Ofdm Communication System Base Band Transmitter Design

Nowadays, wireless communication technology is developing rapidly. As a new kind of physical layer transmission technology, orthogonal frequency division multiplexing (OFDM) gets widespread attention and application, because of its advantages, such as fast transmission rate, high spectrum efficiency etc. With the development of the wireless communication, the amout of the wireless users is growing, and the wireless spectrum resourse becomes very scarely. For the purpose of full use of the spectrum resourse, cognitive radio (CR) has been proposed. By spectrum sensing the authorized users, it decides whether using the authorized frequency band to do the wireless communication or not.National 863 project dynamic spectrum sharing networks finishes the functional demo of the dynamic spectrum sharing at the frequency band from 694MHz to 806MHz, tests and verifies the feasibility of the cognitive radio. The network node is based on the sofrware radio in this project. The features of physical layer wireless transmission link use OFDM technology to achieve, in software of the baseband processing unit. The design and implementation using field programmable gate array (FPGA) of baseband transmitter is the emphasis of this paper.In accordance with the requirements of physical layer, this paper designs the program of OFDM communication system baseband transmitter, including system timing, data transmission link and the interface of baseband controlling radio frequency (RF). System timing provides the time synchronization benchmark for the whole system. Data transmission link finishes the OFDM signal modulation and the interface of digital-to-analog converter. The interface of baseband controlling RF achieves the control of the states of the RF.This paper completes the implementation of OFDM communication system baseband transmitter, in the software radio hardware platform which is self-control. For the three modules: system timing, data transmission link and the interface of baseband controlling RF, dividing detailed function modules achieves timing the system accurately, processing the send data and controlling the RF action correctly. For each module, timing simulation and online testing have been carried on. All parts meet the transmitter desing program through the verified. At the same time, it tests the baseband performance through the dynamic spectrum sharing processor, which ensures the baseband can transport the control orders from the upper layer accurately, processes the data correctly and steadily.This paper also analyzes the reason why the interference cognitive users bring to anthorized users in cognitive OFDM system, and uses the way of adding window in time domain to implement the out-of-band interference of actual system signals. The suppression of the out-of-band power leakage is under the receiver sensitivity. The receiver of the anthorized users is the TV. The result of the suppression doesn't impact the normal transmission of the TV signal.