 2-dB power change. At a distance of 70 m, a distance of 20 m has to be covered before a 2-dB step change occurs (13 seconds of elapsed time). It is therefore plausible that a per packet power control algorithm could be deployed that could be reasonably stable when used in this type of 'gentle mobility' application and that could yield some worthwhile power savings and related link budget benefits. From a PA design perspective, however, it does mean the operating point of the amplifier will be constantly changing and this in itself has an impact on error vector magnitude and harmonics. 4 http://www.cs.colorado.edu/department/publications/reports/docs/CU-CS-934-02.pdf. Interrelationship of the Physical Layer with Other Layers of the OSI Model Table 3.15 Mobility thresholds Transmit power in intervals of 2 dBm Distance (m) Difference (m) 10-12 12-14 14-16 16-18 The ability to power control handsets and access points also provides an opportunity to op- timise the radio system in terms of channel reuse and coverage. This will become increasingly important if voice users become a significant percentage of the offered traffic mix. 3.6.4 Impact of Receive Sensitivity on the Link Budget The link budget is a composite of the TX power, TX accuracy (typical error vector magnitude for an 802.11g transmitter should be <1.5% and <2% for 802.11a, but is often much worse), path loss and sensitivity. Sensitivity is a function of data rate. With 802.11 a and g higher data rates are achieved by using higher-order modulation, every time the modulation state is doubled (for example from BPSK to QPSK), another 3 dB is needed on the link budget, moving from 16 QAM to 64 QAM implies a 6-dB increase. In practice, the fact that on the TX side, EVM tends to get worse with higher-order modulation means that the real implementation losses are higher. Table 3.16 shows some typical receive sensitivity figures (and some claimed sensitivity figures) at different data rates. 3.6.5 Linearity Requirements Table 3.17 compares three generations of ce