3 and to the decoding of trellis codes in Chapter 5. In this chapter, the same technique will be applied to the detection of a sequence of amplitude-modulated pulses.

Stephen B. Weinstein

Papers

Architecture de communication et memoire intermediaire permettant la distribution de services informatiques

Optimum Data Transmission

Forthcoming Special Issues and Mini-Special Issues of IEEE Journal on Selected Areas in Communications for 1984- 1985

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