Introduction to Smart Antennas

Introductory Chapter: Smart Antennas and Beam-formation
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In contrast, if the handset was continuously scanning, the approaching tower could be sensed to negotiate the hand-off before the link is jeopardized. Constantine Balanis.

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In reality, there are no smart antennas, but instead smart antenna systems. This is a critical point from an engineering perspective since it highlights the complexity and challenge of designing modern communication systems. The focus of his presentation was using an adaptive system to steer null points in addition to the beam in an antenna array using a least mean square LMS algorithm.

He began with a simple linear patch array with fixed uniform amplitude weights, since an analytic solution was practical and could be used to validate a simulation setup.

Dr. Michael D. Zoltowski, "From Array Processing to Smart Antennas to MIMO"

However, once the simulation results were verified for confidence, designing a more complex array with weighted amplitudes accompanying the element phase shift was only practical through simulation. While beam steering will create a device centric system by targeting individual users on massive multiple input multiple output MIMO networks, null steering can improve efficiency by minimizing interference to other devices. If you would like to learn more about how PADT approaches antenna simulation, you can read about it here and contact us directly at info padtinc.

You must log in to post a comment. The radiating properties of these antennas are described by a radiation pattern, which is a plot of the radiated energy from the antenna measured at various angles at a constant radial distance from the antenna.

Introduction to Smart Antennas

In the near field the relative radiation pattern shape varies accorging to the distance from the antenna, whereas in the far field the relative radiation pattern shape is basically independent of distance from the antenna. The radiation pattern of a directional antenna is shown in Fig. The plot consists of a main lobe also referred to as major lobe , which contains the boresight and several minor lobes including side and rear lobes.

Between these lobes are directions in which little or no radiation occurs. These are termed minima or nulls. Ideally, the intensity of the field toward nulls should be zero minus infinite d Bs. However, practically nulls may represent a 30 or more dB reduction from the power at boresight.

The direction where the maximum gain occurs is usually controlled by adjusting properly the amplitude and phase between the different elements [59]. The main thrust of these efforts has been to develop arrays that would provide both interference protection and reliable signal acquisition and tracking in communication systems [61]. The radiation characteristics of these arrays are adaptively changing according to changes and requirements of the radiation environment.

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Quick search: within: This issue This series All series. Iftikhar Rasheed , Bilal Asad , M. Online publication date: Jan View Most Downloaded Articles. He has authored over papers, reports, and patents. The purpose of this book is to provide the reader a broad view of the system aspects of smart antennas.

Research on adaptive arrays has involved both theoretical and experimental studies for a variety of applications. Adaptive arrays provide significant advantages over conventional arrays in both commu- nication and radar systems. They have well-known advantages for providing flexible, rapidly configurable, beamforming and null-steering patterns [62].

However, this is often assumed because of its flexibility in using the available array elements in an adaptive mode and, thus, can overcome most, if not all, of the deficiencies in the design of the basic or conventional arrays [63]. Therefore, conventional goals, such as low sidelobes and narrow beamwidth in the array design can be ignored in the implementation of an adaptive array. Nevertheless, much work has drawn attention toward these impairments of adaptive arrays and reported the serious problems, such as grating nulls, with improper selection of element distributions and patterns [64].

An adaptive antenna array is the one that continuously adjusts its own pattern by means of feedback control [9]. The principal purpose of an adaptive array sensor system is to enhance the detection and reception of certain desired signals [62]. The pattern of the array can be steered toward a desired direction space by applying phase weighting across the array and can be shaped by amplitude and phase weighting the outputs of the array elements [65]. Additionally, adaptive arrays sense the interference sources from the environment and suppress them automatically, improving the performance of a radar system, for example, without a priori information of the interference location [66].

In comparison with conventional arrays, adaptive arrays are usually more versatile and reliable. A major reason for the progress in adaptive arrays is their ability to automatically respond to an unknown interfering environment by steering nulls and reducing side lobe levels in the direction of the interference, while keeping desired signal beam characteristics [66]. Most arrays are built with fixed weights designed to produce a pattern that is a compromise between resolution, gain, and low sidelobes.

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However, the versatility of the array antenna invites the use of more sophisticated techniques for array weighting [65]. Particularly attractive are adaptive schemes that can sense and respond to a time-varying environment. The precise control of null placement in adaptive arrays results in slight deterioration in the output SNR. Adaptive antenna arrays are commonly equipped with signal processors which can au- tomatically adjust by a simple adaptive technique the variable antenna weights of a signal processor so as to maximize the signal-to-noise ratio.

At the receiver output, the desired signal along with interference and noise are received at the same time. The adaptive antenna scans its radiation pattern until it is fixed to the optimum direction toward which the signal-to-noise ratio is maximized. In this direction the maximum of the pattern is ideally toward the desired signal.

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However, the output signal-to-interference plus-noise ratio SINR obtained from the array, as the desired and interference signal angles of arrival and polarizations vary, depends critically on the element patterns and spacings used in the array [61]. It utilizes the fact that if some receive antennas are experiencing a low signal level due to fading, also called a deep fade, some others will probably not suffer from the same deep fade, provided that they are displaced in appropriate positions, or in polarity [68].

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Let us now consider the transmission of an information sequence over a frequency non- selective channel. The bit error rate BER is the ratio of the number of bit errors to the total number of transmitted data bits [69]. From Fig.

Introduction to smart antennas

Therefore, fading degrades the performance of a wireless communication system significantly. In order to combat fading, the receiver is typically provided with multiple replicas of the transmitted signal. In this way, the transmitted information is extracted with the minimum possible number of errors since all the replicas do not typically fade simultaneously.

This method is called diversity and is one of the most effective techniques to combat multipath fading. There exist many diversity techniques including temporal, frequency, space, and polarization diversity. A block diagram of a digital communication system with diversity is shown in Fig. The diversity combiner combines the received signals from the different diversity branches. The combiner simply exploits the information embedded in each branch to form the decision variable [26, 70].

In temporal diversity, the same signal is transmitted at different times, where the sep- aration between the time intervals is at least equal to the coherence time, Tc. Therefore, the separated in time channels fade independently and thus, proper diversity reception is achieved.

Frequency diversity exploits the fact that frequencies separated by at least the coherence bandwidth of the channel, Bc , fade almost independently of each other. In this way, the replicas of the transmitted signal are provided to the receiver via separate spatial channels [26, 70].

Wireless LANs with Smart Antennas

It has been shown that a spatial separation of at least half-wavelength is necessary that the signals received from antenna elements are almost independent in a rich scattering, or more precisely in a uniform scattering environment [71]. In antenna diversity, signals received by the different antenna branches are demodulated to baseband with quadrature demodulator and processed with correlator or matched filter detector.

source link The output is then applied to a diversity combiner. This procedure guarantees that fading will be slow and generally not change through a time slot. The option to select the best antenna significantly improves performance [68]. One method of combining in spatial diversity is to weight each diversity branch with its complex conjugate of its own channel gain so that the phase introduced by the channel to be as much as possible removed. The combiner then adds the outputs of this process from each individual branch to form its decision.

This technique is also known as the maximum ratio combiner MRC and is the optimal diversity scheme. However, it needs perfect channel knowledge for maximum performance. Although optimal, MRC is expensive to implement and requires an accurate tracking of the complex fading which is difficult to achieve in practice [26]. It consists of the co-phasing of the signals received from each diversity branch using unit weights before added by the combiner [26]. In switched diversity SC , the decision is made using a branch with SNR larger than a predetermined threshold.

If the SNR drops below this threshold, the combiner switches to another branch that satisfies the threshold criterion.

Another combining scheme is selection diversity SC in which all the branches are monitored simultaneously [70]. The branch yielding the highest SNR ratio is always selected at any one time. The received signal is then multiplied by the complex conjugate of the corresponding branch. The formed decision is based upon this output.

At this point, it would be useful to see the performance of a particular antenna diversity scheme.