Study of Digital Modulation Schemes using DDS 1. Introduction Phase shift keying(psk) is a simple form of data modulation scheme in which the phase of the transmitted signal is varied to convey information. In PSK the frequency of the carrier remains constant. Phase-shift-keying offers a simple way of increasing the number of levels in the transmission without increasing the bandwidth by introducing smaller phase shifts.the most common form of PSK is BPSK. BPSK encodes 0 phase shift for alogic input and a 180 phase shift for a logic 0 input. When PSK is implemented with a quadrature modulator it is refer to as QPSK. BPSK has only two phases, 0 and. All BPSK signal can be represented as φ1 (t) = φ 1 (t) = 2 cos 2πf c t T 2 cos 2πf c t T 0 t T 0 t T Below shows the BPSK signal: Fig1. Binary phase shift keying QPSK has four phases,0, /2,3 /2,2. For a given bit-rate, QPSK requires half the bandwidth of PSK and is widely used for this reason. With QPSK, the carrier undergoes four changes in phase and can thus represent 4 binary bits of data. Although this may seem insignificant initially, a modulation scheme has now been supposed that enables a carrier to transmit 4 symbols of information instead of 2, thus effectively doubling the bandwidth of the carrier. Like BPSK, the waveform of QPSK has the constant envelope and discontinuous phase at symbol boundary. However, it is intuitively clear that QPSK transmits data twice as fast as BPSK does. Also the distance of adjacent points of the QPSK constellation is shorter than that of the BPSK. This is cause more difficulty, in
comparasion to BPSK, to distinguish those symbols, therefore, the error performance is degraded. Fig 2 QPSK signal constellation The GMSK Modulator Baseband block modulates using the Gaussian minimum shift keying method. The output is a baseband representation of the modulated signal. 2. Description of the System In this project, I implemented three modulation techniques to the DDS signal. Two methods have been implemented to generate DDS signal according to the lecture note, Rom look-up table method,pulse output method. The simulation results show later are mainly used Rom look-up table method. During generating DDS signal, four sets of parameter have been implemented, they are (1) N=12,W=10,delta_R=3,na=8 (2) N=12,W=10,delta_R=1,na=8 (3) N=10,W=6, delta_r=4,na=6 (4) N=10,W=6, delta_r=1,na=6 The whole diagram scheme is DDS signal Modulation AWGN channel Demodulation BER plot. The modulation methods are involved in BPSK, QPSK, GMSK. The AWGN channel is working on SNR = 1:18. The theoretical BER values are obtained from Q function. 3. Simulation & Result The simulation involved in selecting appropriate parameters, calculate BER for each modulation scheme, compare the result to the theoretical result. Two sets of simulations have been investigated and run, one is using random integer as signal source, another is using DDS signal generator as signal source. The Fig 1 shows the SER result for BPSK, QPSK, 8-PSK,16-PSK with general random binary input.
Based on Q-function, we can directly get theoretical BER results. The function is shown as below: E b = 0.5* Q ρ b Fig.3 Theoretical result for PM by using Q-function
Fig.4 Phase modulation performance result with random binary input Fig.5 GMSK modulation performance result
Fig.6 Eye diagram for Binary signal source Fig.7 Eye diagram for BPSK modulated signal
Fig.8 Eye Diagram for QPSK modulated signal Fig.9 Performance result for BPSK, QPSK with DDS signal
Fig.10 QPSK modulation with different DDS generator parameter setting 4. Result analysis When using DDS signal with setting as N=12,W=10,delta_r=3,na=8, the BER performance for BPSK is better than using random binary number as signal source. From the plot, we can see the BER performance even close to theoretical results. However, QPSK is not as good as using random binary source, and off theoretical result. The reason might be the inappropriate parameter selection for QPSK. Also from the result, it is obvious that the smaller step size, the better system performance. The reason is because the smaller step size make the signal source more close to real analog signal, therefore, overcome ISI effect and improve the performance. Due to time limitation, lots of idea haven t been implemented yet, the analysis is not enough. I will investigate more cases after I get a chance. 5. Survey on the modulation schemes used in different commercial standard FSK is one of the simplest forms of data modulation. This modulation scheme us easily implemented in a DDS. The product of Analog Device AD9852 has been implemented by this modulation scheme. In the mean while, QPSK, DQPSK have been used by AD9853, QAM scheme has been implemented in AD9856, FM has been used in AD9850.
6. Typos found in lecture Page132 Location: The first paragraph, line 4, sentence: although it is customary to limit r to F / 4.. Modify: r --> F clk Reason: From understanding of the content, here should use F clk (Can you give me extra credit for this issue) Page138 Location: The last second paragraph, line 2, sentence: The effect of using a large number of address lines W and a small number of output bits na and vice versa is shown in figures 4.6 and 4.7 Modify:.is shown in figures 4.7 and 4.6 Reason: From observing the plot and reading the table Reference: [1]Soft Radio, Reed [2] A Technical Tutorial on Digital Signal Generator, Analog Devices clk