JP3183659B2 - Orthogonal frequency division multiplexed signal demodulation method and receiver - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an orthogonal frequency division multiplexing (OFDM) technique, and more particularly to an OFDM technique.
The present invention relates to a method for demodulating an orthogonal frequency division multiplexed signal and a receiver for demodulating a signal.

[0002]

BACKGROUND OF THE INVENTION OFD where the information to be transmitted is modulated in parallel on a large number of precisely positioned adjacent carriers
The M signal has a number of advantages, as is well known, including avoiding or reducing fading due to multipath interference. This is particularly advantageous for use in audio broadcasting (DAB) and for use in video broadcasting (DVB).

[0003] In the conventional OFDM signal demodulation method,
It produces a highly accurate local oscillator signal that is mixed with the received RF signal. This produces an intermediate frequency (IF) signal, which is then sampled.
The samples are then transformed into separate real and imaginary components, which are transformed into the frequency domain. The result is a multiplexed output, each associated with a carrier. In that case, each output represents one of the four possible phase values. (For example, in one example of a typical DAB signal, there are 1536 carriers.)

In this case, if there is an error in the frequency of the local oscillator, a sampling error occurs. Therefore,
Great efforts have been made to improve the accuracy and speed of the locking-in process during tuning. In addition, an analog-to-digital converter (A / D
Efforts have been made to accurately match the sampling times of the (transformer) to the received signal, and to accurately match the Fourier transform process used to transform to the frequency domain to the received code.

[0005]

SUMMARY OF THE INVENTION It is desirable to avoid or mitigate the problems inherent in satisfying the above requirements, and a simpler and less expensive method and apparatus for demodulating an orthogonal frequency division multiplexed signal. Was desired.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and requires timing synchronization for demodulating a carrier modulated by random sampling, in particular, aperiodic or substantially aperiodic sampling. It is an object of the present invention to provide a simple and inexpensive method for demodulating an orthogonal frequency division multiplexed signal and a receiving apparatus capable of avoiding performance.

[0007]

SUMMARY OF THE INVENTION In view of the above, the present invention provides a method for demodulating an orthogonal frequency division multiplexed signal, the method comprising: sampling the signal at substantially non-periodic intervals; A method for demodulating an orthogonal frequency division multiplexed signal, comprising a step of converting the signal into a frequency domain.

[0008] Further, there is provided the method for demodulating an orthogonal frequency division multiplexed signal according to claim 1, wherein the sampled signal is an RF signal.

[0009] The demodulation method of an orthogonal frequency division multiplexed signal according to claim 1, wherein the sampled signal is an IF signal.

The method according to claim 3, wherein the IF signal is derived by mixing the RF signal with the free-running local oscillator signal.

The method according to claim 3, wherein the IF signal is derived by mixing the RF signal with the stabilized local oscillator signal.

The demodulation method of an orthogonal frequency division multiplexed signal according to claim 3, wherein the IF signal is derived by mixing the RF signal with a local oscillator signal locked to the received signal.

The method according to any one of claims 1 to 6, wherein the sampling timing forms an additive random sequence.

The method according to claim 1, further comprising the step of determining the distribution of orthogonal frequency division multiplexed carriers in the spectrum of the signal converted into the frequency domain, and generating output data in consideration of the distribution. 7. The method for demodulating an orthogonal frequency division multiplexed signal according to any one of (1) to (7).

A receiving apparatus for demodulating an orthogonal frequency division multiplexed signal includes means for sampling the signal at substantially non-periodic intervals, and means for converting the sampled signal into a frequency domain. And an orthogonal frequency division multiplexed signal receiving apparatus.

[0016] The receiving apparatus for an orthogonal frequency division multiplexed signal according to claim 9, wherein the sampled signal is an RF signal.

[0017] The receiving apparatus for an orthogonal frequency division multiplexed signal according to claim 9, wherein the sampled signal is an IF signal.

An apparatus according to claim 11, further comprising means for deriving an IF signal by mixing an RF signal with a free-running local oscillator signal.

12. The apparatus for receiving an orthogonal frequency division multiplexed signal according to claim 11, further comprising means for deriving an IF signal by mixing the RF signal with a stabilized local oscillator signal. .

12. The orthogonal frequency division multiplexed signal receiving apparatus according to claim 11, further comprising means for deriving an IF signal by mixing the RF signal with a local oscillator signal locked to the received signal. It is in.

[0021] The receiving apparatus for an orthogonal frequency division multiplexed signal according to any one of claims 9 to 14, wherein the sampling timing forms an additive random sequence.

Further, there is provided means for judging the distribution of orthogonal frequency division multiplexed carriers in the spectrum of the signal converted into the frequency domain, and generating output data in consideration of the distribution. 16. An apparatus for receiving an orthogonal frequency division multiplexed signal according to any one of claims 15 to 15.

According to one aspect of the invention, a received signal
The OFDM signal is demodulated by sampling the (RF or IF signal) at random time intervals, and the resulting sample signal is converted to the frequency domain. Random sampling, especially non-periodic or substantially non-periodic sampling, not only allows demodulation of the modulated carrier, but also makes the sampling
When used to extract the demodulated signal directly from the signal, the need for a local oscillator can be avoided and the IF
The need for an accurate local oscillator can be avoided when sampling the signal. Furthermore, according to the technique of the present invention, the necessity of timing synchronization can be avoided, and only approximate code synchronization is required.

The above-described arrangement of the present invention ensures that the received signal is accurately tuned, and then samples the signal at random time intervals with the prior art technique of sampling the signal at exactly predetermined times. It will be appreciated that there is a major difference in doing so.

In the preferred embodiment of the present invention, the sampling times are t _n = t _n-1 +
Form an additive random sequence of the form e _n .

[0026] In the above formula, t _n represents the n-th sampling time, t _n-1 represents the sampling time of the preceding, and e _n denotes the period value in each section is changed randomly. This type of sequence has poor periodicity,
Therefore, the sampling timing t _n = _n T + en, ie, the sampling interval, is advantageous compared to other sequences used in conventional dithering techniques in which the sampling interval is _equal to the nominal period T plus a randomly determined period. It is. If the signal is periodic, aliasing
= Aliasing noise), which limits the frequency range in which the technique is effective or effective.

[0027]

Embodiments of the present invention will be described below with reference to the accompanying drawings. Embodiment 1 FIG. FIG. 1 shows an OF according to an embodiment of the present invention.
FIG. 2 shows a DM signal receiving apparatus, and FIG. 2 shows a conventional OFDM signal receiving apparatus. First, referring to FIG. 2, a conventional OFDM signal receiving apparatus includes an antenna 11 for receiving a transmitted RF signal. The RF bandpass filter 12 passes the selected multiplex signal and removes other signals. The mixer 13 includes the RF signal and the local oscillator 1
9 to generate a sum signal and a difference signal between the RF signal and the output signal of the local oscillator. The IF bandpass filter 14 attenuates the undesired sideband signal and supplies an IF signal to an analog-to-digital converter (ADC) 15. The ADC 15 samples the filtered IF signal at precise regular time intervals. The sampled signal is then converted from the real signal to an in-phase and quadrature component by a (real-imaginary / quadrature component) converter 16. The in-phase and quadrature components are referred to as an I signal and a Q signal, respectively.

The processor 17 performs a fast Fourier transform (F
FT) to convert the I and Q signals into the frequency domain.

The phase demodulator 18 extracts the phase of the FFT output including the transmitted data. In this manner, for each carrier, phase information representing a modulation code for the carrier is generated by Fourier transform of the I signal and the Q signal.

Code / timing / frequency synchronizer 20
Receives the signal from the phase demodulator 18 and (a) a frequency synchronization signal used to control the local oscillator 19 to ensure accurate tuning of the receiver, and (b) sampling at an appropriate timing. A timing synchronizing signal to be supplied to the ADC 15 for securing and a code synchronizing signal to be supplied to the fast Fourier transform (FFT) processor 17 for (c) locating each transmission code accurately.

Referring now to FIG. 1, which illustrates an embodiment of the present invention. Components corresponding to the circuit components shown in FIG. 2 are denoted by the same reference numerals as those used in FIG.

As is clear from FIG. 1, in the present embodiment, the local oscillator 19, the mixer 13 or the IF band filter 14 is not provided. Therefore, in the circuit configuration of FIG. 1, the filtered RF signal is directly supplied to the ADC.
15 is supplied. The ADC 15 samples the signal at a timing determined by the unequal sampling timing generator 7. The generator 7 is shown in detail in FIG. The sampling timing generator 7
Has a crystal oscillator 9, and an output pulse of the crystal oscillator 9 is sent to a pulse selection circuit 8. The pulse selection circuit 8 also receives the output of the pseudo-random number generator 10. The output of the pseudo-random number generator 10 represents an index that determines which pulses to delete and which pulses to leave in a pulse train or pulse stream. As one example,
Time interval between two successive selected pulse P _n and P _n-1 corresponds to r _n pieces of clock pulses, where, r _n
Represents the generated random number. The selected pulse is output from the generator 7. Thus, the output of the generator 7 is a pulse stream, i.e., a pulse stream, with a random integer number of clock periods between each individual pulse. It should be noted that the term "random" sequence in this specification is used with the intention of including a pseudo-random sequence.

The output of the ADC 15 is supplied to the I / Q converter 1
6. The I / Q converter 16 performs conversion using the above sampling timing, whereby the I signal and the Q signal are converted into a frequency domain by the time-frequency conversion processor 21. In this embodiment, a standard FFT processor cannot be used. This is because standard FFT processors are configured to operate on received samples derived at regular time intervals. Therefore, in the present embodiment, the discrete Fourier transform (DFT) is used as the processor 21.
A processor is used. The sampling timing signal is provided to DFT processor 21 and stored therein for use in the conversion process.

The output of the processor 21 is supplied to the demodulator 18.

FIG. 4 is a spectral diagram showing the output power of the DFT processor 21 measured over a code period in a particular practical example. As can be seen, random sampling results in blurry alias frequencies that look like noise. Such alias frequencies could be reduced by increasing the average sampling rate. However, even with the noise levels shown in FIG. 4, the central high level region shown, which represents the region containing the carrier, can be easily distinguished from other regions. The reason is that the received carrier and the level outside the frequency range of interest are separated by as much as about 20 dB.

FIG. 5 is a diagram illustrating the effect of the alias frequency in the demodulation process. In FIG.
Each point represents the detected phase of the individual modulation code associated with each carrier. In a perfectly tuned ideal system,
A single point value will appear in each of the four quadrants (quadrature). However, due to random sampling, the values in each quadrant (quadrature) are blurred. Nevertheless, as long as no overlap occurs between the point groups, it is possible to demodulate the code without error.

FIGS. 4 and 5 show that the center frequency is 201M.
Hz, carrier spacing 1 kHz, and code duration 1
ms having a random phase for each carrier
A specific example simulating the B1536 carrier signal is shown. To extract the sign, the average sampling frequency 8
Take 8000 aperiodic samples at MHz.
A suitable sampling function for DAB signals is an additive random timing sequence in 2 ns corresponding to an average sampling frequency of 8 MHz, a standard deviation of 0.3 and a greatest common divisor of 1/500 MHz. The above figure shows 2000 (assuming that the actual sampling data is 8 bits and the imaginary sampling data is 8 bits).
The output of the complex point DFT is shown.

For example, Eureka operating in mode I
Similar results can be expected in an implementation such as a receiver suitable for a 147 DAB system. The parameters for this signal are 1 kHz apart, with a code duration of 1246 μs and a guard of 246 μs.
There are 1536 carriers with (protected) sections. The UK Radio Frequency Audio Broadcasting (UK RF DAB) frequency is 220
MHz range. Suitable crystal oscillator frequencies for these parameters are in the region of 500 MHz. Also, the RF bandpass filter 12 preferably has a bandwidth of about 2 MHz.

It is preferable that the RF band-pass filter 12 has a narrow bandwidth in order to avoid a large adjacent signal that generates unclear aliasing that erases a desired signal.

Embodiment 2 FIG. 6 shows another embodiment of the present invention. In this embodiment, the RF signal from the filter 12 is converted to an IF signal, so that the filter 12 need not have an accurately set narrow bandwidth as described above. Mixer 13 receives a signal from free-running local oscillator 19 and generates an IF signal filtered by IF bandpass filter 14. Therefore, ADC1
5 samples the IF signal instead of the RF signal, unlike the embodiment shown in FIG.

Since the local oscillator 19 is a free-running local oscillator, the IF carrier frequency cannot be accurately predicted. However, the number of complex point Fourier transforms may exceed the number of carriers (eg, 2536 for 1536 carriers).
000 complex points Fourier transform), the carrier is within the frequency range of the transform.

It will be appreciated that the location of the transformed carrier within the output of the DFT can vary depending on the local oscillator frequency. Referring to FIG. 7, the vertical line represents the “frequency bin (frequ
ency bins) ". In a conventional receiver, the frequency of the local oscillator is determined by a predetermined bin, for example bin I
Is adjusted so that M carrier waves are located within M from. Accordingly, demodulator 18 generates data based on the contents of these bins. In the present embodiment, the local oscillator 19 is not adjusted. Therefore, determine the carrier distribution
After (eg, offset x represents the position of the lowest frequency carrier), demodulator 18 outputs data based on the contents of the carrier in M + x from bin x.

The frequency spectrum in which the transformed carrier is distributed can be determined in any of a number of ways. For example, each of the frequency outputs of the Fourier transform process can be checked with reference to a threshold, thereby determining the frequency slot containing the transformed carrier. Alternatively, it is possible to check the demodulated data for each frequency slot. A frequency slot containing a randomly distributed phase does not contain a carrier, while a frequency slot whose phase is substantially equal to the four expected phases corresponding to the modulation of the signal is a frequency slot containing a carrier. Conceivable. Further, alternatively, the distribution of the transformed carrier can be determined by checking the contents of the demodulated data. As one particular example, the output of the modulator is checked to determine whether the pattern of the individual modulation codes corresponds to a known transmitted pattern, for example, a phase reference code transmitted in a conventional system. It is possible to decide.

Thus, the demodulator can determine the position of the OFDM carrier in the output spectrum of the DFT processor and use that information to generate demodulator output data, as described above.

It will be appreciated that with the above technique, any variation in carrier distribution can be corrected, eliminating the need to convert individual carriers to specific frequency slots as in the prior art. Will. Such correction also corrects signal distortion of a type that causes a spectral transition (for example, Doppler transition) of the received RF signal. Therefore, such a distribution correction technique is required to deal with the above-described signal distortion, as shown in FIG.
Can be advantageously applied to the embodiment shown in FIG.

Instead of using a free-running oscillator, the local oscillator 19 can be locked to a stable external source to reduce temporal frequency drift, or can be locked to an input received signal to reduce frequency drift as in conventional receivers. It is possible to compensate for the transition.

[0047]

As described above, the present invention relates to demodulation of an orthogonal frequency division multiplexed signal by sampling a received signal at non-periodic time intervals and converting the sampled signal into a frequency domain. Since the signal is demodulated, it is possible to demodulate a carrier modulated by random sampling, particularly non-periodic or substantially non-periodic sampling, and to orthogonal frequency division avoiding the need for timing synchronization. A method for demodulating a multiplexed signal can be provided.

A receiving apparatus for demodulating an orthogonal frequency division multiplexed signal may include means for sampling the signal at substantially non-periodic intervals, and means for converting the sampled signal into a frequency domain. Therefore, it is possible to provide a simple and inexpensive orthogonal frequency division multiplexed signal receiving apparatus.

[Brief description of the drawings]

FIG. 1 is a block diagram of a receiving device according to an embodiment of the present invention.

FIG. 2 is a block diagram of a conventional receiving device.

FIG. 3 is a block diagram showing a sampling timing generator used in the embodiment shown in FIG.

FIG. 4 is a diagram showing a spectrum of an RF signal supplied to a demodulator in the present invention.

FIG. 5 is a diagram of a scattering diagram drawn on an arbitrary scale showing the phase of a signal demodulated in the present invention.

FIG. 6 is a block diagram of a receiving device according to another embodiment of the present invention.

FIG. 7 is a diagram schematically illustrating an output of a discrete Fourier transform processor which is the time-frequency transform processor illustrated in FIG. 6;

[Explanation of symbols]

7 Non-uniform sampling timing generator, 8 pulse selection circuit, 9 crystal oscillator, 10 pseudo-random number generator, 11 antenna, 12 RF bandpass filter, 13
Mixer, 14 IF bandpass filter, 15 A / D converter
(ADC), 16 real-imaginary / quadrature phase component converter, 18 phase demodulator, 19 free-running local oscillator, 21 time-frequency conversion processor (DFT processor).

Continuation of front page (56) References JP-A-10-107629 (JP, A) JP-A-4-140924 (JP, A) JP-A-9-243679 (JP, A) (58) Fields investigated (Int .Cl. ⁷ , DB name) H04J 11/00

Claims (16)

(57) [Claims] 1. A method for demodulating an orthogonal frequency division multiplexed signal, comprising the steps of sampling the signal at substantially aperiodic intervals and transforming the sampled signal into the frequency domain. A method for demodulating a frequency division multiplexed signal. 2. The method for demodulating an orthogonal frequency division multiplexed signal according to claim 1, wherein the sampled signal is an RF signal. 3. The demodulation method of an orthogonal frequency division multiplexed signal according to claim 1, wherein the sampled signal is an IF signal. 4. The method for demodulating an orthogonal frequency division multiplexed signal according to claim 3, wherein an IF signal is derived by mixing the RF signal with a free-running local oscillator signal. 5. The method according to claim 3, wherein an IF signal is derived by mixing the RF signal with a stabilized local oscillator signal. 6. The method according to claim 3, wherein the IF signal is derived by mixing the RF signal with a local oscillator signal locked to the received signal. 7. The demodulation method of an orthogonal frequency division multiplexed signal according to claim 1, wherein the sampling timing forms an additive random sequence. 8. The method according to claim 1, further comprising the step of determining the distribution of orthogonal frequency division multiplexed carriers in the spectrum of the signal converted to the frequency domain, and generating output data in consideration of the distribution. 8. The method for demodulating an orthogonal frequency division multiplexed signal according to any one of claims 7 to 7. 9. A receiver for demodulating an orthogonal frequency division multiplexed signal, comprising: means for sampling the signal at substantially non-periodic intervals; and means for converting the sampled signal into the frequency domain. An apparatus for receiving an orthogonal frequency division multiplexed signal. 10. The apparatus for receiving an orthogonal frequency division multiplexed signal according to claim 9, wherein the sampled signal is an RF signal. 11. The apparatus for receiving an orthogonal frequency division multiplexed signal according to claim 9, wherein the sampled signal is an IF signal. 12. The orthogonal frequency division multiplexed signal receiving apparatus according to claim 11, further comprising means for deriving an IF signal by mixing the RF signal with a free-running local oscillator signal. 13. The apparatus of claim 11, further comprising means for deriving an IF signal by mixing the RF signal with a stabilized local oscillator signal. 14. The apparatus of claim 11, further comprising means for deriving an IF signal by mixing the RF signal with a local oscillator signal locked to the received signal. . 15. The method according to claim 9, wherein the sampling timing forms an additive random sequence.
15. The receiving apparatus for an orthogonal frequency division multiplexed signal according to any one of claims 14 to 14. 16. Determine the distribution of orthogonal frequency division multiplexed carriers in the spectrum of the signal transformed to the frequency domain,
16. The apparatus for receiving an orthogonal frequency division multiplexed signal according to claim 9, further comprising means for generating output data in consideration of said distribution.