Term paper on Smart antenna system

Elements of Smart Antenna System In this section we will be dealing with some of the basic principles behind smart antennas.

Smart Antenna Receiver The purpose of the receiver in smart antenna system is to combine the received signals into one signal which is used as an input to the rest of the receiver components(such as the channel decoding unit for instance). It basically consists of four parts: Array of antennas, Radio unit, Beam forming unit, and Signal processing unit. These parts are illustrated in the figure below:

Figure – Smart antenna receiver Cont… Figure – Smart antenna receiver

Cont… The radio unit consists of down conversion chains and analog-to-digital converters (A/D). In this part down conversion of received signals, from each elements of the array antenna, takes place. Based on the received signals, the signal processing unit calculate the complex weights with which the received signal from each of the array elements is multiplied. Depending on the optimization criterion, the weight calculating mechanisms may differ.

Cont… Switched beam(SB):- the receiver will test all the pre-defined weight vectors (corresponding to the beam set) and choose the one giving the strongest received signal level. Adaptive approach:- is concerned with maximization of the SIR(Signal to Interference Ratio). This is done by computing the optimum weight vector using algorithms such as optima combining, for instance.

Smart Antenna Transmitter The transmission part of the smart antenna system is schematically very similar to the reception part. Here a single input signal is split into many branches, according to the number of array elements. This is clearly shown in the following figure:

Figure - Transmission part of smart antenna system Cont… Figure - Transmission part of smart antenna system

Figure - Transmission part of smart antenna system Cont… Figure - Transmission part of smart antenna system

Cont… These split signals will then be weighted with the complex weights, which are calculated by the signal processing unit, in the beam forming unit. The weights are used to decide the radiation pattern in the downlink direction. In the radio unit D/A and uplink conversions take place.

Antenna Antenna elements are one of the essential components of a smart antenna system. They convert electromagnetic waves into electrical impulses. They have important role in shaping and scanning the radiation pattern and constraining the adaptive algorithm used by the digital signal processing unit.

Array Design The main beam of a larger array can resolve the signals-of-interest (SOIs) more accurately and allows the smart-antenna system to reject more signals-not-of-interest (SNOIs). However, this brings two main disadvantages: Increased cost and complexity of the hardware implementation Increased convergence time for the adaptive algorithms, thereby reducing valuable bandwidth Thus, a careful network analysis is required to resolve these issues.

Figure - linear array with elements along the Y – axis. It is an array with a group of radiating elements configured in a straight line. A linear array of M even elements with uniform spacing placed along the y axis is shown below. Figure - linear array with elements along the Y – axis.

Cont… For M number of identical array elements, the array factor(AF) for the above linear array can be calculated as: Which can be simplified to:

Cont… Where: - Phase excitation of the individual elements - Amplitude excitation of the individual elements d - The spacing between two consecutive array elements

Cont… The amplitude coefficients control the shape of the pattern and the major-to-minor lobe level. The phase excitations control the scanning capabilities of the array. Therefore, an antenna designer can choose different amplitude distributions to conform to the application specifications.

Planar Array It is an array configuration that is well suited for mobile communication. The planar arrays are more attractive, specially for mobile devices, because of their ability to scan in3-D space. It can scan the main beam in any direction of θ (elevation) and φ (azimuth). A planar array of M x N identical elements with uniform spacing positioned symmetrical in the x y-plane is given below:

Figure – Planar array with uniformly spaced components Cont… Figure – Planar array with uniformly spaced components

Cont… The array factor(AF) for this planar array with its maximum along θ0, φ0, for an even number of elements in each direction can be calculated as: where: - amplitude excitation of each individual elements

Antenna Beamforming General functions of smart antenna system: The direction of arrival of all the incoming signals are estimated using DOA algorithm The desired user signal is identified and separated from the rest of the unwanted incoming signals. A beam is steered in the direction of the desired signal while placing nulls at interfering signal directions by constantly updating the complex weights.

Cont… The information obtained by antenna arrays is applied via algorithms processed by (DSP). DSP has two objectives: To estimate the direction of arrival (DOA) of all impinging signals To determine the appropriate weights to ideally steer the maximum radiation of the antenna pattern toward the SOI and to place nulls toward the SNOI.

Direction of Arrival (DOA) Algorithms The DOA algorithm determines the directions of all incoming signals based on the time delays of incoming signals in all directions received by the antenna array. These time delays depend on the antenna geometry, number of elements, and inter element spacing.

Cont… Time delay of planner array

Cont… Illustration of DOA estimation based on time delay information.

Cont… This clearly shows that the DOA can be determined from the knowledge of the time delay between the two elements.

DOA estimation techniques The techniques can be categorized into two. Conventional methods Subspace-based methods

Do not exploit the statistics of the signal Cont… Conventional methods The DOA is determined from the peaks of the output power spectrum obtained from steering the beam in all possible directions. Do not exploit the statistics of the signal They have poor resolution i.e. the width of the main beam and the height of the side lobes limits its ability to separate closely spaced signals.

Subspace Based Methods Cont… Subspace Based Methods These methods, unlike conventional methods, exploit the structure of the received data. MUltiple SIgnal Classification (MUSIC) algorithm and the Estimation of Signal Parameters via Rotational Invariance Technique (ESPRIT). MUSIC deals with the decomposition of covariance matrix into two orthogonal matrices, i.e., signal-subspace and noise-subspace. Assuming that noise in each channel is highly uncorrelated.

Cont… ESPRIT is another DOA estimation technique, based on the fact that in the steering vector, the signal at one element is a constant phase shift from the earlier element. The advantage of subspace based methods over conventional methods is their high resolution ESPRIT has advantage of being computationally less intensive, requires less storage and does not involve an exhaustive search through all possible steering vectors to estimate the DOA.

Adaptive Beam forming Adaptive algorithm process the information of DOA algorithm to ideally steer the maximum radiation of the antenna pattern toward the SOI and place nulls in the pattern toward the SNOIs. For reference (or training) based adaptive beam forming algorithms, (LMS), the adaptive beam forming algorithm does not need the DOA information but instead uses the reference signal, or training sequence.

Cont… Illustration of the basic concept of how the weights are computed to satisfy certain requirements of the pattern.

First the output y(t) of the array due to the desired signal p(t) is: Cont… First the output y(t) of the array due to the desired signal p(t) is: y(t) = Pejω0t ( ˙w1 + ˙w2) ˙w1 + ˙w2 = 1 On the other hand, the output y(t) due to the interfering signal n(t) is given as: y(t) = Ne j (ω0 t−π/4) ˙w1 + Ne j (ω0 t+π/4) ˙w2

Cont…

Cont… Thus, the above values of ˙w1 and ˙w2 are the optimum weights that guarantee the maximum signal-to interference ratio (SIR) for a desired signal at θ0 = 0◦ and an interferer at θ1 = 30◦.

Cont… The plot of array factor obtained on the basis of the weights derived above.

Optimal Beam Forming Algorithms In optimal beam forming techniques, a weight vector that minimizes a cost function is determined. This cost function is inversely associated with the quality of the signal at the array output, so that when the cost function is minimized, the quality of the signal is maximized at the array output.

Cont… One of the most widely used cost function is mean square error (MSE) based function. Where dk represents the desired signal, rxd is cross correlation and Rxx is covariance. To minimize the cost function:

Solving in terms of the weights, w, yields: Cont… Solving in terms of the weights, w, yields: Wopt represents optimal antenna array weight vector that minimizes the cost function.

Least Mean Square (LMS) Algorithm It is an algorithm used to determine the optimal weight vector values. Thus, the LMS algorithm computes the weights iteratively as: Where µ is the step size for the iteration.

Cont… The following figure shows implementation of LMS algorithm. The advantage of LMS is: It is a low complexity algorithm i.e. it requires no direct matrix inversion and no memory.

General Design Procedure Choose a particular antenna element and design it. Designing an array that is going to be used in the smart antenna. Selecting an adaptive algorithm that minimizes the MSE (Mean Square Error).

Cont… Determine the complex weights that scan the beam toward the direction of the SOI (signal of interest) and place the nulls toward the direction of the SNOIs.

Advantages and Disadvantages Increase the useful received signal level and also lower the interference level. Ability to focus energy toward the intended users which results in increased range. Fulfills the security requirement in a better way.

Cont… Disadvantages Requires separate transceiver chains for each of the array antenna element as well as accurate real time calibration. Antenna beam forming requires intensive computation. pattern-adaptive capabilities and reasonable gain features of the smart antenna requires array antenna elements.

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