amplifiers for example and passive devices, filters and discrete devices such as capacitors, inductors and resistors, the modest workhorses of every electrical system. As individual component performance has improved together with packaging design it has been possible to extract more performance at circuit and system level in terms of stability, selectivity, stability and DC to RF and RF to DC conversion efficiency. This has provided the basis for moving to higher frequencies. For example, the availability of low-cost high-quality FR4 printed circuit board material was an essential precondition for realising 800- and 900-MHz transceivers for first-generation mobile phones manufactured in the mid-1980s. The move to higher frequencies allowed for more voice channels to be supported but also allowed wider channel spacing to be realised to support higher per user peak data rates. In first-generation cellular systems channel spacing was either 30 kHz (US) or 25 kHz (Europe and rest of the world). Second-generation GSM systems relaxed the channel spac- ing to 200 kHz, which made network planning a bit easier and relaxed some of the RF component specifications in the RF front end of a mobile phone. The 200 kHz was then multiplexed in to 8 time slots with each time slot optimised for a 12 kbps voice path. Data was not a predominant design requirement but as demand increased, time slots were concate- nated together to produce higher data per user data rates. Third-generation systems increased channel bandwidth either to 1.25 MHz (CDMA in the US) or 5 MHz (UMTS in Europe and Asia) First-generation and third-generation networks achieve user-to-user and channel-to-channel selectivity in the frequency domain (5-MHz channels), time domain (time slots) and code domain. A full treatment of how code domain selectivity is achieved can be found in Chapter of 3 G Handset and Hardware Design2 and will not be repeated here. All we need to know is that as data rates increase code domain selectivity becomes progressively less efficient 