Difference between CDMA and TDMA

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
CDMAphone, 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.
We discuss power control loop design in a later section in the chapter.