Thursday, November 21, 2013

HotNets '13: Full Duplex Backscatter

Authors: Dinesh Bharadia, Kiran Raj Joshi, Sachin Katti

Can we add powerful imaging capabilities to the radios in our gadgets (e.g., smartphones), such as weapon detection, indoor imaging, motion tracking, etc? Radio transmissions produce backscatter which reflects off of objects in the environment around the transmission. This backscatter contains information about the reflectors in the environment by determining the amplitude, delay, and AoA of each reflection.

The key challenges with respect to the targeted radios are the limited available bandwidth (e.g., WiFi at 80 MHz) and dynamic range (e.g., ADCs with only 12 bits of resolution). Limited bandwidth (and therefore lower sampling rate) affects the accuracy of the estimation of the delay (and therefore distance) of the reflectors in the environment. Dynamic range affects how well the system can detect both nearby reflectors (with strong reflections) and far away reflectors (with weak reflections); the strong reflections may completely drown out the weak reflections within the digital representation of the signal.

In order to handle limited bandwidth, whereby multiple reflections can more easily appear within the same sampling interval, angle of arrival can be used to actually determine that there are two (or more) such reflections assuming that the reflectors are spatially diverse.

With respect to limited dynamic range, it is possible to perform successive estimation and cancelation of components in decreasing order of signal power. However, this cancelation must be performed prior to the ADC (and therefore in the analog signal itself) in order to allow the dynamic range to shift with respect to the signal power in the subsequently strongest signals.

In order to evaluate the cancelation method, a WARP radio is used with the transmit port split into three separate wires of different lengths (which correspond to different delays). MATLAB simulations are used to determine the overall accuracy of the backscatter approach, using up to 6 total reflectors (and up to 3 within the same sampling interval). The simulations support this approach's feasibility for a variety of imaging applications.

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Q: Where the antennas the same form factor as that in most cellphones?

A: While the form factor is the same, this technique (for AoA) required 4 antennas in the evaluation. In addition, the spacing requires higher frequencies (5 GHz, 60 GHz) to meet the form factor for smartphones due to antenna spacing requirements.

Q/A: Previous work in RF-self interference cancellation wanted to cancel all reflections, where as in this work we are interested in successively canceling each individual reflection in order to support low-power (far) reflections while using ADCs with limited accuracy.

Q: How many reflections can you detect?

A: We used up to 6 reflections, with 3 in the same sampling interval. However, we have not tested its performance as the number of reflections increases.

Q: If there are several mobile phones that are nearby, can they cooperate to perform some imaging?

A: Cooperation between devices could certainly help.