own right absorbs a significant percentage of the available signal energy (about 20%). HSPA aims to simplify this process and reduce some of the power control signalling and energy overhead by only using the SF16 part of the code tree and dispensing with fast power control. However, as you might have noticed, this takes away one of the desired properties of the OVSF code tree, which is the ability to support lots of users each with multiple simultaneous channels each at a variable data rate. In other words, much of the multiplexing capability of the OVSF code tree disappears if only SF16 is used. The answer used in HSPA is to have a high-speed data-shared channel (HS-DSCH) that can be shared by multiple users with a MAC-driven access control based on 2 millisecond (and later 0.5 millisecond frames) that is not dissimilar to the contention based MAC used in present WiFi systems. The challenge here is that the shared channel requires a new high-speed shared physical control channel (HSDPCCH). This control channel has to carry the channel quality indication (CQI) messages and acknowledgement/negative acknowledgement (ACK/NACK) messages that the MAC needs to decide on admission control and other factors, such as coding overhead, choice of modulation and transmission time interval. This signalling is discontinuous, but when present increases the peak to average ratio of the transmitted (composite) signal and can also encounter relative timing issues with the other (dedicated) control channels. If the high-speed control channel is not correctly detected, no communication takes place, which is potentially a little hazardous. The peak to average ratio can be accommodated by backing off the PA, but this has an impact on coverage (range). In a sense, HSPA has exchanged the code-planning and power-planning challenges inherent Release 99 WCDMA with code-sharing and power-sharing issues. This means that the RF performance of the handset and base station remains as a critical component of overall system performance. Although s