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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 interference
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 interference