JP2000349690A - Code division multiple connection communication method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a transceiver in a simple processing procedure and also in a simple device constitution to suppress the non-desired waves having higher signal levels than a desired wave and to extract only the desired wave by offsetting the carrier frequency of a signal to be multiplexed by an amount equivalent to the symbol transmission frequency. SOLUTION: In a data communication method using a spread spectrum modulation system of a direct spread type, the carrier frequency is offset by an amount equivalent to 1/n symbol transmission frequency to the carrier frequency of another series to be multiplexed if an integrating section where the mutual correlation value is calculated is set in an n-symbol cycle (n: an integer of 2 or more) of a spread code. In a code division multiplex communication system, for example, a transmitter multiplexes and transmits a transmitting signal I of a system 1 and a transmitting signal Q of a system 2 after spreading them. Under such conditions, the non-desired waves of the system 1 belong to the system 2 with the non-desired waves of the system 2 belonging to the system 1 respectively. Thus, it is possible to approximate the mutual correlation value set between the desired and non-desired waves to zero by offsetting the carrier frequency.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spread spectrum communication system, and more particularly, to a code division multiple access communication system which suppresses interference between channels in a direct spread system.

[0002]

2. Description of the Related Art A spread spectrum communication system is a system for performing communication by spreading a spectrum component of a signal to be transmitted over a wide frequency band, and a direct spread system is a PN (Pseudo) system.
The signal is spread using a special waveform called a "random noise" sequence. FIG. 2 is a block diagram of two sets of code division multiple access communication systems using the direct spreading method. The transmitter has two transmission signals I and Q. Each transmission signal is spread by a spreader with a waveform from a PN sequence generator, and the modulated signals of both systems modulated by the modulator are multiplexed. And radiated from the transmission antenna 8. Note that the PN sequence I and the PN sequence Q are different PN sequences. In the receiver, after the transmission signal from the transmitter is received by the receiving antenna 9 and split by the splitter 10,
The signal demodulated by the demodulator of each system is despread by the despreader by the waveform from the PN sequence generator, and the received signal I
And Q will be reproduced.

However, undesired waves other than the desired wave to be despread are also superimposed on the signals despread by the respective despreaders. Despreading is performed by taking the maximum value of the autocorrelation of the PN sequence of the desired signal.
Since there is no PN sequence combination in which the cross-correlation value between the PN sequence of the desired wave and the PN sequence of the undesired wave is always zero, the cross-correlation is interference by the undesired wave. In the prior art, in consideration of such characteristics, multiplexing is performed using a PN sequence whose cross-correlation is sufficiently close to zero. There is almost no interference effect. However, in recent usage forms, there is a case where a non-desired wave having a signal level higher than the signal level of the desired wave is superimposed on the transmitting side,
In the reproduction process on the receiving side, there is a possibility that the cross-correlation value between the desired wave and the non-desired wave becomes larger than the auto-correlation value between the desired waves. In this case, the desired signal cannot be correctly despread, and an error occurs in the received signal. Therefore, if there is a difference in signal levels to be multiplexed, a signal having a large signal level may have an undesired effect on a small signal. The conventional technology cannot solve it. Therefore, an object of the present invention is to realize a transceiver capable of extracting only a desired wave by suppressing interference caused by a non-desired wave having a signal level higher than that of the desired wave, with a simple processing procedure and a simple device configuration.

[0003]

The code division multiplex communication of the present invention suppresses interference from undesired waves. More specifically, this is realized by offsetting the carrier frequency of the signal to be multiplexed by the symbol transmission frequency. Further, the present invention is characterized in that the present invention can be applied to a case where integration for obtaining a correlation value is performed over two or more symbol sections. Specifically, an offset amount as shown in Expression 8 may be given. By offsetting the carrier frequency to be multiplexed by the symbol transmission frequency, the cross-correlation value between the desired wave and the undesired wave becomes zero regardless of the PN sequence or the like. Further, with the extension of the integration section, the carrier frequency offset amount for making the cross-correlation value zero becomes smaller.

[0004]

Next, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram of a code division multiplex communication system according to an embodiment of the present invention. The spreading method is a direct spreading method, the number of multiplexing is two, and the modulation method is a PSK method. The transmitter comprises a multiplier 21, I
PN sequence generator 22, multiplier 23, oscillator 24, multiplier 25, PN sequence generator 26 for Q, multiplier 27, oscillator 28, adder 29, and transmission antenna 30. After the transmission signal Q of the system 2 is spread, it is multiplexed and transmitted. The transmission rates of the system 1 and the system 2 are the same, and the PN prepared for each system is used for the spreading process.
One symbol is spread by a sequence generator using a PN pattern sequence of one cycle. The receiver is receiving antenna 3
1, multiplier 32, oscillator 33, multiplier 34, PN sequence generator 35 for I, LPF (Low Pass Filter)
r) 36, a multiplier 37, an oscillator 38, a multiplier 39, a PN sequence generator 40 for Q, and an LPF 41.
The meanings of the character variables in the figure are as follows. a1
(T) is a transmission signal α, c1 (t) is a PN sequence for I, f1 is a modulation wave frequency for I, ω1 is a modulation wave angular frequency for I, ω2 is a modulation wave angular frequency for Q, and τ is a transmission signal. The time difference between the signal I and the transmission signal Q, σ1 is the oscillator 33 and the PN sequence oscillator for I 35
The time difference σ 2 between the oscillator 38 and the Q PN sequence generator 4
Indicates the time difference between 0. F1 and f2 are given by f2 = f1 + Δf
(Δf is an offset frequency). In the system 1 in the transmitter, the PN sequence for I generated by the PN sequence generator for I 22 is multiplied by the product of the transmission signal I and spread,
The sine wave (cos ω1t) generated by the oscillator 24 and its spread wave are multiplied by the multiplier 23 and modulated. Similar processing is performed in the system 2 in the transmitter, and the transmission signal Q is spread with a Q PN sequence and modulated with a sine wave (cos ω2t). The two modulated waves are multiplexed by the adder 29 and radiated from the transmission antenna 30. The signal radiated from the transmitting antenna 30 is represented by the following equation.

(Equation 1) The modulated wave received by the receiving antenna 31 is distributed to the systems 1 and 2 of the receiver. The modulated wave received by the receiving antenna 31 is distributed to the system 1 of the receiver and the system 2 of the receiver. In system 1 of the receiver, the sine wave (c
osω1t) and the modulated wave are multiplied and multiplied by the multiplier 32 and demodulated.
The PN sequence for I generated by the PN sequence generator 35 for I and the demodulated wave are multiplied by the multiplier 34 to despread. The despread signal passes through the LPF 36 and becomes a received signal I. Similar processing is performed in the receiver system 2, and a signal demodulated with a sine wave (cos ω2t) and despread with a PN sequence for Q passes through the LPF 41 to become a received signal Q.
In the system 1, the equation of the demodulated and despread signal is

(Equation 2) Becomes Here, considering that f1 >> Δf,

(Equation 3) Can be approximated as follows. Ω2τ is a constant. co
Since the term relating to s2ω1t is removed in the subsequent LPF, the equation of the received signal in the final system 1 is:

(Equation 4) Becomes By integrating Equation 4 over one symbol period, a correlation value in the system 1 is obtained. Assuming that the symbol transmission frequency is fs and the symbol period is Ts (= 1 / fs),
The correlation value is as follows. That is, when the first half of Equation 4 is integrated over one symbol period, the autocorrelation shown in Equation 5 is obtained. Here, t0 indicates an arbitrary integration start time.

(Equation 5) Initial synchronization acquisition of spread spectrum communication is mainly performed in a specific section where the same symbol is continuous.
Auto-correlation of the PN sequence for
It becomes the maximum when it becomes = 0. In this section, a1
(t) is a constant at any time. Next, Equation 4
Is integrated over one symbol period, Equation 6
Are obtained.

(Equation 6) In the specific section for initial synchronization acquisition described above, a2 (t + τ)
Is a constant at any time, and Equation 6 can be considered as an equation for integrating the product of the cross-correlation between c1 and c2 and the sine function over one symbol period. The PN code of c1 and c2 is Δ
Since the signal is a signal that randomly changes in a noise cycle with a period shorter than f, the pattern obtained by cutting out a certain section is almost the same regardless of the cutout start time. If the signal level of the undesired wave is higher than the signal level of the desired wave, the cross-correlation value increases in proportion to the level difference, so that if the signal levels are equal, the cross-correlation that could be ignored cannot be ignored. here,

(Equation 7) Assuming that the frequency offset amount is fs, the integral of one cycle of the sine function becomes zero, and as a result, the integral of Expression 6 becomes a value close to zero. In this system 1,
The undesired wave is system 2. Also, in the system 2, if the calculation is performed similarly, the condition of Expression 7 can be obtained. In addition,
The undesired wave becomes system 1. From the above, when a frequency offset that satisfies Equation 7 is given, the cross-correlation value due to the interference between the undesired wave and the desired wave becomes a value close to zero, and the influence of the interference can be suppressed to the minimum. Further, the series of processing contents follow the basic spread spectrum communication method, and does not require special processing or a special device.

In the above proof, the integral interval for obtaining the correlation value is one symbol period, but the integral interval is n.
The carrier frequency offset amount when extended to the symbol period is

(Equation 8) Given by Therefore, when the integration period is increased, the offset frequency is reduced. When increasing the number of multiplexes, it is sufficient to prepare the number of multiplexed oscillators whose oscillation frequencies are offset by a necessary amount. Although the multiplexing is performed in the transmitter in the figures in this specification, the same can be applied to a case where signals from independent transmitters are multiplexed and communicated by a satellite or the like.

[0005]

The first effect is that the influence of interference by undesired waves having a signal level higher than the desired wave can be suppressed. The reason is that by offsetting the carrier frequency, the cross-correlation value between the desired wave and the undesired wave can be made closer to zero. The second effect is that no special processing or additional equipment is required. The reason is that a system can be formed only with components necessary for general spread spectrum communication. Further, by setting the integration interval to n, the offset frequency can be reduced to 1 / n, so that a limited frequency band can be effectively used.

[Brief description of the drawings]

FIG. 1 is a block diagram of a code division multiple access communication system according to one embodiment of the present invention.

FIG. 2 is a block diagram of a code division multiple access communication system using a direct spreading method according to the related art.

[Explanation of symbols]

 Reference Signs List 1 spreader 2 PN sequence generator for I 3 modulator 4 spreader 5 PN sequence generator for Q 6 modulator 7 multiplexer 8 transmitting antenna 9 receiving antenna 10 distributor 11 demodulator 12 despreader 13 PN sequence for I Generator 14 Demodulator 15 Despreader 16 PN sequence generator for Q 21 Multiplier 22 PN sequence generator for I 23 Multiplier 24 Oscillator 25 Multiplier 26 PN sequence generator for Q 27 Multiplier 28 Oscillator 29 Adder 30 Transmitting antenna 31 Receiving antenna 32 Multiplier 33 Oscillator 34 Multiplier 35 PN sequence generator for I 36 LPF 37 Multiplier 38 Oscillator 39 Multiplier 40 PN sequence generator for Q 41 LPF

Claims (1)

[Claims] 1. A data communication method using a direct spread type spread spectrum modulation method, wherein an integration interval for calculating a cross-correlation value is n symbol periods of a spreading code (n is an integer of 2 or more). Wherein the carrier frequency is offset by 1 / n symbol transmission frequency with respect to the carrier frequency of another stream to be multiplexed.