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Direct-sequence spread-spectrum transmissions multiply the symbol sequence being transmitted with a spreading sequence that has a higher rate than the original message rate. Usually, sequences are chosen such that the resulting spectrum is spectrally white. Knowledge of the same sequence is used to reconstruct the original data at the receiving ...
Moreover, for a given noise power spectral density (PSD), spread-spectrum systems require the same amount of energy per bit before spreading as narrowband systems and therefore the same amount of power if the bitrate before spreading is the same, but since the signal power is spread over a large bandwidth, the signal PSD is much lower — often ...
Direct sequence spread spectrum is usually the sending of modified digital bits. For example, 1 digital bit might represent a binary '1' or a binary '0'. Applying DSSS to 1 single message bit will involve constructing some OTHER pseudo-random sequence of bits, having a bit-rate that is usually much higher than the message bit rate.
In a binary direct-sequence system, each chip is typically a rectangular pulse of +1 or −1 amplitude, which is multiplied by a data sequence (similarly +1 or −1 representing the message bits) and by a carrier waveform to make the transmitted signal. The chips are therefore just the bit sequence out of the code generator; they are called ...
Barker codes of length N equal to 11 and 13 are used in direct-sequence spread spectrum and pulse compression radar systems because of their low autocorrelation properties (the sidelobe level of amplitude of the Barker codes is 1/N that of the peak signal). [15]
Unlike random noise, it must be easy to generate exactly the same sequence at both the transmitter and the receiver, so the receiver's locally generated sequence has a very high correlation with the transmitted sequence. In a direct-sequence spread spectrum system, each bit in the pseudorandom binary sequence is known as a chip and the inverse ...
CDMA is a spread-spectrum multiple-access technique. A spread-spectrum technique spreads the bandwidth of the data uniformly for the same transmitted power. A spreading code is a pseudo-random code in the time domain that has a narrow ambiguity function in the frequency domain, unlike other narrow pulse codes. In CDMA a locally generated code ...
A long delay spread with little Doppler spreading can be mitigated with a relatively long MFSK symbol period to allow the channel to "settle down" quickly at the start of each new symbol. Because a long symbol contains more energy than a short one for a given transmitter power, the detector can more easily attain a sufficiently high signal-to ...