User manual

G-2 Appendix G: Optimizing Radio Performance for Hostile Environments
Moseley SL9003Q 602-12016 Revision G
ASSESSING INTERFERENCE
This method is very useful to assess interference at your STL receiver (especially if you do not
have a spectrum analyzer available).
Turn OFF the STL transmitter at the studio. At the receiver from the front panel navigate to QAM
RADIO –> MODEM -> STATUS. The first line entry "QAM Modem" will indicate the RSL
(Received Signal Level) in dBm. With no interference present the RSL will be below –120 dBm,
typically. If this is not the case and RSL is above this level then you are receiving undesired
interference within your STL passband.
For the QAM data to be properly demodulated at the STL receiver the RSL must be greater than
the interference noise floor by the following amounts:
21 dB for 16 QAM
24 dB for 32 QAM
27 dB for 64 QAM
(To determine your QAM mode navigate down 5 more menus under MODEM STATUS until you
read "MODE".) For instance, if your STL is operating in 32 QAM mode (i.e., 32Q) and your RSL
interference is –90 dBm, then the minimum signal that your STL receiver can acquire must be
greater than –66 dBm. Add 10 dB more for fade margin then you will want to see an RSL of at
least -56 dBm.
INTERLEAVER
Bit errors may also result from sources other than traditional RF interference and Gaussian
noise from low signal levels. Some of these noise sources include microphonics, lightening
bursts, ignition noise, and other sources that are basically bursty in its nature. The problem with
bursty noise is it creates large groups of burst errors piled together, which may be too much for
the Reed-Soloman error correction algorithm to correct within a single coded block of data.
To combat this phenomenon an interleaver within the QAM modem is used to spread out the
error bursts over several coded blocks of data. The larger the interleaver factor the longer the
errors are spread out and therefore fewer errors will occur in any coded block for any single
error burst. This allows the error correction algorithm to operate on smaller number of errors
within each block.
The trade off here for increasing interleaving is added delay. Table G-1 shows the correlation
between interleave setting and delay.