ultiframe structure of a PDTCH. Note how the frame structure includes 2 X PTCCH and 2 X IDLE bursts that are in GMSK modulation. Therefore, the 'duty cycle' of TS 7 will always have at least four bursts in the GMSK every 240 ms. Considering this and that each EGPRS MS TS will also operate on PACCH blocks (estimating every second 52 multiframe for sustained data transfer), the average power decrease will be less than 2 dB, depending on the traffic type (i.e. the File Transfer Protocol (FTP) will occupy most RLC blocks with 8-PSK while other traffic will have 'breaks' of GMSK RLC dummy blocks or PACCH signalling blocks). Average Power Decrease Versus Backoff in a Linear PA Figure 2.80 illustrates the concept of a linear PA power amplitude response curve with 0 dB backoff and an average power decrease of 0 dB. Note that the 8-PSK bursts would operate beyond the 3 dB compression point (and therefore experience non-linear distortion) SO an APD of 2 dB is require to ensure this does not happen. Depending on the linearity of the PA near the compression point, the APD at full power (0 dB backoff) would change. For example, mobiles are likely to have an APD of 4 dB or more near the 3 dB compression point due to the use of lower performance amplifiers (less linearity) compared to base station transmitters. Figure 2.81 illustrates how operating at full power (left figure for a 0 dB backoff) requires a 2 dB APD for 8-PSK to operate efficiently (in the linear range) while operating at a 2 dB backoff (right figure) does not require any average power decrease for 8-PSK (APD = 0 dB). Phase-State Vector Diagrams of APD The vector diagram (Figure 2.82) illustrates how amplifier performance directly affects the APD and required backoff. In reviewing the vector state transition diagram, the peak-to-average 'overshoot' is B0(0..3) B1(4..7) B2(8..11) B3(13..16) B4(17..20) B5(21..24) B6(26..29) B7(30..33) B8(34..37) B9(39..42) B10(43..46) B11(47..50) 52 TDMA Frames (240 ms) Figure 2.79 GSM 52 multiframes Radio Network Optimisation Out 