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The supply of spectrum in any given area is not unlimited and no two wireless signals can operate on the same frequency. Should this happen both would interfere with each other and prevent communication.
Governments worldwide appoint regulatory authorities, like the Federal Communications Commission (FCC) in the US, to manage the spectrum in their given country.
These authorities allocate spectrum for different uses, such as TV broadcasting, radio, satellite broadcasting, marine communications, radio-navigation, fixed satellite and space research.
What spectrum is used by mobile operators?
Generally speaking spectrum in the middle of the Ultra High Frequency (UHF) band is used for mobile communications but which exact frequency bands are allocated varies on a country-by-country basis.
In the US for example, the main spectrum bands in use are the 700MHz, 800MHz, 1700MHz, 1950MHz and 2100MHz bands. While in the UK 900MHz, 1800MHz and 2100MHz are the main bands.
From a mobile operator’s perspective, lower frequencies are preferred as they allow radio signal to propagate over a long distance and provide better in-building penetration. This means less capital investment is required as fewer base stations are needed.
Spectrum is typically acquired from auctions conducted by the country’s regulator. These grant them a licence to offer a specific type of mobile service (eg, 2G or 3G) on blocks of spectrum.
Mobile spectrum is auctioned in two types, paired and unpaired. Paired spectrum tends to be the most common and comes in matched blocks of two. It enables uplink and downlink transmissions to be carried out on separate frequencies and is used in Frequency Division Duplexing (FDD) systems. Unpaired spectrum carries out uplink and downlink on the same frequency and is used in Time Division Duplexing (TDD) systems.
In some countries, operators may exchange or sell their spectrum to each other, with regulatory permission, as means of procuring frequencies. Another practice known as spectrum refarming can also be performed by operators to change the usage of a frequency (eg, from 2G to 4G).
Why has spectrum been in the headlines so often?
Spectrum has become a hot topic, particularly in the US, as a result of increasing demand for mobile broadband by consumers using smartphones and tablets. It is estimated that the average smartphone uses over 20 times more data than traditional mobile phones and this explosion in demand is placing increasing strain on mobile operators’ spectrum holdings.
Operators have been calling for more frequencies to be allocated to mobile broadband in order to cope with this tide of user demand. But in the meantime there has been a scramble to acquire the limited amounted of mobile broadband spectrum still available.
One of the highest profile deals so far has been Verizon’s bid in the US to acquire AWS spectrum (1710-1755 and 2110-2155MHz) from the SpectrumCo consortium. While rival AT&T, which failed last year in its bid to acquire T-Mobile USA for its spectrum holdings, has been procuring frequencies from the likes of NextWave Wireless and reportedly ComCast and Horizon-Wi-Com.
Where can new spectrum come from?
There are several areas being explored to boost the allocation for mobile broadband, including spectrum re-allocation and spectrum sharing.
Spectrum re-allocation is where spectrum that was allocated for one service is re-allocated, with regulatory permission, for another purpose. Satellite frequencies are one of the most common bands of spectrum to be eyed for conversion for mobile broadband services. In the US, satellite TV provider Dish Network is awaiting FCC approval for its plans to use the S-band spectrum (2-4GHz) for LTE-advanced mobile broadband services.
Another potential source of frequencies in the US is Wireless Communications Spectrum (2.3GHz), some of which AT&T is looking to acquire from NextWave Wireless. WCS spectrum was first auctioned in 1997 but cannot be used for mobile broadband due to restrictions designed to avoid possible interference with satellite radio transmissions in adjacent spectrum bands.
AT&T and US satellite radio operator Sirius XM have proposed to the FCC that WCS can be used for mobile broadband without interfering with satellite radio spectrum and are awaiting a decision from the regulator.
Research firm Current Analysis predicts that a practice known as spectrum sharing will become inevitable due to a lack of spectrum for mobile broadband. UK regulator Ofcom was among the first to propose the idea of auctioning off spectrum for small cells on a shared spectrum basis and other countries are also studying it as a solution to mobile broadband demand.
It is thought that technology being used to drive white space spectrum technology, which operates on the gaps between television channels, could be used in a sharing capacity. White space devices utilise a database to move wireless signals from one frequency to another to avoid interference with other signals.
