GB2410388A - Temperature compensated power control for mobile transmitter - Google Patents

Abstract

In a mobile terminal for use in a cellular communications network, a sensor (134, fig.6) measures the ambient temperature (step 203) and a temperature related power level offset value (table 3) is added to the initial set transmission power (steps 206, 208). The mobile then determines whether the corrected transmission power exceeds the maximum allowable power (step 210) and, if necessary, limits the power to the maximum (step 212). As soon as the mobile transmits signals, it measures the transmitted power (step 216) and, if the difference between the measured level and the set level exceeds 1db, the mobile updates the power offset correction table (steps 220, 222, 224) for future use.

Description

Mobile Communications This invention relates to the field of mobile

communications. More particularly, but not exclusively, the invention relates to systems and methods for controlling the transmission power for a mobile terminal.

In cellular telecommunications systems, and particularly in Wideband Code Division Multiple Access (WCDMA) systems, the control of transmission power levels of mobile stations or user equipment (UK) is between different UE's and to ensure the quality of the connection.

This is described in more detail in the 3GPP specifications, such as a 3GPP TS 125401; "Universal Mobile Telecommunication System (UMTS); UTRAN Overall Description", 3GPP TS 25.101 "UK Transmission and Reception (FDD) and 3GPP TS 34. 121, "3GPP Technical Specification Group Terminals; Terminal Conformance Specification; Radio transmission and reception (FDD) .

In order to improve the transmission power control, US patent application US 6,591,089 proposes a mobile station with a transmission power control system.

The mobile station has a variable gain amplifier and adjusts the gain of the variable gain amplifier in accordance with a level data signal, so as to transmit an output transmission signal for each slot with transmission power control. Furthermore, the mobile station has a compensation system for the transmission power control. The compensation system partially extracts the output transmission signal over each slot and carries out a logarithmic wave detection of the extracted transmission signal to produce a resultant signal of the wave detection. Then the compensation system calculates for each slot an average error of the resultant signal of the level data signal and corrects the level data signal with reference to the average error to produce a corrected 1, . level data signal as a newly level data signal. When the gain of the variable gain amplifier is adjusted according to this newly level data signal, transmission power control is compensated and the mobile station produces a gain compensated transmission signal as the output transmission signal.

However, a disadvantage of the system described in US 6,591,089 is that the method can not be effective before transmission has already started.

Moreover, delays are occurring to the correction of the transmission power, such that it cannot be ensured that a certain maximal transmission power is not exceeded.

The measured power is always lagging a time slot behind the set power which could allow the transmission power to exceed the maximum allowed transmission power. However, the method allows to indicate that the maximum power has been exceeded and therefore, power can be adjusted for the next time slot.

Another method known in the art is to correct the power levels in dependence on the ambient temperature. It is known that a variation in the ambient temperature affects the transmission power of a UK. Therefore, the transmission power can be controlled by measuring the ambient temperature and applying a predetermined correction to the transmission power. This can for example be achieved by providing a variable gain amplifier which varies the gain of the transmission power according to a predetermined lookup table.

The look up table provides a certain power offset AP for a particular temperature offset from a standard room temperature or any other calibration temperature.

Open loop transmission power control is the ability of the UE transmitter to set its output power to a specific value. According to the 3GPP specification, the UE open loop power is defined as the mean power in a timeslot or ON power duration, whichever is available. Under normal l conditions, the open loop power control tolerance is +/-9 dB, whereas in extreme conditions, the tolerance is+/-12 dB.

Inner loop power control or closed loop power control is the availability of the UE transmitter to adjust its output power in accordance with one or more Transmit Power Control (TPC) commands received in the downlink connection from the base stations.

Although the above described method, i.e. applying corrections in dependence on the ambient temperature, is suitable for both open and closed loop power control, it is not normally precise enough to ensure tight transmission power control requirements, such as for an UMTS system.

It is therefore an aim of the present invention to provide an improved system and/or method for transmission power control.

According to one aspect of the present invention, there is provided a mobile terminal for use in a cellular communications network, the terminal comprising: means for variably setting the transmission power; means for measuring the transmission power; means for storing correction parameters; the terminal adapted i) to set an input parameter such that an estimated transmission power is achieved; ii) to measure the transmission power; iii) if a predetermined deviation is detected or exceeded between the estimated transmission power and the measured transmission power, to determine a correction for said input parameter or transmission power and to store said correction for future use.

In this way transmission power corrections are determined for each UE individually during use of the UK. The corrections are up-to-date, as the UE monitors the measured transmission power during the lifespan of the UE and updates the determined corrections if this is required.

An advantage of the above described system is that precise transmission power control may be achieved in open and closed loop connections. Also, with this method there is no delay experienced before the transmission power control becomes effective.

Another advantage is that the time-consuming calibration against temperature of each UE is done during use of the UE rather than during manufacture.

With the system described a precise transmission power control can be achieved, which is precise enough to conform with the UMTS standards.

According to another aspect of the present invention, there is provided a method of controlling transmission power in a mobile terminal, the method comprising the steps of: i) setting an input parameter such that an estimated transmission power is achieved; ii) measuring the transmission power; iii) comparing the estimated and measured transmission power; iv) if the difference between the estimated and measured transmission power exceeds a predetermined value, determining a correction for said input parameter of transmission power; and v) storing said correction for future use.

According to yet another aspect of the present invention, there is provided a mobile terminal for use in a cellular communications network, the terminal comprising: means for variably setting the transmission power; means for measuring the ambient temperature; means for measuring the transmission power; means for storing correction parameters, the terminal adapted to: i) measure the ambient temperature; ii) set an input parameter such that an estimated transmission power is achieved, and, if the ambient temperature differs from a predetermined temperature by a predetermined amount, taking into account an offset from said input parameter or transmission power; iii) to measure the transmission power; iv) if a predetermined deviation is detected or exceeded between the estimated transmission power and the measured transmission power, to determine a correction for said input parameter or transmission power and to store said correction for future use. s

This invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic front view of a mobile terminal in which the present invention can be implemented; Figures 2, 4, 5 and 9 are diagrams illustrating the transmission power in dependence on the gain amplifier voltage; Figure 3 is a schematic diagram illustrating thermistor compensation; Figure 6 is a schematic diagram illustrating a transmission system including a transmission power control according to one embodiment of the present invention; Figure 7 is a flow chart diagram illustrating open loop power control according to one embodiment of the present invention; and Figure 8 is a flow chart diagram illustrating closed loop power control according to another embodiment of the present invention.

Figure 1 is a schematic illustration of a mobile communication terminal 10. The terminal 10 includes a display 26, microphone 16, speakers 18, a keypad 21 and navigation keys 23.

The UE output power can be described as a linear function with respect to the gain control voltage Vgc. Therefore, the transmission power is determined by two variables, i.e. the slope (denoted as Automatic Gain Control (AGC) slope in the following) and the power intercept of the function describing the output power in dependence on the gain control voltage.

Figure 2 depicts the dependence of the output power (in dBm) from the gain control voltage Vgc, (in volts). The function can be described by the following formula: Vgc = (Po - P ins)/ (AGC slope) ( 1) l The above described method is for example used to set the transmission output power in open loop power control.

The two variables AGC slope and the power intercept can be determined in a calibration step. Then, the required gain control voltage can be evaluated from the calibration data if a certain power Po is to be set.

The calibration data are usually determined at room temperature.

Therefore, the calibration works satisfyingly only if the UE is situated in an ambient temperature which is very close to the room temperature used for calibration. However, at extreme ambient temperatures, the deviation from the function shown in Figure 2 is expected to be large. If now the calibration data obtained at room temperature are used in order to determine the required gain control voltage to achieve a certain required transmission power output, this will lead to incorrect power setting and therefore the possibility of the UE exceeding a maximum allowed power, such as the maximum power according

to the UMTS specification.

The effects of temperature variation can be reduced by correcting the dependence of the power output from the required gain control voltage for a particular measured ambient temperature if the ambient temperature deviates considerably from the operation temperature. This can for example be implemented by using a look up table including UE transmission power offset versus temperature.

In the UK, the temperature is measured by a temperature sensor such as a thermistor. The power offset values in the look up table are obtained in a calibration step which is performed during manufacture of the UK. As calibration is a time-consuming process, the power offset values are usually obtained once, from an average value from a number of UEs and are then applied to all UEs of a particular construction. It is noted that the offset values obtained during the calibration process remain fixed for all UEs. Table I below gives an example for such an correction look up table. In the first column the temperature offset from room temperature is given in degree Celsius. In the second column the according correction values for the transmission power output are given in dB. These power offset values are here denoted with AP. s

TEMP OFSETT [C] POWER OFSETT [dB] -20 AP-20 -10 Ap-,o Room Temp 0 +10 P+ 'o

Table 1

Referring now to Figure 3, an alternative solution to the look up table is described. In this case a thermistor 50 and a resistor 52 is used in a feedback loop wherein gain control voltage Vgc is adjusted to overcome power errors in the UE power transmission. The power control and compensation using such a feedback loop may be provided either in a pure hardware implementation: As the transmitter performance changes with temperature, the thermistor resistance changes to compensate for this effect.

Thus the gain control voltage is adjusted by the thermistor in order to maintain power output. Alternatively, a feedback loop controlled by software may be used. In this case changes in the thermistor output voltage are fed into a controller and a controller determined the required amount of compensation, for example based on a predetermined look up table of power offset against temperature.

The described method helps in reducing power errors. However, with the described method the precision in transmission output power required for UMTS can not normally be achieved. This is mostly due to the fact that the above described corrections method is a compensation which is applied to all Ues of a particular construction. However, not every UE requires the same amount of compensation. Therefore, some UEs might be overcompensated while others might be under compensated, possibly allowing maximum power limits to be exceeded or UEs to be underperforming.

Open loop power control As described above, open loop power is the initial power set by the UE without any control by the base station. The UE determined the required gain control voltage required from equation (1) given above.

Before setting the AGC control voltage, the UE checks whether the AGC value obtained does not exceed the value for the maximum output power as determined via calibration. Hereby the UE takes into account the ambient temperature, and applies the power offset values to the transmission power as required.

However, the set power may not be correct for any of the following reasons: the UE calibration information may be incorrect or corrupted or the UE transmitter components may be malfunctioning.

Closed loop power control In closed loop power control, the transmission output power is controlled via TPC commands from the base station. The base stations can request a power change of +/- ldB, +/- 2dB or +/- 3dB. IN addition, the UE power can also be required to change by a change in the Transport Format Combination (TFC) due to a change in the data rate. According to the UMTS standards, for a power change of ldB, 2dB or 3dB, the error shall be less than 0.5dB, ldB and l.SdB, respectively. Moreover, the UE maximum power or the limit set by the base station shall not be exceeded, For the TFC, change in power maybe as high as 21dB with +/- 6 dB tolerance.

It is possible for a UE to exceed the maximum power allowed due to tolerances of the UE transmitter. UE power can easily change by +/-6 dB, particularly at extreme conditions where component performance can vary significantly. However, this may lead to a transmission power which does not conform with the 3GPP standards.

This problem is worse in open loop power condition where the UE has just started to transmit. There is no reference of the transmitted power available, which the UE could use to gauge what the actual transmission power will be. Open loop power errors are more serious as transmission power approaches maximum allowed power.

Referring now to Figure 5, it is described how the UE may exceed the maximum power. Fig 5 shows again the output power as a function of the gain control voltage. Due to tolerances in the output power, the set power Ps can be in the range between Pl (lowest) and Ph (highest). If the set power is equal to Pl, the transmitter is under achieving, and if the set power is equal to Ph. the transmitter is exceeding the maximum power.

Power detection A UE furnished with a power detector can accurately measure the transmitted power. Usually, a power detector is used to measure accurately the transmission power at the antenna port.

Once power is transmitted, the power detector can be used to measure the actual power and corrections can be made to transmitted power if necessary. However, since power can only be set and transmitted time slot by time slot, the risk of exceeding maximum power still remains.

According to one particular implementation, the detector employed has a dynamic range of about 35dB and allows UE transmission power to be measured accurately from the maximum power of 24dBm down to about 10dBm.

Figure 4 depicts the transmission output power for a UE as a function of measured detector output voltage Vd. Again, the function can be described by two variables, i.e. the power detector slope Spd and the power intercept Pd by the following formula: Po = Spd*Vd + Pd (2) It is noted that this function is only valid in the linear part of the power detectors response.

Power Detection and Correction In the following a first embodiment of the present invention will be described in more detail.

The UE sets the gain amplifier voltage as described above to achieve a certain transmission power output. When estimating the required gain amplifier voltage for a certain transmission power, the UE may also take into account temperature corrections as described above, for example power offset values provided in a look up table. In addition, the UE measures the transmitted power as soon as the UE starts to transmit the signals.

The UE then compares the estimated transmission power with the measured transmission power. If a predetermined deviation, for example of at least ldB is detected by the UK, the UE determines a correction to the transmission power which reduces or disposes of the deviation between the estimated transmission power, taking into account any correction because of the ambient temperature or the like, and the measured transmission power.

The determined correction is stored in the UE for future use, and may in addition directly applied to correct the transmission power output for the next timeslot, such that the UE then transmits signals with the correct required transmission power. The next time the UE needs to estimate the required gain amplifier voltage for a certain transmission power, the correction previously determined is read from memory and applied in order to ensure that the UE transmits the correct or improved power directly from the start of transmission.

Again the UE measures the transmitted power as soon as the UE starts to transmit the signals, and compares the estimated power, including any temperature and previously determined corrections, with the measured transmission power. In this way the corrections can be updated anytime if required, for example if an update is required due to a degradation in one or more of the transmission circuit elements or the like.

In this way corrections individual for each UE are determined during use of this particular UK. These corrections are applied in addition to other corrections such as corrections for an ambient temperature deviating from room temperature (or another calibration temperature), which is determined for all UE at the same time.

If an ambient temperature correction is applied as described above, the corrections may be determined in dependence on the ambient temperature.

If the difference between measured and set power is more than, for example ldB, it appears that the correction in the UE power versus temperature as provided by the look up table is not suitable for this particular UK. This may be the case as the provided look up table is a universal look up table for all UEs, usually determined by a one-off calibration during manufacture of the UEs.

However, in this case the power offset versus temperature table will be modified or supplemented by the corrections determined by measuring the transmission power. If these corrections are determined in dependence on the ambient or operating temperature, a modified or supplemental look-up table is provided.

This procedure results in an amended or a complementary look up table which will now be unique to this UK. The next time the UE operates at this temperature the correct power offset value as determined above will be used. Thus the UE is able to set the transmission power accurately.

Accordingly, this provides a method of UEs to evaluate their own look up tables based on power detector measurements and will eliminate or greatly reduce UEs from exceeding maximum power. In addition, the UE will be able to set the transmission power accurately over a longer time period, as the UE constantly modifies the power correction table should the transmitter performance change over time.

An example of a look up table including the original transmission power offset values and a complementary column for including the corrections measured for a particular UE is shown in tables 2 and 3. As mentioned above, it may be useful to retain the original power offset table values (shown in column 2 of tables 2 and 3). These can then form default values for UE to refer to. When a particular UE is manufactured, the power offset values in the second column of the look up table are set. The table includes a third column which is designed to include the offset corrections.

However, at the time of manufacture the values of the third column are set to zero. Table 2 shows such a look up table.

TEMP OFSETT [C] POWER OFSET [dB] Offset Correction [dB] -20 AP-20 0 -10 AP'0 0 Room Temp 0 0 +10 AP+'o 0 +20 AP+20 0

Table 2

A table including corrections to the power offset values is shown in table 3. The correction are given in column 3 of table 2. These values are determined by power detector measurements and comparison of the estimated and measured transmission power. It is noted that the values are modifiable during future use of the UK.

TEMP OFSETT (C) POWER OFSET dB Offset Correction dB -20 AP 20 -3 -10 AP'0 -1 Room Temp 0 +1 +10 AP+ro +1 +20 AP+20 +2

Table 3

Fig 6 is a schematic outline of a transmission system including a transmission power control system for a mobile station suitable for implementing the present invention. The transmission system includes two power amplifiers 102 and 118, an intermediate frequency (IF) mixer 104, a IF local oscillator 106, two variable gain amplifiers 108 and 114, a radio frequency (RF) mixer 110, a RF local oscillator 112, a band pass filter 116, a directional coupler 120, an antenna 122, an attenuator 132, a logarithmic wave detector 130, a low pass filter 128, a controller 126, a low pass filter 124, a memory 136 and a temperature sensor or thermistor 134.

An intermediate frequency (IF) signal is generated as input into the power amplifier 102, and subsequently delivered to mixer 104. The mixer also receives a local frequency signal from local IF oscillator 106 and mixes the signals such that the IF signal is changed into a RF signal. The RF signal is delivered to variable gain amplifier 108. Subsequently, the amplified signal is fed to the RF mixer 110. Mixer 110 also receives a local frequency signal from RF local oscillator 112. The mixed signal is put into a second variable gain amplifier 114. The signal is subsequently filtered in band pass filter 116, further amplified by power amplifier 118, and fed to directional coupler 120 before it is transmitted by antenna 122. The directional coupler 120 partially extracts the output transmission signal to be transmitted from power amplifier 118 through the antenna 122, and produces an extracted l transmission signal, which is delivered to logarithmic wave detector 130. The logarithmic wave detector 130 carries out logarithmic wave detection of the extracted transmission signal, resulting in a signal which linearly changes corresponding to a decibel value of the extracted transmission signal. The signal is then delivered to low pass filter 128, converted into a digital signal, and delivered to controller 126. The system also includes a temperature sensor 134. The output signal of sensor 134 is also converted into a digital signal and fed into controller 126. The output of controller 126 is converted into an analog signal, is fed into low pass filter 124 and is subsequently used to control the signals in variable gain amplifiers 108 and 114.

With the measured temperature received from sensor 134, the controller accesses a correction table stored in memory 136, and extracts a predetermined power offset AP for the measured temperature. Accordingly, the power is corrected by variable gain amplifiers 108 and 114 according to the power offset AP.

Open loop power correction Referring now to Figure 7, the procedure of controlling power in open loop is described according to a first embodiment of the present invention.

In step 202 the UE evaluates the up link power as described above with respect to Figure 4, and measures the ambient temperature in step 203. If the difference between the ambient temperature and a predetermined "room" temperature is more than 10 degrees Celsius (step204), in step 206 the UE retrieves the power offset values for the detected temperature difference from the look up table. With the retrieved correction value, the UE reevaluates the transmission power in step 208. The process continues in step 210 and the UE determines whether the estimated transmission power, including any temperature corrections, exceeds the maximum transmission power allowed by the UMTS standards. If the maximum power is exceeded, the UE limits the l transmission power to the maximum allowed power in step 212. In step 214, the UE then starts to transmit the signals.

As soon as the UE transmits signals, the UE measures the transmission power at the antenna port in step 216. In step 218, the UE compares the S estimated transmission power to the measured transmission power. If the difference between the estimated and the measured power is greater than ldB (step 220), the UE resets the transmission power such that the estimated transmission power is equal or close to the measured transmission power (step 222). In step 224, the UE stores the determined correction in the offset correction table for future use. The process ends in step 226 by the UE continuing to transmit with the corrected power, if any correction was required.

Closed loop power correction As described above, closed loop power control applies if a link has been established with a base station. The base station can request the UE to change the set power by 1, 2 or 3dB. A change in the TFC can also initiate a power change depending on data rate required. In both cases the UE has to achieve these power changes without exceeding maximum power.

In the following, closed loop correction is described following a request from the base station because of a change in the TFC.

The maximum power "jump" due to a TFC change is 21dB. This "jump" corresponds to a date rate change from minimum data rate to maximum date rate. In the case for such a large power jump the UE should know accurately the power it is transmitting, know accurately the power change required, should establish what the final power will be and limit the set power to the maximum allowable power.

With a power detector the absolute power can be measured accurately, and required power increments can be evaluated by the UE from down link information or a data rate change. Final power can be established from existing transmission power plus the required power change. However, the final power should not exceed the maximum permissible power.

Referring now to Figure 9, it is illustrated how a requested TFC jump can lead to a set power which is greater than the maximum allowed power.

The graph shows the set transmission power in dependence on the gain control voltage. Before the TFC change is requested, the gain amplifier voltage is Vgcl, resulting in a transmission ouput power of Pi. The change in the transmission data rate now requires a jump in the transmission power of 21dB. However, it can be seen from Figure 9, that the maximum allowed transmission power would be reached if the transmission power is increased by 21dB. Therefore, the set power needs to be limited, as will be explained below.

Referring now to Figure 8, the procedure of controlling power in closed loop is described according to a second embodiment of the present invention.

The procedure starts in step 302 by the UE receiving a downlink TPC command from the base station. In steps 304 and 306, the UE evaluates the required power step and the absolute power required to achieve the desiredpower step. In step 308, the UE checks whether that estimated absolute power is greater than the maximum allowed power. If the estimated power is indeed greater than the maximum allowed power, the UE limits the transmission power to the maximum value in step 310. The UE then continues in step 312 by implementing the power step.

As soon as the UE transmits signals, the UE measures the transmission power at the antenna port in step 314. In step 316, the UE compares the estimated transmission power to the measured transmission power. If the difference between the estimated and the measured power is greater than I dB (step 318), in step 320 the UE stores the determined correction in the offset correction table for future use. It is noted that the UE does not directly interfere with the power settings as determined by the base station in closed loop power correction. Instead, the UE updates the offset correction table such that in the future the correct power is transmitted.

The process continues in step 322 by the UE checking whether the measured transmission power is greater than the maximum allowed transmission power. If this is the case, the UE limits the power set to the maximum power in step 324. The process ends in step 326 by the UE continuing to transmit with the corrected power, if any correction was required.

Whilst in the above mentioned embodiments transmission power control for UMTS systems has been described, it is appreciated that the present invention can be applied to other systems like for example GMS or other WCDMA systems in a similar way.

It is to be understood that the embodiments described above are preferred embodiments only. Various features may be omitted, modified or substituted by equivalents, without departing from the scope of the present invention.

Claims (18)

1. CLAIMS: 1. A mobile terminal for use in a cellular communications network,

   the terminal comprising: means for variably setting the transmission power; means for measuring the transmission power; means for storing correction parameters; the terminal adapted i) to set an input parameter such that an estimated transmission power is achieved; ii) to measure the transmission power; iii) if a predetermined deviation is detected or exceeded between the estimated transmission power and the measured transmission power, to determine a correction for said input parameter or transmission power and to store said correction for future use.
2. 2. A terminal according to claim 1, wherein said correction is determined in dependence on variable v.
3. 3. A terminal according to claim 1 or 2, wherein said input parameter is set in dependence on variable v.
4. 4. A terminal according to claim 2 or 3, wherein said variable v is the ambient temperature.
5. 5. A terminal according to any preceding claim, wherein said correction is read and applied in order to set said input parameter in one or more future occasions when the mobile terminal is required to set the transmission power.
6. 6. A terminal according to any preceding claim, wherein said input parameter is the gain amplifier voltage.
7. 7. A terminal according to any preceding claim, wherein said correction is applied for controlling transmission power.
8. 8. A terminal according to any preceding claim wherein, said terminal is adapted to perform open loop power control.
9. 9. A terminal according to claim 8, wherein said terminal is adapted to limit the transmission power to a predetermined maximum transmission power.
10. 10. A terminal according to claim 8 or 9, wherein in step iii), said terminal is adapted to reset the transmission power taking into account the determined correction.
11. 11. A terminal according to any preceding claim, wherein said terminal is adapted to perform closed loop power control.
12. 12. A terminal according to claim 11, wherein said terminal is adapted to control power after the terminal received a TPC command.
13. 13. A terminal according to claim 11 or 12, wherein said terminal is adapted to limit the transmission power to a predetermined maximum transmission power.
14. 14. A method of controlling transmission power in a mobile terminal, the method comprising the steps of: i) setting an input parameter such that an estimated transmission power is achieved; ii) measuring the transmission power; iii) comparing the estimated and measured transmission power; iv) if the difference between the estimated and measured transmission power exceeds a predetermined value, determining a correction for said input parameter of transmission power; and v) storing said correction for future use.
15. 15. A method according to claim 14, wherein said correction is determined in dependence on the ambient temperature.
16. 16. A method according to claim 14 or 15, wherein said correction is read and applied in order to set said input parameter in one or more future occasions when the mobile terminal is required to set the transmission power.
17. 17. A method according to claim 14, 15 or 16, wherein said input parameter is the gain amplifier voltage.
18. 18. A mobile terminal for use in a cellular communications network, the terminal comprising: means for variably setting the transmission power; means for measuring the ambient temperature; means for measuring the transmission power; means for storing correction parameters, the terminal adapted to: i) measure the ambient temperature; ii) set an input parameter such that an estimated transmission power is achieved, and, if the ambient temperature differs from a predetermined temperature by a predetermined amount, taking into account an offset from said input parameter or transmission power; iii) to measure the transmission power; iv) if a predetermined deviation is detected or exceeded between the estimated transmission power and the measured transmission power, to determine a correction for said input parameter or transmission power and to store said correction for future use.