[OpenTRV-dev] Mobile phone detector

[Kevin Wood]

Hi All,

As promised, a few thoughts:

Mobile phones are also battery critical devices that like to sleep as
often as possible. Making 2 such devices detect each other is always going
to be a challenge!

We have quite a few technologies operating at a few different frequencies
in the UK:

GSM/GPRS:
880 - 915 MHz
1710 - 1785 MHz

WCDMA
1920 - 1980 MHz
Some in the GSM bands too

LTE
832 – 862 MHz
1710 – 1785 MHz
2500 – 2570 MHz
3400 - 3800 MHz (TDD)

The above are uplink frequencies (mobile TX) but there will be adjacent
downlink bands where cell sites will be constantly transmitting. Places
like bus stops are a popular location for cell sites, so discriminating
between them and the mobile stations might be a challenge.

With TDD network configurations, cell site transmissions are on the same
channel as mobile transmissions!

What is transmitted is too complex to even try to demodulate, so a basic
rf detector is about all you can do to detect presence.

Such a simple detector is prone to false triggering from any other radio
signal it receives. About the only defence we have is to make it frequency
selective around one or more of the above bands.

During use (speech call or internet activity) the mobile will regularly
transmit. When idle, the timer that determines when it "phones home" is
set in 6 minute intervals, and a typical value would probably result in a
timer running for an hour or two between updates, so an idle phone is not
easy to detect! Especially if you don't intend to sample 100% of the time.

If you're looking for a reasonably strong field strength, the detector
needn't consume any battery power. A simple diode detector fed from a
resonant antenna and tuned circuit could just wake the CPU using an
analogue comparator channel when it "sees" RF.

But! Another issue is that power control of a mobile's transmission is
very tight, again, because battery life of mobile is critical, and also
because, in WCDMA and LTE technologies, received signals for each device
must be as close to equal strength as possible at the cell site in order
that one mobile doesn't swamp the signal from others. For this reason, if
a cell site is close, we will have a double whammy of a strong signal from
the cell site to reject, and low signals from any mobiles transmitting to
it.

That's it for this "brain dump". Hopefully there's some useful stuff there!


Best Regards


Kevin


> Fab, thanks!
>
> All as Deniz says, and with the circuitry to be capable of running on at
> most tens of microwatts average (we might be able to sample for a few
> seconds every few minutes) at ~2.4V to work nicely with some variant of
> our V0p2 board running from 2xAA NiMH or other similar low-power
> microcontroller.
>
> (We could even harvest a small amount of power to inject back into
> batteries or a supercap as a secondary consideration!)
>
> Rgds
>
> Damon
>
>
>> On 8 Oct 2015, at 22:16, Kevin Wood <kevin at the-wood-family.com> wrote:
>>
>> Hi guys,
>>
>> Currently on holiday but my day job is developing systems to test mobile
>> phones.
>>
>> I'll give this some thought.
>>
>> Kevin
>>
>> Hi All,
>>>
>>> Damon has asked me to do a bit of research into using a mobile phone
>>> detector as a presence sensor for the bus shelters.
>>>
>>> The basic idea is an antenna, and an amplifier/filter tuned to the
>>> appropriate frequency. Additionally we'd need some kind of conditioning
>>> to get the output into a microcontroller readable format.
>>>
>>> Does anyone have any thoughts on how practical this would be for
>>> sensing
>>> whether there are people waiting?
>>>
>>> Regards,
>>>
>>> Deniz
>>>
>>> P.S. A quick google for how the detectors work gave me the following
>>> results:
>>> http://www.eeweb.com/project/circuit_projects/cell-phone-detector
>>> http://www.electroschematics.com/1035/mobile-bug-detector-sniffer/
>>> http://www.instructables.com/id/Free-Energy-Cellphone-detector-From-Cellphone-An/
>>> http://www.instructables.com/id/VHF-UHF-RF-Sniffer/?ALLSTEPS
>>> _______________________________________________
>>> OpenTRV-dev mailing list
>>> OpenTRV-dev at lists.opentrv.org.uk
>>> http://lists.opentrv.org.uk/listinfo/opentrv-dev
>>>
>>
>>
>>
>>
>