Figure 3 shows how the speech service quality is
deteriorated when Packet Switched (PS) service is
introduced. In the worst case, where CS traffic of
14.1 Er1 and PS traffic of 9.1 Erl, the amount of bad
quality samples of speech FER >= 1% becomes as
high as 20%. The main reason is that all traffic (CS &
PS) is preferred in the same TCH TRXs. The
interference that is brought by PS (GPRS) traffic
becomes very harmfid for voice traffic when high PS
Mic load. Because DL Power Control increases DL
TxPower when bad quality, the interference gets
higher, lowering DL CR, and thus decreasing the quality of speech.