A Fully Integrated, 291 pJ/bit UWB Dual-Mode Transceiver for cm-Range Wireless Interconnects
Gambini, S.; Crossley, J.; Alon, E.; Rabaey, J.H.;
Berkeley Wireless Res. Center, Berkeley, CA, USA
This paper appears in: Solid-State Circuits, IEEE Journal of
Issue Date : March 2012
Volume : 47 , Issue:3
On page(s): 586 - 598
ISSN : 0018-9200
References Cited: 22
INSPEC Accession Number: 12543295
Digital Object Identifier : 10.1109/JSSC.2011.2177690
Date of Publication : 03 January 2012
Date of Current Version : 21 February 2012
Sponsored by : IEEE Solid-State Circuits Society
ABSTRACT
We present an ultra-wideband transceiver designed for ultra-low-power communication at sub-10 cm range. The transceiver operates at a 5.6 GHz carrier frequency, chosen to minimize path loss when using a 1 cm2 antenna, and can switch its architecture between self-synchronous rectification and low-IF to adapt its power consumption to the channel characteristic in real time. A low-power digital circuit exploits redundancy in the modulation scheme to provide a real-time BER estimate used to close the mode-switching loop. Implemented in 65 nm CMOS, the transceiver consumes 25 μW when transmitting and 245 μW when receiving in low-power mode, plus 45 μW in the clock generator, and only requires an external antenna. Dual-mode operation allows range extension and mitigates interference.
A few comments:
Interesting concept to use spatially the scarce wireless spectrum. This radio is targeted for low power networks which will typically bounce around small bits of data gathered from sensors etc. The idea is to put a temperature sensor bead/tag somewhere and forget about it for 2-3 years. Intermittently, it will wake up, gather data and send a probably 10-bit message to another transceiver in close proximity and through many such "bounces" it will make its way to the central node which will collect all data and collate.
The power dissipation needs to be very low to allow for a transceiver like this to live on a 1cm X 1cm X 0.5cm volume of battery and last for 2-3 years while sending data every ms or so.... Of course, the sensor itself will have its own power-dissipation and that's not taken into account.
Also, the antenna is a paste-on type with an area of 1cm X 1cm. The targeted range is no more than 5cm. They end up getting 1-2 cm in the low-power mode and in the high-power mode they get 3cm. It might be useful to define a Figure of merit which takes into account the range reached? Without it, 290pJ/bit number doesn't really make sense...
A few criticisms:
- It has been argued that we can potentially choose any frequency that gives us a channel response with minimum channel loss. Given that spatial reuse is being targeted, you could argue that you don't care if you step on other radio's shoes. Not very sure about that, even though FC requirements are met.... For example if you have a 802.11n WLAN around, it would'nt be too happy with a temperature sensor around at the same frequency going off at unpredictable intervals....?
- Conversely, what about being blocked by the ambient WLAN signal? They have presented some results but no further system analysis has been presented.
- How do multiple transceivers communicate with each other? (Probably that's another paper?)
- What do you do with a 1-cm range radio?
- How do you get frequency control in place without external components? A Tx and an Rx talking at different frequencies are'nt much use...
I liked the way they have gone about discussing the system constraints. A few good items to remember:
- They have argued that they can use whatever band-width they need. If we accept this, it is quite nice to see, how they have traded off power dissipation ofor band-width. They have used an UWB Tx with a very low duty cycle signaling of data. This allows them to get rid of the high frequency LO which would be expensive power-wise.
- Also, due to this, they have dumped the RF-PLL and proved in another paper that frequency drift does not matter much if it is limited to below the signal band-width.
- Now the Tx is going to be a bursty transmitter which will send a 2ns burst every Tb to signal no inversion of signal. If an inversion is to be signaled, the delay becomes either Tb/2 or 3/2.Tb. During the other times, it can be turned off to save power.
- Due to the coding, we can only have patterns like Tb/2, Tb, ..., Tb, 3Tb/2 or 3Tb/2, Tb, ..., Tb, Tb/2. If we have something like Tb/2, Tb, ..., Tb, Tb/2 or 3Tb/2, Tb, ..., Tb, 3Tb/2, then we know a bit error has happened and this gives us a handy way to figure out the channel quality.
- The Rx is very interesting. Given the channel and coding chosen, it will be the more power hungry circuit element
- An I/Q heterodyne Rx is too expensive for the current use, so instead the incoming RF is mixed with an LO which leads to an IF frequency such that2(pi)f_IF.T_p > pi/2. This will ensure that due to the frequency mismatch, there will be a peak sometime in the bit period which will be detected by the base-band. (Is this applicable only to burst-mode signals of this kind???)
- Normally they use a diode-based envelope detector for signal demodulation, but based on the channel quality, they can go to the mixer based Rx defined in (a) above.
- The timing recovery is absolutely critical and this is the only circuit that stays on all the time. They have explained the timing recovery somewhat but it requires deeper reading/modeling to give really critical comments here.
Overall, while the idea is good and the RF system work is top-class, one does ask as to whether something like this is really useful and how it will work at the higher system level what with issues of inter-operability, hand-shaking and data generation and use not completely clear.
Key take-aways:
- Trade-off band-width for power
- Use of effective coding scheme to get an idea of channel state
- Use of an interesting demodulation scheme in place of an I/Q heterodyne.
