JPH0851408A - Method and apparatus for recovering a reference carrier frequency for demodulating an OFDM modulated signal - Google Patents

Abstract

(57) [Abstract] [Purpose] To reproduce the reference carrier frequency for OFDM modulation signal demodulation with high accuracy and stability. [Structure] On the transmission side, the frequency is f _A (= a · f _d ) + f _IF inside or outside the OFDM modulation signal band.
And f _B (= b · f _d ) + f _IF, where a and b are natural numbers different from each other, and f _d is OFDM.
The carrier frequency interval of, or f _IF is a carrier signal for locking a reference carrier frequency consisting of two frequencies, which are reference carrier frequencies, and is transmitted.
Carrier signal f _A for reference carrier frequency lock consisting of frequency
For each of + f _IF and f _B + f _IF , a reference carrier frequency f as an oscillation output signal of the oscillator, which is obtained from a voltage controlled oscillator (VCO) 105 forming a phase locked loop, is provided.
_{The IF} 108 is multiplied to form the baseband signals (f _A , f _B ), and the correction value ε is calculated from the baseband signal.
(= F _A − (a / b) · f _B ) is generated in the multiplier 119 and the phase comparator 122, and the generated correction value is applied to the control signal input terminal of the voltage controlled oscillator (VCO) 105.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to frequency orthogonal division multiplex modulation (hereinafter referred to as OFDM) suitable for a terrestrial (a term used to distinguish from satellite broadcasting) digital television broadcasting or digital audio broadcasting modulation system. The present invention relates to a method and apparatus for regenerating a reference carrier frequency for demodulating an OFDM modulated signal on the receiving side.

[0002]

2. Description of the Related Art Conventionally, regarding the reproduction of a reference carrier frequency for demodulating an OFDM modulated signal, the frequency deviation of the reproduced reference carrier is corrected by detecting the phase error of the received data. I came.

[0003]

In signal transmission using the OFDM modulation method, in particular, a single frequency network (SFN: Si) that relays and broadcasts the same signal at the same frequency is used.
(ngle Frequency Network) etc., it is necessary to exactly match the transmission frequency of each transmitting station. Therefore, in this case, regarding signal distribution to each retransmission station, it is required that each retransmission station reproduce exactly the same reference carrier frequency as the signal distribution source. With respect to this requirement, it is not sufficient in terms of accuracy to compensate the frequency deviation of the reproduction reference carrier wave by the phase error of the received data.

Therefore, a first object of the present invention is to enable reproduction of a reference carrier frequency having high frequency accuracy and stable over time, when the accuracy of the reference carrier frequency as described above is required. It is to provide a method and apparatus for doing so. A second object of the present invention is to provide a plurality of OFDM
It is intended to realize a system in which an integrated synchronization system is obtained by setting a common carrier signal for locking a reference carrier frequency for a modulation signal. Further, the third object of the present invention is to
It is to stabilize the FFT (Fast Fourier Transform) data sample clock at the time of OFDM modulation signal demodulation processing.

[0005]

In order to achieve the above object, a method of regenerating a reference carrier frequency for demodulating an OFDM modulated signal according to the present invention is such that, on the transmission side, the OFDM carrier is in or out of the OFDM modulated signal band. The frequency is f _A (= a ・
f _d ) + f _IF and f _B (= b · f _d ) + f _IF where a and b are natural numbers different from each other, and f _d is OFDM.
The carrier frequency interval of, or f _IF is the reference carrier frequency locking carrier signal consisting of two frequencies that is the reference carrier frequency and is transmitted, and the receiving side receives the reference carrier frequency locking carrier consisting of the two frequencies. each of the signal f _a + f _IF and f _B + f _IF, as well as baseband signaling by multiplying the reference carrier frequency f _IF as the oscillation output signal obtained the generator from the voltage controlled oscillator which constitutes a phase locked loop, _A correction value ε (= f _A − (a / b) · f _B ) from the baseband signal.
And applying the generated correction value to the control signal input terminal of the voltage controlled oscillator, and at the time of applying the correction value ε, the value of the correction value ε is converged to zero by the action of the phase locked loop. To do.

Further, in the method of the present invention, the reference carrier frequency locking carrier signal transmitted from the transmitting side is set to 3 frequencies or more, and the receiving side receives the reference carrier frequency locking carrier signal of 3 frequencies or more. It is characterized in that two frequencies are selected and used.

Further, the method of the present invention is characterized in that the reference carrier frequency locking carrier signal transmitted from the transmitting side is intermittent in the time direction.

Further, the method of the present invention is characterized in that the frequency of the reference carrier frequency locking carrier signal transmitted from the transmitting side changes in the time direction.

Further, the method of the present invention is characterized in that the level of the reference carrier frequency locking carrier signal transmitted from the transmitting side is made higher than the carrier level of the OFDM modulated signal.

Further, according to the method of the present invention, the reference frequency locking carrier signal transmitted from the transmitting side has a plurality of OFDs.
It is characterized in that it is shared with the M modulation signal.

Furthermore, the apparatus for regenerating the reference carrier frequency for demodulating an OFDM modulated signal according to the present invention has, on the transmitting side, within the OFDM modulated signal band or outside the band.
The frequencies are f _A (= a · f _d ) + f _IF and f _B (= b ·
f _d ) + f _IF where a and b are natural numbers different from each other, and f _d is OFDM
The carrier frequency interval of, or f _IF is the reference carrier frequency locking carrier signal consisting of two frequencies that is the reference carrier frequency and is transmitted, and the receiving side receives the reference carrier frequency locking carrier consisting of the two frequencies. Multiplier means for multiplying each of the signals f _A + f _IF and f _B + f _IF by a reference carrier frequency f _IF obtained from a voltage controlled oscillator forming a phase-locked loop, and a base carrier frequency locking base which is basebanded by the means. Two band pass filters for extracting the band signals f _A and f _B respectively, and a constant a for the extracted base band signal f _B.
/ B by multiplying the constant multiplication means for generating _{(a / b) · f B} , the extracted and the constant multiplied respectively the f _A and obtained (a / b) · f correction value from the _B ε
(= F _A − (a / b) · f _B ), and at least a phase comparator configured to substantially supply the calculation result to the control signal input terminal of the voltage controlled oscillator. It is characterized by.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the accompanying drawings. FIG. 1 shows a block diagram of an example of a reference carrier frequency reproducing apparatus according to the present invention, and FIG. 2 shows a signal spectrum in each part ((A) to (G)) in the figure.

In FIG. 1, first, an OF including an unmodulated carrier signal for locking the two-frequency reference carrier frequency described above.
The DM modulated signal, (received IF signal) 100 (see FIG. 2 (A)) is a band pass filter (BPF) 101 and 10.
3, the OFDM modulated signal 102 (see FIG. 2B) and the reference carrier frequency locking carrier signal (hereinafter simply referred to as frequency locking carrier signal) 104 (see FIG. 2).
(See (E)) and The center frequency of the OFDM modulated signal 102 becomes the IF reference carrier frequency f _IF if there is no frequency shift. The OFDM modulated signal 102 is further separated into two, and an output signal 106 from a voltage controlled oscillator (VCO) 105 that generates an IF reference carrier frequency f _IF and an output signal 107 (here The output signals 106 and 107 mean the same signal), and the signals obtained by shifting the π / 2 phase by the π / 2 phase shifter 112 are multiplied by the multiplexers 109 and 110, and the low pass filters (LPF) 113 and 1 are provided.
The baseband I signal 11
5 (see FIG. 2C) and Q signal 116 (see FIG. 2D) are obtained. Up to this point, there is no difference from the normal OFDM modulation signal demodulation circuit.

On the other hand, the frequency lock carrier signal 104
Is a signal in the baseband band, which is multiplied by the output signal 108 from the VCO 105 described above (again, the output signal 108 means the same signal as the output signals 106 and 107) by the multiplexer 111, and two BPs are added.
The reference carrier frequency lock baseband signals (hereinafter simply referred to as frequency lock baseband signals) f _A and f _B by F 117 and 118 (FIG. 2 (F), respectively).
(See (G)) is extracted. Of these extracted frequency-locking baseband signals, the signal f _A passes through the 1 / N frequency divider 120 and is input to the phase comparator 122. Further, the signal f _B is output from the multiplier 119 by a constant a / b (where a, b
b, as described above, when the carrier frequency interval of the OFDM was _{f d f A = a · f} d, which is a natural number different from each other in a relation of f _B = b · f _d) is multiplied further, f
_{As in} the case of _A, the phase comparator 1 passes through the 1 / N frequency divider 121.
22 is input.

In the phase comparator 21, the control signal for controlling the VCO which receives the outputs of the 1 / N frequency dividers 120 and 121 and generates the reference carrier frequency, that is, the correction value ε (= f _A − (a / B) · f _B ) and calculates and outputs the signal. This output signal is the loop filter 123.
Is applied to the VCO 105 as described above. As a result, a feedback loop is formed, and the value of the correction value ε converges to zero.

Now, the center frequency f _IF of the received IF signal causes a frequency shift Δf ₁ due to the Doppler effect during propagation and becomes f _IF + Δf ₁ , and the VCO oscillation frequency of the receiver also _changes from the IF conversion frequency f _IF to Δf ₂ Deviation f _IF + Δf
Assuming a _two, the baseband signal f _A and f _B for frequency lock, both difference frequency (Delta] f ₁ -.DELTA.f
₂ ) is added (in the multiplexer 111, the shift Δf _{2 in} the positive direction of the VCO oscillation frequency appears as the shift −Δf _{2 in the} negative direction), and the correction value ε is the correction value ε = f _A − (A / b) · f _B = a · f _d + (Δf ₁ −Δf ₂ ) − (a / b) · {b ·
f _d + (Δf ₁ −Δf ₂ )} = {(ba−a) / b)} · {Δf ₁ −Δf ₂ }.

However, as described above, this correction value ε converges to zero by the feedback loop, and as a result,
Despite the frequency deviation of Δf ₁ and Δf ₂ above VC
It is possible to always keep the oscillation frequency of O at f _IF accurately and accurately reproduce the reference carrier frequency for reproducing the OFDM modulated signal.

In the above, the frequency locking carrier signals (f _A + f _IF , f _B + f _IF ) of two frequencies are both O.
Although it is arranged at a higher frequency position than the FDM modulated signal (information signal), this frequency locking carrier signal is OF
It is also possible to separately arrange the both sides of the band of the DM modulated signal. Further, it is also possible to arrange the frequency-locking carrier signal of two frequencies inside the band of the OFDM modulation signal without arranging it outside the band of the OFDM modulation signal. In this case, BPF 101 and 103
Are shared and the signals 102 and 104 can be unified.

Next, another example will be described. 3 is 3
_A carrier signal for frequency locking is used as the frequency-unmodulated carrier (baseband frequencies f _A = a · f _d , f _B = b ·
f _d , f _C = c · f _d , f _d are used as OFDM carrier frequency intervals: a, b, c are integers), and a block diagram of a reference carrier frequency regenerator for regenerating a reference carrier frequency is shown. There is.

In FIG. 3, the received IF signal 200 is B
The OFDM modulation signal 202 and the frequency lock carrier signal 204 are separated by the PFs 201 and 203, respectively. The center frequency of the OFDM modulated signal 202 becomes the IF reference carrier frequency f _IF if there is no frequency shift. The OFDM modulated signal 202 is further separated into two,
An output signal 20 from the VCO 205 that generates an IF reference carrier frequency f _IF for each of the separated modulated signals.
6 and the output signal 207 by π / 2 phase shifter 212
/ 2 phase-shifted signals are input to multiplexers 209, 2
10 by multiplying by 10 and passing through LPFs 213 and 214, respectively, to obtain baseband I signal 215 and Q signal 2
Get 16. The above is the same as the process of obtaining the I signal and the Q signal described with reference to FIG.

On the other hand, the frequency lock carrier signal 204
_{(F a + f IF, f} b + f IF, f c + f IF) is, VCO2
The output signal 208 from 05 is multiplied by the multiplexer 211 to form a signal in the base band, and the three BPFs 217, 218, and 219 extract the frequency lock base band signals f _A , f _B , and f _C , respectively. Each of these extracted signals is input to a level detection circuit (Det) 220, 221 and 222 to detect the level, and detection outputs 223 and 2 corresponding to the level are output.
24 and 225 are output.

Baseband for frequency lock extracted above
Signal f out of signal_AGoes through the 1 / N frequency divider 227
It is input to the phase comparator 229. Also, the signal f_BMultiply
A / b times by the frequency divider 226, and the 1 / N frequency divider 22
It is input to the phase comparator 229 through 8 This phase comparison
The device 229 uses the correction value ε as in the above-described embodiment._AB(= F _A
-(A / b) · f_B) Is output and the loop
Input to VCO control signal generation circuit 234 via filter 230
I will be forced.

As described above, a circuit for outputting a value proportional to the correction value ε _AB from the frequency lock baseband signals f _A and f _B through the loop filter 230 is corrected by the frequency lock baseband signals f _A and f _B. Value output circuit 23
Set to 1. Similarly, the frequency lock baseband signal f
_A circuit for outputting a value proportional to the correction value ε _AC from _A and f _C through the loop filter is a correction value output circuit 232 based on the frequency lock baseband signals f _A and f _C , and a frequency lock baseband signal f _B , From f _C , the correction value ε
A circuit that outputs a value proportional to _BC through the loop filter is a correction value output circuit 233 based on the frequency lock baseband signals f _B and f _C.

Baseband signal for each frequency lock
Of the three correction value output circuits 231 and 232 by the combination of
And the respective corrected output ε from 233_AB, Ε_ACAnd
And ε _BCTo generate a signal (control signal) to be supplied to the VCO.
Input to the VCO control signal generating circuit 234. VCO
In the control signal generation circuit 234, for each frequency lock
Base band signal level detection signals 223, 224 and
225 and 225, the level below a certain threshold
By ignoring the correction by the baseband signal,
Of the baseband signals for wave number lock, the level is
Secured carrier signal combinations (selection of combinations
V) such that the correction value obtained from
So that the CO control signal 235 is applied to the VCO 205
To do. According to this, the above-mentioned frequency lock key for two frequencies is used.
Carrier signal is sent from the transmitter and used as a reference carrier.
Obtained reference carrier frequency compared to the case of reproducing frequency
Accuracy is improved, and if the frequency lock key is
Signal f_A+ F_IFThe level of
Even if it decreases, the frequency locking carrier signal f that remains
_B+ F_IF, F_C+ F_IFTo generate the VCO control signal.
By doing so, stable reproduction of the reference carrier frequency is performed.
It becomes possible. In addition, it is not limited to 3 frequencies
Transmit carrier signal for wave frequency lock from the transmitter
It goes without saying that the system may be configured as described above. This
, Depending on the number of transmitted signals and frequency position
It is expected that some effect will occur.

In the above description, the OFDM modulation signal and the frequency locking carrier signal coexist at the same time, but this is not always necessary.
It may be transmitted intermittently in the time direction. This example will be described with reference to FIGS. 4 (a) to 4 (c).

In FIGS. 4A to 4C, when the frequency locking carrier signal is represented by f _A and f _B and the OFDM modulation signal (information signal) is represented by OFDM, FIG.
In addition to the FDM modulation signal, the frequency lock carrier signal frequency is fixed and constantly transmitted (concurrent concept). In the figure (b), the frequency lock carrier signal frequency is fixed and the OFDM signal is time-divided. The method of transmitting by multiplexing with, and FIG. 6C is a method of constantly transmitting while changing the frequency of the frequency-locking carrier signal in addition to the OFDM modulated signal. Of course, the same figure (b)
, (C) at the same time, while changing the frequency of the frequency lock carrier signal with time, OF
A method in which the DM modulated signal is multiplexed and transmitted may be used.

The carrier level of the frequency lock carrier signal can be made higher than the carrier level of the OFDM modulated signal for information transmission, and the reference carrier frequency reproduction on the receiving side can be stabilized.

FIG. 5 shows another spectrum form of the frequency lock carrier signal and the OFDM modulation signal according to the present invention. Frequency lock carrier signals f _A , f
_B , f _C , ---- are multiple OFDM modulated signals OFDM ₁ ,
By making it common to OFDM ₂ , OFDM ₃ , and so on, it is possible to make the synchronization system common by a plurality of OFDM modulated signals within a specific band. This allows integration between each OFDM signal (eg, OFDM ₁ and O
It is possible to enhance system flexibility such as integration of FDM ₂ into a wideband OFDM1 channel) and separation (for example, OFDM ₁ frequency separation to perform a plurality of services). Further, the frequency-locking carrier signal can be arranged at both upper and lower ends of the OFDM modulation signal group or in the OFDM modulation signal group.

In the above, by specifying the frequency of the frequency lock carrier signal or the time interval of intermittent transmission thereof, it is possible to specify the receivable receiver.

Further, by using the reference carrier frequency regenerated by the present invention, it is possible to frequency lock the FFT data sample clock during the OFDM modulation signal demodulation process and stabilize it.

[0031]

As described above, according to the present invention, even when there is a frequency shift due to the Doppler effect during propagation or a reference carrier frequency shift in the device on the receiving side, it is highly accurate and stable. It becomes possible to reproduce the reference carrier frequency and the FFT data sample clock.

[Brief description of drawings]

FIG. 1 shows a block diagram of an example of a reference carrier frequency recovery device according to the present invention.

FIG. 2 shows a signal spectrum in each portion ((A) to (G)) in the reference carrier frequency reproducing device shown in FIG.

FIG. 3 shows a block diagram of another example of a reference carrier frequency recovery device according to the present invention.

FIG. 4 shows a relationship in the time direction between a reference carrier frequency locking carrier signal and an OFDM modulated signal according to the present invention.

FIG. 5 shows another spectrum form of a carrier signal for locking a reference carrier frequency and an OFDM modulated signal according to the present invention.

[Explanation of symbols]

100 reception IF signal 101 OFDM modulation signal BPF 102 OFDM modulation signal 103 Frequency lock carrier signal BPF 104 Frequency lock carrier signal 105 Voltage controlled oscillator (VCO) 106, 107, 108 VCO output signal 109, 110, 111 Multiplexer 112 π / 2 phase shifter 113, 114 LPF 115 OFDM signal baseband I signal 116 OFDM signal baseband Q signal 117 Frequency lock baseband signal f _A BPF 118 Frequency lock baseband f _B BPF 119 Multiplier 120,121 1 / N frequency divider 122 Phase comparator 123 Loop filter 124 VCO control signal (correction value ε) 200 Received IF signal 201 BPF for OFDM modulation signal 202 OFDM modulation signal 203 Frequency Carrier signal BPF 204 frequency lock carrier signal 205 voltage controlled oscillator (VCO) 206, 207, 208 VCO output signal 209, 210, 211 multiplexer 212 π / 2 phase shifter 213, 214 LPF 215 base of OFDM signal Band I signal 216 OFDM signal baseband Q signal 217 Frequency lock baseband signal f _A BPF 218 Frequency lock baseband signal f _B BPF 219 Frequency lock baseband signal f _C BPF 220, 221, 222 Level Detection circuit 223, 224, 225 Level detection signal 226 Multiplier 227, 228 1 / N frequency divider 229 Phase comparator 230 Loop filter 231 Frequency lock baseband signals f _A , f _B correction value output circuit 232 Frequency ro Correction value output circuit 233 based on the base band signals f _A and f _C for correction, and a correction value output circuit 234 based on the base band signals f _B and f _C for frequency locking 234 VCO control signal generation circuit 235 VCO control signal (correction value ε)

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kenichi Tsuchida 1-10-11 Kinuta, Setagaya-ku, Tokyo Inside the Broadcasting Technology Laboratory, Japan Broadcasting Corporation (72) Makoto Sasaki 1-1-10 Kinuta, Setagaya-ku, Tokyo Issue Broadcasting Technology Institute of Japan Broadcasting Corporation (72) Inventor Shigeru Yamazaki 1-10-11 Kinuta, Setagaya-ku, Tokyo Inside Broadcasting Technology Laboratory of Japan Broadcasting Corporation

Claims (7)

[Claims] 1. A method of recovering a reference carrier frequency for demodulating an OFDM modulated signal at a receiving side, wherein the transmitting side has a frequency f _A (= a · f) within or outside the OFDM modulated signal band. _d ) + f _IF and f _B
(= B · f _d ) + f _IF where a and b are natural numbers different from each other, f _d is a carrier frequency interval of OFDM, and f _IF is a carrier signal for locking a reference carrier frequency consisting of two frequencies which are reference carrier frequencies. Then, on the receiving side, the received carrier signals f _A + f _IF and f _B for locking the reference carrier frequency having the two frequencies are received.
Each of + f _IF is multiplied by the reference carrier frequency f _IF as an oscillation output signal of the oscillator, which is obtained from a voltage controlled oscillator forming a phase locked loop, to generate a base band signal, and from the signal converted to the base band signal. A correction value ε (= f _A − (a / b) · f _B ) is generated, and the generated correction value is applied to the control signal input terminal of the voltage controlled oscillator. Is converged to zero by the action of the phase-locked loop, a method for recovering a reference carrier frequency for demodulating an OFDM modulated signal. 2. The method of reproducing the reference carrier frequency according to claim 1, wherein the reference carrier frequency locking carrier signal transmitted from the transmitting side has three or more frequencies, and the receiving side receives the three frequencies. A method for reproducing a reference carrier frequency for demodulating an OFDM modulated signal, wherein two frequencies are selected and used from the above carrier signals for locking a reference carrier frequency. 3. The method for recovering a reference carrier frequency according to claim 1 or 2, wherein the reference carrier frequency locking carrier signal transmitted from the transmitting side is intermittent in the time direction. A method of recovering a reference carrier frequency for signal demodulation. 4. The method of reproducing the reference carrier frequency according to claim 1, wherein the reference carrier frequency locking carrier signal transmitted from the transmitting side has a frequency changing in the time direction. O characterized by
A method for recovering a reference carrier frequency for demodulating an FDM modulated signal. 5. The method of reproducing the reference carrier frequency according to claim 1, wherein the level of the reference carrier frequency locking carrier signal transmitted from the transmitting side is higher than the carrier level of the OFDM modulated signal. A method for reproducing a reference carrier frequency for demodulating an OFDM modulated signal, characterized in that 6. The method of reproducing the reference carrier frequency according to claim 1, wherein the reference carrier frequency locking carrier signal transmitted from the transmitting side is shared by a plurality of OFDM modulated signals. A method for recovering a reference carrier frequency for demodulating an OFDM-modulated signal, comprising: 7. An apparatus for regenerating a reference carrier frequency for demodulating an OFDM modulated signal at a receiving side, wherein the transmitting side has a frequency of f _A (= a · f) within or outside the OFDM modulated signal band. _d ) + f _IF and f _B
(= B · f _d ) + f _IF where a and b are natural numbers different from each other, f _d is a carrier frequency interval of OFDM, and f _IF is a carrier signal for locking a reference carrier frequency consisting of two frequencies which are reference carrier frequencies. Then, on the receiving side, the received carrier signals f _A + f _IF and f _B for locking the reference carrier frequency having the two frequencies are received.
+ F _IF is multiplied by a reference carrier frequency f _IF obtained from a voltage controlled oscillator forming a phase locked loop, and a reference carrier frequency locking base band signal f _A and f _B basebanded by the means. The two band pass filters to be respectively extracted and the extracted base band signal f _B are multiplied by a constant a / b (a /
b) · f _B and a constant multiplying means for generating said respective obtained is the extracted and the constant multiplier f _A and (a / b) · f correction value from the _{B ε (= f A - (} a / b)
· F _B) is calculated, and is characterized in that the calculation result comprises at least a phase comparator configured to substantially supply the control signal input terminal of the voltage controlled oscillator OFD
A device for reproducing a reference carrier frequency for demodulating an M-modulated signal.