JPS637692B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention uses TDMA (time division multiple access:
(Time Division Multiple Access) modulator.

The TDMA system to which the present invention is applied includes a small capacity multidirectional multiplex communication system. In the small-capacity multidirectional multiplex communication system, a master station transmits time-division multiplexed signals to multiple slave stations all at once, and each slave station extracts only the signal at the time allotted to it. Each slave station synchronizes with the master station clock and transmits in bursts to the master station during the time period assigned to it. At the master station, each slave station makes sure that the burst signals from each slave station are lined up in an orderly manner. I have to. Therefore, each slave station can easily identify and reproduce the received signal based on the master station clock. Furthermore, since this system has a small transmission capacity, the clock frequency is low, a linear modulation system is adopted, and the band limitation of the data signal is mainly performed at the baseband.

FIG. 1 shows a conventional TDMA modulator that obtains burst modulated waves in such a multidirectional multiplex communication system. Two columns of baseband data signals (I CH
and Q CH) are band-limited by a low-pass filter 4 and then fed to a linear modulator 3 where the transmit local oscillator 1 is fed via a radio frequency band (RF) switching circuit 2. quadrature amplitude modulation of the carrier wave from. The RF switching circuit 2 controls the supply of carrier waves to the modulator 3 on and off in response to a burst control signal so that the modulated wave output from the modulator 3 is sent out in bursts only during the time allotted to the local station. do.

When using a conventional TDMA modulator in which the carrier wave is turned on and off abruptly using a switching circuit, the following problems arise. In other words, since a linear modulation system is adopted, only a simple wideband filter that suppresses interference between transmitting and receiving within its own station is installed in the RF band. Therefore, when steep burst switching is performed, even if high frequency components occur due to discontinuities in the rising and falling waveform transition parts, the RF
Since there is no suppression effect by the filter, if there are adjacent channels or parallel lines at nearby frequencies, harmonic components will fall into the receiving system of the system and cause interference. The conditions become particularly severe when the reception level of the desired wave decreases due to fading or the like.

Based on the above considerations, an object of the present invention is to provide a TDMA modulator that reduces the amount of interference caused by burst switching to other systems with nearby frequencies.

According to the present invention, low-pass filtering means band-limiting a baseband data signal, means for linearly modulating a carrier wave with the output of the low-pass filtering means, and means for supplying the carrier wave to the linear modulation means. is coupled to the input side of the low-pass filtering means, and is input to the low-pass filtering means such that a burst-like modulated wave is obtained at the output of the linear modulation means in response to a first control signal. and a first switching means for switching the baseband data signal to be transmitted.

The present invention will be described in detail below with reference to the drawings.

FIG. 2 is a block diagram showing an embodiment of a TDMA modulator according to the present invention, in which parts having the same functions as in FIG. 1 are given the same numbers. This TDMA
The modulator differs from the conventional one shown in FIG. 1 in that the input data signal is switched on and off by a burst control signal in a baseband switching circuit 5, and then passed through a low-pass filter 4. Bandwidth is limited. Therefore, if the linear modulator 3 operates ideally, the same effect as performing on/off switching in the RF band and then limiting the band using an RF band filter can be obtained. Therefore, this configuration
In TDMA modulators, unnecessary spectrum broadening due to burst switching is suppressed by a low-pass filter, so the effect on nearby systems is the same as in continuous mode.

FIG. 3 is a block diagram showing another embodiment of the TDMA modulator according to the present invention. In order to obtain a burst modulated wave, the baseband band switching circuit 5 is
It is characterized in that switching is performed by both the RF band switching circuit 2 and the RF band switching circuit 2. In this case, the linear modulator 3 is imperfect and there is a carrier wave leak, and even if the carrier wave is not completely suppressed, the residual component is transferred to the RF
Further switching is performed using band switching circuit 2,
A desired amount of suppression can be obtained. In addition, in Fig. 3, the carrier wave supplied from the transmitting local oscillator 1 to the RF switching circuit 2 is switched on and off, but the same result can be achieved even if an RF switching circuit is provided at the output of the linear modulator 3 and switching is performed. You can expect good results.

FIG. 4 is a diagram showing an example of the timing relationship between the data signal and burst control signals 1 and 2 in the TDMA modulator shown in FIG. The timing at which the baseband band switching circuit 5 turns on and off is at the code conversion point of the data signal, but after that,
When switched in the RF band, the data signal is band-limited by the low-pass filter 4, so it has a smooth waveform response. Therefore, the timing at which the RF band switching circuit 2 is turned on and off is set, for example, at the time when the envelope of the modulated wave turned on and off in the baseband crosses zero. By selecting appropriate on/off timings in this manner, RF band switching can be performed while the power of the modulated wave is small, resulting in fewer waveform discontinuities and the generation of harmonics.

FIG. 5 is a block diagram showing still another embodiment of the present invention. The RF band switching circuit 2 operates linearly with respect to the baseband drive signal, band-limits the burst control signal with a low-pass filter 6, and performs on/off switching using a control signal with a smooth waveform response. I do. The burst modulated wave output waveform obtained in this way is a product of the band-limited data signal and the band-limited burst control signal, and therefore does not have steep waveform discontinuities. Therefore, the generation of harmonic components of the spectrum can be reduced. In addition, RF
The same effect can be expected even if the position of the switching circuit is the output of the modulator 3.

FIG. 6 is a diagram showing a specific comparison example of characteristics between the conventional TDMA modulator shown in FIG. 1 and the TDMA modulator according to the present invention. In the figure, a-1 and a-2 show a conventional TDMA modulator output waveform and a receiving filter output envelope waveform as an interference wave falling into another system whose frequency interval (ΔF) is twice the baud rate (fp). Similarly b-1 and b-
2 is the TDMA modulator output waveform of FIG. 2 according to the invention, c-1 and c-2 are FIG. 3 according to the invention, and d-1 and d-2 are the TDMA modulator output waveforms of FIG. The adjacent channel receive filter output envelope waveform is shown. Here, the transmitting low-pass filter that limits the band of the data signal is a 50% roll-off filter that corrects the characteristics of the receiving Gaussian filter, and the receiving filter is the time-bandwidth product BT = 1 (B
is a frequency bandwidth, T is a pulse width) in combination with a Gaussian filter and a sharp cut filter with BT=2. Furthermore, in the example shown in FIG. 5 of the present invention, it is assumed that the burst control signal is band-limited by a Gaussian filter with BT=1. Figure 6 shows the burst transition from on to off, t=...,-
The data up to 2T, -T, 0 is transmitted as a burst signal, and the data after t=T is switched off at the timing of t=T/2. Further, in the example shown in FIG. 3 of the present invention, the timing of the burst control signal 2 is delayed by 0.375T from the burst control signal 1, and the carrier leakage of the modulator is 20 dB.

With a conventional TDMA modulator, the worst DU ratio between the desired signal and the maximum value of the envelope waveform of the interference wave that appears in the reception filter output of an adjacent channel 2 fp apart is 13 dB, but with the modulator of the present invention, both
It can be seen that the improvement is more than 26dB, which is about 13dB or more.

As explained above, according to the present invention, by reducing the waveform discontinuity at the rise and fall of the burst modulated wave, it is possible to use adjacent channels or nearby frequencies without providing a narrow band filter in the RF band. The amount of interference with other systems can be reduced. Furthermore, as explained in FIGS. 3 and 4, by using the baseband switching circuit in combination, the on/off suppression ratio required for the RF switching circuit can be reduced.
Although the configuration may seem complicated, it is more economical because the microwave band circuit is simpler.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one configuration example of a conventional TDMA modulator, FIG. 2 is a block diagram showing an embodiment of a TDMA modulator according to the present invention, and FIG. 3 is a block diagram showing another embodiment of a TDMA modulator according to the present invention. FIG. 4 is an explanatory diagram showing the timing relationship of the burst control signals supplied to the TDMA modulator shown in FIG. 3; FIG. 5 is a block diagram showing a further embodiment of the TDMA modulator according to the invention. FIG. 6, a block diagram showing an example, is a diagram showing an example of comparing characteristics of a conventional TDMA modulator and a TDMA modulator of the present invention.

Claims (1)

[Claims] 1. Low-pass filtering means for band-limiting a baseband data signal, means for linearly modulating a carrier wave with the output of the low-pass filtering means, and supplying the carrier wave to the linear modulation means. and means coupled to the input of the low-pass filtering means for providing a burst of modulated waves at the output of the linear modulation means in response to a first control signal. a first switching means for switching the baseband data signal input to the TDMA;
modulator. 2. Second switching, wherein the linear modulation means switches either the output of the linear modulation means or the carrier wave from the carrier wave supply means in response to a second control signal synchronized with the first control signal. A TDMA modulator according to claim 1, characterized in that it comprises means. 3. The TDMA modulator according to claim 2, wherein the first and second control signals have different timings. 4 First step to limit the band of the baseband data signal
means for linearly modulating a carrier wave with the output of the first low-pass filtering means; means for supplying the carrier wave to the linear modulation means; switching means for switching either the output of the linear modulation means or the carrier wave from the carrier wave supply means so that a burst modulated wave is obtained at the output of the linear modulation means; and the control signal supplied to the switching means. and second low-pass filtering means for band limiting the TDMA modulator.