JPH0832487A - Spread spectrum communication system - Google Patents

Abstract

PURPOSE:To provide the spread spectrum communication system provided with amplitude information similar to that of the QAM modulation system through it is a modulation system keeling a fixed amplitude. CONSTITUTION:A QAM coding section 12 maps input data onto I, Q phase planes and changes each phase and each amplitude. Spread sections 15, 16 apply spread spectrum processing to the I, Q by the same spread code. Multipliers 18, 19 multiply a sin component and a cos component of a local signal 17 and a synthesizer 20 synthesizes the products. A 1/2 distributer 22 distributes the synthesized product into I and Q components and multipliers 23, 24 multiply a local signal with the I and Q components and the product is digitized and given to correlation devices 30, 31, in which the amplitude component is discriminated and its correlation output is demodulated by a QAM demodulation section 35.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used for transmitting digital data in wireless communication or wire communication,
In particular, it relates to a spread spectrum communication system using direct spread.

[0002]

2. Description of the Related Art Conventionally, various modulation systems have been used for wireless data communication, and among them, the spread spectrum communication system has attracted attention as a new communication system. In Japan as well, use in the 2.4 GHz ISM band (industrial scientific medical band) has been approved, and future development is expected.

Among such spread spectrum communication systems, the direct spread system has already been partially put to practical use. An example of its basic configuration is shown in FIG. 9, and its spectrum is shown in FIG. In the transmission system, information data 1 is modulated by an information modulator 2. This is commonly called primary modulation. The spread spectrum modulator 3 secondarily modulates the information modulated signal by the spread code from the code generator 4 to form a wide band signal, which is transmitted from the transmitting antenna 5 as an RF signal.

On the other hand, in the receiving system, the RF signal is received by the receiving antenna 6 and despread by the spread spectrum demodulation section 8. As the code for despreading, the same spreading code used at the time of transmission is used to perform despreading. As the despreading method, there are a passive type despreading method and an active type despreading method. In this figure, the despreading method is an active type, and a synchronous code is sent from the synchronizing circuit unit 7 to perform despreading. It is carried out. Thereby, the secondary modulation signal is demodulated. After that, the information demodulation unit 9 performs general demodulation to demodulate the primary modulation component, and outputs data 10 as information. FIG. 10 shows the frequency spectrum after the primary modulation and the frequency spectrum after the secondary demodulation at this time.

In the spread spectrum communication system, FIG.
As shown in 0, in order to expand the band B of the primary modulated wave by n times for communication, a transmission band of n × B is required. Try to calculate the required transmission band.

[0006] Suppose that, when transmitting at 256 kbps using direct spread spectrum with a spreading factor of n = 127, if BPSK is used for primary modulation, a transmission band of about 64 MHz is required, and QPSK (Quadrature Phase Shift) is required for primary modulation.
Keying: Quadrature phase shift keying) requires a transmission band of about 32 MHz.

By the way, the bandwidth recognized in the ISM band in Japan is 26 MHz, and in order to effectively use this frequency band, it is necessary to use a modulation system with higher frequency efficiency because of the above. come.

On the other hand, 8PSK is used for the primary modulation.
Phase modulation method such as modulation method or 16PSK modulation method,
16QAM modulation system or 64QAM modulation system (QAM: Qu
adrature Amplitude Modulation). These are methods well known as means for narrowing the band in general communication.

[0009]

As described above, by narrowing the primary modulation band, it is possible to effectively use the ISM band of 26 MHz, but these systems have the following problems.

First, there is a phase modulation method.
There is a problem that the code error rate characteristic deteriorates rapidly as the number of phases increases. This characteristic is shown in FIG. It can be seen that the code error rate is relatively low in the two-phase modulation and the four-phase modulation, but the characteristics are greatly deteriorated as the number of phases is increased to eight-phase modulation and sixteen-phase modulation. Therefore, in reality, a large number of phases is rarely used.

Next, the QAM modulation system, which has less deterioration in the code error rate characteristic than the phase modulation system, is being put to practical use in a microwave fixed line for terrestrial use.
This characteristic is shown in FIG.

However, the QAM modulation system has information in the amplitude term, and if the circuit has a non-linear portion, the amplitude information is destroyed. Therefore, a circuit having a linear characteristic must be used for the transceiver. As a result, the device becomes expensive. Also, when the reception level changes greatly like in a mobile object, a large dynamic range is required, and in reality it becomes difficult to maintain linearity over the entire range. Further, the amplitude value becomes unstable even with frequent fading, which causes an error and is difficult to apply to a moving body.

The present invention was devised in view of such circumstances, and provides a spread spectrum communication system capable of providing amplitude information similar to that of the QAM modulation method while being a modulation method which maintains a constant amplitude. It is intended to be provided.

[0014]

In a spread spectrum communication system according to the present invention, input data is divided into blocks of several bits, and 2 ⁿ states are converted into I and Q components of a signal phase according to the number of bits n. Corresponding to the corresponding phase diagram, I,
In a QAM modulation / demodulation system that changes the phase and amplitude of each Q, the transmitter side has a spreading means for spreading each I phase and Q phase with the same spreading code, and the spreading code is partly spread according to the amplitude component. The receiver side separates the received signal into I and Q components, inputs each to the correlator that correlates with the same spreading code as the transmitter side, and determines the amplitude component from the output of the correlator. It is characterized in that it is then demodulated.

[0015]

Since the QAM modulation method is used, the bandwidth at the time of primary modulation is narrow, and as a result, a large spreading gain can be obtained in a limited band and high speed transmission can be performed. Furthermore, since the amplitude value is obtained by the correlation of the codes, there is no information in the amplitude component of the carrier and the information is propagated only in the phase component, even though it is a QAM modulation method, and therefore there is almost no deterioration even when using a circuit having nonlinear amplification. Does not happen. Further, the present invention can be applied to a field such as mobile communication having a large level fluctuation, which has been difficult to apply conventionally.

[0016]

Embodiments of a spread spectrum communication system according to the present invention will be described below in detail with reference to the drawings.

[First Embodiment] FIGS. 1 and 2 are circuit block diagrams of a spread spectrum communication system according to the first embodiment. FIG. 1 shows a transmitter side and FIG. 2 shows a receiver side. Hereinafter, description will be given with reference to the drawings.

First, on the transmitter side, information data is input from the data input terminal 11 and is mapped by the QAM encoding section 12 to the I and Q phase planes. That is, the input data is divided into blocks of several bits, and the number of bits n
2 ⁿ states are made to correspond to the phase diagram corresponding to the I component and the Q component of the signal phase, and the respective phases and amplitudes are changed. Then, the I and Q modulation information 13 and 14 are input to the respective spreading units 15 and 16. These spreading units 15 and 16 spread the I phase and the Q phase with the same spreading code. In the spreading units 15 and 16, the modulation information 1
3 and 14, the number of chips corresponding to the amplitude information is matched with the specified code according to the amplitude value and modulated. For example, when a code of 127 chips is used, if the amplitude value is 1, all 127 chips are modulated with a matched code, and if the amplitude value is 0.35, 127 × 0.35 = 44. At 45, modulation is performed with a code that matches 44 chips. This means that the spreading code is partially changed according to the amplitude component.

The I and Q signals thus spread are
The sin and cos components of the local signal 17 are multiplied in multipliers 18 and 19 and up-converted into mutually orthogonal IF signal components. After that, the multiplexer 20
The signals are combined into a IF signal and transmitted as an IF signal.

On the other hand, on the receiver side, the signal input section 2
The IF signal received from 1 is divided into two by the two-way divider 22 into I and Q components, which are respectively multiplied by the sin and cos components of the local signal 25 by the multipliers 23 and 24, and down-converted into a baseband signal. It Then, through the filters 26 and 27, the comparators 28 and 2
1 bit is quantized by 9 or the like. The digitized I and Q signals are input to correlators 30 and 31 that correlate with the same spreading code as the transmitter, and are output as correlation outputs. The carrier & clock recovery circuit 32 determines the phase information of the carrier and the timing of the correlation based on the correlation output. Then, the correlation outputs are latched by the latch circuits 33 and 34, and the QA is extracted from the despread signal.
The M demodulation unit 35 performs QAM demodulation and outputs the obtained data output 36.

In the above, the correlation output outputs as many amplitude values as there are codes. That is, the amplitude value 127 is output if all 127 match 127 chips, and the amplitude value 44 is output if 44 match. If this system uses 16QAM, the amplitude value is 1
(127), 0.34 (44), -0.34 (-4
4) and -1 (-127) take four values, so that the amplitude determination can be performed by setting the determination threshold value in between. As a result, the amplitude information can be obtained from the signal having a constant amplitude at the carrier stage.

As described above, a QAM signal having amplitude information can be demodulated from the QPSK-modulated signal. As a result, even if a circuit having a non-linear portion is used, the normal Q
It does not deteriorate like AM signals and can be applied to applications requiring a wide dynamic range such as mobile communications.

Although QPSK modulation is used in the description of the present embodiment, it is possible to use QPSK modulation, OQPSK, MSK, GMSK, etc.
The present invention can be applied to any modulation method as long as it performs modulation orthogonal to the I and Q axes.

Further, in this embodiment, the I component and the Q component are separated by using the two dividers, but this is an example.
Even if a degree divider is used or a local signal is provided with a 90 degree phase shifter, the generality is not lost at all.

As for the correlator, a 1-bit digital correlator is shown here, but even if a multi-bit digital correlator or an analog correlator is used, the essence of the present invention is not lost and can be properly implemented.

[Second Embodiment] Next, a second embodiment will be described with reference to FIG. FIG. 3 shows only the demodulation side. This is an example in which the present invention is applied to a despreading method of spread spectrum called DLL, and is an example of application to a so-called active despreading method ("Spread spectrum communication system" by Mitsuo Yokoyama, Science and Technology Publishing Company. ……reference).

The IF signal input from the signal input unit 37 is 3
It is distributed and input to the multipliers 38, 39 and 40, respectively. To the multipliers 38, 39 and 40, three systems of signals 51, 52 and 53 shifted by half a chip from the DLL control circuit 50 are input. These are an early signal, a late signal, and a punct, respectively.
called ual signal.

The multiplier 39 inputs the input signal and the late signal 52.
The late narrowband signal 42 multiplied by and is input to one input terminal of the differential amplifier 48 as a signal 46 via the envelope detector 44, and the early signal obtained by multiplying the input signal and the early signal 51 by the multiplier 40. The narrow band signal 43 is input to the other input terminal of the differential amplifier 48 as a signal 47 via the envelope detector 45. And differential amplifier output 4
The DLL control circuit 50 is controlled by 9. That is, the DLL control circuit 50 is controlled so that the chip is delayed if the early side signal is large and the chip is advanced if the late side signal is large. As a result, the early signal 51 and lat are always
The punctual signal 53 in the middle of the e signal 52 is controlled so that maximum correlation can be obtained. And
The punctual narrowband signal 41, which is obtained by multiplying the input signal and the punctual signal 53 by the multiplier 38, is despread (secondary demodulated) and becomes a signal that is only primary modulated.

At this time, although the input signal and the punctual narrowband signal 41 are the same IF signal, the puncttu
The al narrowband signal 41 is despread and narrowed into a narrow band, and is converted into a QAM signal having an amplitude term by the correlation with the code by which the punctual signal 53 is multiplied.
That is, 127 chips of the punctual signal 53 have an amplitude value according to the number of codes matched on the transmitter side, and the modulated signal has amplitudes in the I and Q terms, respectively. That is, it is converted into a general QAM modulated wave. Therefore, after this, the QAM demodulation unit 54 causes the general QA
A received signal can be obtained by demodulating similarly to the M signal, and the data output 55 is performed.

As described above, the present invention can be applied not only to the passive type, but also to the active type spectrum despreading, and the advantages thereof are the same as those of the first embodiment.

Next, various modified embodiments of the present invention will be described.

In the first embodiment, spreading of I and Q phases is performed with the same code. However, in the present invention, the same code need not be used, and the I and Q phases may be spread by different codes. Generally, in the case of a modulation method such as QAM, the carrier synchronization is I phase or Q phase.
Since the carrier is locked in synchronization with either of the phases, there is no distinction between the I phase and the Q phase.
Determine the phase and Q phase. However, when the I phase and the Q phase are spread by different codes, the I phase and the Q phase can be uniquely determined. This may be applied to passive despreading or active despreading.

As shown in FIG. 4, the structure of the data to be transmitted is such that the packet 100 has a structure including a preamble part 101 and a data part 102 in which previously known data is stored. Having the known preamble part in this way makes it possible to clarify the data part and to utilize this known preamble part for correlation. Since the data transmitted in the preamble part is known,
When it is applied to the QAM modulation method, it is possible to predict what kind of phase and amplitude it will have.

-1. In the QAM demodulation, the calculation of the reference amplitude value and the carrier reproduction are important parameters, but by using the known reference in the preamble part, the calculation of the amplitude value and the carrier reproduction can be performed at high speed. You will be able to do it.

-2. Further, fluctuations in the propagation path can be considered as a cause of a great change in the phase and amplitude of the received signal, but under a use environment where there is no fluctuation such that the phase and amplitude change suddenly in the same packet, By using the amplitude value calculated in the preamble part and the carrier reproduction to form an open loop, the circuit can be simplified.

-3. Further, it is necessary to take the correlation timing from the correlation output and control the despreading as a characteristic of the spread spectrum. However, in the modulation method in which the amplitude changes like the QAM modulation method, the correlation output changes. Therefore, it takes a long time to synchronize the codes. Therefore, high-speed code synchronization can be achieved by performing correlation while predicting the amplitude value using the known signal of the preamble part.

As the data to be transmitted, similarly to the above, known data has a data structure in which it is transmitted as a preamble portion. Then, for the modulation of the preamble part, a general phase modulation such as QPSK is used as the primary modulation. In this case, the transmitter side can be realized by making the amplitude term of the QAM modulator maximum or constant.

-1. Since the data to be transmitted is known in the preamble part and the amplitude term is constant, it is possible to predict what phase it has. this is,
Compared with, the synchronization can be performed at a higher speed because the amplitude term is constant.

-2. Also, by using the amplitude value calculated in the preamble part and the carrier reproduction in the same manner as in (2), the circuit can be simplified.

-3. Furthermore, even when the correlation is obtained, since the amplitude is constant, the correlation can be easily obtained as compared with, and the code can be synchronized at a higher speed.

In the above description, phase modulation without an amplitude component is used in the preamble part, and known data is sent in this preamble part. However, known data is transmitted by performing phase modulation in the preamble part. The amplitude value can be calculated and the carrier can be reproduced without doing so. First, for the amplitude term, only phase modulation is used, so
The amplitude is always constant regardless of the content of the data, and therefore the amplitude value can be calculated. Next, since the phase term also has a constant amplitude, it is possible to identify the phase by a tangent calculation method (tan ⁻¹ (Q / I)) used for normal phase modulation. Therefore, it is characterized in that not the known data but the ID and various information are sent to the preamble portion.
Although the method for determining the phase is a little more complicated than that of, the amount of data that can be sent can be increased and the efficiency of use of radio waves can be improved by sending various information to the preamble part.

Third Embodiment Next, a third embodiment will be described. 5 and 6 are circuit block diagrams of the spread spectrum communication system according to the third embodiment. FIG. 5 shows the transmitter side and FIG. 6 shows the receiver side. Hereinafter, description will be given with reference to the drawings.

First, on the transmitter side, information data is input from the data input terminal 11 and is mapped by the QAM encoding unit 12 on the I and Q phase planes. That is, the input data is divided into blocks of several bits, and the number of bits n
2 ⁿ states are made to correspond to the phase diagram corresponding to the I component and the Q component of the signal phase, and the respective phases and amplitudes are changed. Then, the I and Q modulation information 13 and 14 are input to the respective spreading units 15 and 16. These spreading units 15 and 16 spread I and Q with the same spreading code. In the spreading units 15 and 16, the modulation information 13,
According to 14, the number of chips corresponding to the amplitude information is matched with the specified code according to the amplitude value and modulated.
This partly changes the spread code depending on the amplitude component,
That's what it means. The I and Q signals thus spread are multiplied by the sin and cos components of the local signal 17 in multipliers 18 and 19 and up-converted into mutually orthogonal IF signal components. After that, the signals are multiplexed by the multiplexer 20 and output as an IF signal.

On the other hand, known, constant or random data is input from the other signal input section 56. This is a pilot signal used for timing of despreading or for carrier reproduction or calculation of amplitude value. This data is spread by another spreading code in another spreading unit 57, and then is multiplied by the local signal 59 which is the same as the local signal 17 in the multiplier 58, and becomes the pilot signal 60. The pilot signal 60 is combined with the signal from the combiner 20 by a combiner 61, and is output as an IF signal 62.

On the receiver side, the IF signal received from the signal input section 21 is divided into two by the two-way divider 63,
One is input to the two-way divider 22 and the other is applied to the multiplier 66.
Is input to The two-way divider 22 divides the I-component and the Q-component into two, and the multipliers 23 and 24 respectively multiply the sin and cos components of the local signal 65 to down-convert to a baseband signal. After that, 1-bit quantization is performed by the comparators 28, 29, etc. via the filters 26, 27. The digitized I and Q signals are input to correlators 30 and 31 that correlate with the same spreading code as the transmitter, and are output as correlation outputs.

On the other hand, the signal input to the multiplier 66 is multiplied by the local signal 65 and input to the correlator 71 via the filter 69 and the comparator 70 to obtain a correlation output.
The spreading code of the correlator 71 is the same as that of the spreading unit 57 so that the correlation can be obtained. Based on the correlation output of the correlator 71, the carrier & clock recovery circuit 72
The phase information of the carrier and the timing of correlation are determined. Then, the correlation outputs are latched by the latch circuits 33 and 34, and the QAM demodulation unit 35 outputs the QA
The M data demodulation is performed and the obtained data output 36 is performed. Reference numeral 73 is a timing signal.

In this case, the carrier & clock recovery circuit 7
No. 2 operates based on the pilot signal, but since this pilot signal is only phase-modulated and not QAM-modulated, it can be synchronized at a higher speed than in the first embodiment. This has the same effect as. Moreover, since the pilot signals are sent at the same time, the ratio of the data part in the packet is large (up to 100%) compared to the case where the preamble part is sent, and it is possible to improve the utilization efficiency of radio waves. This is because the multiplicity of spread spectrum is used, and if the codes are different, the mutual correlation does not exert an influence. Therefore, by making the code of the pilot signal orthogonal to the code for QAM modulation, the influence on the QAM signal can be eliminated.

[Fourth Embodiment] Next, a fourth embodiment will be described. 7 and 8 are circuit block diagrams of the spread spectrum communication system according to the fourth embodiment. FIG. 7 shows the transmitter side and FIG. 8 shows the receiver side. The basic configuration is the same as in FIGS.

On the receiver side, a latch circuit 75 and a demodulation section 76 are added to the output side of the correlator 71, and the data output 7
Do 7. 74 is a timing signal. In the case of this embodiment, the use efficiency of the radio wave can be improved by including the ID and data in the pilot signal.

[0050]

As described above, the present invention uses the phase modulation method in which the carrier amplitude is constant and applies the autocorrelation characteristic of spread spectrum to transmit the amplitude information. As a result, QA
Modulation equivalent to M modulation can be performed, and the same signal can be transmitted in a narrow band while maintaining the same spreading factor. Further, since there is no information on the amplitude of the carrier, the carrier amplitude can be applied to a circuit having a non-linear portion, and as a result, a wide dynamic range can be realized, which facilitates application to mobile communication and the like.

[Brief description of drawings]

FIG. 1 is a circuit block diagram of a transmitter side of a spread spectrum communication system according to a first embodiment of the present invention.

FIG. 2 is a circuit block diagram on the receiver side of the first embodiment.

FIG. 3 is a circuit block diagram on a receiver side of a second embodiment.

FIG. 4 is a diagram showing a data structure used in a modified example.

FIG. 5 is a circuit block diagram of a transmitter side of a third embodiment.

FIG. 6 is a circuit block diagram on the receiver side of a third embodiment.

FIG. 7 is a circuit block diagram on the transmitter side of a fourth embodiment.

FIG. 8 is a circuit block diagram on a receiver side of a fourth embodiment.

FIG. 9 is a block diagram showing a basic configuration of a conventional spread spectrum communication system.

FIG. 10 is a diagram showing a frequency spectrum when spectrum spreading is performed.

FIG. 11 is a characteristic diagram showing a PSK code error rate.

FIG. 12 is a characteristic diagram showing a code error rate of a modulation method having 16-value change points.

[Explanation of symbols]

 12 ... QAM encoding unit 13 ... I information 14 ... Q information 15, 16 ... Spreading unit 17 ... Local signal 18, 19 ... Multiplier 20 ... Multiplexer 22 ... 2 Distributor 23, 24 ... Multiplier 30, 31 ... Correlator 32 ... Carrier & clock recovery circuit 35 ... QAM demodulator 38-40 ... Multiplier 41 ... Punctual narrowband signal 42 ... Late narrowband signal 43 ... Early narrow band signal 44, 45 ... Envelope detector 48 ... Differential amplifier 50 ... DLL control circuit 51 ... Early signal 52 ... Late signal 53 ... Punctual signal 54 ... QAM demodulation unit 57 ... Spreading 58: Multiplier 59: Local signal 60: Pilot signal 61: Multiplexer 63 ... Two distributors 65 ... Local signal 71 ... Correlator 72 ... … Carrier & clock recovery circuit 76 …… Demodulator

Claims (2)

[Claims] 1. The input data is divided into blocks of several bits, and 2 ⁿ states are made to correspond to a phase diagram corresponding to I and Q components of a signal phase according to the number n of bits, and the phases of I and Q respectively. In a QAM modulation / demodulation system that changes the amplitude and the amplitude, the transmitter side has a spreading means for spreading each of the I phase and the Q phase with the same spreading code. On the machine side, the received signal is separated into I and Q components, each is input to a correlator that correlates with the same spreading code as on the transmitter side, and the amplitude component is judged and demodulated by the output of the correlator. A spread spectrum communication system characterized by the above. 2. The spread spectrum communication system according to claim 1, wherein the transmitter side prepares a separate spreading code in addition to the signal to be transmitted, and transmits the signal spread by the different spreading code. It has a means for multiplexing and transmitting the spread signal of the signal, and on the receiver side, in addition to the despreading means for demodulation, another despreading means corresponding to the other spreading code is provided. Spread spectrum communication system.