ST2115
Scientech Technologies Pvt. Ltd.
41
Background:
As suggested in the previous experiment that DSSS has a great potential for being a
prominent multiple access technique owing to the presence of a unique PN sequence
per user. Larger the size of this PN sequence per data bit, larger could be the number
of users who can utilize a particular communication channel simultaneously. Also to
remember, that every PN sequence should be orthogonal to each other in order to
establish a fair communication between the users.
In actual practice, more than one modulation techniques are involved in the complete
communication process. One is spreading the data with a high frequency chip (PN
Sequence) and the other is RF modulation of this Chipped data in order to make it
appropriate for transmission. Generally two techniques are used for RF modulation
viz. BPSK and QPSK. ST2115 uses BPSK technique for RF modulation.
Acquisition or recovery of data from spreaded and RF modulated signal is really a
complicated process. The complications arise due to the fact that RF modulated signal
get phase shifted due to various interfering properties of environment. Also some
phase shift is always inherently present in any analog modulation and demodulation
process. Now after analog demodulation we recover our chipped data. But this
chipped data is now not in synchronization with the original chipped data. In order to
recover data from this chipped data, we have to synchronize this chipped sequence to
the PN sequence for proper desreading. There are various circuits and proposal for
achieving synchronization.
ST2115 uses a delay compensated PN sequence for despreading when RF modulation
process is involved.
Procedure:
Refer to the above figure 4.1 while configuring setup for the experiment.
1.
Switch data switches to 1 or 0 as per your choice of binary data pattern.
2.
Connect any two of the four taps viz. A, B, C or D to the inputs of EX-OR
gate of PN Sequence generator. Connect 240 KHz clock signal on board to the
clock input of the PN sequence generator.
3.
Now switch ‘On’ the power supply and observe the output of Binary Data
generator and PN sequence generator. Since the data generator frequency used
here is 30 KHz and that of PN Sequence Generator is 240 KHz, and hence
there are 8 PN sequence bits per Data bits for spreading the binary signal.
4.
There are two outputs of PN Sequence Generator shown on the board. One of
the outputs is for spreading the binary data signal and the other one is for
dispreading the coded signal to recover back the original data(when BPSK
modulation is used for RF modulation of Spreaded signal)
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