JPH11355241A - Symbol synchronization circuit - Google Patents

Abstract

(57) [Summary] In a conventional symbol synchronization circuit, in a multipath environment, multipath distortion at the rear end of a previous symbol section is superimposed on a GI section of an OEDM symbol. The rear end section is not affected by multipath distortion from the previous symbol. Therefore, the autocorrelation between the two sections has a problem in that the accuracy is deteriorated, the peak position is ambiguous, and the peak is shifted backward. SOLUTION: A symbol synchronization circuit for controlling a symbol phase matching position to be shifted by N samples before an actual maximum value detection position using a parameter N value prepared for each reception power, and Try to solve the problems below.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a radio communication apparatus, and more particularly to an OFDM (Orthogonal Frequency Division).
Multiplex) The present invention relates to a symbol synchronization circuit in a modem.

[0002]

2. Description of the Related Art In general, OFDM technology is attracting attention for its high multipath resistance, and is being developed as a terrestrial digital broadcasting standard. Advantages of OFDM are that it has high resistance to multipath delay, does not require a complicated adaptive equalizer, has a smaller channel spacing than a single carrier due to orthogonal carriers, provides high frequency use efficiency, and has a flat spectrum. It has been mentioned that it has a Gaussian amplitude distribution, so that it is easy to reduce the influence of interference on other systems. Recently, it has attracted attention as a general-purpose wireless access method suitable for high-speed transmission.

[0003] In an OFDM receiver, a reception FFT is used.
In order to cut out a section, first, it is necessary to synchronize symbols and set a time window. In the case of OFDM, each symbol can be separated after FFT, and a conventional symbol synchronization technique by detecting a zero cross or an envelope cannot be applied to a modulated waveform before FFT.

[0004] The OFDM receiver can acquire the initial synchronization of the symbol phase by detecting the phase of FFT-Window at which the autocorrelation value of the received waveform becomes maximum. Since the guard interval section (hereinafter referred to as GI section) of the OFDM signal is a copy of the last part of the IFFT in the symbol section, the autocorrelation at the same phase is obtained by sliding the delay of the FFT-Window width. It has the property of being maximal. Therefore, it is possible to track the symbol phase without using a special pilot symbol sequence. However, since the influence of multipath is large in the GI section, there is a method of preparing a special pilot symbol sequence having an FFT-Window length as an auxiliary means at the time of initial acquisition. That is, by expanding the moving average of the autocorrelation from the GI section length to the FFT-Window section length, the ratio of the area affected by the multipath can be relatively reduced, and the decrease in the autocorrelation accuracy can be reduced. It is a method.

[0005] Basically, a scheme using a pilot symbol sequence that is less affected by multipath during initial synchronization acquisition is suitable. At the time of synchronization tracking in user data, a correlation detection method using a GI section is used to reduce overhead. Is suitable. Also, matched filters are more suitable for speeding up than sliding correlation,
If the SN is not very good, the determination tends to be ambiguous, and the circuit scale is disadvantageously increased.

FIG. 3 shows a symbol synchronization circuit in a conventional OFDM receiver. In FIG. 3, 20 is the estimated F
FT-Window width delay means, 21 multiplication means, 2
2 is a moving average means for calculating an autocorrelation by a moving average of an estimated FFT-Window width, 23 is a maximum value holding means for monitoring the output of the moving average means 22 for each sampling, and recording the maximum value, 24 A difference means 25 for comparing the output of the moving average means 22 with the value recorded in the past maximum value holding means 23, 25 averages the comparison result of the difference means 24 over many symbol sequences and ensures that the maximum value is obtained. An inter-symbol average update judging means for judging that there is, and controlling the updating of the recorded value of the maximum value holding means 23, and a symbol phase control means 26 for adjusting the symbol clock phase when the maximum value holding means 23 updates the maximum value.

Hereinafter, the operation of the symbol synchronization circuit shown in FIG. 3 will be described with reference to FIG. Here, it is assumed that the deviation of the FFT-Window width due to the transmission / reception frequency error has been separately corrected by the AFC means, and that the input received signal has been AGC-processed in the preceding stage.

FIG. 4 shows an OFDM reception signal having two consecutive symbols and an FFT-Wind estimated by delay means 20.
10 shows a state where the symbol phase is delayed by an ow width, and shows a method of detecting a symbol phase by correlation in a GI section.

After multiplying the two signals by the multiplying means 21 of FIG. 3, the auto-correlation is calculated by the moving average means 22 for each sampling. Since the guard interval section (hereinafter referred to as a GI section) of the OFDM signal is a copy of the last part of the IFFT in the symbol section, the constraint time of the moving average is set to be equal to or longer than the GI section.
Are observed. The autocorrelation value calculated by the moving average means 22 is compared with the past maximum value held by the maximum value holding means 23 by the difference means 24. Using the difference result, the inter-symbol average update determination unit 25 determines the maximum value update, and updates the contents of the maximum value holding unit 23. In the determination by the inter-symbol average update determination means 25, an instruction for performing the average operation of the symbol interval period and updating the maximum value until the phase determined to be the maximum value is observed with a predetermined number of symbols. By performing processing so as not to output, jitter in the synchronization maintaining operation can be reduced. The inter-symbol average update determination means 25 instructs the update of the maximum value,
It instructs the symbol phase control means 26 to shift the symbol timing.

[0010]

As a feature of the OFDM system, first, as described above, the GI section is a copy of the last part of the IFFT in the symbol section, and the FFT-Window width of an arbitrary symbol sequence is obtained. It has autocorrelation. Furthermore, in order to suppress out-of-band side lobes of the transmission spectrum, a method is provided in which a transition region of a roll-off characteristic by a time domain filter is provided at the beginning and end of a symbol section.

However, the following problems will occur in the conventional method and configuration due to the above-described features, with reference to FIG.

First, in a multipath environment, GI
Multipath distortion at the rear end of the previous symbol section is superimposed on the section, but the rear end section of the symbol is not affected by multipath distortion from the previous symbol. Therefore, the autocorrelation between these two sections has a degraded accuracy, the peak position becomes ambiguous, and tends to shift backward.

Next, by processing in the roll-off transition section,
Since the signal component at the boundary of the symbol section is impaired, the correlation in the boundary region is greatly reduced under the low SN condition, so that the peak position becomes more ambiguous. For this reason, there is a problem that the required number of symbols for averaging is lengthened due to the low SN condition, and the symbol detection position is shifted backward in a multipath environment.

The present invention has been made in consideration of the above-mentioned conventional problems, and can improve the problem that the symbol detection position shifts backward in a multipath environment in a scheme of performing symbol synchronization using autocorrelation in a GI section. An object of the present invention is to provide a symbol synchronization circuit that can improve an error rate.

[0015]

In order to achieve the above object, the present invention uses a parameter N value prepared for each received power and sets a symbol phase N samples ahead of an actual maximum value detection position by N samples. Aiming to solve the problem under the multi-path condition by controlling the alignment position, and
Assuming a coexistence form of the OFDM scheme with the existing TDMA radio access scheme, the OFDM symbol synchronization circuit is initialized with control link frame synchronization information established by a modulation / demodulation scheme other than OFDM prior to channel establishment. .

The present invention improves the problem that the symbol detection position shifts backward in a multipath environment and the problem that the required number of symbols for averaging becomes long under low SN conditions using such means. can do.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention according to claim 1 of the present invention is directed to an OFDM receiving apparatus, in which a repetition characteristic of a received waveform is detected by autocorrelation, and an average position of a phase at which the maximum value is detected is determined. Therefore, for the circuit means for establishing information symbol synchronization, a symbol which is shifted in phase from the average position where the maximum value is detected by an adjustment amount proportional to the reception power detected by the separate means. This is a synchronization circuit and has an effect of reducing a backward shift of symbol synchronization due to multipath distortion.

According to a second aspect of the present invention, an OFD
In the coexistence type radio access scheme in which the radio control channel is established by the existing single carrier scheme prior to the channel establishment of the M scheme, the OFDM is succeeded by using the matching word synchronization established by the single carrier scheme as a time index. A symbol synchronization circuit for estimating the maximum value detection position at the beginning of the channel establishment procedure of the system and initializing the symbol synchronization circuit means, and reduces the time required for initial synchronization acquisition under low SN conditions. Having.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. (Embodiment 1) FIG. 1 shows a configuration diagram of a symbol synchronization circuit according to Embodiment 1 of the present invention.

In the figure, 1 is an estimated FFT-Window
w width delay means, 2 multiplication means, 3 estimation FFT-Wi
Moving average means 4 for calculating the autocorrelation based on the moving average of the n width, 4 monitors the output of the moving average means 3 for each sampling, and a maximum value holding means for recording the maximum value, and 5 a maximum value holding means 4 Difference means 6 for comparing the output with a value recorded in the past performs an averaging operation on the comparison result of the difference means 5 over many symbol rows, and determines a maximum value position based on the averaging operation. The inter-symbol average update determination means 7 for controlling the update of the recorded value of the maximum value holding means 4 and correcting the symbol phase at the same time, and the maximum value position detected by the inter-symbol average update determination means 6
N pre-shift control means for shifting to the previous position by N samplings in accordance with the separately detected received power, and 8 a symbol phase control means for adjusting the symbol clock phase based on the controlled symbol phase signal of the N pre-shift control means 7. .

The first embodiment is characterized in that N preshift control means 7 is provided as a configuration. In the present invention, the received power is measured using the RSSI measurement result based on the AGC error voltage or the like. Under a multipath condition with a large delay spread, a floor phenomenon occurs in which the bit error rate does not drop below a certain value due to intersymbol interference.
Since this problem has a greater effect as the received power increases, the parameter N proportional to the measured received power is used.
Set.

When the symbol phase is controlled based on the average position where the maximum value of the autocorrelation is detected in the conventional symbol synchronization circuit, control is performed to pre-shift the symbol phase by the parameter N. As N is larger, the influence of multipath distortion due to delay spread is less likely to occur, and the influence of intersymbol interference due to symbol phase shift appears instead. However, in the case of the 0FDM system, a difference occurs when the shift is backward. Interference occurs between symbols, which greatly degrades the signal. However, in the case of a forward shift, the symbols are within the same symbol and the influence is small.
Therefore, the present invention can reduce the influence of the floor phenomenon by controlling the parameter N by the received power and pre-shifting the symbol timing by N samples from the detection position.

Embodiment 2 Hereinafter, a symbol synchronization circuit according to Embodiment 2 of the present invention will be described.

FIG. 2 shows the configuration of the symbol synchronization circuit according to the second embodiment of the present invention. 9 in the figure is the estimated FFT
A window width delay means, 10 is a multiplication means, 11 is a moving average means for calculating an autocorrelation by a moving average of the estimated FFT-Window width, 12 is an output of the moving average means 11 which is monitored for each sampling, and the maximum value is obtained. A maximum value holding means 13 for recording a value, a difference means 13 for comparing the output of the moving average means 11 with a value recorded in the maximum value holding means 12 in the past, and a reference numeral 14 The averaging operation is performed over the symbol sequence, and the maximum value position is determined based on the averaging operation.
The inter-symbol average update determining means 15 for controlling the update of the recorded value of the symbol and correcting the symbol phase at the same time as the maximum value position detected by the inter-symbol average update determining means 14 according to the reception power separately detected. N pre-shift control means 16 for shifting to the previous position by N samplings, and symbol phase control means 16 for adjusting the symbol timing based on the N shift control of the controlled N pre-shift control means 15.

The above configuration is the same as that of the symbol synchronization circuit of the first embodiment. In the second embodiment, an OFDM modulation / demodulation method is added to the above configuration in an existing radio access system based on the single carrier TDMA method. It is characterized by having done.

That is, an OFDM channel or a non-OFDM
An existing TDMA frame synchronizer 17 that establishes symbol synchronization and frame synchronization in advance by the modulation and demodulation method, and an arrival propagation time of a radio wave is estimated from frame synchronization information obtained by the existing TDMA frame synchronizer 17, and arrives over the same propagation path. OFDM symbol position estimating means 18 for estimating the symbol phase of a received signal of the OFDM scheme
The inter-symbol average update determining means 1 uses the symbol phase estimated by the FDM symbol position estimating means 18 as an initial value.
It is characterized in that a symbol position initializing means 19 for initializing the control of No. 4 is added.

In the symbol synchronization circuit shown in FIG. 2, in the present invention, assuming a coexistence form of the existing TDMA radio access system and OFDM system, the control link is set by the existing single carrier and the channel is set by the OFDM modulation / demodulation system. Consider a realization form.

In such a system, first, a reception frame synchronization using a matching pattern is established at the time of setting a radio link by using an existing single carrier system. The symbol phase once established by the single carrier scheme can be used as a time index for symbol synchronization when switching to the multicarrier scheme at the time of channel setting, unless the terminal position moves at high speed. Also,
Since the arrival time of the received wave can be estimated from the position of the matching pattern, the OF
Optimal initialization can be performed on the symbol synchronization circuit for DM.

[0029]

As is apparent from the above description, the present invention solves the conventional problems. In particular, in a system in which symbol synchronization is performed by using autocorrelation in a GI section, a symbol is obtained under a multipath environment. The problem that the detection position shifts backward can be improved, and the bit error rate can be improved.

Further, assuming a coexistence mode of the existing TDMA radio access system and OFDM system, in an implementation mode in which a control link is set by an existing single carrier and a channel is set by an OFDM modulation / demodulation system, especially in a low SN condition In this case, there is an advantage that the initial acquisition time of symbol synchronization can be reduced by reducing the required number of symbols for averaging.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a symbol synchronization circuit according to a first embodiment of the present invention;

FIG. 2 is a configuration diagram of a symbol synchronization circuit according to a second embodiment of the present invention;

FIG. 3 is a configuration diagram of a conventional symbol synchronization circuit.

FIG. 4 is an operation explanatory diagram of the conventional symbol synchronization circuit shown in FIG. 3;

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 Estimated FFT-Window width delay means 2 Multiplication means 3 Moving average means 4 Maximum value holding means 5 Difference means 6 Intersymbol average update determination means 7 N preshift control means 8 Symbol phase control means 9 Estimated FFT-Window width delay means DESCRIPTION OF SYMBOLS 10 Multiplication means 11 Moving average means 12 Maximum value holding means 13 Difference means 14 Inter-symbol average update determination means 15 N preshift control means 16 Symbol phase control means 17 Existing TDMA frame synchronizer 18 OFDM symbol position estimation means 19 Symbol position initialization means

Claims (2)

[Claims] In an OFDM receiving apparatus, circuit means for detecting repetition characteristics of a received waveform by autocorrelation and establishing information symbol synchronization with an average position of a phase at which the maximum value is detected is provided. Wherein the phase is shifted forward from the average position at which the maximum value is detected by an adjustment amount proportional to the reception power detected by a separate means. 2. Prior to establishing an OFDM channel,
In the coexistence type radio access system in which the radio control channel is established by the existing single carrier system, by using the matching word synchronization established by the single carrier system as a time index, the O
A symbol synchronization circuit characterized in that the maximum value detection position is estimated at the beginning of a channel establishment procedure of the FDM system, and symbol synchronization circuit means is initialized.