METHOD OF, AND APPARATUS FOR, THE TRANSMISSION OF A PLURALITY OF SIGNALS

Abstract: The present invention relates to a method of, and apparatus for, the transmission of a plurality of signals. It relates particularly, but not exclusively, to the simultaneous transmission and distribution of Coded Frequency Division Multiplexing (COFDM) signals in a Cellular Access to Broadband Services and Interactive Telvision (CABSINET) microcell. The method, and apparatus, for the transmission of a plurality of signals, include the steps of a) receiving a plurality of input signals; b) attenuating the said signals so as to provide suitable signals for enabling other components to operate in a preferred operating region; c) band-pass filtering the said signals; d) translating the frequency of the said signals to provide modified signals in a predetermined bandwith; e) attenuating the modified signals; f) band-pass filtering the modified signals; g) amplifying the modified signals; h) band-pass filtering the modified signals; and i) transmitting the modified signals to a plurality of receivers, so that each reciever, when suitable tuned, receives a separate transmitted signal. An advantage of the invention, over existing systems, is that the expensive and disruptive process of installation of cables is avoided.

Description: METHOD OF, AND APPARATUS FOR, THE TRANSMISSION OF A PLURALITY OF SIGNALS The present invention relates to a method of, and apparatus for, the transmission of a plurality of signals. It relates particularly, but not exclusively, to the simultaneous transmission and distribution of Coded Frequency Division Multiplexing (COFDM) signals in a Cellular Access to Broadband Services and Interactive Television (CABSLNET) microcell. Transmission and distribution of digital video, for example to set-top boxes, in private homes (or elsewhere) is increasing due to schemes such as video on demand. In the past, such video signals have been transmitted via cable. However, technology is now available to transmit video signals via RF means to a number of end users. This development offers the advantage of not having to lay cables to individual private homes, which is labour intensive and causes environmental damage. Digital video signals are now able to be broadcast, via a base-station, (which transmits signals) to a number of micro-cells. These micro-cells may include a radio-repeater and a number of set-top boxes adapted to receive the transmitted digital video signals. UK Patent GB 2 318 947 describes an intelligent digital beamformer and phased antenna array for installation in a satellite. The system provides multiple active beams within a footprint region for communications with a user on the earth surface. There is no reference to use the beamformer in a system which is capable of simultaneous transmission and distribution of COFDM signals. US Patent 5 428 600 describes a system which simultaneously extracts, in the frequency domain, a narrow band BFSK carrier (received from Transmitter A) and de-spreads or correlates a wide-band CDMA signal (received from Transmitter Bi). The two signals are combined, using a phase modulator, to produce a frequency spectrum of a wide-band CDMA signal and BFSK carrier signal. The BFSK carrier is then used by receiving station B to assist with data recovery and demodulation. An aim of the system descπbed in the aforementioned US Patent is to extract earner and data extraction. An aim of the present invention is to provide a method of, and apparatus for, the transmission and distribution of coded orthogonal frequency division multiplexed signals in order to transmit digital video signals to a plurality of end users According to a first aspect of the invention there is provided a method for the transmission of a plurality of signals, the method including the steps of: a) receiving a plurality of input signals; b) attenuating the signals so as to provide suitable signals for enabling other components to operate in a preferred operating region; c) band-pass filtering the said signals, d) translating the frequency of the said signals to provide modified signals in a predetermined bandwith, e) attenuating the modified signals; f) band-pass filtering the modified signals; g) amplifying the modified signals; h) band-pass filtering the modified signals; and I) transmitting the modified signals to a plurality of receivers, so that each receiver, when suitably tuned, receives a separate transmitted signal. Preferably input signals are COFDM signals. Most preferably the signals are in the microwave region of the electromagnetic spectrum. Step g) of the method may include amplification of the modified signals, followed by attenuation of the modified signals, and further amplification of the modified signals The method may also include the steps of providing static discharge protection and antenna port mismatch protection. One way of achieving this is to provide a DC block at a suitable point in the circuit. The invention can be used to receive and transmit additional signals used for the synchronisation of, for example, data symbol rates and centre frequencies. These additional signals may be COFDM signals. According to a second aspect of the invention there is provided an apparatus for the transmission of a plurality of signals, the apparatus including- a) means for receiving a plurality of input signals; b) means for attenuating the said input signals so as to provide suitable signals for enabling other components to operate in a preferred operating legion; c) means for band-pass filtering the said signals; d) means for translating the frequency of the said signals to provide modified signals in a predetermined bandwidth; e) means for attenuating the modified signals; f) means for band-pass filtering the modified signals; g) means for amplifying the modified signals; and h) means for transmitting the modified signals to a plurality of receivers, so that each receiver, when suitably tuned, receives a separate transmitted signal. The apparatus may also include dc signal blocking means for protecting the internal RF circuitry from static or other discharges. Preferably the means for attenuating the input signals is an input switched intermediate frequency (i.e., generally higher in frequency than the data frequency, but lower in frequency than the carrier) attenuator, operating in the 0 dB to -70 dB range. Preferably the means for translating the frequency of the signals is a double balanced transmit mixer. Preferably antenna port mismatch protection means such as a circulator may also be included in the apparatus. The means for amplifying the signal may be an rf high power amplifier. Preferably the means for transmitting the COFDM signal is a dual polarisation antenna. First and second oscillator means may also be included in the apparatus. The first and second oscillator means may be a phase-locked oscillator and a high-stability oven controlled crystal oscillator, respectively. Preferably the band-pass filter means is a low insertion loss band-pass filter such as, for example, an inter-digital filter. A power supply means is provided in use and this may include: a linear power supply which may be used to provide a regulated power supply; and a switched mode power supply which may also be used to provide a regulated power supply. An embodiment of the invention will now be described, by way of example only, with reference to the accompanying Figures, in which:- Figure 1 shows a block diagram of a 5.8 GHz radio repeater; and Figure 2 shows a block diagram of a system incorporating the repeater. A 5.8 GHz radio repeater 2 having five stages is shown. It is capable of simultaneous transmission and distribution of fourteen Coded Orthogonal Frequency Division Multiplexed (COFDM) signals and one signalling channel in a Cellular Access to Broadband Services and Interactive Television (CABSLNET) microcell. The repeater may be modified to handle more or less signals. Refemng in detail to Figure 1, the 5.8 GHz radio repeater includes an input switched intermediate frequency (IF) attenuator 10 (0 dB to -70 dB range), an 815 MHz band-pass filter 14 (FL1), an up-converting mixer 16, a high stability Oven Controlled Crystal Oscillator 36 (OCXO), a 4970 MHz phase locked oscillator 34 (PLO) , a 4970 MHz bandpass filter 32 (FL2), 5785 MHz band-pass filters 20, 28 (FL3, FL4), a 5785 MHz pre- amplifier 22, a 5785 MHz high power amplifier 26 and a 5785 MHz circulator 30 Following LF attenuation and band-pass filtering with FL1, up-conversion of the 755 MHz to 875 MHz band to the 5725 MHz to 5845 MHz band is performed by the double balanced transmit mixer 16. The mixer output is filtered with FL3, a low insertion loss band-pass filter 20 such as, for example, an mter-digital filter. This filter 20 suppresses the local oscillator (LO) feedthrough from the mixer, and selects the upper sideband. The transmit buffer 22 pre-amphfies the selected sideband, easing the gain requirement for the 5785 MHz RF high power amplifier FL4, a low insertion loss band-pass filter such as, for example, an inter-digital filter, is used to further suppress both the transmit LO leakage and the undesired sideband. The RF high power amplifier 26 boosts the transmit signal to a level of approximately +20 dBm (dB signal gain with respect to 1 mW) per channel, as measured at an antenna output SMA connection port. To satisfy the heavy supply current requirement of the high power amplifier 26, a switched mode power supply 44 is used. Following FL4, a circulator 30 and DC block 38 are used to provide antenna port mismatch protection and static discharge protection, respectively. The 8 dBi (dB gain with respect to an isotropic radiator) dual polarised directive antenna 40 consists of a linear array of stacked co-planar micro-strip patch antennae. The antenna 40 achieves an azimuth coverage of about 90 degrees. The phase-locked oscillator (PLO) provides a tuneable ( ±2%) 4970 MHz LO signal from a + 15 V 400 mA DC supply. The PLO 4970 MHz output is derived from a high performance 82.833333 MHz Oven Controlled Crystal Oscillator (OCXO). This OCXO has a temperature stability of ±0.025ppm over -20 to +60°C, ageing ±lppm per year and pre-set frequency ±0.2ppm. The 4970 MHz PLO produces over +10 dBm of output drive power from a 0 dBm input drive power. Phase noise performance is better than -96 dBc/Hz at 1 KHz offset. Output spurious signals are better than -73dBc (dB signal gain with respect to the signal carrier) at 275KHz, and output harmonics are better than -70dBc. FL2, a low insertion loss bandpass filter such as, for example, an inter-digital filter, is used to suppress undesired LO output harmonics. Following band-pass filtering with FL2, the 4970 MHz LO signal is applied to the up-converting mixer LO input port. Two separate linear power supplies are used to provide regulated supplies to the OCXO and PLO. A switched mode power supply (SMPS) is used to provide a regulated supply to both the pre-amplifier and high power amplifier. All power supply units are 230 N ac 50/60 Hz mains driven and provide satisfactory ripple rejection and load regulation. Tables 1 to 3 give the electrical, environmental and mechanical specifications, respectively, of the 5.8 GHz radio repeater of Figure 1 for the transmission and distribution of 14 COFDM signals. Table l - Electrical Specifications Table 2 - Environmental Specifications Figure 2 is a diagrammatical, schematic view of a system comprising two users A and B and a repeater 2. In practice there may be many more users, but for simplicity only two are shown. User A (the so called Head End) can communicate to user 3 (the Consumer) via repeater 2 in order, for example, to play interactive television games or perform other two- way communications. Although the invention has been described by one embodiment only, it will be appreciated that variation may be made to the aforementioned embodiment without departing from the scope of the invention. For example, the transmission of 5.8 GHz signals to fourteen receivers only has been discussed. However, other transmission frequencies and numbers of receivers are possible by varying, respectively, the OCXO 36 and PLO 34 centre frequencies and the bandwidth of the band-pass filters 14, 20 and 28. For example, for 28 users, the bandwidth of the filters 14, 20 and 28 may be increased to 300 MHz. Similarly, although the invention has been described operating in a transmission mode, it may operate equally well in a receiving mode. Therefore the features of the invention may be incorporated into a receiver.

Claims: Claims 1. A method for the transmission of a plurality of signals, the method including the steps of: a) receiving a plurality of input signals; b) attenuating the said signals so as to provide suitable signals for enabling other components to operate in a preferred operating region; c) band-pass filtering the said signals; d) translating the frequency of the said signals to provide modified signals in a predetermined bandwith; e) attenuating the modified signals; f) band-pass filtering the modified signals; g) amplifying the modified signals; h) band-pass filtering the modified signals; and i) transmitting the modified signals to a plurality of receivers, so that each receiver, when suitably tuned, receives a separate transmitted signal. 2. A method according to claim 1 wherein the signals are coded orthogonal frequency division multiplexed signals. 3. A method according to claim 2 wherein the signals are in the microwave region of the electromagnetic spectrum. 4. A method according to any preceding claim wherein the step of static discharge protection is provided. 5. A method according to any preceding claim wherein the step of antenna port mismatch protection is provided. 6. Apparatus for the transmission of a plurality of signals, the apparatus including: a) means for receiving a plurality of input signals; b) means for attenuating the said input signals so as to provide suitable signals for enabling other components to operate in a preferred operating region; c) means for band-pass filtering the said signals; d) means for translating the frequency of the said signals to provide modified signals in a predetermined bandwidth; e) means for attenuating the modified signals; f) means for band-pass filtering the modified signals; g) means for amplifying the modified signals; and h) means for transmitting the modified signals to a plurality of receivers, so that each receiver, when suitably tuned, receives a separate transmitted signal. 7. Apparatus according to claim 6 further including dc signal blocking means for protecting the internal RF circuitry from static discharges. 8. Apparatus according to claim 6 wherein the means for attenuating the input signals is an input switched intermediate frequency attenuator. 9. Apparatus according to claim 8 wherein the input switched intermediate frequency attenuator operates in the 0 dB to -70 dB range. 10. Apparatus according to claim 6 wherein the means for translating the frequency of the signals is a double balanced transmit mixer. 11. Apparatus according to claim 6 further including antenna port mismatch protection means. 12. Apparatus according to claim 11 wherein the antenna port mismatch protection means is a circulator. 13. Apparatus according to claim 6 further including at least one oscillator means. 14. Apparatus according to claim 13 wherein the oscillator means is a phase-locked oscillator. 15. Apparatus according to claim 13 wherein the oscillator means is a high-stability oven controlled crystal oscillator. 16. Method substantially as herein described with reference to the Figures. 17. Apparatus substantially as herein described with reference to the Figures.

Inventors: HARKER BRETT JAMES, HARKER, BRETT, JAMES
Applicants: CENTRAL RESEARCH LAB LTD, HARKER BRETT JAMES, CENTRAL RESEARCH LABORATORIES LIMITED, HARKER, BRETT, JAMES
Application year: 2000
Main classification: H04B7
IPC codes: H04B7, H04L27