SSC P485 PL Transceiver IC

July 1998

11

Revision 5

24000828

ADVANCE INFORMATION

Spread Spectrum Carrier

TM

Technology

Spread Spectrum Carrier

TM

(SSC) Technology is a method of spread spectrum communications suitable for both

point-to-point or carrier sense multiple access (CSMA) networks. Historically, spread spectrum communication

systems have been used for secure communications and/or to overcome narrow-band impairments in the

communications medium. Spread spectrum receivers generally require an initial period of time to synchronize

with the carrier, so they have not been appropriate for CSMA networks. Spread Spectrum Carrier Technology is

a method by which a series of short, self-synchronizing, frequency swept "chirps" act as a carrier. The chirps are

always of the same known pattern and detectable by all of the nodes on the network. The chirp ranges in

frequency from 100 to 400 kHz over a duration of 100 µs. The chirp is swept from approximately 200 kHz to

400 kHz and then from 100 kHz to 200 kHz. Figure 8 illustrates the SSC power line chirp.

Figure 8. Spread Spectrum Carrier Chirp

Preamble Encoding

Two modulation schemes are used for symbol transmission by the physical layer. Amplitude Shift Keying (ASK)

is used in the preamble of the message packet. ASK modulation uses SUPERIOR and INFERIOR states to

encode symbols. A SUPERIOR state is represented by the presence of a chirp and an inferior state by the

absence of a chirp. Because the transmitter is quiet during inferior states, superior states transmitted by other

devices contending for the channel can be detected during the preamble of the packet. An example of ASK

modulation is shown in Figure 9. Note that in the preamble, the duration of a symbol is slightly longer than in the

body of the packet. A preamble symbol is 114 µs in length. Symbols in the sync sequence and the Packet Body

are 100 µs in length. Please note the “Chirp” is ALWAYS 100µs in length and is followed by 14 µs of quiet time

during the preamble.

Data Encoding

Phase Reversal Keying (PRK) utilizes two phases of the SUPERIOR state, SUPERIOR S1 and SUPERIOR S2,

which are 180° out of phase with one another, to modulate the encoded data. This modulation technique is more

robust than the ASK technique, because it allows the P485 to correlate and track each symbol rather than just

those encoded as SUPERIOR states. Figure 9 shows an example of PRK.