CDMA

An important difference between TDMA and CDMA is that in TDMA, the duty cycle of the RF amplifier is a product of the number of time slots used. AGSM handset using one time slot has a duty cycle of 1/8. Maximum output power of a 900 MHz GSM phone is 2 Watts. Effectively, the average maximum power available across an eightslot frame is therefore 250 mW. In CDMA, the handset is continuously transmitting but at a maximum of 250 mW. The total power outputs are therefore similar. In a TDMA phone, the RF burst has to be contained within a power/time template to avoid interference with adjacent time slots. The RF output power of the TDMA handset is adjusted to respond to changes in channel condition (near/far and fading effects) typically every 500 ms. In an IS95 CDMA phone, power control is done every 1.25 ms, or 800 times a second. This is done to ensure that user codes can be decorrelated under conditions of relatively stable received signal strength (energy per bit over the noise floor). Failure to maintain reasonably equivalent Eb/Nos (energy per bit over the noise floor) between code streams will result in intercode interference. Traditionally the power control loop in an IS95 CDMA phone requires careful implementation