With the advancement of FPGA technology, the integration of digital communication techniques with FPGAs has become a significant trend in modern digital communication systems. To make high-speed communication systems more compact, cost-effective, and energy-efficient while improving device reliability, QPSK (Quadrature Phase Shift Keying) digital modulation is widely used due to its high spectral efficiency, excellent spectral characteristics, and strong anti-interference performance. By implementing QPSK modem code using Verilog on the Xilinx FPGA platform and utilizing its built-in IP cores, the system demonstrates full modulation and demodulation capabilities, offering advantages such as high integration and easy software upgrades.
QPSK technology is extensively applied in digital communication systems because of its robust anti-interference capability, good error performance, and efficient use of spectrum. As very large-scale integrated circuits have evolved, FPGAs have become increasingly popular in digital communication systems. Various methods for implementing QPSK on FPGAs have been proposed. In this paper, Direct Digital Frequency Synthesis (DDS) is implemented on an FPGA, where phase modulation is achieved by controlling the output phase of the DDS signal (carrier). Except for D/A conversion, all other processes can be realized on the FPGA. The QPSK modulated signal is a carrier-suppressed signal. Unlike some continuous-phase modulated signals, the reference carrier cannot be directly extracted using a conventional phase-locked loop or narrow-band filter. However, the carrier phase changes only take on a finite number of discrete values, so the phase information of the reference carrier is embedded within the signal. Nonlinear processing is then used to eliminate the modulation information, generating a component that maintains a certain relationship with the original carrier's phase. This allows for signal purification and recovery of the suppressed carrier, thereby completing coherent demodulation. The QPSK modem designed in this paper features compact size, low power consumption, high integration, strong software portability, and strong interference resistance, aligning with future trends in communication technology.
1. Basic Principle of QPSK Modulation
Quadrature Phase Shift Keying (QPSK) is a special case of M-ary Phase Shift Keying (MPSK) that uses four different phases of the carrier to represent digital information. Since each phase represents two bits of data, each symbol is referred to as a two-bit symbol. The first bit is denoted as 'a', and the second bit as 'b'. These two bits are usually arranged using Gray code. When ab is 00, the carrier phase is 0°; when ab is 01, it is 90°; when ab is 11, it is 180°; and when ab is 10, it is 270°.
The generation of a QPSK signal can be done through either a phase modulation method or a phase selection method. Given the practicality and widespread use of the phase modulation method, this paper adopts that approach. A block diagram of the QPSK signal generated using phase modulation is shown in Figure 1. The serial-to-parallel converter splits the input binary sequence into two parallel bipolar sequences. These sequences are labeled as 'a' and 'b', respectively, and each pair forms a two-bit symbol. The bipolar pulses of 'a' and 'b' are then bi-phase modulated using two balanced modulators for the in-phase and quadrature carriers. The outputs from these modulators are combined to produce the final QPSK signal. The phase encoding logic is as follows: when the two-bit symbol ab is 11, the output phase is 315°; when ab is 01, it is 225°; when ab is 00, it is 135°; and when ab is 10, it is 45°, as illustrated in the table below.
[Image: Block diagram of QPSK modulation]
Data vs. Phase
Traditional QPSK analog modulators typically use quadrature modulation. While all-digital modulators also employ this method, they replace analog components with digital signal processing algorithms. QPSK uses four distinct carrier phases to represent two bits of information. There are two main approaches: phase selection and quadrature modulation. The phase selection method is further divided into two types, A and B. In this paper, phase selection method B is used, as shown in Figure 1.
[Image: QPSK Modulation Block Diagram]
Figure 1: QPSK Modulation Block Diagram
2. MATLAB Simulation of QPSK Modulation
There are many MATLAB codes available online for QPSK modulation simulation. The author of this article has written a simple code that illustrates the basic principle of modulation. Below is the code, which includes clear comments and should be studied alongside the demodulation part.
```matlab
clear all;
clc;
M = 4;
Ts = 1;
Fc = 5;
N_sample = 8; % Number of samples per sine wave
N_num = 300; % 100 bits for I and Q channels
Dt = 1 / Fc / N_sample;
t = 0:dt:N_num*Ts - dt;
T = dt * length(t);
Noise = (1/500) * randn(1, length(t)) * 32767; % Add noise
Py1f = zeros(1, length(t));
Py2f = zeros(1, length(t));
for PL = 1:1:700 % Increase noise intensity
D1 = sign(randn(1, N_num));
D2 = sign(randn(1, N_num));
d = [d1; d2];
D = reshape(d, 1, []);
Gt = ones(1, Fc * N_sample);
S1 = sigexpand(d1, Fc * N_sample);
S2 = sigexpand(d2, Fc * N_sample);
B1 = conv(S1, Gt);
B2 = conv(S2, Gt);
S1 = B1(1:length(S1));
S2 = B2(1:length(S2));
St_qpsk_1 = S1 .* round(cos(2*pi*Fc*t)*32767);
St_qpsk_2 = S2 .* round(sin(2*pi*Fc*t)*32767);
St_qpsk = St_qpsk_1 + St_qpsk_2;
% Power spectrum of the transmitted signal
% plot((abs(fft(St_qpsk, 2048))).^2);
end
```
This simulation helps in understanding the behavior of QPSK signals under different conditions and serves as a foundation for further analysis and implementation.
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