Study On The Key Technology Of OFDM System In WMAN

Orthogonal frequency division multiplexing (OFDM) is a kind of multi-carrier digital modulation technology. OFDM is highly robust against inter-symbol interference (ISI) and multi-path fading effect. It has attracted considerable attention as a suitable technique for high-data-rate wireless communication. OFDM is very sensitive to synchronization error and channel estimation error. How to accomplish synchronization and channel estimation in IEEE802.16 OFDM system is a crucial problem that needs to be tackled. This paper focuses on the research of dealing with this crucial problem and designing of OFDM system with both hardware and software. The main research and innovation of the paper include:1. On the basis of analyzing IEEE802.16 protocols, taking into account the special frame structure of IEEE802.16 based system, the paper explores a time-delay related frame synchronization algorithm, which is based on training symbols. A new kind of frame synchronization algorithm, employing a special weighted sequence, is put forwarded. The proposed algorithm, with much reduced computational complexity and less sensitive to frequency offset, achieves better performance in precise synchronizing. The paper also discusses symbol timing synchronization algorithm. Making use of the redundant information included in orthogonal sub-carriers, the paper puts forward a frequency offset estimation algorithm with improved performance and reduced complexity, where the "virtual carrier" is exploited. The simulation results demonstrate that the proposed algorithm achieves the same precision as that of super-high resolution subspace MUSIC algorithm, but with greatly reduced computational complexity of subspace decomposition processing. Moreover, the algorithm makes very efficient useof the data, and its pull-in range covers 0-2π . Even in the environment with lowSNR, the proposed algorithm presents superior synchronization performance. All the features make it satisfy IEEE802.16 OFDM system very well. Considering the necessity of timing recovery in the environment with low SNR and high BER, the paper proposes a unified timing recovery algorithm, where the Viterbi-like algorithm isemployed. Simulation results show that the algorithm achieves a significant performance gain over many now-existing algorithms with suffering from either less efficient or too much time delay. It can work very well even at the condition of detector BER greater than 10(-2). The characteristics of the algorithm make it suitable for 802.16 WMAN applications.2. The paper investigates both block-type pilot linear minimum mean square error algorithm and IEEE802.16 OFDM channel estimation analysis. Simulation results confirm the block-type pilot structure is more suitable for IEEE802.16 system. An optimal pilot setting algorithm, based on space frequency domain, is explored and the pilot chart is presented. The proposed algorithm finds its application in MIMO-OFDM. Simulation results show the algorithm presents a good approximation to the perfect channel state information (CSI) curve, with about 2dB difference in SNR. The simulation results also demonstrate the algorithm achieves fairly high stability in the environment of fast fading, performance of the proposed algorithm is better that of pilot training channel estimation.3. Taking into account the characteristics of IEEE802.16 OFDM system, OFDM system simulation platform is constructed in Matlab, exploiting simulink as a tool. OFDM modulation and demodulation simulation system are configured on LabVIEW platform. All the system signal processing is simulated, including defining system parameters, designing modulation model, and verifying all the proposed algorithms under different environments, such as white Gaussian noise, multi-path fading, with or without guard interval etc. Bit error performance is evaluated. The research provides valid theoretical basis for practical OFDM system performance evaluation.4. Taking advantage of software radio, the paper designs a hardware platform with both 256-IFFT/FFT and 512-IFFT/FFT OFDM schemes co-existing in one platform. You can predetermine one from the two schemes to carry out almost the same model function with different system performance and parameter setting.5. Referring to IEEE802.16 standard, the paper proposes a design method for generating signals and frames suitable for laboratory investigations implemented in laboratory environment.6. Based on 6701evm digital evaluation card, combined with analogy front-end, the paper designs a DSP software model to deal with baseband signal processing. An overall OFDM scheme, with modulation and demodulation function, is accomplished. Both experimental test and performance analysis demonstrate that the proposed scheme is feasible.7. The paper proposes a software digital modulation model, where you can predetermine either 16QAM constellation mapping or QPSK constellation mapping to fulfill the function. According to the selected modulation scheme, you can set up all kinds OFDM links with different features. Before setting up these links, you can select either Radix-2 or Radix-4 modulation mode to accomplish IFFT/FFT software modulation model. In order to accelerate out-band OFDM power spectrum decreasing, a rising cosine roll-off window is added, where the roll-off factor β is software selectable.8. Combined with the hardware platform, a software Viterbi decoding method, based on chained data architecture, is proposed. The method is suitable for convolutional decoding of 6701evm platform. Applying this method to demodulation mode can greatly enhance the capability of the system to eliminate multi-path fading.