A two-dimensional （2-D） polynomial regression model is set up to approximate the time-frequency response of slowly time-varying orthogonal frequency-division multiplexing （OFDM） systems. With this model the estimation of the OFDM time-frequency response is turned into the optimization of some time-invariant model parameters. A new algorithm based on the expectation-maximization （EM） method is proposed to obtain the maximum-likelihood （ML） estimation of the polynomial model parameters over the 2-D observed data. At the same time, in order to reduce the complexity and avoid the computation instability, a novel recursive approach （RPEMTO） is given to calculate the values of the parameters. It is further shown that this 2-D polynomial EM-based algorithm for time-varying OFDM （PEMTO） can be simplified mathematically to handle the one-dimensional sequential estimation. Simulations illustrate that the proposed algorithms achieve a lower bit error rate （BER） than other blind algorithms.
为解决一些基于分簇的密钥方案中簇头节点被捕获时会对其它节点产生严重的安全威胁问题,基于对称多项式和LU矩阵提出一种高效的基于分簇的LPKPM(key pre-distribution management scheme using LU matrix and polynomial)方案。将对称多项式的LU矩阵分解成L矩阵和U矩阵,对簇头节点进行密钥预分配,提高簇头节点的安全阀值。理论分析和模拟结果表明,该方案相比其它方案有较好连通性和安全性,降低了存储消耗。
