BPSK-Binary Phase Shift Keying
Aim: To generate a binary phase shift keyed signal; band limiting.
To study synchronous demodulation –phase ambiguities
In phase shift keying (PSK), the phase of a carrier is changed according to the modulating
waveform which is a digital signal. In BPSK, 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 degree. A Binary Phase Shift Keying(BPSK) signal can
be defined as
Where b(t) = +1 or -1, fc is the carrier frequency, and T is the bit duration.
The received signal has the form = , where is the phase shift
introduced by the channel. The signal b(t) is recovered in the demodulator. If synchronous
demodulation is used, the waveform is required at the demodulator.
The recovered carrier is multiplied with the received signal to generate
The signal is passed through a low pass filter and high frequency component is dumped. If
the channel is noisy; some of the demodulated bits will be in error.
Observations and remarks:
A phase change of 180 degree at every bit transition is observed.
The multiplier output after modulation and original sequence. Note that it is mulitplied with
The demodulated sequence and the original sequence. A small delay is notable.
On increasing the frequency of input we get following output
The graph of demodulated signal before and after switching phase changer.
On increasing the frequency the delay is clearly visible.
Also on changing the DECISION MAKER, we got error in retreiving bit sequence, after
proper tuning we get the sequnece correctly