Application And Research Of Digital Watermarking Technology In Wireless Transmission Based On OFDM

Due to its high band-width efficiency, its robustness to multi-path, its high transmission speed and its ability of flexible modulation on subcarriers in accordance with the channel conditions, Orthogonal Frequency Division Multiplexing (OFDM) has found its wide applications in high speed data transmission wireless systems, such as digital video broadcast, digital television, wireless local area network and will play a key role in the forth generation mobile communication systems. Compared with former modulations, OFDM is more suitable for multi-media communication. Huge amounts of information, such as audio, data and video can be transported through wideband channel with high fidelity. However, in a highly dynamic and open environment, how to transmit data speedily and accurately without being illegally utilized is particularly important.Traditional watermarking algorithms have dealt with embedding the watermark signal directly into the media to be protected. Combining the watermarking technique with the OFDM modulation technology, this scheme presents kinds of watermarking algorithms. In this algorithms, embedding and extracting are realized at the wireless baseband waveform of the physical layer. The robustness of the algorithms is behaved as the capacity of resisting wireless channel interference.The major achievements of this paper are as follows: (1) The digital watermark technology is introduced from several aspects, such as basic frame, algorithm, evaluation criteria and so on. (2) Spread spectrum digital watermarking theory is studied and spread spectrum technique has been applied in image watermarking algorithms by using two irrelevant PN sequences to spread the spectrum of the watermark signal. Watermark information can be extracted by correlation detection. (3)This paper analyzes the principles and methods of implement of orthogonal frequency division multiplexing technique, focusing on the function of the cyclic-prefix how to resist the multi-path time-delay. Finally, under the premise of ideal synchronization of OFDM frames, the author designs an OFDM transmission/receiving system and applies a great deal of experimental results to analyze the transmission performance of OFDM system in wireless multi-path channel.(4) A kind of spread spectrum watermarking algorithm is presented. In this algorithm, watermarking information is embedded in the constellation mapping data of frequency domain of the OFDM signal. Watermarking information is isolated from carrier information by the principle that inverse constellation mapping can adjudicate the characteristic of input signal correctly in the allowance error scope, and then watermarking information can be restored by correlation demodulation.(5) A kind of digital watermarking algorithm which based on time domain OFDM parallel symbols cyclic shift is presented. The watermarking signal "1" corresponds to OFDM parallel symbols offsetting a time unit to the left. Signal "0" corresponds to OFDM parallel symbols offsetting a time unit to the right. Making use of the synchronization technique of OFDM system, the starting time of symbols can be discovered, and then watermarking signal can be extracted. The two algorithms are experimented on wireless interference in the end.Experimental results show that the two algorithms are both strong by Gaussian white noise disturbance. The former algorithm can assure the robustness against multi-path time-delay but is frail to peak-value-clipping. The latter has a high performance to resist peak-value-clipping disturbance but is weak to multi-path time-delay.