Information Technology Reference
In-Depth Information
Figure 1: Power spectral density versus frequency (DSSS).
spread spectrum techniques are largely used to give them robustness against
interferences.
The spectrum spreading on the whole ISM band is carried out in two main
ways:
•
The
direct sequence spread spectrum
(DSSS) is used in the Universal
Mobile Telecommunications System (UMTS) for cellular phones. It
reduces the power spectral density by modulating the signal with a well-
known binary sequence at a higher rate (chip-sequence), thus making the
signal itself more similar to a background noise (Figure 1).
•
The
frequency hopping spread spectrum
(FHSS) transmits the signal in
different time slots using a different carrier in each time, following a
given hopping sequence (Figure 2).
It is hard to say what is the better way for digital data transmission over
the ISM band. For sure, a DSSS modem is more complex than an FHSS one,
but DSSS systems present a smaller overhead.
Figure 2: Power spectral density versus frequency (FHSS).
