Q.What is spectrum and why do we need it?
A.Spectrum is the radio waves that mobile signals are broadcast over. It comes in different frequencies which are useful for different tasks – some work well for voice calls and other parts for data usage such as mobile video. It is increasingly important for operators to own spectrum that can handle data traffic thanks to the explosion in smartphones.

Q.Who will the auction Benefit most?
A.The government owns the spectrum so UK PLC stands to gain through the sale. Last time around the government netted £22.5bn, although this time it is likely to be closer to £4bn.

Q.What outcome does ofcom want?
A.The watchdog is committed to maintaining four players in the market – Vodafone, O2, Everything Everywhere and 3 – which it says will ensure competition. The smallest operator, 3, is vulnerable, so Ofcom will ensure it has its share of the 4G spectrum. It is also tasked with making sure one operator does not come out of the auction with significantly more spectrum than its rivals. It has previously faced criticism for this.