Smart Antenna In Wireless Communications

Smart Antenna In 4G Wireless Communications

4G Communications 

Introduction: 

Smart antennas have been around for about 40 years which were first used in RADAR applications in the form of phased array. Smart antennas are currently used in wireless communication systems to provide interference reduction and enhanced user capacity, data rates. smart antenna is an antenna array system that is aided by a processing system that process the signals received by the array or transmitted by the array using suitable array algorithms to improve wireless system performance.

In mobile communication systems, capacity and performance are usually limited by two major impairments. They are multi-path and co-channel interference. Multipath is a condition which arises when a transmitted signal undergoes reflection from various obstacles in the propagation environment. This gives rise to multiple signals arriving from different directions. Since the multi-path signals follow different paths, they have different phases when they are arrive at the receiver. The result is degradation in signal quality when they are combined at the receiver due to the phase mismatch .Co channel interference is the interference between two signals that operate at the same frequency. In cellular communication the interference is usually caused by a signal from a different cell occupying the same frequency band.

 

An antenna array consists of a set of distributed antenna elements:

 Dipoles, monopoles or directional antenna elements arranged in certain geometry (linear, circular or rectangular grid) where the spacing between the elements can vary. The signals collected by individual elements are coherently combined in a manner that increases the desired signal strength and reduces the interference from other signals .The antenna arrays have input or output as RF signals in the analog domain ,in the receive mode , the RF signals are converted to digital domain by analog to digital converters (ADCS) and in transmit mode , the baseband digital signals are converted to RF using digital to analog converters (DACS) .The down conversion from RF to baseband or up 

conversion from baseband to RF can involve the use of IF signals. The baseband signals received from each antenna is then combined using the "smart" algorithms in a digital processing section . The digital processing section can be implemented on a microprocessor or a DSP OR FPGA."Smart" algorithm implementation usually is a software code unless implemented in an ASIC OR FPGA. 

Classification of smart antennas:

Based on the signal processing technique smart antennas can be grouped into four basic types based on 

i) Beam forming is the digital processing has ability to shape the 

radiation pattern for both reception and transmission and to adaptively steer beams in the direction of the desired signals and to put nulls in the direction of the interfering signals .  

ii) Diversity combining a major limiting factor in wireless communication is multi-path fading where the amplitude of received signal fluctuates over time the occurrence of a deep fade where the signal amplitude becomes very small can impair the communication link for a conventional or signal antenna system .this diversity in the received signal , for the same transmitted information ,is exploited by smart antenna processing

schemes. Many simple algorithms weight, such as maximal ratio combining, and selection diversity have been developed to take advantage of using antenna arrays to exploit diversity reception in wireless system. 

iii) Space Time Equalization is multi-path fading in wireless communication can also introduce frequency distortion to received signal .by introducing temporal processing in each antenna element to remove the effect of frequency distortion and doing a spatial combining described above results in mitigating channel induced frequency selective fading and providing antenna gain . schemes are called space time adaptive processing (STAP) or equalization .

 iv) Multiple Input Multiple Output (MIMO) requires array processing at the transmitter and receiver .there are two different types of MIMO schemes: one uses spatial multiplexing (data is converted from serial to parallel and transmitted simultaneously over multiple antenna elements). The receiver also uses multiple antenna elements to receive the signal and applies a maximum likelihood (ML) algorithm to retrieve the simultaneously transmitted symbols and the other uses space time coding using diversity combining techniques to combat fading. 

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