Spread spectrum receiving apparatus

Abstract: There is provided a receiving apparatus for receiving a signal which was spread spectrum diffused, comprising converting means for converting a frequency of the received signal; and arithmetic operating means for arithmetically operating the signal converted by the converting means and a diffusion code, wherein the converting means converts a frequency of the received signal in accordance with an output of the arithmetic operating means, so as to improve receiving characteristics of a spread spectrum receiving apparatus.

Description: The invention relates to a spread spectrum receiving apparatus for receiving a spread spectrum signal. Hitherto, in a spread spectrum communication using a convolver, as shown in Fig. 3, a communication partner is determined by collations of diffusion codes CODE 1 to CODE n which terminals T 1 to T n and T 1 ' to T n ' have as shown in Fig. 3. Namely, an apparatus cannot communicate with a terminal equipment having another code and can communicate with a terminal equipment of only the same code. Fig. 4 is a block diagram briefly showing the above principle. In Fig. 4, a reception wave inputted from an antenna (not shown) is down converted into a desired IF frequency and is supplied to one input terminal 2a of a correlator 1. A reference signal which has previously been diffused by, for example, the diffusion code CODE 1 is inputted from a reference signal generation unit 4 to another input terminal 2b of the correlator 1. Only in the case where the diffusion code which the received signal from the input terminal 2a has is equal to CODE 1 , namely, when it is the same code as the diffusion code which the reference signal has, a correlation peak as shown in the diagram is outputted from an output terminal 3 of the correlator 1. The transmission data can be demodulated on the reception side by using such a correlation peak. Even when the received signal using a code different from the code CODE 1 which the reference signal has is inputted to the input terminal 2a, no correlation peak is outputted and the data cannot be demodulated. In the above conventional example, however, there is a problem such that when the frequency of the reference signal is deviated from the frequency of the received signal, the value of the correlation peak decreases. In addition, the number of diffusion codes cannot be set to an infinite value and the diffusion codes need to be mutually orthogonal with each other as much as possible. Therefore, the number of codes which can be used is limited and the number of terminals which can be used in the same band is also limited. It is an object of the invention to improve receiving characteristics of a spread spectrum receiving apparatus, for example to provide a spread spectrum receiving apparatus which can correctly receive a signal even when a frequency fluctuates. It is known from EP-A-500079 to provide a receiver for a spread spectrum signal in which an oscillator used for frequency conversion is controlled by an error signal derived from the outputs of two correlators supplied with relatively phase shifted converted signals. According to the present invention there is disclosed a receiving apparatus for receiving a spread spectrum signal diffused according to a diffusion code, the apparatus comprising; converting means operable to frequency convert the spread spectrum signal to obtain a converted signal at a reception frequency (F 1 ' + ΔF 1 ); control means operable to control the converting means; demodulating means operable to demodulate the converted signal using a corresponding diffusion code; correlator means operable to correlate the converted signal with a reference signal diffused according to the corresponding diffusion code so as to generate a synchronising signal; and synchronising means operable to synchronise operation of the demodulating means in accordance with the timing of the synchronising signal; characterised by the control means being operable to control the converting means such that the synchronising signal generated by the correlator means is maintained at a predetermined level. A corresponding method is disclosed in accordance with claim 7. The above and other aspects, features and advantages of the present invention will become apparent from the following detailed description and the appended claims with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing a construction of a receiver in the first embodiment of the invention; Fig. 2 is a block diagram showing a construction of a receiver in the second embodiment of the invention; Fig. 3 is a block diagram showing a constructional example of a conventional communication system; Fig. 4 is a block diagram for explaining a principle of a multi-dimensional connection using a conventional convolver; Fig. 5 is a block diagram showing constructions of communication systems in the first and second embodiments of the invention; Fig. 6 is a schematic diagram for explaining characteristics of convolvers in the firstand second embodiments; Figs. 7A and 7B are schematic diagrams showing situations upon demodulation of the received signals in the first and second embodiments; Fig. 8 is a block diagram showing a specific system construction of the first embodiment; and Fig. 9 is a block diagram showing a specific second system construction in the firstand second embodiments of the invention; DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS In the first embodiment, a correlator output of a receiver is detected and an output frequency of a local oscillator of a reception unit is controlled in accordance with a detection output, thereby suppressing the output frequency to the optimum oscillating frequency even when the oscillating frequency of the local oscillator fluctuates due to changes in temperature and the like. On the other hand, even for a change in transmitting frequency of a transmitter, a stable correlation output can be obtained. Fig. 1 is a block diagram showing a construction of the receiver in the first embodiment. The receiver comprises: a receiving antenna 10; a low noise amplifier 20; a mixer 21 for down conversion; an AGC amplifier 22, a correlator output amplifier 23; a band pass filter 24; a detector 25; an A/D converter 26; a control unit 27; an IC 28 for PLL (phase locked loop); a loop filter 29; a VCO (voltage controlled oscillator) 30; a sync unit 39; a demodulation unit 40; the correlator (convolver) 1 as an analog device; a local oscillator 12; a mixer 13; and a code generator 14. The operation in the construction as mentioned above will now be described. A diffusion signal (center frequency f 1 + Δf 1 ) which was inputted from the antenna 10 is amplified by the low noise amplifier 20 and is converted into an IF (intermediate) frequency (f 1 ' + Δf 1 ) by the mixer 21 for down conversion. The converted IF signal is supplied to one input terminal of the correlator 1. A diffusion signal of a center frequency (f 1 ' + Δf 1 ) is supplied to another input terminal of the correlator 1 through the local oscillator (f 1 ' + Δf 1 ) 12, mixer 13, and code generator 14. A portion C surrounded by an alternate long and short dash line corresponds to a local oscillator (oscillating frequency f 0 ) having a construction of a PLL and locks the oscillating frequency of the VCO 30 to a desired frequency by using the IC 28 for PLL and the LPF 29 on the basis of set data S 1 which has been preset by the control unit 27. For example, when the frequency of the transmitter is slightly deviated by (f 1 + Δf 1 ) or the frequency f 0 of the local oscillator of the receiver is changed by the temperature or other factors, there is a possibility such that a correlator output decreases because the input frequency of the correlator 1 differs or that the proper level cannot be maintained. In consideration of such a problem, according to the embodiment, by providing the detector 25 at the output stage of the correlator 1, the correlator output is detected and a detection output is A/D converted by the A/D converter 26 and the converted digital signal is supplied to the control unit 27. The optimum level of the detection output has been preset into the control unit 27. The set data which is transmitted to the PLL is controlled so as to keep the optimum level. Due to this, the signal is generated from the local oscillator by a frequency that is slightly different and the correlator output can keep a proper level. In the above first embodiment, the frequency of the local oscillator for down conversion of the receiver has been controlled in accordance with the output of the correlator. In the second embodiment, as shown in Fig. 2, the local oscillator (portion D surrounded by an alternate long and short dash line in Fig. 2) for the reference signal is set so as to have a PLL construction as shown in the diagram in accordance with the output of the correlator 1. The oscillating frequency is controlled by the set data from the control unit 27. The set data S 1 and the set data S 2 are the different data. Oscillating frequencies of the VCO 30 and a VCO 131 are quite different. Fig. 5 is a block diagram showing constructions of communication systems in the first and second embodiments of the invention. In Fig. 5, terminals T 1 to T n and K 1 to K n communicate with terminals T 1 ' to T n ' and K 1 ' to K n ', respectively. The terminals T 1 , T 1 ', K 1 , and K 1 ' execute the communication by using the diffusion code CODE 1 . Similarly, the terminals T 2 , T 2 ', K 2 , and K 2 ' perform the communication by using the diffusion code CODE 2 . Further, the terminals T n , T n ', K n , and K n ' execute the communication by using the diffusion code CODE n . According to the spread spectrum communication system, generally, in the case where a plurality of terminals of the same diffusion code exist, the communication is impossible. According to the apparatus of the embodiment, however, in case of using the same code, the multiplex communication is executed by changing the transmitting frequency by only Δf. Namely, an up frequency f 1 and a down frequency f 2 are allocated to the communication between the terminals T 1 to T n and the terminals T 1 ' to T n '. For instance, a frequency of (the up frequency f 1 ) + Δf is allocated to the terminals K 1 to K n , while a frequency of (the down frequency f 2 ) + Δf is allocated to the terminals K 1 ' to K n '. In this manner, the transmitting frequency of the terminals using the same code is made different by only Δf. In brief, although the terminals T 1 and T 1 ' and the terminals K 1 and K 1 ' have the same diffusion code, the frequencies which are used are different in each case by Δf. The reason why it is possible to discriminate even in case of the same code by changing the frequency as mentioned above is based on the characteristics of the correlator, for example, convolver. Fig. 6 is a schematic diagram for explaining the characteristics of the convolver 1 (used as a correlator 1). The signals which have the frequencies f 1 and f 2 and were diffused by the diffusion code in which the same code had been inverted on the time base are inputted to the two input terminals 2a and 2b of the convolver 1, respectively. Now, assuming that a length of convolver 1 is set to L and a transfer speed of a surface acoustic wave which propagates on the convolver 1 is set to V a (for example, V a = 3500 m/sec), a condition such that the correlator output can be taken out to the output terminal 3 is expressed by |f 1 - f 2 | < 2πV a /L. Namely, now assuming that the frequency (diffused by the diffusion code) of the reference signal is set to f 2 and the frequency of the received signal is set to f 1 , when a difference between f 1 and f 2 is equal to or larger than 2πV a /L (Hz), no correlator output is outputted from the output terminal 3. Therefore, even when the diffusion code by which the reference signal was diffused and the diffusion code by which the received signal was diffused are the same code CODE 1 , since the frequencies are different, the correlation peak is not obtained and even in case of the same code, the discrimination can be performed. Figs. 7A and 7B are schematic diagrams showing situations upon demodulation. Fig. 7A shows the case where the diffusion code of the received signal coincides with the diffusion code of the reference signal and their frequencies also coincide. Fig. 7A shows a state in which the correlator output is derived from the output terminal 3. On the other hand, Fig. 7B shows the correlator output from the output terminal 3 in the case where the frequency of the received signal is away from the frequency of the reference signal by only Δf (Δf > 2πV a /L). Although the diffusion codes are set to the same code CODE 1 , since the frequencies are away from each other by only Δf, no correlator output is obtained. In the embodiment, therefore, in spite of the fact that the terminals T 1 , T 1 ', K 1 , and K 1 ' having the code CODE 1 and the terminals T 2 , T 2 ', K 2 , and K 2 ' having the code CODE 2 possess the same codes CODE 1 and CODE 2 , respectively, by changing their frequencies in each case by Δf, the terminals can be discriminated without mutually interfering. The terminal T 1 can communicate with the terminal T 1 ', the terminal K 1 can communicate with the terminal K 1 ', and the like. In this manner, the communication can be executed. Fig. 8 is a block diagram showing a specific system construction of the embodiment. In the diagram, A denotes constructions of the transmission units of the terminals T 1 and K 1 . B denotes constructions of reception units of the terminals T 1 ' and K 1 '. For example, in the terminal T 1 , the diffusion signal having the center frequency f 1 propagates into the air by the antenna 9 through the amplifier 7 by the local oscillator 5 of the frequency f 1 , the mixer 6, and the code generator 8 having the code CODE 1 . Similarly, the diffusion signal of the center frequency f 1 + Δf which was diffused by the same diffusion code CODE 1 is generated from the terminal K 1 . The reference signal generation units 4 in the reception units B of the terminals T 1 ' and K 1 ' have the reference signals of the frequency f 1 ' and (f 1 ' + Δf) and discriminate the received signals from the terminals T 1 and K 1 by the frequency. That is, although all of the terminals T 1 , K 1 , T 1 ', and K 1 ' use the same diffusion code CODE 1 , since the frequencies of the reference signals which are inputted to the correlators 1 are different, they are not interfered with each other. Fig. 9 is a block diagram showing a specific second system construction of the embodiment. Fig. 8 relates to the case where in the terminals T 1 ' and K 1 ', the frequencies of the local oscillators 12 in the reference signal generation units 4 in the reception units B are the frequencies which were fixed every terminal. Fig. 9 differs from Fig. 8 with respect to a point that the frequency is made variable by using a synthesizer 15. Thus, the communication with a plurality of terminals having the same code can be executed by switching the frequency. In this case, the initial value of the set data S 1 of the control unit 27 is switched in accordance with a frequency of the synthesizer 15. Although the invention has been described with respect to specific embodiments, the invention is not limited to the foregoing embodiments but many modifications and variations are possible within the scope of the appended claims of the invention.

Claims: A receiving apparatus for receiving a spread spectrum signal diffused according to a diffusion code (CODE 1 ), the apparatus comprising; converting means (21) operable to frequency convert the spread spectrum signal to obtain a converted signal at a reception frequency (F 1 ' + ΔF 1 ); control means (27) operable to control the converting means; demodulating means (40) operable to demodulate the converted signal using a corresponding diffusion code (CODE 1 ); correlator means (1) operable to correlate the converted signal with a reference signal diffused according to the corresponding diffusion code so as to generate a synchronising signal; and synchronising means (39) operable to synchronise operation of the demodulating means in accordance with the timing of the synchronising signal; characterised by the control means being operable to control the converting means such that the synchronising signal generated by the correlator means is maintained at a predetermined level. Apparatus as claimed in claim 1 wherein the correlator means comprises a convolver. Apparatus as claimed in claim 2 wherein the convolver comprises a surface acoustic wave device. Apparatus as claimed in any preceding claim wherein the converting means comprises an oscillator means (30) and a mixer means (21) operable to mix the spread spectrum signal with an output signal from the oscillator means; wherein the control means is operable to control an output frequency (f o ) of the output signal in dependence upon the level of the synchronising signal. Apparatus as claimed in claim 4 wherein the reception frequency is an intermediate frequency. Apparatus as claimed in any preceding claim comprising generating means (12, 13, 14) operable to generate the reference signal, the generating means comprising a second oscillator means (12) and a second mixer means (13) operable to mix the output of the second oscillator means with a code signal encoded in accordance with the corresponding diffusion code; and wherein the control means is further operable to control the second oscillator means in dependence upon the synchronising signal. A method of receiving a spread spectrum signal diffused according to a diffusion code, comprising the steps of; frequency converting the spread spectrum signal by operation of a converting means (21) to obtain a converted signal at a reception frequency (F 1 '+ΔF 1 ); controlling the converting means; demodulating the converted signal using a corresponding diffusion code; correlating the converted signal with a reference signal diffused according to the corresponding diffusion code so as to generate a synchronising signal; and synchronising the demodulating in accordance with the timing of the synchronising signal; characterised by the controlling step controlling the converting means such that the synchronising signal generated by the correlation step is maintained at a predetermined level. A method as claimed in claim 7 wherein the converted signal is correlated by means of a convolver. A method as claimed in claim 8 wherein the convolver comprises a surface acoustic wave device. A method as claimed in any of claims 7 to 9 wherein the converting step comprises mixing the spread spectrum signal with an output signal from an oscillator means (30); wherein the controlling step controls an output frequency (f o ) of the output signal in dependence upon the level of the synchronising signal. A method as claimed in claim 10 wherein the reception frequency is an intermediate frequency. A method as claimed in any of claims 7 to 11 comprising generating the reference signal by mixing the output of a second oscillator means (12) with a code signal encoded in accordance with the corresponding diffusion code; and wherein the second oscillator means is controlled in dependence upon the synchronising signal.

Inventors: SAITO KATSUO, SAITO, KATSUO, SAITO, KATSUO, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1993
Main classification: H04B1
IPC codes: H04B1