The challenges of severe Doppler effects in high-speed railway are considered. By building a cooperative antenna system; an algorithm of joint channel estimation and Doppler frequency offset （DFO） estimation is proposed based on Ricean channel model. First, a maximum likelihood estimation （MLE） algorithm for DFO is designed, show- ing that the Doppler estimation can be obtained by estimating moving velocity of the train and the path loss with the exploitation of pilots that are placed inside the frame. Then a joint detection algorithm for the receiver is proposed to exploit multi-antenna diversity gains. Last, the theoretical Crammer Rao bound （CRB） for joint channel estimation and DFO estimation is derived. The steady performance of the system is confirmed by numerical simulations. In particular, when the Ricean fading channel parameter equals 5 and the velocities of train are 100 m/s and 150 m/s, the estimation variances of DFO are very close to the theoretical results obtained by using CRB. Meanwhile, the corresponding sig- nal to noise ratio loss is less than 1.5 dB when the bit error rate is 10-5 for 16QAM signals.
Vehicular ad hoc networks(VANETs)aim to get the intelligent communication for vehicular networks,they can extraordinarily improve the security and efficiency issues of road traffic.Data dissemination is the base of communication,it acts as an important role in VANETs system.In this paper,we survey main results of data dissemination in vehicle to vehicle(V2V)communications,and expound this review on three parts:routing protocols,mobility model and security,all of which deeply affect the transmission performance in VANETs.We present the main representative research techniques and achievements in each category,and at last,further conclude the research trends and challenges for data dissemination in VANETs.
针对传统多输入多输出(MIMO)系统检测算法先检测的子流分集度较低以及错误传播的问题,提出了一种改进的迭代降维并行检测算法.该算法在每次迭代内对第1个子流遍历取值,其余子流采用排序连续干扰消除(OSIC)算法进行检测,在每次迭代结束时仅输出分集度最高的首子流的估计值,在迭代间通过干扰消除降低待检测子流的维度.仿真结果表明:该算法能以较低的复杂度代价获得逼近最大似然检测算法的差错概率性能;在4×4、QPSK调制的MIMO系统中,相对于传统的OSIC算法,文中算法在误比特率为10-3时获得了9.3 d B的增益.
