Various mechanisms have been developed over time to mitigate these effects. First- generation cellular systems for example used a combination of handover algorithms and power control to respond to changing channel conditions. Second-generation cellular systems had (and still have) a combination of frequency-domain (frequency-hopping) and time-domain (channel-coding) averaging techniques that are used to provide additional performance gain over and above first-generation systems. Digital compression of voice, images and video also increased the user's perception of performance gain. Third-generation systems retain all of the above techniques but add in a FFT transform at the physical layer. This has been enabled by an increase in DSP capability that has allowed us to process signals orthogonally in both the time and frequency domain. The technique is known generically as orthogonal frequency division multiple access (OFDMA). The DSP capability is needed both to perform the transform but also to deliver sufficient linearity in the RX (receive) and TX (transmit) path to preserve the AM (amplitude-modulated) characteristics of the signal waveform that has an intrinsically higher peak-to-average ratio than other waveforms. OFDM techniques are used in guided media; for example on copper access ADSL and VDSL lines. OFDMA techniques are used in unguided media, for example terrestrial broadcasting, WiFi and mobile broadband to deliver higher data rates and throughput consistency. The consistency comes from the ability of OFDMA to mitigate or at least average out the slow and fast fading effects of the radio (or to a lesser extent free-space optical) channel. OFDMA Making Telecoms Work flattens the difference between guided and unguided media. The technique is particularly effective in systems with a return signalling channel that includes mobile broadband. Because unguided media offers more deployment flexibility and is faster to deploy and because it offers more user functionality it has captured more traffic than the pu