GB2281461A

GB2281461A - Control of output power in radio transmitters using look-up table

Info

Publication number: GB2281461A
Application number: GB9317900A
Authority: GB
Inventors: Ray Lea; Derek Murray; Mark Robertson
Original assignee: Nokia Telecommunications Oy
Current assignee: Nokia Oyj
Priority date: 1993-08-27
Filing date: 1993-08-27
Publication date: 1995-03-01
Also published as: GB9317900D0

Abstract

Apparatus for controlling the output power level of a transmitted signal comprises a read only memory (ROM) 20 having stored therein information concerning desired output power levels. Access to the ROM 20 is governed by a microcontroller 22 to which are fed three signals, a first signal 24 representative of a required power level, a signal 26 representative of frequency of a frequency synthesiser and a signal 28 representative of temperature. The ROM 20 supplies a desired power output level signal, compensated for frequency and temperature variations, to an attenuator 12 which modifies the input signal fed to an amplifier 16 producing the transmitted signal. <IMAGE>

Description

Control of Output Power in Radio Transmitters This invention relates to controlling output power in radio transmitters.

The conventional approach to controlling the output power in a radio transmitter is to use a closed loop system, as illustrated in Figure 1 of the accompanying drawings. A sample of the detected radio frequency output is compared with a reference voltage, which is an analogue signal representative of a desired output power level. The resulting difference signal is used to control an attenuator at the input of an amplifier, thus completing the control loop. This approach has particular disadvantages for cellular radio conforming to the GSM (Groupe Systeme Mobile) standard, namely: (a) a linearised detector and a linearised attenuator are required to accommodate the large dynamic range (b) the fast power ramping required between timeslots in GSM makes the control loop difficult to design.

Extra poles within the loop lead to overshoot.

According to the invention apparatus for controlling the output power of a radio transmitter comprises memory means having stored therein information concerning desired output power levels and adjusting means for adjusting the output power level in accordance with the information derived from the memory means.

Preferably the apparatus includes means for varying the output power to compensate for variations in temperature and for variations in frequency of a frequency synthesiser of the radio transmitter.

In a preferred embodiment the memory means comprise a read only memory controlled by a microcontroller having three inputs, namely a first input representative of a desired power level, a second input representative of the frequency and a third input representative of temperature, the microcontroller determining the address in the ROM from which the desired power level, compensated for frequency and temperature, is retrieved in accordance with the combination of the three inputs, the compensated power level signal being applied to the adjusting means.

The means for adjusting the output power level is preferably an attenuator controlling the input to an amplifier which provides the transmitter output signal.

Apparatus according to the invention may be incorporated in the transmitter of a base station and/or the transmitter of a mobile handset in a radio telephone system.

Hence, the invention overcomes the problems of the prior art by employing open loop control. This removes the need for a detector and means that the attenuator can be nonlinear.

Apparatus according to the invention will now be described by way of example with reference to Figures 2 and 3 of the accompanying drawings, in which: Figure 2 is a block circuit diagram of the apparatus, and Figure 3 is a timing diagram indicating power level changes at the transition between time slots.

Referring to Figure 2, the input power is supplied to the input 10 of an attenuator 12 which is controlled by a signal on a lead 14 and the output of which is supplied to an amplifier 16 which in turn provides the transmitter output power at 18. The control signal on lead 14 is derived from an open loop control arrangement.

A read only memory (ROM) 20 contains the power ramp information required to give the correct transition between the power levels of two successive time slots. At different addresses in the ROM are stored different power levels. The appropriate address, and therefore the appropriate required power level, is selected by a microcontroller 22 which selects a ROM address in dependence upon a blend of three input signals, namely a first digital signal 24 representing a power select level, a second digital signal 26 representing the frequency from the frequency synthesiser and a third digital signal 28 representative of the temperature of the amplifier, or possibly ambient temperature. The signal 28 is derived from a thermal probe 30 placed near to the amplifier 16, the signal from which is converted into digital form in a converter 32.The second and third signals 26, 28 therefore provide compensation for frequency variations and thermal changes, respectively.

The radio transmitter of Figure 2 is intended to conform to the GSM standard and to operate in time division multiple access (TDMA) employing eight time slots in each frame of transmitted data. Figure 3 shows how the requested power level signal 24 may vary at the transition between two time slots 34, 36 in a typical frame of transmitted data. A guard band 38 is provided between the time slots 34, 36 and the requested power level signal, which is digital, is shown stepping down from a higher level in the earlier time slot 34 to a lower level in the later time slot 36. When such a change in the requested power level occurs, a new output power level is selected from the stored values in the ROM, with compensation for frequency and temperature variations. This new output power level is converted into analogue form in a converter 40, filtered at 42 and fed on the lead 14 to the attenuator 12 as the control signal determining the output power of the transmitted signal for that time slot.

Claims

1. Apparatus for controlling the output power of a radio transmitter, the apparatus comprising memory means having stored therein information concerning desired output power levels and adjusting means for adjusting the output power level in accordance with the information derived from the memory means.

2. Apparatus according to claim 1 and including means for varying the output power to compensate for variations in temperature and for variations in frequency of a frequency synthesiser of the radio transmitter.

3. Apparatus according to claim 2, wherein the memory means comprise a read only memory controlled by a microcontroller having three inputs, namely a first input representative of a desired power level, a second input representative of the frequency of the frequency synthesiser and a third input representative of temperature, the microcontroller determining the address in the ROM from which the desired power level, compensated for frequency and temperature, is retrieved in accordance with the combination of the three inputs, the compensated power level signal being applied to the adjusting means.

4. Apparatus according to any of the preceding claims, wherein the adjusting means is an attenuator which controls the input to an amplifier which provides the transmitter output signal.

5. Apparatus according to any of the preceding claims and forming the transmitter of a base station in a radio telephone system.

6. Apparatus according to any of claims 1 to 4 and forming the transmitter of a mobile handset in a radio telephone system.

7. Apparatus according to any of the preceding claims, wherein the radio transmitter uses time division multiple access (TDMA) and wherein a desired power level is read out of the memory means for each time slot.

8. Apparatus for controlling the output power of a radio transmitter, the apparatus being constructed and arranged substantially as herein particularly described with reference to Figures 2 and 3 of the accompanying drawings.