Received Signal Strength Indicator

  
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Received Signal Strength Indicator

Postby joey » Sun May 01, 2016 9:20 am

Hi,

I wonder if you could expand slightly on your use of the RSSI value. I'm retrieving the RSSI of the last received packet sent between a pair of eRA-Connect2-Pi-900s, and plotting it in a graph to monitor the connection. From what I gather RSSI can be represented differently per manufacturer and technology, some represent it as a measure of RF received power, others seem to factor in a received quality figure usually between 0-100, some represent it in dBs some just a notional % quality figure. Having setup a few experiments I'm also finding that the best (highest) RSSI I receive is 63 and that's when the 2 modules are practically next to each other.

I suppose what I'm after is exactly how RSSI is represented in easyRadio and whether or not it has a unit of measure e.g. dBm?

I absolutely love the USB Raspberry Pi compatible ERA units! Keep up the hard work!
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Re: Received Signal Strength Indicator

Postby Antman » Tue May 03, 2016 5:21 pm

Hi Joey,

The RSSI figure is in HEX. You can get this HEX value by sending the command ER_CMD#T8 which returns the last valid packet or the ER_CMD#T9 command to get a live RSSI figure. Once you have the HEX value you need to convert to a decimal value and refer to our RSSI chart on page 18 of the latest easyRadio advanced data sheet Rev 3.6. You will also need to know the bandwidth the eRA module is in (The default is 100KHz).

With the decimal value, known bandwidth and frequency (400 or 900) of the ERA modules you can find the dBm figure by using the charts.

Let me know how you get on.

Regards
Richard
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Re: Received Signal Strength Indicator

Postby joey » Tue May 10, 2016 7:42 pm

Thanks for the reply Richard.

I am currently outputting the decimal value to an array in Python and displaying the value in a basic GUI.

With regards the graph in the advanced data sheet; I'm assuming you are taking the decimal value, the selected bandwidth and frequency then putting this through an algorithm to provide the dBm values on the graph? Or are you comparing these figure against known lab results?

I'm using a 900MHz unit, currently at default bandwidth (100KHz) and operating at power level 9 (7dBm). Although my GUI allows these to be altered.

Basically I'd like to display the dBm RSSI value rather than the decimal figure..
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Re: Received Signal Strength Indicator

Postby Antman » Wed May 11, 2016 4:08 pm

Hi Joey,

To fill you in some more. The graph is taken directly from the CC1020 RF Chip data sheet which we use in the eRA900TRS RF module. You can google this is if you wish as there is an application note on RSSI for the CC1020. It is left in its RAW form which when compared to the graph will give the actual dBm measurement on the input to the module.
We do nothing to this value just simply provide it which you call up from the ER commands T8 and T9. There is no algorithm, other than what can be made by observation from the graph.

I hope this helps.

BR

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Re: Received Signal Strength Indicator

Postby joey » Wed May 11, 2016 9:02 pm

Thanks for the reply Richard.

I thought I'd post a link to the RSSI Application note for the CC1020RF chip for anyone else who might be after this in the future: http://www.ti.com/lit/an/swra062/swra062.pdf

I've had a read through, and from what I can gather:

The RSSI measurement can be referred to the power at the RF input pin by using the following equation:

P = 1.5 x (RSSI-3) x (VGA_SETTING - RSSI_Offset [dBm])

where:

RSSI is the digital value read from the RSSI register converted to decimal.
VGA_SETTING is the maximum VGA gain and is set by VGA3.VGA_SETTING [4:0]. The VGA gain is programmable in steps of approximately 3 dB.
RSSI_Offset depends on the channel filter bandwidth used due to different VGA settings.


I feel I'm really close to being able to write a python function to give me the live power in dBm, but I have a few final questions (then I promise to stop hassling you!! :D )

With regards to the VGA_SETTING and RSSI_OFFSET [dBm], is this something you set on the chip? I've located "table1" on the CC1020 reference PDF listed above and have found an entry relating to:

Fc: 868 MHz
Receiver channel filter bandwidth [KHz]: 102.4
RSSI Offset: 92
VGA_Setting (Decimal): 14


So I suppose I'm trying to establish whether you know what the VGA_SETTINGs and RSSI_OFFSETs are set to on the eRA900TRS when set to default bandwidth values etc.

I'm assuming the 102.4kHz above refers to your 100kHz bandwidth default?

As above I'm using a 900MHz unit with the following settings:

Band plan: 869.7MHz
Bandwidth 100KHz (default)
Power level 9 (7dBm)

Thanks in advance for any guidance you can offer.
Regards,
Joey
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Re: Received Signal Strength Indicator

Postby WirelessMike » Thu May 12, 2016 2:46 pm

Hi,

I am interested to know what you are hoping to do with the RSSI 'measurements' ? I hope that it is not to be relied upon for ranging or location applications ? If you are using it for 'Carrier Sense' in a CSMA application you will lose sensitivity (range) if the threshold is set too high or you will be troubled by continuous interupts if the threshold is set too low.

Bear in mind that RSSI stands for 'Received Signal Strength INDICATOR'. As you will have probably observed this 'signal' will vary immensely due to relative positions of transmitter and receiver, their antennas and objects in the intervening and surrounding space.

It will be affected by absorption, reflection and additive/subtractive 'multipath' effects. In addition any RF noise (interference) that is within the pass band of the receiver will cause errors. Any electronic device that generates signals whose harmonics may extend into the UHF region such as clock oscillators, fast logic signals, switch mode power supplies will influence the 'reading'.

It is informative to scan a band of channels and observe the different background noise on each channel. A cheap 'scanner' receiver is useful for this purpose and in addition you can use the worlds best DSP processor (your ears) to hear the noise and the signals.

As you may gather I am not too keen on the use of RSSI other than a rough 'n' ready 'Good, Fair and Poor' display !
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Re: Received Signal Strength Indicator

Postby joey » Sat May 14, 2016 1:43 pm

Hi Mike,

I'm experimenting using RSSI measurements to generate a very rough RSSI heat map of a fixed site - one eRA900TRS as fixed position Tx which puts up a carrier, the other unit is used to take RSSI measurements at various locations (GPS mapped) around the area, as part of a uni project.

Fully aware of the absorption and multipath issues - analysis features heavily in the project.

Any thoughts regarding the RSSI_Offset and VGA_Setting I mention above?

Best Regards,
Joey
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Re: Received Signal Strength Indicator

Postby WirelessMike » Thu May 19, 2016 9:31 am

Joey,

I wish you well with your project and would be most interested to know how it proceeds. What you are trying to do is fine in theory but very difficult in practice !

Have a think about using multiple transceivers within range of each other and measuring the signal strengths from each to every other node in the network. This will provide a much more accurate 'map' - albeit one that changes dynamically as objects move within the fields.

The RSSI function is provided 'as is' but we do not endorse it with extended technical support.

The suggested TI App note provides further information.

Mike
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