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Pages: 1 (448 words) Published: November 2, 2014
Theory and Terminology Note
Frequency Hop Spread Spectrum vs. Direct Sequence Spread Spectrum

Direct Sequence Spread Spectrum (DSSS):
The DSSS encoder spreads the data across a
broad range of frequencies using a mathematical
key. The receiver uses the same key to decode
the data.

1

As a general rule, FHSS can resist interference
from spurious RF signals ten times better than
DSSS.

928 MHz

The frequency range is
divided into channels. The
data is transmitted on these
randomly ordered channels.

3

. . . . . . . . . . . . . . . . . . . . 25 26 27

Frequency

928 MHz

The narrowband signal is converted into this
DSSS signal for transmission.
Direct Sequence
Spread Signal

902 MHz

Frequency

928 MHz

The DSSS signal uses a much lower
power density than narrowband inference.

When broadband interference is present,
however, the resulting decoded broadband
interference can give a much higher noise floor,
almost as high as the decoded signal.

Narrow Band
Interference Signal

Direct Sequence
Spread Signal

902 MHz

Frequency

928 MHz

Decoded Signal

Power

For this reason, DSSS works best for large
data packets in a low to medium interference
environment, but not as well in higher
interference industrial applications.

2

902 MHz

While narrowband and DSSS transmissions use
the same total power to send data, DSSS uses a
lower power density (power/frequency), making
it harder to detect. DSSS also sends redundant
copies of the encoded data to ensure reception.
Narrowband interference appears to the receiver
as another narrowband transmission. When the
total received signal is decoded, the wider band
transmission (DSSS encoded data) is decoded
back to its original narrowband format while the
interference is decoded to a lower power density
signal, thereby reducing its effects.

Frequency

Power

Frequency hopping technology works best
for small data packets in high...
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