10-12-2012, 02:30 PM
Spread Spectrum Modulation Techniques
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To spread the information signal over a wider bandwidth.
Useful for multiuser access
The spreading of the signal Is controlled by PN sequence.
These signals are demodulated at receiver by cross-correlation with locally generated PN sequence.
Inherent interference rejection capability
These signals can be recovered when it is jammed by narrowband interferer. It affects the small portion of the signal and can be removed by filter.
It eliminates frequency planning since all cells can use the same channel.
Initially used in military application.
Encryption is also used for hiding.
Used in CDMA
Pseudo-Noise (PN) Sequences
Direct sequence spread spectrum:
For direct sequence spread spectrum (DSSS), each bit in the original signal is represented by multiple bits in the transmitted signal, using a spreading code.
A10-bit spreading code spread s the signal across a frequency band that is 10 times greater than a 1-bit spreading code.
One technique for direct sequence spread spectrum is to combine the digital information stream with the spreading code bit stream using an exclusive-OR (XOR).
The XOR obeys the following rules:
O xor O=O, O xor 1=1 ,1 xor O=1 ,1 xor 1 = 0
in example an information bit of 1 inverts the spreading code bits in the combination, while an information bit of zero causes the spreading code bits to be transmitted without inversion.
It has a wider bandwidth than the information stream. In this example, the spreading code bit is clocked at four times the information rate.