Mobile broadband for fixed use – a significant player ?

Mobile broadband is often in the news, not least because the growth in the market is quite fast. At least part of the growth may be down to users signing up to 12 or 18 month contracts in order to get a “free” laptop, printer or other gadget.

Carter’s “Digital Britain” interim report also dangles mobile broadband as at least part of the solution to the “universal” broadband conundrum for fixed customers. But how much of a contribution can it really make ?

There are two limits on the available capacity of a mobile or wireless broadband system – how many MHz of spectrum is available to the service provider and how efficient the technology is in terms of Mbits/s per MHz. OFCOM is fairly active in trying to make more MHz of spectrum available, either by taking it away from 2G operators O2 and Vodafone in the 900 MHz band or by making available new “digital dividend” bandwidth released by the switch to Digital TV, for example the 800 MHz band consultation announced today.

Note that the bandwidth in MHz here is the channel width, and not the operating frequency. A pair of 5 MHz bandwidth channels (one for download, one for upload) in the 900 MHz frequency band could be at 880-885 MHz and 925-930 MHz, for example. Currently the 900 MHz  band can only be used in the EU for 2G GSM voice/data however there are moves to allow 3G services in this band as the lower frequency gives greater range (less base stations per covered area) and better building penetration.

Developments in mobile broadband continue to offer higher headline speeds like 7.2 Mbits/s but we seldom hear much about the total capacity of the system and hence the likely average user experience.

To illustrate this point, consider current 3G mobile products operating in the 2100 MHz band. This spectrum in the UK was auctioned off and the two largest allocations were won by Three and Vodafone who each have two paired blocks of 15 MHz. In other words Three or Vodafone can each deploy a maximum of 15 MHz of bandwidth downstream in any particular location. 

So what is the capacity of 15 MHz of 3G bandwidth ? Split into three channels of 5 MHz each the real world capacity or “sector throughput” using current HSPA technology is estimated by Motorola to be 3 x 2.25 = 6.75 Mbits/s total capacity using the whole bandwidth of one of the larger players. If 10 users were to download at the same time they would see an average throughput of 675 kbits/s which is well below Carter’s touted 2 Mbits/s USO – the headline speed would be 2M or higher but it would only ever be seen by one user on a base station at once.

Moving up to HSPA+ technology with clever MIMO aerials may uplift the sector throughput at 5MHz to 3.7 Mbits/s but that is still only a total of 11 Mbits/s available to all the users in perhaps a 5km radius from a rural base station. If this is going to be of practical value then it will need to be deployed in a pretty sparsely populated area, or many more masts with restricted range will have to be used to provide the capacity to match user aspirations.

Going to 4G with LTE technology, which is just entering its test phase today, could see the sector throughput rise to 7.8 or perhaps 12 Mbits/s with a 5 MHz bandwidth, a useful improvement but still woefully inadequate for a decent number of users expecting to actually use their connection at the same time.

 I hope this illustrates the limitations of wireless / mobile broadband – there’s a limited amount of spectrum available, and you can only get so many Mbits/s of data per MHz of bandwidth. Compare and contrast this with telephone line or fibre optic systems which can deliver several Mbits/s or higher down each line at the same time without significantly compromising the capacity available to other lines. They are as different as chalk and cheese and wireless or mobile broadband is never going to deliver high speed multi-Mbits/s services to the masses, which is why you have low GB/month contracts and high charges per extra GB on mobile broadband – the capacity is very scarce so it is consequently expensive and heavy use has to be deterred.

Tags: 3G, 4G, LTE, mobile, Wireless