13-04-2012, 11:35 AM
SPREAD SPECTRUM SYSTEM
[attachment=20054]
The spread spectrum technique was developed initially for military and intelligence requirements.
The essential idea is to spread the information signal over a wider bandwidth to make jamming and interception more difficult.
In our project, we will look some spread spectrum techniques and multiple access technique based on spread spectrum
Concept of Spread Spectrum
Input is fed into a channel encoder
Produce an analog signal with a relatively narrow bandwidth around some center frequency.
Further modulated using a sequence of digits known as a spreading code or spreading sequence.
The spreading code is generated by a pseudonoise, or pseudorandom number generator.
The effect of this modulation is to increase significantly the bandwidth (spread the spectrum) of the signal to be transmitted.
At the receiver, the same digit sequence is used to demodulate the spread spectrum signal.
The signal is fed into a channel decoder to recover the data.
DSSS Using BPSK
Direct Sequence Spread Spectrum – DSSS
Each bit in the original signal is represented by multiple bits in the transmitted signal, using a spreading code.
The spreading code spreads the signal across a wider frequency band in direct proportion to the number of bits used.
A 15-bit spreading code spreads the signal across a frequency band that is 15 times greater than a 1-bit spreading code.
DSSS Performance
The information signal has a bit width of T, which is equivalent to a data rate of 1/T. the spectrum of the signal depending on the encoding technique, is roughly 2/T.
Similarly, the spectrum of the PN signal is 2/Tc.
The amount of spreading that is achieved is a direct result of the data rate of the PN stream.
Code Division Multiple Access
CDMA is multiple access scheme that allows many users to share the same bandwidth
3G (WCDMA), IS-95
Basic Principles of CDMA
Each user is assigned a unique spreading code
The processing gain protects the useful signal and reduces interference between the different users
CDMA for DSSS
the unwanted signal energy remains spread over a large bandwidth and the wanted signal is concentrated in a narrow bandwidth.
The bandpass filter at the demodulator can therefore recover the desired signal.
[attachment=20054]
The spread spectrum technique was developed initially for military and intelligence requirements.
The essential idea is to spread the information signal over a wider bandwidth to make jamming and interception more difficult.
In our project, we will look some spread spectrum techniques and multiple access technique based on spread spectrum
Concept of Spread Spectrum
Input is fed into a channel encoder
Produce an analog signal with a relatively narrow bandwidth around some center frequency.
Further modulated using a sequence of digits known as a spreading code or spreading sequence.
The spreading code is generated by a pseudonoise, or pseudorandom number generator.
The effect of this modulation is to increase significantly the bandwidth (spread the spectrum) of the signal to be transmitted.
At the receiver, the same digit sequence is used to demodulate the spread spectrum signal.
The signal is fed into a channel decoder to recover the data.
DSSS Using BPSK
Direct Sequence Spread Spectrum – DSSS
Each bit in the original signal is represented by multiple bits in the transmitted signal, using a spreading code.
The spreading code spreads the signal across a wider frequency band in direct proportion to the number of bits used.
A 15-bit spreading code spreads the signal across a frequency band that is 15 times greater than a 1-bit spreading code.
DSSS Performance
The information signal has a bit width of T, which is equivalent to a data rate of 1/T. the spectrum of the signal depending on the encoding technique, is roughly 2/T.
Similarly, the spectrum of the PN signal is 2/Tc.
The amount of spreading that is achieved is a direct result of the data rate of the PN stream.
Code Division Multiple Access
CDMA is multiple access scheme that allows many users to share the same bandwidth
3G (WCDMA), IS-95
Basic Principles of CDMA
Each user is assigned a unique spreading code
The processing gain protects the useful signal and reduces interference between the different users
CDMA for DSSS
the unwanted signal energy remains spread over a large bandwidth and the wanted signal is concentrated in a narrow bandwidth.
The bandpass filter at the demodulator can therefore recover the desired signal.