The rapid time-variation of a fading multipath environment can impair the performance of multiple-input multiple-output orthogonal frequency division multiplexing (MIMO OFDM). This paper proposes a pilot placement method for MIMO OFDM systems under time-varying channels with the guard band. The time-varying channel is described by complex exponential basis expansion model (BEM). We discuss the least square (LS) channel estimation to obtain the minimum mean square error (MSE) and derive the pilot allocation that can satisfy the minimum MSE with regard to guard band in time-varying channels. It is shown that optimal pilot clusters can distribute non-uniformly in frequency domain and minimize the MSE. We generalize our scheme over G OFDM symbols and compare it with comb pilots. It is demonstrated that the proposed approach is more effective than previous work. Simulation results validate our theoretical analysis

In today's world, many public and private services are provided virtually on the Internet. Due to the increasing dynamism and development of computer networks, intrusion detection systems, as one of the hottest topics in network security, has become an attractive area of research for researchers. The intrusion detection system tries to categorize the activity of the connections into two categories, normal and abnormal. In intrusion detection system, each connection is described based on a set of features, and decisions about whether that connection is normal or abnormal are made using those features. The act of determining the norm or abnormality of a connection is called classification. In this article, a method based on combined classification is proposed to detect zero-day attacks. One of the most important innovations in this method is using a new version of the GRASP feature selection algorithm, which is used to diversify the base classifiers. In this method, an attempt is made to produce a subset of different features that have high accuracy; and variety to be used in the assembly stage.Experimental results showed that the method used to create feature subsets has high quality.