Question

A binary data stream 111010010 is to be transmitted using DPSK. Determine the encoded data and phase of the DPSK o/p .

Answer 1

In digital transmission in order to send data we use phase shift keying in which face of the carrier signal changes to transmitt data. Phase shift keying provides various phases which corresponds to the discrete data to be transmitted in the form of bits.

Differential Phase Shift Keying

It is one type of phase modulation where data tranmission occured by changing the carrier phase.Here the modulated signal is relative to the previous signal element. So when a binary bit sequence is transmitted the sequence is encoded in such a way that the next bit depends on previous bit.  

The encoded bit sequence generated according to the following conditions,  

1. If the symbol bit is 1 then there is no transition in the sequence

2. If the symbil bit is 0 then consider next bit if it is 1 then there is transition in the sequence otherwise no transition

In the encosing a next bit depends on previous bit.  

So consider the given sequence,  

1

1

1

0

1

0

0

1

0

1. Consider the first bit which is 1, if 1 is the symbol bit then there will be no transition so the encoded bit will be also 1,
2. Consider the second symbol bit which is 1 and since it is 1 there will be no transition and encoded bit will be also 1.
3. The third symbol bit is also 1 so there will be no transition in the encoding so the encoded bit will be also 1.
4. Next bit is 0 and since the next bit to 0 is 1 and when symbol bit is 0 there will be a transition, so 0 changes to 1 in the encoded sequence.
5. The fifth symbol is 1 and there will be no transition. So encoded bit 5 will be 1.
6. Consider next symbol bit which is 0 and the next bit is also 0 then there will be no transition so encoded bit will be 0.
7. Next symbol bit is 0 and the bit next to it is 1, so there will be transition and encoded bit will be 1.
8. Next symbol bit is 1 and no transition in the encoding so the encoded bit will be 1.
9. The last symbol bit is 0 and there will be tansition as there is not next bit. So the encoded bit will be 1.

So the differential encoded sequence will be ,

1

1

1

1

1

0

1

1

1

Now we have to consider the phase of the encoded sequence,

When the bit is 0 then phase is not reversed and when bit is 1 phase is reversed.  

So the phase sequence of differentialy endcoded data will be,

Let initial phase of sequence be 0,  

0

0

0

0

0

The waveform will be,