GB2093301A

GB2093301A - Improvements in or Relating to Mobile Radio Receivers

Info

Publication number: GB2093301A
Application number: GB8204461A
Authority: GB
Original assignee: British Telecommunications PLC
Current assignee: British Telecommunications PLC
Priority date: 1981-02-17
Filing date: 1982-02-16
Publication date: 1982-08-25
Also published as: GB2093301B

Abstract

Fading of a received signal in a mobile radio receiver is corrected by apparatus which includes mixers 21, 22 (Fig. 1) for mixing the received pilot signal with a locally generated signal to produce orthogonal signals I, Q. The signals I, Q are processed by a circuit 28 according to a given relationship to produce signals A, B. The signals A, B are used to modulate the received signal in mixers 15, 16. The outputs of the mixers 15, 16 are combined to produce a signal corrected for fading both in amplitude and frequency. In a further embodiment (Fig. 3) the signals A and B are coupled via modulators 30, 31 to the mixers 15 and 16. The modulators 30 and 31 are modulated by the locally generated pilot signals from 24. <IMAGE>

Description

SPECIFICATION Improvements in Or Relating to Mobile Radio Receivers This invention relates to mobile radio receivers. In particular the invention relates to the correction of fading in receivers of mobile radio systems.

Rapid fading of the received signal is a serious problem for mobile radio systems which employ any form of amplitude modulation. The fading results from the simultaneous reception of a number of replicas of the transmitted signal, the replicas having different amplitudes and phases and reaching the receiver from different directions. As the receiver (or transmitter) moves, the resultant received signal varies in amplitude and frequency. The amplitude fluctuations usually follow a Rayleigh distribution whilst the frequency fluctuations are bounded by the Doppler frequency which is equal to the ratio of the speed of the motion to the signal wavelength.

Slow fluctuations in the amplitude and frequency of the received signal can be corrected by feedback automatic gain control (AGC) and automatic frequency control (AFC), but feedback systems are limited in their speed of response by stability problems. In order to cope with rapid fading a feed forward correction system is required. An example of such a system is described in International Patent Application No. WO 81/00495.

The present invention provides an improved feed forward correction technique in which amplitude and frequency correction can be carried out simultaneously.

According to one aspect of the present invention there is provided apparatus for correcting fading in a radio receiver of a mobile radio system arranged to operate with radio signals of the type which comprise a single sideband with a pilot signal, said apparatus comprising an input for the received signal, a first path from said input for the received signal, said first path including first and second sections having first and second mixing means, a second path from said input for the pilot signal, said second path including third and fourth mixing means each arranged to receive the pilot signal, said third mixing means being arranged to combine the pilot signal with a locally generated signal whose frequency is approximately equal to that of the pilot signal to produce a first difference signal, said fourth mixing means being arranged to combine the pilot signal with the locally generated signal phase shifted by approximately 900 to produce a second difference signal, means for combining the first and second difference signal according to a predetermined relationship to produce third and fourth signals, means for coupling the third signal to said first mixing means, means for coupling the fourth signal to said second mixing means, and means for summing the outputs of the first and second mixing means.

The first path may include a delay element to provide a delay corresponding to the delay introduced by the second path.

The first path may include a 900 phase splitter, one output from which is connected to the first mixing means, and the other output from which is connected to the second mixing means.

The third and fourth mixing means may each be connected to the combining means by a low pass filter.

The apparatus may include an oscillator for generating said locally generated signal. The output of the oscillator may be connected to a 900 phase splitter one output from which is connected to the third mixing means and the other output from which is connected to the fourth mixing means.

In an alternative arrangement the coupling means includes fifth and sixth mixing means the fifth mixing means being arranged to combine the third signal with one of the locally generated signals and the sixth mixing means being arranged to combine the fourth signal with the other of the locally generated signals. In this case the first and second mixing means receive in phase components of the received signal.

According to another aspect of the present invention there is provided a method of correcting fading in a radio receiver of a mobile radio system arranged to operate with radio signals of the type which comprise a single sideband with a pilot signal, said method comprising mixing the pilot signal with a locally generated signal whose frequency is approximately equal to that of the pilot signal and with the locally generated signal phase shifted by approximately 900 to produce first and second difference signals, combining the first and second difference signals according to a predetermined relationship to produce third and fourth signals, mixing said third and fourth signals with first and second components respectively of said received signal to produce fifth and sixth signals, and summing the fifth and sixth signals. The summed signal is the received signal corrected for fading.

The invention will be described now by way of example only with particular reference to the accompanying drawings. In the drawings: Figure 1 is a block schematic diagram of apparatus in accordance with the present invention; Figure 2 is a diagram showing signal spectra; Figure 3 is a block schematic diagram of another form of apparatus in accordance with the present invention, and Figure 4 is a spectra diagram illustrating the operation of the present invention.

Referring to Figure 1 apparatus for correcting fading in a mobile radio system comprises an input 10 for the received signal. A first path 11 extends from the input 10 and comprises a delay circuit 12 which is connected to a wideband 900 splitter 14. The splitter 14 has two outputs one of which is connected to a first mixer 1 5 and the other of which is connected to a second mixer 1 6. One output from the splitter 14 is the received signal and the other is the received signal phase shifted by 900. The outputs of the mixers 1 5 and 16 are connected to a summing circuit 1 8.

A second path 20 extends from the input 10 and includes two synchronous demodulators 21, 22 connected in parallel. Each demodulator 21, 22 is arranged to receive as one input the pilot of the received signal. The demodulator 21 receives as a second input the output of an oscillator 23 by way of a 900 phase splitter 24 and the demodulator 22 is arranged to receive the output of the oscillator phase shifted by 900.

The output from each demodulator 21, 22 is connected by a respective low pass filter 26, 27 to a circuit 28. The circuit 28 is designed to process the outputs from the demodulators 21, 22 according to a predetermined expression to provide outputs A and B which are applied to the mixers 5 and 16 respectively.

The circuit of Figure 1 is arranged to correct fading in a receiver of a mobile radio system which uses a single sideband signal with an accompanying pilot. The pilot may be situated either above, below or within the speech sideband as illustrated in the three spectra of Figure 2. The circuit resolves the fading-induced modulation of the signal as two orthogonal signals I, Q in Figure 1. These signals are processed to produce signals A and B which are then used to modulate the received signal to remove the fading induced modulation on that signal.

The operation of the circuit will now be explained in detail. Consider the transmitted signal of the system to consist of an unmodulated pilot of angular frequency w, and a speech sideband represented by a series of discrete angular frequencies WI ... WN.

Thus the transmitted signal is:

The effect of fading may be represented by complex modulation of this signal producing both amplitude and phase modulation. The received fading signal S1 at the input 10 is given by:

where M(t) and çS(t) are the amplitude and phase modulation terms representing the effects of fading.

The oscillator 23 is designed to operate at the nominal pilot frequency and its output is split into two quadrature components cos wot and sin o,t which drive the synchronous demodulators 21, 22 respectively. The resulting signals 1, Rafter passage through the filters 26, 27 are: M(t) cos cos s(t) (3) 2 M(t) Q= sin X(t) (4) 2 All higher frequency products from the demodulators 21,22 are removed by the filters 26, 27.

The circuit element 28 processes the signals I, Q to produce two new signals A, B as follows:-

The signals A and B are applied to the mixers 1 5 and 16 respectively.

A sample of the received signal S1 is delayed by the delay element 12 and split into two mutually quadrature components before being applied to the two multipliers 1 5, 16. The magnitude of the delay introduced by delay element 12 is equal to the delay introduced by the filters 26, 27 and the processing in the circuit 28. The resulting outputs C, D of the multipliers 1 5, 1 6 are:

The signals C, D are summed by the summing circuit 1 8 to produce the output S2 which is given by:

Comparing S2 with the transmitted signal of equation (1) it can be seen that the effects of fading have been removed.

The system described above can be applied to any of the single sideband signals represented by the spectra shown in Figure 1 provided that the minimum frequency spacing between the pilot and the nearest frequency component of the speech spectrum is greater than the sum of the maximum Doppler frequency and the cut-off frequency of the filters 26, 27. This is to ensure that the signals I, 0 consist only of the difference frequency components resulting from the mixing of the pilot frequency term in equation (2) with the locally generated pilot frequency from oscillator 23.

There are other ways in which the present apparatus may be arranged. By way of example oife arrangement will now be described. This is shown in Figure 3, and is intended to operate on a single sideband signal with a pilot at the carrier frequency, as shown in Figure 4a. The received signal is converted to an intermediate frequency and amplified in accordance with normal receiver practice, and is finally demodulated by an oscillator which is offset from the carrier frequency by 5 kHz to produce the spectrum shown in Figure 4b. The pilot is at 5 kHz and the speech spectrum is inverted.

The operation of the circuit of Figure 3 up to the production of the signals A, B is as described previously with reference to Figure 1. The signals A, B are then applied to modulators 30, 31 and modulated by the locally generated 5 kHz pilot signal from oscillator 23 to produce the new signals X, Y respectively, which are applied to the multipliers 1 5, 1 6. This arrangement avoids the need for the wideband 900 splitter 14 shown in Figure 1. The summed output at S2 contains the desired speech spectrum and an inverted speech spectrum as shown in Figure 4c; also present are harmonics resulting from the use of a square wave oscillator 23. These harmonics and the inverted speech spectrum are removed by a band pass filter 33 to yield the speech spectrum shown in Figure 4d as the output signal S3.

Some or all of the functions shown in Figure 3 may be implemented by digital signal processing accompanied by appropriate analogue to digital and digital to analogue conversion. By way of example, all the functions shown within the dotted boundary in Figure 3 have been implemented on an Intel 2920 Signal Processor.

The circuit element 28 of Figure 3 also generates signals G and F representing the magnitude and frequency of the pilot signal. These signals are used for automatic gain control (AGC) and automatic frequency control (AFC) respectively. The signals G and F are derived from the signals I, Q, A and B according to the following expressions: G=l2+Q2 (10)

where In, ln+1 and On, Qn+1'represent successive samples of the signals I and 0 respectively.

Apparatus in accordance with the present invention is particularly suitable for operation at frequencies above 100 MHz especially frequencies in the range 100 MHz to 1 HGz.

Claims

1. Apparatus for correcting fading in a radio receiver of a mobile radio system arranged to operate with radio signals of the type which comprise a single sideband with a pilot signal, said apparatus comprising an input for the received signal, a first path from said input for the received signal, said first path including first and second sections having first and second mixing means, a second path from said input for the pilot signal, said second path including third and fourth mixing means each arranged to receive the pilot signal, said third mixing means being arranged to combine the pilot signal with a locally generated signal whose frequency is approximately equal to that of the pilot signal to produce a first difference signal, said fourth mixing means being arranged to combine the pilot signal with the locally generated signal phase shifted by approximately 900 to produce a second difference signal, means for combining the first and second difference signals according to a predetermined relationship to produce third and fourth signals, means for coupling the third signal to said first mixing means, means for coupling the fourth signal to said second mixing means, and means for summing the outputs of the first and second mixing means.

2. Apparatus as claimed in Claim 1 wherein the first path includes a delay element to provide a delay corresponding to the delay introduced by the second path.

3. Apparatus as claimed in Claim 1 or Claim 2 wherein the first path includes a 900 phase splitter, one output from which is connected to the first mixing means, and the other output from which is connected to the second mixing means.

4. Apparatus as claimed in any preceding claim wherein the third and fourth mixing means are each connected to the combining means by a low pass filter.

5. Apparatus as claimed in any preceding claim including an oscillator for generating said locally generated signal.

6. Apparatus as claimed in Claim 5 wherein the output of the oscillator is connected to a 900 phase splitter one output from which is connected to the third mixing means and the other output from which is connected to the fourth mixing means.

7. Apparatus as claimed in Claim 1 or Claim 2 wherein the coupling means includes fifth and sixth mixing means the fifth mixing means being arranged to combine the third signal with one of the locally generated signals and the sixth mixing means being arranged to combine the fourth signal with the other of the locally generated signals.

8. Apparatus as claimed in any preceding claim wherein the radio signals have frequencies in the range 100 MHz to 1 GHz.

9. A method of correcting fading in a radio receiver of a mobile radio system arranged to operate with radio signals of the type which comprise a single sideband with a pilot signal, said method comprising mixing the pilot signal with a locally generated signal whose frequency is approximately equal to that of the pilot signal and with the locally generated signal phase shifted by approximately 900 to produce first and second difference signals, combining the first and second difference signals according to a predetermined relationship to produce third and fourth signals, mixing said third and fourth signals with first and second components respectively of said received signal to produce fifth and sixth signals, and summing the fifth and sixth signals.

10. A method as claimed in Claim 9 wherein the radio signals have frequencies in the range 100 MHzto 1 GHz.

11. Apparatus for correcting fading in a radio receiver substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

1 2. A method of correcting fading in a radio receiver substantially as hereinbefore described.