Comtech EF Data SLM-5650A Installation And Operation Manual page 304

C.9 Direct Sequence Spread Spectrum (FAST Option)
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 .378
•
LL BPSK .451
•
LL BPSK .541
•
HP BPSK 1/3
•
HP BPSK 1/2
The Spreading Factors currently supported are:
•
1(Spreading Off),
•
2 through 512 up to a maximum chip rate of 64 Mcps.
The user can select between two built-in pseudo-random PN sequences (polynomials) and 10,000
Gold Code Sequences per polynomial 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.
Appendix C C–10
SLM-5650A Satellite Modem
Revision 10
MN-SLM-5650A