DE19533382C1 - GMSK modulated analogue signal generation system - Google Patents

Abstract

The system provides a GMSK modulated analogue output signal of limited bandwidth from a supplied digital input signal, using a Bessel filter of eighth order, directly supplied with the frequency modulated input signal in serial digital form. The Bessel filter has a lowpass transmission characteristic, with the analogue output signal obtained at its output and is used as an input filter for transmission of a limited bandwidth analogue signal.

Description

The invention relates to a method for generating a band-limited analog output signal from a digital Input signal and a device for transmitting a band-limited analog signal with an input filter.

For the transmission of digital signals they are conventional modulation methods, such as amplitude or frequency modulation, because of the steep flan The required bandwidths of the digital signals not suitable. If you wanted digital signals using amplitude transmit or frequency modulation, either would have to Bandwidth of the transmission channel should be chosen so large that the number of channels available is increasing strongly  would reduce, or when transmitting digital signals over channels with a range of around usual today 3 kHz, the rectangular shape of the signals would ver grind that a high error rate is expected got to.

Modulation methods have therefore been developed with digital signals with a high frequency over channels le can be transmitted with low bandwidth. Derar procedures are in "Sonnde / Hoekstein: Entry into the digital modulation: basics, applications and Measurement technology, Franzis-Verlag GmbH & Co. KG, Munich, 1992 " described. The described methods, which are also called Baseband modulation are known, in particular at the transmission of signals in digital form, the means as a result of rectangular pulse trains, used.

For the modulation of digital signals, so-called called continuous-phase modulation methods det. These methods have the advantage that phase jumps, who have a high bandwidth requirement, who avoided the. The best known phase continuous modulation Procedures are the so-called FSK (Frequency Shift Keying) - Procedure using the FFSK (Fast Frequency Shift Shifting) procedure ren, which also as MSK (Minimum Shift Keying) procedure is called, and the GMSK (Gaussian minimum shift Keying) procedure. The procedures are based on the same principle zip of frequency shift keying, but have a difference modulation index. Is used for bandwidth bandwidth limit with the MSK process a Gaussian filter, you get that GMSK method, which is preferred at higher bit rates, such as 4800 bps (bits per second) or 9600 bps is applied.  

The procedure is common that the Digi to be transmitted talsignal is supplied to an arithmetic unit in which a signal stored in digitized form is read out cyclically and is output as an analog signal after a digital / analog conversion. Such arithmetic units are known under the name "Direct Digital Synthesizer (DDS)". The addresses required for reading out the stored data are calculated as a function of the digital signal. In Fig. 3, a block diagram of a schematic arrangement of a direct digital synthesizer is given.

The core of the digital synthesizer shown in Fig. 3 is a memory ROM, in which a plurality of discre ter values of a sine wave are stored. The data output of the memory is connected to a digital / analog converter. An analog signal is formed in the digital / analog converter from the discrete values in the memory.

The addresses for addressing the memory are formed in an arithmetic logic unit ADR. The digital input signal D _E is applied to the input of the arithmetic unit ADR. Depending on whether a signal with a high or low signal level is present at the input of the arithmetic logic unit ADR (high or low), the arithmetic logic unit generates consecutive addresses, or addresses which do not follow one another, but rather each have free address spaces between two generated addresses, which were skipped. In the first case, the discrete values stored in the memory ROM are successively read out without interruption, in the second case, for example, only every second or third value. The arithmetic unit ADR, the memory ROM and the digital / analog converter are controlled by a common clock. If, for example, only every second value is read from the memory instead of each value at a constant clock frequency, the frequency of the analog output signal doubles.

A digital synthesizer can thus be used to modulate a digital signal in a simple manner by frequency shift keying (FSK). Because of the discrete values read from the memory and the frequency switching caused by the digital input signal, the voltage output from the digital / analog converter is not sinusoidal, but has many harmonics. To limit the output signal S _A , the voltage output by the digital / analog converter is therefore passed through a low-pass filter TP.

Although modules are also known, such as the modem module FX 489 from CML Semiconductor Products, which can perform the function described above, the current method is quite laborious. This also has a negative impact on costs.

Due to the previously known methods for the transmission of digital data are the transmission devices, in most Cases transmitter, quite elaborately trained. That's the one gear part of a transmitter according to the modulation type of the digital signal. That means you can rule a transmitter only for a certain type of input signal use.

To remedy this disadvantage, two-way radios are known che have multiple input channels, which differ on digital signal types, such as GMSK, FSK or FFSK-modulated signals are aligned. A derar term training of transmission devices is complex and therefore has a negative impact on costs.

It is an object of the invention, a method mentioned above ren or an initially mentioned device of kind of training that it or they spend less dig and cheaper to perform or forth is to be delivered.

The solution to this problem results from the features of characterizing part of claims 1 and 6 respectively. Advantageous developments of the invention result from the subclaims.

According to the invention, the digital input signal is in digital form without the interposition of further assemblies a filter with an eighth order Bessel transfer function leads, the band-limited analog output signal on Output of the low-pass filter is obtained. It has over surprisingly shown that a serial digital input signal nal is changed by a Bessel low pass so that it is in the Form one with a direct digital synthesizer corresponds to the th signal. Instead of the elaborate processing the digital signal is single in a function block led over a Bessel low-pass filter.

The invention enables nth order filters for all phase-continuous modulation process so dimensio kidney that for a given signal / noise ratio smallest bit error rate is achieved. For this the benefit energy for a transmission channel with a defined band width and channel spacing maximized. Doing so will be iterative the transmission properties for n-th order filters  regarding filter degree and cutoff frequency examined and optimal set. The filter is set so that it is a has critical damping and the group delay is constant is. This results in a linear phase in the passageway rich. In addition, a is particularly advantageous symmetrical swing-in and swing-out behavior achieved.

Especially when using the Gaussian minimum shift Keying (GMSK) procedure, which is used in practice at higher Data rates such as 4800 bps (bits per second) or 9600 bps is applied, which corresponds directly above a Bessel filter led the digital input signal by conventional means, such as by means of of a known modem module, modulated signal. A special treatment of the input signal is in use of the inventive method for GMSK modulation of Higher bit rate digital signals not required.

At lower bit rates, such as 1200 bps or 2400 bps, which are regularly modulated using the FSK method, the input signal must be fed to a signal preprocessing before being fed to the low-pass filter. The intermediate signal formed by the preprocessing speaks a frequency-modulated signal and has a rectangular course with a first frequency during the presence of a high signal level and a rectangular course with a second frequency during the presence of a low signal level, the second frequency being one integer multiples of half the first frequency and greater than the first frequency. The second frequency can also be lower than the first frequency if this is expedient. If the intermediate signal formed in this way is given to a Bessel low-pass filter, a signal curve is obtained at the output of the Bessel low-pass filter that speaks to the analog modulation signal formed in a conventional manner, for example by means of a known modem module FX 469 . Here too, the complex function of FSK modulation or FFSK (Fast Frequency Shift Shifting) modulation can essentially be replaced by the use of a Bessel filter. Only a frequency multiplication of the digital input signal is required.

The inventive modulation of a digital sig nals not only get the benefit of a simpler one Modulation circuit and thus cheaper manufacture, but also a better use of the transmission box nals. The cut-off frequency of the low-pass filter only depends depends on the bandwidth of the transmission channel. There on the Low pass square wave signals are given for the transm the step response of the low pass bend. With a step response, the bandwidth of the fil optimally exploited.

Since this is almost independent of the modulation method, can the filter, for example, in the data input of a Two-way radio can be installed without the need for a certain modulation methods is required. The The transmission device according to the invention therefore provides that the input filter is a low-pass filter with linear Group runtime and critically adjusted damping is. The logic circuit of the preprocessing to generate the digital intermediate signal can be easily, at for example outside the radio, in the digital part respectively. It is also possible that signaling procedures, such as five-tone sequences according to ZVEI or similar, which corresponds essentially to an FSK modulation chen, by using the method according to the invention be improved. Here, instead of the audio frequency  Square wave signal with the same frequency used. Since the Cutoff frequency of the filter after the highest transmission NEN tone frequency directed, but by the now used Square-wave signals also reduce the bandwidth of the filter fully used audio frequencies increases here through transmission security.

If when demodulating FSK and FFSK signals, the were modulated according to the method according to the invention, Zero crossings detected and their distances evaluated demodulation is also clear worse signal-to-noise ratio possible because of the edges steepness of the analog signal always maximum with respect to the Available bandwidth. Hereby degraded bit error rate and transmission security elevated. This has a beneficial effect on the usable realm range of a transmitter.

For GMSK modulation, the filtering applies instead of using a Gaussian filter, which is not directly can implement analog filter, with a filter according to the invention hardly changes the actual signal is recognizable over time. The demodulation process is there by not touching it.

Another great advantage of the method according to the invention rens is that active and passive low-pass filters with Bessel Characteristic with commercially available filter design tools are easy to create.

Measurements on a setup for a company radio channel nöml (non-public mobile land radio) in the 20 kHz channel grid as a transmission channel with a Bessel filter with 3 kHz bandwidth have shown that the bandwidth Period (BT) product for GMSK at 4800 bps of 0.5  improved to 0.625 and at 9600 bps from 0.3 to 0.3125. The improvement is mainly due to that the entire bandwidth of the transmission channel is out can be used.

A comparison of the step response of a Bessel filter with the calculated step response of a Gaussian filter and the Comparison of the impulse response of the Bessel filter to one 1-bit pulse at a transmission speed speed of 9600 bps (104.2 µs) with the calculated pulse response from a Gaussian filter gave an almost complete answer Accordance.

Further details, features and advantages of the present the invention result from the following description Exercise a particular embodiment with reference on the drawing.

It shows:

Fig. 1 is a block diagram of an inventive voltage Anord for GMSK modulation,

Fig. 2 is a block diagram of an inventive voltage Anord for FSK or FFSK modulation,

Fig. 3 is a block diagram of a conventional arrangement for GMSK, FSK and FFSK modulation,

Fig. 4, a digital input signal and the corresponding modulated output signal of a modem module when using the GMSK-method at 4800 bps,

Fig. 5, the output signal shown in Fig. 4 ent speaking eye pattern,

Fig. 6 shows the digital input signal of FIG. 4 and the corresponding GMSK-modulated output signal in accordance with the inventive method, at 4800 bps,

Fig. 7, the entspre in Fig. 6 shown sponding output signal eye pattern,

Fig. 8, a digital input signal and the corresponding modulated output signal of a modem module when using the GMSK-method at 9600 bps,

Fig. 9, the. Output signal shown in Figure 8 ent speaking eye pattern,

Fig. 10, the digital input signal of FIG. 8 and the corresponding GMSK-modulated output signal in accordance with the inventive method, at 9600 bps,

Fig. 11 to that shown in Fig. 10 output signal shown ent speaking eye pattern,

Fig. 12, a digital input signal and the corresponding modulated output signal of a modem module when using the FSK method at 1200 bps,

Fig. 13 to that shown in Fig. 12 output signal shown ent speaking eye pattern,

Fig. 14, the digital input signal of FIG. 12 and the corresponding FSK-modulated output signal in accordance with the inventive method, at 1200 bps,

Fig. 15, the output signal shown in Fig. 14 ent speaking eye pattern,

Fig. 16, a digital input signal and the corresponding modulated output signal of a modem module with application of the FFSK-method at 2400 bps,

Fig. 17, the. Output signal 16 shown in Fig ent speaking eye pattern,

Fig. 18, the digital input signal of FIG. 16 and the corresponding FFSK-modulated output signal in accordance with the inventive method, at 2400 bps,

Fig. 19, the output signal shown in Fig. 18 ent speaking eye pattern, and

Fig. 20 is a transmission device of the invention in a schematic block circuit.

As can be seen in FIG. 1, a digital s input signal D _{E is} supplied with a low-pass filter B-TP with Bessel characteristics. The modulated output signal GMSK method is obtained at the output of the Bessel filter.

FIG. 4 shows an input signal D _E referred to as a useful data stream, below which there is an output signal referred to as a modulation signal. FIG. 5 shows the eye pattern corresponding to the output signal S _A.

In comparison, in FIG. 6, the input signal D _E referred to as the useful data stream, an output signal referred to as the analog modulation signal, shows the sharper edges of the output signal. In Fig. 7 the eye signal corresponding to the output signal is shown. As a comparison with the eye pattern shown in Fig. 5 reveals, the "eyes" are opened further, since the channel bandwidth is better utilized.

In FIG. 8, a method known as useful data input signal D _E is shown below a designated output user data stream input signal D _E designated shown as the modulation signal a as an analog modu lationssignal designated output useful data designated input signal _E at which an be as a modulation signal drawn output signal eye pattern shown.

In comparison, in Fig. 14 under the input signal D _E referred to as the useful data stream, an output signal referred to as the analog modulation input signal D _E and the output signal S _A , the run of the intermediate signal S _Z required when using the FSK modulation is arranged. Because of the time shift due to the signal delay, the corresponding edges have been provided with arrows. The frequency of the intermediate signal S _Z is 1200 Hz when there is a high signal level of the input signal D _E and 1800 Hz when there is a low signal level of the input signal D _E. In FIG. 15 the eye pattern corresponding to the output signal is shown.

In Fig. 16 a, referred to as user data stream input signal _E is shown, at which an be recorded as a modulation signal output Modembau stone using a conventional FFSK modulation generated at 2400 bps. In Fig. 17, the output signal S _A corresponding eye pattern is shown.

In comparison to this, an output signal referred to as an analog modulation signal was generated in FIG. 18 under the input signal D _E designated as the useful data stream. In FIG. 18, the sharply formed edge can be clearly seen the output signal. Arranged between the input signal D _E and the output signal S _A run the intermediate signal S _Z required when using the FSK modulation. Because of the time shift due to the signal running time, the corresponding edges were provided with arrows. The frequency of the intermediate signal S _Z is at a high signal level of the input signal D _E presence of 1200 Hz and in the presence of a low signal level of the input signal D _E 2400 Hz. In FIG. 19, corresponding to the output signal eye pattern is shown.

As can be seen in FIG. 20, a transmission device U according to the invention has a low-pass filter B-TP in its input with a linear group delay and a critically set damping. This makes it possible to use the transmission device, which in most cases should be a Sen, universally. Instead of using one transmitter with multiple input channels or multiple transmitters with one input channel each, it is now possible to use a single transmitter for a wide variety of modulated signals. For example, digital signals that are to be modulated by means of the GMSK method can be sent directly to the transmitter, whereas signals that are to be modulated by means of the FSK method have to be subjected to frequency modulation in the computer before, for example they are given to the transmitter according to the invention. Since the frequency modulation takes place on the digital level of the signal, this is usually not a problem. The uniform configuration of the transmitter according to the invention has a very favorable effect on the production costs and considerably simplifies the maintenance of the transmitter.

Claims (1)

1. A method for generating a GMSK-modulated bandbe limited analog output signal (S _A T) from a digital input signal (D _E ), characterized in that the input signal (D _E ) in serial digital form directly to an eighth order Bessel filter (B -TP) is supplied, the band-limited analog output signal (gene output signal (S _A ) from a digital input signal (D _E ), characterized in that the input signal (D _E ) is frequency-modulated and then in serial digital form directly to a Bessel Eighth order filter (B-TP) is supplied, the band-limited analog output signal (gen a band-limited analog signal (S _Ü ) with an input filter (B-TP), characterized in that the input filter is a Bessel filter of the eighth order.