Q-Flex Satellite Modem Installation and Operating Handbook
7-15
LinkGuard™
provides a menu option for setting a power spectral density threshold,
above which the modem will automatically alarm to indicate that a significant source of
interference has been detected. The
LinkGuard™
Rx traffic warning alarm is displayed
locally on the modem and can be accessed remotely via the modem’s web server, SNMP
traps and physical contact relays. Automated 24x7 interference detection is therefore
provided without the need for an operator to be present.
LinkGuard™
detects various forms of interference including tonal interferers (such as
CW), radars, Wi-MAX, jammers and other modulated carriers including adjacent
interfering carriers. If there is more than one source of interference then it will detect and
display all of them.
When
Paired Carrier
is being used (where two carriers are deliberately superimposed on
top of each other) then
LinkGuard™
is capable of detecting and displaying any
interference under both of the carriers.
For existing links, even ones that do not use Paradise modems, a
LinkGuard™
-capable
modem can be set up to receive the same Rx signal in order to simply monitor the link for
interference (so long as it can lock to the carrier). This is a useful and convenient way of
quickly confirming suspected cases of interference while keeping the service running.
Note that even when the level of interference is severe enough to prevent the
demodulator from locking, the spectrum graph will continue to show the interference
along with the wanted signal (although it will not be able to differentiate between them at
this point and will show a single composite spectrum).
LinkGuard™
does not necessarily replace other interference detection equipment but it
does provide a useful new first line of defense. It works with all carriers including DVB-
S2/S2X.
7.7 FastLink Low-latency LDPC
FEC design is a trade-off between good BER performance and low latency. TPC has low
latency but BER performance can be 1dB or more worse than conventional LDPC. (For
example, TPC 8QAM ¾ at 5e-8 BER has an Eb/No of 6.4dB compared to LDPC at
5.5dB.) Conventional LDPC latency can be many times that of TPC. (For example, LDPC
16QAM ¾ at 64kbps has a latency of 395ms compared to TPC latency of 47ms, which is
a factor of over eight different.)
With satellite delay in addition, LDPC can exhibit over ½ second one-way delay for very
low data rates. Low-latency LDPC FECs are becoming available that provide comparable
BER performance to LDPC with latency nearer to TPC. FastLink low-latency LDPC from
Paradise does precisely this. FastLink also offers an extensive range of modulations
ranging from BPSK to 64QAM (including both 8PSK and 8QAM, 16APSK and 16QAM
and 32APSK).
One of the key advantages of FastLink is its flexibility. As mentioned, FECs offer a trade-
off between BER performance and latency. While the default settings for FastLink
achieve this balance, two other modes are also available. The first mode optimises
performance for particularly low latency, at the cost of a slight increase in Eb/No required
to achieve a specific BER. The second mode optimises BER performance and is suitable
when latency is not an issue (e.g. when higher data rates are being used). This flexibility
