Background Wireless two-way communication systems transmit and receive signals within predetermined frequency bands. These frequency bands are set by international standards authorities.

The frequency bands are themselves sub-divided into channels of predetermined width.

A two-way communication unit, for example a portable or mobile radio or a mobile telephone, transmits within a particular one of these channels. Normally either the user controls which channel is used, or the system assigns a channel to the two-way communication unit when that unit needs to transmit to another unit.

The communication unit's transmission must give sufficient signal strength in the assigned channel for the recipient to be able to detect the transmission. The transmission must however result in only very little signal strength being transmitted in the channels adjacent to the assigned channel.

The term'Adjacent Channel Coupled Power', or ACCP, is defined as the amount of power transmitted by the communication unit in the channel adjacent to its assigned channel. The'Adjacent Channel Coupled Power Ratio', or ACCPR, is defined as the ratio of the signal strength transmitted by the communication unit in the adjacent channel to that which it transmits in the assigned channel. The ACCPR is normally quoted in decibels, in fact being a negative number of the form-50dB. A low ACCP equates to a high negative number for the ACCPR.

International standards define the maximum signal strength which a communication unit is permitted to broadcast in channels which are adjacent to the assigned channel. This

maximum can be conveniently defined as a limit to the radio's ACCPR. Known communication units require complex and expensive components to meet the twin objectives of high signal strength in the assigned channel and low signal strength in adjacent channels, i. e. to have a high ACCPR.

The Globale Systeme Mobile (GSM) system in Europe uses a broad frequency spectrum made up of a large number of channels. The European GSM system uses the traditional Gaussian Minimum Shift Keying (GMSK) signal modulation scheme. With such a large number of available channels, the communication units need only use every second channel. Because the GSM system uses only every second channel, there is no problem with a communication unit broadcasting significant signal strength in the two channels immediately adjacent to the channel assigned to it. These channels are unused.

Communication units which are employed in the European GSM system can therefore operate with relatively poor ACCPR performance.

One goal of communications systems is more efficient use of the available radio spectrum bandwidth. This leads to one of the design aims for communication units being that they achieve a high ACCPR, i. e. a low ACCP.

Summary of the Invention The invention concerns a transmitter encoder for the transmission of GMSK signals. The transmitter encoder comprises a digital data source, a first filter unit with a low-pass characteristic, an integrator, a sine component extractor for calculating the sine component of the output of the integrator, the sine component extractor being connected to a second low-pass filter, and a cosine component extractor for calculating the cosine component of the output of the integrator, the cosine component extractor being connected to a third low-pass filter. In this arrangement, the digital data source, the first filter unit and the integrator are connected in series and the output of the integrator is supplied to both the sine and cosine component extractors.

The output of the second filter may provide the Q output of the encoder and the output of the third filter may provide the I output of the encoder. Furthermore, the second and

third low-pass filters may be linear-phase Finite-Impulse Response filters, and the second and third low-pass filters may have co-efficients which provide an attenuation of >60dB to frequencies outside of a 3.125 kHz band.

The encoder's digital data source may provide a digitised representation of a voice signal for transmission, or a digitised representation of both a voice signal and other data for transmission.

A transmitter encoder in accordance with the invention may advantageously be used in a mobile or portable telephone or radio.

Brief description of the drawings Figure 1 shows a prior art arrangement for the transmission of GMSK digital signals.

Figure 2 shows the transmitted power plotted against frequency for the circuit arrangement of figure 1.

Figure 3 shows an arrangement for the transmission of GMSK digital signals in accordance with the present invention.

Figure 4 shows the transmitted power plotted against frequency for the circuit arrangement of figure 3.

Detailed description of the preferred embodiment Figure 1 shows a prior art arrangement for the transmission of GMSK digital signals.

The elements shown on figure 1 are: Reference sign 1: A source of digital data. This data may be a digitised representation of a voice signal for transmission. The digital data source may however provide a digitised representation of both a voice signal and other data for transmission.

Reference sign 2: A Gaussian filter. This filter has a low-pass-characteristic with a Gaussian transfer function.

Reference 3: An integrating stage. The output of the integrator is fed to two subsequent stages in parallel, which are labelled as 4 and 5.

Reference 4: A sine component extractor which takes the sine of the signal input to it.

The output of this stage is the Q component of the signal for transmission.

Reference 5: A cosine component extractor which takes the cosine of the signal input to it. The output of this stage is the I component of the signal for transmission.

The arrangement of figure 1 has relatively poor ACCPR performance. This performance is illustrated in figure 2. Figure 2 in fact shows the frequency spectrum of a 4.8kBit/s GMSK signal generated by the encoder implementation shown in figure 1.

A point on the curve of figure 2 has been highlighted with a small square box. This point is located at just over 3 kHz from the frequency of peak power. The frequency of peak power is shown as zero Hz on figure 2. The power at the point indicated by the square is approximately 33dB below that at the frequency of peak power. Such a small fall-off in power indicates a relatively poor ACCPR performance for the arrangement of figure 1.

In practice, a communication unit attempting to transmit or receive in a channel adjacent to that used by the communication unit illustrated in figure 1 would experience substantial interference.

There is a need to improve the performance of the arrangement of figure 1.

In order to be able to support traditional voice communication with an acceptable audio quality, the use of a digital modulation scheme is required. Digital modulation schemes with a bandwidth efficiency larger than 1 Bit/Hz/s require a linear radio frequency (RF) power amplifier (PA) because of their non-constant envelope characteristic. There are however currently unresolved drawbacks of linear transmission (Tx) line-ups. These are, for example, low efficiency, complexity, cost, battery lifetime. These drawbacks make transmission of digital modulation schemes with non-constant envelope characteristics impractical. For this reason, a constant envelope modulation scheme is desirable.

GMSK is the most bandwidth-efficient constant envelope modulation scheme currently known. Therefore an improvement to the arrangement of figure 1 should be sought which still employs the GMSK modulation scheme.

There is a particular need, at least on the north American market, for the next generation of two-way communication devices to support a new digital narrowband specification with a 6.25kHz channel bandwidth.

A minimum Baud rate of 4.8 kBaud and an ACCPR of-60dB is the required minimum standard that needs to be met by the next generation radios. Figure 2 shows conclusively that the arrangement of figure 1 is unable to provide this. The 3 kHz point in figure 2 is a base-band measurement. A bandwidth of 6.25 kHz at radio frequency corresponds to a bandwidth of 3.125 kHz at base-band, i. e. if shown on axes corresponding to those of figure 2.

Figure 3 shows an arrangement in accordance with the present invention. The arrangement of figure 3 shows a block diagram of a modified GMSK encoder. Reference numerals 1-5 in Figure 3 indicate circuit components whose general function has already been explained in connection with the elements in figure 1 which bear the same reference numerals.

The encoder of figure 3 has been modified with respect to the arrangement shown in figure 1 by adding a linear phase Finite-Impulse Response (FIR) filter structure with a low-pass characteristic. In effect, there is a linear phase FIR filter structure after each of the component extractors 4 and 5. These are illustrated in figure 3 as elements 6 and 7 respectively.

The FIR filters'coefficients are calculated in such a way that the main energy of the GMSK signal is passed through unattenuated, whereas the frequency components reaching into the neighbouring channels get attenuated to below the-60dB threshold.

This filter structure is the key design that achieves a GMSK constant envelope modulation scheme suitable for use in future 6.25 kHz channel spacing radio systems.

The encoder of figure 3 is able to fulfil the required ACCPR specification whilst simultaneously keeping the constant envelope characteristic of the GMSK signal.

In particular, the arrangement of figure 3 meets the requirements set for new digital narrowband two-way communication devices on the North American market. Similar standards of performance are likely to be specified in other world markets.

The encoder of figure 3 keeps all the advantages of the current non-linear power- amplifier (PA) approach, whilst delivering superior ACCPR performance. No arrangement of comparable performance for the future 6.25kHz market is currently known.

Figure 4 shows the measured frequency spectrum generated by the modified GMSK encoder structure shown in figure 3.

Figure 4 shows a plot of power versus frequency for the encoder of figure 3. A point on the curve at approximately 3 kHz has been highlighted with a small square box, similarly to the illustration of figure 2. However, this point lies at a far lower power level than does the corresponding point on figure 2. This indicates that either of the neighbouring channels to that on which an encoder as shown in figure 3 was transmitting would receive only a very low signal level from that encoder. The ACCPR ratio would in fact be very high, and much higher than for the arrangement of figure 1.

The arrangement of figure 3 is particularly applicable to 6.25 kHz channel spacing radio systems. However, the encoder could be used in other systems if appropriate filter codes were developed for the FIR filters 6 and 7. The person skilled in the art would be able to develop such codes in a known manner, which is not explained here for reasons of conciseness. In general terms, a transmitter encoder in accordance with the present invention may be employed in a mobile-or portable telephone or radio.