ursts on demand (we return to this topic again in Chapter 5). Table 2.14 Battery density comparisons Ni-Cads Lithium Ion Zinc Air Lithium Thin Film Wh/kg Making Telecoms Work Additionally, most of the techniques required to achieve high per user peak data rates reduce DC to RF conversion efficiency because they require more linearity. This applies particularly to RF power amplifiers. LTE mitigates this to some extent by deploying a single- carrier implementation of OFDM on the uplink. This reduces the amplitude of the modulated waveform but requires a discrete Fourier transform and an inverse fast Fourier transform on the transmit path and a fast Fourier transform, a maximum-likelihood sequence equaliser and inverse discrete Fourier transform to be performed in the base station receive path. It moves the agony around rather than removing it. 2.14 Chapter Summary All physical layer options have benefited from improved component performance combined with an increase in DSP capability. This has allowed for a wider range of frequencies to be used more aggressively both in the optical and RF domain and over copper cable and twisted-pair networks and peak data rates have therefore largely increased in parallel for all delivery options. Fibre clearly offers the most bandwidth but is only economic when that bandwidth can be shared amongst multiple users or if a user has relatively extreme bandwidth requirements and the willingness to pay for that bandwidth. Copper still has a considerable way to go in terms of peak data rate capability and has the advantage of using a delivery medium whose cost has been thoroughly amortised. Wireless at RF frequencies is not bandwidth limited and can potentially match cable and copper delivery bandwidth but only if DC to RF and RF to DC conversion efficiency can be improved as a route to realising acceptable data duty cycles (time between battery recharge). Joules per bit rather than bits per Hz will be the big challenge going forward. The options for meeting this particular challenge ar