CXO

Peter Cochrane's Blog: The big mobile phone signal fail

One bar or two? Believe it or not, industry has not thought to standardise how signal is displayed

Written in a London coffee shop and dispatched to silicon.com via a mystery wi-fi connection with lots of signal.

You might think it would be an obvious thing to have but there is no adopted standard for the derivation and display of signal strength on mobile phones.

If you thought that measuring radio signal strength is easy, you'd be wrong. It is fraught with practical difficulty and uncertainty. We are dealing with electromagnetic fields that are distorted and displaced by people, things and fixed and mobile structures. And we are talking about very low energy levels.

Even a strong mobile phone signal is only of the order 10 to the power of -8 or about 1/100,000,000,000 of the power of a domestic microwave oven. And it is in a spectrum crowded by many different signal sources at higher and lower levels.

So we are inevitably always dealing with averages that offset slow and rapid variations in propagation and signal interactions. A visualisation of this complexity for a small section of the spectrum is given below as an example:

graphic depicting complexity of spectrum

Spectrum complexity is shown above
(Image credit: Peter Cochrane)

So, line up as many different mobile phones as you can muster and be prepared for surprisingly wide differences in the number of bars displayed.

In most operational equipment we have to take, for economic and size reasons, an indirect signal measure as opposed to some direct measurement involving dedicated circuitry. This indirect signal measure is often taken at a convenient processing point where a voltage level can be detected and operated on by an algorithm that gives us a simple bar display.

Sadly, industry has chosen to standardise neither the precise method nor the algorithm, and the indication displayed can vary widely from mobile to mobile.

Certainly, some mobiles are better than others, and some have serious design flaws involving attenuation by the human hand or head. In at least one case a direct physical contact between hand and antenna is possible, and this contact does no good at all and falls into the category of a serious design goof-up.

In their desire for iPod-like cases with smooth edges and sleek lines, designers have moved antennas inside the case. Spiky and telescopic protrusions are now a thing of the past. This design choice may make things look nice but it does nothing positive for performance.

Antennas have not only become invisible, they have shrunk. As a consequence, there is no doubt that performance has suffered. But those design decisions are the trade-off between an engineering ideal, customer desire, market leadership, and design constraints.

What does all this add up to? You and I experiencing dropped calls, periods of unintelligible speech, and rather more cell towers visible than we might otherwise need. But that's real-life engineering, never ideal or perfect, but it does the job.

So, as a calibration point all we can say is that one bar is not good and five bars will probably work well. Not very accurate I know but it seems to do the job for a near £5bn market that is generally satisfied.

About Peter Cochrane

Peter Cochrane is an engineer, scientist, entrepreneur, futurist and consultant. He is the former CTO and head of research at BT, with a career in telecoms and IT spanning more than 40 years.

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