ons are a mix of standard and reverse duplex for reasons that we discuss in Chapter 5. 4.3.2 Band Allocations, Guard Bands and Duplex Separation. The Impact on Transceiver Design, Cost and Performance The choice of frequency, the guard bands between band allocations and the duplex separation of the uplink and downlink within each individualband all have a profound influence on the architecture of the phone and the active and passive devices used in the phone. Additionally, legacy spectral allocations may need to be supported in some handset frequency plans and future repurposed UHF TV allocations other than the 700 and 800 MHz bands may need to be accommodated. These 'wide-area' cellular radio transceiver functions may also need to physically coexist with local area (WiFi) and personal area transceivers and with easily desensitised receive only functions such as GPS or ATSC/DVB TV. Digital filtering techniques and architectural inno- vations, for example direct conversion receivers, translational (GSM) and polar loop (EDGE and WCDMA) transmit architectures, have been developed that minimise the present RF component count and RF component cost implications of multiband and multimode handsets. As a result, it would be reasonable to assume that RF component costs represent a declining percentage of the BOM of a modern cellular handset. However, RF costs have remained relatively stable as a percentage of the total BOM over time and are likely to remain SO. This is due to the increase in RF complexity. It might have also been expected that operator and user expectations of RF performance and functionality, for example higher average data rates would also have encouraged more highly specified RF front ends, but this has not been the case. Figure 4.14 gives an indication of the typical value split between functions in 2007, in this case in an ultralow-cost handset. We have said that the RF BOM is staying relatively constant over time as a percentage (between 7 and 10%) of the overall BOM of the phone and that this is tr