WIT2410
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2000- 2005 Murata
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2
M-2410-0000 Rev G
fade can be described as a frequency-selective notch that
shifts in location and
intensity over time as reflections change due to motion of the radio or objects within
its range. At any given time, multipath fades will typically occupy 1% - 2% of the
2.4 GHz band. This means that from a probabilistic viewpoint, a conventional radio
system faces a 1% - 2% chance of signal impairment at any given time due to
multipath.
Spread spectrum reduces the vulnerability of a radio system to interference from
both jammers and multipath fading by distributing the transmitted signal over a
larger region of the frequency band than would otherwise be necessary to send the
information. This allows the signal to be reconstructed even though part of it may be
lost or corrupted in transit.
Figure 1
Narrowband vs. spread spectrum in the presence of interference
1.2. Frequency Hopping vs. Direct Sequence
The two primary approaches to spread spectrum are direct sequence (DS) and
frequency hopping (FH), either of which can generally be adapted to a given
application. Direct sequence spread spectrum is produced by multiplying the
transmitted data stream by a much faster, noise-like repeating pattern. The ratio by
which this modulating pattern exceeds the bit rate of the baseband data is called the
processing gain, and is equal to the amount of rejection the system affords against
narrowband interference from multipath and jammers. Transmitting the data signal
as usual, but varying the carrier frequency rapidly according to a pseudo-random
pattern over a broad range of channels produces a frequency hopping spectrum
system.
