Comtech EF Data SLM-5650A - Manual (Page 404)

SLM-5650A Satellite Modem Revision 8
Appendix D MN-SLM5650A
D–10
The High Performance (HP) code provides the best performance but worst latency. This code is
available from 32 kbps up to 46 Mbps depending on MODCOD. The HP code is compatible with
the CDM-625 LDPC code.
See Appendix B. OPERATIONS REFERENCE for more details on the data rate and symbol
rate ranges supported in each code.
D.9 Direct Sequence Spread Spectrum (DSSS)
As On-The-Move systems with very small dishes (e.g., VSATs) become more popular, there is an
increased interest in Spectrum Spreading. To meet this need, Direct Sequence Spread Spectrum
(DSSS) is optionally available in the SLM-5650A.
DSSS works by modulating the normal carrier with a pseudo-random PN sequence (chips) at a
much higher chip rate. The chip rate is determined by the carrier’s symbol rate and a multiplying
(spreading) factor.
The SLM-5650A offers spreading in the following LDPC MODCODs:
ULL
BPSK
1/2
LL
BPSK
.382
LL
BPSK
.456
LL
BPSK
.547
HP
BPSK
1/2
The Spreading Factors currently supported are 1(Spreading Off), 2, 4, 8, 16, 32, 64, and 128 up to
a maximum chip rate of 23 Mcps. The user can select between four built-in pseudo-random PN
sequences so that multiple spread carriers can occupy the same bandwidth in a Code-Division
Multiple Access (CDMA) fashion.
When using large spreading factors, the SLM-5650A also exhibits a large tolerance to jamming –
up to 24 dB – and a Low Probability of Detection/Interception. With DSSS enabled, the lower
data rate limit has been reduced to 8 kbps. The SLM-5650A Base Modem Web Server Interface
provides a Web page to facilitate anti-jamming applications. This page allows the user to
configure the modem parameters so that a fixed occupied bandwidth can be maintained.
Lastly, there is no loss in performance with spreading enabled.
D.10 Extended Temperature Option
Extended temperature range is -32° to 50° C. When operating from -32° to 0° C, Tx power
accuracy is +/- 1.5 dB.
A heater is installed on the bottom of the chassis. This 300-Watt heater operates at 120 or
240V AC.
When temperatures are below 0° C, allow a 10-minute warm up period upon power-up before the
modem boots. To ensure performance specifications, allow an additional 15 minutes, for a total of
25 minutes of recommended warm-up time.