Binary Phase Shift Keying ( BPSK ) Demodulation

Theory


The BPSK generation and demodulation parts of the set-up can be represented by the block diagram in Fig.1. The second Multiplier and the Tuneable Low-pass filter module are used to implement a product detector to recover the digital data from the BPSK.



Fig.1 Diagram of set up of this experiment.

In phase shift keying (PSK), the phase of a carrier is changed according to the modulating waveform which is a digital signal. In BPSKD, the transmitted signal is a sinusoid of fixed amplitude. It has one fixed phase when the data is at one level and when the data is at the other level, phase is different by 180 °. A Binary Phase Shift Keying ( BPSKD ) signal can be defined as

The demodulation of a BPSK modulated signal, the carrier must be in phase and frequency synchronism with the carrier available at the transmitter. The carrier for the purpose of demodulation can either be transmitted along with the modulated signal or extracted from the modulated signal at the receiver. Two commonly used techniques of carrier recovery are signal squaring and Costas loop.

Demodulation of a BPSK signal can be considered a two-stage process.
  1. Translation back to baseband, with recovery of the band limited message waveform
  2. Regeneration from the bandlimited waveform back to the binary message bit stream.

Translation back to baseband requires a local, synchronized carrier.

Stage 1
Translation back to baseband is achieved with a synchronous demodulator. This requires a local synchronous carrier. In this experiment a stolen carrier will be used.

Stage 2
The translation process does not reproduce the original binary sequence, but a band limited version of it. The original binary sequence can be regenerated with a detector. This requires information regarding the bit clock rate. If the bit rate is a sub-multiple of the carrier frequency then bit clock regeneration is simplified.