C o m m i ssion ing M anu al
E val u at i n g L i n k Q u al i t y
28.3. Carrier-to-Interference Ratio
A common performance measure of radio communication systems for
unlicensed
bands
is the carrier-to-interference ratio (C/I). The C/I is important to determine the
maximum allowed interference level for which the system will still work. C/I is the
ratio of the power in the carrier to the power of the interference signal. CIR is
normally expressed in dB. The C/I is a very useful measure in communications
because it is not sufficient to simply have a strong signal (i.e. RSSI), since a weak
signal in the presence of weak noise may well be as useful as a strong signal in the
presence of strong noise.
To calculate the desired RSSI level, you need to measure the interference level
at
the
BSR
using the Spectrum Analyzer. When the Spectrum Analyzer graph displays
“spikes”, this generally indicates interference. Note that normal “noise” level is
approximately –112 dBm (referred to as
thermal noise
).
Airspan recommends using frequencies that are approximately 28, 20, and 12 dB
above interference levels to effectively operate in 8-, 4-, and 2-level FSK (4 Mbps/3
Mbps, 2 Mbps, and 1.33 Mbps/1 Mbps), respectively, as listed in the table below.
Table
28-2: ASWipLL minimum C/I requirements per FSK mode
FSK
Rate mode
C/I
8-level FSK
4 Mbps and 3 Mbps
28 dBm
4-level FSK
2 Mbps
20 dBm
2-level FSK
1.33 Mbps and 1 Mbps
12 dBm
Note:
ASWipLL can operate in 2-, 4-, and 8-level FSK with signal strengths greater
than -90, -83, and -75 dBm, respectively.
For example, if the noise level measured by the spectrum analyzer is -70 dBm (see
Figure 28-3), the RSSI level at the SPR should be as follows to ensure a good link
with the BSR:
8-level FSK: RSSI > -42 dBm (28 dBm minus –70 dBm)
4-level FSK: RSSI > -50 dBm (20 dBm minus –70 dBm)
2-level FSK: RSSI > -58 dBm (12 dBm minus –70 dBm)
01040311-07
Airspan Networks Inc.
28-5
