Why ultrawideband needs Bluetooth

Wireless USB dongles and hubs appearing this year will be the first products to implement ultrawideband (UWB) links. The government is committed to amending legislation to allow their use by 21 August.

They will help to reduce cable tangles, but data rates are likely to be significantly lower than the targeted 480Mbits/sec. UWB on devices such as digicams and PDAs, on which power consumption will be critical, seems likely eventually to exploit a new partnership with frugal Bluetooth.

UWB vendors are reticent about power consumption. But a paper by Tom Siep, head of global standards at Bluetooth pioneer Cambridge Silicon Radio (CSR), says the transmit drain of first-generation devices is about a half-ampere (500mA) – and that even in a low-power wait state, which can last hours, they draw 20mA.

UWB’s power drain when receiving is even higher, says Siep.

Bluetooth uses 11mA transmitting and 0.18mA waiting. But it can transmit at only 3Mbits/sec. Siep calculates that it would drain 16mA hours (mAh) sending a 2GB file, compared with just 6.11mAh on a 400Mbit UWB link.

However, if you factor in a 12-hour wait, UWB takes more than 13 times the power – 246mAh compared with Bluetooth’s 18.5mAh. By allowing Bluetooth to do the waiting and setup, calling in UWB for the data transfer, the total drain would be just 7.72mAh (for full calculations, see paper at http://tinyurl.com/yt8fd8).

It is this combination that CSR and others are developing. Bluetooth has the added advantage of having device discovery and pairing features built in.

One possible snag is that it may be more efficient to send a movie as a file rather than to stream it. “Streaming is by far the more difficult communications task. Lots of things get easier – including power budgeting – when the task is not time-critical,” Siep said.