JPH0715478A - Modulator - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a modulator capable of reducing the amount of calculation of the root Nyquist filter, reducing the power consumption, and reducing the price. In a modulator for multi-carrier transmission that combines and transmits signals modulated by a plurality of sub-carriers, frequency band limiting filters 5 to 8 for suppressing leakage power between sub-carriers, and adjacent channels Interpolation filters 19 and 20 for suppressing the leakage power between are provided separately, and the sampling frequency of the frequency band limiting filter is set lower than the sampling frequency of the interpolation filter. sub·
Since the suppression of the leakage power between carriers and the suppression of the leakage power between adjacent channels are performed separately for each filter, the order of each filter can be set small, and the total value of the calculation amount of each filter decreases. In addition, the frequency band limiting filter 5
The operating clock frequency of ~ 8 can be made smaller than that of the interpolation filters 19 and 20, so that the frequency band limiting filter 5 ~
The calculation amount of 8 further decreases.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a modulator used for transmission in a digital mobile communication system using multiple carriers, and more particularly to a simplified structure for reducing leakage power to adjacent channels. is there.

[0002]

2. Description of the Related Art In a digital mobile communication system, a multi-carrier ASK (amplitude modulation) system has been put into practical use in order to increase the transmission capacity per radio channel. In this system, one radio channel is composed of a plurality (N) of sub-carriers, and each sub-carrier is amplitude-modulated according to the signal, and the transmission capacity can be expanded N times. .

The 4ASK modulation, which is one of the modulation systems, is a system in which four signals are simultaneously transmitted by using four subcarriers, and a conventional modulation device that executes this is as shown in FIG. Input terminal 1 for inputting the transmission data
, A gray encoder 2 that generates four types of gray code impulses according to the input data of 2 bits of transmission data, and a parallel symbol string (X
1, X2, X3, X4), and outputs the serial / parallel converter 3, a symbol converter 4 that outputs the input symbol strings in combination, and a symbol that is output from the symbol converter 4. FIR low-pass band-limiting filters (hereinafter referred to as “root Nyquist filters”) 35, 36, 37, 38 for removing intersymbol interference between columns and interference between channels are provided.

The apparatus further includes tables 9, 10, 11, and 12 for performing sub-carrier modulation on the output of each root Nyquist filter, each root Nyquist filter output and each table. Multipliers 13, 14, 15, 16 that multiply the values, and the output of multiplier 13 and the multiplier
An adder 17 for adding the output of 14; an adder 18 for adding the output of the multiplier 15 and an output of the multiplier 16; a DA converter 21 for converting the outputs of the adders 17, 18 into an analog signal; 22 and D
Analog that removes harmonic components from the output of A converters 21 and 22.
Filters 23 and 24, synthesizer 25 that produces in-phase and quadrature-phase carriers, and analog filter 23
Of the output of the synthesizer 25 and the in-phase carrier of the synthesizer 25, and the analog multiplier 27 of the output of the analog filter 24 and the quadrature carrier of the synthesizer 25, and the outputs of the multipliers 26 and 27. It has an analog adder 28 and a transmission output end 29.

This modulator is configured so as to satisfy the specifications of FIG. 7, and has a leakage power of -60 dB or less with respect to adjacent radio channels (radio channels used by other transmission devices) 25 kHz apart. Is standardized to be. Therefore, the root Nyquist filter 35-
As shown in FIG. 9, 38 is composed of an FIR filter having an order as large as 600th order.

Further, in this modulator, the DA converter 21,
The frequency of the operating clock (output sampling frequency) of the DA converters 21 and 22 (output sampling frequency) is selected to 72 kHz in consideration of the ease of realization of the analog filters 23 and 24 that follow 22. Therefore, the sampling frequencies of the root Nyquist filters 35 to 38 are similarly set to 72 kHz.

This modulator uses signals (X1, X2, X
(3, X4) is modulated with four sub-carriers, and equation (1)
It is transmitted in the form of the modulated signal shown in.

S (t) = X ₁ (t) × cos (ω _c −3Δω) t + X ₂ (t) × cos (ω _c −Δω) t + X ₃ (t) × cos (ω _c + _{△ ω) t + X 4 (} t) × cos (ω c + 3 △ ω) t (1) where the omega _c is the carrier frequency, also, △ omega = 2
It is π × 2.25 kHz.

In order to obtain the modulated signal of the equation (1), the modulator operates as follows. First, when transmission data is input from the transmission data input terminal 1 at a transmission rate of 32 kb / s, the gray encoder 2 generates impulses which are quaternary weighted for every 2 bits at 16 kHz intervals. The serial / parallel converter 3 sequentially separates the impulses generated by the Gray encoder 2 into four corresponding to the number of sub-carriers, and outputs them as ₄ Kbaud symbol strings X ₁ , X ₂ , X ₃ , X ₄ . .

The symbol converter 4 combines these symbol strings to obtain X ₁ + X ₄ , X ₂ + X ₃ , X ₁ -X ₄ , X ₂ -X ₃
Generate the symbol sequence of. This symbol conversion output is
Root Nyquist filters 35, 36, 37, 38 respectively
The band is limited by and output at 72 kHz intervals.

FIG. 10 shows the spectral characteristics of the root Nyquist filters 35-38. As shown in this figure, the attenuation amount in the stop band is -60 dB or less.

For the operation output of the root Nyquist filter 35, 3Δω = 6.7 given from Table 9
The in-phase component of 5 kHz is multiplied by the multiplier 13. Further, the arithmetic output of the root Nyquist filter 36 is multiplied by the in-phase component of Δω = 2.25 KHz given from the table 10 in the multiplier 14. The outputs of the multipliers 13 and 14 are added by an adder 17, then converted into an analog signal by a DA converter 21, processed by an analog filter 23, and expressed by the following equation (2). It is output as I (t).

I (t) = {X ₁ (t) + X ₄ (t)} × cos (3Δωt) + {X ₂ (t) + X ₃ (t)} × cos (Δωt) (2) , For the operation output of the root Nyquist filter 37, 3Δω = 6.75k given from Table 11
The quadrature component of Hz is multiplied by the multiplier 15, and the output of the root Nyquist filter 38 is multiplied by the quadrature component of Δω = 2.25 kHz given from the table 16 by the multiplier 16. To be done. This multiplier 15 and 16
The outputs of are added by the adder 18, and then the DA converter 22
Converted to an analog signal by the analog filter 24
And is output as Q (t) represented by the following equation (3).

Q (t) = {X ₁ (t) −X ₄ (t)} × sin (3Δωt) + {X ₂ (t) −X ₃ (t)} × sin (Δωt) (3 ) These I (t) and Q (t) are quadrature-modulated by the carrier generated by the synthesizer 25 and then added by the adder 28,
It is output as S (t) represented by the following equation (4).

[0015] S (t) = I (t ) × cos (ω c t) + Q (t) × sin (ω c t) (4) S (t) of the formula (4) has the formula ( By substituting 2) and equation (3), it is confirmed that it is the same as the modulated signal S (t) of equation (1).

FIG. 11 shows the output spectrum of this modulator. The frequency bands of the four sub-carriers are separated from each other, and are attenuated to -60 dB or less in the other frequency bands, and the standard of adjacent channel leakage power of -60 dB or less is satisfied.

As described above, the conventional modulator has 600
Using the following four root Nyquist filters, 600
As shown in FIG.
Has realized a multi-carrier 4ASK modulator that satisfies the specifications of.

[0018]

However, the conventional multi-carrier ASK modulator uses a root Nyquist filter having a large order of 600th order in order to attenuate the adjacent channel leakage power to meet the standard. Therefore, the amount of calculation becomes very large, high-speed calculation is required to perform real-time processing, and it is difficult to reduce the power consumption and cost of the device.

The root Nyquist filter of the conventional multi-carrier ASK modulator has a function of suppressing leakage power between sub-carriers and suppressing leakage power to an adjacent channel, and therefore has severe characteristics. Is required.

By the way, even if the leakage power between sub-carriers transmitted from one transmitter is set to be larger than the leakage power between radio channels transmitted from different transmitters, the accuracy of modulation / demodulation does not deteriorate. Does not happen.

The present invention addresses these problems and solves the problems in the conventional modulator.
It is an object of the present invention to provide a modulator capable of reducing the amount of calculation of the Nyquist filter, reducing the power consumption, and reducing the price.

[0022]

Therefore, in the present invention, in a modulator for multi-carrier transmission that combines and transmits signals modulated by a plurality of sub-carriers, leakage power between sub-carriers is suppressed. The frequency band limiting filter and the interpolation filter that suppresses the leakage power between adjacent channels are separately provided, and the sampling frequency of the frequency band limiting filter is set to be lower than the sampling frequency of the interpolation filter.

[0023]

Therefore, since the frequency band limiting filter that suppresses leakage power between sub-carriers and the interpolation filter that suppresses leakage power between adjacent channels are separated, the order of each filter can be set small. As a result, the total value of the calculation amounts of the filters is reduced as compared with that of the conventional modulator.

Further, by dividing the filters in this way, the operating clock frequency of the frequency band limiting filter can be made smaller than that of the interpolation filter, and by this multi-rate conversion, the calculation amount of the frequency band limiting filter can be reduced. It can be lowered further.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1, a 4ASK modulator according to an embodiment of the present invention is a root Nyquist filter whose main role is to remove intersymbol interference of each symbol string output from a symbol converter 4. It is provided with 5, 6, 7, and 8 and interpolation filters 19 and 20 inserted in front of the DA converters 21 and 22 mainly for removing interference between channels. Other configurations are the same as those of the conventional device (FIG. 8).

In the modulator of the embodiment, the specifications are set as shown in FIG. 7 and the analog filters 23 and 24 following the DA converters 21 and 22 are easily realized, as in the conventional device. In consideration of the characteristics, the operation clock frequency (sampling frequency) of the DA converters 21 and 22 is selected to be 72 kHz.

The root Nyquist filters 5-8 are
As shown in FIG. 2, the specification is composed of a band limiting FIR filter of the 88th order, and the sampling frequency is D
36 kHz, which is half the sampling frequency of A converters 21 and 22
It is set to z.

The interpolation filters 19 and 20 are also the same as those shown in FIG.
As shown in FIG. 5, the sampling frequency is composed of a 27th-order band-limiting FIR filter, and the sampling frequency is set to the DA converters 21 and 22.
It is set to 72 kHz which is the same as the sampling frequency of.

In the modulation device of the embodiment, the operation until the symbol train is generated from the symbol converter 4 is the same as the conventional device. First, the transmission data input terminals 1 to 3
When transmission data is input at a transmission rate of 2 kb / s, the Gray encoder 2 generates impulses which are quaternary weighted for every 2 bits at 16 kHz intervals, and the serial / parallel converter 3 generates the Gray encoder 2. The impulses are sequentially separated into 4 corresponding to the number of sub-carriers, and 4 Kba
The ud symbol strings X ₁ , X ₂ , X ₃ , and X ₄ are output.
Next, the symbol converter 4 combines these symbol strings to generate X ₁ + X ₄ , X ₂ + X ₃ , X ₁ -X ₄ and X ₂ -X ₃ symbol strings.

The output of the symbol converter 4 is band-limited by the 88th-order root Nyquist filters 5, 6, 7, and 8, and output at 36 kHz intervals.

The spectrum of the output data of the 88th root Nyquist filters 5-8 is shown in FIG.

For the operation output of the root Nyquist filter 5, 3Δω = 6.7 given from Table 9
The in-phase component of 5 kHz is multiplied by the multiplier 13, and the in-phase component of Δω = 2.25 KHz given from the table 10 is multiplied by the multiplier 14 to the operation output of the root Nyquist filter 6. The outputs of the multipliers 13 and 14 are added by the adder 17.

On the other hand, the calculated output of the root Nyquist filter 7 is 3Δω given from the table 11.
= 6.75 kHz quadrature phase component is multiplied by the multiplier 15, and for the operation output of the root Nyquist filter 8, Δω = 2.25 given from the table 12
The quadrature phase component of kHz is multiplied by the multiplier 16, and the outputs of the multipliers 15 and 16 are added by the adder 18.

The spectrum of the output data of the adders 17 and 18 is shown by the solid line in FIG. In this figure, the waveforms appearing on the left and right of the center sub-carrier modulation output pattern are patterns of 36 kHz repetition.
The sub carrier leakage power at this time is -30.
It becomes 2 dB.

Band limits are applied to the outputs of the adders 17 and 18 at intervals of 36 kHz by the interpolation filters 19 and 20, respectively, and the result is output at intervals of 72 kHz.
The filter characteristics of the interpolation filters 19 and 20 are shown by dotted lines in FIG. 4, and the outputs of the interpolation filters 19 and 20 are as shown by solid lines in FIG.

The outputs of the interpolation filters 19 and 20 are DA converters.
21, 22 and analog filters 23, 24 (the filter characteristics of the analog filters 23, 24 are shown by the dotted lines in FIG. 5) are processed and converted into signals of the spectrum shown in FIG.
Analog signal I output from analog filter 23
(t) and Q (t) are quadrature-modulated with the carrier signal generated by the synthesizer 25 and output from the output end 29 in the form of a modulated signal S (t). This I (t) is represented by the above-mentioned formula (2), Q (t) is the formula (3), and S (t) is the formula (1).
Are each represented by.

As shown in FIG. 6, the modulator of the embodiment satisfies the standard of adjacent channel leakage power of −60 dB or less.

As described above, the modulator of the embodiment uses two kinds of filters, that is, the 88th-order root Nyquist filter and the 27th-order interpolation filter having the specifications of FIG. 2 in order to satisfy the standard of FIG. As a result, the sub-carrier leakage power has been successfully reduced to -30 dB or less, and the adjacent channel leakage power has been successfully reduced to -60 dB or less at 25 kHz separation.

The calculation amount of this modulator is 88 × 4 = 352 times in the root Nyquist filter section and 27 × 2 = 54 times in the interpolation filter section, which is a total of 406 product-sum operations.
It is extremely small compared to the amount of calculation of the conventional modulator. Therefore, the power consumption and the price of the device can be reduced accordingly. (Note that the order 88 of the root Nyquist filter is a value when the sampling frequency is set to 36 kHz, and the sampling frequency is 72 kHz.
If set to, the order is 88 × 2 = 176. At this time, the total number of product-sum operations is 758. )

As is apparent from the above description of the embodiments, the modulator of the present invention reduces the total calculation amount of the filters by the subordinate structure of the root Nyquist filter and the interpolation filter, and・ By setting the sampling frequency of the Nyquist filter smaller than the sampling frequency of the interpolation filter,
The amount of calculation is further reduced. In the modulator of the embodiment, the amount of calculation is reduced to ⅓ due to the subordinate structure of the two filters, and it is reduced to ⅙ by performing the multi-rate conversion together.

Due to this reduction in the amount of calculation, the modulator of the present invention can achieve low power consumption and low cost.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a 4ASK modulator according to an embodiment of the present invention,

FIG. 2 is a diagram showing the specifications of a filter used in the 4ASK modulator of the embodiment;

FIG. 3 is a spectrum diagram of a root Nyquist filter output in the modulator,

FIG. 4 is a spectrum diagram of a sub-carrier modulation output in the modulator,

FIG. 5 is a spectrum diagram of an interpolation filter output in the modulator,

FIG. 6 is a spectrum diagram of an analog filter output in the modulator,

FIG. 7 is a diagram showing the specifications of a 4ASK modulator,

FIG. 8 is a block diagram showing the configuration of a conventional 4ASK modulator.

FIG. 9 is a diagram showing filter specifications used in a conventional 4ASK modulator.

FIG. 10 is a spectrum diagram of the root Nyquist filter output of the modulator,

FIG. 11 is a spectrum diagram of a modulation output in the modulator.

[Explanation of symbols]

 1 Transmission data input terminal 2 Gray encoder 3 Serial / parallel converter 4 Symbol converter 5-8, 35-38 Root Nyquist filter 9-12 Table for performing subcarrier modulation 13-16 Multiplier 17 , 18 Adder 19, 20 Interpolation filter 21, 22 DA converter 23, 24 Analog filter 25 Synthesizer 26, 27 Analog multiplier 28 Analog adder 29 Transmit output terminal

Claims (1)

[Claims] 1. A modulator for multi-carrier transmission for combining and transmitting signals modulated by a plurality of sub-carriers, comprising: a frequency band limiting filter for suppressing leakage power between the sub-carriers; and an adjacent channel. A modulation device, wherein an interpolation filter for suppressing leakage power between the two is provided separately, and a sampling frequency of the frequency band limiting filter is set to be lower than a sampling frequency of the interpolation filter.