GB2289386A - Radio transmitter and method of control of transmit power increase - Google Patents

Abstract

A radio transmitter is described, having power control for increasing and decreasing the power of a transmit signal for use in, but not limited to, a time division multiple access (TDMA) radio system. The transmitter has frequency control means (11, 56) for tuning the transmitter to transmit a signal on a selected radio channel, power control means (13, 26) for increasing and decreasing the power of the signal for transmission in preselected time slots to and from a peak value, and control means (20) for causing the power control means to commence increasing the power while the frequency control means tunes the transmitter to transmit outside the selected radio channel and for causing the frequency control means to tune the transmitter to transmit the signal in the selected radio channel when the power has been increased to the peak value. By ramping up from outside the channel, a slower ramp-up can be achieved, giving rise to less interference. <IMAGE>

Description

Radio Transmitter and Method of Control of Transmit Power Increase Field of the Invention This invention relates to a radio transmitter and in particular to a transmitter having power control for increasing and decreasing the power of a transmit signal for use in, but not limited to, a time division multiple access (TDMA) radio system.

Background to the Invention One of the most likely external influences on the selection of TDMA versus continuous transmission (e.g. FDMA applications) for future digital systems is the possibility of interference into non-radio electronic equipment; i.e. electromagnetic compatibility (EMC) problems. If it is shown that TDMA transmissions are likely to cause greater possibilities of interference with operation with industrial, domestic, automotive and medical equipment, these detrimental factors may override any technical superiority of the mode.

The likelihood of interference from TDMA results from its modulated carrier being switched on and off many times a second in defined time slots. If radio frequency emissions (RF) are induced into other electronic equipment, there is a possibility of rectification of this RF, leading to a shift of DC potential in sensitive stages of the other electronic equipment. The rapid rise and fall of RE induced into such other electronic equipment causes a change in induced rectified potential at the repetition rate of the TDMA transmission: this rapid change is the primary cause of undesirable effects within the equipment; e.g. an audible buzz at the repetition rate in audio equipment.

There is also a secondary effect due to amplitude modulation at the symbol rate of a digital transmission if a constant envelope modulation method is not chosen.

It has been known for many years that transmission of information by on-off keying of the carrier, such as manually keyed Morse code carrier wave (CW) transmissions, causes less EMC interference if the rise and fall time of the CW waveform is shaped such that its rate of risking or falling is slowed.

This both reduces the harmonic content of any rectified RF induced into electronic equipment if the rectified RF is AC coupled and can reduce or remove the transients due to high pass filtering characteristics of series capacitors feeding the input impedance of a following stage.

This effect is distinct from the benefits of reduction of transmitted bandwidth of the wanted keyed signal, which is also a benefit.

It is a problem, however, that long rise and fall times in a TDMA system require long guard times between slots which in turn reduce the proportion of time available for modulation and traffic.

Summarv of the Invention According to a first aspect of the present invention, a radio transmitter is provided comprising: frequency control means for tuning the transmitter to transmit a signal on a selected radio channel, power control means for increasing and decreasing the power of the signal for transmission in preselected time slots to and from a peak value, and control means for causing the power control means to commence increasing the power while the frequency control means tunes the transmitter to transmit outside the selected radio channel and for causing the frequency control means to tune the transmitter to transmit the signal in the selected radio channel when the power has been increased to the peak value.

This invention applies the benefits of increased risetime to a rapidly repeated TDMA transmission, to reduce the effects induced into other electronic equipment. This is achieved by using the existing unused guard bands between frequencies (i.e. frequencies mid way between channel centres) to slowly bring a transmitter up to its desired output level or near that level, and back down again. The guard bands already exist, being defined by limitations in receiver filter shape and transmitter modulation bandwidth: they are currently unused spectrum.

If a TDMA system is chosen with four or more slots of transmission per second, the (frequency) guard bands between channels are used to ramp the transmitter up to full power over the slot prior to the one chosen for transmission. Then, in the guard time between timeslots, the carrier frequency of the transmitting radio is moved to centre it on the wanted transmission frequency prior to transmission of any modulation. The guard times also already exist, and are currentlv used to quickly bring transmitters up to full power.

Brief Description of the Draxvings Fig. 1 shows a transmitter in accordance with a preferred embodiment of the invention.

Fig. 2 shows details of the RF section of the transmitter of Fig. 1.

Fig. 3 shows a three-part time diagram illustrating time slots, transmit power and frequency plotted against time.

Detailed Description of the Preferred Embodiment Referring to Fig.1, the transmitter comprises, in its preferred embodiment, a base band input 10, for receiving a speech or data signal for transmission, a radio frequency (RF) section 11 having an output 12 coupled to a controllable attenuator 13, coupled in turn to a power amplifier 14 and antenna 15. The amplifier 14 is provided with a feedback loop comprising coupler 16, feedback line 17 and comparator 18. The output of comparator 18 controls the value of attenuation in attenuator 13.

A microprocessor 20 is provided having a transmit sense input 21, a transmit output 22, a power level control data bus output 23 and a clock output 24. These are connected to, respectively, a level detector 25, an enable input of a shaping ROM 26, an address input of the shaping ROM 26 and a RAMP control circuit 27 and a six-bit counter 28. An eight-bit data output of the ROM 26 passes to a D/A converter 30, the analog output of which is connected to the positive input of a comparator 18.

Elements 10 to 13 of the circuit of Fig. 1 are described in European Patent No.0369135 of Motorola Inc. Additionally, microprocessor 20 has a frequency control output data bus 35 coupled to RF section 11.

Referring to Fig. 2, details of RE section 11 are shown. It comprises, coupled to the input 10, a modulation and intermediate frequency (IF) circuit 50. This circuit may be arranged to modulate the base band input 10 using GMSK or other modulation as is well known in the art. The circuit may have an IF reference injection mixer as is well known in the art or alternatively direct conversion can be used. The output of the modulation at IF stage 50 is coupled to a mixer 51, which in trnn provides an RF signal to an intermediate amplifier 52 providing an output on output 12. Coupled to the mixer 51 is a phase-locked-loop comprising a reference signal generator 53, a voltage controlled oscillator (VCO 54), a phase comparator 55, a divider 56 and a damping element 57. These elements together form a typical radio synthesizer. The signal generated by the VCO is divided in divider 56 by a factor (N) and compared with the reference oscillator signal from oscillator 53 in the phase comparator 55. The damping element 57 may be a low pass filter and may have a controllable time constant (illustrated by a dotted line from the microprocessor 20) and has the effect of eliminating unstable oscillations and smoothing the operation of the VCO.

The frequency control output 35 of the microprocessor 20 is coupled to the divider 56 for loading of a selected divider factor N into the divider 56.

By the microprocessor 20 loading different divider factors into divider 56, the microprocessor can control the frequency of the RF signal supplied to the attenuator 13. In addition or alternative to the controlling the divider factor in divider 56, the microprocessor can provide frequency control in an IF stage in circuitry 50 (as illustrated by the dotted line from microprocessor 20).

The operation of the apparatus will be described with reference to Fig.

3. At the bottom of Fig. 3, a number of time slots are shown numbered N-2, N-l, N, N+1, N+2. Between the time slots (eg. time slot 100) are guard times (eg. guard time 101). In prior art arrangements, these guard times are used for ramping up and ramping down the transmitter power by supplying a ramp signal from shaping ROM 26 to D/A converter 30 and controlling attenuator 13.

In the present arrangement, the transmitter power burst profile is shown by the curve 110 in Fig. 3. It can be seen that, for transmission during time slot N, ramping begins at a much earlier instant, at time 111, which is shown as co-inciding with the guard time 102 preceding the time slot 103 preceding the time slot in question. The exact time 111 of commencement of ramping is not important. Ramping could commence during the preceding time slot 103. The ramping down of curve 110 commences at the end of time slot 100 and continues through the following time slot and finally ends in the guard time 101 following that time slot. Again the exact duration of ramping down is not important.

Referring to the upper part of Fig. 3, a number of channels 120, 121 and 122 are shown. In the present case channel 121 is the wanted channel.

between channel 121 and 120 is a guard band 123. Between channel 121 and upper frequency adjacent channel 122 is another guard band 124. The channel spacing between the centre of the wanted channel 121 and the centre of the upper frequency adjacent channel 122 is, for example 25kHz. The same channel spacing exists between the wanted channel 121 and the lower frequency adjacent channel 120. Each of these guard bands typically has a width of up to 7 kHz.

Curve 130 represents the centre frequency of a signal being transmitted by the transmitter of Fig. 1. Initially the centre frequency resides in the centre of guard band 123. At point 111 on the transmit power curve 110, the centre frequency still lies in the guard band 123. At the end of time slot 103, during guard time 104, the microprocessor 20 supplies a new value to the divider 56 causing a change of frequency. The frequency change is a 12.5 kHz change, bringing the transmit frequency to within the centre of the wanted channel 121. The damping factor of damping circuit 57 of Fig. 2 dictates the time taken for the transmitted signal to reach the centre of the wanted channel. This factor is chosen to match the available time provided in guard time 104.Alternatively, the frequency change may be effected in some other means, such as by varying the frequency output of the IF stage 50. The change may be effected in one or a number of steps which may permit the rate of frequency change to be varied.

When the transmitted signal has reached its desired transmit frequency at the commencement of time slot 100 (the wanted time slot), traffic is supplied to input 10 and is transmitted through the antenna 15. At the end of time slot 100, the microprocessor 20 reverses the process by supplying the original division factor to divider 56, causing the transmit frequency to leave the centre of the wanted channel and revert to the centre of the guard band 123 during the guard time immediately following the wanted time slot 100. At the same time, ramping down of the power commences until the transmitted power dies away to 0.

Instead of returning to guard band 123 on ramp-down, the frequency can be changed to that of the guard band 124, thereby alternating between the upper and lower adjacent guard bands.

By bringing the transmitter up to power in the guard band between the wanted channel and an adjacent channel, a suitably slow rise time is achieved. During the guard time between slots "N-1" and "N", the radio frequency is changed to centre it on the wanted channel (without modulation).

The radio transmits in slot N; then is moved off frequency in the guard time between slot "N" and "N+1"; then gradually ramped down during slot "N+1".

Considering the case of a four slot TDMA system, slot "N+2" is the next information slot in the sequence of four from "N-2" and so on. For this reason, the ramping up and down occurs in time slots allocated to different transmitters. The lowest number of slots possible would be three per frame; but this would not permit the transmitting radio to receive in any slot for synchronisation or data reception purposes, unless two frequency operation with a duplexer is used. If a four slot system is used, only one slot (equivalent to "N+2", "N-2" on the diagram) is available for reception if no duplexer is used. If a greater number of slots is sent per frame, the transmitter could be turned on and off over more than one slot. Conversely, less than one slot (i.e. a portion only of a slot) may be chosen for ramping up and down to maximise transmitter efficiency.

Fig. 3 is not to scale. The shape of both the power rise and frequency shift is be optimised to suit the parameters of the system, consistent with minimising adjacent channel interference and maximising EMC protection.

The power rise may be continuous during the frequency shift. As transmitter efficiency is effectively being traded off against EMC performance, there will be an optimum shape of rise for each circumstance.

It may be preferable for the transmitter to rise to either the lowest level obtainable under modulation; or even to a lower level (e.g. -6 to -lOdBc) whilst in the frequency guard band, to minimise adjacent channel interference.

It may also be advantageous for the receiving radio IF filter to contain a notch at exactly the half channel offset frequency to minimise adjacent channel interference from radios ramping up or down in the guard band. It may also be advantageous for mobile radios to derive some form of frequency correction from a base station, to ensure ramp up and down at the optimum frequency to minimise adjacent channel interference.

With suitable analysis of the power rise during the slow ramp up or down, it may be possible to use information obtained to assist in linearization of the power amplifier if a linear modulation is in use.

It would be preferable on a system wide basis to assign only one of the two available (high and low frequency) guard bands for use, to minimise transmitters interfering whilst ramping up and down.

A possible drawback with the arrangement described in a operation of a TDMA system is the possibility of interference into either the wanted channel or the adjacent channel by the carrier being slowly ramped up. Due to the slow speed of the ramp, the bandwidth of the rising/falling transmitter will be minimal. The measure of interference will therefore be limited by the rejection in IF filters in receiving radios.

Provided that the receiver rejection at the half channel offset (i.e.

frequency in use for ramping) equals or exceeds the adjacent channel power specification for an operating modulated transmission, no greater interference will be encountered by adjacent channel units than that would be encountered whilst receiving a wanted transmission adjacent to an operational TDMA transmitter. Thus if the adjacent channel power specification for a system is 60dB, the receiver filter requirements would be 60dB rejection at a half channel offset; which should be readily available; and can be met easily with a filter conforming to the adjacent channel test receiver requirements of ETS300113.

Furthermore, the requirements for receiver filter rejection can be relaxed if the transmitter is ramped to a lower power in the guard bands, and brought to full power whilst being brought onto centre frequency. For example, if the transmitter was ramped to -lOdBc, the requirements of the receiver filter could be relaxed by 10dB; this would place a -50dB requirement if specified adjacent channel power whilst operating was -60dB.

Claims (11)

Claims

1. A radio transmitter comprising frequency control means for tuning the transmitter to transmit a signal on a selected radio channel, power control means for increasing and decreasing the power of the signal for transmission in preselected time slots to and from a peak value, and control means for causing the power control means to commence increasing the power while the frequency control means tunes the transmitter to transmit outside the selected radio channel and for causing the frequency control means to tune the transmitter to transmit the signal in the selected radio channel when the power has been increased to the peak value.

2. A radio transmitter according to claim 1, wherein the frequency control means comprises an input for receiving a frequency change signal and damping means for providing a gradual change in transmitter tuning in response to the frequency change signal.

3. A radio transmitter according to claim 1, wherein the frequency control means comprises an input for receiving a frequency change signal and the control means is arranged to supply a stepwise frequency change signal for providing a stepwise change in transmitter tuning.

4. A method of operating a radio transmitter comprising the steps of: tuning the transmitter to transmit a signal outside a selected radio channel and controlling the power of the signal to increase the power from a low value, increasing the power of the signal to a peak value and substantially simultaneously retuning the transmitter to transmit within the selected radio channel when the power has been increased to the peak value.

5. A method according to claim 4, wherein the step of tuning the transmitter to transmit outside the channel comprises tuning the transmitter to transmit within a frequency guard band between the channel and an adjacent channel.

6. A method according to claim 4 or 5, further comprising the steps of: decreasing the power of the signal from the peak value to the low value and substantially simultaneously retuning the transmitter to transmit outside the selected channel when the power has been decreased to the low value.

7. A method according to any one of claims 4 to 6, wherein the transmitter is operated to transmit in time slots and wherein the power is increased to its peak level at the start of a wanted time slot and the transmit frequency is adjusted to be within the selected radio channel at the start of the wanted time slot.

8. A method according to claim 7, wherein the commencement of power increase is at a time within a preceding time slot.

9. A method according to claim 8 wherein the retuning of the transmitter is carried out in a guard time between the preceding time slot and the wanted time slot.

10. A method according to claim 7 wherein the time slots are arranged in frames such that each Nth slot belongs to a frame, where N is at least 3.

11. A method according to any one of claims 4 to 10 wherein the step of increasing the power of the signal comprising increasing it to a level intermediate the low value and the peak value before commencing retuning the transmitter.

A radio transmitter comprising: frequency control means for tuning the transmitter to transmit a signal outside a selected radio channel and within said channel selectively power control means for increasing and decreasing the power of the signal for transmission in preselected time slots to and from a peak value, and control means for controlling the frequency control means and the power control means to operate the radio selectively in a first mode in which the transmitter is tuned outside said channel and the power of the signal is at a low but increasing value and a second mode in which the transmitter is tuned within said channel and the power of the signal is at the peak value.