Article - Issue 44, September 2010
Innovation Watch - Speckled mobiles
The effect: Has your mobile handset ever disconnected itself during a call as you turn your head? That tiny movement on the edge of a reception area can cause a disconnection.
The cause: Wireless signals that power mobiles experience shadows and patterns from obstructing objects just like any other radiation. But the tiny ripples in reception strength are mostly due to ‘multi-path’ effects where fragments of signal reach the handset by a multitude of routes, bouncing off buildings and trees. These signals ‘interfere’ with each other like waves to give peaks and troughs of signal strength.
Similar effects can be seen using light, which is also electromagnetic radiation but with a much shorter wavelength. Laser light characteristically shows as fine-grained brightand- dark sandy pattern called ‘laser speckle’, but speckle can also be seen in sunlight on anodised aluminium. Look closely and it is coloured, with the graininess different for each colour. The graininess ‘washes out’ under white light on rougher surfaces like paper, though with a very narrow-band laser it can still be seen.
A solution: With wireless, ‘white light’ can be simulated by using broad band radio illumination that makes the graininess wash out. A technique called ‘ultra wide band’ wireless uses this approach, but wholesale transfer to an ultra-wide-band approach would be difficult. Another idea is to spread the effective position of the handset by fitting it with several antennas – this is called ‘MIMO’ (multi-input-multi-output) by wireless engineers. This is now starting to be used, mostly for WiFi systems at present.