The message is a bi-polar sequence, and so the output of the first multiplier is a binary phase shiftkeyed - BPSK - signal. The second multiplier is the spreading modulator, using a pseudo randombinary sequence (PRBS), which in this context is referred to as a pseudo-noise (PN) sequence. Thetwo multipliers could be replaced by a single multiplier, its input sequence being the modulo-twoaddition of the message and the PN sequence, but this modification will not be implemented.The ‘channel’ is elementary in the extreme - band limiting could be inserted, but the delay wouldthen complicate the alignment of the receiver. The adder serves to introduce noise.Not included is any form of carrier/clock acquisition circuitry. The necessary signals will be stolenfrom the transmitter. Note that the PN clock will be a sub-multiple of the carrier, so only onesignal need be recovered. Not only must the two PN generators be synchronized; their alignmentis also necessary. This is not a trivial condition to achieve in practice.The receiver output would need to be ‘cleaned up’, and restored to bi-polar digital format (TIMSwould use the DECISION MAKER) but this processing is not included. If a short sequence is usedas the message source, visual comparison of the recovered message with that sent is sufficient forthe purpose of this experiment.
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