JPH0541719A - Frequency offset detection device for automatic frequency control circuit - Google Patents

Abstract

(57) [Summary] [Purpose] (1) No need for a table for the arctangent function,
(2) Large frequency offset can be corrected, (3) S / N ratio does not deteriorate due to multiplication processing, (4) Memory for storing matched filter output is unnecessary, and (5) Transmission line characteristics change with time. According to this, a frequency offset detection circuit of an automatic frequency control circuit is obtained whose characteristics are less deteriorated. [Structure] A received signal and a known sequence are input, the received signal is subjected to phase rotation, then the transmission path characteristics estimated from the signal and the estimated value of the received signal estimated from the known sequence, and the received signal subjected to phase rotation In the likelihood generation circuit that generates the error power of M, the first likelihood generation circuit 1, the second likelihood generation circuit 2, the Mth likelihood generation circuit 3, and the Mth likelihood generation circuit 3 that give different phase rotations, respectively. A frequency offset output circuit 4 which inputs the likelihoods and outputs the frequency offset amount.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency offset detecting device for an automatic frequency control circuit (hereinafter referred to as an AFC circuit) used for digital data transmission.

[0002]

2. Description of the Related Art Before describing the prior art, the technical background will be described. First, FIG. 5 shows a modulator / demodulator for explaining the mechanism of carrier frequency offset generation. In communication for modulation / demodulation, the modulation circuit 2
2 and the transmission side carrier wave generation circuit 23 modulates the data on the transmission side carrier wave and transmits the data through the transmission side antenna 24, and the signal received through the reception side antenna 27 is a demodulation circuit 25 and a reception side carrier wave generation circuit. The signal is demodulated by the carrier wave again by 26. Here, if the same frequency as the carrier wave on the transmission side cannot be created on the reception side, a carrier frequency offset occurs.

The function of the AFC circuit is to (a) bring the carrier frequency offset close to zero or (b) remove the frequency offset from the received signal having the carrier frequency offset by some means. Regarding method (a),
A received signal satisfying the Nyquist criterion described later has been widely studied.

However, these techniques cannot be applied to a transmission line which does not satisfy the Nyquist criterion, in other words, which has intersymbol interference. Further, the method (b) has been actively studied in recent years as an effective method for a storage batch processing demodulator that temporarily stores received signals in a memory or the like and then collectively demodulates using the entire input signal.

Next, the phase reverse rotation used in the accumulation batch processing demodulator will be described. Consider a case where a signal in which a phase rotation of a radian is applied for each sample to the received signal r (n) of the symbol rate is received. here,
The received signal r _OFF (n) that has received the frequency offset at the sample time n can be expressed as r _OFF (n) = exp {jan} r (n). In addition, j shows √-1. The received signal r _OFF (n) that has received this frequency offset is temporarily stored in the memory. Next, the frequency offset is estimated by some means from the received signal r _OFF (n) that has received the frequency offset. In other words, b is obtained as the estimated value of a.

The AFC circuit reads the received signal r _OFF (n) that has received the frequency offset from the memory, applies the phase reverse rotation of b radians for each sample, and then r _AFC (n) = exp {-jbn} r _OFF (N) = exp {j (ab) n} r (n) is output. Here, if a = b, then r _AFC (n) = r (n), and the carrier frequency offset can be removed. Although the concept of this technique can be applied to all carrier frequency offset cancellation, the problem is how to estimate b, and the present invention relates to this estimation method of b.

Burst used in the TDMA communication system will be described. A burst is a block of data such as a packet, and is usually composed of a known sequence known in advance on the receiving side and a data sequence for transmitting information.

Next, intersymbol interference will be described. Figure 6
A generation model of intersymbol interference is shown in. The reception signal r (n) at each time is composed of a single transmission information I (n) and a plurality of transmission information following it,

[0009]

[Equation 1]

It can be expressed as Here, f _i is a weighting coefficient (transmission path characteristic), L is the number of subsequent symbols, and w (n) is noise. The Nyquist criterion is satisfied when f _i = 0 (i ≠ 0). Further, when f _i ≠ 0 (i ≠ 0), it is said that intersymbol interference has occurred. When such intersymbol interference occurs, the operation of the conventional AFC circuit becomes difficult.

The transmission line characteristics will be described. The transmission line characteristic corresponds to the above f _i . As an example, a method of estimating the transmission path characteristic from a known sequence (training sequence) x (n) (n = 0, 1, ... N−1, but repeated patterns of L symbols or more are added to both ends) will be described. If the cross-correlation between the known sequence x (n) and the received signal r (n) is P _i ,

[0012]

[Equation 2]

Have the following relationship: Here, * indicates a complex conjugate. X and the cross-correlation vector created from this P _i
The Wiener-Hoff equation is obtained from the inverse covariance matrix of (n), and the transmission line characteristic f is obtained by solving this equation.
_i can be estimated. If the autocorrelation of x (n) is close to random, P _i approximately matches the transmission line characteristic f _i .

Next, FIG. 4 shows "MLSE having frequency offset correction function in TDMA digital mobile communication".
Configuration of Receiver "(Sect. B-II, 11, pp. 736
FIG. 4 is a block diagram of a frequency offset detection circuit for the conventional AFC circuit shown in FIG.

In the figure, 5 is a received signal input terminal, 6 is a known sequence input terminal, 7 is a frequency offset amount output terminal,
Reference numeral 10 is a transmission path estimation circuit for estimating the transmission path characteristic, 17 is a matched filter circuit, 18 is an M-th power circuit for multiplying an input signal by M, 19 is an averaging circuit for averaging signals, 20 is an arctangent calculation circuit, and 21 is It is a frequency offset calculation circuit that calculates a frequency offset from the output of the arctangent calculation circuit.

The operation of the above configuration will be described. The transmission path estimation circuit 10 estimates the transmission path characteristics from the reception signal input from the reception signal input terminal 5 and the known series input from the known series input terminal 6. Matched filter circuit 1
Reference numeral 7 reverses the transmission characteristic with respect to time, and creates a matched filter circuit output with the complex conjugate taken as the tap coefficient of the FIR filter. The M-th power circuit 18 multiplies the output of the matched filter circuit by M to remove the influence of the phase of the transmission signal. The averaging circuit 19 averages the M multiplied outputs. The arctangent calculation circuit 20 outputs the arctangent of the ratio of the real part to the imaginary part of the average circuit output. The frequency offset calculation circuit 21 inputs the arctangent and outputs the frequency offset.

Here, in the above-mentioned conventional example, the following problems occur. (1) Since the calculation of the arctangent calculation circuit 20 is complicated, a table for the arctangent function is usually required. (2) When the burst length becomes long, the phenomenon that the phase makes one rotation due to the frequency offset occurs, and it becomes difficult to correct a large frequency offset. (3) Since the signal is multiplied in the M-th power circuit 18, noise power increases, so that the SN ratio deteriorates and the characteristics are adversely affected. (4) It is necessary to temporarily store the output of the matched filter in the memory or the like in order not to duplicate the calculation in the matched filter circuit 17. (5) If the transmission line characteristics change with time in a burst, the output characteristics of the matched filter become erroneous and the characteristics deteriorate.

[0018]

Since the frequency offset detection circuit of the conventional automatic frequency control circuit is configured as described above, (1) a table for the arctangent function is necessary, and (2) a large table. It is difficult to correct the frequency offset, (3) the signal is multiplied so that the SN ratio deteriorates and the characteristics are adversely affected, (4) the matched filter output needs to be temporarily stored in a memory, etc. (5) There is a problem that the characteristics deteriorate when the transmission path characteristics change with time.

The present invention has been made to solve the above problems, and provides a frequency offset detection circuit of an automatic frequency control circuit capable of solving the above problems (1) to (5). The purpose is to do.

[0020]

SUMMARY OF THE INVENTION A frequency offset detecting device for an automatic frequency control circuit according to the present invention includes an automatic frequency control circuit for correcting a frequency offset of a carrier wave on a transmitting side and a carrier wave on a receiving side. In a frequency offset detection circuit of an automatic frequency control circuit that outputs a quantity, a phase rotation circuit that inputs a received signal and repeatedly applies phase rotation to the received signal to rotate the phase in a predetermined time for each sample. , A transmission line estimation circuit that inputs the received signal that has undergone phase rotation and a known sequence to estimate the characteristics of the transmission line, and a reception signal estimation circuit that inputs the transmission line estimated value and the known sequence and outputs the estimated value of the received signal When,
A plurality of likelihood generation circuits each including a received signal subjected to phase rotation and an error power calculation circuit that inputs an estimated value of the received signal and outputs error power are provided with different amounts of phase rotation for each sample. A frequency offset output circuit for inputting a plurality of likelihoods output from the plurality of likelihood generating circuits and outputting a frequency offset amount is provided.

Further, in the frequency offset detection device of the automatic frequency control circuit having the above-mentioned configuration, the past frequency offset output holding circuit for holding the amount of frequency offset output from the frequency offset output circuit from the present to the past, and the frequency offset output holding circuit And a frequency offset calculation circuit that inputs a circuit output and outputs a frequency offset amount.

[0022]

A frequency offset detecting apparatus for an automatic frequency control circuit according to the present invention prepares a plurality of different carrier frequency offsets, and applies a plurality of different frequency offsets to a received signal and a plurality of different known series. Select the most suitable frequency offset from the carrier frequency offsets.

Further, since the new frequency offset is calculated based on the past frequency offset that is once held, fluctuation due to noise is absorbed by the averaging process.

[0024]

EXAMPLES Example 1. FIG. 1 is a block diagram showing an embodiment of the present invention. In FIG. 1, parts that are the same as or equivalent to those in FIG. 1
Is a first likelihood creating circuit that creates a likelihood from a signal obtained by subjecting the received signal to phase rotation, and 2 is a first likelihood creating circuit that performs a similar process after applying a different phase rotation from the first likelihood creating circuit 1. 2 likelihood generating circuit, 3 is the M-th likelihood generating circuit that performs similar processing after applying different phase rotation to the first likelihood generating circuit 1 and the second likelihood generating circuit 2. Is a frequency offset output circuit that outputs a frequency offset corresponding to the most suitable phase rotation amount from the M likelihoods.

FIG. 2 is a block diagram showing the likelihood generating circuit in the above embodiment. In FIG. 2, parts that are the same as or equivalent to those in FIGS. 1 and 4 are assigned the same reference numerals and duplicate explanations are omitted. 8 is a phase rotation circuit that always gives a predetermined phase rotation, 9 is an error power calculation circuit that outputs error power of two inputs, 10 is a transmission line estimation circuit, 11 is a known series and transmission line characteristics It is a received signal estimation circuit that estimates a received signal from

Next, the operation will be described. It should be noted that duplicate description is omitted for the same or corresponding parts as in the conventional example. First, the operation of M likelihood preparation circuits prepared will be described with reference to FIG. The phase rotation circuit 8 constantly applies rotation of a predetermined phase amount to the reception signal input from the reception signal input terminal 5. The transmission path estimation circuit 10 estimates the transmission path characteristics for the received signal that has undergone this phase rotation. The reception signal estimation circuit 11 has a known sequence input terminal 6
The received signal is estimated from the known sequence and the transmission line characteristics input from. The error power calculation circuit 9 outputs the error power of the received signal estimated to have undergone the phase rotation and the estimated received signal.

Example 2. Further, FIG. 3 is a block diagram of an embodiment in which a configuration for averaging fluctuations due to noise is added to the first embodiment, and in FIG. 3, the same or corresponding parts as in FIG. 1, FIG. 2 and FIG. The same reference numerals are given and duplicate description is omitted. Reference numeral 13 is a past frequency offset output holding circuit that temporarily holds the frequency offset output, and 14 is a frequency offset calculation circuit that inputs the once held frequency offset output and calculates the frequency offset.

Next, the operation will be described. It should be noted that duplicate description is omitted for the same or corresponding parts as in the conventional example. In the present embodiment, the frequency offset output from the frequency offset output terminal 7 in the first embodiment is input from the frequency offset input terminal 15, and the past frequency offset output holding circuit 13 holds the frequency offset from the present to the past. .. The frequency offset calculation circuit 14 inputs the frequency offset from the present to the past, outputs the frequency offset calculated by substituting it into a certain function from the frequency offset calculation value output terminal 16.

An example of the operation of the frequency offset calculation circuit 14 will be shown. The frequency offset input is f _IN (n), the frequency offset calculation value output is f _OT (n), and the following equation can be considered. f _OT (n) = kf _IN (n) + (1−k) f _TO (n−1) where k is a constant (0 ≦ k ≦ 1). With this configuration, fluctuations in f _IN (n) due to noise can be absorbed by the averaging process.

As described above, according to the present invention, the frequency offset can be corrected if the phase does not make one revolution in the known series in the burst. On the other hand, in the conventional example, if the phase is 1/4 rotation or more during the burst, it is impossible to correct the frequency offset, and the present invention makes it possible to greatly expand the correction amount of the frequency offset. Further, the present invention can be combined with an adaptive equalizer, and can follow a temporal change in transmission line characteristics.

[0031]

As described above, according to the present invention, a plurality of different carrier frequency offsets are prepared, and a plurality of different carrier frequency offsets are prepared from the sequence in which the plurality of frequency offsets are applied to the received signal and the known sequence. Since the most suitable frequency offset is selected from among (1) the table for the arctangent function is not necessary, (2) large frequency offset can be corrected, (3) the SN ratio does not deteriorate due to multiplication processing, ( 4) There is no need for a memory to store the output of the matched filter, and (5) there is little deterioration in the characteristics due to temporal changes in the transmission line characteristics.

Further, since the new frequency offset is calculated based on the past frequency offset that is once held, there is an effect that fluctuations due to noise are absorbed by the averaging process.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a block diagram showing in detail the likelihood generation circuit of FIG.

FIG. 3 is a block diagram showing functions added to the embodiment of FIG.

FIG. 4 is a block diagram showing a conventional example.

FIG. 5 is an explanatory diagram of generation of carrier frequency offset.

FIG. 6 is an explanatory diagram of intersymbol interference.

[Explanation of symbols]

 1 1st likelihood creation circuit 2 2nd likelihood creation circuit 3 Mth likelihood creation circuit 4 Frequency offset output circuit 5 Received signal input terminal 6 Known sequence input terminal 7 Frequency offset output terminal 8 Phase rotation circuit 9 Error Power calculation circuit 10 Transmission line estimation circuit 11 Received signal estimation circuit 12 Likelihood output terminal 13 Past frequency offset output holding circuit 14 Frequency offset calculation circuit 15 Frequency offset input terminal 16 Frequency offset calculation value output terminal

Claims (2)

[Claims] 1. A received signal is input to a frequency offset detection circuit of an automatic frequency control circuit that outputs the amount of carrier frequency offset to be corrected to an automatic frequency control circuit that corrects the frequency offset of carrier waves on the transmission side and the reception side. Then, a phase rotation circuit that repeatedly gives a predetermined time to the received signal to rotate the phase temporally for each sample, and a received signal that has undergone the phase rotation and a known sequence are input, and the characteristics of the transmission line are input. A transmission line estimation circuit for estimating
Received signal estimation circuit that inputs the channel estimation value and known sequence and outputs the estimated value of the received signal, and error power calculation circuit that inputs the received signal that has undergone phase rotation and the estimated value of the received signal and outputs the error power And a plurality of likelihood generation circuits each consisting of and with different amounts of phase rotation for each sample.
A frequency offset detection device for an automatic frequency control circuit, comprising a frequency offset output circuit which inputs a plurality of likelihoods output from the plurality of likelihood generation circuits and outputs a frequency offset amount. 2. The frequency offset detection device for an automatic frequency control circuit according to claim 1, wherein the frequency offset output holding circuit holds the amount of frequency offset output from the frequency offset output circuit from the present to the past, and the frequency offset. A frequency offset detecting device for an automatic frequency control circuit, comprising: a frequency offset calculating circuit that receives an output of an output holding circuit and outputs a frequency offset amount.