Frequency synchronization of stations of a communication system

Abstract: A narrow band modulation UHF/Microwave communication system comprises a transmit/receive base station having a plurality of transmitter for transmitting information at respective UHF/Microwave channel frequencies to a plurality of remote transmit/receive stations, in which the transmitters at the base station are served in common by a single master oscillator and in which a single reference frequency signal is derived from the master oscillator at the base station and transmitted to all of said remote stations where it serves to provide for frequency locking of the oscillators at said stations to produce synchronisation between the frequencies at the base station and the remote stations.

Description: This invention relates to communication systems and relates more specifically to such systems utilising narrow band modulation techniques at UHF/Microwave frequencies. The present invention is directed to a more efficient use of the frequency spectrum embracing the frequencies utilised in the above systems. Conventional UHF radio systems universally use frequency modulation (FM) for speech transmission purposes and frequency or phase shift keying (FSK or PSK) for the transmission of other data. Frequency channel spacing in such systems at frequencies above 300MHz is usually at least 25KHz, the actual spacing being dictated by the stability requirements for the local receiver and transmitter oscillators. The system must remain operative within an acceptable tolerance of the nominal channel frequency for a range of different supply voltages, temperatures and over a predetermined time from the initial setting of the transmitter/receiver. Although some retuning of the transmitter/receiver has been acceptable in the past, nowadays it is highly desirable that the equipment remains on frequency for life (e.g. five years). Some exemplary figures illustrating the problem of frequency shift in UHF radio systems are as follows:- (a) If a total acceptable shift for frequency modulation (FM) or frequency shift keying (FSK) operation is 1KHz and the transmission frequency is 1GHz then the total frequency shift from all causes over a period of 5 years will be one part in one million. This represents the frequency shift limit achievable with good quality crystals for frequency control. When it is further realised that this very small frequency shift limit must also include the transmit/receive station offsets and spread between equipments the shift limit is clearly very stringent. Consequently, more bandwidth efficient systems such as very narrow band multi-channel FM (e.g. channel spacing of 12.5KHz or 6.25KHz) systems would likely be impractical and single sideband systems which require even better stability would definitely not be possible. There is no real prospect of significant improvement in frequency control crystal stability and although more expensive control crystals are more stable their stability is improved only by a very small factor. In United States Patent Specification No. 4317220 there is proposed a transmission system in which oscillators at one or more slave stations are synchronised from a pilot signal frequency included in the modulation signals on a carrier signal transmitted from a master station, while in German Patent Specification No. 2951288 it is proposed that all the transmitters of a transmission system are synchronised from a broadcast frequency standard signal. The present invention seeks to overcome all of the above-mentioned crystal stability problems at only small cost and the invention is applicable to narrow band frequency modulation and frequency shift keying systems as well as true single sideband systems occupying the minimum possible bandwidth. According to the present invention a narrow band modulation UHF/Microwave communication system comprising a transmit/receive base station having a plurality of transmitters for transmitting information at respective UHF/Microwave channel frequencies spaced over a band of frequencies to a plurality of remote transmit/receive stations, carrier signals for said transmitters at the base station being derived from a single master oscillator, is characterised in that a single reference carrier frequency signal at a frequency spaced from said band of frequencies is derived from said master oscillator and is transmitted unmodulated to all of said remote stations where it serves to provide for frequency locking of the oscillators at said stations to produce synchronisation between the frequencies at the base station and the remote stations. The oscillators at the remote stations may include oscillators for the generation of signal frequencies for the transmission of information to the base station. In carrying out the present invention the reference frequency signal will be well spaced from the closest transmission channel frequencies by so-called guard bands. By way of example the present invention will now be described with reference to the accompanying drawings in which:- Figure 1 shows a block schematic diagram of a transmitter/receiver base station in a UHF/Microwave communication system: Figure 2 shows a block schematic diagram of one of a number of transmitter/receiver remote (e.g. mobile) stations of the communication system and operatively associated with the base station of Figure 1; and, Figure 3 is a frequency spectrum diagram in respect of the system depicted in Figures 1 and 2. Referring to Figure 1 of the drawings, the base station illustrated comprises a transmitter section TXSB which embodies a plurality of transmitters, such as the transmitters TXB1 and TXB2 in the present example, for transmitting signals at UHF or Microwave frequencies in closely spaced frequency channels (e.g. channel spacing of 12.5KHz or 6.25KHz) to respective remote (e.g. mobile) stations, such as the remote station depicted in Figure 2 of the drawings. The information or data to be transmitted to the remote station from the base station will be fed in analogue and/or digital form to a signal modulator circuit MODB which produces output signals for modulating the UHF/Microwave frequency signals of frequency F1 and F2 produced by the transmitters TXB1 and TXB2. The signal frequencies F1 and F2 produced by the transmitters TXB1 and TXB2 may be synthesised or otherwise produced as multiples (or divisions) of the output from a master crystal oscillator MOB. The frequencies F1 and F2 may be of the order of 1GHz having a narrow channel spacing of 12.5 or 6.25KHz for example. The duly modulated UHF signals of frequencies F1 and F2 after being fed through power amplifying stages PSB are applied to a diplexer and transmitter/receiver antenna structure DTRB. In the present example the modulated signal of frequency F1 will be received by the receiver section RSR of the remote station shown in Figure 2. The modulated signal of frequency F1 will be received via a diplexer transmitter/receiver antenna arrangement DTRR after which the signal will be detected and amplified by the front end stage FSR of the receiver section RSR. The output from the stage FSR is fed to a receiver RXR1 including a local oscillator having the same frequency F1 as the carrier wave of the received signal. The output from the receiver RXR1 is then applied to a demodulator DEMR which produces data output in analogue and/or digital form. In accordance with the principle of the present invention the frequency F1 of the local oscillator output of the receiver RXR1 is controlled by a slave oscillator SOR which in turn is controlled either continuously or periodically, as appropriate, in accordance with frequency locking data stored in a digital data storage device FLD. At this juncture it is convenient to revert to Figure 1 of the drawings from which it will be seen that the transmitter section TXSB of the base station includes in addition to the transmitters TXB1 and TXB2 a third transmitter TXB3 which is also controlled by the master oscillator MOB and which produces a signal of frequency F0. This frequency F0 is shown on the frequency spectrum diagram of Figure 3 and, as can be seen, it is well separated from the other channel frequencies, such as the frequencies F1 and F2, which have narrow frequency spacing (e.g. 6.25KHz) relative to adjacent channels (e.g. F3 and F4). The frequency bands between the frequency F0 and the nearest channel frequencies (e.g. frequencies F1 and F2) are conveniently referred to as guard bands and serve to ensure frequency locking of the oscillators at the remote stations on to the master oscillator at the base station. The signal of reference frequency F0 produced by the transmitter TXB3 is not modulated for the transmission of data to remote stations but is transmitted in response to control signals applied to the transmitter for the purpose of synchronisation at all of the remote stations to and from which the base station transmits/receives data. In the case of the remote station shown in Figure 2 the signal frequency F0 (i.e. reference frequency) is received by the receiver RXR2 after being amplified etc. by the received front end stage FSR. The frequency of the local oscillator of the receiver RXR2 is controlled by the slave oscillator SOR for the receivers RXR1 and RXR2. The frequency of the slave oscillator SOR is in turn controlled by frequency locking data device FLD for the receiver RXR2 thereby achieving frequency locking of the slave oscillator SOR relative to the master crystal oscillator MOB in the base station transmitter section TXSB. In this way the expected normal drift in the frequency F0 of the master oscillator over a period of time will not adversely affect the operation of the system. The slave oscillator SOR in the receiver section RSR of the remote station which is controlled by the frequency locking data stored in the store FLD also controls the frequency of a transmit slave oscillator SOT in a transmitter section TXSR of the remote station. The transmit slave oscillator SOT controls the frequency F1¹ of signals produced by the transmitter TXR1 and modulated by the output from a modulator MODR in response to an analogue and/or digital data input. The control of the frequency F1¹ may be achieved through a local oscillator or synthesiser multiple/division of the slave oscillator output from the oscillator SOT. These modulated transmitted signals are fed through power stages PSR to the diplexer transmit/receive antenna arrangement DTRR. At the base station these modulated signals of carrier frequency F1¹ are received by the diplexer transmit/receive antenna arrangement DTRB and fed to the front end stages FSB of receiver section RSB which includes two receivers RXB1 and RXB2. Both of these receivers have local oscillators associated therewith which are controlled by a slave oscillator SOB which in turn has a frequency controlled by the master oscillator MOB. Thus, any drift in the frequency F1¹ due to drift in the frequency F0 of the master oscillator MOB will be compensated for in the transmitter section of the remote station and in the receiver section of the base station. The output from the receiver RXB2 is demodulated by a demodulator DEMB from which an analogue and/or digital data output is derived. The other receiver RXB1 will receive incoming signals of frequency F2¹ from a different remote station (not shown) which will also be synchronised with the master oscillator frequency by means of the reference frequency signal of frequency F0 transmitted either continuously or periodically by the base station as hereinbefore described. As previously mentioned the reference frequency signal F0 for achieving synchronisation between the base station equipment and the remote stations may be continuous or it may be transmitted at periodic intervals. In this connection, amateur radio practice where single sideband operation at 1296MHz is quite commonplace would suggest that even for a single sideband system in accordance with the present invention using low cost crystals re-tuning would only need to be performed at intervals of minutes or tens of minutes. However, in the system now being proposed re-tuning is entirely automatic and transparent to the user. As regards the addition of the reference frequency control receiver at the remote stations the additional equipment required is minimal and as far as the use of the frequency locking data storage and frequency locking loop at the receiver of the remote station is concerned, this represents a very small overhead in chip area and power terms and moreover allows cheap crystals to be used, the only requirement being that the crystals are capable of achieving the initial lock on to the transmitted reference frequency F0. The guard banding previously referred to will ensure that this in fact is accomplished. In practice, a microprocessor control scanned routine across the nominal and guard frequencies will achieve lock-up at first switch on within milliseconds. Problems of alternative schemes such a "pilot-tone" single side band or "transparent tone in band" (TTIB) are avoided. Problems such as selective fading are unlikely to occur in the system now proposed since update data will only be required infrequently and where this data is not available old data can be used until the lock is re-acquired.

Claims: A narrow band modulation UHF/Microwave communication system comprising a transmit/receive base station having a plurality of transmitters (TxB1, TxB2) for transmitting information at respective UHF/Microwave channel frequencies spaced over a band of frequencies to a plurality of remote transmit/receive stations, carrier signals for said transmitters (TxB1, TxB2) at the base station being derived from a single master oscillator (MOB) characterised in that a single reference carrier frequency signal (FO) at a frequency spaced from said band of frequencies is derived from said master oscillator (MOB) and is transmitted unmodulated to all of said remote stations where it serves to provide for frequency locking of oscillators (SOR, SOT) at said stations to produce synchronisation between the frequencies at the base station and the remote stations. A narrow band modulation UHF/Microwave communication system as claimed in Claim 1 in which the reference carrier frequency signal (FO) is transmitted periodically to the remote transmit/receive stations. A narrow band modulation UHF/Microwave communication system as claimed in Claim 1 or Claim 2 wherein at a remote station the reference carrier frequency signal is utilised to control the frequency of a slave oscillator (SOR) in the receiver part of said remote station. A narrow band modulation UHF/Microwave communication system as claimed in Claim 3, in which the slave oscillator (SOR) controls the local oscillator of each channel frequency signal receiver of the receiver part (RSR) of said remote station. A narrow band modulation UHF/Microwave communication system as claimed in Claim 3 or Claim 4, in which the slave oscillator (SOR) in the receiver part (RSR) of said remote station controls a slave oscillator (SOT) in the associated transmitter part (TxSR) of the same station, the latter slave oscillator (SOT) controlling a local oscillator in the transmitter part (TxSR) of the remote station. A narrow band modulation UHF/Microwave communication system as claimed in Claim 5, in which the master oscillator (MOB) of the base station controls a slave oscillator (SOB) in the receiver part (RSB) of the base station which in turn controls the local oscillators of respective receivers for receiving signals transmitted from remote stations.

Inventors: SAUL PETER HENRY, SAUL, PETER HENRY
Applicants: PLESSEY SEMICONDUCTORS LTD, PLESSEY SEMICONDUCTORS LIMITED
Application year: 1990
Main classification: H04B7
IPC codes: H04B7