Method and apparatus for compensating code channel power in a transmitter
It has now been found that in some transmitter implementations which transmit multiple code channels, for example CDMA transmitters, the observed power for each code channel at the output relative to the other code channels is dependent upon the total transmit power due to non-linearities in the transmitter. Methods and apparatus are provided which use pre-set digital gains or digital gain adjustments to compensate for the non-linearities as a function of total transmit power such that at the output, the desired relative powers are observed.
1. A method of calibrating a compensation element in a transmitter having an output, the method comprising:
configuring the transmitter to transmit in each of a plurality states;
for each of the plurality of states, setting a transmit power value to each of a plurality of transmit powers in a range of transmit powers supported by the transmitter;
for each of the plurality of states and each of the plurality of transmit powers:
while measuring code channel powers of each activated code channels at a port of the transmitter output:
determining a first digital gain for each activated code channel, representative of a nominal gain to produce a code channel power at a port of the transmitter output;
adjusting digital gains of each activated code channel until desired values of total output power are measured at the output port;
determining a difference between an adjusted digital gain resulting in the desired value of total output power and the first digital gain to obtain a differential digital gain for each activated code channel;
saving the differential digital gains, for use in mapping to the differential digital gains, in conjunction with a nominal gain, to desired values of total output powers.
2. The method of claim 1 wherein saving the differential digital gains comprises generating a look-up table for each of the plurality of states for mappings of total output powers to differential digital gains.
3. The method of claim 1 wherein saving the differential digital gains comprises generating a look-up table of mappings of total output powers to differential digital gains in which different columns of the look-up table comprise differential digital gains representing the differential digital gains for each of the plurality of states.
4. The method of claim 1 , wherein the code channels are CDMA code channels.
5. The method of claim 1 used for calibrating a single transmitter.
6. The method of claim 1 used for calibrating a batch of transmitters that do not have significant unit to unit variation.
7. A computer-readable medium having computer readable instructions stored thereon for implementation by a computer processor for implementing a method comprising:
configuring the transmitter to transmit in each of a plurality states;
for each of the plurality of states, setting a transmit power value to each of a plurality of transmit powers in a range of transmit powers supported by the transmitter;
for each of the plurality of states and each of the plurality of transmit powers:
while measuring code channel powers of each activated code channels at a port of the transmitter output:
determining a first digital gain for each activated code channel, representative of a nominal gain to produce a code channel power at a port of the transmitter output;
adjusting digital gains of each activated code channel until desired values of total output power are measured at the output port;
determining a difference between an adjusted digital gain resulting in the desired value of total output power and the first digital gain to obtain a differential digital gain for each activated code channel;
saving the differential digital gains, for use in mapping to the differential digital gains, in conjunction with a nominal gain, to desired values of total output powers.