and achieves progressively less baseband gain. LTE takes a different approach with channel bandwidths that scale from 1.4 MHz (as a multiple of the 200 kHz GSM channel raster) to 5, 10, 15 or 20 MHz channels. Advanced LTE will take this further still with channel bonding to give per user channel bandwidths of up to 100 MHz. As with ADSL and VDSL, higher per user data rates create intersymbol interference problems and time-domain equalisation becomes computationally expensive and introduces too much delay. ISI is mitigated in LTE by dividing the carrier into orthogonal frequency subcarriers. As with VDSL, user bits can be mapped on to frequency subcarriers that have favourable channel conditions at particular moments in time. 25 3G Handset and Network Design, Belcher and Varrall, John Wiley and Son ISBN 0-471-22936-9, first published 2003. Physical Layer Connectivity We could at this point go in to an extensive explanation of how OFDM is implemented in modern mobile broadband systems, but this has already been done in very excellent depth by Harri Holma and Antti Toskala in LTE for UMTS Evolution. Rather than attempt to repeat something that has been already done very well we use the rest of this chapter to reflect on the general merits (and demits) of OFDM applied at RF frequencies and use WiFi rather than mobile broadband as an example. The specifics of realising OFDM in a mobile broadband transceiver are addressed in Chapter 5. All we need to know at the moment is that the LTE transform takes a channel that can be anything between 1.4 and 20 MHz and produces subcarriers at 15 kHz that are grouped together as 12 subcarriers to create a resource block with a bandwidth of 180 kHz, very similar to GSMs carrier bandwidth. The theory (and largely the practice) is that the high per user data rates that are achievable using wider channel bandwidths of 5 MHz or more can be delivered without high levels of ISI, which would require computationally intensive time-domain equalisation. The equalisation would also introduce a