Murata Electronics North America 5811M Manual Do Utilizador

 

© 2003 Cirronet

 Incorporated 

3 

M-5811-0008 Rev -

 

 

 

Narrowband vs. spread spectrum in the presence of interference

 

 

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. 

 
 
 
 
 

 

 

Figure 2 

Forms of spread spectrum 

 

One disadvantage of direct sequence systems is that due to spectrum constraints and the 
design difficulties of broadband receivers, they generally employ only a minimal amount 
of spreading (typically no more than the minimum required by the regulating agencies).  
For this reason, the ability of DS systems to overcome fading and in-band jammers is 
relatively weak.  By contrast, FH systems are capable of probing the entire band if 
necessary to find a channel free of interference.  Essentially, this means that a FH 
system will degrade gracefully as the channel gets noisier while a DS system may 
exhibit uneven coverage or work well until a certain point and then give out completely.    
 
Because it offers greater immunity to interfering signals, FH is often the preferred 
choice for co-located systems.  Since direct sequence signals are very wide, they tend to 
offer few non-overlapping channels, whereas multiple hoppers may interleave with less