| Summary: | 碩士 === 大同大學 === 通訊工程研究所 === 94 === The Radio frequency spectrum is human’s extremely precious but limited resource. Under the original analog system, the use of broadcasting and communication Radio frequency spectrum already have been revealed to be very crowded. Recently, with the amazingly quick development of computer technology and semiconductor industry, digital broadcasting and communication are also led to a speedy way. The massive uses of Radio frequency spectrum are already closely linked with people's daily life, and the Radio frequency spectrum utilization is no longer confined to television, radio, or phone calls. Different kinds of demand and function make the use of Radio frequency spectrum more complex and crowded. Therefore, to increase the efficiency of Radio frequency spectrum and frequency interferes mutually, problems such as these appear much more important. Also, we should think about and find the solution.
The technology of OFDM (Orthogonal Frequency Division Multiplexing) has been under research for over 26 years. Recently, due to the mature skills of DSP (Digital Signal Processing) and VLSI (Very Large Scale Integrated circuits), some existing OFDM flaws have been overcome. Simultaneously, the adoption of FFT (Fast Fourier Transform) algorithms eliminated arrays of sinusoidal oscillators and coherent demodulators required in parallel data system to lower the implementation of the technology cost. Therefore, the OFDM technology has been widely used in digital broadcasting and communication, such as DAB、 DVB、DMB、 ISDB-T、 HD Radio (IBOC) 、 DRM、 DVB-H、HDTV and IEEE 802.11a、 802. 11g、 802.16d、806.16e、 802.20 and so on.
OFDM has many merits, such as high spectral efficiency, robustness to channel
fading, immunity to impulse interference, uniform average spectral density, resistance to
multi-path fading and less non linear distortion. The crack of OFDM symbol and its discontinuous boundary, if it has not been processed, is easy to create the out of band power dispersion (Out of Band Power Leakage). It easily disturbs the adjacent channel and also reduces the communication or service quality of broadcasting; Therefore， we must avoid adjacent channel interference with the aim to reduce the problem of out of band power dispersion and find the solution.
In this thesis, we utilized the concept of “Symbol Boundary Smoothing” and proposed a method of reducing out of band power dispersion in DAB. We added some computation data into “Null Sub-carriers” while still accommodating the original system. Therefore we do not have to change its existing transmitting and receiving system. We used the results of “Tail-tracking Approach” and ”Zero-forcing Approach” through further studying, then proposed ”Combination Approach”, with the aims of improving out of band power dispersion and reducing adjacent channel interference .
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