This is not true. When you have a time slot, you transmit the
same amount of power if you are talking or not in GSM. There is
a feature called DTX, Discountinous Transmission. However, your
voice quality suffer for the benefit of less interference.
In CMDA, once you have a Traffic Walsh Code (like timeslot) in
CDMA, the power transmit and receive is variable according to
speech activity, thus requires less power and less interference.
Once you have a timeslot in GSM, it is not totally yours, you
share it with another user in frequency re-use concept. This
is called co-channel interference, and if your filter is bad
at the cell site, you got adjacent channel interference as well.
GSM is less interference tolerate, less time dispersion tolerate.
CDMA can actually combine multipath delay in time to turn them
into gains rather than loss. A feature that GSM can never do
due to 200khz narrowband signal.
Both GSM and CDMA need filter to separate TX and RX, with 45Mhz
separate, you don't need a expensive filter for either one.
1) due to the nature of GSM (TDMA) it will actually use less power when transmitting and recieving. Lets take a specific example. If we hold both powers at max AMPS transmit (28dbm) then the TDMA pulsed transmit would take less power. If your PA is only on for 1 out of 10 time slots then you would take max 1/8th the power right? in that example I assume that there is some penalty paid for the power up and down cycles. The lowest rate that the CDMA system transmits on is 1/8 rate. At that point you still spend 1/6th the power. Besides you will not be at 1/8th rate when you are speaking, only when listening and no
information is passed.
2) Due to the nature of GSM (TDMA) the PA that they use is less power hungry and much cheaper. If you have a slotted system then you can afford 2 things, 1) more noise, and 2)less isolation. In a CDMA system you have to have some type of filtering to seperate the transmission of Recieve and Transmit signals. This filtering costs tons in the way of transmit power and power in general. In GSM you have no need for this filtering, your time slot is yours, either recieve or transmit whatever you want. The corollary to this
is that when you have your own slot you need not worry about the noise you are transmitting inside your band. Noone else is affected so who cares? With CDMA (a noise limited system) ANY noise that you add to the system that is not directly associated to your communication will lower the capacity of the entire system. This causes the PA to be more complex and bigger, more lossy, more power hungry, etc... |
