Spread spectrum communication apparatus

Abstract: A spread spectrum communication apparatus, which can convert a specific pattern into a pattern of which spectrum becomes uniform even if the specific pattern is input, comprises process means for processing parallel data using a plurality of spread codes, and communication means, connected to said process means, for communicating the parallel data processed by the plurality of spread codes, whereby a transfer quality can be improved.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a spread spectrum communication apparatus which performs a code-division multiple communication by using different spread codes. Related Background Art Hitherto, in a spread spectrum communication system utilizing a direct spread manner, a baseband signal having an extremely wide bandwidth as compared with original data is generated from a baseband signal of an ordinarily transferred digital signal at a transmission side by using a spread code series such as a pseudo noise code (PN code) or the like. Further, thus signal is converted into an RF (ratio frequency) signal by performing such a modulation as a PSK (phase shift keying), an FSK (frequency shift keying) or the like, and then transferred. At a reception side, a reception signal is converted into a narrow-band signal having a bandwidth corresponding to the original data by performing an inverse spread. In the inverse spread, a correlation with the reception signal is obtained by using a spread code same as that of the transmission side. Then, an ordinary data demodulation is performed to reproduce the original data. Like this, in the spread spectrum communication system, since a transmission bandwidth is extremely wide for an information bandwidth, only an extremely low transference speed can be realized as compared with an ordinary narrow-band modulation system, on condition that the transmission bandwidth is limited to a certain width. In order to solve the above-mentioned problem, such a method as a code-division multiplexing system has been proposed. In this system, a high-speed information signal is converted into low-speed parallel data, then the converted parallel data are respectively spread-modulated by different spread code series and are added to others. Thereafter, the added data is converted into the RF signal and transferred, thereby realizing a high-speed data transference on the condition of constant transmission bandwidth, without decreasing a spread ratio of spread modulation. Fig. 3 is a block diagram showing the structure of a transmitter in this system. Input data is converted into n parallel data in a serial/parallel converter 301. Converted data are respectively multiplied by different n spread code outputs from a spread code generator 303 in n multipliers 302-1 to 302-n to be converted into wide-band spread signals of n channels. Then, outputs from the multipliers 302 are added to others in an adder 304 and then output to a high-frequency stage 305. The added baseband wide-band spread signal is converted into a transmission frequency signal having an appropriate central frequency and then transmitted through a transmission antenna 306. Fig. 4 is a block diagram showing the structure of a receiver. A signal received at an antenna 401 is appropriately filtered and amplified in a high-frequency signal process unit 402 to be converted into an intermediate frequency signal. The intermediate frequency signal is distributed into channels which respectively correspond to n spread code generators connected in parallel. In each channel, an input signal is detected correlatively with each of outputs from spread code generators 404-1 to 404-n, each output corresponding to the channel in correlators 403-1 to 403-n, then the inverse spread is performed. The inverse spread signals are synchronized for the respective channels in synchronization circuits 405-1 to 405-n to coincide a code phase with a clock in each spread code generator. The inverse spread signals are further demodulated in demodulators 406-1 to 406-n to reproduce data, respectively. Then, the reproduced data are converted into serial data in a parallel/serial converter 407 to reproduce the original information. However, in the above-mentioned prior art, when specific patterns are sequentially input, there occurs such a problem as a spectrum of a multiplexed high-frequency signal does not become uniform and becomes weak in noise. "The article Comparison of MC-CDMA with DS-CDMA using Frequency Domain and Time Domain Rake Receivers' by R.A. Stirling-Gallacher et al. (Wireless Personal Communications 2:105-119, Netherlands, 1995) discloses an orthogonal OFDM/CDMA system having no interleaver or guard interval. In this system, one data symbol is duplicated into N parallel copies. Each branch of the data stream is then multiplied by a chip from a spreading code of length N . The outputs of these multipliers then modulate a set of N carriers. EP-A-0622920 discloses a spread spectrum receiving apparatus comprising a reproducing circuit to reproduce a carrier signal from a reception signal and a diffusion code, a demodulation circuit to demodulate the reception signal into a baseband signal on the basis of the carrier signal reproduced by the reproducing circuit, and a judgement circuit to judge the reception data from the baseband signal from the demodulation circuit. US-A-5425050 discloses a transmission scheme in which a first class of data is transmitted digitally at a very low error rate, and a second class of data is subjected to a Spread Spectrum (SS) processing. In this SS processing, a wideband video signal is divided into a multiplicity N of separate components of correspondingly narrower bandwidth and lower data rate; each sample of each component is multiplied by a different pseudo-random sequence of length N, thus being spread to full channel bandwidth; the N product signals are added and transmitted. According to the inventors, SS processing allows one to adjust the received components in order to obtain a signal of uniform spectrum and noise-like statistical characteristics. EP-A-0682423 discloses a multiple-code division multiple access (MC-CDMA) system allowing a user at a radio transmitter unit to dynamically change its source data bit rate." SUMMARY OF THE INVENTION An object of the present invention is to improve a transference quality. Another object of the present invention is to provide a spread spectrum communication apparatus which capable of converting, even if a specific pattern is input, such the specific pattern into a pattern of which spectrum becomes uniform. Still another object of the present invention is to improve a transference quality without relatively increasing a circuit scale. The above and other objects of the present invention will become apparent from the following description of embodiments and the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing the structure of a transmitter in a first embodiment of the present invention; Fig. 2 is a block diagram showing the structure of a receiver in the first embodiment; Fig. 3 is a block diagram showing the structure of a conventional transmitter; Fig. 4 is a block diagram showing the structure of a conventional receiver; Fig. 5 is a block diagram which indicates a carrier reproduction circuit; Fig. 6 is a block diagram which indicates a baseband demodulation circuit; Fig. 7 is a block diagram which indicates an input data converter in the first embodiment; Fig. 8 is a block diagram which indicates an output data converter in the first embodiment; Fig. 13 is a block diagram which indicates an output data converter in a fourth embodiment of the present invention; and Fig. 14 is a block diagram which indicates an input data converter in the fourth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 is a block diagram showing the structure of a transmitter in a first embodiment of the present invention. Fig. 2 is a block diagram showing the structure of a receiver in the first embodiment. In Fig. 1, at first, an input data converter 101 converts data which is input in serial into n first parallel data, then a predetermined one-to-one mapping conversion is performed to convert the n data into n second parallel data. Multipliers 102-1 to 102-n multiply the obtained parallel data by n spread codes output from a spread code generator 103. The spread code generator 103 generates different (n+1) spread codes (including a spread code PNO for synchronization). An adder 104 adds the spread code PNO for synchronization which is output from the spread code generator 103 and n outputs from the multipliers 102-1 to 102-n. A high-frequency stage 105 converts an output from the adder 104 into a transmission frequency signal. A transmission antenna 106 transmits an output from the high-frequency stage 105 as a radio wave. In Fig. 2, a reception antenna 201 receives the radio wave. A high-frequency signal process unit 202 performs such a process as a filtering or an amplification for a received high-frequency signal. A synchronization circuit 203 captures and maintains a synchronization for a spread code and a clock at a transmission side. A spread code generator 204 generates (n+1) spread codes which are the same as those of the transmission side by a code synchronization signal and a clock signal which are input from the synchronization circuit 203. A carrier reproduction circuit 205 reproduces a carrier signal from the spread code PNO for carrier reproduction output from the spread code generator 204 and an output from the high-frequency signal process unit 202. A baseband demodulation circuit 206 performs a demodulation of baseband by using an output from the carrier reproduction circuit 205, an output from the high-frequency signal process unit 202 and n spread codes PN1 to PNn being outputs from the spread code generator 204. An output data converter 207 performs an inverse conversion of the one-to-one mapping conversion to n to first parallel demodulation data being outputs from the baseband demodulation circuit 206 to produce second parallel demodulation data, to which a parallel/serial conversion is performed. In the above constitution, at the transmission side, input data is first converted into n first parallel data the number of which is equal to a code-division multiple number, by the input data converter 101. Then, the parallel data are converted into second parallel data in accordance with a predetermined one-to-one mapping conversion rule. The input data converter 101 has, for example, a structure shown in Fig. 7. Input serial data is first converted by a serial/parallel converter 701 into n parallel data the number of which is equal to the code-division multiple number. Then, the parallel data are input to a data converter 702. The data converter 702 outputs data of odd order of the input parallel data as it is, and outputs data of even order after inverting its polarity by an inverter. Accordingly, it is possible to convert a pattern of all "1" or all "0" which ordinarily frequently appears in serial input data into such an alternate pattern as "1010...". As a result, it is possible to avoid non-uniformity of transmission spectrum which is produced by multiplexing the pattern of all "1" or all "0" as it is. It should be noted that the inversion of polarity of parallel data is not limited to data of even order. By performing the inversion to data of which optional number is equal to or greater than 1 in all data of parallel data and less than the number of all data, the same effect can be obtained. As to a selection what data should be inverted, it is determined based on spread codes PNO to PNn and frequently-transmitted data. That is, the above determination is performed such that a spectrum of a signal which is obtained by spreading the frequently-transmitted data with the spread codes PNO to PNn and synthesizing them becomes as possible as uniform. On the other hand, the spread code generator 103 generates the different (n+1) spread codes PNO to PNn having the same code period. Among these codes, the spread code PNO is used for the synchronization and the carrier reproduction, and is not modulated by the parallel data but is directly input to the adder 104. The remaining n spread codes are modulated by n parallel data in multipliers 102-1 to 102-n to be input to the adder 104. The adder 104 lineary adds input (n+1) signals and outputs an added baseband signal to the high-frequency stage 105. It should be noted that the data converter 702, the multipliers 102, the spread code generator 103, the adder 104 can be gathered in a table conversion memory which outputs an output of the adder 104 upon inputting an output of the serial/parallel converter 701. The baseband signal is then converted into a high-frequency signal having an appropriate central frequency at the high-frequency stage 105 to be transmitted through the transmission antenna 106. At a reception side, a signal received at the reception antenna 201 is appropriately filtered and amplified in the high-frequency signal process unit 202 and is output in such a state as the signal is a transmission frequency band signal as it is, or in such a state as the signal is converted into an appropriate intermediate frequency band signal. The signal is input to the synchronization circuit 203, in which a spread code synchronization and a clock synchronization for a transmission signal are established by using a reference spread code PNO input from the code generator 204, and the code synchronization signal and the clock signal are output to the spread code generator 204. The reference spread code PNO is a code obtained by time-inverting a spread code for synchronization at the transmission side, in a case where a convolver is used in the synchronization. The synchronization circuit 203 and the code generator 204 entirely constitute a kind of phase-lock-loop. In such a state as the synchronization is not established, since there is a phase difference between a correlation peak signal and a spread code start signal, a spread code clock is advanced (or delayed), whereby the phase difference between a spread code component for the synchronization included in a reception signal and the reference spread code is slowly decreased. When both phases are coincided with each other, that is, after the phase difference becomes "0", this phase difference is controlled to become "0". After establishing the synchronization, the spread code generator 204 generates the spread codes PNO to PNn of which clocks and spread code phases are coincided, for spread codes at the transmission side. Among these codes, the spread code PNO for synchronization is input to the carrier reproduction circuit 205. In the carrier reproduction circuit 205, a reception signal converted into a transmission frequency band or an intermediate frequency band being an output from the high-frequency signal process unit 202 is inverse spread by the spread code PNO for synchronization to reproduce a carrier of the transmission frequency band or the intermediate frequency band. As the carrier reproduction circuit 205, a circuit utilizing the phase-lock-loop, for example, shown in Fig. 5 is used. In Fig. 5, the reception signal is multiplied by the spread code PNO for synchronization in a multiplier 501. After establishing the synchronization, clocks and code phases of the spread code for synchronization are coincided with those of the reference spread code for synchronization, in the reception signal. Since the transmission-side spread code for synchronization is not modulated by data, so that an inverse spread is performed in the multiplier 501 to obtain an output in which a carrier component appears. The output is then input to a band-pass-filter 502 to eliminate the carrier component and is output. Then, the output is input to the phase-lock-loop consisting of a phase detector 503, a loop filter 504 and a voltage control oscillator 505, and a signal phase locked with the carrier component. Thereafter, a signal of which phase is locked into the carrier component output from the band-pass-filter 502 is output from the voltage control oscillator 505, as a reproduction carrier. The reproduced carrier is input to the baseband demodulation circuit 206, in which a baseband signal is produced from this reproduction carrier and an output from the high-frequency signal process unit 202. The baseband signal is distributed to n branches, which are then inverse spread every code-division channel by the spread codes PN1 to PNn being outputs from the spread code generator 204 and are data demodulated. , The baseband demodulation circuit 206 is, for example, constituted as shown in Fig. 6. In Fig. 6, an input reception signal is multiplied by the reproduction carrier in a multiplier 601, and the reception signal is converted into the baseband signal with eliminating a needless signal by a low-pass-filter 602. The baseband signal is converted into a digital signal having a resolution of one or plural of bits in an A/D converter 603 having a reproduction clock as a sample period. The digital signal is distributed to n branches. In each branch, a most significant bit (code bit) of the digital signal is exclusive ORed with each of the spread codes PN1 to PNn being outputs from the spread code generator 204 in exclusive OR circuits 604-1 to 604-n to be input to adders 605-1 to 605-n together with other bits. In the adders 605-1 to 605-n, the input signals are added to outputs of registers 606-1 to 606-n at each reproduction clock pulse to be output to the registers 606-1 to 606-n, respectively. The registers 606-1 to 606-n are reset when a head bit of each spread code is input, thereafter, an added result of the product of the reception signal by the spread code is input for one period of spread code. Therefore, when a final bit for the one period of spread code is input, a correlation value with each spread code for one period and the reception signal is to be stored in the registers 606-1 to 606-n. By performing a data judgement to the correlation value in following judgement circuits 607-1 to 607-n, n parallel demodulation data can be obtained. The inverse conversion of the one-to-one mapping conversion, which mapping conversion is performed in the input data converter 101, is performed to the demodulated n parallel demodulation data in the output data converter 207. Then, the n parallel demodulation data are converted into serial data to be output. As the output data converter 207, a circuit, for example, shown in Fig. 8 is used. The parallel demodulation data are input to a data inverse converter 801. The data inverse converter 801 outputs data of odd order of the input parallel data as it is, and outputs data of even order after inverting its polarity by the inverter. Accordingly, the inverse conversion of the one-to-one mapping conversion, which was performed at the transmission side, is performed, and the data of before conversion is reproduced. Then, the parallel data being outputs from the data inverse converter 801 are converted into serial data by a parallel/serial converter 802 and is output as a demodulation data sequence. When an error is occurred in reception data, since a calculation among the reception data is never performed in the above sequential mapping conversion and the inverse conversion, an error of unity symbol does not influence other symbol, as a result, a communication quality is not deteriorated. (Second Embodiment) Figs. 13 and 14 show a configuration of an input data converter 101 and an output data converter 207 in a fourth embodiment, in the above-mentioned case. Other configurations are common to those of Figs. 1 and 2. A data converter 702A and a data inverse converter 801A consist of exclusive OR circuits, respectively. In the present embodiment, the data converter 702A is set to output data of odd order of the input parallel data as it is, and output data of even order after inverting its polarity, in response to setting data from a setting unit 702B. An operator can arbitrarily change a conversion rule of a one-to-one mapping conversion externally by using the setting unit 702B. The data inverse converter 801A has been preset, by a setting unit 801B, to perform an inverse conversion of the one-to-one mapping conversion, which is performed by the data converter 702A at a transmission side. In the present embodiment, the data inverse converter 801A outputs data of odd order of the input parallel data as it is, and outputs data of even order after inverting its polarity. Accordingly, the inverse conversion of the one-to-one mapping conversion, which was performed at the transmission side, is performed. Therefore, data of before converting at the transmission side is reproduced. It should be noted that an inverse conversion rule of the data inverse converter 801A is set by the operator in the setting unit 801B in accordance with setting data set in the setting unit 702B at the transmission side. As explained above, the present invention has been described based on the preferred embodiments. However, the present invention is not limited to the above-mentioned embodiments, so that various modifications are possible within the scope of the appended claims.

Claims: A spread-spectrum communication apparatus comprising: processing means (101, 102, 104, 206, 207) for processing parallel data using a plurality of spread codes, each of the parallel data being associated with a corresponding spread code, and communication means (105, 202), connected to said processing means, for communicating the parallel data processed by the plurality of spread codes, characterized in that said processing means includes inversion means (702, 801, 702A, 801A) for inverting predetermined bits of data included in a plurality of the parallel data so that the spectrum of a signal, which is communicated by said communication means, becomes uniform. An apparatus according to claim 1, characterized in that selection of which data should be inverted is determined on the basis of said spread codes and of the frequently-transmitted data. An apparatus according to claim 1, characterized in that said processing means includes setting means (702B, 801 B) for desirably setting bits to be inverted by said inversion means (702A, 801 A). An apparatus according to claim 1, characterized in that said processing means includes baseband-conversion means (601,602) for converting parallel data received by said communication means into a baseband signal, and inverse-spreading means (604) for inverse-spreading the baseband signal by the plurality of spread codes. An apparatus according to claim 1, characterized in that said processing means (101) includes serial-to-parallel conversion means (701) for converting input serial data into output parallel data, said serial-to-parallel conversion means being placed before said inversion means.

Inventors: KATO ICHIRO, KATO, ICHIRO, KATO, ICHIRO, C/O CANON K.K.
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1996
Main classification: H04B1
IPC codes: H04B1, H04J13