DSP Applications and Student Projects - Digital Signal Processing and Applications with the C6713 and C6416 DSK - page 377

10.11 PHASE SHIFT KEYING—BPSK ENCODING AND
DECODING WITH PLL

See also the two projects on binary phase shift keying (BPSK) and modulation
schemes in Sections 10.12 and 10.13. This project is decomposed into smaller mini-
projects as background for the final project. The final project is the transmission of
an encoded BPSK signal with voice as input and the reception (demodulation) of this
signal with phase-locked loop (PLL) support on a second DSK. All the files associ-
ated with these projects are located in separate subfolders within the folder PSK .

10.11.1 BPSK Single-Board Transmitter/Receiver Simulation

BPSK is a digital modulation technique that separates bits by shifting the carrier
180 degrees. A carrier frequency signal is chosen that is known by both the trans-
mitter and the receiver. Each bit is encoded as a phase shift in the carrier at some
predetermined period. When a 0 is sent, the carrier is transmitted with no phase
shift, and when a 1 is sent, the carrier is phase-shifted by 180 degrees [36-39].

CCS Component
The necessary files for this project are on the CD in BPSK _ sim within the folder
PSK . Figure 10.20 shows the C source program BPSK_sim.c that modulates a bit
stream of 10 bits set in the program. Since there is no carrier synchronization,
demodulation is performed by the same program on the same DSK board.
Build this project as BPSK _ sim . Connect the DSK output to the input to verify
the demodulation of the transmitted sequence. Run the program. The demodulator
program prints the demodulated sequence within CCS. Verify that it is the same as
the sequence set in the array encodeSeq to be encoded.
The array buffer stores the entire received vector that can be plotted within
CCS. Select View Time/Frequency. Use buffer as the address, 190 as

the acquisition and display size, 8000 as the sample rate, and a 16-bit signed integer
format. Figure 10.21 a shows the CCS plot of the received sequence: {1, 0, 1, 1, 0, 0,
0, 1, 0, 1} as set in the program. Note that when the received sequence changes from
a 0 to a 1 or from a 1 to a 0, a change of phase is indicated in the positive and
negative y axis, respectively. Change the sequence to be encoded in the program to
{0, 1, 0, 0, 1, 1, 1, 0, 1, 0} and verify the CCS plot in Figure 10.21 b .

Æ graph Æ

MATLAB Component
The MATLAB program BPSK_sim.m is also included on the CD. It simulates the
modulation and demodulation of a random bit stream. Run this MATLAB file and
verify the plots in Figures 10.22 a and 10.22 b for signal-to-noise ratios (SNR) of 0.5
and 5.0, respectively. They display the transmitted and received waveforms of a
random bit stream. The SNR can be changed in the program. The MATLAB
program also displays the decision regions and detection, as shown in Figures 10.23 a
and 10.23 b , for SNRs of 0.5 and 5.0, respectively. With small values of SNR, the