GB2163325A - Voice and data transmission for R.F. channel - Google Patents

Abstract

A method of multiplexing a frequency shift keyed (FSK) signal and at least one other signal into a common signal path comprising generating the FSK signal in the form of upper and lower sidebands of a carrier frequency, said sidebands being separated by a frequency band encompassing said carrier frequency and transmitting said sidebands together with the other signal, said other signal occupying at least part of said frequency band between the FSK sidebands. The other signal is conveniently a frequency modulated voice channel.

Description

SPECIFICATION Voice and data transmission for R.F. channel This invention relates to a method and apparatus for multiplexing a single voice channel and N data channels or N data channels into one signal path.

Conventionally the available frequency spectrum for radio, or any other transmission medium, is divided into contiguous channels, Fig. 1. This method, frequency division multiplexing (FDM), permits the simultaneous use of spectrum by several independent signals.

In the transmission of data, methods such as wideband coherent frequency shift keying (FSK) provide improved transmission compared with narrowband methods but, with conventional channelling, reduce the number of signals a given bandwidth can support.

However for a random FSK data sequence the signal energy is concentrated into two separated bands of frequency. These are centred about the carrier but displaced by the deviation frequency ( + Af), the occupied bandwidth of each energy band being proportional to twice the data rate, Fig. 2, and hence some radio spectrum is unused. This ivention proposes a channelling system which gives the best spectral efficiency under these circumstances. This invention proposes a channelling system which gives the best spectral efficiency under these circumstances and allows simultaneous demodulation of the voice and data spectra.

According to the present invention there is provided a method of multiplexing a frequency shift keyed (FSK) signal and at least one other signal into a common signal path comprising generating the FSK signal in the form of upper and lower sidebands of a carrier frequency, said sidebands being separated by a frequency band encompassing said carrier frequency and transmitting said sidebands together with the other signal, said other signal occupying at least part of said frequency band between the FSK sidebands.

The invention also provides an apparatus for multiplexing a frequency shift keyed (FSK) signal and at least one other signal into a common signal path, the apparatus including means for generating the FSK signal in the form of upper and lower sidebands of a carrier frequency, said sidebands being separated by a frequency band encompassing the carrier frequency, and means for transmitting said sidebands together with said other signal occupying at least part of said frequency band between the FSK sidebands.

Embodiments of the invention will now be described with reference to Fig. 3-10 of the accompanying drawings, in which: Figure 3 illustrates the frequency spectrum of a transmission according to the invention, Figure 4 illustrates an arrangement for the baseband multiplexing of a number of FSK data channels, Figure 5 illustrates an arrangement for generating an FSK signal at an offset frequency and multiplexing it with a voice channel before translation to r.f., Figure 6 illustrates an alternative arrangement to that of Fig. 5, Figure 7 illustrates multiplexing of several FSK data channels each having two different deviation frequencies, Figure 8 illustrates a combined FSK data demodulator and voice receiver, and Figure 9 illustrates an integrated implementation of a combined data and voice receiver.

In transmissions according to the invention the data signal power spectra are placed symmetrically around the carrier and the gap in between this spectra can be used by other signals Fig. 3. In its realisation this technique also provides a useful reduction in the number of separate radio frequency oscillators required. If a binary data input is converted into an FSK signal comprising two sidebands each with a nominal deviation of i Af from a centre or carrier frequency fc then, assuming Af to be large enough, there will be a gap in the r.f. spectrum between the sidebands. Normally the minimum bandwidth of an FSK sideband is twice the data bit rate, e.g. for data being transmitted at 1 Kbit/s each sideband will have a frequency bandwidth of 2KHz.To provide a usable gap in a 25KHz channelled system for, say, a voice channel with the characteristics of a 1 2.5KHz channelled system, Af should be a minimum of 9.375KHz.

Our co-pending British patent application No. 8421028 (J.M. Baker-3) discloses how a binary data input can be converted into an FSK signal at baseband and then translated to r.f. for transmission. Fig. 4 shows how a number of data channels can be multiplexed.

Each of the N binary data channels is fed to an individual FSK generator comprising a 180 phase switch 1 and to a separate 7r/2 (90 ) phase shifter 2 which, in response to the binary data logic levels, produces a pair of f signals in quadrature. The resultant pairs of quadrature signals are separately summed.

The combined pairs of signals in quadrature are then applied to a single sideband (SSB) modulator 3. It will be noted that the data channels are combined at baseband before being translated to r.f. for transmission. The SSB modulator is of the type described in application No. 8421028 (J.M. Baker-3), comprising first and second mixers to which the baseband signal(s) are applied in quadrature, the other inputs to the mixers being the r.f. carrier local oscillator also applied in quadrature. The mixer outputs are then summed.

Fig. 5 shows how an FSK data channel can be multiplexed with a voice channel to utilise the spectrum as shown in Fig. 3. The binary data is applied to a 180 phase switch 1 to effect phase inversion of the output Af of oscillator 4. The oscillator output is also applied to a fixed ir/2 (90 ) phase shifter 2. The data is now presented as a pair of qudrature signals each of frequency Af to respective adders 5,6. The voice signal is fed to respective phase conversion circuits 7,8 to provide outputs cos f(t) and sin f(t), where f(t), is the voice input. These voice signals are then applied to the adders 6,6. The combined voice and data signals are then fed to the SSB modulator 9 where they are translated tb r.f.

as previously described.

It will be appreciated that the elements of Figs. 4 and 5 can be combined so that several data channels, each with a different deviation frequency Af, can be combined with a single voice channel occupying the centre of the r.f.

transmission band with the data channels spaced out on both sides.

When multiplexing a single data channel with a voice channel it is convenient to use the same oscillator to frequency modulate both channels before translation up to r.f., as shown in Fig. 6.

Whilst it has been assumed that each data channel has the same deviation frequency Af about the carrier frequency, it is possible to generate the quadrature signals with different deviation frequencies representing the binary '1' and '0' conditions respectively. Fig. 7 shows how a number of such data channels can be multiplexed, with or without a control voice channel. Each data channel has an overall bandwidth of 2Af but its sidebands are spaced asymetrically about the carrier frequency. Generation of such FSK spectra will normally require two separate deviation oscillators, one of each sideband.

Fig. 8 shows a combined data demodulator and voice receiver for signals transmitted by the arrangement of Fig. 4. The incoming signals are first mixed with a local oscillator frequency to bring them from the r.f. carrier frequency down to an intermediate frequency.

The voice channel is then demodulated in a conventional manner from the IF. For each data channel the received signals at IF are mixed in quadrature with a further local oscillator running at the intermediate frequency, in what is otherwise known as a direct conversion or Zero IF receiver. The outputs of each pair of mixers are then bandpass filtered, the passband in each receiver being different, to correspond to the original deviation frequency of the data signal. The bandpass filtered signals are then passed through limiting amplifiers to produce square waves which are in quadrature. Finally the two square waves are demodulated by logic, which in the simplest case comprises a clocked flip-flop. One square wave is applied to the flip-flop D input and the other to the clock input. The flip-flop output is the original binary data.

Fig. 9 shows an integrated implementation of a combined voice and data receiver, where the two functions are totally integrated onto a single chip. In this case the incoming signals are first mixed in quadrature with a local oscillator signal at the carrier frequency. For the voice channel the quadrature outputs are low pass filtered and then demodulated in the manner disclosed in British patent No.

1530602 (I.A.W. Vance-1). Diode detectors are used in a feedback mode around each amplifier to provide gain control. The filtered quadrature outputs are also fed to a normalising circuit including mixers and an adder, the output of which is fed forward to effect normalisation of the audio output. The zero IF mixer outputs are also fed to bandpass filters ;for each data channel and processed to recover the data in the same manner as in Fig.

Claims (8)

1. A method of multiplexing a frequency shift keyed (FSK) signal and at least one other signal into a common signal path comprising generating the FSK signal in the form of upper and lower sidebands of a carrier frequency, said sidebands being separated by a frequency band encompassing said carrier frequency and transmitting said sidebands together with the other signal, said other signal occupying at least part of said frequency band between the FSK sidebands.

2. An apparatus for multiplexing a frequency shift keyed (FSK) signal and at least one other signal into a common signal path, the apparatus including means for generating the FSK signal in the form of upper and lower sidebands of a carrier frequency, said sidebands being separated by frequency band encompassing the carrier frequency, and means for transmitting said sidebands together with said other signal occupying at least part of said frequency band between the FSK sidebands.

3. An apparatus according to claim 2 wherein, the means for generating the FSK signal comprises an oscillator having an out put the frequency of which is thr FSK deviation frequency, switching phase shift means responsive to a binary data input to effect 180 phase switching of the oscillator output applied thereto, a fixed 90 phase shift means to which the oscillator output is also applied whereby the outputs of the switched and fixed phase shift means together provide a pair of signals at the deviation frequency into side bands of the radio frequency carrier.

4. An apparatus according to claim 2 or 3, wherein the at least one other signal is a frequency modulated voice channel signal.

5. An apparatus according to claim 2, 3 or 4, including means for generating separate FSK signals at baseband for a plurality of data channels, the FSK signals for each channel having a different deviation frequency, means for summing said separate FSK signals with a frequency modulated voice channel at baseband, said summed signals then being transmitted at r.f.

6. An apparatus according to claim 4, including frequency modulating means to which the voice channel signal is applied to provide quadrature signals, the quadrature voice signals being summed with the FSK signals in quadrature for translation to r.f. for transmission.

7. An apparatus for multiplexing a frequency shift keyed data signal and another signal substantially as described with reference to Figs. 3-7 of the accompanying drawings.

8. A method of multiplexing signals substantially as hereinbefore described.