FAQ -- WiMAX -- what are RSSI and CINR all about?
Most recent updates:
-- 12/10/2011 added more info about CINR values
-- 08/02/2012 added info about CINR R1 and R3
-- 20/07/2012 added info about preambles and sub-carriers (also corrected an error regarding CINR R1 and R3)
RSSI = received signal strength indicator
which is basically "level of received signal" strength in dBm
CINR = Carrier to Interference+Noise Ratio
which is basically "signal to noise ratio" in dB
RSSI from -50 to -85 dBm
-- you want RSSI to be closer to -50 than to -85
CINR from 10 to 30 dB
-- you want CINR to be closer to 30 than to 10
-- any value of 20 or more can be considered a "good signal"
-- any value of 15 or less will probably trigger your modem to try to swap to another tower
Some modems don't just show a single CINR value but show R1 and R3 values separately:
CINR R1 and R3 VALUES:
-- the R1 and R3 refer to "re-use 1" and "re-use 3" respectively; this is a method of interference reduction which is part of the OFDMA specification ... it describes two different methods of using the wireless bandwidth
-- R1 references the level of performance (data transfer) ... you want it to be >15 dB to prevent "tower hopping"
-- R3 references how your connection is established ... it should normally be about 3dB-4dB higher than R1
-- you want BOTH of these to be as high as possible (closer to 30)
-- if you can get R1 to be 25dB or greater this will allow the best possible modulation to be used by the modem
-- if R3 is higher than R1 by 5dB or more it may indicate that there is some interference in the area (e.g. multipath problems) which is why physically relocating the modem ("moving the modem around") may improve performance
CINR is measured on the first OFDM Symbol, called “Preamble”. An OFDM symbol is a small timeslice. Each symbol consists of 1024 sub-carriers in the frequency domain. Each preamble is modulated by the base station on every 3rd sub-carrier. So there are 3 possibilities for unique preamble modulation.
The network has been designed to use a different preamble sub-carrier combination in cases of likely frequency clashes with other sites. This gives an additional “Reuse 3” on the preamble and this is important for measuring and acquiring the site correctly. In other words: CINR R3 is for connection establishment!
CINR R1 is measured across ALL 1024 sub-carriers of the preamble. CINR R3 is only measured across the MODULATED 33% of subcarriers of the preamble.
If another site on the same frequency interferes at the customer location, the CINR R1 metric is likely to be MUCH LOWER than the CINR R3 metric. This is because the other site is very likely to use one of the other 2 available CINR modulations, so that the preambles don’t clash. The preamble technically only uses 33% of the band, however the user traffic is carried on ALL subcarriers.
The implication of this is that the CINR R1 is the important metric in trying to estimate how clean a signal is and how much throughput one can get out of it.
For home gateway users, I downloaded some info from the manufacturer's website and posted it here:
There's a receive sensitivity table there which gives you some idea of what protocols (and therefore what bandwidth) is achievable at certain values.