25-04-2012, 11:48 AM
Mobile Computing Seminar Technologies and standards in wireless communication Summary
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Radio Transmission Techniques
We rst have a look at techniques for transmitting information reliably over radio waves in the GHz
spectrum
Three techniques in radio transmission are presented Frequency Hopping Spread Spectrum Direct Se
quence Spread Spectrum and Orthogonal FrequencyDivision Multiplexing They all try to achieve resis
tance against narrowband interference caused by other radio sources
Frequency Hopping Spread Spectrum FHSS
In Frequency Hopping the transmission band is divided into dierent frequency channels typically
and a logical channel ie a channel on which two or more devices communicate hops periodically from
one frequency channel to another with a pseudorandom hopping sequence
Direct Sequence Spread Spectrum DSSS
Also in Direct Sequence the frequency band is divided into fewer but larger
channels but no hopping
is used To compensate for noise on a channel a technique called chipping is implemented each bit is
converted into a redundant bit pattern called chip sequence whereby an nchip code spreads the signal by
a factor of n If some interference destroys part of the chip sequence the original bit may still be recovered
from the remaining chips
Orthogonal FrequencyDivision Multiplexing OFDM
The general idea of OFDM is to split a highrate data stream into lowerrate streams which are trans
mitted simultaneously over a number of subcarriers These subcarriers are orthogonal to each other in
the sense that when listening to one subcarrier the others dont interfere hence the name Orthog
onal FrequencyDivision Multiplexing By using this technique multipath delay spread and intersymbol
interference are considerably decreased because only low bit rate streams are employed