Re: GSM voice quality
Well, BTS does not decrease its power when trafic increases. In fact when no of users increase in any given cell then no. of traffic channels assigned by that BTS increase. Thus TX power of BTS increase.
BTS will lower down the TX power when mobile is near to the cell. Moobile will tell BTS about the recieved power and tell BTS to lower down the TX power. This is called the power control and it's a very importanat aspect of CDMA systems.
Near-Far Effect:- This was the problem faced by early CDMA systems.
When no.of users closer to the BTS increase, there will be a Noise floor increase at the BTS because of the large no. of users on uplink. TO overcome this noise floor, the mobiles which are at the cell boundary will increase their TX power.If the nearby users still increase then far users will not be able to TX power after a certain limit ( MAX Tx power of a mobile which is fixed ≈+24 dBm in CDMA). Thus far users will not be able to communicate with the BTS. This is called the Near-Far effect. To overcome this problem power control is used in CDMA.
Power Control:- The purpose of power control is that the powers of signals received from all the mobiles (near and far) should be same at the BTS.
Added after 10 minutes:
To Vfone
Even though GSM is a TDMA system, when the no. of users in surrounding cells increase , the BER (bit error rate) will increase which will make the GSM voice quality poor.
Also at high traffic , BTS allocates half rate traffic channel(TCH/HR (6.5 kbps) instead of TCH/FR (13kbps)) to the users. Since vocoder rate is decreased in this case, voice quality gets affected adversely.