ome of the functional complexity at the PHY (physical) radio level has been moved to the MAC (medium access control) level, the effectiveness and efficiency of the MAC is dependent on the careful measurement and interpretation of the CQI and ACK/NACK responses. The 7-dB step change in power that occurs when the CQI and/or ACK/NACK signalling is transmitted can trigger AM/PM distortion. This may cause phase errors that in turn will compromise CQI measurements or disrupt the ACK/NACK signalling. This will probably be the determining factor limiting coverage and will probably require some conservative cell-geometry factors (signal versus noise rise across the cell) in order to maintain the signalling path (without which nothing else happens). The requirement for a more complex and flexible multiplex can be met either by having a rather overcomplex code structure and/or an overcomplex MAC. Either or both can be problematic both from a radio planning and/or a handset/base station design perspective. Making Telecoms Work 3.3.6 CDMA/OFDM Hybrids as a Solution This seems to imply that something else needs to be done to allow these wide-area radio systems to deliver data rates that meet user's likely future data-rate expectations. The options are either to increase cell density and/or to increase the sensitivity, selectivity and stability of the handsets and base stations - preferably both. In 1XEV, (including the most recent 1XEV-DO Revision A) handset enhancements are based on implementing receive diversity and advanced equalisation. Base-station enhance- ments include implementing 4-branch receive diversity (two pairs of crosspolarised spatially separated antennas) and pilot interference cancellation - a mechanism for getting unwanted signal energy out of the receive path. A similar evolution path exists for HSPA. Advanced receiver techniques were first specified in Release 6 together with an enhanced uplink (HSUPA). Release 7 included standardised multiple antenna/MIMO proposals. The downlink evolution road map for 1X