Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Patent
1998-02-05
2000-02-29
Vo, Don N.
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
4552262, H04B 702, H04L 102
Patent
active
060318775
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates, in general, to communication systems and is particularly applicable to communication systems using an adaptive beamforming technique.
SUMMARY OF THE PRIOR ART
The use of adaptive antennas (AA) in communication systems (particularly frequency division multiplexed (FDM) systems, such as the pan-European digital cellular Global System for Mobile (GSM) communication and alternate code-division multiple access (CDMA) systems) is becoming increasingly attractive because such adaptive antennas offer general improvements in system performance, and especially handling (traffic) capacity. As will be appreciated, a high degree of beam accuracy is achieved in an adaptive antenna system by accurately varying the phase and amplitude (magnitude) components of a transmitted wave. More specifically, phases and magnitudes of a set of transmitted waves, emanating from an array of antenna elements of a transceiver, are varied by "weighting" individual elements in the array such that an antenna radiation pattern (of a base site, for example) is adapted (optimised) to match prevailing signal and interference environments of a related coverage area, such as a cell.
Adaptive transmit beamforming in duplex communication systems requires that beamforming coefficients (i.e. the "weighting" factors) are adjusted in response to previously received channel information, which received information may occur in either an up-link or down-link for the system. In fact, when specifically considering a GSM base station, beamforming coefficients for a traffic mode must be calculated (estimated) within a period of four time-slot durations (namely a time of 4.times.15/26 milliseconds (ms), nominally 2.3 ms), whereas the period for calculating beamforming coefficients at a mobile unit may, in fact, be of shorter duration. Unfortunately, when one considers the amount of processing required to calculate (estimate) these beamforming coefficients, this limited period of time places severe constraints on an achievable accuracy. Indeed, upon receipt of a signal, information contained within the signal (typically) must be sampled, stored and then demodulated (by synchronisation and equalisation processes). Additionally, transmit weights must be formed from the received signal and then applied to data for transmission prior to loading and modulation of this data.
Furthermore, the limited time available for processing is further eroded by the problems inherently associated with such beamforming mechanisms, which problems principally result from: (i) the beamforming coefficients (weights) being frequency dependent (bearing in mind that the up-link and down-link resources usually operate at different frequencies, such that a frequency transposition and a phase-error correction is required); and (ii) a time dependent fluctuation in channel environment caused by a relative movement between a mobile unit and a fixed base station. In the latter respect, the effects of a time variation may be mitigated to some extent by averaging several received slots weights, for example, but this form of time correction is rather coarse.
With respect to selection of beamforming coefficients in typical communication systems (and as will be understood), an optimum selection (corrected, of course, for differences between the up-link and down-link frequencies) is provided by the Wiener solution: received signal vector at n branches (i.e. n antenna elements); a vector of optimum weights for the n branches; desired signal vector, s, that is sent during a defined training sequence of a burst; E[x*x.sup.T ]; columns and vice versa; and
The beamforming coefficients necessarily calculated for a succeeding frame of information must be estimated from historic received signals because correlation matrices R.sub.xx and r.sub.xd are not available directly (inasmuch as one cannot know what these correlation matrices are until such time as a signal relating to these matrices has been received). In this respect, an estimation R.sub.xx. (de
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patent: 5796779 (1998-08-01), Nussbaum et al.
Creps Heather L.
Motorola Inc.
Phu Phuong
Vo Don N.
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