JPH03159440A - Phase modulation circuit - Google Patents

Abstract

PURPOSE:To considerably reduce a memory capacity required for the generation of a base band signal by time-dividing an input symbol sequence into K-number of sequences, reading filter output waveforms with respect to respective sequences from a memory and synthesizing K-number of the waveforms so as to generate the corresponding base band signals. CONSTITUTION:In a switch 33, the sequence is time-divided into two odd and even sequences. The symbols of the odd time sequence and the even time sequence are respectively inputted to shift registers 35 and 39, the contents of respective shift registers are set to be addresses and the base band waveforms in a two symbol sections are read from waveform ROM 36 and 40. The waveforms read from waveform ROM 36 corresponding to the odd time sequence are phase-rotated in a phase rotary machine 37 by +pi/4, are added with the waveforms read from waveform ROM 40 corresponding to the even time sequence in an adder 38. Then, the corresponding base band signals 1(t)+jQ(t) can be obtained in an output terminal 42 through a digital/analog converter 41.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a phase modulation circuit that generates a narrowband phase modulated wave by phase modulating a carrier wave using a band-limited baseband signal.

[Prior Art] A phase modulated wave (PSK) has many sidelobe components, and band limitation is required to prevent interference with adjacent channels.

On the other hand, in mobile communications, for example, a frequency synthesizer for multi-channel access is used in the transmission system and the transmission frequency is changed, so band restriction for spectrum shaping cannot be performed at the final stage after power amplification. Therefore, this processing is normally performed in the baseband band before transmission frequency conversion. That is, a configuration is adopted in which a baseband signal whose band is limited by a low-pass filter is generated from an input symbol sequence, and a carrier wave is modulated to generate a narrowband phase modulated wave.

By the way, in order to prevent intersymbol interference from occurring at the time of code determination on the receiving side, it is necessary to remove the out-of-band spectrum by shaping the waveform using, for example, a filter with squared cosine characteristics, but this is not possible with normal analog circuits. It is difficult to obtain such a transmit spectrum with a filter that uses .

Therefore, in the conventional configuration, a filter response corresponding to an input symbol sequence is calculated in advance and stored in a ROM (read-only memory), and a signal read from the ROM using the input symbol sequence as an address is used as a baseband signal, and a carrier wave A method of phase modulating the

Here, π/4 shift 4-phase differential phase keying (QDPSK)
The case will be specifically explained with reference to FIG.

For convenience, 2-bit fl is used for 4-phase phase modulation.
symbol, and the transmitted symbol sequence as (a. bl%
) (n = −. −2. −1.0 , +1,
+2. ...).

In FIG. 4, in π/4 shift}QDPSK, as is well known, four signal points indicated by white circles and four signal points indicated by black circles are taken alternately for each symbol. therefore,
Both the black and white circles have four phases and are separated by π/2 in phase, and the combination of transmission symbols (OO), (1
0), (01), and (l1) are taken.

In this way, differential phase keying (DPSK) carries information on changes in phase, and the interval t of the nth symbol is
(n-0.5)T≦t<(n+0.5)T. Note that 1/T is the symbol transmission rate.

In such a π/4 shift QDPSK, a quadrature modulator shown in FIG. 5 is used.

That is, the signal space shown in FIG. 4 is regarded as a two-dimensional space of I (in-phase component) and Q (quadrature component), and the quadrature modulator 5
1, two signals I (t) and Q (t) (hereinafter referred to as (referred to as "I series" and "Q series").
is input. In each multiplier 55 and 56 of the orthogonal modulator 51, the baseband signal of each series is used as a carrier wave cos (2π
f, t) and the carrier wave si obtained by rotating the phase by π/2
By product-modulating each of n(2πf, t) and adding them in an adder 57, a modulated wave corresponding to the input symbol sequence is output. Here, the code 5th is π
/2 phase rotator, and fC is the carrier frequency.

In this case, the ■ series and the Q series take on five values.

In other words, regarding the signal points represented by black circles and the signal points represented by white circles, in the Q direction there are signal points 1, O and 2, 3 and 7, and 4.
and 6, 5, and even in the ■ direction signal points 7, 0 and 6,
There are five values: 1 and 5, 2 and 4, and 3. However, the whole is π/
8 If the signal point is rotated clockwise, for example, in the Q direction, the signal points can be set to four values: 1 and 2, 0 and 3, 4 and 7, and 5 and 6.

[Problem to be solved by the invention]

By the way, the output of the filter 53 composed of ROM is
Assuming that there is an influence from the preceding and following N/2 symbols (however, there is no intersymbol interference at the code determination time point t=nT), we will have 5-value signal points for each of the total (N+1) symbols, including the code determination time point. and the ROM address is 5
Only (81) is required. Therefore, the number of ROM addresses for each series is, for example, 511 when N=10, and the ROM capacity becomes enormous.

That is, as the number N of interfering symbols increases, R
OM'! The amount of J increases exponentially.

SUMMARY OF THE INVENTION An object of the present invention is to provide a phase modulation circuit that can handle baseband signals necessary for quadrature modulation with a small capacity ROM.

[Means to solve the problem]

FIG. 1 is a block diagram showing the basic structure of the system of the present invention.

The present invention provides a phase modulation circuit that generates a waveform from an input symbol sequence and phase-modulates a carrier wave using a band-limited baseband signal. , a memory for outputting a baseband waveform corresponding to the input symbol sequence, and a synthesizing means for synthesizing each baseband waveform and outputting a baseband signal corresponding to the input symbol sequence.

[For production]

If the number N of interference symbols is small, the ROM capacity can be small.

Therefore, in the present invention, in order to equivalently reduce the number N of interfering symbols, the input symbol sequence is time-divided into K sequences, the filter output waveform for each sequence is read out from the memory (ROM), and the K The waveforms are synthesized and the corresponding baseband signal is generated. That is, the number of interfering symbols in each sequence in this case is N/K, and the memory capacity can be significantly reduced. Note that when N/K is not an integer, the minimum integer exceeding N/K is used.

[Example] Hereinafter, an example of the present invention will be described in detail based on the drawings.

In this embodiment, a case of π/4 shift QDPSK that can effectively reduce the ROM capacity will be described.

At the signal points of π/4 shift I-QDPSK shown in FIG. ,
7, and signal points 0, 2, 4, and 6 at even time points.

That is, when divided into odd-numbered time points and even-numbered time points, they can be separated into two four-phase phase signal points shifted by π/4 on the phase plane. Therefore, in the case of QDPSK, the waveform corresponding to the odd time series is generated in the same way as the waveform for the even time series, and then phase rotation is given by π/4, and the waveforms of the odd and even time series are combined. By doing so, the corresponding baseband signal can be obtained.

Here, for ■ series, time division and waveform generation interval ( (
2n-1.5)T≦t<(2n+1.5)T) is shown in FIG. The waveform generation intervals for even and odd time series are twice as long as when they are not divided.

In this way, in π/4 shift QDPSK, the number of divisions K=2
For example, as shown in Figure 4, even time series can be represented by white circle signal points, and odd time series can be represented by black circle signal points, so the black circle signal points can be rotated by π/4 to match the white circle signal points. Then, the white circle signal point becomes binary for both I and Q. In other words, both the odd and even time series are binary series (±1)
Can be done.

Therefore, the number of ROM addresses required for the I-series modulation baseband signal is 2×2h when N=10, making it possible to significantly reduce the ROM capacity. Furthermore, if the number of divisions is further increased to K=4, the number of ROM addresses becomes 4×23 when N-10, which can further be reduced to 1/4.

FIG. 3 is a block diagram showing one embodiment of the structure of the part corresponding to the present invention.

Note that to simplify the explanation, symbol sequences are treated as complex numbers. That is, the input symbol sequence is a, +jbn,
The baseband signal is represented by I(t)jQ(t), and the thick lines in the figure are composed of two lines representing complex real numbers and imaginary numbers. For example, input symbol sequence a, +jb,
The thick line passing through is a1 and b. Two signals flow. Therefore, an actual circuit has a two-series circuit structure: a real number series and an imaginary number series.

In FIG. 3, input symbol sequences a. 10jb,,
The signal is input to the differential encoder 32 from the input terminal 3l, converted into a symbol sequence I n + J Q Fl, and input to the switch 33.

One output of the switch 33 is input to a shift register 35 via a phase rotator 34, and the output is input to a waveform ROM.
36 address input, and the read baseband waveform is input to the adder 38 via the phase rotator 37. Further, the other output of the switch 33 is input to the shift register 39, the output thereof becomes an address input of the waveform ROM 40, and the read baseband waveform is input to the adder 38.

The output of the adder 38 is digital/analog conversion #i41
The baseband signal 1−(t)+j Q D corresponding to the input symbol sequence is taken out to the output terminal 42 via
) is obtained.

Next, the operation of each part based on the above configuration will be explained.

In the differential encoder 32, the input symbol sequence a+JbnC
an+bn=±1) is input, I ,,+jQ+s=
(a n+j b n) (I n-t +jQa
−+) is performed, and In +J
Q n is output. However, the initial state is ■. +jQ
Let o=1+゛j.

The switch 33 divides the time into two sequences: odd (output when 2n+l) and even (output when 2n). The symbols of the even time series have a value of ±1.

Regarding the odd time series, since it corresponds to the black circle signal point in Fig. 4, the phase rotator 34 applies a phase rotation of -π/4, and converts it into a ±1 symbol by making it correspond to the white circle signal point. do.

Odd time series and even time series symbols are input to shift registers 35 and 39, respectively, and baseband waveforms of two symbol intervals are read out from waveform ROMs 36 and 40, respectively, using the contents of each shift register as an address. The waveform read from the waveform ROM 40 corresponding to the odd time series is phase rotated by +π/4 by the phase rotator 37, and added to the waveform read from the waveform ROM 40 corresponding to the even number time series by the adder 38. and the corresponding baseband signal 1 (t) +jQ (
t) is available at the output terminal 42.

The baseband signal output from this output terminal 42 is
This becomes the input of the quadrature modulator 51 shown in FIG.

Note that the differential encoder 32, switch 33, and phase rotator 34 can also be realized by arithmetic processing by a microprocessor. Alternatively, a configuration may be adopted in which the calculation results are written in a ROM table in advance to obtain outputs of an odd time series and an even time series (±1) according to the input symbol series.

〔Effect of the invention〕

As described above, according to the present invention, the memory capacity required for controlling the baseband signal required for modulation can be significantly reduced, and the circuit scale can be easily reduced. ,

[Brief explanation of the drawing]

Figure 1 is a block diagram showing the principle structure of the present invention. FIG. 2 is a diagram showing the relationship between time division and waveform generation sections. FIG. 3 is a block diagram showing the configuration of an embodiment of the part corresponding to the present invention. FIG. 4 is a diagram explaining the signal space of π/4QDPSK. FIG. 5 is a block diagram showing the structure of a quadrature modulator used in π/4QDPSK. 31...Input terminal, 32...Differential encoder, 33...
・Switch, 34, 37... Phase rotator, 35, 39
...Shift register, 36, 40...Waveform ROM,
38... Adder, 41... Digital/analog converter゜, 42... Output terminal, 51... Quadrature modulator,
52... Differential encoder, 53... Filter, 54...
- Waveform generator, 55, 56... Multiplier, 57... Adder, 58... π/2 phase rotator. Figure 1

Claims (1)

[Claims] (1) In a phase modulation circuit that generates a waveform from an input symbol sequence and phase-modulates a carrier wave using a band-limited baseband signal, there is a switch that time-divides the input symbol sequence into multiple sequences, and a switch that divides the input symbol sequence into multiple sequences, and A phase modulation circuit comprising: a memory that outputs a corresponding baseband waveform; and a synthesis means that synthesizes each baseband waveform and outputs a baseband signal corresponding to an input symbol sequence.