JP2002199035A - Receiver and method for compensating frequency error - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low-cost radio receiver capable of fast detecting synchronization even when a carrier offset exists. SOLUTION: This receiver for receiving a transmission signal including a prescribed digital synchronous code that is modulated by a transmitter carrier frequency and transmitted by radio and synchronizing with the transmission signal by detecting the synchronous code consists of a local oscillator (40), a demodulator (15) for demodulating the received transmission signal from a carrier frequency into a baseband on the basis of a receiver local frequency from the local oscillator, analog-to-digital converters (23 and 30) for digitizing a demodulated analog signal and outputting the digital signal, complex multipliers (44 and 46) for multiplying the digital signal by such a compensation rotation complex coefficient that can compensate the error between the transmitter carrier frequency and the receiver local frequency to obtain a 1st multiplication output and also multiplying the digital signal by the conjugate complex number of the compensation rotation complex coefficient to obtain a 2nd multiplication output, and an adder (42) for adding at least the 1st and 2nd multiplication outputs and outputting the added multiplication outputs to a correlator (10).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a receiver and a method for compensating for a frequency error. It is about the method.

[0002]

2. Description of the Related Art In radio communication using a spread spectrum system such as CDMA, for example, it is necessary to synchronize symbol boundaries between a transmitter as a base station and a receiver as a mobile station. Therefore, the transmitter transmits a spread code [A ₀ to A _N−1 ] as a predetermined synchronization code before data transmission. on the other hand,
The receiver includes a correlator, and can detect synchronization by correlating a synchronization code included in the received signal with a spreading code prepared in the receiver.

FIG. 1 is a diagram showing an example of the configuration of a conventional correlator 10 for detecting synchronization in a receiver. A signal received from the antenna of the receiver, demodulated by the quadrature demodulator, and subjected to analog-to-digital conversion is input to the input terminal 1. In general,
The spread spectrum receiver samples the in-phase component and the quadrature component of the received signal, and treats the signal as a complex signal I + jQ having the in-phase component I as a real part and the quadrature component Q as an imaginary part. It is assumed that such a complex signal is input to the input terminal 1 in FIG.

[0004] The received signal input to the input terminal 1 contains a spread code [A ₀ to A _N-1 ] which is a predetermined synchronization code. The received signal is input to each multiplier as shown in the figure via delay elements each having a delay of one chip period T. In each multiplier, tap coefficients [C ₀
~ C _N-1 ] is multiplied by the received signal. Here, the despreading code C _n is the complex conjugate of _{A n, C n = A n} * (n = 0, 1,
…, N-1).

[0005] The outputs of the multipliers are added to each other and then input to the power measuring unit 2. The power measuring unit 2 calculates the complex power (I ² +
Q ² ) is calculated, and the measured correlation power value is output from the output terminal 3.

As described above, the spreading code at the transmitter is
A _n = a _{[A 0 ~A N-1]} , since the despreading code C _n by the correlators _{10 = [C 0 ~C N-} 1] is in the complex conjugate of A _n, the symbol of the transmission signal At the boundaries, a strong correlation is obtained.

FIG. 2 is a graph showing an output example of the output terminal 3 in an ideal state when N = 31. The vertical axis represents the output of the output terminal 3 and the horizontal axis represents the sampling time. A strong peak appears at the transmission symbol boundary at the sample time = 0. Thus, by detecting the peak of the correlation power value, the synchronization of the symbol boundary can be detected.

[0008]

In practice, there is an error (carrier offset) between the carrier frequency ω _c of the transmitter and the local oscillation frequency in the quadrature demodulator of the receiver.
And the synchronization performance of the correlator described above is significantly degraded.
If a carrier offset exists, the received signal undergoes a phase rotation on the complex plane in the demodulator. For example,
Δω for each frequency value of carrier offset and spreading code for
A _n, when the despreading code to A _n ^*, the peak S ₀ of the correlation value (the output of the power measurement unit 2 of FIG. 1)

[0009]

(Equation 1) Becomes Here, a is the amplitude of the spreading code and the despreading code. The magnitude of S ₀ decreases as Δω moves away from ₀ , and finally reaches S ₀ when ± 2π / NT (rad / sec).
= 0. This can be seen from the fact that the integral of the exponential function exp (jθ) for one cycle becomes 0.

The graph of FIG. 2 is an output example of the output terminal 3 when N = 31 and the carrier offset Δω is 2π / NT (rad / sec). This carrier offset 2π / NT is
This is a large carrier offset such that the phase is rotated by 2π during one symbol period (31 chip periods). Therefore, FIG.
As shown in the figure, the worst case (Δω = ± 2π / NT (rad / sec)
), The correlation power value becomes 0, and it becomes impossible to detect a symbol boundary. But carrier offset Δ
High-precision oscillators that can reduce ω are expensive.

In order to solve this problem, there has been a conventional synchronous detector configured as shown in FIG. In FIG. 4, a transmission signal received from an antenna 14 is demodulated to a baseband by a quadrature demodulator 15. The demodulated in-phase and quadrature-phase signals are analog-to-digital converted by A / D converters 28 and 30, respectively, and supplied to the correlator as I and Q signals. Quadrature demodulator 1
5 performs a demodulation operation based on the frequency from the local oscillator 20, but if the local oscillation frequency of the local oscillator 20 is shifted from the carrier frequency of the transmitter by Δω, the above-described problem occurs. Attempts have been made to rotate the received signal at a constant frequency -ω _k in the quadrature demodulator and to input it to the correlator 10. To do so, the oscillation frequency (ω _c + Δω) of the local oscillator 20 is shifted by -ω _k . Usually, before synchronization is established, the carrier detector cannot know the carrier offset amount Δω.
Three types of ω _k , 0 and + ω _k are tried, and the correlation between the received signal and the spread code is obtained for these. If Δω = ω _k , then a large output is obtained from the correlator 10 and synchronization can be established.

However, in the conventional synchronous detector, for example, three types of -ω _k , 0 and + ω _k must be tried.
It took a long time to establish synchronization. When the carrier frequency is high and the transmission bandwidth is narrow, the carrier offset becomes relatively large with respect to the transmission bandwidth, which has been a particular problem.

An object of the present invention is to provide an inexpensive receiver that can detect synchronization at high speed even when a carrier offset exists.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 5 shows how the quadrature demodulated signal is supplied to the correlator. The signal to be transmitted may be any digitally modulated wireless communication signal including a predetermined synchronization code, such as QPSK, BPSK, and QAM.
The transmission signal received from the antenna 14 is transmitted to the quadrature demodulator 1
5 demodulates to baseband. The demodulated in-phase and quadrature-phase signals are converted from analog to digital by A / D converters 28 and 30, respectively, and output as I and Q signals. The quadrature demodulator 15 performs a demodulation operation based on the frequency from the local oscillator 40. However, if the local oscillation frequency of the local oscillator 40 is shifted from the carrier frequency of the transmitter by Δω, the above-described problem still remains. Occurs.

In this embodiment, the oscillation frequency of the local oscillator 40 is not shifted. A / D converter 2
Before the complex signal I + jQ output from 8, 30 is input to the correlator 10, a certain multiplication is performed to perform offset compensation.

The complex signal I + jQ is, for example, divided into three, rotated at a plurality of different angular frequencies, and the sum is input to the correlator 10. For example, consider -ω _k , 0, + ω _k as a plurality of different frequencies. As shown in FIG. 5, the multiplier 44 performs exp
(-jw _k T _n ). In the multiplier 46, the complex signal I
+ jQ is multiplied by exp (+ jw _k T _n ). Adder 42
Add the two multiplication result outputs and the complex signal I + jQ as they are, and input the added output to the correlator 10.
The configuration of the correlator 10 may be the same as in the related art. Assuming that the complex signal I + jQ is R _n and the input signal to the correlator 10 is W _n , W _n is represented by the following equation.

[0017]

[Number 2] _{W n = R n x exp (} -jw k T n) + R n + R n x exp (+ jw k T n)
(n = 0, 1, ... , N-1) if if positive direction to present a carrier offset [Delta] [omega [Delta] [omega is close to omega _k, the first term (output from the multiplier 44) contribute to the correlation value . If in the negative direction is present carrier offset -Derutaomega [Delta] [omega is close to omega _k, (the output from the multiplier 46) the third term contributes to the correlation value. When the carrier offset Δω is close to 0, the signal of the second term (through) contributes to the correlation value. Although the other two outputs become noise, synchronization can be detected from the peak obtained from one output.

FIGS. 7 and 8 show that ω _k = π / NT (rad / sec)
An example of the calculation of the correlation power value is shown below. FIG. 7 shows a case where no carrier offset exists (Δω = 0).
Is an example where a carrier offset of Δω = + 2π / NT (rad / sec) is added. From FIG. 8, it can be seen that a peak of the correlation power value that has not occurred in the example of FIG. 3 appears.
Also, from FIG. 7, it can be seen that the peak of the correlation power value occurs even when there is no carrier offset, as in the example of FIG.

Further, the compensating multiplier of the present invention may be configured as a single unit as shown in FIG. In this embodiment, as an operation of rotating the complex signal I + jQ, the complex signal I +
It should be multiplied by 1 + 2cosω _k t to jQ. 2cosω _k t is, exp
This is because it is equivalent to (−jw _k t) + exp (+ jw _k t). In this embodiment, it is sufficient to perform the operation of “complex number × real number” instead of “complex number × complex number”, so that the processing amount can be reduced and the circuit can be simplified.

[0020]

According to the embodiment of the present invention, as described above, the received complex signal I + jQ is rotated at a plurality of different angular frequencies and the sum thereof is input to the correlator. Can be established in a short time and at low cost. Therefore, when the present invention is applied to portable wireless telephone communication, it is possible to provide a wireless terminal (such as a portable telephone) that can be used in a short time after power-on.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a configuration example of a conventional correlator that performs synchronization detection in a receiver.

FIG. 2 is a graph showing a correlation output value of the output terminal 3 in an ideal state, and shows a peak.

FIG. 3 is a graph showing a worst case correlation output value of an output terminal 3, which indicates that a symbol boundary cannot be detected.

FIG. 4 shows a configuration of a receiver as a conventional synchronization detector.

FIG. 5 shows a configuration of a receiver as a synchronization detector according to an embodiment of the present invention.

FIG. 6 shows a configuration of a receiver including a compensation multiplier according to another embodiment of the present invention.

FIG. 7: No carrier offset (Δω = 0)
In the case, a calculation example of the correlation power value obtained by the embodiment of the present invention is shown.

FIG. 8 shows a carrier offset Δω = + 2π / NT (rad / se
An example of calculation of a correlation power value obtained by the embodiment of the present invention in the case of c) will be described.

[Explanation of symbols]

 40 local oscillator 15, quadrature demodulator 28,30 analog / digital converter 44,46,48 multiplier 42 adder 10 correlator

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5K004 AA05 AA08 FH08 JH05 5K022 EE02 EE21 EE36 5K047 AA02 AA13 BB01 CC01 EE02 EE04 GG34 GG37 HH01 HH03 HH15 HH42 MM13

Claims (6)

[Claims] 1. A receiver for receiving a transmission signal containing a predetermined digital synchronization code modulated by a transmitter carrier frequency and wirelessly transmitted, and detecting the synchronization code to synchronize with the transmission signal. T: a local oscillator; a demodulator for demodulating the received transmission signal from a carrier frequency to a baseband based on a local frequency of a receiver from the local oscillator; digitizing the demodulated analog signal and outputting a digital signal An analog-to-digital converter; multiplying the digital signal by a compensated rotation complex coefficient capable of compensating for an error between the transmitter carrier frequency and the receiver local frequency to obtain a first multiplied output, and conjugate the compensated rotation complex coefficient A complex multiplier for multiplying the digital signal by a complex number to obtain a second multiplication output; at least the first multiplication output and the second square calculation An adder that adds the force and outputs the result to a correlator. 2. The receiver according to claim 1, wherein:
The receiver, wherein the adder adds the first multiplied output, the second multiplied output, and the digital signal. 3. A receiver for receiving a transmission signal containing a predetermined digital synchronization code modulated by a transmitter carrier frequency and wirelessly transmitted, and detecting the synchronization code to synchronize with the transmission signal. T: a local oscillator; a demodulator for demodulating the received transmission signal from a carrier frequency to a baseband based on a local frequency of a receiver from the local oscillator; digitizing the demodulated analog signal and outputting a digital signal Analog-to-digital converter; multiplies the digital signal by a real-valued compensation coefficient obtained by adding a compensation rotation complex coefficient and its conjugate complex number that can compensate for an error between the transmitter carrier frequency and the receiver local frequency and 1 And a multiplier for outputting to a correlator. 4. A receiver for receiving a transmission signal including a predetermined digital synchronization code modulated by a transmitter carrier frequency and wirelessly transmitted, and detecting the synchronization code to synchronize with the transmission signal. A method for compensating for an error between the transmitter carrier frequency and a receiver local frequency, comprising: demodulating the received transmission signal from a carrier frequency to baseband based on the receiver local frequency from a local oscillator; Digitizing the demodulated analog signal and outputting a digital signal; multiplying the digital signal by a compensation rotation complex coefficient capable of compensating for an error between the transmitter carrier frequency and the receiver local frequency; Output, and multiplying the digital signal by a conjugate complex number of the compensation rotation complex coefficient to obtain a second multiplied output; Adding the first multiplied output and the second multiplied output and outputting the sum to a correlator. 5. The method according to claim 1, wherein the adding step is a step of adding the first multiplied output, the second multiplied output, and the digital signal. Method. 6. A receiver for receiving a transmission signal containing a predetermined digital synchronization code modulated by a transmitter carrier frequency and wirelessly transmitted, and detecting the synchronization code to synchronize with the transmission signal, A method for compensating for an error between the transmitter carrier frequency and a receiver local frequency, comprising: demodulating the received transmission signal from a carrier frequency to baseband based on the receiver local frequency from a local oscillator; Digitizing the demodulated analog signal and outputting a digital signal; adding a compensated rotation complex coefficient and its conjugate complex number that can compensate for an error between the transmitter carrier frequency and the receiver local frequency, and 1; Multiplying said digital signal by said real-valued compensation coefficient and outputting it to a correlator.