• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 2 answers
  • Subscribers 52 subscribers
  • Views 2472 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

CC2540: RSSI vs 'Received Power'; TI Composition of RSSI ?

USACoder
Prodigy 60 points
Part Number: CC2540
Other Parts Discussed in Thread: CC2650MODA

Is anyone aware of an 'Application Note' or other documentation of exactly how the RSSI values reported by CC2540MA are constructed ?   Do they relate directly to specific dB values seen at the receiver ?   Are the RSSI values just the 'received power at receiver' adjusted by the Transmit Power contained in the advertisement message from the Bluetooth beacon ?    Is there some difference between the reported RSSI value and measured power at the receiver ?

Please advise if you can.

Thanks in advance.

 

  • 0
    TI__Guru** 106705 points
    Hi,

    The RSSI level reported by the radio is the actual power level, in dBm, seen by the receiver.

    Cheers,
    Fredrik
  • 0 in reply to Fredrik K
    Prodigy 60 points

    Fredrik,

    Thanks for advising.    Does there happen to be an App Note or other official documentation that would make an official statement on the point?

    I ask because apparently IEEE 802.11 (below) states that there is no standardized relationship between RSSI and any physical parameter  ... and I was thinking that perhaps TI is adjusting the Power Measured at Receiver by the known receiver antenna, circuit, and chip losses and other hardware losses in order to more accurately reflect true 'pure path loss' for proximity calculation purposes.

    On the other hand, the Bluetooth SIG specification appears (below) to say that " 'RSSI' is the received signal strength, in dBm, of the packet received" an apparently clear physical relationship.   

    If RSSI on the CC2540 is simply 'actual power level at receiver', in dBm, are there known receiver antenna, circuit, and chip losses for CC2540, CC2540MA, CC2540MODA to make the appropriate adjustments to determine 'pure path loss' for proximity calculation purposes ?

    Please advise, and thanks so much for your replies.

    Regards.

    From Wikipedia:

    In 802.11 implementations[edit]

    In an IEEE 802.11 system, RSSI is the relative received signal strength in a wireless environment, in arbitrary units. RSSI is an indication of the power level being received by the receive radio after the antenna and possible cable loss. Therefore, the higher the RSSI number, the stronger the signal. Thus, when an RSSI value is represented in a negative form (e.g. −100), the closer the value is to 0, the stronger the received signal has been.

    …

    There is no standardized relationship of any particular physical parameter to the RSSI reading. The 802.11 standard does not define any relationship between RSSI value and power level in milliwatts or decibels referenced to one milliwatt. Vendors and chipset makers provide their own accuracy, granularity, and range for the actual power (measured as milliwatts or decibels) and their range of RSSI values (from 0 to RSSI maximum).[2] One subtlety of the 802.11 RSSI metric comes from how it is sampled—RSSI is acquired during only the preamble stage of receiving an 802.11 frame, not over the full frame.[3]

    …

    Received channel power indicator[edit]

    For the most part, 802.11 RSSI has been replaced with received channel power indicator (RCPI). RCPI is an 802.11[3] measure of the received radio frequency power in a selected channel over the preamble and the entire received frame, and has defined absolute levels of accuracy and resolution. RCPI is exclusively associated with 802.11 and as such has some accuracy and resolution enforced on it through IEEE 802.11k-2008. 

    References[edit]

    1. Martin Sauter (2010). "3.7.1 Mobility Management in the Cell-DCH State". From GSM to LTE: An Introduction to Mobile Networks and Mobile Broadband (eBook). John Wiley & Sons. p. 160. ISBN 9780470978221. Retrieved 2013-03-24.
    2. Lui, Gough; Gallagher, Thomas; Binghao, Li. "Differences in RSSI readings made by different Wi-Fi chipsets: A limitation of WLAN localization". Localization and GNSS (ICL-GNSS), 2011 International Conference on. Retrieved 24 June 2013.
    3. 3.     "IEEE 802.11-2012". IEEE. 2012-03-29. Retrieved 2013-02-11.

     Copyright Wikipedia(C) and others(?) for topic 'Received signal strength indication'.

     

    Supplement to Bluetooth Core Specification | CSSv7, Part A

    1.5 TX POWER LEVEL

     

    1.5.1 Description

     

    The TX Power Level data type indicates the transmitted power level of the packet containing the data type. The TX Power Level should be the radiated power level. The TX Power Level data type may be used to calculate path loss on a received packet using the following equation:

     pathloss = Tx Power Level – RSSI

    where “RSSI” is the received signal strength, in dBm, of the packet received.   For example, if Tx Power Level = +4 (dBm) and the RSSI on the received packet is -60 (dBm) then the total path loss is +4 – (-60) = +64 dB. If a second packet were received at -40 dBm with a Tx Power Level data type = +15 dBm the resulting pathloss would be +55 dB. An application could use these pathloss values to choose which device it thinks might be closer (the one with the lower pathloss value).   Unfortunately, due to fading and varying antenna, circuit, and chip characteristics, these resulting pathloss values will have uncertainty. Some of the uncertainty (for example, due to fading) may be able to be removed if multiple packets are received from the same device.  

    Note: When the TX Power Level data type is not present, the TX power level of the packet is unknown.

    Copyright Bluetooth SIG©

  • 0 in reply to USACoder
    TI__Guru** 106705 points
    The CC2540 datasheet lists RSSI parameters in dBm, which means that it is the actual power level. All the RF performance numbers are done trhough conducted measurements with RF instruments connected directly to the output of the balun and RF filter (refer to the CC2540EM reference design for details).

    The CC253x/4x User Guide provides more details on how the RSSI is measured. Furthermore the BLE Stack provides all the required calculations to provide the correct value to the application SW.

    RSSI is thus the power delivered to the input of the reference RF balun and matching/filter.

    CC2540MA and CC2540MODA are not valid part names. Are you really asking about the CC2650MODA?

    Cheers,
    Fredrik