frequency) filtering both in the handset and the base station. 3.2.7 The Need to Deliver Cost Reduction and Better Performance However, user expectations of performance increase over time. User data rates in first- generation analogue systems were typically 1200 or 2400 bits per second, GSM data rates are tens of kilobits, 3G data rates are (supposed to be) hundreds of kilobits and have to compete in the longer term with local area WiFi systems delivering tens of megabits and personal area systems delivering hundreds of megabits (potentially WiFi at 60 GHz). The performance of a radio system can be measured in terms of the radio system's sensitivity, selectivity and stability. Sensitivity is the ability of the radio system to extract a wanted signal from the noise floor. Improved sensitivity translates into improved range and/or an ability to support higher data rates. In terms of the user experience, sensitivity equals coverage and capacity. Selectivity is the ability of the radio system to extract a wanted signal in the presence of unwanted signals from other users. As with sensitivity, improved selectivity translates into improved range and capacity. However, by relaxing the RF channel spacing over time, we have thrown away some of the selectivity inherent in narrowband radio systems SO have the need to replicate this in some other way. In parallel, users expect to receive voice and nonvoice services in parallel SO we have the need to support multiple data streams per user, which implies a need to provide additional per user channel to channel selectivity. Stability is the ability of the radio system to perform consistently over temperature over time, which in turn is dependent on the short- and long-term accuracy of the frequency and time reference used in the transceiver. The move to higher frequencies in the microwave band has increased the need for a more accurate frequency reference, but this has been offset by the relaxation in RF channel spacing. The combination of higher data rates and the need to deli