GB2282298A - A cell enhancer for simulcast radio transmission - Google Patents

Abstract

A cell enhancer for simulcast radio transmission comprises a receiver (11), a first intermediate frequency mixer (14), a second intermediate frequency mixer (16) and a transmitter (18) for retransmission of the signal at substantially the same frequency as that at which it is received. The first and second intermediate frequency mixers comprise first (20) and second (21) controllable oscillators and a control circuit (22 - 28) for maintaining a controlled frequency separation within said channel between the frequencies of signals generated by said oscillators so that areas of signal cancellation in cell overlap regions are rapidly moved backwards and forwards. <IMAGE>

Description

A CELL ENHANCER FOR SIMULCAST RADIO TRANSMISSION Field of the Invention This invention relates to a cell enhancer for simulcast radio transmission, that is to say apparatus comprising a receiver and a transmitter for retransmission of a signal received at the receiver in the same radio channel.

Background to the Invention Using present technology, the service area of an individual radio transmitter can be increased by simultaneous broadcasting using synchronized wire lines see for example GB2001230A and WO90/04889. In the process, a second transmitter location is constructed. The two transmitter stations must be equipped with highly stable oscillators, in order substantially to eliminate interference in the overlapping areas of the transmitters. In addition, the low-frequency signals supplied by wire must be compensated in phase and delay.

"Cell enhancers" are another possible method of increasing the size of service areas by simultaneous broadcasting. See for GB2259430A.

In the case of a cell enhancer the signal transmitted by a fixed broadcasting station is received, suitably filtered, amplified and retransmitted. Note that this does not involve any frequency conversion to an IF position. The transmitted signal is therefore an exact image of the received signal. It is obtained with amplification by a fixed amount. Of course, the insulation between the receiving and transmitting antennas must be considerably greater than the maximum amplification of the cell enhancer, to avoid self-excitation.

Practical tests shown that the insulation must be more than 15dB relative to the maximum amplification of the amplifier.

This technique is already used in order to adequately serve buildings, dead spots and the like. This inevitably results in cancellations, due to the rigidity of the frequency and phase of the connections in the overlapping regions of the fixed radio station and the cell enhancer, but these are tolerated.

Cancellation occurs in overlapping regions wherever the signal strengths are approximately equal, and the phases of the signals are rotated through 1800 owing to differences in the delay of the broadcast signals.

As a result of a very slight frequency shift of the simultaneous transmitters relative to one another (a few Hz), these cancellation areas can be very rapidly moved backwards and forwards inside the overlapping area, so that there is only slightly impaired reception in the receiver of the mobile or portable unit. In conventional simulcast radio networks, however, this slight frequency difference can be maintained only for relatively short periods (about 6 months) by continuous monitoring and adjustment of frequency, even when highly stable oscillators are used in the fixed radio stations.

There is a need for an improved cell enhancer.

Summarv of the Invention According to the present invention, a cell enhancer is provided for simulcast radio transmission, comprising a receiver for receiving a radio frequency signal within a given radio channel, a first intermediate frequency mixer, a second intermediate frequency mixer and a transmitter, for retransmission of the signal in substantially the same channel. The first and second intermediate frequency mixers comprise first and second controllable oscillators and a control circuit for maintaining a controlled frequency separation within said channel between the frequencies of signals generated by said oscillators.

The radio channel may havee a bandwidth of, say, 25KHz. Within this channel, a very much smaller separation, for example 5 - 30Hz, is maintained between the frequencies of the signals generated by the oscillators. Since the frequency of the transmitter oscillator is controllable with respect to the frequency of the receiver oscillator, for example they are both derived from the same reference oscillator, there is no danger that ageing and other factors will cause the transmit frequency to be exactly the same as the receive frequency.

Brief Descrintion of the Drawing The figure shows a block diagram of a cell enhancer in accordance with a preferred embodiment of the invention.

Detailed Descrintion of the Preferred Embodiment Referring to the drawing, a receive antenna 10 is shown connected to a receive band pass filter 11 and in turn connected to a preamplifier 12. The output of amplifier 12 is connected via a controllable attenuator 13 to a mixing network 14. The output of the mixing network is connected to a ceramic IF band pass filter 15, which is connected to a further mixing network 16, in turn connected to a transmit filter 17, a power amplifier 18 and a transmit antenna 19. Coupled to the mixing networks 14 and 16 are first and second voltage controlled oscillators 20 and 21 respectively.

Coupled to these oscillators are receive and transmit phase locked loops 22 and 23 respectively. Coupled to both phase locked loops is a reference oscillator 25. An input to receiver PLL 22 is provided for control of that PLL.

This input is also connected to a summer circuit 27, which is connected to the transmitter PLL 23. The summer circuit 27 has an adjustable input 28 connected to dual-in-line switches (not shown).

The operation of the circuit is as follows. Antenna 10 is a directional antenna. A signal transmitted by another base station (not shown) is received on this antenna, filtered in filter 11 and amplified in preamplifier 12. The signal emerging from amplifier 12 is a high frequency signal which is mixed to an intermediate frequency in mixing network 14. A suitable IF frequency is, for example, 10.7MHz. The resulting signal at this frequency can be filtered in a quartz filter 15. The filtered IF signal is supplied to the mixer 16 which mixes it with a transmitter VCO frequency to provide a new HF frequency. This is filtered by filter 17 and amplified by amplifier 18 before transmitting in directional antenna 19.

The IF injection signal from VCO 20 which is mixed in mixing network 14 and the IF injection signal provided by VCO 21 which is mixed in mixing network 16 are derived as follows.

Reference oscillator 25 supplies the same reference frequency to PLLs 22 and 23. Each of these PLLs has a frequency divider (not shown) which divides the reference oscillator frequency, compares its phase with the output of the respective VCO and controls the frequency of the respective VCO accordingly. The frequency dividers of the PLLs are controlled by i,lput 26 in the case of PLL 22 and by the output of summer 27 in the case of PLL 23.

Input 26 defines a given IF frequency for mixing network 14. Input 28 has the effect of adding between 0 and 30Hz to this selected frequency, so that the input to PLL 23 defines a VCO frequency between 0 and 30Hz above or below the frequency of VCO 20. A preferred frequency difference between the VCOs 20 and 21 is 5Hz, VCO 21 being 5Hz higher in frequency than VCO 20.

In this way, the output from mixing network 16 is 5Hz higher than the input to mixing network 14. The increased frequency signal is transmitted through antenna 19 (after amplification). There is 15dB separation between the antennas 19 and 10, so that the output of antenna 19 is not reamplified at the input.

With this arrangement, overlapping areas of the individual transmitters (antenna 19 and the other base station which is transmitting the signal to antenna 10) do not have constant cancellation nulls, but rather there are fades which move backwards and forwards in the overlapping area corresponding to the frequency shift.

To prevent self-excitation of the equipment, the decoupling between the transmitter output and the receiver input must be greater than the total amplification of the system (minimum 15dB). This is achieved by selection of directional antennas with a high forward and back ratio and by a corresponding layout of the antennas. The combined amplification of preamplifier 12, attenuator 13 and amplifier 18 and the other elements in the chain may have a total amplification of: antenna coupling minus 15dB.

The adjustable attenuation component 13 can be used to adjust the amplifier for various physical locations.

The arrangement has the advantage that it does not require highly stable oscillators. Therefore the arrangement is not subject to natural frequency changes due to temperature or ageing in the fixed radio station or in the cell enhancer. The frequency ratios remain constant under all conditions. No adjustment work is required throughout the entire lifetime of the equipment. There is no need for phase or delay components for compensation. The expense of maintenance and the overall cost of a simulcast system can be considerably reduced. A number of stations can be cascaded, e.g. to provide railway lines with simulcast coverage.

The return speech channel (mobile-to-fixed radio station) is obtained by conventional receiver selection and appropriate satellite receivers at the locations of the cell enhancers. This technique can be used both in simplex and duplex radio networks.

Claims (9)

Claims

1. A cell enhancer for simulcast radio transmission, comprising a receiver (10, 12) for receiving a radio frequency signal within a given radio channel, a first intermediate frequency mixer (14), a second intermediate frequency mixer (16) and a transmitter (18, 19), for retransmission of the signal in substantially the same channel, wherein the first and second intermediate frequency mixers comprise first (20) and second (21) controllable oscillators and a control circuit (22-28) for maintaining a controlled frequency separation within said channel between the frequencies of signals generated by said oscillators.

2. A cell enhancer according to claim 1, wherein the control circuit comprises first (22) and second (23) phase locked loops for controlling the first and second oscillators respectively and a common reference oscillator (25) for supplying a reference frequency to the phase locked loops.

3. A cell enhancer according to claim 2, wherein the control circuit further comprises a control input (26) for supplying a first reference control signal representative of said first intermediate frequency to said first phase locked loop (22) and a second reference control signal representative of said second intermediate frequency signal to said second phase locked loop (21) and means (27) for offsetting said first and second control signals relative to each other.

4. A cell enhancer according to claim 3 further comprising a settable input (28) for providing a settable offset between said first and second control signals.

5. A cell enhancer according to any one of the preceding claims, further comprising an intermediate frequency filter coupled between said first and second mixers.

6. A cell enhancer according to any one of the preceding claims wherein the controlled frequency separation is between 1 and 30 Hertz.

7. A cell enhancer according to any one of claims 1 to 6 wherein a first directional antenna is coupled to the receiver and a second directional antenna is coupled to the transmitter and a predetermined separation attenuation ratio is maintained between the antennas and wherein amplifier means (12, 18) are provided in at least one of the receiver and the transmitter wherein the amplifier means and associated elements (11, 13, 14, 15, 16, 17) coupling the first antenna to the amplifier means and coupling the amplifier means to the second antenna have a total amplification ratio no greater than the predetermined attenuation ratio.

8. A cell enhancer according to claim 7, further comprising an adjustable attenuator associated with the amplifier means for adjusting the total amplification ratio to match the antenna attenuation ratio.

9. A cell enhancer for simulcast radio transmission, comprising: a receiver for receiving a radio frequency signal within a given radio channel having a given nominal frequency and a given bandwidth, a first mixer coupled to the receiver comprising a first controllable oscillator for injecting into said radio frequency signal a first injection signal having a first injection frequency substantially below said nominal frequency to provide an intermediate frequency signal, a second mixer coupled to receive the intermediate frequency signal from the first mixer, comprising a second controllable oscillator for injecting into said intermediate frequency signal a second injection signal, where the second injection signal has a second injection signal frequency sufficiently similar to said first injection signal frequency to upconvert said intermediate frequency signal to approximately said nominal frequency, thereby providing a substantially reproduced radio frequency signal within the bandwidth of said channel, and wherein said second injection signal frequency is separated from said first injection signal frequency by a controlled separation, transmitter means for retransmitting said reproduced radio frequency signal and shielding means for shielding the transmitter means from the receiver means.