Pulse or digital communications – Repeaters – Testing
Patent
1995-02-16
1998-08-11
Tse, Young T.
Pulse or digital communications
Repeaters
Testing
375208, H04B 1707
Patent
active
057937966
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to apparatus for use in equipment providing a digital radio link using direct sequence spread spectrum between a fixed and mobile radio unit.
Equipment for providing such a radio link is described in Great Britain Patent Application Number 9304901.3. This application describes the use of Wiener-like filters for providing good estimates of the amplitude of the inphase I, the quadrature phase Q, components of, for example, a spread spectrum pilot signal.
Spread spectrum signals provide the possibility of obtaining excellent immunity), to multipath fading through resolving the individual, time separated, multipath components and optimally combining them. The common approach for achieving this is to use a `Rake` receiver as is familiar to those versed in the art. Such a receiver assigns de-spreading correlators to each of the dominant multipath components and synchronizes them for maximum de-spread energy. For each of the Rake `fingers` the phase and amplitude of the de-spread components is estimated and used to apply optimum amplitude weighting and phase alignment prior to addition (combining). The weighted sum of the multipath components will experience considerably less fading than any of the individual components so that a diversity gain is obtained.
One problem with the normal implementation of such a receiver is that the delay spread of the signal must be perpetually searched to determine whether a Rake finger should be assigned to a new, stronger, multipath component. If the channel is changing rapidly, significant loss in performance can result from there being too long a delay between a stronger multipath component appearing and it being assigned to a Rake finger.
Additionally some channels may consist of a large number of quite small multipath components. If all or most of these are not assigned to a Rake finger then a significant amount of signal energy may be wasted for the purposes of reception although it will still appear as interference to the reception of the spread spectrum transmission sharing the carrier frequency in a Code Division Multiple Access (CDMA) sense. The problem with these low level components is that, unless they are being measured continuously, it is not possible to assess with adequate accuracy, when and at what level they should be included in the combining output. The requirement to allow inclusion of a large number of multipath components could lead to the need for permanent provision of hardware for a large number of Rake fingers and a very fast searcher to assign the Rake fingers efficiently and rapidly.
Conventionally, every Rake finger would require a spread spectrum decorrelator, nominally timed aligned to the multipath component. Two identical spread spectrum decorrelators, assigned in time symmetrically either side of the first correlator would be required. The time offset of these correlators will be typically plus or minus one quarter or one eighth of a chip interval. The chip interval is the spread bit period, ie. bit period/spreading factor. The sampling of the signal at four or eight times the chip rate becomes essential. These correlators together with the first form of code lock loop in which the energy on the three correlator outputs are compared and the timing of the code generators for the correlators are adjusted so that the energy on the centre correlator is always maximised, as familiar to those versed in the art. A phase estimator (typically a phase lock loop) to obtain the carrier phase of the signal for the purposes of phase compensation prior to combining, and, an amplitude estimator to apply optimum weighting to the signals prior to combining would also be required. The major complexity in terms of operations is in the three spectrum decorrelators.
The searcher would typically consist of a number of sliding correlators arranged together to cover the overall delay spread of the signal, perhaps sliding their timing in steps of half chips. If optimal performance in assigning Rake fingers to ve
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Great Britian Patent Application No. 9304901.3.
Chandler David Peter
Hulbert Anthony Peter
Roke Manor Research Limited
Tse Young T.
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