Pulse or digital communications – Receivers – Interference or noise reduction
Patent
1995-04-10
1998-07-21
Vo, Don N.
Pulse or digital communications
Receivers
Interference or noise reduction
375299, 455137, 455273, H04B 710, H04L 102
Patent
active
057844186
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to digital audio broadcast systems and in particular to systems which operate by transmitting signals from a network of transmitters.
BACKGROUND
A digital audio broadcasting system is currently being developed by a consortium of European manufacturers called Eureka 147. This has proposed a coded orthogonal frequency division multiplexed (COFDM) digital audio broadcast (DAB) modulation scheme. COFDM is a wide band modulation scheme which is specifically designed to cope with problems of multipath reception. The basic idea of COFDM is to take the digital signal (data) to be transmitted and divide it between a large number of adjacent carriers and divide it between time intervals so that only a small amount of data from a sample is carried at any one time. Error correction routines are used to correct for any errors in received signals. The carriers are arranged such that adjacent ones are mathematically orthogonal. This enables their side bands to overlap so that signals can be received without adjacent carrier interference and consequently a smaller bandwidth is necessary to transmit a plurality of carriers.
The signal is transmitted in bursts of data. Each burst is formed by distributing bits of the signal over the carriers (usually by quadrature phase shift keying) and then taking the Fast Fourier Transform (FFT) of the carriers. The resultant spectron is modulated by a further carrier, thus forming a broad band signal (DAB ENSEMBLE) for transmission. At a receiver the reverse process takes place.
In conventional AM/FM broadcasting a network of transmitters is provided. This is known as a "diversity" transmission system. Adjacent transmitters broadcast the same radio station on frequencies which have a sufficiently large gap between them for here to be no distortion in intermediate or "mush areas" between the transmitters. This is because there is no synchronisation of the transmission signals and therefore reception in such areas would be severely distorted if both transmitters were using the same frequency but were out of phase.
In a COFDM DAB modulation scheme each signal occupies a larger portion of the spectrum than a conventional AM/FM signal occupies because of its division onto a plurality of carriers which are used to form a "DAB ensemble". Because of this, even if signals on adjacent transmitters are spaced by the same amount as they would be in an AM/FM distribution network, there will still be interference in the intermediate or mush areas.
SUMMARY OF THE INVENTION
It has been appreciated that this problem can be overcome by synchronising the transmitters and arranging for them each to transmit identical program modulation and each to use the same frequency. Using such an arrangement, the signals from each transmitter will add constructively at a receiver, thereby enhancing rather than degrading reception. For such operation to be successful it is essential that all the transmitters radiate substantially identical carrier frequencies and that these carriers carry identical modulation (the DAB ensemble).
Therefore, the present invention features means for receiving a first digital broadcast signal and reference information in a first frequency band, means for receiving a second digital broadcast signal in a second frequency band, and means for frequency-converting the first digital broadcast signal into the second frequency band, as directed by the received reference information.
BRIEF FIGURE DESCRIPTION
The invention will now be described in detail by way of example with reference to the accompanying drawings in which:
FIG. 1 shows a transmission and reception system embodying the present invention;
FIG. 2 shows a source transmitter for transmitting a signal to a satellite in an embodiment of the invention;
FIG. 3 illustrates circuitry for use fin a terrestrial transmitter embodying the invention;
FIG. 4 shows an embodiment of the invention for use in a block of flats apartments to improve reception; and
FIG. 5 schematically illustrates f
REFERENCES:
patent: 4317220 (1982-02-01), Martin
patent: 4972410 (1990-11-01), Cohen et al.
patent: 5170413 (1992-12-01), Hess et al.
patent: 5345601 (1994-09-01), Takagi et al.
F. Muller-Romer "Digitale terrestrishce Sendernetze fur Horfunk und Fernsehen", Fernseh und Kino Technik, vol. 45, No. 11, 1991, pp. 575-583.
V.M. Kolesnikow et al. "Eine Neue Konzeption Fur Den Digitalen Terrestrischen Horrundfunk", vol. 34, No. 6, Nov. 1990, RTM Rundfunktechnische Mitteilungen, pp. 276-278.
A. Weisser "Implications of DAB on Sound Contribution and Distribution Circuits", 1st International Symposium on DAB -1992 Proceedings, Jun. 1992, pp. 127-136.
Laven Philip Andrew
Parnall Simon
Sykes John Prailsford
British Broadcasting Corporation
Neuner George W.
Vo Don N.
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