Multi-level quadrature amplitude modulator system with fading compensation means

Description: TECHNICAL FIELD (1) Industrial application The present invention relates to a transmission line distortion compensation method when a multi-level quadrature amplitude modulation method is applied to a fading line in which the transmission line fluctuates drastically.

(2) Conventional Technology In a digital radio line, especially a land mobile communication line, the envelope and phase of the received wave fluctuate due to the influence of fading.

Conventionally, in such a line, a frequency modulation or phase modulation method that does not include information in the envelope has been adopted in consideration of the fluctuation of the envelope by 20 dB or more.

However, in order to further improve the frequency utilization efficiency, it is necessary to apply a multi-level quadrature amplitude modulation method in which information is also included in the amplitude.

The multi-level quadrature amplitude modulation method is a modulation method conventionally used in a transmission line having very gentle transmission line fluctuations such as a microwave line. Further, the compensation of the transmission line distortion in that case has been performed by reproducing the phase fluctuation by AGC (Automatic Gain Controller) for the envelope distortion and by using the PLL (Phase-Locked Loop) for the phase fluctuation and detecting the phase fluctuation.

(3) Problems to be Solved by the Invention The above method has been an effective method for a fixed micro line in which transmission line fluctuations are gradual. However, in the case of a line such as a land mobile communication line in which the transmission line fluctuates extremely, the following problems occur.

1) Envelope fluctuations are so severe that AGC cannot completely compensate for envelope distortion.

2) The threshold value for data judgment cannot be set properly in the part where the envelope distortion cannot be completely compensated.

3) Since the phase fluctuation is also severe, the phase distortion of fading cannot be completely compensated by the PLL.

Therefore, in order to apply the multi-level quadrature amplitude modulation method to a communication line in which the transmission line fluctuates sharply, a new transmission line distortion compensation method that solves all the above points is required.

(4) Means for Solving Problems When compensating for transmission line distortion in a communication line with severe transmission line fluctuations, it is necessary to compensate while predicting or measuring the transmission line fluctuations.

In the present invention, the transmission / reception unit has the following configuration for compensation for transmission line distortion.

1) The transmission unit transmits a known symbol once for every N information symbols (N is a natural number) for measuring transmission line distortion. Therefore, when transmitting the same amount of information, the transmission band is (N + 1) / N times that of the conventional method. FIG. 1 shows a frame configuration when a frame symbol is inserted.

2) The receiver first detects the frame symbol.

3) Since the frame symbol is a known signal, it is used to measure the distortion of the transmission line in the frame symbol.

4) The transmission line distortion other than the frame symbol (the symbol in which information is transmitted) is estimated based on the transmission line distortion measured by the frame symbol.

5) Compensate for the distortion of the received signal based on the transmission line distortion estimated in 4).

6) Furthermore, based on the information in 4), the threshold value for data judgment is calculated.

7) Based on the results of 5) and 6), the transmitted symbol is estimated and the data is reproduced by decoding the signal.

(5) Action FIG. 2 shows a 16QAM signal space diagram (a signal point of a complex baseband signal shown on a complex plane) as a typical example of the multi-level quadrature modulation method. 16QAM is a method of arranging signals at equal intervals on a complex plane as shown in FIG. In addition, the amount of information contained in one symbol in the M value QAM is K = log.₂(M) It is a bit. Therefore, in the case of FIG. 2, one symbol contains 4-bit information.

The configuration of the transmitter is shown in FIG. First, the serial / parallel conversion unit (1) divides the data by K bits, and then the baseband signal generation unit (2) converts the data into the corresponding complex baseband signal.

Next, the frame synchronization insertion unit (3) inserts a frame symbol (known) once for every N information symbols for transmission line distortion measurement.

When the transmission line distortion is measured by the frame symbol, it is necessary to increase the S / N of the frame symbol in order to improve the estimation accuracy. Therefore, as a frame symbol, a point that gives the maximum amplitude (Fig. 2, A, B, C, D) is appropriate. In the following, it is assumed that point A (3 + j 3) is used as a frame symbol.

After that, the band is limited by the transmission filter unit (4), modulated by the quadrature modulation unit (5), the power is amplified by the amplification unit (6), and then transmitted from the antenna unit (7).

The 16QAM transmission signal x (t) after performing the above operation is described by the following equation.

x (t) = a_I(T) cos (ωt) -a_Q(T) sin (ωt) (1) where a_I(t): In-phase component of transmission baseband signal a_Q(t): Orthogonal component of the transmission baseband signal ω: Transmission angular frequency. Where a_I(t), a_Q(t) is a waveform whose band is limited by the transmission filter unit (4).

The configuration of the receiving unit is shown in FIG.

The antenna unit (11) receives the noise, the reception filter unit (12) removes out-of-band noise, and then the AGC unit (13) amplifies the noise to an appropriate level.

The received signal y (t) after being amplified by the AGC unit (13) is described by the following equation.

y (t) = r (t) s_I(T) cos (ωt + θ (t))-r (t) s_θ(T) sin (ωt + θ (t)) (2) However, r (t): Envelope fluctuation due to the transmission line θ (t): Phase fluctuation due to the transmission line. Also, s_I(t), s_Q(t) is the baseband waveform after the band is limited by the reception filter unit (12).

After that, the carrier wave is reproduced by the synchronous detection unit (14), and synchronous detection is performed using the carrier wave to obtain a received complex baseband signal u (t).

Here, in carrier wave reproduction, it is assumed that only the center frequency of the reception input is reproduced and the phase fluctuation due to the transmission line is not reproduced. Therefore, the received complex baseband signal u (t) after synchronous detection is u (t) = u._I(T) + j · u_Q(T) = r (t) exp (j · θ (t)) (s_I(T) + j · s_Q(T)) = C (t) (s)_I(T) + j · s_Q(T)) (3). However, c (t) is a complex transmission line distortion due to the transmission line, and c (t) = c._I(T) + j · c_Q(T) = r (t) exp (j · θ (t)) (4). Therefore, u (t) includes the transmission line distortion together with the transmission symbol.

From this signal, the clock is first reproduced in the clock reproduction unit (15). The clock can be obtained not only from the complex baseband signal but also from the envelope of the received wave.

Next, the frame detection unit (16) detects the timing of the frame symbol.

Since the frame symbol is a signal point having the maximum amplitude, u (t) periodically includes the one having the maximum amplitude. Therefore, by detecting the timing, the timing of the frame symbol can be detected.

The transmission line distortion estimation unit (17) estimates the transmission line distortion from the reception baseband signal at the frame timing. The method is as follows.

First, the reception timing of the l-th frame symbol is t_I= L (N + 1) T_sAnd. Where T_sIs one symbol length. Received complex baseband signal u (t) at that time_I) Is u (t) from Eq. (3)_I) = U_I(T_I) + J u_Q(T_I) = (3 + j 3) c (t_I) (5). Therefore t = t_IEstimated value of c (t) in <img file="000003.tif" id="000003" he="010" wi="080" img-format="tif" img-content="drawing" />Is <img file="000004.tif" id="000004" he="010" wi="080" img-format="tif" img-content="drawing" />Is sought.

on the other hand <img file="000005.tif" id="000005" he="010" wi="080" img-format="tif" img-content="drawing" />Sets c (t) to (N + 1) T_sCorresponds to sampling at (sec) intervals. Therefore, the frame symbol insertion interval ((M + 1) T_s) Is set to be less than or equal to the Nyquist interval of c (t), and the estimation value of c (t) in the information symbol can be obtained by using the interpolation method.

There are several methods of interpolation, such as Newton's formula and Gauss's formula. Here, as an example, a method using a quadratic Gaussian complementary formula will be described.

t = (N + 1) T_s, 2 (N + 1) T_s, 3 (N + 1) T_sIs the reception timing of the frame symbol, and the transmission line distortion at that time is c.₁, c₂, c₃And. Further, it is assumed that the insertion interval of the frame symbol is sufficiently smaller than the Nyquist interval determined by the band of c (t).

In that case, 2 (N + 1) T_st 3 (N + 1) T_sC (t) in can be interpolated by a quadratic function as follows.

<img file="000002.tif" id="000002" he="035" wi="130" img-format="tif" img-content="drawing" />When the transmission line fluctuation is very slow with respect to the symbol rate, it is also possible to first smooth the frame symbol, improve the S / N, and then interpolate.

After that, the estimated value of the obtained transmission line distortion <img file="000006.tif" id="000006" he="010" wi="080" img-format="tif" img-content="drawing" />Is transferred to the transmission line distortion compensating unit (18), which is used to compensate the transmission line distortion in u (t).

There are two possible transmission line compensation methods.

One is <img file="000007.tif" id="000007" he="010" wi="080" img-format="tif" img-content="drawing" />When, in the transmission line compensation unit (18) <img file="000008.tif" id="000008" he="010" wi="080" img-format="tif" img-content="drawing" />This is a method in which both the envelope and the phase are compensated by calculating, and the judgment threshold value of the data is set to 0, ± 2, ± j · 2.

The other is in the transmission line compensation unit (18). <img file="000009.tif" id="000009" he="010" wi="080" img-format="tif" img-content="drawing" />Compensate only the phase by calculating, and set the judgment threshold of the data, <img file="000010.tif" id="000010" he="010" wi="080" img-format="tif" img-content="drawing" />Is the method.

After that, the baseband signal with the transmission line distortion compensated <img file="000011.tif" id="000011" he="010" wi="080" img-format="tif" img-content="drawing" />And the threshold information is transferred to the decoding unit (19), the transmission symbol is reproduced, and the K-bit information included in the symbol is reproduced. This information is converted into serial information by the parallel / serial conversion unit (20) and output as playback data.

(6) Effects of the Invention By using the present invention, it becomes possible to apply multi-level quadrature modulation to land mobile communication, which has been difficult in the past, and the frequency utilization efficiency is greatly improved.

On the other hand, in the present invention, since the frame symbol is inserted to measure the transmission line distortion, the power allocated to the information symbol is reduced as compared with the conventional method.

The transmission band becomes (N + 1) / N times.

The problem is that.

Conventionally, differential coding has been performed to compensate for the phase uncertainty of the regenerated carrier. On the other hand, in the present invention, since the frame symbol is used, absolute phase detection is possible for each signal point. Therefore, it is possible to improve the error rate characteristic as compared with the conventional method. This ratio is greater than the reduction in power allocated to the information symbol, so there is no problem.

Further, since the degree of improvement in frequency utilization efficiency by using multi-level quadrature modulation is larger than the degree of decrease in frequency utilization efficiency by inserting a frame symbol, there is no problem.

[Simple explanation of drawings]

FIG. 1 is a frame configuration diagram when a frame symbol is inserted, FIG. 2 is a signal space diagram of 16QAM, FIG. 3 is a configuration diagram of a transmission unit, and FIG. 4 is a configuration diagram of a reception unit. 1 ... Serial / parallel conversion unit, 2 ... Baseband signal generation unit, 3 ... Frame synchronization insertion unit, 4 ... Transmission filter unit, 5 ... Quadrature modulation unit, 6 ... Amplification unit, 7 ... Antenna unit, 11 ... Antenna unit, 12 ... Reception filter unit, 13 ... AGC unit, 14 ... Synchronous detection unit, 15 ... Clock reproduction unit, 16 ... Frame detection unit, 17 ... Transmission line distortion estimation unit, 18 ... Transmission line distortion compensation unit, 19 ... Decoding unit, 20 ... Parallel / serial conversion unit.