| In accordance with the provisions of FCC, the concept of Ultra-WideBand is not limited to pulse transmission, but can be extended to a continuous transmission technology, as long as the absolute signal bandwidth is greater than the 500MHz. OFDM-UWB system distributes the high-speed data by parallel/serial conversion to a number of sub-channels of relatively low transmission rate. The bandwidth of each channel signal is 528MHz, which meets the definition of absolute bandwidth of FCC. Since the Period of each channel symbol increases correspondingly , the undesirable impact to the system can be reduced, which is caused by the channel multipath delay spread generated from time-delay spread in wireless channels.Thus eliminate the inter-symbol interference (ISI) caused by multipath as much as possible. Carrier channels in an OFDM system are mutual Orthogonal, so avoid the inter-subcarrier interference. Spectrums are mutually overlapping, which also increase the utilization of spectrum. Currently OFDM-UWB has been proposed for the IEEE802.15.3a ultra-wideband standard, wireless USB PHY standard, ECMA-368 standard, as well as ECMA-369 standard. Bluetooth-SIG adpots OFDM-UWB as their on-air interface for high speed wireless in 2006.Synchronization problems in OFDM-UWB communication system mainly conclude carrier frequency synchronization, symbol timing synchronization and sampling synchronization. Carrier frequency synchronization means the synchronization between the receiver and transmitter and that between the sub-carrier frequency synchronization, which will make a direct impact on the sub-carrier orthogonality, resulting in inter-carrier interference (ICI). Symbol timing synchronization refers to how to find the correct symbol start position at receivers, so as to do error-free demodulation to data. When timing errors estimated make the FFT symbol window beyond the borders, ISI and ICI will work. Sampling synchronization is to estimate and compensate the sampling frequency asynchronism between the transmitors A/D and receivers A / D. sampling synchronization error will lead to ICI among sub-carriers sampled. Although wide bandwidth decreased the sensitivity to frequency offset, the demands of high transfer rate and fast frequency-hopping make it more difficult to achieve carrier frequency synchronization. The captureing speed and accuracy requirement enhance difficulties to timing synchronization. Sampling synchronization makes much less impact to the system. Therefore, this article focuses on the study of symbol timing and carrier frequency offset estimation algorithms for OFDM-UWB system.OFDM-UWB system adopts the scheme of pilot mode, so this paper studies on data-aided algorithms. Data-aided symbol timing synchronization algorithms for OFDM-UWB system could be devided into two categories: correlations peak detection method and subtraction of polarity method. Correlations peak detection method is proposed by Schmidl and Cox. It is constructed by two parts of the same training sequencys. Symbol timing is done by finding the time index of correlation peak. After Schmidl and Cox proposed the method, a large number of derivative algorithms emerged, which optimized Shmidl and Cox algorithm in various aspects. For the existance of multipath fading, the first component may not be the strongest. Therefore, the correlations peak detection algorithm and its derivatives have serious misjudgment rate in the serious multi-path of UWB systems. Subtraction of polarity methods are proposed to overcome the effects of multipath fading. They use the polarity of pilot symbols to do symbol timing estimation. These kinds of algorithms are simple with high accuracy. They are designed for TFI1, 2, so work no well in TFI3, 4, of which the pilot structure is different. Therefore the realization of algorithms are limited to pilot modes.In allusion to the weakness to multipath of the correlations peak detection method and the limitation to pilot modes of polarity Subtraction methods, a method of correstions polarity extraction is proposed. The algorithm utilizes the characteristic of both TFI1, 2 and TFI 3, 4 that pilot polarities turn in a confirm rule. The cross-correlation between received signals and known pilot symbols are done firstly. Then the cross-correlations with that between an adjacent symbols and pilot symbols are calculated, real parts of the results of which reflect the signal polarities, as well as the moment ploarities alternate. Multiply the real part and an adjacent one. According to the alternation among polarities of polits, When the product turns to negative from positive, it can be determined that it is the symbol timing time. Simulations are done under TFI1, 2 and TFI3, 4 modes to evalue timing accuracy of the algorithm. Meanwhile, MSE are contrasted with energy difference method and corrrelations subtraction method. It can be concluded from simulation results that all of these three algorithms achieved a certain timing accuracy. The proposed one performances better then the other two. In the environment adopting TFI3, 4, some degree of superiority are also be shown. All three algorithms are poor performance in anti-noise ability, which is the subject to study further.In carrier frequency offset estimation, the best linear unbiased estimation (BLUE) algorithm, the mean algorithm, and the maximum likelihood (ML) algorithm are main categories. They all could achieve a good estimation accuracy, but with high complexity. Fully understanding the above algorithms, this paper aims at reducing algorithms complexity on the basis of accuracy. An algorithm using two CAZAC sequences of half cycle as the pilot symbols is proposed. In the conditions of ideal noise-free environment, the first half cycle of received signal is same as the second half cycle. In practical communication environment, the phase deviation between the first half and the second half of cycle is the symbol frequency offset estimation. Therefore the estimation can be carried out in a single symbol instead of two symbols in the mean algorithm. In order to fully consider multi-path effects, the paper proposes a semi-cross- contrast scheme, which contrasts a half of the cycle of two adjacent symbols. And then extends the estimation to all the received symbols. Although the algorithm is contrasted between two symbols, the complexity of shifting operations and addition only about 50% of the original algorithm. Finally, the proposed algorithm is analyzed by the derivation CRLB, complexity, and simulation results. The analysis show that the improved algorithm performances close to the original algorithm. The differency is in the order of magnitude of 10-7, while complexity reducs larger than 30%. |
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