Synchronization In Digital Modulation Systems Based On Software Defined Radio

Orthogonal frequency division multiplexing (OFDM) is a special form of multi-carrier modulation, where in the frequency domain the channel is divided into many orthogonal sub-channels, the frequency spectrum of each sub-channel overlaps with another, which not only reduces the interference between sub-channels but greatly improve the spectrum utilization. Since the signal bandwidth of each sub-channel is narrower than the coherence bandwidth of the whole channel, even if the channel is in frequency selective fading, for each sub-channel, it's relatively flat and non-frequency selective, which can greatly reduce the Inter-symbol interference (ISI). Due to its advantage such as great anti-multipath capability and high spectrum utilization, OFDM applies to high-speed data transmission in multi-path wireless and frequency selective channels. Now, OFDM is widely used in wireless local area network (WLAN), broadband wireless access systems, and it will be written as a mainstream physical layer technology in 4G standards.On the other hand, there are some drawbacks of OFDM, a major one of which is its sensitive to synchronization errors. OFDM requires strict orthogonality between sub-carriers in frequency domain, sub-carrier frequency deviation will cause serious Inter-channel interference (ICI), which will greatly reduce the performance of the system. The transmitter of OFDM system uses the inverse fast Fourier transform (IFFT) to modulate the parallel data to each sub-carrier, while the receiver demodulates down the data from each sub-carrier using fast Fourier transform (FFT), therefore, whether the receiver can determine the right location of OFDM symbols also affects the performance of the system.In this thesis, we study the synchronization algorithms in OFDM systems, especially the carrier frequency synchronization and symbol synchronization, and validate the performance of the algorithms on GNU Radio platform. Firstly, we analyze the characteristics of wireless channel, including its large-scale fading and small-scall fading, then we describe the principle of OFDM and its strengths and weaknesses. When the data is transmitted in burst transmission mode, synchronization by single carrier training sequence in the frame can also get good results, so it is very important to study the synchronization of single carrier synchronization. In this paper, we analyze the case of digital PLL-based synchronization of single carrier QPSK, and on this basis, we expound the synchronization of OFDM, analyze the impact on the performance due to carrier frequency offset and time synchronization error. Then we introduce some common OFDM synchronization algorithms, simulate the algorithms using MATLAB, design a suitable system structure and validate the performance on GNU Radio platform.