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3G as an alternative to home broadband?


In a desire to develop competition in broadband provision to the home, many have hoped that wireless might one day evolve sufficiently to become a viable alternative. But the technologists were generally sceptical. Wireless, they noted, did not have the capacity and tended to lag behind the speed of fixed connections. Previous attempts to deliver wireless broadband, from Ionica to the more recent UK Broadband, have all either failed, or remained small-scale activities. Yet suddenly, almost out of nowhere, 3G data card sales are rocketing and for many wireless does appear to be becoming a viable alternative to fixed line broadband. What is going on here?


The answer is that 3G is a viable alternative for some – in particular those with relatively low total data volumes and who do not want a fixed line to their home. On the data side most 3G cards have a limit to the data per month unlike home broadband which is typically unlimited (albeit with some “fair usage” clause). So, 3 are currently offering 5GBytes/month for £15 on contract with 10p/MByte additional charge if this limit is exceeded. That equates to around 160Mbytes/day. For downloading emails and web surfing that is probably plenty for most, but audio and video streaming could quickly eat through that (audio streaming at 100kbits/s equates to around 45Mbytes/hour, video streaming at 500kbits/s would use up the daily entitlement in 40 minutes). As the BBC iPlayer becomes ever more popular, many predict a rapid increase in the average data consumption to the home. (In passing, it is worth noting that 160Mbytes equates to around 1,800 voice call minutes per month. Current offers are £20 for 500 voice call minutes so data is being offered at around a quarter of the voice call price making VoIP attractive.)


The other question is whether the household wants to have a fixed phone line. If they decide they do, and pay the line rental, then the additional cost of broadband on this line is typically well below £15 and provides higher data rates, unlimited volumes and often mostly free calls (except to premium rate and overseas numbers). For such a household, mobile broadband is not particularly attractive (except as a way of accessing the network when on the move). However, many people do not want a phone line. Those who are renting, students and those who spend little time at home often do not want to feel tied to a fixed line. Such individuals already make use of mobile as their only means of voice communications and extending this to mobile data clearly looks sensible.


Finally, there is the question of network capacity. Cellular networks have enhanced their capacity with HSDPA which broadly enables higher data rates for those with good coverage while excluding those with very poor coverage. This makes it very difficult to analytically derive cell capacity. Instead, Qualcomm and others have modelled and measured typical scenarios and concluded that data rates in the region of 1.2 – 1.5Mbits/s per cell can be supported. So if all the data users tried to access their 160Mbytes between, say, 8pm and 10pm, the cell could, at best support around 8 subscribers per carrier. If voice traffic is also to be carried then this would be lower. Assuming around 10,000 cells covering the UK, each with 3 sectors, then the total subscriber numbers per operator per carrier would be in the region of 240,000. Of course, if users actually averaged less than their allotted allocation per day then more could be supported – as is likely the case. So perhaps up to a million users per operator might be feasible, especially if additional spectrum is acquired allowing more carriers to be deployed, giving perhaps as many as five million across all operators.


So we can conclude that cellular is not a viable replacement for broadband for all – it does not have the capacity for more than around 25% of UK households. It is also only attractive to a certain class of user who would typically not have a fixed line to the home and it may become less attractive if video streaming becomes the norm. But for a substantial subset of the market it looks ideal.

A measurement device in your pocket

The recent news item that a US university had used mobiles to track the movement of thousands of individuals generated a lot of interest in the press. The study, reported in Nature, took anonymised data from a cellular operator as to the movements of their users and concluded from this that humans are creatures of habit, travelling the same routes to the same locations most of the time. This may not come as a great surprise to anyone who commutes to work each day and might not seem to be a significant advance for science but it does potentially give some indication of what more may be to come.


Nokia Eco-sensor concept

A Nokia Eco-Sensor Concept Phone (more)


Gathering data of most sorts – for example on the air quality throughout a country – can be very expensive. But costs can be much reduced, and the volume of data massively increased, by harnessing the daily movements of millions of mobile phones carried everywhere by most of us as part of our daily travels. The mobile is unique in being a device that either knows its own location, or which the network can locate, and which can input, process and transmit data. With our example of air quality measurements we could imagine clipping a small sensor onto the bottom of the mobile phones of volunteers. This might periodically sample the air quality and then the mobile might send a short data message back to the network. The network would then add the cell location to the message and pass this onto the agency conducting the trial. For very little cost, detailed information which was frequently being updated could be generated.


The list of possibilities is likely to be extensive. Ofcom is using around 50 mobile phones with Wi-Fi capabilities to test the Wi-Fi data rates available throughout London. Phones with microphones could test noise levels, deduce what TV programme their owner was watching to derive audience research data and much more. Phones docked in cars with vibration sensors could send back information on road quality and traffic speeds could be estimated from their position. The position of entrants out on a course for a cycling or running event could be tracked by the organisers to help them run the event smoothly. It seems likely that many specialists would like measurements of some sort in their specific areas of interest and many hobbies would benefit from more information.


This sort of application might raise privacy concerns and in some cases might require additional hardware or software to be added to the phone. But many users might be willing to agree to help if they thought that the information would benefit them – perhaps by leading to a better environment. In the near future you might leave your phone on for more reasons that just to receive incoming calls.

Hexagons in 3D – Is it time to update the defining image of the cellular industry?

If you had to pick a single iconic image to represent  the world of the mobile operator , it would have to be the hexagon. Much used in the early marketing literature of operators, the hexagon provided a simple representation of the area covered by a base station, and helped to illustrate how a limited set of frequencies could be reused in order to serve an unlimited number of users. This is the central ‘magic’ of cellular networks.


Hexagons define frequency re-use in outdoor macrocellular networks

 Hexagons defining outdoor cellular coverage areas


More formally, a hexagon defines the region which contains points which are closer to one base station site than to any other  if the base stations are arranged in a regular grid.  Assuming uniform wave propagation conditions, it therefore shows the coverage area of the base station in the centre of the hexagon, i.e. locations where a mobile would receive and deliver a stronger signal to this base station than to any other. The hexagon is a special case of a Voronoi polygon, which contains the closest locations to any random selection of points.



 Voronoi Polygons for Random Points (Base Station Sites)


Real-world propagation conditions are never like that, of course; in practice the coverage area of a given base station is very irregular indeed. Nevertheless, the hexagon provides a useful idealisation – its six sides give an indication of the  number of sources of interference which need to be considered when working out the total capacity of a basic cellular system.


So what’s new? Today, cellular systems are undergoing a period of renewal. Well over two-thirds of voice traffic occurs inside buildings and it’s likely that data services will occur even more inside buildings. This means that mobile networks need to do more than provide coverage to a 2D plane – they need to consider the third dimension. Re-use of radio resources vertically is inevitable, whether using Wi-Fi access points or femtocells. A 2D map can be coloured without reusing colours in adjacent shapes using just four colours, so 4 frequencies  can be reused without limit to avoid interference between adjacent cells. In 3D, the number rises greatly adding to the complexity. [Note: I haven’t yet been able to find the 3D equivalent of the four-colour theorem – I’d be fascinated to hear if anyone knows the answer]


So the question arises: what is the equivalent of the hexagon in three dimensions? In the jargon, we are seeking a space-filling polyhedron.  There exist various exotic candidate shapes (anyone for rhombo-hexagonal dodecahedra?). However, we don’t simply want a polyhedron which fills the space, but one which corresponds to a 3D version of the Voronoi polygon, enclosing the points closest to the antennas.


If the antennas in a building are on a regular grid across each floor of the building, with antennas directly above and below each other on successive floors, then the Voronoi polygon is simply the humble cube.



Cubic Honeycomb

Inside a cubic lattice


If the antennas on successive floors are offset between floors, so that an antenna is at the midpoint of its four nearest neighbours on the floor above, then a rather more interesting shape results. This arrangement is known to crystallographers as a body-centred cubic arrangement, for which the Voronoi polygon is the truncated octahedron. This has 8 regular hexagonal faces, 6 regular square faces, 24 vertices and 36 edges.




 The Truncated Octahedron



So there are 14 adjacent interference-creating cells surrounding each antenna:


A Lattice of Truncated Octahedra



Finally, we can contemplate arranging the antennas in a face-centred cubic pattern. The Voronoi polygon in this case is the rhombic dodecahedron, with 12 rhombic faces.




Rhombic Dodecahedron


The lattice in this case looks like this:

Lattice of rhombic dodecahedra



Of course, these patterns don’t relate closely to the reality of in-building propagation any more closely than the real world of outdoor macrocell planning. In particular, the high propagation losses involved in penetration through walls and floors will distort the relevant shapes hugely.


Nevertheless, doesn’t an industry which has changed so much deserve a new defining image? Perhaps the truncated octahedron could fit the bill !


The Truncated Octahedron