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I noticed (I have some radios I was testing stuff with) that signal quality (not necessarily strength) seems to change from room to room.

The frequency of my radio is 2.4GHz, bit rate is 100kHz.

It can't be delay spread because the rooms aren't large enough for that to really play a role. No doppler, because the nodes aren't moving....

For the record, I have a large room (probably about 5 x 10m, 3m high ceilings) in which the signal seems to be of better quality than in a small hallway.

Any thoughts?

Edit: Intrasymbol interference? The carrier is 2.4GHz, so the period is quite small...

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  • \$\begingroup\$ maybe somebody was operating a microwave cooker and some rooms are closer to it than others. \$\endgroup\$
    – Andy aka
    Commented Nov 29, 2013 at 0:04
  • \$\begingroup\$ How do you define signal quality and how are you measuring the receive signal strength? \$\endgroup\$
    – alex.forencich
    Commented Nov 29, 2013 at 1:15
  • \$\begingroup\$ I define signal quality as link quality indicator (LQI) here. \$\endgroup\$
    – Mewa
    Commented Nov 29, 2013 at 2:07

2 Answers 2

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You're probably seeing some form of multipath interference. After the signal is transmitted, it can bounce around quite a bit and the receiver can end up getting several overlapping copies of the original signal. Depending on their time delays, this can manifest itself in several ways. First, if the offset is less than a wavelength, then it can cause destructive interference and it will appear as a decrease in receive signal strength. If the offset is more than a wavelength, then it can cause intersymbol interference where one part of a transmitted symbol bleeds into the next, causing the receiver to misinterpret the received symbols and producing a garbled output.

Also, if you are using a system like wifi, the overall link quality is also dependent on being able to receive signals from all of the other hosts connected to the router as only one host is allowed to transmit at a time. A host is supposed to listen before transmitting to make sure that nobody else is transmitting on the channel. If two hosts that can't receive signals from each other transmit at the same time, the signals will interfere at the router and both hosts will have to try again. It sounds like you aren't using wifi, though, so this may not be an issue.

How are you measuring the receive power?

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  • \$\begingroup\$ I thought intersymbol interference occurred when the delayed signal took longer to arrive than the symbol duration? My data rate is 100kHz, so one bit lasts about 10us, which means the signal would have to travel like 3 km before it would cause intersymbol interference. I measure received power using RSSI, and the signal quality (which is what I'm actually interested in in this question) using LQI on a CC2500 radio. \$\endgroup\$
    – Mewa
    Commented Nov 29, 2013 at 2:07
  • \$\begingroup\$ Actually, intersymbol interference is when the delay is less than one symbol duration. I'm drawing a blank on what it is called when the delay is more than a symbol duration, though. ISI of less than a symbol can be corrected for to some extent using different kinds of equalization. Longer delays basically show up as noise unless a more complicated receiver is used. RSSI is a measurement of signal power while LQI also factors in receive errors. It's likely that in one location more multipath copies of the message are being received that produce enough ISI to generate more errors. \$\endgroup\$
    – alex.forencich
    Commented Nov 30, 2013 at 1:59
  • \$\begingroup\$ Wow, what the heck was I thinking? Well thanks! That's what I thought until someone told me it couldn't be ISI and then I spent a day trying to figure out what's going on. \$\endgroup\$
    – Mewa
    Commented Nov 30, 2013 at 2:07
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Where is the transmitter? What kind of receiving antenna? How directive are these antennas? Signals transmitted at 2.4 GHz are affected by walls depending on their composition and the angle at which the signal has to travel through the wall to reach the receiver. Instructions for any wi-fi router have guidelines for where to install the router to minimize these effects as well as interference from wireless phones and other potential sources of interference. Depending on the geometry, you could also get multi-path effects as the signal bounces around a room as well as effects due to the directivity patterns of your antennas.

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  • \$\begingroup\$ I use two TI radios (CC2500) and I record RSSI and LQI from the exchanged data. I know RSSI is measured even though no exchange happens. I am curious about LQI here in particular. The antenna patterns are here: katalog.we-online.de/pbs/datasheet/7488910245.pdf, if you are curious. \$\endgroup\$
    – Mewa
    Commented Nov 29, 2013 at 2:02

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