Linear Power Amplifier
Application:
The final output RF power amplifier in the forward-link transmitter in the IS-95 base station is an LPA. The input to the LPA is a sum of scaled Walsh code words which have been masked by long-code masks and spread by the PN codes. To retain their orthogonality when they are received at the mobiles, the Walsh code words must be amplified with a minimum of distortion. Any distortion in the LPA, or other circuitry, before the correlator in the mobile receiver, causes increased interference power at the correlator output. This interference reduces link quality and leads to reduced capacity and coverage. In IS-95, an alignment test is run on the LPA to ensure that it is sufficiently linear. The linearity of the LPA must be maintained over the range of powers transmitted on the IS-95 forward link.
Linear Power Amplifier
Example:
At maximum load, the LPA in IS-95 typically puts out an average of about 39 dBm (about 8 Watts). Because the sum of scaled, masked, and PN spread Walsh code words which modulates the input to the LPA has a widely varying envelop, the LPA is operated at an average power level which is much lower than the peak or maximum power level at which it can operate and still have the required level of linearity. This practice is called "backing off" the LPA. Since the total power available to the forward link in limited by the power of the LPA at maximum-load, power must be allocated very carefully among the pilot, Synchronization, paging, and traffic channels. For instance, if the pilot uses 2 Watts, the synch channel .5 Watts, and the paging channel 1 Watt, the power available to all the traffic channel equals 4.5 Watts. At an average of 250 mWatts per traffic channel, the forward-link capacity is limited to 18 simultaneous speakers by the linearity of the LPA. Over-driving the LPA leads to distortion which degrades all users.
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