Are we paying a power price for wireless?

-April 01, 2014

Wireless is everywhere, no question about that. Whether it is to recharge your mobile device via a wireless pad embodying one of the several competing standards, or a basic wireless link such as ZigBee or Bluetooth in their various guises, or even a Wi-Fi IEEE 802.11.x link in a small network, the lure of life without wires (meaning cables and connectors) seems pretty attractive.

And why not go wireless? Those wired links, whether for power transfer or data connectivity, represent another set of physical openings into a product's enclosure, headaches with lost or twisted cables, loss of freedom to roam, and reduced flexibility.

But as engineers, it's important to be honest about the cost of bringing forth the wireless life, and deciding where and when that cost is worth the benefits. In general, a wireless connection - whether to supply operating/recharging power or provide a data link - takes more circuitry on both ends than a wired one. Even if that circuitry's cost in BOM and footprint is of little or no consequence, you have to consider the cost in power dissipation.

But that's only part of the power differential between wired and wireless: there's also source-to-receiver link efficiency. It's very unlikely that a wireless recharging link can be as efficient as a piece of wire between the recharging module and a smartphone, and even wireless links with deep sleep states have tricky wake-up power sequencing and associated dissipation.

This bike speedometer seemed to me an unnecessary application for wireless - until I installed it and saw that even a short, direct, low-cost wired link can benefit from an upgrade to wireless (photo from Cateye Co., Ltd.).

Sometimes in our rush to jump on the latest trend, we gloss over the tradeoff that every choice forces. It may be that the virtues of wireless are well worth it compared to any negatives, and consumers may find these benefits very appealing and the downsides quite acceptable. That's fine; after all, no one uses a wired TV remote control anymore as the wireless IR-based one makes so much more sense.

But I find it ironic that with all the market emphasis and associated design effort concerning low-power design, whether for "green" reasons or to increase run time, we'd opt blindly for a recharge or data link approach which may undercut all of those efforts in many situations. Don't misunderstand me, I'm not saying "forget about wireless except in special cases;" all I am saying is think about where it makes sense in the context of the situation.

In fact, I am a convert to wireless where I never thought it made sense at first - and was I wrong. I had a basic bike speedometer with the sensor on the fork and the readout on the handlebar, connected by a short length of wire. It seemed to me that wireless in such a short-distance, well-defined situation was unneeded and overkill. But that short length of wire was a pain to route along the headset and frame, was always getting snagged when riding or putting the bike in the car, and was a general nuisance.

When that speedometer unit finally failed, I replaced it with a wireless one which cost only about $15 more than its wired equivalent. Long story short: bicycling life has been a pleasure since that change, even though I now have to put batteries in both the fork's sender and the handlebar readout, rather than just in a handlebar unit alone.

What's your view on wired versus wireless for charging and for short-distance data links? Do you have mixed or strong feelings?

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