JPH05160805A - Spread spectrum communication system - Google Patents

Abstract

PURPOSE:To prevent synchronizing step-out or unestablishment of synchronism caused when the correlation is tentatively larger than the autocorrelation by changing a clock phase difference tau of each station little by little. CONSTITUTION:Communication is implemented among transmission stations T1-Tn and reception stations R1-Rn by using different spread codes P1(t)-Pn(t). Let a maximum band among loop bands Lw1-LWn of spread code tracing systems 111-11n of each reception station be LWm a larger frequency difference FD than the maximum band LWm is given among each of clock frequencies FC1-FCn of each of the spread codes P1(t)-Pn(t) used for each transmission station.

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 specifically to clock frequencies of spread codes (similar noise signals) p ₁ (t) to pn (t) used in respective transmitting stations. The present invention relates to a multiplex communication system in which mutual influences when performing multiplex communication are reduced by making FC _{1 to} FCn different.

[0002]

2. Description of the Related Art A spread spectrum communication system spreads a signal in a wide frequency band and transmits it. Since the power per unit hertz is small, it does not interfere with other communication, and noise and transmission line characteristics are reduced. In recent years, it has been widely used in communications using a noisy line, which has features such that it is not easily affected.

In this communication system, different spreading codes p ₁ (t) to pn (t) having a small correlation with each other can be used to perform multiplexing at the same frequency.
For example, in FIG. 5, the transmitting station T ₁ and the receiving station R ₁ communicate using the spreading code p ₁ (t), and the transmitting station T ₂ and the receiving station R ₂ use the spreading code p ₂ (t) in the same frequency band. ) Is used for communication. In this case, conventionally, the spread codes p ₁ (t) to p
The clocks of n (t) are the same (f = F ₀ ).

Transmission signal X _{1 of the} transmission stations T ₁ and T ₂
(T) and X ₂ (t) are expressed as follows.

[0005] The transmitting station _{T 1 X 1 (t) =} Ad 1 (t) p 1 (t) cos (W c t) ··· (1) the transmitting station _{T 2 X 2 (t) =} Ad 2 (t) _{p 2 (t) cos (W} c t) ··· (2) where Acos (W _c t) is the _{carrier, d 1 (t), d} 2
(T) is transmission data.

[0006] At this time, the received signal Y ₁ at the receiving station R ₁
(T) is as follows.

[0007] receiving station _{R 1 Y 1 (t) =} Ad 1 (t) p 1 (t) cos (W c t) + Ad 2 (t) p 2 (t) cos (W c t) + n (t ) (3) However, n (t) is noise.

In this case, considering a case where there is a phase difference of τ between the spreading code on the receiving side and the transmitting side, it means that the receiving station R ₁ uses the spreading code p ₁ (t + τ) to despread. The correlation value r (τ) of is expressed as follows. r (τ) = p ₁ (t) p ₁ (t + τ) + p ₂ (t) p ₁ (t + τ) + n (t) p ₁ (t + τ) (4) The first term in the right side of the above equation 4 the desired signal component, i.e. the spreading code p ₁ autocorrelation r ₁₁ of the (t) (τ), and the second term interference signal component, i.e. the spreading code p ₁ (t) and the spread code p ₂ (t)
The cross-correlation r ₁₂ (tau), also the third term respectively represent the noise component. When a general M-sequence code is used as the spread signal, the autocorrelation r _ii (τ) is such that energy is concentrated and a clear peak appears when synchronization of the spread code is established as shown in FIG. The absolute value of the cross-correlation r _i x (τ) for received signals having different signs is small at any phase. In addition, i represents any one of the transmitting stations T _{1 to} n, and x represents any _one of the receiving stations R _{1 to} n.

However, as can be seen from FIG. 7, the cross-correlation r _i x (τ) depends on the spreading codes p _i (t) and px.
Although the value changes depending on the phase τ of (t), in a spread spectrum communication system that directs random access, it is common for each station to have an independent clock source (signal source) CS, so the frequency of each clock source is different. Τ changes slowly at a speed Δf corresponding to stability. For example, when the frequency of the spreading code (chip speed) is 10 MHz and the stability of the clock source CS is 10 −6, Δf is about 10 Hz, and the value of the cross correlation r _i x (τ) changes at this speed. .. Therefore, if there is a near-far problem, for example, when the other transmitting station T _x is far from the own transmitting station R _{1 as} viewed from the receiving station R _i , or if there is a change in transmission level or the like, it has a certain degree. In the phase difference τ, a phenomenon may occur in which the cross correlation r _i x (τ) becomes larger than the maximum value of the autocorrelation r _i x (τ).

Therefore, in view of the total power of the interference signal, it is said that out-of-synchronization occurs although the original spread code p ₁ (t) can be sufficiently kept in synchronization by the processing gain of spread spectrum. There was such a problem.

[0011]

Means for Solving the Problems] Therefore, in the present invention, when the LWm what each synchronization tracking system maximum loop bandwidth LW ₁ ~LWn in the receiving station, is used At a respective transmission station Between the clock frequencies FC _{1 to} FCn of the spreading codes p ₁ (t) to pn (t), the maximum bandwidth LW
A frequency difference FD larger than m is given.

That is, conventionally, in order to solve this problem,
I have been trying to use a spreading code p _i (t) with a small cross-correlation r _i x (τ), further stabilizing the clock frequency, and other measures. However, in these solutions, the code selection width is limited, and the circuit is complicated and the cost is inevitable.

Therefore, when the clock frequencies FC _{1 to} FCn are set as in the present invention, the phase difference τ of the interference signals at the receiving stations R _{1 to} Rn becomes the maximum value LWm of the synchronous tracking system loop band, that is, the tracking. It changes faster than the maximum possible speed.

Therefore, the synchronous tracking system of each of the receiving stations R _{1 to} Rn does not follow this change. In other words, the peak value of the cross-correlation r _i x (τ) at each station is averaged and the interference signal is It is close to white noise. Therefore, the level never exceeds the autocorrelation r _ii (τ), and the loss of synchronization does not occur.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 is a block diagram showing an embodiment of a system according to the present invention. In the figure, the transmitting station T ₁ and the receiving station R ₁ communicate using the spread signal p ₁ (t), and the transmitting station T ₂ and the receiving station R _{2 in} the same frequency band use another spreading code p ₂ ( Although the case where communication is performed using t) is illustrated, in the description, expansion will be discussed up to the case where other transmitting / receiving stations perform communication using spreading codes p3 (t) to pn (t), respectively. Each spread code is represented by p ₁ (t) and its clock source is represented by CS ₁ .

Clock sources CS of transmitting stations T ₁ and T _2
1 generates clock signals of frequencies F ₁ and F ₂ , respectively,
Loop bandwidth LW of the synchronous tracking system of each receiving station R ₁ -R ₂
Letting LWm be the maximum one among _{1 to} LW ₂ , each spreading code p ₁ used in each transmitting station T _{1 to} Tn.
Clock frequencies FC _{1 to} FCn of (t) to pn (t)
, A frequency FD larger than the maximum band LWm is given. For example, between the clock frequencies F ₁ and F ₂ of the transmitting stations T ₁ and T ₂ , | F ₁ −F ₂ | = FD, where FD> LW
The relationship m is given. Frequency difference FD is maximum band L
A value slightly higher than Wm is sufficient.

FIG. 2 shows a schematic configuration of each transmitting station T ₁ . In the figure, PNG _i is a spread code generator, and clock source C
In accordance with the clock signal of a frequency f = F ₁ supplied from the S _1, the spread code P _i (t) is generated, CG in carrier oscillator, for generating a carrier wave Acos (W _c t). MD ₁ ,
MD ₂ is a modulator, and modulator MD ₁ is transmission data d _i
The carrier wave is modulated at (t), and the modulator MD ₂ spreads (modulates) it with the spread code p _i (t). This gives X ₁ = A
A transmission signal of d _i (t) p _i (t) cos (W _c t) is generated. EX-OR is an exclusive OR circuit.

FIG. 3 shows a schematic configuration of each receiving station R ₁ . The main operation of the receiving station in this configuration is the same as the conventional one, for example Dlxon RC "Spread Spectrun Systems"
Since it is described in detail in John Wiley 1976 and is already known, a brief description will be given here.

The signals Ad _i to be received on each receiving station R _{i
(T) p i (t)} cos (W c t), the interference signal is a signal from another transmitting station I (t) and the noise signal n (t) is supplied. A band-pass filter (BPF) 11 removes out-of-band components from the supplied signal. 12
Is a synchronous tracking system, and the received signal Ad _i (t) p _i (t) co
s (W _c t) to generate a spreading code p _i (t) in synchronization with the spread signal ^ p _i (t) of the. The signal of ^ is originally placed on p. It cannot be expressed here, so put it before p.
In the following state, ^ p _i (t) = p _i (t
+ Τ).

Spreading code ^ p generated by the synchronization tracking system 12
_i (t) is multiplied by the received signal and Ad _i (t) r
_{11 (t) cos (W c} t) is extracted. Interference signal I
(T) is despread, and most of it is a bandpass filter (BP).
F) Blocked at 13. The signal that passes this is (A /
2) cos (W _c t) after have been multiplied, the high frequency components in the low-pass filter (LPF) 14 is removed by the carrier component is canceled, the data component d _i (t) is extracted.

The outline of the synchronization tracking system 12 is shown in FIG. The synchronization tracking system 12 includes a spreading code generator 21, a sliding correlator 31, and a delay lock loop 41. In the spread code generator 21, the spread code generator pNG ₁ generates a spread signal ^ p _i (t) according to the clock signal F ₁ supplied from the voltage controlled oscillator VCO.

The sliding correlator 31 performs initial synchronization pull-in, and the determination circuit 32 (Thre
shold) output is kept low. Switch circuit S
W is a predetermined voltage V while this low level signal is supplied.
c is supplied to the voltage controlled oscillator VCO. While the predetermined voltage Vc is being supplied, the voltage controlled oscillator VCO generates a clock signal F ₁ + ΔFx which differs from the original clock rate by a predetermined value ΔFx.

As a result, the phase τ of the reception side spread signal ^ p _i (t) changes at the speed ΔFx with respect to the transmission side spread signal p _i (t). The cutoff frequency of the low-pass filter 33 ₁ is set slightly higher than this so that this ΔFx can pass.

When the phases τ do not match, the input signal A
_{d i (t) p i (} t) cos (W c t) and the spread code ^ p _i
(T) has no correlation, and the output of the low-pass filter 33 ₁ remains at a low level, but the clock FC ₁ is deviated by ΔFx. Therefore, the phase τ between the two at the cycle T = 1 / ΔFx.
Repeats approaching and leaving.

In this process, when the phase difference becomes smaller than the predetermined value and the output of the amplitude detector 33 exceeds the threshold value, the decision circuit 32 supplies a high level signal to the switch circuit SW, and in response thereto, The switch circuit SW supplies the output signal of the delay lock group 41 to the voltage controlled oscillator VCO.

The delay lock group 41 includes a sliding lock.
The reception side expansion which is brought close to a range by the correlator 31
Scatter ^ p_i The phase of (t) is set to the spread signal p on the transmission side._i 
(T) is completely matched, and the spreading code ^ p_i T from (t)
Spread code with advanced phase by only ^ p _i (T−T) is the multiplier 4
2, spreading code ^ p_i Expansion with the phase delayed by T from (t)
Scattered signal ^ p_i (T + T) is supplied to the multiplier 45.

The outputs of the multipliers 42 and 45 are supplied to amplitude detection circuits 44 and 47 via bandpass filters 43 and 46, and their outputs are combined by a combining circuit 48. The loop filter 49 takes out frequency components of a predetermined value or less, but the cutoff frequency of the loop filter 49 is set to a value that can cover the deviation amount Δf related to the frequency stability of the clock, and its output is It is supplied to the voltage controlled oscillator VCO via the switch circuit SW.

On the multiplier 42 side, the reception side spread signal ^ p _i
When (t) is slightly behind the transmitting side, the correlation is obtained and the output of the amplitude detector 44 becomes high, while the multiplier 45
On the side, when the spread signal on the receiving side ^ p _i (t) is slightly advanced with respect to the transmitting side, the correlation is obtained and the output of the amplitude detector 44 becomes high. Output of combining circuit 48, that is, loop filter 4
The output of 9 is a combination of these outputs, and the upper and lower sides of this output are controlled by the voltage controlled oscillator circuit VCO so that the phase shift of the generated reception side spread signal ^ p _i (t) is reduced.
Act on.

When the phase difference between the two is eliminated, the output of the loop filter 49 is stabilized there, and the state where the phases match each other is maintained.

The loop band LW ₁ of the synchronization tracking system 12 is mainly determined by the cutoff frequency of the loop filter 49.

In the present invention, each receiving station R ₁ synchronization tracking system 1
When the maximum of the two loop bands LW ₁ is LWm, the clock frequencies FC ₁ to Tn of the transmitting stations T _{1 to} Tn are
A frequency difference FD larger than the maximum band LWm is given during FCn.

At the time of initial pull-in, or when the synchronization is maintained, the transmitting station T ₁ of its own station due to a near-far problem or the like.
Signal X _i (t) of another transmitting station Tx
In this case, since a frequency difference of at least FD or more is given between the clock FC _{1 of the} local station and the FCx of the other station in this case, this other station spreading code p
The cross-correlation r _i x (τ) with x (t) is also the frequency wave FD
It changes at the above speed.

In the present invention, the loop band LW ₁ of the synchronization follow-up system 12 is lower than this frequency difference FD, so that the synchronization follow-up system 12 becomes larger even if the other station signal Xx (t) becomes larger. , The cross-correlation r _i x (τ) with it changes at a speed equal to or higher than the speed at which it can follow itself, so it does not follow this. In other words, the cross-correlation r at each station
The peak value of _i x (τ) is averaged, and the interference signal I (t)
Is close to white noise. Therefore, the level does not exceed the autocorrelation r _ii (τ), and out-of-sync does not occur.

[0034]

As described above, according to the present invention, when the maximum _one of the loop bands LW _{1 to} LWn of the synchronous tracking system of each receiving station is LWm, each spread signal p _i of each transmitting station is set. Clock frequencies F _{1 to} Fn of (t) to pn (t)
The frequency difference FD larger than the maximum band LWm is given during the period.

As a result, the cross-correlation r _i x at each station
The peak value of (τ) is averaged, and the interference signal I (t) becomes close to white noise. Therefore, the level does not exceed the autocorrelation r _ii (τ), and the loss of synchronization does not occur.

Therefore, the degree of freedom in selecting the spread code p _i (t) is increased as compared with the conventional case. Further, since it is sufficient to change the setting of the frequency of the clock source, there is no increase in cost and circuit complexity.

[Brief description of drawings]

FIG. 1 is a system diagram showing an embodiment.

FIG. 2 is a block diagram showing the configuration of a transmitting station according to the embodiment.

FIG. 3 is a block diagram showing the configuration of a receiving station according to the embodiment.

FIG. 4 is a block diagram showing a configuration of a synchronous tracking system according to the embodiment.

FIG. 5 is a system diagram showing a conventional example.

FIG. 6 is a diagram showing an example of autocorrelation r ₁₁ (τ).

FIG. 7 is a diagram showing an example of cross-correlation r ₁ x (τ).

[Explanation of symbols]

 T transmitter station R receiver station p (t) spreading code 11 synchronous tracking system

Claims (1)

[Claims] 1. In a spread spectrum communication system in which different transmitting signals having small correlation values are used to perform communication between respective transmitting stations and receiving stations, a loop band LW ₁ ~ LWn
Of the maximum bandwidth LWm, the maximum bandwidth LW is set between the clock frequencies FC _{1 to} FCn of the spreading codes used in the transmitting stations.
A spread spectrum communication system characterized in that a frequency difference FD larger than m is given.