JP2001203663A - Orthogonal frequency division multiplex transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an orthogonal frequency division multiplex transmission system that can stably transmit/receive a reference carrier clock signal and a symbol clock signal with a simple circuit configuration without the need for provision of a special synchronization period. SOLUTION: The OFDM transmission system consists of means 6-12 that transmit an OFDM transmission signal by inserting a reference pilot carrier signal of a reference setting level L1 to a guard interval period of the OFDM transmission signal and inserting a reference pilot carrier signal of a level L2 less than the reference setting level L1 to other periods than the guard interval period in succession to the former reference pilot carrier signal, of a means 13 that receives the signal from the transmission means, and of means 17-19 that recover a reference carrier frequency to obtain base band I, Q signals of the OFDM signal by means of a PLL loop consisting of a voltage controlled oscillator that utilizes the received signal and locks the frequency of the received signal.

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) transmission system, and more particularly to a system for synchronously generating a symbol clock and a sample clock of a transmitter and a receiver.

[0002]

2. Description of the Related Art The OFDM system is a digital modulation / demodulation system capable of setting up a large number (approximately 256 to 1024) of subcarriers in one channel band and efficiently transmitting a video signal and an audio signal.

The OFDM frequency spectrum is as shown in FIG. A sum of all carriers digitally modulated with the effective symbol period ts as a basic unit is called an OFDM transmission symbol.

[0004] The actual transmission symbol is usually shown in FIG.
As shown in (1), the guard interval period tg is added to the effective symbol period ts. The waveform of the guard interval period is obtained by repeating the signal waveform at the end of the effective symbol period. On the receiving side, information is extracted from the remaining part without using the signal in the guard interval.

This applies a window to the received signal,
This means that fast Fourier transform (FFT) is performed to demodulate a signal for an effective symbol period. In other words, in order to demodulate an OFDM modulated wave, how to process carrier synchronous reproduction on the receiving side and accurate extraction of an effective symbol period in order to FFT received data is an important technical issue. I have.

In the conventional OFDM modulation / demodulation system, an unmodulated pilot carrier as shown in FIG. 2 is inserted separately from the OFDM signal carrier on the transmitter side, the frequency and phase of this pilot carrier are detected, and the OFDM subcarrier is detected. A method of performing quadrature demodulation by synchronizing the entire frequency and phase has been proposed (1992 NHK Technical Research Institute Open Research Proceedings pp.28-36).

[0007] In the above example, the reference pilot carrier outside the OFDM modulation spectrum band is set, but the pilot carrier may be arranged in the OFDM modulation spectrum band. Then, in order to perform effective symbol synchronization, a non-modulated pilot carrier different from that for reference carrier reproduction is required.

In Japanese Patent Laid-Open Publication No. Hei 10-303852, a dedicated synchronizing signal section is also set on the transmission format, a pilot carrier signal is intermittently transmitted in this section, and the pilot carrier signal is extracted on the receiving side. A method of synchronously reproducing a reference carrier frequency has also been proposed. Also, consider a transceiver system having a configuration as shown in FIG. 2 using the OFDM transmission method.

As shown in FIG. 3, the transmitters 1 to n each transmit an intermittent carrier spectrum OFDM signal having a frequency relationship in which the carrier spectra do not overlap with each other. The receiver receives all the transmission signals from the transmitters 1 to n at the same time and performs signal processing collectively.

In order for the receiver to collectively receive the transmission signals from the transmitters 1 to n, it is necessary that the transmission signals of the plurality of transmitters 1 to n are synchronized with each other. Become. As a method of synchronizing the transmitters for that purpose, a method shown in FIG. 3 can be considered.

[0011] The receiver transmits a continuous single-wave or plural-wave reference pilot carrier signal to the transmitters 1 to n for synchronous reproduction of the reference carrier frequency and the symbol clock. The transmitters 1 to n reproduce the reference carrier frequency and the symbol clock by using the received unmodulated carrier signal, and the transmitters 1 to n transmit the synchronized OFDM modulated signals.

[0012]

The conventional method of intermittently transmitting a pilot carrier signal from a transmitter in a synchronization signal section and synchronously reproducing a reference carrier frequency in a receiver requires a dedicated synchronization section. , The transmission capacity is reduced accordingly.

The extracted reference carrier signal is OFD
It is necessary to hold the M data section and make the continuous signal by aligning and adding the phases. However, it is not easy to hold the OFDM data section which is 10 times or more of the synchronization signal section accurately and to generate the repetitive signal. In particular, it is not suitable for demodulation of multi-level QAM.

The intermittent transmission of the reference carrier is equivalent to the ASK (Digital Amplitude Modulation) system, which is not a continuous single spectrum but has a side-lop and a wide spectrum.
Interference with the DM data area also occurs.

Next, regarding a method of synchronizing a plurality of transmitters, it is necessary that the receiver receives a weak radio wave and simultaneously transmits a synchronization signal to the plurality of transmitters. In the receiver, the reception of the OFDM data and the transmission of the synchronization signal are performed simultaneously. Therefore, the synchronization transmission signal often has a much stronger electric field strength than the reception data signal.
There is a possibility that the DM reception signal receives strong interference of the synchronous transmission data and the error rate is deteriorated.

For this purpose, a method of setting a pilot carrier outside the OFDM data band and removing it with a BEF (Band Emissive Filter) in the receiver is conceivable. However, a BEF that sufficiently reduces interference is manufactured. Have difficulty. For this reason, the occupied bandwidth is widened, and the problem of considerably lowering the frequency use efficiency occurs. Further, in order to suppress the occupied bandwidth, it becomes more difficult to set a reference pilot carrier in the OFDM band.

[0017]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention is directed to a transmitting unit for transmitting signals by an orthogonal frequency division multiplexing modulation method and receiving signals from the transmitting unit. In the orthogonal frequency division multiplexing transmission system including a receiving unit, the transmitting unit inserts a reference pilot carrier signal of a reference setting level into a guard interval period constituting an orthogonal division frequency multiplexing transmission signal of the orthogonal division frequency multiplexing modulation method. Then, during a period other than the guard interval, a transmission unit that inserts and transmits a reference pilot carrier signal whose amplitude level is less than the reference set level, continuously with the reference pilot carrier signal, A receiving unit that receives the transmission signal transmitted from the transmitting unit; and a reference signal output from the receiving unit. The PLL loop constituted by locking the lot carrier signal frequency by a voltage controlled oscillator,
A quadrature frequency division multiplex transmission system, comprising: reproduction means for reproducing a reference carrier frequency for obtaining baseband I and Q signals of the quadrature division frequency multiplex signal. The invention of claim 2 is an orthogonal frequency division multiplexing transmission system that transmits by an orthogonal frequency division multiplexing modulation system and includes a transmission unit and a reception unit that receives a signal from the transmission unit. A reference pilot carrier signal of a reference set level is inserted in a guard interval period constituting an orthogonal division frequency multiplex transmission signal of a multiplex modulation scheme, and during a period other than the guard interval, an amplitude level is continuously set with the reference pilot carrier signal. Has a transmission means to insert and transmit a reference pilot carrier signal less than the reference setting level The receiving unit includes a receiving unit that receives a transmission signal from the transmitting unit, a level detecting unit that detects a level change of the signal received by the receiving unit, and a symbol detected by the level change detecting unit. A quadrature frequency division multiplexing transmission system characterized by comprising means for synchronizing symbols by means of a PLL loop comprising a voltage-controlled oscillator by locking this signal from a timing signal; Claim 3
According to the invention, in an orthogonal frequency division multiplexing transmission system that performs transmission by an orthogonal frequency division multiplexing modulation method, the orthogonal frequency division multiplexing transmission system includes a plurality of transmission units and a reception unit that receives transmission signals from the plurality of transmission units, Receiving means for receiving transmission signals from the plurality of transmission units, and inserting a reference pilot carrier signal of a reference setting level into a guard interval period constituting a transmission signal of the orthogonal division frequency multiplexing modulation method, other than the guard interval And transmitting means for inserting and transmitting a reference pilot carrier signal whose amplitude level is lower than the reference set level continuously with the reference pilot carrier signal, and Extracts the reference carrier signal from the synchronous transmission signal received by each transmitter and reproduces the reference carrier frequency And a means for generating an orthogonal frequency division multiplexing transmission signal synchronized with the reproduced reference carrier signal, and the signal from the transmitting means of the receiving section is transmitted to the plurality of transmitting sections. There is provided an orthogonal frequency division multiplex transmission system characterized by transmitting simultaneously, and a fourth aspect of the present invention provides an orthogonal frequency division multiplex transmission system for performing transmission by an orthogonal frequency division multiplex modulation scheme. A transmission unit and a reception unit that receives transmission signals from the plurality of transmission units; the reception unit includes a reception unit that receives transmission signals from the plurality of transmission units; and the orthogonal division frequency multiplex modulation scheme. A reference pilot carrier signal of a reference set level is inserted in a guard interval period that constitutes a transmission signal of the reference signal. Transmitting means for inserting and transmitting a reference pilot carrier signal whose amplitude level is less than the reference setting level, in succession to the pilot carrier signal, and wherein each of the plurality of transmitting units has a transmitter Means for detecting a level change of the received synchronous transmission signal, and means for locking the signal from the symbol timing signal detected by the level change detecting means and performing symbol synchronization by a PLL loop comprising a voltage controlled oscillator. The orthogonal frequency division multiplexing transmission system characterized in that the signal from the transmitting unit of the receiving unit is configured to be simultaneously transmitted to the plurality of transmitting units. is there.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the orthogonal frequency division multiplex transmission system of the present invention will be described below with reference to the drawings. As shown in FIG. 8, an orthogonal frequency division multiplexing (OFDM) signal is a combination of a number of digitally modulated waves f1, f2,..., Fn orthogonal to each other, and this period is referred to as an effective symbol period ts and an effective symbol period ts. One symbol is formed in the guard interval period tg in which the signal waveform at the end of the symbol period is repeated.

OFDM with guard interval period
In the case of a signal, the signal during the guard interval is
The signal in the effective symbol period is used cyclically and repeatedly (see FIG. 1A).

Next, an embodiment of the orthogonal frequency division multiplexing transmission system according to the present invention constructed based on the above principle will be described with reference to the drawings. FIG. 1 shows an embodiment of the relationship between an OFDM transmission signal and an unmodulated pilot carrier signal of a reference carrier frequency transmitted from the transmission side of an OFDM transmission / reception apparatus according to the present invention. One embodiment of the system is shown in the block diagram of FIG. 4, and another embodiment is shown in the block diagram of FIG.

In FIG. 1, in the transmission synchronization signal, a pilot carrier having a reference set level L1 exists in a guard interval period of an OFDM transmission signal, and a pilot carrier having a level L2 lower than the reference set level exists in an OFDM data period. Is configured.

OFDM for guard interval period
If the transmission synchronization signal is configured such that a pilot carrier of level L2 less than the reference set level is present in the data period, the receiver removes the guard interval period and performs the OFDM demodulation process. Does not affect the OFDM data demodulation performance.

Further, since the level of the synchronization signal is lower in the OFDM data transmission signal period than in the guard interval period, it is possible to suppress the deterioration of the OFDM demodulation performance.

On the other hand, although there are fluctuations in the level due to fluctuations in the transmission path, etc., since the synchronization signal itself is always continuous,
Synchronization signal completely 0 (null) during OFDM data transmission period
As compared with the conventional method, the performance of reference carrier synchronous reproduction can be improved, and the spread of the spectrum of the pilot carrier signal can be suppressed.

The level ratio (L1 / L2) of the synchronization signal between the guard interval period (level L1) and the OFDM data transmission period (level L2) depends on the transmission modulation method (QPSK, n-value QAM, etc.) and the required BER (bit). Error rate)
Etc., and each of them sets according to the situation.

Further, since the timing of the level change coincides with the symbol timing, the symbol synchronization signal can be reproduced without needing another pilot carrier for symbol synchronization.

Next, an embodiment of the orthogonal frequency division multiplex transmission system will be described below with reference to FIG.
One embodiment of the orthogonal frequency division multiplex transmission system of the present invention is an ECC input circuit 1, an inverse Fourier transform (IFFT) circuit 2, a guard interval adding circuit 3, a master clock circuit 4, a D / A conversion circuit 5, a quadrature modulator. 6, carrier frequency oscillator 7, PLL circuit 8, AGC amplifier 9, adder 10, frequency converters 11, 14, transmitter 12, receiver 13, quadrature demodulator 15, BPF 16, reference carrier regenerator 17, guard interval detection It comprises an A / D converter 20, a guard interval remover 21, an FFT QAM decoder 22, and an ECC output circuit 23.

The information data is input to the ECC input circuit
C is added. The data is output by the IFFT circuit 2 as real (I) and imaginary (Q) components.

These output signals are supplied to a D / A converter 5 via a guard interval adding circuit 3 and are converted into analog signals. A necessary clock signal is supplied from the master clock (MCK) circuit 4 to the IFFT circuit 2, the guard interval adding circuit 3, and the D / A converter 5.

From the carrier frequency oscillator 7 to the PLL circuit 8
Generates the reference carrier frequency set. The generated reference carrier frequency is supplied to the AGC amplifier 9, and the gain of the AGC amplifier 9 is controlled in accordance with the timing clock sent to the guard interval adding circuit 3, and the synchronous transmission as shown in FIG. Generate a signal. The transmission synchronization signal generated by the AGC amplifier 9 is
The intermediate frequency signal generated by the quadrature modulator 6 and the adder 1
0, and the next frequency converter 11 and transmitter 1
2 is transmitted.

Next, the operation of the receiving side of the embodiment of the orthogonal frequency division multiplex transmission system of the present invention will be described below. On the receiving side, the received signal received by the antenna is returned to an intermediate frequency signal by the receiver 13 and the frequency converter 14. From the signal returned to the intermediate frequency, only the reference pilot signal is extracted by the BPF 16. The extracted reference pilot signal (see FIG. 1B) is supplied to a reference carrier reproduction circuit 17, where the reference carrier frequency is reproduced. This reference carrier frequency is supplied to the quadrature demodulator 15.

FIG. 6 shows an embodiment of the reference carrier reproducing circuit 17. One embodiment of the reference carrier reproducing circuit 17 of FIG. 6 includes a phase comparator 24, a secondary loop filter 25, and a voltage controlled oscillator (VCO) 26,
The reference (synchronous) pilot carrier signal extracted from F16 is supplied to the phase comparator 24 and the VCO
(26) The reference carrier signal is output to the quadrature demodulator 15 from (26).

The reference (synchronous) pilot signal extracted by the BPF 16 is also sent to a guard interval detecting circuit 18, and an output of the guard interval detecting circuit 18 is supplied to a PLL circuit (PLL 2) 19. P
The LL circuit 19 prevents erroneous operation due to transmission line fluctuation and sudden noise, and supplies the symbol clock signal to the guard interval removing circuit 21.

FIG. 5 shows the guard interval detection circuit 1
8 shows a specific example. The guard interval detection circuit 18 shown in FIG. 5 includes a detection circuit 42 and a comparator 43, receives the extracted synchronous pilot signal, performs detection, and outputs a symbol timing clock signal. OF frequency-converted by the frequency conversion circuit 14
The DM transmission signal is supplied to the next quadrature demodulator 15,
It is demodulated into real and imaginary baseband I and Q signals.

The demodulated baseband I and Q signals are A
The signal is converted into a digital signal by the / D converter 20. Guard interval removing circuit 21, FFT (QAM decoder) 2
2. OFDM decoded data is obtained by the ECC output circuit 23.

Next, another embodiment of the orthogonal frequency division multiplex transmission system of the present invention will be described below with reference to FIG. In another embodiment of the orthogonal frequency division multiplex transmission system of the present invention, a receiving OFDM demodulation unit 28, a master clock oscillator 29, an RF unit 30,.
A synchronization pilot carrier generation unit 31, a pilot carrier transmission antenna 32, and a data reception antenna 33;
Each of the transmitters 1 to n (34) includes a transmission OFDM modulator 35, an RF unit 36, a BPF 37, a reference carrier regenerator 38, and a guard interval detector 39.

The receiver 27 has a pilot carrier transmitting antenna 32 in addition to a normal data receiving antenna 33 for synchronizing the transmitters 1 to n (34). The reception OFDM demodulation unit 28 obtains a reference carrier clock and a symbol clock signal from the master clock 29.

Further, both clock signals are output from a synchronous pilot carrier generation unit 31 by a synchronous signal shown in FIG.
That is, a reference pilot carrier of the reference set level L1 exists in the guard interval period of the OFDM transmission signal, and a reference (pilot) carrier of the level L2 lower than the reference set level exists in the OFDM data period. A synchronization signal is generated and transmitted from the reference (pilot) carrier transmitting antenna 32 to the transmitters 1 to n (3
4) Each reference (pilot) carrier receiving antenna 40
Sent to.

The pilot carrier signal received by the pilot carrier receiving antenna 40 is extracted by the BPF 37 via the RF unit 36, and is extracted by the reference carrier reproducing unit 38.
In the guard interval detector 39, the reference carrier clock signal and the symbol clock signal are reproduced.

The reproduced reference carrier clock signal and symbol clock signal are sent to the transmission OFDM modulator 35, and the transmitters 1 to n (34) transmit the synchronized OFD signals, respectively.
M transmission signals can be transmitted.

[0041]

As described above, according to the orthogonal frequency division multiplex transmission system of the present invention, it is not necessary to provide a special synchronization section, and a stable reference carrier clock signal and symbol clock signal can be obtained with a simple circuit configuration. Can be transmitted and received.

Further, according to the present invention, since the transmission level of the transmission pilot signal is lower than the specified value in the data reception signal period with respect to the guard interval period, the interference between the reception signal and the pilot signal is suppressed. Can be demodulated.

Further, according to the present invention, even when a plurality of transmitters are operated synchronously, interference between a received signal and a pilot signal in the receiver can be suppressed, and a highly accurate reference carrier clock and symbol clock can be obtained. I can do it.

[Brief description of the drawings]

FIG. 1 shows a configuration of an embodiment of a synchronization signal according to the present invention and OFD
FIG. 3 is a diagram illustrating a relationship with an M transmission frame.

FIG. 2 is a diagram showing a synchronous relationship between a plurality of transmitters and one receiver.

FIG. 3 is a diagram showing a data processing frequency relationship on a spectrum between a plurality of transmitters and one receiver.

FIG. 4 is a diagram showing a block configuration of an embodiment of the OFDM demodulator according to the present invention.

FIG. 5 is a diagram showing a block configuration of an embodiment of a guard interval detection circuit of the present invention.

FIG. 6 is a diagram showing a block configuration of an embodiment of a reference carrier reproducing circuit of the present invention.

FIG. 7 is a diagram showing a block configuration of another embodiment of the OFDM demodulator according to the present invention.

FIG. 8 is a diagram showing an OFDM frequency spectrum diagram and an orthogonal state.

[Explanation of symbols]

Reference Signs List 1 ECC input circuit 2 IFFT circuit 3 Guard interval addition circuit 4 Master clock (MCK) circuit 5 D / A conversion circuit 6 Quadrature modulator 7 Carrier frequency oscillator 8 PLL circuit (PLL1) 9 AGC (automatic gain control) amplifier 10 Adder 11, 14 Frequency converter 12 Transmitter 13 Receiver 15 Quadrature demodulator 16, 37 BPF 17 Reference carrier regenerator 18 Guard interval detector 19 PLL circuit (PLL2) 20 A / D converter 21 Guard interval remover 22 FFTQAM decoding Device 23 ECC output circuit 24 phase comparator 25 secondary loop filter 26 voltage controlled oscillator (VCO) 27 receiver 28 reception OFDM demodulation unit 29 master clock oscillator 30, 36 RF unit 31 pilot carrier generation unit 32 pilot carrier transmission Transmission antenna 33 data reception antenna 34 transmitters 1 to n 35 transmission OFDM modulator 38 reference carrier regenerator 39 guard interval detector 40 pilot carrier reception antenna 41 data transmission antenna 42 detection circuit 43 comparator L1 reference pilot signal of guard interval period Level L2 Level of reference pilot signal in effective symbol period tg Guard interval period ts Effective symbol period

Claims (4)

[Claims] 1. An orthogonal frequency division multiplexing transmission system, comprising: a transmission unit for performing transmission by an orthogonal frequency division multiplexing modulation system, and a reception unit for receiving a signal from the transmission unit, wherein the transmission unit includes the orthogonal division frequency A reference pilot carrier signal of a reference set level is inserted in a guard interval period constituting an orthogonal division frequency multiplex transmission signal of a multiplex modulation scheme, and during a period other than the guard interval, an amplitude level is continuously set with the reference pilot carrier signal. Comprises transmitting means for inserting and transmitting a reference pilot carrier signal lower than the reference setting level, wherein the receiving section receives a transmission signal transmitted from the transmitting means, and the receiving means PL constituted by a voltage-controlled oscillator by locking a reference pilot carrier signal frequency output from Loop by orthogonal division frequency multiplexed signal baseband I, orthogonal frequency division multiplexing transmission system characterized by being configured and a reproducing means for reproducing a reference carrier frequency for obtaining the Q signal. 2. An orthogonal frequency division multiplexing transmission system, comprising: a transmission unit for performing transmission by an orthogonal frequency division multiplexing modulation system, and a reception unit for receiving a signal from the transmission unit, wherein the transmission unit comprises: A reference pilot carrier signal of a reference set level is inserted in a guard interval period constituting an orthogonal division frequency multiplex transmission signal of a multiplex modulation scheme, and during a period other than the guard interval, an amplitude level is continuously set with the reference pilot carrier signal. Comprises transmitting means for inserting and transmitting a reference pilot carrier signal lower than the reference setting level, wherein the receiving section receives a transmitting signal from the transmitting means, and receives the signal by the receiving means. Level detecting means for detecting a level change of the detected signal, and the level change detected by the level change detecting means. From down Bol timing signal, PLL composed of a voltage controlled oscillator to lock this signal
An orthogonal frequency division multiplex transmission system comprising: means for performing symbol synchronization by a loop. 3. An orthogonal frequency division multiplexing transmission system for performing transmission by an orthogonal frequency division multiplexing modulation method, comprising: a plurality of transmission units; and a reception unit for receiving transmission signals from the plurality of transmission units. Receiving means for receiving transmission signals from the plurality of transmission units, and inserting a reference pilot carrier signal of a reference set level in a guard interval period constituting a transmission signal of the orthogonal division frequency multiplexing modulation method, the guard interval And transmitting means for inserting and transmitting a reference pilot carrier signal having an amplitude level lower than the reference set level, the transmission means being continuous with the reference pilot carrier signal. Section extracts the reference carrier signal from the synchronous transmission signal received by each transmitter and reproduces the reference carrier frequency And a means for generating an orthogonal division frequency multiplexing transmission signal synchronized with the reproduced reference carrier signal. The signal from the transmitting means of the receiving unit is transmitted to the plurality of transmitting units. Orthogonal frequency division multiplex transmission system characterized by transmitting simultaneously. 4. An orthogonal frequency division multiplexing transmission system for performing transmission by an orthogonal frequency division multiplexing modulation method, comprising: a plurality of transmission units; and a reception unit for receiving transmission signals from the plurality of transmission units. Receiving means for receiving transmission signals from the plurality of transmission units, and inserting a reference pilot carrier signal of a reference set level in a guard interval period constituting a transmission signal of the orthogonal division frequency multiplexing modulation method, the guard interval In a period other than the above, the transmitting means for inserting and transmitting a reference pilot carrier signal whose amplitude level is less than the reference setting level, which is continuous with the reference pilot carrier signal, comprises: The transmitting unit includes means for detecting a level change of the synchronous transmission signal received by each transmitter, and the level change detecting means. Means for performing symbol synchronization by means of a PLL loop comprising a voltage-controlled oscillator by locking this signal from the detected symbol timing signal. , An orthogonal frequency division multiplex transmission system, wherein the signals are simultaneously transmitted to the plurality of transmission units.