JPH0219663B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention is based on GMSK (Gaussian Filtered)
Minimum Shift Keying), TFM (Tamed
Fsequency Madulation), CCPSK (Compact
Spectrum Constant envelope Phase Shift
Input code string when using waveform memory, which is ROM memory, to generate orthogonal modulated waves in quadrature modulators used in digital FM (frequency modulation, FSK) systems used in mobile communications, such as Since the modulation waveform differs depending on the code sequence, a quadrant management circuit is provided that controls which modulation waveform is output depending on the code sequence. (For the above-mentioned methods of digital FM, see Reference 1...Journal of the Institute of Electronics and Communication Engineers 1982.
(Refer to February 2015, p. 192-198 Technology Outlook) (Conventional technology) Conventionally, when generating digital FM waves using orthogonal modulation, it has been stated in the literature that quadrant management is necessary, but there is no specific method. does not seem to have been disclosed. (Specific Object of the Invention) In the orthogonal modulator used for digital FM, since the action of the orthogonal modulator is linear, the modulated wave cannot be obtained immediately from the input digital code string by linear operation. For this reason, the modulated wave is generally a non-linearly converted waveform stored in ROM, and the modulated wave is read out from this waveform memory ROM according to the input code. The modulating wave must be different depending on the influence. Therefore, it is necessary to control the output modulated wave according to the input code string. The quadrant management circuit according to the present invention realizes this. Here, the basics of the present invention will be explained. The quadrature modulator will be explained later with reference to FIG. 1, and the quadrant management circuit will be explained later with reference to FIG. (1) As is well known, the modulated wave of digital FM (FSK) is expressed by the following equation. v(t)=cos [ω _c t+θ(t)] ...(A) ω _c is the angular frequency of the carrier wave, and θ(t) is the phase change related to the information signal, which is a fundamental signal of digital FM. As mentioned above, there are various methods of FSK that are suitable for mobile radio, but for example, MSK
(Minimum Phase Shift Keying) is a narrowband
A type of FSK in which the frequency shifts discretely to ±ΔK, but the phase shifts continuously. In FSK, when the modulation index is chosen to be 0.5, the signal waves corresponding to the codes are orthogonal to each other, and it is called MSK.It is well known that the power spectrum has good concentration, and it is also the original form of the GMSK system, so let's take the case of MSK as an example. This will be explained below. In the case of MSK, θ(t)=±
πt/2T (T is the time of one time slot of the code=bit period as shown in FIG. 2), and the phase shift after T seconds is therefore ±π/2. The phase θ(t) continuously advances by 90° when the information signal is “1” and becomes “0”.
When , there is a continuous delay of 90°. This relationship is illustrated in FIG. 4, which shows an example of input digital code and phase changes. In Figure 4, the dashed line is
In the case of MSK, the solid line shows each example in the case of GMSK and CCPSK. It is known that on this phase plane, the chain of codes and the change in phase can be associated by appropriately dividing the code length. For example, the example of the code and phase change announced in the CCPSK system is the 5th example.
As shown in the figure, this is the case of a 3-bit code string. (Reference 2: Okai et al., Narrowband Constant Envelope Digital Phase Modulation Method, Technical Research Report of the Institute of Electronics and Communication Engineers (CS79-133) p.55-62). The following relationship exists between the phase change in FIG. 5 and the phase change in FIG. 4 for the code string. In time slot 2, the code string is 101, so Figure 5d
This results in a phase change below . In time slot 3, the code string is 011, so the phase change is as shown in the upper part of FIG. 5c. It will be seen that in this way, the phase changes for the continuous code strings shown in FIG. 4 can be determined from the 3-bit code strings and the eight types of phase change patterns shown in FIG. 5 corresponding to the 3-bit code strings. (2) Reason why phase plane management is necessary Since the “0” in time slots 2 and 9 in Figure 4 has “1” before and after it, resulting in a code string of 101, the phase waveform on the lower side of Figure 5 d can be It is possible to read out and maintain the continuity of the phase plane. However, in time slot 2, the phase changes between π/2 and 0, while in time slot 9, the phase changes between 0 and -π/2. This means that if we consider sine waves and cosine waves, the cosine wave will have the same waveform in response to this phase change, but the sine wave will have a waveform with reversed polarity. Therefore, when reading out a sine wave and a cosine wave using a quadrature modulator, even if the phase changes are the same, accurate modulated waves cannot be obtained unless the quadrant in which the instantaneous phase of the time slot of interest exists is accurately known. Therefore, it is necessary to monitor the code series and manage quadrants accordingly. As explained above, even time slots with the same code sequence should be output to the quadrature modulator.
Since the waveforms of the sine wave and the cosine wave differ depending on the quadrant in which the instantaneous phase exists, uncertainty in the output waveform can be removed by adding a quadrant management circuit. (Structure and operation of the invention) When a quadrature modulator is used to generate a digital FM wave by orthogonal modulation, the quadrant management circuit of the present invention can be used to easily read a modulated wave from a waveform memory ROM using a simple circuit. A practical quadrature modulator can be realized. First, the drawings will be explained. Figure 1 is an example of the configuration of a conventional quadrature modulator, and Figure 2 is the
The phase plane diagram and output waveform diagram in the ideal state of the quadrature modulator shown in the figure, FIG. 3 is a waveform diagram of each part of FIG. 1, FIG. FIG. 8 is a time chart of the circuit shown in the figure, and is a diagram showing an example of the configuration of a quadrature modulator provided with a quadrant management circuit according to the present invention. In the following, for the sake of simplicity, the case of MSK will be explained, assuming that the number of bits to monitor input data is 3. In FIG. 1, D is a delay element for one data, that is, one bit, and the waveform memory 1 outputs a sine wave and a cosine wave of a phase plane corresponding to the phase change shown in FIG. 5 with respect to input data. However, the waveform memory stores the waveform of each component sampled by an integer multiple of the data clock (time for one time slot), and reads out the waveform for one data starting from the data start point. and memory table, counter, D/A converter and low frequency filter (LPF)
It is made up of. Further, in the figure, 2 is an OSC (local oscillator), 3 is a 90° phase shifter, 3 is a multiplier, and Σ is an adder. Before explaining the operation of each part in FIG. 1, FIG. 2 will be explained. If the input code string is as shown in Figure 2 a, when reading out a waveform corresponding to "101" and the central "0" of the input data, if the phase plane of b is "a-b-c". “b” and “d-e-
In the case of "f", "e" exists in different quadrants on its phase plane, so the output waveforms are different as shown in Figure c. In order to deal with such a case, the output waveform is controlled by the quadrant management circuit shown in Fig. 6, which will be explained later.
→1 When the input sign changes, the quadrant in which the signal exists in the phase plane does not change. It can also be seen that if the number continues from 1 to 1, the number of quadrants increases, and if it continues from 0 to 0, the number of quadrants decreases. A quadrant management circuit makes such a determination. Next, the operation of each part shown in FIG. 1 will be explained using the waveforms of each part shown in FIG. The input data in FIG. 3a is the same as that in FIG. 2, and (1, a) means that the data is 1 and the phase plane is a. In FIG. 1, an input data string passes through two delay elements and determines a specified table of the waveform memory table in the waveform memory using a data sequence of three codes as described above. This predetermined table stores waveforms corresponding to the input data string. This waveform is sequentially read out by a clock having a speed that is an integer multiple of the data clock (in FIG. 3, it is shown as a read clock b that is six times faster than the data clock). Data is read by specifying a read address in the memory using the output of a counter operated by a read clock that is reset at a data switching point. This read output is D/A converted and output. (as in Figure 3d). The output after D/A conversion is a staircase waveform, so a low-pass filter (LPF) is used.
Only the low-frequency components are extracted and converted into a continuous wave, which is input to the multiplier of the modulator (see Fig. 3e and Fig. 8). The quadrature modulator of FIG. 1 operates on the following principle.
Let the angular frequency of the carrier wave component from OSC2 be ω ₀ ,
If the signal on the lower path is C _U (t), then C _U (t) = sinω ₀ t ...(1) Similarly, the signal on the upper path (on the phase shifter 3 side) is C _L
(t) is C _L (t) = cosω ₀ t ...(2) Also, the waveform of the sine component read from the waveform memory B _S
(t) is 0t<T, B _S (t)=sin(π/2Tt)...(3) Similarly, the cos component waveform B _C (t) is B _C (t)=cos(π/2Tt)... …(4) Therefore, the modulation output S(t) is S(t)=B _C (t) C _L (t) − B _S (t) C _U (t) = cos (π/2Tt) cosω ₀ t− sin(π/2Tt) sinω ₀ t =cos(ω ₀ +π/2Tt) (5) From equation (5), it can be seen that the modulated output of the orthogonal modulator shown in FIG. 1 has a constant amplitude. Note that waveform memory 1
In this example, as mentioned above, the waveform for the center bit in the 3-bit continuous data string is stored. Next, the quadrant management circuit of the present invention will be explained using the embodiment shown in FIG. When the code sequence shown in FIG. 2 is input to the quadrature modulator shown in FIG. 1, the phase plane states shown in FIG. ,
Since the data located before and after that is "1", the circuit shown in FIG. 1 cannot distinguish between the states b and e. However, for b and e, the phase plane in which they exist is between 0 and π/2, and e is between π/2
~π. Therefore, the sine component and cosine component for each phase plane are different from each other as shown in FIG. 2c. In Figure 6, D ₁ and D ₂ are each 1-bit delay circuits or elements, and 4 is a binary system.
5 is an EX-OR circuit (modulo 2 addition circuit), and 6 is a reversible (U _p /Doun abbreviated as U/D) counter that counts clock input.
It is controlled by the U/D control signal from AND4 and the enable signal from the EX-OR circuit. FIG. 7 is a time chart of the circuit shown in FIG. In this figure, a is a symbol corresponding to the bit of interest shown in the phase plane of FIG. 2b, and b is the input code string shown in FIG. 2a. The data input _, that is _, the input code string, is input to the _delay element string shown in _FIG . It is input to an AND circuit 4 and an EX-OR circuit 5 which perform control. Among these, the AND circuit 4 controls the U/D counter to count up by 1 only when the sign of the bit of interest and the preceding bit are both "1". The relationship between d and c in FIG. 7a corresponds to this example. At this time, the output of the AND circuit 4 becomes "1" at bit d, as shown in FIG. 7c. Similarly, when g ₁ and g ₂ are "0", the U/D counter decreases by 1. On the other hand, the EX-OR circuit 5 becomes "1" only when the two input data are different, inhibiting the counting operation of the U/D counter 6 only at that time, and the output remains unchanged. Therefore, if the bit we are looking at now is d in Figure 7a, the preceding bit is c, and since c and d have the same sign.
The output of EXOR5 becomes "0". At this time, AND
Since the output of the circuit 4 is "1", the U/D counter 6 performs up-counting, and the output appears as "1" at the A terminal shown in the figure. Note that A and B are U/D.
The output from the output terminal of the counter 6 is used as a quadrant control signal for the waveform memory 1, as shown in FIG. When the A terminal becomes “1”, A=0, B
It shows that the state is one quadrant (π/2 radian) advanced from the state of =0. In this way, when there are two consecutive bits of code "1", the position where data changes in the phase plane increases by π/2 (see Figure 2b), and vice versa when there are two consecutive bits of "0". π/
The quadrant control signals A and B can be generated so that the quadrant does not change when the number decreases by 2 and continues like "1 and 0" or "0 and 1". Here, the correspondence between the quadrants and the A and B outputs is established as follows.

[Table] For example, if both A and B go from 0 to A=1 and B=0, the phase of equations (3) and (4) increases by π/2 sin(π/2Tt+(π/2)=cos(π /2Tt) cos(π/2Tt+(π/2)=-sin(π/2Tt) Calculations are made and stored in the waveform memory (ROM), and by sequentially selecting combinations of "A, B", "g ₁ , g ₂ , g ₃ ", the quadrant and phase in which the specified data bit exists can be determined. The method of change is specified. Therefore, a total of 4 × 8 = 32 patterns, 4 types of sine components and 4 types of cos components in each quadrant of each of the 8 types of phase changes in Fig. 5, are stored in the ROM. This pattern ( The waveform) is 2 ⁵ = 32, that is, any one type is selected by the 5-bit control line. Fig. 8 is a diagram showing an example of the configuration of a quadrature modulator equipped with the quadrant management circuit of the present invention. The part surrounded by the dashed dotted line is the same quadrant management circuit as in Figure 6, and the other parts are the same as the orthogonal modulator in Figure 1.However, the D/A converter and LPF on the output side of waveform memory 1 are the same as those in Figure 6.
In the case of the figure, it is explained that it is provided in the waveform memory as described above. As shown in FIG. 8, the invention can be realized by specifying a specific memory table of the waveform memory 1 to be read out using the signals A and B from the quadrant management circuit and the input data string. It is possible to eliminate uncertainties such as those mentioned in the section on purpose. In addition, the explanatory formulas (1) to (5) of the orthogonal modulation are
The explanation was given using the waveform of the MSK method, but as shown in Figure 4, the same applies to waveforms other than MSK, but B _S (t), B _C
(t) waveform becomes more complex. In this case B _S
(t), B _C (t) are the function of phase change as θ (t), then B _S (t) = sin θ (t), B _C (t) = cos θ (t), and these can be expressed as equation (5). By substituting, we get the first equation (A). The change in θ(t) at this time is given in the form shown in FIG. (Effect of the invention) Using a quadrature modulator to generate digital FM waves,
If the quadrant management circuit according to the present invention is used when reading a modulated wave from a waveform memory, a quadrature modulator can be easily configured with a simple circuit. In the description of the invention and in FIG.
Although the system is taken as an example, the circuit of the present invention can also be applied to systems such as TFM and CCPSK as long as the modulation index is digital FM of 0.5. Note that when the modulation index is 0.5, if the code is "1" continuously, the phase advances by π/2 during 1 bit,
Since the phase has a period of 2π, the number of states for managing the phase plane may be 4 as described above. When the modulation index is other than 0.5, there are as many states to manage on the phase plane as the number divided by 2 by the modulation index, but this can be done by changing the number of U/D counter stages and the number of bits monitoring the input data string. Therefore, the present invention can be easily extended. In this way, the circuit of the present invention can manage quadrants according to the input digital code string, and the U/D
Correct modulated waves can be obtained by outputting from the waveform memory ROM in accordance with the quadrant control signal that is the output of the counter.

[Brief explanation of drawings]

Figure 1 is a configuration example diagram of a conventional quadrature modulator, Figure 2 is a phase plane diagram and output waveform diagram of the quadrature modulator in an ideal state, Figure 3 is a waveform diagram of each part of Figure 1, and Figure 4 is a digital A correspondence diagram of input codes and phase changes in FM. Figure 5 is a diagram of phase changes due to code combinations in the case of a 3-bit code string.
FIG. 6 is a configuration example diagram of a quadrant management circuit according to the present invention;
FIG. 7 is a time chart of the circuit shown in FIG. 6, and FIG. 8 is a diagram showing a specific configuration example when the quadrant management circuit of the present invention is attached to a quadrature modulator. 1... Waveform memory, 2... Oscillator, 3... π/2 radian phase shifter, 4... AND circuit, 5... EX-OR circuit,
6...Up/down counter, _D1 , _D3 ...1 bit delay circuit.

Claims (1)

[Claims] 1. When using a read-only memory (ROM) that serves as a waveform memory when generating orthogonal modulated waves in a quadrature modulator used in a digital frequency modulation transmitter, which modulation method is used depending on the code sequence of the input code string? A quadrant management circuit for a quadrature modulator that controls whether a waveform is output from the ROM, which delays the serial input code one bit at a time to generate parallel codes of a predetermined number of bit strings, and also supplies the output to the ROM. a series circuit of delay elements; an AND gate that receives the bit of interest in the parallel code and its preceding bit and generates an output of "1" only when both of these values are "1" of the binary code; EX-OR (exclusive) generates an output “1” only when two bits are different from each other.
OR) circuit, counts 1 only when the output of the AND gate is "1", subtracts 1 when the bit of interest in the input code and its preceding bit are both "0", and counts 1 when the output of the AND gate output is “1”
In this case, the status quo is maintained, and the reversible counter supplies a control signal for quadrant control of the waveform generated by the ROM by inputting its binary output to the ROM together with the parallel code of the bit string input to the ROM. A quadrant management circuit for a quadrature modulator, comprising: