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CC2530 RSSI VALUE

LAKSHMANAN KUMAR RANGARAJ
Expert 1795 points
Other Parts Discussed in Thread: CC2530

Hi ,

 I am using cc2530 device.I am trying to measure the Rssi value to distance (RSSI = 1/d2).I getting the value from the end device   afIncomingMSGPacket_t pkt->rssi.

The Rssi value is not consistent with distance.for example for

1 meter distance the RSSI is 44dbm 

2 Meter distance the RSSI is 47dbm

3 meter distance the RSSI is 56dbm

4 meter distance the RSSI is 45 dbm

5 meter distance the RSSI is 45 dbm

 

i am measuring the rssi in the indoor with Line of Sight. My question is it becoz of the antenna ? if so where can i get the antenna details ?? can you please advice us.

Is there any API to get the transmitted RSSI ???

Appreciate your earliest reply.

Regards

Lakshman.

 

  • 0
    Prodigy 30 points

    I have same problem.

    in range of 3~4 m,  RSSI increase when distance are increase.

    someone help me!!

  • 0
    TI__Mastermind 19350 points

    Hello - This subject has been explored in a lot of other posts - do none of those even come close to helping you to answer your question? Please just search the forum for "RSSI" and for "LQI" and see all of the existing discussions ... perhaps you can add onto the one that is close to what you are experiencing? Here is just one to get started:

    http://e2e.ti.com/support/low_power_rf/f/155/p/16430/63242.aspx#63242

     

  • 0
    TI__Genius 14940 points

    This is more likely an issue with the test environment rather than the radio or antenna.  Think of it this way, if you are not in an open field or free space environment (or an anechoic chamber to mimic such) you will get reflected radiated energy from just about everything around you.  This reflected energy will constructively and destructively add or detract from the energy at any given point depending on the phase relationship at the point where the direct radiated signal and reflected signals sum together.  This can have surprising effects on the signal strength seen by the radio.  Thus, depending on where you are in this phase relationship, you can see an increase in signal strength with increased distance between the antennas which is counter intuitive.

    Another way of looking at it is to realize that at 2.4GHz, the wave length is approximately 12.5cm.  Since a standing wave (like which is effectively setup in any reflective environment) at this frequency will have two minima and maxima in each wave length then it is easy to see that in a linear meter of length at 2.4GHz there will be as many as 16 maxima and minima.  Now take an orthogonal dimension so that you have a surface one square meter in size and you get 16x16=256 maxima and minima possible within the surface of that square meter.  Finally take a third orthogonal dimension to achieve a cubic meter and you end up with 16^3=4096 possible maxima and minima in that cubic meter of space.  Thus, in a typical room, office, lab, or other similar closed space you can easily have tens of thousands of maxima and minima from which your receiving antenna can be placed.  Since there are two nulls per wavelength, you can go from a maxima to a minima in as little as 3.125cm or less than 2 inches of movement.

    I did some range experiments in a hallway one time where steel studs were used behind the sheet rock.  In this hallway, it didn't matter how far down it I went, the signal was approximately the same because the steel studs made the hallway act like a waveguide.  However, depending on where I moved the receiving radio, I could modify the RSSI value by as much as 40dB in the space of only a few centimeters of movement.  An empirical proof of the above description.

    This is but one reason why RSSI is difficult to work with when using it to measure distance.  It is really only valid in free space unless you fully understand the environment you are working in.

    Jim Noxon

  • 0 in reply to Jim Noxon
    Prodigy 10 points

    Thanks Jim. From the pratical experiance even in the free space the signal strength is varying due to multi path.So finally i did the linear regression of the recorded data to find the path loss exponent.

     

    Regards

    Lakshman.

     

  • 0 in reply to LAKSHMANAN KUMAR RANGARAJ81523
    TI__Genius 14940 points

    The regression is one part of the process.  Please don't get me wrong, there are plenty of systems out there that use RSSI to measure distance but it always takes a bit more than just sampling the RSSI.  Understanding what can affect the RSSI measurements is key to being able to take samples which provide reliable results with any degree of certainty.  Generally these systems utilize such things as frequency diversity, antenna diversity, filtering of oversampled data, etc.  When you look at the various methods to make such a system work, you see that in the end they all do some form of up front dithering to manage the local minima/maxima and also some back end filtering to increase the resolution.  It all adds up to a delay between the sampling process and the actual measurement and the greater the accuracy required the larger the delay will be.  Don't forget the knowledge of RSSI resolution varying with distance (it gets worse as the distance grows).

    All of the above should indicate to you that there's more to measuring distance from RSSI than merely rearranging the Friis equation and reading a sample from a register.  In general, RSSI distance measurement is combined with several other mechanisms to both increase it's accuracy and decrease the lag of the system overall.  These additional mechanisms can include accelerometers and gyros for quick movement tracking, TOF (Time of Flight) methods to account for accuracy loss at greater distances, GPS systems for initial global position determination, time referencing, and sanity checks when available, etc.  All of these things combine to create fairly robust systems but the final system requirement will be determined by what your design goals dictate are acceptable accuracy and delay parameters.

    Jim Noxon

  • 0
    Prodigy 35 points

    Could anyone let me know, What is the API or the  Function used to measure and calculate the RSSI in Simple application code?

    How the distances are calculated in the code i.e.logic used, and displayed finally on the hyper terminal?

  • 0 in reply to uma srinath1
    Intellectual 545 points

    how we can convert RSSI into distance ???

    wat is the formula for convert RSSI to distance ???

    am using cc2530 with smart RF

  • 0 in reply to uma srinath1
    Prodigy 35 points

    Any updates on this? Could anyone provide information on this.?

  • 0 in reply to Jim Noxon
    Prodigy 35 points

    Hello Jim,

    Could you please explain the logic and the functional code how the RSSI has been calculated and mesaurements are done with respect to DB and Distance calculations.

  • 0 in reply to uma srinath1
    Prodigy 230 points

    Dear all,

    Did you really read the whole explanation above? :)

    Its impossible to get accurate distance values just by measuring the RSSI value! 

    Best Regards,

    Gomes

  • 0 in reply to Jim Noxon
    Prodigy 10 points

    Hi,

    How can i identify the node location and in which direction it is placed using CC2530.

    Is there any angle of arrival is there to find the position.

    help me with your valuable thoughts.

    Thanks,

    sathish