JPH06112981A - Digital modulation circuit - Google Patents

Abstract

PURPOSE:To reduce the quantity of hardware by shifting a phase of a modulation waveform signal at odd number order modulation and summing the result onto a modulation waveform signal at an even order modulation time so as to minimize number of storage capacity of a ROM and number of adders. CONSTITUTION:Input digital signals I, Q are converted to + or -1 levels by a signal point arrangement circuit 51. Then a demultiplexer 52 demultiplexes the signals into signals Ie, Qe, Io, Qo at even order and odd number order modulation. The demultiplexed modulation signal is inputted to waveform shaping filters 53-56, in which the signal is converted into a modulation wave signal. Then an output signal from the waveform shaping filters 55, 56 with respect to the signal at odd number modulation time is rotated for its phase by 45 deg.(pi/4) with a 45 deg. phase correction device 57. Thus, the modulation signal waveform at an odd number modulation time for the pi/4 shift QPSK modulator is obtained. The resulting waveform is added to a modulation signal waveform at an even order modulation time, then the waveform shaping function of the pi/4 shift QPSK modulation is attained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital modulation circuit, and more particularly to a π / 4 shift quadrature phase shift key (QPSK) used in digital mobile radio.
The present invention relates to a modulation circuit applied to a modulation system.

[0002]

2. Description of the Related Art In recent years, in order to improve the reliability of mobile communication,
The introduction of digital modulation schemes for radio is being considered. The digital modulation method is a method in which a carrier signal is modulated by digital data and placed on a radio wave. Compared with the conventional analog method, the transmission efficiency is better, the resistance to noise is better, the communication quality is better, and the protection against interference, eavesdropping, etc. is provided. It has advantages such as easy and reliable.

A modulator for placing data on a carrier wave comprises a waveform shaping filter for generating a modulated wave form from the data and a mixer for multiplying the carrier wave signal by the modulated wave form to obtain a modulated wave signal. As a method of forming the former waveform shaping filter, there is a method by digital signal processing which is easy to be an LSI and is suitable for downsizing and power saving. Above all, read-only memory (ROM: Read Only Me
The digital filter method using mory) is often used. Known examples of this method are disclosed in, for example, JP-A
61-234153, "Digital Modulator". As a preparation for the description of the present invention, a method of forming a waveform shaping filter of a digital modulator using a ROM will be described.

FIG. 1 shows an example of a waveform shaping filter using a ROM. In the figure, 1 ₁ , 12 ₂ , ~, 1 _N are delay elements, 2 ₀ , 2 ₁ , 2 ₂ , ~, 2 _N-1 , 2 _N are coefficient multipliers,
3 ₁ , 3 ₂ , ~, 3 _N-1 , 3 _N are adders, and 4 is a ROM. Input data x _n is input to the delay elements 1 ₁ , 12 ₂ , ..., 1 _N , and the data series x _n , x _n-1 , x _n-2 , ..., x _N-1 , x _N.
To get The coefficient multipliers 2 ₀ , 2 ₁ , 2 ₂ , ~, 2 _N-1 , 2 _N apply filter coefficients a ₀ , a ₁ , a ₂ ,
~, A _N-1 , a _N , and adders 3 ₁ , 3 ₂ , ~, 3 _N-1 ,
When integrated with 3 _N , output data y _n is obtained. That is, the output y _n is

[0005]

Y _n = a ₀ x _n + a ₁ x _n-1 + a ₂ x _n-2 + ... + a _N-1 x _{n-N + 1} + a _N x _nN (1) Equation 1 is a difference equation expressing the operation of the digital filter, and the characteristics of the filter are coefficients a ₀ , a ₁ , a ₂ , ..., a
It is determined by _N-1 , a _N.

The coefficient multiplier and adder shown in FIG. 1 can be realized by a ROM if the number of bits of the input data x _n is small and the number of filter taps (the number of filter coefficients) is small.

The waveform shaping filter shown in FIG.
FIG. 2 shows a configuration example of a digital modulator applied to the modulation method. In the figure, 21 is a signal point arrangement circuit, and 22 and 23.
Is a waveform shaping filter, 24 and 25 are mixers, and 26 is a signal adder. FIG. 2A is a phase arrangement diagram of QPSK modulation, in which two pieces of digital data (I, Q) = ((0,0),
Four phases (π / 4, 3π / 4, 5π / 4, 7) of the carrier signal phase with respect to (1, 0), (1, 1), (0, 1))
Modulate by switching to π / 4). In practice, a quadrature modulation circuit format is used as shown in FIG. That is, the 2-bit digital data I, Q having a digital value (0, 1) is converted into a signal amplitude value of (1, -1) by the signal point arrangement circuit 21 and then passed through the waveform shaping filters 22, 23. , The mixers 24 and 25 multiply the two carrier signals cos ωt and sin ωt having orthogonal phases to obtain a signal adder 2
Add and combine in 6.

Although the QPSK modulation method has been described above as an example, the digital modulation method used for mobile communication is π /
There is a 4-shift QPSK modulation method. This modulation method, as shown in FIG. 3, includes the signal points (white circles) of the normal QPSK modulation of FIG.
This is a modulation method in which (c) the black circles) are switched and transmitted every symbol period. As an example of implementing this modulator using a ROM, the presentation number B-338 “symbol tap RO” presented at the 1992 IEICE Spring Conference.
There is a π / 4 shift QPSK baseband signal generator using the M division method.

[0009]

DISCLOSURE OF THE INVENTION A digital modulator using a ROM as described in the above-mentioned prior art, which has a π /
When trying to realize the 4-shift QPSK modulation method, the
As can be seen from (a), five I and Q amplitude levels (−√2, −1, 0, 1, √2) are required. If this is realized by a digital filter using a ROM, the result shown in FIG.
As shown in. In the figure, 41 is π / 4 shift QPSK
Signal point arrangement circuits for modulation, and 42 and 43 are waveform shaping filters. Since the output of the signal point arrangement circuit 41 has 5 levels, information of 3 bits per modulation signal is required, and the number of ROM address bits of the waveform shaping filters 42 and 43 is increased to increase the ROM storage capacity. This leads to an increase in wear scale. Conversely speaking, only a waveform shaping filter with a small number of taps can be constructed.

In order to avoid this problem, in the π / 4 shift QPSK modulation described in the section of the prior art, the ROM is divided into a plurality of pieces, and the outputs of the respective ROMs are summed by an adder to realize this. With this method, the storage capacity of the ROM can be suppressed to a scale that is easy to realize, but the number of ROMs increases and the adder also has R
As many as the number of OMs are required, and the total amount of hardware also becomes large.

An object of the present invention is to use a conventional ROM for .pi.
/ 4 shift QPSK digital modulator, ROM
(EN) A method of constructing a waveform shaping filter that minimizes the storage capacity and the total number of adders is provided, and the hardware amount of a digital modulator is gradually reduced.

[0012]

In order to achieve the above object, attention is paid to the characteristics of π / 4 shift QPSK modulation, and devised so that the signal point arrangement can be expressed by two amplitude levels (1 bit). That is, from FIG. 3, π / 4 shift QPSK
It can be seen that the modulation signal point arrangement for modulation is such that the QPSK modulation signal point is modulated while rotating by π / 4 (45 degrees) at each modulation time point. Alternatively, QPSK at the even-numbered modulation points
The same modulation point arrangement as that used for modulation is used, and the modulation point arrangement for QPSK modulation is shifted by π / 4 at the time of odd-numbered modulation. Therefore, at the time of odd-numbered modulation,
As shown in (c), when an input signal is considered with an axis inclined by 45 degrees such as I ′ and Q ′, and when the output of the waveform shaping filter is synthesized, the data at the odd-numbered modulation time point is inclined by 45 degrees and the even-numbered modulation time point is inclined. Add and synthesize with the waveform shaping output of. That is, a demultiplexer that divides the modulated data into odd-numbered and even-numbered data, four ROM waveform shaping filters, a phase corrector that rotates the output of the odd-numbered waveform shaping filter by 45 degrees, an odd-numbered and even-numbered data A π / 4 shift QPSK digital modulator can be configured by an adder that adds the ROM filter outputs of

In this way, it is necessary to divide the input signal into odd-numbered and even-numbered ones, but it is sufficient that each of them has the same signal point arrangement and the same arrangement as the QPSK modulated wave, and the number of input signal levels of the ROM waveform shaping filter is 5 (3 bits) to 2 (1
A bit). Since the number of signal bits of the N tap waveform shaping filter decreases, the number of address bits is 2N.
Bit reduced. For example, with a 5-tap filter, 10
Bits are reduced and ROM capacity is drastically reduced to 1/1024. Along with this, the number of divided ROMs also decreases, and the number of adders for adding ROM outputs also decreases. Thus π / 4
In the shift QPSK digital modulator, it is possible to realize a configuration in which the ROM capacity and the number of adders can be reduced and the hardware amount can be gradually reduced.

[0014]

As described above, in the digital modulator of the present invention,
In addition to the ROM waveform shaping filter, a demultiplexer,
Although an extra phase corrector and adder are required, the ROM capacity of the waveform shaping filter can be greatly reduced. Since this decrease is very large, it is more than enough to compensate for the increase in hardware due to the additional circuit required.

Further, the waveform shaping filter has even-numbered I,
Since it is necessary for Q data and for odd-numbered I and Q data respectively, a total of four filters are required. This is
Although the number of ROM filters will increase, since the I and Q filters have exactly the same coefficients, they can be used in time multiplex and the number can be reduced. In this case, the address of the ROM only increases by one bit, and the adder can also be used multiple times, so that further hardware reduction can be realized.

[0016]

FIG. 5 is a block diagram of the first embodiment of the present invention. In the figure, 51 is a signal point arrangement circuit, 52 is a demultiplexer, 53, 54, 55 and 56 are waveform shaping filters, 57 is a phase corrector, and 58 and 59 are signal adders.

The input digital signals I and Q are converted by the signal point arrangement circuit 51 into amplitude values of ± 1. This conversion is performed according to the signal point arrangement of QPSK modulation shown in FIG. Next, the demultiplexer 52 separates the signals Ie, Qe, Io, and Qo at the even and odd modulation times. The separated modulation signals are respectively subjected to the waveform shaping filter 5
3 to 56, and converts into a modulated waveform signal. The filters 53, 54 and 55, 56 are filters having exactly the same coefficient, and only data at even or odd time points are input, but the output signal outputs data at any time point. The output signals of the waveform shaping filters 55 and 56 with respect to the signals at the odd-numbered times of modulation have a phase of 45 degrees (π) by the 45-degree phase corrector 57.
/ 4) Rotate. In this way, the modulation signal waveform at the odd-numbered modulation time point of the π / 4 shift QPSK modulation wave is obtained. By adding and synthesizing this with the modulation signal waveforms at the even-numbered modulation points by the adders 58 and 59, the waveform shaping function of π / 4 shift QPSK modulation can be achieved. In order to finally obtain a modulated wave, the carrier wave modulation circuit (mixers 24, 25 shown in FIG. 2 is used.
And the signal adder 26). This part is QPSK
Since it is exactly the same as the case of the modulator, it is omitted.

FIG. 6 shows an example of the configuration of the phase corrector used in the embodiment of FIG. In FIG. 6, 61 and 62 are adders and subtractors, 63 and
64 is a coefficient multiplier. When the I and Q signals are expressed by complex numbers, they become I + jQ. Where the phase angle of the complex number is φ
= Arctan (Q / I) Therefore, the signal x _I + jx _Q
By rotating -π / 4 phase,

[0019]

Y _I + jy _Q = (x _I + jx _Q ) × exp (−jπ / 4) = (x _I + jx _Q ) × (cosπ / 4−jsinπ / 4) = (√2 / 2) × (1 −j) × (x _I + jx _Q ) = (√1 / 2) × ((x _I + x _Q ) + j (x _Q −x _I )) (2), which can be realized by the configuration shown in FIG. I understand. Although the coefficient multiplier is used in FIG. 6, this portion can be included in the waveform shaping filters 55 and 56 in FIG.

As can be seen from the above description, according to this embodiment, the modulator of the π / 4 shift QPSK modulation system shown in FIG. 3A has the same structure as the QPSK modulator of FIG. 2A. It can be realized by using a waveform shaping filter. That is, as in the conventional example of FIG.
The hardware can be realized by using a ROM having a small storage capacity without increasing the number of address bits of the OM.

In the first embodiment shown in FIG. 5, four waveform shaping filters are required, and additional circuits such as a phase corrector and an adder are required. ROM waveform shaping filter 5
Since 3, 54 and 55, 56 have the same coefficient, these hardware can be realized in one ROM by using them in multiple. A second embodiment of the present invention using this idea will be shown below.

In FIG. 7, reference numeral 70 is a ROM filter, and 71 and 72.
Is a waveform shaping filter, 73 is a phase corrector, and 74 is a demultiplexer. Signals Io and Q at the time of odd-numbered modulation
Since the coefficient of the waveform shaping filter is the same as that of o, it can be processed by one ROM if multiple uses are made. further,
The same ROM also realizes the function of the phase corrector 73. Since the phase corrector can be configured by the adder and the coefficient multiplier as described with reference to FIG. 6, it can be easily understood that it can be configured by the waveform shaping filter realized by the ROM shown in FIG. In order to reduce the number of output bits of the ROM, the output terminal is multiplexed with I and Q data. Therefore, the demultiplexer 74 separates the I and Q outputs, and the signal adders 58 and 59 add the data at the even-numbered modulation points. Thus, according to the embodiment described in FIG. 7, the π / 4 shift QPSK modulator in which the number of ROMs is reduced by one and the phase corrector is also reduced can be configured.

The method based on the above-described multiplexing can also be used for the waveform shaping filter at the time of even-numbered modulation. This embodiment is shown in FIG. In the figure, 80 is a ROM filter, 81 and 82 are waveform shaping filters, 83 is a multiplexer, 84 is a signal adder, and 85 is a demultiplexer. The coefficients of the waveform shaping filter are the same for the signals Ie and Qe at the even-numbered modulation time, so that they can be shared by one ROM by multiple use. At this time, RO
The outputs are time-multiplexed to reduce the number of output bits of M. That is, the multiplexer 83 is used to multiplex the outputs of the waveform shaping filters 81 and 82. Further, addition and synthesis with the output of the waveform shaping filter at the time of odd-numbered modulation is also performed by the signal adder 84 while being multiplexed, and if the final output is separated by the demultiplexer 85, the adder for addition and synthesis is also 1
You can do it individually.

As described above, according to the present invention, the π / 4 shift QP
The embodiment for reducing the ROM capacity of the ROM waveform shaping filter of the SK modulation digital modulator has been described. Although the present invention requires some additional circuits, the reduction of the ROM capacity is performed by a power of 2, so that the reduction in hardware is sufficient even if the increase in additional circuits is supplemented. Further, by additionally using the waveform shaping filter, the additional circuit can be incorporated in the ROM.

[0025]

According to the present invention, π / 4 shift QPSK
When a modulation type digital modulator is implemented by digital processing using a ROM, the ROM capacity is increased in the conventional method, so that the number of adders for adding and synthesizing the ROM and the ROM output is increased and the hardware amount is increased. The problem can be solved. Although some additional circuits other than the ROM are required, the reduction in the ROM capacity according to the present invention does not pose a problem because it is sufficient to supplement the hardware amount of these additional circuits. Furthermore, by using multiple waveform shaping filters, the number of ROMs and additional circuits can be incorporated in the ROMs, and further hardware reduction can be achieved.

[Brief description of drawings]

FIG. 1 is a block diagram of a waveform shaping filter.

FIG. 2 is a block diagram of a QPSK digital modulator.

FIG. 3 is an arrangement diagram of signal points of a π / 4 shift QPSK modulation method.

FIG. 4 is a partial configuration diagram of a π / 4 shift QPSK modulator.

FIG. 5 is a block diagram of a first embodiment of the present invention.

6 is an explanatory diagram of a phase corrector used in the embodiment of FIG.

FIG. 7 is a block diagram of a second embodiment of the present invention.

FIG. 8 is a block diagram of a third embodiment of the present invention.

[Explanation of symbols]

51 ... Signal point arrangement circuit, 52 ... Demultiplexer, 5
3, 54, 55, 56 ... Waveform shaping filter, 57 ... Phase corrector, 58, 59 ... Signal adder.

Claims (4)

[Claims] 1. A modulation circuit for obtaining a modulated wave signal of a π / 4 shift four-phase phase shift keying system, wherein the modulated signal is divided into even-numbered and odd-numbered signals at the time of modulation, and each is divided by a waveform shaping filter, After being converted into a modulation waveform signal, the phase of the modulation waveform signal at the odd-numbered modulation time point is shifted by π / 4 and then added and synthesized with the modulation waveform signal at the even-numbered modulation time point to obtain the modulation waveform signal. Digital modulation circuit. 2. A digital modulation circuit according to claim 1, wherein said waveform shaping filter is constituted by a digital waveform shaping filter using a read-only memory, and the waveform shaping filter is multiplexed with an in-phase component and a quadrature component. 3. The circuit according to claim 2, which shifts the phase of the output of the waveform shaping filter at the time of odd-numbered modulation by π / 4,
A digital modulation circuit that performs batch processing using a read-only memory that constitutes the waveform shaping filter. 4. The waveform shaping filter output at the even-numbered modulation time point is added and synthesized with the waveform shaping filter output at the odd-numbered modulation time point while the in-phase and quadrature component are multiplexed, and then converted into the in-phase and quadrature component. A digital modulation circuit that separates and obtains a modulated waveform signal.