Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Patent
1992-10-29
1996-01-30
Chin, Stephen
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
375281, 375308, 375348, 370 21, H04K 110, H04L 2728
Patent
active
054886325
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the transmission and reception of information and particularly to the transmission and reception of information in digital form at frequencies which are hostile from the point of view of interference from other signals. More particularly, the invention is described in relation to transmitting in or close to the frequency bands of existing ultra-high frequency (UHF) TV signals.
2. Related Art
There exist channels in the UHF TV Spectrum which are not used as part of the frequency planning rules--these are known as the "taboo" channels. To understand these taboo channels it is necessary to have a brief understanding of the way in which the UHF TV band is planned.
The following description is given in the context of the frequency plan adopted in the United Kingdom. It will be understood by a person skilled in the art that, for similar reasons to those discussed below, taboo channels exist in the frequency plans of other countries and that the techniques described below for avoiding interference in a new low power service in a taboo channel may be applied in those countries also (with suitable alterations taking into account the different respective channel bandwidths/channel spacings and sub-carrier frequencies in those countries).
The frequency plan in the United Kingdom consists of 51 main transmitter stations covering some 90% of the population using horizontal polarisation. There then exists 950 small low power relay stations filling the main coverage gaps these use vertical polarisation. Each main transmitter station has a certain coverage area and needs perhaps 20 relay stations for gap filling.
The relay stations in a given coverage area of a single main station have restrictions on the frequencies to which they may be assigned because of the frequency planning taboos. Some of the taboos came about as the result of limited technology when the original UHF plan was designed back in 1961.
In the United Kingdom television channels are assigned 8 MHz segments of the frequency spectrum. If it is desired to broadcast television signals in channel number N, then a first pair of taboo channels (adjacent) arise at channel numbers N.+-.1 because, with receiver technology as it was in 1961, the receiver filters accepting channel N could not reject frequencies used by channels N.+-.1. Two other pairs of taboo channels (local oscillator and image channel) also arise at channels numbers N.+-.5 and N.+-.9 respectively because of the heterodyning process used to demodulate received television signals. If a first receiver were to receive a broadcast signal at one of channels numbers N.+-.5 then during the demodulation process frequencies would be generated at the receiver which would propagate and could interfere with operation of a nearby receiver attempting to demodulate a broadcast signal at channel number N.
There is now interest in exploiting these taboo channels in a way which does no cause interference to the existing television service. The present invention may be utilized for this purpose. More generally, the invention may be applied to enable the transmission of relatively low power signals in frequency bands subject to interference from other transmissions.
Transmissions in the taboo channels may take place without causing interference to existing relay stations in surrounding coverage areas which use the same frequencies providing very low power transmissions are used in the taboo channels. This criterion can be met by using digital modulation which enables transmitter power to be very much reduced without significantly reducing the coverage area. Typically, a digital signal may be transmitted using the methods of the present invention with 30 dB less power for approximately the same coverage as analogue amplitude modulation (AM). However, when sharing the UHF band at such low levels of transmitted power the digital signal is very vulnerable to interference from the much higher power levels of the existing services.
A proposal
REFERENCES:
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EBU Review, No. 224, Aug. 1987, (Brussels, BE), M. Alard et al.: "Principles of modulation and channel coding for digital broadcasting for mobile receivers", pp. 168-190, see sections 1, 2.1, 2.2; FIGS. 1, 2.
Wescon Conference Record, vol. 24, Anaheim, Calif., US, 16-18 Sep. 1980, U. A. von der Embse: "High dynamic range FFT processors for multiple channel receivers" pp. 5/4 1-13, see abstract; introduction; p. 5/4.3 left-hand col., lines 29-53; FIGS. 2, 3, 12-14.
IEEE Transactions on Consumer Electronics, vol. 35, No. 3, Aug. 1989, (New York, US), B. Le Floch et al.: "Digital sound broadcasting to mobile receiver", pp. 493-503, see sections 1, 4, 7.
IEEE Global Telecommunications Conference & Exhibition Conference Record, Dallas, Tex., US, 27-30 Nov. 1989, IEEE (New York, US), J. C. Rault et al.: "The coded orthogonal frequency division multiplexing (COFDM) Technique, and its application to digital radio broadcasting towards mobile receivers", pp. 428-432.
Gledhill Jeffrey J.
Mason Arthur G.
Chin Stephen
National Transcommunications Limited
Vo Don
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