Study On Synchronization Technique In Broadband OFDM System And Its FPGA Realization

OFDM (Orthogonal Frequency Division Multiplexing) has been widely applied in BWA (Broad-band Wireless Access) systems due to its high bandwidth efficiency, good performance in anti multi-path fading and narrow-band interference suppression as well as other prominent advantages. Furthermore it has already become a research hotspot in radio communication field as the key technology in future mobile communication systems. However, OFDM is very sensitive to frequency offset, timing error and phase noise, a tiny synchronization offset may lead to the deterioration of the BER (Bit Error Rate) performance of system or even communication failure. Therefore synchronization performance exerts a direct influence on data transmission in broadband wireless OFDM system.IEEE 802.11a based WLAN system has been widely recognized, in which OFDM is employed as the PHY (Physical) layer access technology to enhance its high performance. Without loss of generality, study and realization of IEEE 802.11a based synchronization technologies can be easily applied to other OFDM based BWA systems.The development history and research actuality of OFDM technology has been reviewed in this dissertation. The basic principle of OFDM technology, distinguishing feature and technique preponderance are also expatiated in detail. In addition, a detailed theoretical analysis and simulation are employed to discuss the effect of timing offset, carrier offset, sampling offset and other non-idea factors on OFDM system. Some simple but practicable OFDM synchronization algorithms are presented based on the special structure of IEEE 802.11a based OFDM PPDU frame in both time and frequency domain. Simulations are also used to make a detailed analyze and comparison of the estimation performance of algorithms and BER performance of the system pre-and post error compensation. Based on the simulation results, some new methods are proposed to improve the acquisition range and estimation accuracy of some synchronization algorithms, such as the frequency domain timing offset estimation and so on.According to the performance requirements of the sync-system, a stepped time-frequency domain synchronization system is developed on the basis of full consideration to the performance of algorithms and realization complexity. CORDIC based complex phase extraction method is adopted to effectively guarantee the extraction accuracy of CFO (Carrier Frequency Offset). And a multiple peak value searching strategy in time domain is also proposed to effectively improve the reliability of OFDM symbol timing. A frequency domain long training symbol based joint estimation and compensation method of timing offset and channel frequency response is designed to ensure the estimation performance while significantly reduce the complexity of the system design. And meanwhile, IEEE 802.11a based PHY transceiver BBP (Base Band Processor) is also designed to construct a complete set of experimental OFDM based wireless transmission platform with the proposed OFDM synchronization system using ALTERA Cyclone & Cyclone II serials FPGA chips together with MAX2829 RF transceiver and other hardware devices.Finally, indoor wireless data transmission is implemented to test the OFDM frame detection, symbol timing, CFO (Carrier Frequency Offset) estimation, timing offset estimation and the decoding out of the OFDM PPDU frame after pilots based synchronize processing in frequency domain, and to verify the validity of the proposed OFDM synchronization system and the reliability of wireless transmission for the developed broadband OFDM system.