JPH05344172A - Frequency offset compensating system - Google Patents

Abstract

PURPOSE:To prevent the degradation of an error rate from being generated by the phase rotation of a transmitter/receiver by compensating the frequency offset of a multi-phase modulation system while multiplying it to a received signal in the form of a conjugate complex number with the estimated value of a phase rotation amount. CONSTITUTION:A signal 72 detected at an optimum sampling point is inputted to a phase compensation circuit 8 and after performing the adding operation of an N-multiplied number over N symbols (N is an arbitrary integer), the estimated value of the phase rotation amount due to frequency offset is calculated. In this case, the N symbols are used for estimating the phase rotation caused by frequency offset, a multiplied signal 71 containing information is turned to a signal 91 hourly delayed just for the N symbols by a delayer 9. Then, the signal 91 is multiplied to an output signal 81 of the compensation circuit 8 and turned to a multiplied signal 101 removing the phase rotation caused by frequency offset. Thus, the frequency offset as the maximum factor of the error rate caused by phase rotation can be easily compensated and the oscillation frequency of the transmitter/receiver can be stabilized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a digital car telephone,
The present invention relates to a frequency offset compensation method for all digital mobile radio communication systems such as digital mobile phones and digital cordless phones.

[0002]

2. Description of the Related Art In recent years, as digitalization of land mobile communication systems such as car telephones and cordless telephones has been proposed, research on various digital radio technologies,
It is being developed and put to practical use. Especially, in the next generation digital car phones, cordless phones, and mobile phones, π / 4 shift QPSK
It has been decided that the scheme will be adopted.

In the case of a land mobile radio communication system, its transmission line is a Rayleigh fading channel in which the amplitude and phase of the signal fluctuate drastically with time, and it is necessary to recover the carrier wave and phase of the received signal in this channel. It becomes extremely difficult to operate the synchronous detection method.
Therefore, a differential detection method that detects the phase difference between two symbols that does not require carrier regeneration is effective in the Rayleigh fading channel, but in the case of the differential detection method, the phase due to fading or frequency offset is used. Rotation is the biggest factor in degrading the error rate. This frequency offset is mainly due to the instability of the frequency oscillator incorporated in the transceiver of the base station and the mobile station, and various frequency offset compensation techniques have been researched and developed so far. In each case, the device is complicated, and the maximum frequency offset amount that can be compensated is limited due to the uncertainty of the phase that occurs when estimating the phase rotation due to the frequency offset.

Further, in order to suppress the frequency offset to a small level, it is necessary to make the frequency oscillators of the base station and the mobile station highly accurate, which has a drawback of increasing the cost of the apparatus.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to eliminate the frequency offset compensation in the phase modulation method and the limitation of the maximum frequency offset amount that can be compensated.

[0006]

SUMMARY OF THE INVENTION A feature of the present invention is the multiphase phase modulation (MPS) used in digital mobile communication systems.
K) In the differential detection method for demodulating a signal, the phase rotation due to the problematic frequency offset is multiplied by the number M of information phases of the modulation signal to obtain N symbols (N symbols).
Is an arbitrary integer), the phase component of the sum of the M-multiplied signals is extracted, and frequency division is performed to perform estimation.

However, the method proposed here is effective only when the phase rotation due to the frequency offset is in the range of -π / M to π / M, and is estimated for the frequency offset that gives a phase rotation larger than this. The uncertainty of the phase of the phase value becomes a problem. That is, the estimated phase rotation is
Since it is derived with a value in the range of −π to π, the phase rotation amount of π / M or more included in the received signal before M multiplication is M
It cannot be detected by multiplication. Therefore, the present invention solves the above problems by performing double differential encoding on the information phase data sequence transmitted by the transmitting side, and thereby the received signal can be accurately demodulated regardless of the frequency offset amount. I am trying.

[0008]

[Configuration and Operation of the Present Invention] (1) Frequency Offset Compensation Method First, the sampling value at the sampling point t = iTs (the sampling point is estimated in the clock recovery unit) of the differential detection output r (t) of the received signal is r (i ), R (i) to M
If the multiplied value is s (i), the sampling value s (i) at t = iTs is given by s (i) = (r (i)) M (1). Thus, by multiplying by M, MP
It is possible to remove the information phase component of the SK modulated wave and extract only M times the phase rotation amount due to the frequency offset.

Next, letting S (f) be the frequency response of s (i),
S (f) is given by the following equation as the discrete Fourier transform of s (i).

[Equation 1] However, N represents the number of symbols for which the discrete Fourier transform (DFT) is performed, and k represents the order of harmonics.

Here, since the phase rotation due to the frequency offset is regarded as constant over N symbols, it appears as a direct current component of f = 0 Hz on the frequency axis. Therefore, S (f)
DC component S (0) of

[Equation 2] Therefore, the estimated value Δω-Ts of the phase rotation amount due to the frequency offset is

[Equation 3] Can be asked as

Finally, by performing the following calculation using this estimated value, the phase rotation amount due to the frequency offset contained in the received signal can be removed. z (i) = r (i) exp (-j Δω-Ts) = R (i) exp (ΔωTs + θ (i)) exp (-jΔω-Ts) = R (i) exp (jθ (i))・ Exp (-j (Δω- Δω-) Ts) (5) where R (i) is the amplitude component of differential detection output r (i), θ (i) is the phase difference information at sampling point t = iTs, and Δf =
Δω / 2π represents the true frequency offset amount.

As is apparent from the equation (5), when the estimated offset amount Δω− is substantially equal to the true frequency offset amount Δω, the following equation holds. z (i) ≈ R (i) exp (jθ (i)) (6) Therefore, the phase difference information θ (i) can be accurately detected from the equation (6).

(2) Phase Uncertainty Removal Method When the estimated value of the phase rotation amount is obtained by the above method,
The uncertainty of the estimated value due to multiplication by M becomes a problem.
That is, the frequency offset amount Δω of the received signal is π
If it is larger than / MTs, the phase rotation amount estimated by the above method has phase uncertainty. This is expressed using the following formula. First, when the amount of phase rotation is expressed by ΔωTs = α + π / M (7), M ΔωTs = M α + π (8) by multiplying by M.

Here, the estimated value Δω-Ts of the phase rotation amount is
Since it can be obtained as a value in the range of −π <Δω-Ts <π from the equation (4), it is estimated that Δω-Ts = α−π / M from the equation (7). As a result, the differential detection output in which the phase rotation due to the frequency offset is corrected by Eq. (5) has (Δω-Δω
-) Phase error of Ts = 2 π / M occurs and the error rate is almost 1/2.
Will be. Therefore, as a countermeasure against this, a new method for performing double differential encoding on the information phase data series is newly introduced,
The reason why the uncertainty of the phase included in the estimated value of the phase rotation amount due to the frequency offset can be removed theoretically is shown below.

First, the input information phase data series is θ = {θ
0, θ1, θ2, ..., θi}, the information phase φi after primary differential encoding and the information phase ψi after secondary differential encoding are respectively expressed by the following equations. φi = φi-1 + θi (9) ψi = ψi-1 + φi (10) Here, if the transmitted signal is Si (t) = A cos (ωct + ψi) (11), the baseband delay detection with harmonic components removed The output is Bi (t) = Si (t) × Si-1 (t-Ts) = A cos (ωct + ψi) A cos (ωc (t-Ts) + ψi-1) = A2 / 2 ・ cos (ωcTs + ψi − ψi -1) It becomes (12). However, the noise component is ignored in order to clarify the formula. Normally, in the differential detection method, ωc and Ts are set in advance so that ωcTs = 2nπ, but if the frequency offset of Δω occurs due to the instability of the frequency oscillator, then Eq. (12) becomes Eq. (13) Like Bi (t) = A2 / 2 · cos (ΔωTs + ψi − ψi-1) (13) Furthermore, from Eq. (9), Bi (t) = A2 / 2 · cos (ΔωTs + φi) (14), and the estimation of ΔωTs is Bi If the multiplication method of (t) is used, Mφi becomes 2nπ, and the estimated value for M ΔωTs can be obtained. Here, if Δω-Ts = α + π / M (15), the M multiplication estimated phase using the multiplied output becomes Mα + π, and the estimated value is M Δω-Ts = 2π− (Mα + π) from the relationship of modulo 2π. =-(Π-M α) (16) After all, by dividing the equation (16) by M, the estimated value D of the amount of phase rotation due to the frequency offset is obtained as D = − (π−M α) / M (17). Finally, when Eq. (14) is compensated using D, Bi '(t) = A2 / 2 · cos (ΔωTs + φi −D) = A2 / 2 · cos (α + π / M + φi + π / M-α) = A2 / 2 · cos (2π / M + φi) (18), and the output phase after differential detection is φ'i = φi + 2π / M (19). Here, by performing the inverse operation of the first-order differential encoding of the equation (9), the judgment output of the information phase data sequence is θi = φ'i−φ'i-1 = (φi + 2π / M) -(Φi-1 + 2π / M) = φi-φi-1 (20), and the frequency offset compensation can be accurately performed with the uncertainty of the estimated phase removed.

(3) Double differential coding system in π / 4 shift QPSK system The system shown in (2) above is a double differential coding system for the normal phase modulation (MPSK) system. Modulation method is π
Also in the case of the / 4 shift QPSK system, the phase uncertainty included in the estimated value of the phase rotation amount of the frequency offset compensation system of the above (1) is removed by performing the double differential encoding as follows. be able to.

First, as shown in FIG. 1, two bits are assigned to one symbol by serial-parallel conversion of a binary information data sequence, and then information phase 0, which corresponds to the figure,
π and ± π / 2 are mapped to each symbol. Next, after the first-order differential encoding φi = φi-1 + θi (21) is applied to the information phase data series θi, the differential phase φi given by the equation (21) is substituted into the following equation. , Π / 4 shift QPSK signal subjected to double differential encoding can be generated. Ii = 1 / √2 ・ {(Ii-1-Qi-1) cos φi-(Ii-1 + Qi-1) sinφi} Qi = 1 / √2 ・ {(Ii-1 + Qi-1) cos φi + (Ii-1-Qi-1) sinφi} (22)

After transmitting the modulated signal subjected to the double differential encoding as described above and compensating for the phase rotation by using the frequency offset compensation method of the above (1), the reverse operation of the equation (20) is performed. The transmission data sequence can be correctly detected by performing the differential decoding θi − = φi −φi-1 (23), which is an operation.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 2 and 3 show block diagrams of the present invention. FIG. 2 shows a block diagram of a double differential encoding method for removing the uncertainty of the phase generated when estimating the phase rotation amount due to the frequency offset. In FIG. Data series θ = {θ0, θ1
, Θ2, ..., θi} are input to the first-order differential encoder 2 and are output as the differential encoded signal 21 given by the equation (8) or the equation (20). Furthermore, this differentially encoded signal 21
Is input to the second-order differential encoder of 3, and (9) and
It is transmitted as the information signal 31 that has been subjected to the double differential encoding given by equation (10) or equation (22).

FIG. 3 shows a block diagram of a differential detection-differential decoding system with a frequency offset compensation function for compensating for phase rotation due to frequency offset. In Fig. 3, the output signal of the intermediate band (IF) filter 4 of the received signal 41
43 is multiplied by the signal 44 one symbol before, which is the output of the one-symbol delay element 13 and is then input to the low-pass filter 5 as a product signal 51. This low-pass filter removes harmonic components from the product signal 51 and outputs only the baseband signal. Next, per symbol
After inputting the output signal 52 of the low-pass filter to the 6 A / D converters that perform Ns times of sampling, the output 61 is input to the 7 clock regenerator to detect the optimum sampling value. The signal detected at this optimum sampling point
72 is input to the phase compensation circuit 8 and is subjected to the M multiplication running operation shown in the equation (1) and the addition operation over N symbols shown in the equation (3), and then the phase rotation amount due to the frequency offset. Can be estimated as equation (4). Here, since N symbols are used to estimate the phase rotation due to the frequency offset, the product signal 71 containing information is delayed in time by N symbols by the delay circuit of 9 and then the phase compensation circuit of 8 is designated as 91. Output signal 81 is multiplied,
The product signal 101 given by equation (5) from which the phase rotation due to the frequency offset has been removed is input to the decision device of 10 and judged. Further, the discriminator output 102 is input to 11 differential decoders as an information phase signal subjected to differential encoding, and (1
After being subjected to the operation represented by the equation (9), it is output as the differentially decoded parallel information data series 111. Finally,
The differential decoder output 111 is input to 12 parallel-serial converters and decoded as a serial information data sequence 121.

[0021]

As described above, according to the present invention, in the differential detection system which is the demodulation system of the digital phase modulation system, it is possible to easily compensate the frequency offset which is the largest factor of the error rate deterioration. Further, the uncertainty of the estimated phase rotation amount, which is a problem in the multiplication operation, can be easily removed by adding the double differential encoding to the information phase signal without adding a complicated circuit.

[Brief description of drawings]

FIG. 1 shows mapping of phase information to each symbol.

FIG. 2 shows a block diagram of a dual differential encoding scheme.

FIG. 3 shows a block diagram of a differential detection-differential decoding system with a frequency offset compensation function.

[Explanation of symbols]

 1 Data Sequence 2 1st Order Differential Encoder 3 2nd Order Differential Encoder 4 Band Pass Filter 5 Low Pass Filter 6 A / D Converter 7 Clock Regenerator 8 Phase Compensation Circuit 9 Delay Device 10 Judgmenter 12 Parallel / Serial converter

Claims (3)

[Claims] 1. A differential detection system for demodulating a multi-phase phase modulation (MPSK) signal in which information is added to a phase difference between two symbols of a transmission signal for transmission, and the delayed detection signal is multiplied by an information phase number M of the modulation signal. Added to N symbols (N is an arbitrary integer),
The phase component of the sum of the M multiplied signals is extracted and divided by M to estimate the phase rotation amount due to the frequency offset, which is the deviation from the true carrier frequency included in the received signal, and the calculated phase is obtained. A frequency offset compensation method that removes the phase rotation due to the frequency offset caused by the frequency instability of the oscillator of the transceiver by multiplying the estimated value of the amount of rotation on the received signal in the form of a conjugate complex number. 2. The frequency offset compensating method according to claim 1, wherein in order to remove the uncertainty of the estimated phase rotation amount, the differential encoding is performed twice on the transmission information phase data sequence. Differential encoding. 3. The dual differential encoding system according to claim 2, wherein when a π / 4 shift QPSK system is used as a modulation system, 1 is added to an information phase data sequence transmitted in a normal QPSK system.
Next differential encoding and adding π / 4, then π / 4 shift QPSK
A π / 4 shift QPSK double differential encoding method that performs secondary differential encoding for modulated waves.