JP2001285246A - Carrier frequency error detection circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve a problem of a conventional carrier frequency error detection method that it has been unsuitable for large scale integration LSI where downsizing and small capacity are requirements because a carrier frequency error is derived through the use of arctangent calculation, which cannot easily be realized by a logic circuit but can be realized by a ROM, in the case of the conventional carrier frequency error detection method utilizing a correlation between a guard period and a latter half of a valid symbol period of a copy source in an OFDM demodulator that demodulates an OFDM modulation signal having the guard period formed by cyclicly copying the latter half of the valid symbol period. SOLUTION: The carrier frequency error detection circuit of this invention employs an absolute value circuit and a division circuit to replace the arctangent calculation with an approximated calculation that can be realized by using a logic circuit so as to facilitate the LSI.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a carrier frequency error detection circuit in an OFDM demodulator used for a terrestrial digital broadcast receiver or the like.

[0002]

2. Description of the Related Art In recent years, orthogonal frequency division multiplexing (hereinafter referred to as OFDM (Orthogonal Frequency Divided)) has been used in digital audio broadcasting for mobile objects and digital terrestrial television broadcasting.
ision Multiplex)) A transmission system is drawing attention.

The OFDM transmission system is a system in which a large number of subcarriers (hereinafter, referred to as subcarriers) orthogonal to each other are modulated by digital data to be transmitted, and the modulated waves are multiplexed and transmitted. This method has a feature that when the number of subcarriers to be used increases to several hundreds to several thousands, the symbol period of each modulated wave becomes extremely long, so that it is hardly affected by multipath interference.

FIG. 6 is a block diagram showing a conventional OFDM carrier frequency error detection circuit.

[0005] An input terminal 1 receives an in-phase detection axis signal (I signal) and a quadrature detection axis signal (Q signal), which are received by a tuner and passed through a quadrature demodulation circuit and an A / D converter.

[0006] The input I and Q signals are divided into two, one is directly input to the correlator 3, and the other is delayed by the effective symbol period delay memory 2 and then input to the correlator 3.

The I and Q signals output from the correlator 3 are input to a moving average circuit 4 which continuously outputs the average value of the guard period width.

The I and Q signals output from the moving averaging circuit 4 hold in advance the arc tangent calculation result for the input, and the ROM 1 functions as an arc tangent calculator.
Input to 0. The output of the ROM 10 is input to the carrier frequency error calculator 11 and outputs a result for each symbol period. This output is the carrier frequency error.

The operation of detecting the carrier frequency error will be described.

FIG. 4 is a waveform diagram showing an OFDM modulation signal. In OFDM, since transmission data is dispersed and modulated over several hundred to several thousand subcarriers, the modulation symbol rate of each subcarrier is extremely low, and one symbol period is extremely long. For this reason, it is less likely to be affected by multipath. Further, by setting the guard period before the effective symbol period, the effect of multipath interference can be effectively removed. The guard period is formed by cyclically copying the latter half of the effective symbol period.
If the delay time of multipath interference is within the guard period, by demodulating only the signal during the effective symbol period during demodulation,
Intersymbol interference due to delayed adjacent symbols can be prevented.

The supplied I signal and Q signal are input to a delay memory 2 and a correlator 3. The correlator 3 calculates and outputs a correlation coefficient between the signal delayed by the effective symbol period by the delay memory 2 and the directly input signal. The output of the correlator 3 takes the moving average 4 over the guard period width,
The carrier frequency error calculator 11 calculates the carrier frequency error based on the result of the arc tangent calculation based on 0.

As shown in FIG. 5A, the OFDM signal has guard periods G1, G2,... Added at the beginning of each effective symbol period S1, S2,. G1, G2, ...
Are copies of G1 ', G2',... In S1, S2,. Therefore, when the effective symbol period is delayed, the timings of the delayed signals G1, G2,... And the timings of G1 ′, G2 ′,. Since Gn and Gn 'are in a copying relationship, the signal correlation during this period is high. In other periods, OFDM
Since the signal is a noise signal as shown in the figure, the correlation is low.

When there is no carrier frequency error, FIG.
As shown in (c), the I signal of the correlation and moving average output is a signal having a positive peak in the symbol period cycle, and the Q signal is almost always a zero signal. On the other hand, for example, when the carrier frequency error is 1/4 of the OFDM carrier interval, FIG.
As shown in (d), the I signal is almost 0, and the Q signal is a signal having a negative peak in a symbol period cycle.

If the symbol boundary values of the four moving average I and Q signals are used, a carrier frequency error of ± 1/2 of the carrier interval can be detected.

If the symbol boundary values are plotted on the IQ plane as shown in FIG. 7, it can be seen that the phase shift corresponds to the carrier frequency error on the basis of no error.

To determine the phase shift, an arc tangent calculation is performed using the symbol boundary value, and the result is made to correspond to the carrier interval.
Carrier frequency error can be detected. That is, the carrier frequency error is

[0017]

(Equation 1)

Is derived.

The arc tangent calculation is difficult to realize by a logic circuit, and is usually realized by a lookup table method using a ROM.

However, using a ROM requires
It is not suitable for an LSI that requires a reduction in size and capacity.

[0021]

The conventional circuit requires a ROM for arc tangent calculation, and is not suitable for LSI implementation.

[0022]

According to the present invention, an arc tangent calculation is replaced with an approximate calculation which can be realized by a logic circuit using an absolute value circuit and a division circuit.

[0023]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of a symbol period detection circuit according to the present invention.
The same components as those described above are denoted by the same reference numerals.

An input terminal 1 receives an in-phase detection axis signal (I signal) and a quadrature detection axis signal (Q signal) that have been received by a tuner and have passed through a quadrature demodulation circuit and an A / D converter.

The input I and Q signals are divided into two, one is directly input to the correlator 3, and the other is input to the correlator 3 after being delayed by the effective symbol period delay memory 2.

The I and Q signals output from the correlator 3 are input to a moving average circuit 4 that continuously outputs the average value of the guard period width.

The I and Q signals output from the moving average circuit 4 are input to an absolute value circuit 5, where the absolute values of the I and Q signals are output. At this time, the absolute value circuit 5 sends the I and Q signals of the I and Q signals to the absolute value circuit 5 to the carrier frequency error calculation circuit.
Position information on the Q plane is also output. The I and Q signals output from the absolute value circuit 5 are input to the division circuit 6, where division is performed. At this time, the divisor and the dividend are assigned so that the division result is 1 or less. The output of the dividing circuit 6 and the position information from the absolute value circuit 5 are input to a carrier frequency error calculator 7, and the result is output for each symbol period. This output is the carrier frequency error.

As described in the description of the related art, the carrier frequency error is calculated based on I, I at the symbol boundary of the output of the moving average circuit 4.
The phase shift between the Q signal phase and the case without error corresponds to the carrier frequency error.

Conventionally, in order to obtain the phase shift, an arc tangent calculation based on the symbol boundary I and Q signals is performed, and the result is made to correspond to a carrier interval to detect a carrier frequency error.

In the present invention, in order to replace the arc tangent calculation with a simple calculation, the absolute values of the I and Q signals output from the moving average circuit 4 are calculated, and a is set so that 0 <= b / a <= 1. And b. As can be seen from FIG. 2, it can be seen that (π / 4) b / a under this condition can be approximated to atan (a, b).

As shown in FIG. 3, (π / 4) b / a is
The phase shift corresponds to the phase shift in the direction of the arrow depending on which area of the IQ planes 1 to 8 the I and Q signals are. However, it is easily possible to convert the phase shift from the reference.

In the present invention, a carrier frequency error is detected by associating the converted value with a carrier interval.

For example, if the symbol boundary values of certain I and Q signals are (Ia, Qa) in FIG. 3, the carrier frequency error at that time is:

[0035]

(Equation 2)

Is derived.

Since the above-described formula for deriving the carrier frequency error is only the four arithmetic operations, it can be easily realized by a logic circuit.
As a result, it is easy to implement an LSI that requires a reduction in size and capacity.

[0038]

As described above, according to the present invention,
A carrier frequency error detection circuit can be realized without using a ROM, and the LSI of the OFDM demodulation circuit can be easily realized.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of an OFDM carrier frequency error detection circuit according to the present invention.

FIG. 2 is a diagram showing replacement of an atan curve with a straight line according to the present invention.

FIG. 3 is a diagram illustrating a method of detecting a carrier frequency error according to the present invention.

FIG. 4 is a waveform diagram showing a general OFDM modulation signal.

FIG. 5 is a waveform diagram showing the principle of general carrier frequency error detection.

FIG. 6 is a block diagram showing a conventional OFDM carrier frequency error detection circuit.

FIG. 7 is a waveform diagram showing a conventional carrier frequency error detection method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 I / Q signal input terminal 2 Delay memory 3 Correlator 4 Moving average circuit 5 Absolute value circuit 6 Division circuit 7, 11 Carrier frequency error calculation circuit (this invention) 8 Carrier frequency error data output terminal 9 I / Q signal output terminal 10 Arctangent ROM

Claims (1)

[Claims] 1. An OFDM demodulator for receiving an orthogonal frequency division multiplex modulation signal having an effective symbol period and a guard period having a waveform that coincides with a part of the effective symbol period, receives an I signal and a Q signal after quadrature detection. And the I
Delay means for delaying the signal and Q signal by an effective symbol period; a correlator for calculating a correlation coefficient between the I signal and Q signal and the delay output; and an I signal and Q at the correlator output
A moving average circuit for continuously outputting an average value of the guard period width of the signal;
An absolute value circuit for calculating an absolute value of a signal; and a division circuit for dividing an I signal and a Q signal of the output of the absolute value circuit, and performing a carrier frequency error calculation using the output of the division circuit. Carrier frequency error detection circuit.