riod, design engineers also had to deliver additional band support. Phones were initially single band (900 MHz), then dualband 900/1800 MHz (from about 1995), then triband 900/1800/1900 (from about year 2000), then triband, and then (2005 to 2007), quadband (850/900/1800/1900). 4.4.3 The Impact of Volume Thresholds and Maturity Performance Thresholds - A UMTS Example The fact that GSM has more or less reached its development limit in terms of RF performance provided one of the motivations for moving to UMTS. UMTS traded additional processing overhead to achieve what can be rather oversimplistically described as 'bandwidth gain (anal- ogous to the benefits that broad band FM delivered over narrowband AM systems in the 50 years between 1940 and 1990). Bandwidth gain can be translated into more capacity (more users per MHz of allocated spectrum and/or higher data rates per user) and/or coverage. There are other RF potential costs benefits, for example the wider channel spacing (5 MHz rather than 200 kHz) relaxes the need for channel to channel RF frequency stability. However, similar rules on volume and maturity performance thresholds apply. In 2002, the first UMTS phones barely managed to meet the conformance specification of -117 dBm. Figure 4.15 shows the results of reference sensitivity test on four presently available phones (2006/2007). The best device is over 5 dB better than the conformance specification. Note the Telecommunications Economies of Scale -114 dBm -115 dBm -116 dBm -117 dBm -118 dBm -119 dBm -120 dBm -121 dBm -122 dBm -123 dBm Figure 4.15 Sensitivity measurements on four UMTS phones. Reproduced with permission of Spirent Communications - originally published in RF Cost Economics for Handsets Study May 2007. 3 difference between the best and worst handsets was about 5 dB (the worst handset only just passed). So five years after market introduction, the best handsets available were 5 dB better than specification. In other words both GSM and UMTS handsets improved their sensitivity by 1 dB per year