In this updated lecture for UK-based members of the Institute of Electrical Engineers, David Brunnen and Don Pearce tackle the practicalities of achieving high spectral efficiency and point to the emergence of new standards such as HC-SDMA.

If we look back to the origins of wireless communication we can gain a sense of how much scientific endeavour has enabled us to exploit and improve these technologies.  Putting the past century into perspective also strengthens our view of the future –  pushing back the horizons of development and enabling further generations of wireless wizardry.

When wireless first appeared it must indeed have seemed magical – but by today’s standards it was hugely inefficient.  When Marconi conducted his first transmissions in 1895, the energy from his spark gap transmitter occupied a huge band of radio spectrum.  By 1901, the first Transatlantic transmission blanketed an area of millions of square miles, and was capable of sending a pitifully small amount of information – in Samuel Morse’s binary  code of dots and dashes.

Using that 1901 technology, only about 50 separate conversations, whether broadcast or personal communication, could be accommodated on the surface of the earth.  In theory we could now conduct a million conversations in the usable radio  spectrum in just one location, assuming that the entire usable spectrum was used for such communications.  This is of course purely hypothetical - all of the spectrum is simply not available - but it illustrates the contrast with Marconi's magic.  Should we now be amazed or surprised at current capabilities?  What should we now expect in years to come?

Looking at the improvement in the way we use the spectrum we can compare the number of "conversations" (voice or data) that can theoretically be conducted over a given area in all of the useful radio spectrum – and it turns out, according to Dr Marty Cooper, that this number has doubled every two-and-a-half years for the past 104 years.  The cumulative improvement in the effectiveness of personal communication total spectrum utilization has been over a trillion times in the last 90 years, and a million times in the last 45 years.

To understand the scope for further improvement it is worth pausing to consider how the improvements of the past 45 years have been achieved. Broadly the sources of enhanced efficiency in spectral utilization can be classified under four headings.

Firstly there is more usable spectrum – or rather we have developed technologies to work at ever higher frequencies. The earliest wireless communications used Long Waves – very low frequencies – that bounced happily around the world’s atmosphere – and now we can utilise microwave and infra-red frequencies that are increasingly laser-sharp in their directness between two points.

Secondly we should acknowledge the contribution of Frequency Division – the ability to accurately and practically divide the spectrum into ever more narrowly defined slices – reducing interference outside of the intended frequency and removing the approximations of ‘tuning in’.

Thirdly we have gained from a range of Modulation techniques – FM, Time Division Multiplexing, SSB and the use of ‘spread spectrum’ – the channel hopping antics that we only ever notice on our mobile phones when it goes awry.

Of the 1 million times improvement over the past 45 years we can allow that say 25 times comes from being able to play with more spectrum. We can credit Frequency Division and the various Modulation techniques each with a further 5 times improvement in the number of data or voice conversations.

This still leaves however a need to explain the remaining 1600 times improvement to multiply up to make the million – and this comes quite simply from Spatial Division – our ability to use the spectrum across smaller and smaller geographic patches and thus to be able to re-use that spectrum a relatively short distance away. This, of course, is most obvious in the early, some would say primitive, cellular systems that have become endemic in many parts of the world.

This notion of re-using spectrum is, for the future of wireless communications, even greater than reflected in these figures. We are unlikely to see further gains from Frequency Division and Modulation techniques. We are up against fundamentals such as Shannon’s Law – the limitation on information that can be carried in a given bandwidth with a given signal-to-noise ratio. And barely a day goes by without a BBC Radio 4 presenter apologising for the sound quality of a mobile phone link.

But, there are no such limitations on the spatial re-use of radio spectrum - and progress in this field is evident not least from the emergence of new Standards such as the recently-approved HC-SDMA – High Capacity Spatial Division Multiple Access – in the evolution from analogue to digital and via various Time, Frequency and Code Division techniques.   If we can create a reliable broadband wireless connection between any two points – and we defined those points as being separated by only a few feet – the potential, again in theory, exists to increase the effectiveness of spectrum use by 10 million times over today's capabilities.  And there is no greater challenge than to resolve the old trade-off between delivery of true broadband capacity and provision of full wide-area mobility.

This paper, first given as a lecture to the IEE North Hampshire Branch in 2004 and updated in 2006 to reflect recent developments, explores the challenge and identifies the emerging technologies – those now entering public service – that can take us way beyond the limits and inhibitions of current systems and address the new realisation that spectrum, as a scarce resource, should be conserved and utilized with the maximum possible efficiency if we are not to deprive future generations of continuing improvements in wireless communications.

The paper highlights both the challenges and the opportunities that surround the complex world of wireless communications. It does however say little of the dramatic impact that this type of technology will have on everyday business operations, productivity, convenience and the liberating effect of being unwired but never disconnected. The development of these technologies is a fascinating blend of art and science – of innovative endeavour and the risks that face those who dare to make a difference. The Pioneer Project in Belfast captures this spirit – seeking to communicate the reality of new wireless broadband technologies to audiences that may not naturally question ‘the way we do things around here’.

It also underscores a curious facet of our supposedly globally interconnected information society – with its implied free-flow of knowledge and understanding.   The technology being reviewed in Belfast today as ‘pioneering’ is already in everyday commercial use on the far side of the world.  It is fully-conformant with the HC-SDMA  ATIS/ANSI Standard.  It is well-positioned to fit within the emergent IEEE802.20 standard and has been developed in the USA over the past 7 years partly as a next step beyond an earler technology, also virtually unknown in Europe, that is already delivering a portable wideband service to more than 50 million customers through over 300,000 base stations across China and Japan.

The development of technologies for mobile wireless broadband communication, and the art of communicating that vision around the world, both represent challenges that are now being addressed. The authors hope that this paper will help raise awareness of the urgent need for policies that encourage the highest possible spectral efficiency and the conservation of that scarce resource.

A full pdf version of the entire lecture is available on request.

© ABFL Groupe Intellex 2004 and 2006
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