Re: [Discuss-gnuradio] Ranging (measure distance) between two radios

[Kunal Kandekar]

I think it is going to be a challenging project. Note that I only happen to know a bit about ranging, but I have no experience at all implementing this on GNU Radio/USRP, so I may be off-base at some points.

Your basic idea (correlating against a PN sequence) could work well as a first step (detecting time of arrival, but it is only one component of what would be needed. Time of arrival alone will not give you the distance without the precise time of transmission.

Your options are Time of Arrival (ToA)/Time Difference of Arrival (TDoA), Angle of Arrival (AoA), Received Signal Strength (RSS)-based distance estimation, or Two-Way ToA/Ranging (TW-ToA). It seems to me that you essentially have to use TW-ToA, basically measuring the "time of flight" of the signal to and from both radios like a ping. You cannot use ToA or TDoA because they assume the time of transmission is precisely known, and you would not have sufficiently synchronized clocks. (Unless you use the same clock reference for both USRPs, in which case its not really practical for ranging since both USRPs are basically stuck next to each other.) For AoA you would need directional antennas, and it does not give range anyway. For RSS-based distance estimation, you would need a lot of training data to correlate RSS readings with distances.

So suppose you need to measure the time difference in both directions, i.e. radio A transmits at T1, radio B detects and responds, radio A detects the response at T2, and range ~= c*(T2 - T1)/2. This is where I think you will have complications... the processing delay introduced by GNU Radio while detecting and responding will lead to severe inaccuracies, because at the speed of light even a fraction of a microsecond results in an error of dozens of meters. Using in-band signaling and transmit and receive timestamps could alleviate this problem... However, each USRP should know what the processing delay is between receiving a ping and responding to it. Maybe it can be kept constant, so that one USRP does not need to inform the other one how much processing time was spent? Just a thought.

As for references, I think it may be worthwhile to look at IEEE 802.15.4a. It defines a precision wireless ranging protocol using TW-ToA, to an error of about ~1m. It may be helpful to look at that. Googling for "IEEE 802.15.4a ranging" turned up a few interesting papers. The MERL report looked pretty good.

Kunal

On Tue, Jun 7, 2011 at 10:28 AM, Mario Ruz <address@hidden> wrote:

Hello,

This is my first post in the community. I have been thinking about doing this question, but after having reviewed some of the theory with digital signal processing, usrp architecture and some approaches for positioning/raning, I think I need to ask to the expert community, mainly to find a line of work and to know the restrictions for the application we want to carry out.

I would like to measure the distance between two radios at short distances (we have two UN-210 and two XCVR2450 or 1 WBX and 1 DBSRX2, let's say to 1 m, 2m....up to for example 20 meters), starting from off-line experiments and finishing with real time measures in line of sight. My questions are:

- What would be, from your experience, the best approach to do this (transmitted signal and digital processing in the receiver part), what accuracy should I expect in line of sight?

I am looking mainly for a starting point, the signal structure to transmit and the receiver structure. For example, one of the thoughts I have is a correlation in the receiver of a pseudorandom (spread spectrum) transmited signal, this approach could serve to measure time of arrival by measuring the delay of the pseudo-noise code signal, when comparing it with an exact copy at the receiver. Another approach I have seen is the estimation of the Channel impulse response (CIR), that maybe (as far as i know) could serve also for estimating distances. I have seen that there is some code to do a correlation inside the FPGA of the USRP1.

So far my references are (which maybe can be helpul for those who are working in the same field):

- "Advancing Wireless Link Signatures for Location Distinction". Here, they do not carry out ranging/positioning, but a change in the transmitter position is detected. In my case (in the case ranging is not possible), to detect if a distance is crossed would be my objective  (e.g. the transmitter go closer than 1 meter to the receiver).

- "RF Ranging for Location Awareness". In this dissertation they use an arquitecture similar to the USRP hardware.

As you can see what I am looking for is for some sort of radar/ranging application and some starting point. Also, my questions are very generic for now and I apologize for it, mainly because I am still looking how to focus the problem.

Then, any help (references, similar work done) will be very useful.

Kind regards,

Mario

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