ADAPTIVE ANTENNA FOR WIRELESS COMMUNICATION APPLICATIONS in .NET

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9.3 ADAPTIVE ANTENNA FOR WIRELESS COMMUNICATION APPLICATIONS
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The demand for mobile and xed wireless communication continues to grow, making subscriber capacity and reliability of wireless systems a critical issue. Ef cient temporal processing, such as advanced source coding, channel coding, modulation, equalization, and detection techniques, can help alleviate this problem. However, more dramatic improvements may be achieved by exploiting the spatial dimension using a smart antenna system and multiple access techniques.
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Multiple access refers to the simultaneous coverage of numerous users by manipulating the transmission and reception process of signals in time, frequency, code, and space domains. In time division multiple access (TDMA), each user located in the area of service obtains or transmits information in a certain period of time called a time slot. In frequency division multiple access (FDMA), the frequency bandwidth is divided into segments, which are then portioned among different users located in certain service area. In code division multiple access (CDMA), each user obtains a unique random sequence of bits, that is, a unique code, generated by a generator of special random sequences. The information waveform is spread after modulation by this code over the entire frequency bandwidth, which is allocated to all users serviced by the network. The receiver uses the same code to detect the signal with information corresponding only to the desired user by rejecting other users (having other codes) and noises (multiplicative, additive, and due to interference) that exist in the communication channel. In cellular communications, there is another access called space division multiple accesses (SDMA), which is usually used by a division of each cell in sectors using directed antennas to serve each user located in the corresponding sector. The latest form of SDMA usually employs adaptive antenna arrays based on digital beamforming technology [19,24 31]. Here, we will consider the applications of adaptive antennas in different networks on the basis of these four types of multiple access communication [91 96]. Let us rst start with the applications of adaptive antenna in terrestrial communications. 9.3.1. Adaptive Antennas for Outdoor Wireless Communications The important aim in using adaptive antenna arrays is to reject the multiplicative noise caused by multipath fading, slow or fast, to decrease the time-delay effect occurred because of the multipath phenomena, and nally to eliminate the cochannel interference that occurs between subscribers allocated in the same frequency band (in CDMA) or that share the same time frame [75]. To overcome
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FIGURE 9.22. Three typical antenna positions with respect to rooftops level.
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these impairments, an array technology consisting of multiple input and multiple output (MIMO) channel is usually used (see Fig. 9.1). Using M antenna elements, a signi cant increase of antenna gain is achieved plus a diversity gain against multipath fading, which depends on the correlation of the fading among the antenna elements. To provide a low correlation (i.e., diversity gain) between elements, there are several basic ways that can be considered: spatial, polarization, time, frequency, and angle diversity [23,75]. For spatial diversity, the antenna elements are separated far enough for low fading correlation among them. The required separation depends on the obstructions surrounding the antenna such as buildings, trees, hills, and so on. There are three typical situations in the urban environment scene: when the base station (BS) antenna is higher than the overlay pro le of the buildings (Fig. 9.22a); the BS antenna is at the same level as the height of the buildings (Fig. 9.22b); and when the BS antenna is lower than the overlay pro le of the buildings (Fig. 9.22c). Depending on the number of obstructions (scatterers) surrounding the terminal antennas, the angle-of-arrival of the total signal at the receiver will spread dramatically. Thus, for the BS antenna sketched in Figure 9.22a, when only few obstructions surround the user antenna, the angular spread may be only few degrees [75], whereas for the situation in urban areas sketched in Figure 9.22b, the angular spread can exceed ten and more degrees. In these situations, a horizontal separation of antenna elements of 10 20 wavelengths is required. In the third situation shown in Figure 9.22c, the angular spread can reach up to 360 and the antenna element spacing only of a
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