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Abstract-In this paper, the application of Artificial Neural Network (ANN) with back-propagation algorithm and weighted Fourier method are used for the synthesis of antenna arrays. The neural networks facilitate the modelling of antenna arrays by estimating the phases. The most important synthesis problem is to find the weights of the linear antenna array elements that are optimum to provide the radiation pattern with maximum reduction in the sidelobe level. This technique is used to prove its effectiveness in improving the performance of the antenna array. To achieve this goal, antenna array for Wi-Fi IEEE 802.11a with frequency at 2.4 GHz to 2.5 GHz is implemented using Hybrid Fourier-Neural Networks method. To verify the validity of the technique, several illustrative examples of uniform excited array patterns with the main beam are placed in the direction of the useful signal. The neural network synthesis method not only allows to establish important analytical equations for the synthesis of antenna array, but also provides a great flexibility between the system parameters in input and output which makes the synthesis possible due to the explicit relation given by them.

This paper presents the capacity performance of multiple antennas for wireless communication systems. Multiple antennas structures can be classified into single-input multiple-outputs (SIMO), multiple-inputs single output (MISO), and multiple-inputs multiple-outputs (MIMO) systems. Assuming that the channel is unknown at receiver, capacity expressions are provided for each structure. Our results also show that increasing the number of transmitting and receiving antennas for a wireless MIMO channel does indeed improve the channel capacity that can be obtained. __________________________________________________________________________________________ IntroductionDuring the past decade, Wireless Communications and especially mobile communication systems have grown considerably. The radio communication systems must meet a growing number of users and no less demand for new services. New digital technologies are currently being studied to determine the fourth generation of mobile communication systems with the objectives to provide high spectral efficiency and a wide variety of services. In this context, communication systems MIMO (Multiple Input Multiple Output) based on the use of an antenna array at the transmitter and receiver are able to offer high-speed transmission with a minimum quality of service guarantee. The intense research on MIMO systems was inspired by seminal works by Telatar [11] and Foschini and Gans [12] that showed a dramatic linear increase in channel capacity with the number of antennas [13]. MIMO systems allow us to operate two distinct dimensions of a radio link; the first being the Diversity and the second being the Capacity. In this paper we will focus on the Capacity of MIMO channels because it is a performance measure for digital communication systems and it is the maximal transmission rate for which a reliable communication can be achieved. This paper is organized as follows. Presentation of SISO, MISO, SIMO and MIMO Systems (Section II). Then we will present Capacity for all systems and we will concentrate to MIMO channels capacity in (Section III) and we will finish (Section IV) by a comparison between different multiantenna systems and the gain on capacity by increasing the number of antennas. MIMO Systems (SISO-MISO/SIMO-MIMO)

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