Communication system

DESCRIPTION

The invention relates to a communication system comprising at least one transmitter arranged for transmitting a transmission signal modulated to a carrier wave frequency and at least one receiver arranged for calibrating to the carrier wave frequency of a transmitter for the purpose of demodulating the transmission signal.

For many communication applications, such as control functions in the home for passing on meter readings and the like, also known as "domotics", for indicating the detections from various types of sensors, such as temperature sensors, moisture sensors, gas sensors and the like, and for so-called intelligent environment applications, the use of communication equipment which has a minimum power consumption and which preferably operates at minimum cost is required.

The data to be transferred in these applications is generally emitted at a relatively low bit rate (a few kilobytes per second with a duty cycle of less than

0.1 %). Spreaded receivers, also referred to as "Residential Gates" (RGs) in professional literature, are used for receiving and collecting the emitted data. Generally, an RG can communicate with another RG and/or with a local computer network via a (generally wire-bound) data network arranged for exchanging data at a relatively high speed. Wireless communications between one or more RGs and/or a local computer network are also possible, of course. In practice the distance between a transmitter and a receiver or RG is generally in the order of 10 m, and transmitting powers of a few mW are used.

In practice, communication between transmitters and receivers may, for example, be effected by means of specially designated transmission bands, also known in professional literature as the "Industrial, Medical and Scientific" (ISM) bands ranging between 902-928 MHz and between 2400-2438.5 MHz. No licence is required for communication in these bands.

In addition to a minimum power consumption and low costs, the dimensions of the transmitters must preferably be so small that they can be used for various applications. To obtain small dimensions, it must be possible to realise a transmitter entirely in semiconductor technology.

An important impediment to the provision of a transmitter that is fully integrated in semiconductor technology is the need for a reference signal with a very precise frequency, in the order of a few parts per million (ppm) for generating a local oscillator control signal for the transmitter. In practice such a degree of precision for the oscillator inevitably necessitates the use of a transmitter crystal, whose dimensions are relatively large in comparison with those of a semiconductor circuit, however. Furthermore, transmitter crystals are comparatively expensive. Also other technological solutions, such as the so-called "Bulk Acoustic Resonator" (BAR) are not suitable for realising a communication system with transmitters which have relatively small dimensions and which operate at low cost.

It is therefore a first object of the invention to provide a reliable communication system between at least one transmitter and at least one receiver, wherein at least the transmitter can be configured as a fully integrated semiconductor circuit without there being a need to use voluminous crystals or other resonators, such as the aforementioned BAR.

In order to accomplish that object, the invention provides a communication system of the kind referred to in the introductory paragraph, which is characterised in that the bandwidth of the transmission signal emitted by a transmitter is so much smaller than the transmission bandwidth that is available for a communication link between a transmitter and a receiver that the frequency of the transmission signal emitted by the transmitter is within the available transmission bandwidth upon variation of the carrier wave frequency within a predetermined range, and wherein the receiver is arranged for calibrating to a carrier wave frequency emitted by the transmitter.

The solution according to the invention is based on the perception that when the transmission bandwidth that is available for a transmitter is such that variations in the carrier wave frequency remain within the transmission bandwidth that is available for the transmitter, a less precise local oscillator signal may be used in the transmitter. A necessary condition for this is that the receiver be arranged for calibrating to a carrier wave frequency emitted within the available transmission bandwidth by the transmitter for demodulating the received transmission signal.

In other words, by making available to a transmitter a transmission bandwidth that is larger than strictly necessary for the purpose of transferring the

information at a desired rate and with a desired capacity, in the transmitter a local oscillator exhibiting a relatively large drift in the carrier wave frequency may be used in the solution according to the invention, providing that the receiver comprises means for calibrating to the varying carrier wave frequency within the transmission bandwidth.

Although this renders the receiver according to the invention more voluminous and complex as regards circuitry than a receiver arranged for communication with a transmitter provided with a local minimum drift oscillator, i.e. a drift in the aforesaid range of a few ppm, this does not constitute an appreciable drawback for the communication system according to the invention, because generally no stringent requirements as regards the physical volume of the receivers are made. A practical communication system comprises a great deal more transmitters than receivers, so that a saving on the costs of a large number of transmitters largely offsets the slightly higher costs of a relatively small number of receivers.

In a practical embodiment of the communication system according to the invention, the predetermined range within which the carrier wave of a transmitter is allowed to vary may amount to a few tenths of one per cent of the carrier wave frequency, which requirement is of course much less stringent than a precision of a few parts per million, as in the prior art, in which case a crystal or other solutions, such as a BAR, must be used. The boundaries within which the carrier wave frequency of a transmitter is allowed to vary are furthermore determined in part by the maximum bit error rate (BER) that the receiver is capable of reconstructing yet. In an embodiment of the communication system according to the invention, the transmitter and the receiver form a so-called "Frequency Hopping Spread Spectrum" (FHSS) system for operation in the ISM frequency bands, wherein the transmission signal bandwidth of a transmitter is smaller than the transmission bandwidth of a respective ISM band. For example, an FHSS system comprising 50 channels, each having a bandwidth of 50 kHz and a separation between adjacent channels of 100 kHz for use in the 902-928 MHz ISM band. The carrier wave frequency of the transmitter may vary within the range of 915 MHz ± 9.3 MHz in that case.

It will be understood that the inventive concept may also be used for

forms of communication other than FHSS in the ISM band and/or with other numbers of channels and transmission signal bandwidths than those mentioned by way of example in the above.

Another difficulty that arises with communication systems for the aforesaid applications is the realisation of a minimum power consumption of the transmitter. This means that inter alia the calibration procedure, in which the receiver calibrates to the carrier wave frequency received from a transmitter, must be limited as regards the length of time thereof.

In another embodiment of the communication system according to the invention, the transmitter is to that end provided with time control means for transmitting a transmission signal for a predetermined length of time, and a receiver is provided with a local oscillator and with detection means for detecting the magnitude and sign of a frequency deviation of the signal from the local oscillator in relation to a transmission signal received from a transmitter. In yet another embodiment of the communication system according to the invention, the time control means are arranged for repeated transmission of an unmodulated transmission signal of relatively short duration.

This embodiment has the advantage that a further reduction of the power consumption in the transmitter can be realised by transmitting an unmodulated transmission signal for calibrating the receiver.

In a preferred embodiment of the communication system according to the invention, which is suitable for full integration of a receiver in semiconductor technology, the detection means comprise first and second mixing circuits and low- pass filter means for mixing and low-pass filtering the received unmodulated transmission signal and the local oscillator signal, for providing in-phase-(l)- and quadrature-(Q)-mixing signals, modulus means for determining the absolute value of the frequency deviation from the I- and/or Q-mixing signal and sign means for determining the sign of the frequency deviation from the I- and Q-mixing signal.

The sign means are used for determining whether the carrier wave frequency of the transmitter is higher or lower than the frequency of the local oscillator signal from the receiver, and the modulus means are used for determining the absolute value of the frequency deviation.

In an embodiment of the invention which is relatively inexpensive and which is relatively simple as regards circuitry, the modulus means comprise

reference frequency clock means and counter and calculating means for counting the number of reference clock periods during at least a half period of the I and/or Q mixing signals and for calculating the modulus of the frequency deviation from the counted number and the frequency of the reference clock means. To enable a relatively precise determination of the absolute value of the frequency deviation, the reference clock means of the communication system according to the invention are arranged for operating at a substantially higher frequency than the maximum allowable frequency difference between the carrier wave frequencies of a transmitter and a receiver. To determine the sign of the frequency deviation, the sign means of yet another embodiment of the communication system according to the invention are arranged for detecting a rising or falling edge in the I- and Q-mixing signals, for detecting a signal transition of the I- and Q-mixing signals through a signal symmetry level thereof and for providing the sign of the frequency deviation from an evaluation of time-successive transitions of the I- and Q-mixing signals through the signal symmetry level and the signal edge that occurs with each transition.

In an embodiment of the communication system according to the invention, a receiver is provided with correction means for correcting the signal delivered by its local oscillator with the detected frequency deviation for the purpose of calibrating the receiver to the transmission signal received from a transmitter.

In an embodiment, the local oscillator in the receiver is preferably a signal-controlled oscillator, such as a voltage or current controlled oscillator, wherein the correction means are arranged for generating a control signal from the absolute value and the sign of the determined frequency deviation for controlling the local oscillator for the purpose of providing a local oscillator signal of the receiver calibrated to the carrier wave frequency of a transmitter.

In another embodiment of the invention, which is suitable for full integration of a receiver in semiconductor technology, the correction means comprise synthesiser means arranged for generating an I- and Q-offset frequency signal from the absolute value and the sign of the detected frequency deviation, with third, fourth, fifth and sixth mixing circuits, first and second summation circuits and high-pass filter means for mixing the high-pass filtered I- and Q-offset frequency signal and the local oscillator signal and for summing the I- and Q-signals obtained by said mixing for the purpose of providing a local I- and Q-oscillator signal for the

receiver calibrated to a transmission signal received at the carrier wave frequency of a transmitter.

It will be understood that in addition to the advantage of less stringent requirements as regards the precision of the oscillator signal generated by the transmitter, a receiver having a structure according to the invention also corrects for frequency deviations caused by process variations in the semiconductor circuits of the transmitter and the receiver due to ageing, temperature influences, etc.

The communication system according to the invention is not limited to simplex communication between a transmitter and a receiver. In an embodiment suitable for duplex communication, a transmitter is provided with receiving means and a receiver is provided with transmission means for transmitting an information signal to the transmitter in a receiver condition calibrated to the carrier wave frequency of the transmitter.

After calibration to the transmission signal received from a transmitter, the frequency of the carrier wave oscillator signal of the transmitter is known in the receiver. Assuming that the receiving means in a transmitter operate on a frequency derived from the carrier wave oscillator signal, the transmission means in a receiver can transmit at precisely the frequency at which the receiving means in a transmitter operate. Further calibration or synchronisation steps are not required. This possibility can be employed for sending an acknowledgement of receipt (ACK) to the transmitter, for example, which increases the field of applications and the "Quality of Service" (QoS) of the wireless connection.

According to a constructionally simple and inexpensive embodiment of the invention suitable for duplex communication, a transmitter comprises a local oscillator which is operatively connected as an oscillator for generating the carrier wave frequency and as a local oscillator for demodulating a received information signal.

A special advantage of the invention is that the receiver does not need to have any foreknowledge of the frequency deviation of the signal from a transmitter in order to be able to receive this signal, whilst a sufficiently short calibration or acquisition time can nevertheless be achieved by means of the above- described circuits.

In yet another embodiment of the invention, in order to accelerate the calibration process in the receiver even further, which is in turn advantageous

with a view to reducing the power consumption of a transmitter, a receiver is provided with a look-up table for storage of the detected frequency deviation of a transmitter therein. Since the frequency deviation of a transmitter is fairly stable in practice, a look-up table for a respective transmitter only needs to be updated sporadically.

The receiver can for example recognise a transmitter from a call number or the like. Other solutions may be used for this purpose as well, such as a specific information signal emitted by a transmitter or recognition of specific processing properties of a transmitter. According to yet another embodiment of the communication system according to the invention, a further intended saving on the energy consumption as a whole can be achieved, for example, by using the information signal for transferring information regarding the quality of the signal received at a receiver from said receiver to a transmitter and by providing a transmitter with means that change the signal transmission properties of the transmitter in response to the information received.

The information signal may also be an acknowledgement signal, for example, and a transmitter may comprise means for stopping the transmission of the transmission signal in response to the received acknowledgement signal. The invention further provides a transmitter as well as a receiver arranged as disclosed in the foregoing.

It has been explained in the foregoing that a short calibration or acquisition time of a receiver is advantageous with a view to maximally reducing the power consumption of a transmitter. To that end, the invention provides a quick detection method to be implemented both in hardware and in software for determining the frequency deviation of a received transmission signal, which method comprises the steps of: a) providing in-phase-(l)- and quadrature-(Q)-radio frequency signals by mixing the first and second radio frequency signals; b) providing I- and Q-mixing signals by low-pass filtering of the I- and Q-radio frequency signals of step a); c) detecting a rising or falling edge in the I- and Q-mixing signals; and d) detecting a signal transition of the I- and Q-mixing

signals through a signal symmetry level thereof; wherein a first sign value is delivered if, viewed in time: e) the l-mixing signal undergoes a signal transition with a falling edge followed by a signal transition of the Q-mixing signal with a falling edge; f) the l-mixing signal undergoes a signal transition with a rising edge followed by a signal transition of the Q-mixing signal with a rising edge; g) the Q-mixing signal undergoes a signal transition with a falling edge followed by a signal transition of the l-mixing signal with a rising edge; h) the Q-mixing signal undergoes a signal transition with a rising edge followed by a signal transition of the I-mixing signal with a falling edge; and wherein a second sign value is delivered if, viewed in time: i) the l-mixing signal undergoes a signal transition with a falling edge followed by a signal transition of the Q-mixing signal with a rising edge; j) the l-mixing signal undergoes a signal transition with a rising edge followed by a signal transition of the Q-mixing signal with a falling edge; k) the Q-mixing signal undergoes a signal transition with a falling edge followed by a signal transition of the l-mixing signal with a falling edge;

I) the Q-mixing signal undergoes a signal transition with a rising edge followed by a signal transition of the l-mixing signal with a rising edge.

To determine the magnitude of the frequency deviation, the invention further provides a method wherein the number of reference clock periods of reference clock means is counted during at least a half period of the I- and Q- mixing signals, wherein the magnitude of the frequency deviation of a received transmission signal is calculated from the counted number and the frequency of the reference clock means.

It has been found that even under the worst signal conditions the method only needs to be carried out twice at most for determining the frequency deviation with a precision of a few ppm. It will be understood that this method may be carried out a few times in succession, if necessary, in order to obtain an even more optimum result, providing that a signal is received from the transmitter. The fact that repetition of the method leads to an increased power consumption of the transmitter must be taken into account, however, because the actual transmission of data cannot be started yet during these repetitions.

Generally, the time during which a transmitter needs to emit a signal in order to make it possible to determine the frequency deviation of the transmitter carrier wave signal with sufficient precision in the receiver is about 100 microseconds. In the applications referred to in the introduction this corresponds to about 1/10 the period of a single symbol, based on a data transmission rate of 1 kb.

The invention will now be explained in more detail by means of the description below of a number of preferred embodiments, to which embodiments the invention is by no means limited, however.

Figure 1 schematically shows an example of a communication system according to the invention.

Figure 2 schematically shows an example of the bandwidth distribution in communication system according to the invention.

Figure 3 schematically shows an embodiment of a frequency calibration circuit for use in the communication system according to the invention. Figure 4 schematically shows a further embodiment of a frequency calibration circuit for use in the communication system according to the invention.

Figure 5 schematically shows the principle of an embodiment of means for determining the value of the frequency deviation between the local oscillator signal in a receiver and the carrier wave frequency of the transmission signal from a transmitter according to the invention.

Figure 6 graphically shows the operation of the means of figure 5.

Figure 7 schematically shows the principle of an embodiment of means for determining the sign of the frequency deviation between the local oscillator signal in a receiver and the carrier wave frequency of the transmission signal of a transmitter according to the invention.

Figure 8 graphically shows the operation of the means of figure 7.

Reference number 1 in figure 1 indicates a communication system according to the invention, comprising a number of spread receivers 2, also called "Residential Gates" (RG), with which a large number of transmitters 3 can communicate wirelessly as indicated at 4. In the simplest embodiment of the invention, the communication system 1 comprises only receivers 2 and only transmitters 3. In a more sophisticated embodiment of the invention, the communication systems is arranged (also) for operation with components arranged as transceivers.

The receivers 2 are generally connected, via a fixed data link 5 suitable for relatively high-speed data transmission, to a data processing system 6, illustrated in the figure by a server 7 and workstations 8 connected thereto via a bus

10. Reference number 9 indicates a high-speed wireless data link between the server 7 and a receiver 2.

The wireless data link 4 between a transmitter 3 and a receiver 2 is generally designed for processing relatively low bit rates, in the order of a few kbit/sec. In practice the transmission range of a transmitter 3 is a few metres. All this in order to realise a minimum power consumption in a transmitter 3. In typical applications of the communication system 1 according to the invention, the transmitters 3 are arranged for passing on meter readings and the like in the home, also known as "domotics", for indicating the detections from various types of sensors, such as temperature sensors, moisture sensors, gas sensors and the like, and for so-called intelligent environment applications. Figure 2 illustrates the transmission signal emitted by a transmitter

3 upon use of the "Industrial Medical and Scientific (ISM)" frequency band between 902-928 MHz. The frequency is plotted on the horizontal axis. The transmission signal from a transmitter has a bandwidth B ₁ - of 7.4 MHz in the example that is shown in figure 2, and in principle the entire ISM band between 902 MHz and 928 MHz, i.e. a transmission bandwidth B ₀ of 26 MHz, is available for the communication link between a transmitter 3 and a receiver 2.

In the selected example, the carrier wave frequency f _c of a transmitter according to the invention in relation to the ideal carrier wave frequency of f ₀ 915 MHz, which lies in the centre of the ISM band, may vary to a degree equal to ± 9.3 MHz. This relatively wide margin directly translates back into the precision of the local oscillator for controlling the transmitter 3, of which less stringent requirements may be made in comparison with a communication system in which the transmission bandwidth B _τ of the transmission signal emitted by a transmitter takes up substantially the entire transmission bandwidth B ₀ for the communication link. The communication system according to the invention is preferably so dimensioned that the carrier wave frequency f _c of a transmitter may vary in the order of a few tenths of one per cent of the carrier wave frequency. In the illustrated example, the allowable variation is maximally about 1 %. In practice, the requirement made as regards the precision of a local oscillator in the case of a variation of a few

tenths of one per cent is still very low in comparison to the communication system that is known in professional literature, in which the variation may not amount to more than a few ppm.

In the example that is shown in figure 2, a transmission signal according to the regulations of the "Federal Communications Commission" (FCC) is used, being a "Frequency Hopping Spread Spectrum" (FHSS) system, which employs the so-called non-coherent "Binary Frequency Shift Keying" (BFSK) modulation, comprising 50 hopping channels C ₁ , C ₂ , ... C ₅₀ , which is a minimum requirement imposed by the FCC. In this system, each channel has an information bandwidth B, of 50 kHz, with a channel separation B _s between adjacent channels of 10O kHz.

It will be understood that the invention is not limited to FHSS systems with BFSK modulation in the ISM band, but that same may also be used on other frequency bands suitable for data communication. In order to be able to receive a transmitter 3 with a varying carrier wave frequency, the receivers 2 in the communication system 1 according to the invention are arranged for calibrating to the carrier wave frequency f _c emitted by a transmitter 3. Reference number 13 in figure 1 indicates detection means in a transmitter 2 for detecting the magnitude and sign of the frequency deviation of the signal of a local oscillator 12; 39 in a receiver 2 in relation to a transmission signal received from a transmitter 3. A transmitter 3 may be provided with time control means 11 in that case for transmitting a transmission signal for a predetermined length of time that is sufficiently long to enable detection of the relevant frequency deviation by means of the detection means 13 in a receiver 2. In a preferred embodiment of the communication system according to the invention, the time control means 11 are arranged for repeated transmission of an unmodulated transmission signal of relatively short duration. From the viewpoint of energy consumption in a transmitter, the transmission of an unmodulated transmission signal is most efficient, and the duration of said transmission of an unmodulated transmission signal is geared to the time that the detection means 13 need for detecting the frequency deviation of the transmission signal from a transmitter 3 in relation to the local oscillator 12; 39 in the receiver 2.

Time control means 11 suitable for use in a transmitter 3 are feasible in many variations thereof for those skilled in the art, and consequently they

require no further explanation.

Figure 3 schematically shows an embodiment of detection means 13 i.e. a frequency calibration circuit according to the invention for use in a receiver 2 according to the invention. A signal from a transmitter 3 that is received by a receiver 2 is supplied to an input of a first mixing circuit 17 and an input of a second mixing circuit 18 via an input 15 of a radio frequency amplifier 16, referred to as "Low Noise Amplifier" (LNA) in professional literature.

The local oscillator 12, which is a signal-controlled local oscillator, such as a voltage-controlled oscillator (VCO), provides an in-phase output signal I ₀ and a quadrature output signal Q ₀ , which signals are supplied to a further input of the first mixing circuit 17 and a further input of the second mixing circuit 18, respectively.

From the received input signal and the in-phase component of the local oscillator signal, the first mixing circuit 17 produces a first in-phase mixing signal l _m , which mixing signal is supplied to an input of a first low-pass filter 19. From the input signal received by the receiver 2 and the quadrature component of the local oscillator signal, the second mixing circuit 18 produces a quadrature mixing signal Q _m , which is supplied to an input of a second low-pass filter. The output signals of the first and the second low-pass filter 19, 20 are a measure of the frequency deviation between the signal generated by the local oscillator 12 and the signal received from a transmitter 3, and these output signals are supplied to modulus means 21 for determining the absolute value of the frequency deviation and to sign means 22 for determining the sign of the frequency deviation, i.e. higher (+) or lower (-) than the signal generated by the local oscillator 12.

In the embodiment that is shown in figure 3, the modulus means 21 determine the absolute value of the frequency deviation from the l-mixing signal, which is by no means a limitative aspect of the invention. The modulus means 21 may also determine the absolute value of the frequency deviation between the received signal and the signal of the local oscillator 12 from the output signal of the second low-pass filter 20.

The output signal 23 of the modulus means 21 and the output signal 24 of the sign means 22 are supplied to correction means 25 for generating a control

signal 14 therefrom, which control signal is used for controlling the local oscillator 12 in such a manner that the frequency deviation between the local oscillator signal in the receiver 2 and the signal received from a transmitter 3 is reduced to zero as much as possible, so that the receiver 2 is calibrated to the transmitter 3 for further processing or demodulation of the signal transmitted by a transmitter. These further processing means are not shown in figure 3 for the sake of simplicity. Correction means 25 suitable for the purpose of the invention are known to those skilled in the art and require no further explanation.

In another embodiment of the detection means 13, i.e. the frequency calibration circuit according to the invention as shown in figure 4, the correction means are configured as frequency synthesiser means 26 for generating from the absolute value of 23 and the sign 24 of the detected frequency deviation an in-phase and a quadrature-offset frequency signal 35, 36 to be supplied to the first the second mixing circuit 17, 18. To that end the synthetiser means 26 first supply the I- and Q-offset frequency signals 35, 36 to a first and a second high-pass filter 31 , 32, respectively. A third mixing circuit 27 and a fourth mixing circuit 28 are provided for mixing the high-pass filtered l-offset frequency signal with the l-component I ₀ and the Q- component Q ₀ of the local oscillator signal, to the inputs of which mixing circuits the high-pass filtered l-offset frequency signal and the l-component I ₀ of the local oscillator 39 and the high-pass filtered l-offset frequency signal and the Q- component Q ₀ of the local oscillator 39 are supplied. The Q-component Q ₀ of the local oscillator signal and the high-pass filtered Q-offset frequency signal of the synthetiser means 26 are supplied to the input of a fifth mixing circuit 29 and the I- component I ₀ of the local oscillator signal and the high-pass filtered Q-offset frequency signal of the synthetiser means 26 are supplied to the input of a sixth mixing circuit 30. The output signals of the third and the fifth mixing circuit 27, 29 are summed in a first summation circuit 33, and the output signals of the fourth and the sixth summation circuit 28, 30 are summed in a second summation circuit 34. That is, the output signal of the third mixing circuit 27 is deducted from the output signal of the fifth mixing circuit 29 in the first summation circuit 33, and the output signal of the fourth mixing circuit 28 is deducted from the output signal of the sixth mixing circuit 30 in the second summation circuit 34. The output signal 37 of the first summation circuit 33 is supplied to an input of the first mixing circuit 17, and the

output signal 38 of the second summation circuit 34 is supplied to an input of the second mixing circuit 18. The signals 37 and 38 correspond to the l-component l _c and the Q-component Q _c , respectively, of the constructed carrier wave signal x τ _c " of a transmission signal from a transmitter 3 as received by a receiver 2. With the circuit of figure 4 the frequency of the local oscillator 39 need not be adapted, but frequency calibration is obtained by suitably mixing the signal from the local oscillator 39 and the frequency signal generated by the synthetiser means 26. It will be understood that the local oscillator 39 need not be a signal-controlled oscillator, but that it may operate at a fixed frequency. As a result, the occurrence of transition phenomena upon calibration of a receiver 2 to the signal received from a transmitter 3 resulting from the tuning of the local oscillator is advantageously prevented. Suitable synthesiser means 26 for generating I- and Q- offset frequency signals for reducing the frequency deviation to practically zero are known per se to those skilled in the art and require no further explanation. Figure 5 schematically shows the schematic diagram of an embodiment of the modulus means 21 according to the invention, comprising reference frequency clock means 40, counter means 41 and calculating means 42. The counter means 41 have an enabling input 42, to which the output signal from the first low-pass filter 19 or the second low-pass filter 20 is applied. The output signal of the counter is supplied to a first input of calculating means 43, to a second input of which the output signal of the reference frequency clock means 40 is supplied. The calculating means 43 are arranged for determining the frequency deviation between the signal of the local oscillator 12; 39 in a receiver 2 and the signal received from a transmitter 3 on the basis of the count number provided by the counter means 41 and the frequency of the reference clock means. Calculating means suitable for this purpose are known to those skilled in the art and require no further explanation.

The operating principle of the modulus means 21 is graphically illustrated in figure 6. Reference number 46 in the lower half of figure 6 indicates in time the output signal of the first or the second low-pass filter means 19, 20 that is supplied to the enabling input 42 of the counter means 41. It is assumed that this signal has a square wave-like nature, which can be obtained, if necessary, by connecting suitable conversion means for converting a sinusoidal input signal into a

square wave-like output signal. Further assume that the counter means 41 are enabled for the time that elapses between a rising edge 47 and a falling edge 48 of the square wave signal 46. During this time, the counter means 41 count the number of periods of the reference frequency clock means 40, whose output signal is shown in the upper half of figure 6. Based on the number counted by the counter means 41 , the calculating means 43 can then readily determine the frequency of the square wave signal 46, which corresponds to the frequency deviation of the signal received by a receiver 2 and the signal generated by the local oscillator 12; 39 thereof.

To obtain a sufficiently precise determination of the frequency deviation, the reference frequency clock means 40 are arranged for operation at a substantially higher frequency than the maximum allowable frequency difference between the carrier wave frequencies of a transmitter 3 and a receiver 2.

Now let us consider the communication system according to the example that is shown in figure 2. In this example, the modulus of the maximum frequency deviation is 9.3 MHz. A sufficiently precise determination of the frequency deviation can be obtained in that case by using reference frequency clock means 40 that operate at a frequency which is higher by a factor of 10-100 than the maximum allowable frequency deviation, i.e. a frequency between 90-900 MHz, for example. All this depending inter alia on the frequency range of the counter means 41. Reference number 44 indicates time control means in a receiver 2 that are controlled by the modulus means 21 , via an output 45 thereof, for controlling switching means 49 at the output of the LNA 16. While the frequency deviation is being calculated, the switching means 49 are maintained in the open (nonconducting) position for some time via the time control means 44. After calibration of a receiver 2, the time control means 49 are switched into the closed (conducting) position for demodulation of a transmitter signal or a receiver transmission signal, as the case may be.

To determine the sign of the frequency deviation, the sign means 22 of an embodiment of the invention are arranged for detecting a rising or falling edge in the I- and Q-mixing signals, i.e. the output signals of the first low-pass filter 19 and the second low-pass filter 20, for detecting a signal transition of the I- and Q- mixing signals in relation to or through a signal symmetry level of the I- and Q-mixing signals, wherein the sign of the frequency deviation follows from an evaluation of time-successive transitions of the I- and Q-mixing signals through the signal

symmetry level and from the fact whether a rising or a falling edge is involved.

Figure 7 schematically shows a circuit diagram of the sign means 22, comprising a circuit 50 for determining the signal symmetry level 5 from the low- pass filtered I- and Q-mixing signals 46 and 56, respectively (see figure 3 or 4), and a comparator circuit 51 for determining which edge, i.e. the rising edge or the falling edge, of a respective mixing signal goes through the signal symmetry level 5 supplied to a control input 52 of the comparator means 51 by the circuit 50.

The relation between the rising or falling edges of the low-pass filtered I- and Q-mixing signals and the sign of the frequency deviation is schematically illustrated in figure 8.

The upper half of figure 8 shows the Q-mixing signal 56 of the second low-pass filter 20, and the lower half of figure 8 shows the l-mixing signal 46 on the output of the first low-pass filter 19. The signal symmetry level for the I- and Q-mixing signals is indicated by S. In figure 8, four situations or four columns can be distinguished.

That is, a first situation indicated "0", in which the Q-mixing signal 56 passes through the signal symmetry level S with a falling edge. A second situation indicated "π/2", in which the l-mixing signal 46 passes through the signal symmetry level S with a rising edge. A third situation indicated "π", in which the Q-mixing signal 56 passes through the signal symmetry level S with a rising edge, and a fourth situation indicated "3π/2", in which the l-mixing signal 46 passes through the signal symmetry level S with a falling edge. It will be understood that figure 8 repeats itself over time from the situation ¹¹ O", etc.

It can now be demonstrated that the sign value of the frequency deviation, i.e. higher than (+) or lower than (-) the local oscillator frequency of the receiver 2, can be determined from an analysis of the respective signal transitions of the I- and Q-mixing signals 45, 56 in figure 8.

Let us to that end consider the first column "0" in figure 8. At the transition of the l-mixing signal 46 through the signal symmetry level S with a falling edge, indicated by 61 , followed in time by a signal transition of the Q-mixing signal with a falling edge, indicated by 62, the frequency deviation has a first sign value, viz. the value plus (+) in the illustrated example. In column "π" in figure 8, the frequency difference has the first sign value, i.e. the value plus (+), with a signal transition of the l-mixing signal 46 through the signal symmetry level S with a rising

edge, indicated by 63, followed by a signal transition of the Q-mixing signal 56 with a rising edge, indicated by 64. The same applies when in column "π/2" the Q-mixing signal 56 undergoes a signal transition with a falling edge, indicated by 65, followed by a signal transition of the l-mixing signal 46 with a rising edge, indicated by 66, whilst in the case of a signal transition in column "π/2" of the Q-mixing signal 56 with a rising edge, indicated by 67, followed by a signal transition of the l-mixing signal 46 with a falling edge, indicated by 68, the frequency difference has the first sign value, i.e. the value plus (+) in the example of figure 8.

In all the other cases, in column "0" in the case of a signal transition through the signal symmetry level 5 of the l-mixing signal 46 with a falling edge, indicated by 69, followed by a signal transition of the Q-mixing signal 56, indicated by 90, the frequency difference has a second sign value, a negative (-) value in the example of figure 8. The same applies to a signal transition in column "π" of the I- mixing signal 46 with a rising edge, indicated by 71 , followed by a signal transition of the Q-mixing signal 56 with a falling edge, indicated by 72, and to a signal transition in column "π/2" of the Q-mixing signal 56 with a falling edge, followed by a signal transition of the l-mixing signal 46 with a falling edge, indicated by 73 and 74, respectively, whilst also in the case of a signal transition in column "3π/2" of the Q- mixing signal 56 with a rising edge, followed by a signal transition of the l-mixing signal 46 with a rising edge, indicated by 75 and 76, respectively, the frequency difference has a second sign value, i.e. minus (-) in the illustrated example.

On the basis of this analysis, the sign means 22 can determine the sign of the frequency deviation. The comparator means 51 can be suitably configured for this purpose, using digital components, as known to those skilled in the art. It is also possible to use processor means in the form of a microprocessor or a microcontroller for this purpose, if desired. The method of determining the sign of the frequency deviation can be suitably realised in the form of software in that case.

It will be understood that the sign means 22 may also be realised in other suitable ways for use in a communication system according to the inventive concept, i.e. calibrating a receiver 2 to the carrier wave frequency f _c transmitted by a transistor 3, wherein the transmission signal B ₁ - transmitted by the transmitter 3 is so much smaller than the transmission bandwidth B ₀ that is available for a communication link between a transmitter 3 and a receiver 2, that the frequency of the transmission signal transmitted by the transmitter 3 will still lie within the

available transmission bandwidth B _c upon variation of the carrier wave frequency f _c of the transmitter 3 within a predetermined range.

Although the description above is consistently based on the use of a transmitter 3 and a receiver 2, the communication system according to the invention may also be advantageously adapted for communication between transceivers (not shown), because as soon as a receiver 2 has been calibrated to the carrier wave frequency of a transmitter 3, the receiver 2 in question can transmit a signal to the transmitter 3 in question at the calibrated frequency for receipt by a receiver in the transmitter 3 in question, wherein the use of a single local oscillator in the transmitter 3 will suffice for both the transmitting part and the receiving part thereof.

According to an embodiment of the invention, in order to accelerate the calibration process, a receiver 2 may be provided with a look-up table for storage of the detected frequency of a transmitter 3. On the basis of this look-up table, the receiver 2 may be directly pre-set to a frequency as close to the known frequency of the transmitter 3 as possible in that case, for example via the synthetiser means 26, upon receipt of a transmission signal from a respective transmitter 3. This is schematically illustrated in broken lines in figure 4.

It will be understood that the invention is not limited to the embodiments as described and illustrated herein, and that many additions and modifications are possible to those skilled in the art without departing from the inventive concept as defined in the appended claims.