RF Simulation Demo: DPSK

[ExamplePrograms]

Overview

Differential Phase Shift Keying (DPSK) is a digital modulation scheme that uses bit patterns to change the phase of a wave as opposed to setting it to a new value. As a result, the demodulator determines the changes in the phase of the received signal rather than the phase itself, making it easier to implement than PSK.

Theory

Phase shift keying that is used for digital transmission in which the phase of the carrier is discretely varied (a) in relation to the phase of the immediately preceding signal element and (b) in accordance with the data being transmitted. Differential Phase Shift Keying is an alternative form of digital modulation where the binary input information is contained in the difference between two successive signaling elements rather than the absolute phase. With DPSK, it is not necessary to recover a phase-coherent carrier. Instead, a received signaling element is delayed by one signaling element time slot and then compared with the next received signaling element. The difference in the phase of the two signaling elements determines the logic condition of the data.
 
The phase relationship between signaling elements for BPSK is the optimum signaling format, referred to as antipodal signaling, and occurs only when two binary signal levels are allowed and when one signal is the exact negative of the other. Because no other bit-by-bit signaling scheme is any better, antipodal performance is often used as a reference for comparison. The PSK signal is converted to a DPSK signal with two rules:

1. a 1 in the PSK signal is denoted by no change in the DPSK
2. a 0 in the PSK signal is denoted by a change in the DPSK signal

The sequence is initialized with a leading 1. An example of the pattern is thus:

PSK        0 1 0 0 1 1 0 1
DPSK   1 0 0 1 0 0 0 1 1

Coherent detection is also necessary for the correct demodulation of narrow-band FSK. We have already noted that the form of the modulation signal gives rise to a bandwidth that is larger than that required by the Nyquist limit, and that has appreciable energy outside this bandwidth. This is in part due to the abrupt changes in the phase of the signal in FSK and PSK. By combining coherent detection with appropriate smoothing of the phase change, narrow band FSK can become very bandwidth efficient, approaching the Nyquist limit, and have low side-lobe levels. This form of modulation, which is a combination of FSK and PSK, is termed minimum-shift-keying (MSK), or fast-FSK (FFSK).

To illustrate the way in which the complexity of demodulation determines the
method used we consider an example:

The modem is used to connect computer equipment to the PSTN to communicate with other equipment. The PSTN has a pass-band from 400Hz to 3400Hz; its bandwidth is 3000 Hz. Modems must be inexpensive if they are to be widely available. Variations in amplitude are common on telephone lines and ASK is not suitable. Incoherent FSK is widely used for low-rate modems. In all, four frequencies are used, two to transmit and two to receive. The bandwidth available per frequency is 750Hz, and with some simple smoothing baud rates up to 1200 baud are possible. For rates above this value PSK is necessary, and rates up to 2400 baud are possible. Bandwidth is given as, BW = 2/T Where, T=2Tb. Hence, bandwidth for DPSK is BW = Fb. Thus the minimum bandwidth is DPSK is equal to Fb, the maximum base band signal frequency.

Exercise

1) Place the Mod_Create Tx Sequence VI in block diagram to generate the bit sequence by specifying the guard bits, sync bits, message bits, and PN order in the front panel.

2) Place the Generate System Parameters VI in block diagram to generate the system parameter coefficients by specifying the samples per symbol, M-PSK, PSK deviation, and symbol phase continuity.

3) Place the Generate Filter Coefficients VI in block diagram to generate the Pulse shaping filter coefficients by specifying the modulation type, pulse shaping filter, pulse shaping filter samples per symbol, matched samples per symbol, filter parameter, and filter length. Also enable the differential phase shift keying option in the front panel.

4) Place the Modulate PSK function VI in block diagram and wire the corresponding terminals.

5) Place the Add AWGN function VI in block diagram to introduce noise to the modulated output.

6) Place the Demodulate PSK function VI in block diagram and wire the corresponding terminals.

7) Place the Generate Synchronization Parameters VI in block diagram and wire the output to input of the Demodulate PSK by specifying the sync. bit in the front panel.

8 ) Place the Bits to Digital waveform VI to observe the generated bits and recovered bits in digital waveform graph.

9) Place the Bit Error Rate function VI in block diagram to measure the BER.

10) Place the constellation Diagram indicator in front panel to visualize the recovered bits.