JPH0583218A - Orthogonal frequency division multiplex system demodulator - Google Patents

Abstract

PURPOSE:To reduce the occurrence rate of a symbol error in the demodulator for the communication system adopting an orthogonal frequency division multiplex system(OFDM system). CONSTITUTION:When a received wave IN is subjected to orthogonal detection by an orthogonal detector 1, an LPF 11 extracts only a DC component from its output. A delay detection circuit 12 delays and detects the output of the LPF 11 and the result of detection is sent to a sign deciding circuit 13 and an error detecting circuit 14. The sign deciding circuit 13 decides a sign based on the result of detection and sends the result of decision to the error detection circuit 14. The error detection circuit 14 detects a phase error resident in the result of detection and in the result of decision. An averaging circuit 15 controls the frequency of a carrier oscillator 2 to be equal to the frequency of a received carrier based on the detection error by the error detection circuit 14.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a demodulator in a communication system using an orthogonal frequency division multiplexing system (hereinafter referred to as an OFDM system), and more particularly to an automatic frequency control (hereinafter referred to as AFC) system of a carrier oscillator for demodulation. It is a thing.

[0002]

2. Description of the Related Art Conventionally, as a technique of this kind, there is one described in the following documents, for example.

Literature; EBU REVIEW-TECHNICAL, [224] (1987
-8) (France) by R. Lassaille and M. Alard "Principles of Modulation and Channel Coding for Digital Broadcasting Formau Vara's Receivers (Principles of
modulation and channelcoding for digital broadcast
ing) ”P.168-190 As described in the above-mentioned document, the OFDM method is intended for use in digital audio stereo broadcasting.
It is a technology mainly researched and developed in Europe. OFD
The modulation output on the transmitter side used in a communication system using the M method is generally expressed by the following equation (1). However, mq (t); qTs-Δ / 2 ≦ t <(q + 1) T
Modulation output at time t of S-Δ / 2 Ts; clock period (symbol interval) Δ; guard interval time T; time T obtained by T = Ts-Δ q; qth time when time is counted at Ts interval Φq, k; quadrature phase modulation of the kth carrier (hereinafter,
Modulation phase when QPSK); amplitude of each QPSK modulated wave ω 0; angular frequency of wireless carrier N; number of carriers in OFDM system Generally, in OFDM system, for example, a plurality (L) of digitized programs are Multiple for each program (2
M) symbols, one symbol is QPSK-modulated with two symbols as one set, and this is performed for all symbol sets to obtain M × L QPSK modulated waves. At this time, in the equation (1), N = ML. Further, for one program, M carriers are assigned at L intervals as the value of k in the equation (1).

On the other hand, as a demodulation method of a demodulator in the OFDM method, a synchronous detection method, a differential detection method, etc. can be considered. Here, the differential detection method will be described. FIG. 2 is a functional block diagram showing a configuration example of a delay detection type demodulator in the conventional OFDM system.

This demodulator has a quadrature detector 1 for quadrature detecting the received wave IN with a quadrature detection carrier S2 and outputting a detection signal S1 composed of an in-phase detection component S1a and a quadrature detection component S1b. Is connected to a carrier wave oscillator 2 that outputs a quadrature detection carrier wave S2. To the output side of the quadrature detector 1, a symbol reproduction circuit 3 for generating a timing signal S3 and the like from the detection signal S1 and a correlator 4 for extracting a specific frequency component from the detection signal S1 based on the timing signal S3 are connected. ing.

On the output side of the correlator 4, its output signal S4
The differential circuit 5 for performing the differential calculation is connected, and the serial converter 6 is connected to the output side thereof. The serial converter 6 parallelizes the output signal S5 of the differential circuit 5
It is a circuit that performs serial conversion and outputs an output signal S6 having a serial code.

Next, the operation of the demodulator shown in FIG. 2 will be described. A reception wave IN sent from a transmitter (not shown) via a propagation path is input to the quadrature detector 1. This received wave IN is
Usually, the propagation path causes distortion, propagation delay, and the like, but here, in order to simplify the explanation, it is assumed that they are not affected. Then, the received wave IN has the same formula as the formula (1). Quadrature detection carrier wave S2 oscillated by carrier wave oscillator 2
Is input to the quadrature detector 1. Since the quadrature detector 1 performs quadrature detection, the quadrature detection carrier S2 is two signals that are orthogonal to each other. Here, it is represented by a complex number like the following equation (2). cq (t) = e- ^{j (ωct + θc)} (2) where, t and q are the same as in formula (1) ωc; angular frequency of the quadrature detection carrier θc; between the reception wave (1) and the quadrature detection carrier Phase difference The quadrature detector 1 multiplies the received wave IN by the quadrature detection carrier S2, that is, multiplies the equations (1) and (2) to obtain the detection signal S1 (= Yq (T))
Is output. The detection signal Yq (t) is composed of an in-phase detection component S1a usually called the I channel and a quadrature detection component S1b usually called the Q channel, and the in-phase detection component S1a is the real part of the equation (3), the quadrature detection component S1b. Corresponds to the imaginary part of equation (3).

When Δω = ω0−ωc in the equation (3), the equation (4) is obtained. The symbol reproduction circuit 3 reproduces the symbol frequency (clock frequency) 1 / Ts included in the equation (4), and from there, the frequency k / T of each carrier wave (where k = 0,
, ..., N-1), and the time (timing) for performing the operation of the correlator 4 is created, and their timing signals S3
Is supplied to the correlator 4.

Since the correlator 4 extracts only a specific frequency component from the detection signal S1 (= Yq (t)) expressed by the equation (4), the frequency s / T (where s; 0, 1, ... , N
(Integer of −1) is multiplied by equation (4), and for the q-th symbol, from time t = qTs to t = qTs +
Integrate during T. Then, the output signal S4 of the correlator 4 at the frequency number s is given by the following equation (5). For example, assuming Δω = 0 (that is, the reception carrier wave and the quadrature detection carrier wave S2 have the same frequency), the equation (5) is as follows. Wq, s = A · e ^{j (Φq, s−θc)} (6) Therefore, only the modulation component of k = s can be obtained. However, when Δω ≠ 0, the equation (5) does not become so simple, and the influence from the component of k ≠ s appears. At this time, if ΔωT can be made extremely small, it can be considered that the output of equation (6) can be obtained.

Now, considering that one receiver selects and demodulates one program (channel), the number of s may be M at L intervals. Therefore, M correlators 4 are required. From these correlators 4, (6)
The M output signals S4 (= Wq, s) represented by the equation are
It is output to the differential circuit 5.

The differential circuit 5 performs a differential operation on the output of the correlator one symbol before and the current output. If the value one symbol before is Wq-1, s, then Wq-1, s = A · e ^{j (Φq-1, s-θc) If the} differential output signal S5 is Dq, s, then Dq, s = ^{Wq, s · W * q-} 1, s = a 2 · e j (Φq, s-Φq-1, s) ··· (7) ^{However, W * q-1, s} ; of Wq-1, s Conjugate complex number Therefore, if the code is differentiated in the modulator,
It can be seen from Equation (7) that the code is demodulated. (7)
Since M output signals S5 represented by the equation are obtained, the serial converter 6 inputs M to the modulator by performing parallel / serial conversion of M (actually, M sets of complex numbers) parallel symbols. The output signal S6 having the serial code is output.

[0012]

However, the conventional OFDM demodulator has the following problems.
In the demodulation process described above, Δω = 0 or the value of ΔωT is very small and can be ignored, but there are cases where the value of ΔωT cannot be made so small that it can be ignored. For example, this corresponds to the case where the carrier wave oscillator 2 whose frequency stability is not so high is used or the radio frequency is high. In this case, a leak from another channel occurs in the equation (6), which affects the subsequent symbol demodulation operation and increases the probability of a symbol error in the symbol demodulation.
Generally, in OFDM communication, an advanced error control method is adopted, and some symbol errors are repaired by the error control method. However, the occurrence of the error often causes an error exceeding the error control capability. However, it has been difficult to obtain a demodulator that is technically sufficiently satisfactory.

An object of the present invention is to provide an OFDM demodulator which solves the problem of the symbol error occurring when the frequency of the quadrature detection carrier wave is different from the frequency of the reception carrier wave (Δω ≠ 0), which is a problem of the prior art. Is provided.

[0014]

In order to solve the above problems, the present invention provides a carrier wave oscillator that oscillates at a frequency substantially equal to a received carrier wave in a received wave of a communication system using the OFDM system, and the received wave. In the OFDM demodulator having a quadrature detector that performs quadrature detection by the output of the carrier wave oscillator, the following means are taken.

That is, a low-pass filter (hereinafter referred to as an LPF) that extracts only the DC component from the output signal of the quadrature detector.
A delay detection circuit that delay-detects the output signal of the LPF; a sign determination circuit that determines the sign of the output signal of the delay detection circuit; and a phase of the output signal of the delay detection circuit and the output signal of the sign determination circuit. An error detection circuit that detects an error and an averaging circuit that averages the output signal of the error detection circuit for a long period are provided, and the frequency of the carrier wave oscillator is controlled based on the output signal of the averaging circuit. ..

[0016]

According to the present invention, since the OFDM demodulator is constructed as described above, when the received wave is quadrature detected by the quadrature detector, the DC component of the output signal of the quadrature detector is detected by the LPF. Only the signal is taken out and sent to the differential detection circuit. In the differential detection circuit, the output signal of the LPF is differentially detected and the detection result is given to the code determination circuit and the error detection circuit. The sign judging circuit judges the sign of the detection result and gives the judgment result to the error detecting circuit.

The error detection circuit detects a phase error between the detection result and the determination result and gives the error signal to the averaging circuit. The averaging circuit averages the error signals for a long time and controls the frequency of the carrier oscillator so that the frequency of the carrier oscillator becomes equal to the frequency of the received carrier. This reduces the occurrence of symbol errors. Therefore, the above problem can be solved.

[0018]

EXAMPLE FIG. 1 shows an example of the present invention.
FIG. 3 is a functional block diagram of a demodulator of a differential detection system used in a communication system using a DM system, and elements common to those in the conventional FIG. 2 are denoted by common symbols.

This demodulator has the same structure as the conventional demodulator shown in FIG.
An AFC unit 10 for automatically controlling the frequency of the carrier wave oscillator 2 is added.

The AFC section 10 has an LPF 11 that allows only the DC component of the detection signal S1 to pass therethrough, and a delay detection circuit 12 is connected to the output side thereof. The differential detection circuit 12 is L
This is a circuit that delay-detects the output signal S11 of the PF 11 and outputs the delay-detection signal S12 based on the timing signal S3 from the symbol reproduction circuit 3. The output side thereof is connected to the code determination circuit 13 and the error detection circuit 14. ing.

The code determination circuit 13 is provided with the delay detection signal S12.
Is sign-determined and the judgment signal S 13
Is a circuit for outputting to. The error detection circuit 14 is a circuit that detects a phase error between the differential detection signal S12 and the determination signal S13 and outputs the phase error signal S14, and the averaging circuit 15 is connected to the output side thereof. The averaging circuit 15
An averaging signal S15 for controlling the frequency of the carrier wave oscillator 2 by averaging the phase error signal S14 for a long time.
To a carrier wave oscillator 2, and is composed of a moving average circuit, an LPF having a long time constant, and the like.

Next, the operation of the demodulator of this embodiment will be described. When the quadrature detection carrier S2 is supplied from the carrier wave oscillator 2 to the quadrature detector 1, the quadrature detector 1 receives the received wave IN
Of the in-phase detection component S1a and the quadrature detection component S
The detected signal S1 composed of 1b is supplied to the correlator 4 and the LPF 11
Supply to. The detection signal S1 can be expressed by equation (4).

The LPF 11 detects the detection signal S1 (= Yq
(T)) has a characteristic of passing only the DC component (that is, the component of k = 0 in the equation (4)) and attenuating the other components, and the output signal S11 (= Yq, 0 (t)) is output to the differential detection circuit 12. Yq, 0 (t) = A · e ^{j (Φq, 0 + Δωt-θc)} + Zq, k (8) where components other than Zq, k; k = 0 leak through LPF 11 Delay detection circuit 12 is a circuit that performs delay detection with a DC component one symbol before and a current DC component, and is a signal delayed by a delay time T1 (which is almost equal to the time Ts of one symbol, but does not necessarily match). And the current signal are subjected to delay detection, and the timing signal S3 obtained by the symbol reproduction circuit 3 causes the time t = qTs to t = qTs + T.
During time t = td, the differential detection signal S12 (= Pq) represented by the following expression (9) is output to the code determination circuit 13 and the error detection circuit 14. Pq = A ² · e ^{j ((Φq, 0-Φq-1,0) + ΔωT1)} + A · Zq, k, d · e ^{-j (Φq-1,0 + Δω (td-T1) -θc)} + A Z ^* q-1, k, d ^* ej ^{(Φq, 0 + Δωtd-θc)} + Zq, k, d ^* Z ^* q-1, k, d (9) where Zq, k, d A value obtained by sampling Zq, k at t = td Z ^* q−1, k, d; a conjugate complex number of a value obtained by sampling a leak component delayed by one symbol at t = td. The code determination circuit 13 detects the delay detection signal S12. The phase
It is determined which of 0, π / 2, π or −π / 2 is the closest. If the phase is Θq, the code determination circuit 13
Therefore, the determination signal S13 (= Qq) expressed by the following equation (10)
Is output to the error detection circuit 14. Qq = A ² · e ^{j Θq} (10) Here, ΔωT <1 can be realized relatively easily by limiting the frequency stability of the carrier oscillator 2. Further, the characteristics of the LPF 11 can be selected so that the absolute values of the amplitudes of Zq, k, d and Zq-1, k, d are equal to or less than the amplitude A of the QPSK modulated wave. in this case,
The expression (9) can be approximated by the following expression (9-1). ^{Qq≈A 2} · e ^{j (Φq, 0-Φq-1,0)} (9-1) In the modulator, in the case of the differentially encoded QPSK system,
Φq, 0 −Φq-1,0 is 0, π / 2, π or −π / 2
, Θq = Φq, 0-Φq-
1,0 is obtained. Θq = Φq, 0 due to external noise
-Φq-1, 0 may not hold, but it is considered to be a rare example.

The error signal circuit 14 has a delay detection signal S12.
The phase error between the determination signal S13 and the determination signal S13 is detected and the phase error signal S14 is output to the averaging circuit 15. The second to fourth terms of the equation (9) are expected to be considerably smaller than the first term, so if expressed as δq, the phase error signal S1
4 (= Sq) is expressed by the following equation (11). Sq = Pq · Q ^* q = A ⁴ · e ^{j ΔωT 1} · e ^{j ((Φq, 0 −Φq-1,0) −Θq)} + ^δq · A ² · e ^{-j Θq} (11) Where, Assumption, that is, Θq = Φq, 0-Φq-1,
^Assuming that ⁰ and ^δq can be ignored, Sq = A ⁴ · e ^{j ΔωT1,} and it can be seen that a phase error proportional to the angular frequency difference Δω is detected. However, that assumption may not hold and it is necessary to perform the following averaging.

The averaging circuit 15 averages the phase error signal S14 (= Sq) of the equation (11) for a long period of time. A and ΔωT1 in the first term of the equation (11) are constant values for a long time. The first term (Φq, 0-Φq-1, 0) -Θq sometimes becomes non-zero as described above, but it is generally 0,
The long-term average may be almost 0. Also,
Although the value of the second term of the equation (11) is originally quite small,
Furthermore, considering that the signal is modulated by a random signal, the canceling effect works for a long period of time, resulting in a very small value. Therefore, the averaging circuit 15 outputs the control signal S15 (= Sav) represented by the following expression (12) to the carrier wave oscillator 2. Sav = A ⁴ · e ^{j ΔωT1} (12) Here, if ΔωT1 <1, then Sav = A ⁴ · (1 + jΔωT1) (13), and the imaginary part of the control signal Sav has an angular frequency difference. A signal proportional to is obtained. By inputting this signal to the carrier wave oscillator 2 and controlling the frequency in the direction of increasing the angular frequency of the oscillator 2, Δω becomes smaller at the next time, and (13)
The absolute value of the imaginary part of the expression tends to decrease, and the frequency is automatically controlled until the imaginary part finally becomes 0 (that is, Δω is 0).

As described above, in this embodiment, since the AFC unit 10 is added to the OFDM demodulator to control the frequency of the carrier oscillator 2, it is possible to reduce the symbol error, which is preferable. It becomes possible to communicate. Furthermore, it becomes possible to select the frequency stability of the carrier oscillator 2 in consideration of economy, whereby the carrier oscillator 2 can be selected.
The circuit configuration can be simplified and the OFDM system can perform highly accurate demodulation operation at a high radio frequency.

The present invention is not limited to the above embodiment,
Various modifications are possible. Examples of such modifications include the following.

(A) In the above embodiment, the demodulator of the differential detection system has been described, but the AFC unit 10 of the above embodiment can be applied to demodulators of other systems that perform quadrature detection. (B) The AFC unit 10 shown in FIG. 1 is not limited to an individual circuit that uses an integrated circuit or the like.
It can also be executed by program control of a signal processor (DSP) or the like, which simplifies the circuit configuration.

[0029]

As described in detail above, according to the present invention, since the AFC function is added to the conventional OFDM demodulator, it is possible to reduce the symbol error and perform a good demodulation operation. Become. Further, the frequency stability of the carrier oscillator can be selected in consideration of economical efficiency, which simplifies the circuit configuration of the carrier oscillator. Moreover, it is possible to perform highly accurate demodulation operation of the OFDM system at a high radio frequency.

[Brief description of drawings]

FIG. 1 is a functional block of an OFDM demodulator showing an embodiment of the present invention.

FIG. 2 is a functional block diagram of a conventional OFDM demodulator.

[Explanation of symbols]

 1 Quadrature detector 2 Carrier wave oscillator 3 Symbol reproduction circuit 4 Correlator 5 Differential circuit 6 Serial converter 10 AFC section 11 LPF 12 Delay detection circuit 13 Code judgment circuit 14 Error detection circuit 15 Averaging circuit

Claims (1)

[Claims] 1. A carrier oscillator that oscillates at a frequency substantially equal to a received carrier in a received wave of a communication system using an orthogonal frequency division multiplexing system, and a quadrature detector that quadrature detects the received wave by the output of the carrier oscillator. In the demodulator of the orthogonal frequency division multiplexing system having, a low-pass filter that extracts only a DC component from the output signal of the quadrature detector, a delay detection circuit that delay-detects the output signal of the low-pass filter, and the delay A code determination circuit that determines the sign of the output signal of the detection circuit, an error detection circuit that detects a phase error between the output signal of the delay detection circuit and the output signal of the code determination circuit, and the output signal of the error detection circuit An averaging circuit for averaging for a period is provided, and the frequency of the carrier oscillator is controlled based on the output signal of the averaging circuit. Demodulator Osogonaru frequency division multiplexing to symptoms.