GB2088172A

GB2088172A - Improvements in or Relating to Circuit Arrangements for the Modulation of Signals Transmitted by Fixed Stations of Single Frequency Mobile Radio Systems

Info

Publication number: GB2088172A
Application number: GB8135095A
Authority: GB
Original assignee: Italtel SpA; Italtel Societa Italiana Telecomunicazioni SpA
Current assignee: Italtel SpA
Priority date: 1980-11-20
Filing date: 1981-11-20
Publication date: 1982-06-03
Also published as: IT8026106A0; ES507119A0; IT1134372B; ES8308456A1; DK512881A; FR2494527A1; DE3146037A1

Abstract

The fixed part of a single frequency mobile radio system is made up of a plurality of stations that transmit the same useful signal operating on the same nominal frequency. The signal transmitted by each station is modulated in frequency or phase by a pseudo-random or gaussian noise signal from a noise generator GR with a frequency limited to less than the frequency band occupied by the useful signal. The modulating noise signal can be superimposed upon the useful signal in the basic band or else can modulate the carrier applied to the modulator M. <IMAGE>

Description

SPECIFICATION Improvements in or Relating to Circuit Arrangements for the Modulation of Signals Transmitted by Fixed Stations of Single Frequency Mobile Radio Systems The present invention relates to a circuit arrangement which may be particularly simple and which permits a reduction in the frequency stability required from the transmitters of the fixed stations of a mobile radio system of single frequency operation.

In mobile radio systems used solely for signalling (paging), for signalling and conversation, and more generally, wherever there is a service area covered by several appropriately distributed transmitters operating on the same radio channel, the problem arises of guaranteeing that, in those areas where two or more transmitters are received with almost the same field strength, the difference between the carrier frequencies that are received be contained within precise limits.

The lower limit, which is a few tens of hertz, is such as to limit the duration of the RF signal "cancellation" by interference to a fraction of the signalling period used.

The higher limit, that is a few hundred hertz, is such as to prevent the beats and distortions caused by the carriers from falling within the useful signal band.

One solution of the abovementioned problem consists in assigning to the interfering transmitters slightly different nominal frequencies so as to keep the beats within the aforesaid limits.

However this solution has the practical inconvenience of requiring good frequency stability of the various transmitters. For example, in order to restrict beats between 20 Hz and 500 Hz, the maximum permissible frequency error of the transmitters would have to be of about 8C Hz, which corresponds to a relative stability of +5x 10-7 in the 150 MHz range.

A new modulation scheme has recently been proposed in which all the stations have the same frequency and stability of the order of the maximum difference allowed: in an n frequency system, the signals transmitted by the individual stations are modulated in frequency by sinusoidal signals of equal amplitude and with respective phase differences which are multiples of 2rr n In the overlapping zones, the fields of two fixed stations can be isofrequential only for a very short time, so that, even if the two fields are in opposite phase and annul or cancel each other, the fraction of useful signal lost is so limited that it does not interfere with the intelligibility of the information (sound or signal) received.

However, for the correct operation of such a modulation scheme, it is necessary that the phase ratio between the sinusoidal modulating signals be maintained constant or, in other words, that the oscillators present in all the fixed stations be maintained in synchronism.

This result may be obtained either by using oscillators of good stability in all the fixed stations, or by distributing a synchronisation signal or a sinusoidal signal, from which the modulating signal may be obtained locally by means of phase adjusters, to all the stations.

Both these solutions turn out to be very costly.

The synchronisation or sinusoidal signal requires a separate distribution network, which is distinct from that used to send the useful signal. In fact, so as to allow easy separation of the useful signal from the modulating sinusoidal signal in reception, the frequency of the latter is maintained much lower than the band occupied by the useful signal and cannot pass through the network that carries the useful signal.

According to the invention, there is provided a circuit arrangement for the modulation of a signal transmitted by each fixed station of a mobile radio system whose fixed stations operate on the same nominal frequency, comprising a pseudo-random or random noise generator, the output signal of which is arranged to frequency- or phasemodulate the signal radiated by the fixed station.

Such a circuit arrangement does not require a special network to carry a synchronisation signal to the fixed stations.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which Figures 1 to 3 are block diagrams of three preferred embodiments of the invention. In the drawings, like reference numbers refer to like parts.

As shown in Figure 1, a useful signal s is supplied to low frequency circuits BF, the output of which is added to a pseudo-random (or gaussian) noise signal, generated by a generator GR and limited in frequency by a filter F to its components having a much lower frequency than those of the signal s. The sum signal is supplied as a modulating signal to the modulator M, whose output is frequency-multiplied or frequencyconverted to the transmission frequency in the block C/M and radiated by an aerial A.

Many circuit arrangements are already known in the technical field that make it possible to realize the pseudo-random (or gaussian) noise generator GR and the filter F. These may be grouped into two main categories.

The first is based upon the use of digital techniques and includes the presence of a shift register with n cells having appropriate feedback.

The register output signal is filtered by means of a digital filter F made up, in one frequently used embodiment, of a network of weighed resistances connected to the n cells of the register. The second category of gaussian noise generators is based upon analogue type techniques and essentially comprises a very noisy component (for example a Zener diode) followed by a high-gain amplifier and by an analog low-pass filter F.

In the embodiment of Figure 2, the pseudorandom (or gaussian) noise is supplied to the modulation input of a local oscillator OL, so as to frequency- or phase-modulate the carrier supplied to the modulator M.

According to a modification illustrated in Figure 3 and valid only for conversion transmitters, the pseudo-random noise modulates (in frequency or phase) the local oscillator OL associated with a converter C.

Claims

1. A circuit arrangement for the modulation of a signal transmitted by each fixed station of a mobile radio system whose fixed stations operate on the same nominal frequency, comprising a pseudo-random or random noise generator, the output signal of which is arranged to frequencyor phase- modulate the signal radiated by the fixed station.

2. A circuit arrangement as claimed in claim 1, in which the pseudo-random or random noise signal and a useful signal are supplied to a mixer whose output signal is supplied to the input of a modulator.

3. A circuit arrangement as claimed in claim 1, in which the pseudo-random noise signal is supplied to the modulation input of a local oscillator.

4. A circuit arrangement as claimed in claim 3, for a conversion transmitter, in which the pseudorandom noise signal is supplied to the modulation input of the local oscillator which is associated a converter of the transmitter.

5. A circuit arrangement as claimed in any one of the preceding claims, in which the output of the noise generator is connected to a low-pass filter, with a cut-off frequency lower than the frequency band occupied by a useful signal to be transmitted.

6. A circuit arrangement substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

7. A fixed radio station including a circuit arrangement as claimed in any one of the preceding claims.

8. A mobile radio system including a fixed radio station as claimed in claim 7.