Phased Array Antennas For Mobile Earth Computer Science Essay

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Historically parabolic antennas have been used, but as the performance with applications such as radar and satellite systems has become more demanding, the antenna systems are being replaced with phased array systems where directivity can be controlled dynamically using electronic or DSP techniques.

List of Figures

Figure 2.1. Two element array with far-field observation…………………………………4

Figure 3.1. Mobile earth station system architecture……………………………….……..6

Figure 3.2. Adaptive linear phased array antenna

with equally spaced N-elements……………………………………………………………..7

Figure 4.1. Satellite with frequency re-use………………………………………………...10

List of Tables

Table 2.1 Features that control the radiation pattern of an array………………………...

Glossary

Abbreviation Definition

AF Array Factor

BW Bandwidth

EMW Electromagnetic Wave

FNBW First-Null Beamwidth

HPBW Half-Power Beamwidth

EIRP Equivalent Isotropic Radiated Power

G/T Gain/Noise Temperature

VHF Very High Frequency

MHZ Mega Hertz

Chapter 1.0 Introduction

A phased array antenna is a multi-element structure, where the radiation beam width and direction can be controlled electronically; this report will discuss how the technology operates and cover some background theory.

The performance capacity of phased array antenna will be analyzed and applied to a mobile earth station scenario; its ability to combat the effects of multi-path fading in a mobile satellite communications link will be discussed.

The performance of the phased array antenna will be compared to an omni-directional antenna also in the same mobile earth station scenario.

Finally the report will look at some of the main applications of phased array antennas, in satellite and radar systems.

2 Phased array antenna technology

"A radio antenna may be defined as the structure associated with the region of transition between a guided wave and a free-space wave, or vice versa. Antennas convert electrons to photons1, or vice versa [1]."

A phased array antenna is comprised of multiple antenna elements; they are driven coherently and have controllable phase at each of the elements, in order to scan a beam to given angles in space. The amplitude at each element can also be controlled to provide pattern shaping. Arrays can allow more precise control of the radiation pattern, therefore resulting in lower side lobes or pattern shaping. However, the main reason for using arrays is to have a directive beam that can be positioned electronically [2].

2.1. Early development of phased array antennas

During the second World War, many radar systems were developed based on antenna arrays where the element positions could give rise to a wanted or desired antenna radiation patterns, if beam steering was required the antenna would have to be mechanically moved in the required direction so as to direct the antenna main beam.

Much research work on radar systems was driven by the need of the USA to track and detect Russian satellites. The special radar group at Lincoln Laboratories played a key role in research in this field and in the late 1950's, much successful work was carried out in in developing antenna elements and phase control techniques, where further success in the development of phased array antennas was achieved [3].

2.2. Phased array antenna technology

The entire field strength of an array structure can be found by adding the field strength of each individual array element; it is assumed that electrical current is the same in all the elements, and that any mutual coupling between antenna elements can be neglected [4].

It has being reported by Pozar D.M in a paper [5] if the array is uniform (uniform spacing between elements, uniform amplitude, and phase progression and identical elements) then it is valid to neglect mutual coupling between elements.

The generation of a directive radiation pattern is achieved where the field strength of the individual elements will have to constructively add in the wanted direction and to destructively subtract in the other direction.

There are 4 key features of an antenna array which can control the array radiation pattern in the far-field.

Physical distance between antenna elements

Geometric configuration of antenna elements

Amplitude excitation of the individual antenna elements

Phase excitation of the individual antenna elements

Table 2.1 Features that control the radiation pattern of an array.

2.4 Two element array

Two infinitesimal dipole antennas will be considered, the elements are arranged in a linear horizontal plane, the entire field strength in the far-field region can be found using equation 2.1.

(2.1)

Where and are the respective electric field strengths of elements 1 and 2.

Equation 2.1 Total far field electric field strength of 2 horizontally arranged dipole antennas.

Figure 2.1. two element array with far-field observation

The driving current phase difference between each element is given by β and the magnitude of current driving each element is the same across all elements, note this is for the far-field conditions.

From figure 2.2 the far-field, angles and distances can be written as.

(2.2)

(2.3)

(2.4)

(2.5)

This leads to simplifying equation 2.1 which is now written as equation 2.6.

(2.6)

As equation 2.6 indicates the complete field strength of the array is the field strength of a single element which is positioned at the origin, which when multiplied by a value referred to as the array factor (AF) gives the total field strength.

The array factor AF is a very important factor in the design of phased array antennas for the 2 element array it is given by equation 2.7.

(2.7)

The AF given as a normalized value:

(2.8)

The physical separation distance d and excitation phase β between each antenna elements control the complete field strength of the array. Therefore the field strength can be determined through what is termed pattern multiplication which is valid where identical antenna elements are used. It is the product of the far-field of a single antenna element, determined at a central reference point, then multiplied with the AF.

This is shown in equation 2.4.

(2.9)

The method to find AF for two identical elements can be generalized to for N-elements, the reader is recommended to refer to [6] Balanis 3rd edition chapter 6 for a complete theoretical analysis of phased array antennas.

3 Performance

3.1 Introduction

Radio signals transmitted via satellite are subjected to various impairments which may cause the received signal to be attenuated severely. In a mobile satellite link (mobile earth station to satellite or vice versa) signal attenuation is mainly due to the free space loss, shadowing, multipath propagation and signal interference [7].

A phased array antenna will be used to improve the network reception in a multipath channel.

Figure 3.1.mobile earth station system architecture.

3.2. Multipath fading and phased array antennas

Previous researchers have proposed an adaptive phased array antenna which is based on a non-linear method, an "on-off" adaptive algorithm to adjust and control the phase angle of driving current at each element in the antenna system in order to control automatically its directivity pattern to enhance the received signal [8].

The algorithm attempts to maximize output power of the array in the estimated direction of the wanted signal.

Figure 3.2. adaptive linear phased array antenna with equally spaced N-elements.

In figure 3.2 the antenna elements are driven with a controllable phase this enables the beam of the antenna to be directed.

If we assume that K narrow-band correlated signals are incident on the array. The complex signal at the ith antenna element can be written as follows:

(3.1)

Where Ak is the complex valued attenuation constant determined by the propagation path, s(t) is the complex envelope ni(t) , is a random Quantity which denotes the thermal noise at the Ith antenna element and Φk is the electrical phase between the adjacent antenna elements Which can be written as :

(3.2)

Where Φk represents the arrival angle from the broadside of the array, λ the carrier wavelength and d the inter-element spacing.

As shown in figure, the signal Xi (t) from each element is multiplied by a weighting coefficient wi. The wi (i=1,…N) represents complex variable weights in phase which are controlled under a new adaptive algorithm.

3.3 Mobile satellite earth station network performance

A phased array antenna with its ability to direct its main beam, can improve the performance of a mobile satellite communications network, it improves the resilience against co-channel interference and also multipath fading, thus giving a better QOS, it provides a reduced BER and outage probability. The ability to form many beams can be used to service many network users in simultaneously, giving improved spectral efficiency; the ability to adapt beam shapes to match traffic conditions can reduce the handoff rate, which increases efficiency.

An array with beam forming capability in wanted directions and nulls in others is able to mitigate the delayed signals. It can in transmitting mode focus the transmitted energy in the wanted direction, which helps to reduce multipath reflections, which then reduces delay spread. In the receiving mode, the antenna array achieves compensation in multipath fading by the use of diversity combining.

3.4 Co-channel Interference

An antenna array also has the ability to provide spatial filtering, to reduce the co-channel interferences.

In the transmit mode, it can focus the radiated wave by forming a highly directional beam in a smaller area where the receiver is expected to be. This means there is less interference from other directions, where the beam is not pointing. Co-channel interference in transmit mode can also be reduced by forming beams with nulls in the directions of other receivers. This system reduces transmitted energy in the direction of co-channel receivers and requires receiver position information. The reduction of co-channel interference in the receive mode is a major feature of phased array antenna. It does not require prior information about co-channel interferences.

Chapter 4 Phased array antenna application

4.1 Radar systems

Radar was first was first used in the 1930's at the start of the 2nd World War. At

this time the frequencies used where VHF in the 100 MHz to 200 MHz region,

The physical sizes of the antennas were tens of centi-metres. As Radar technology improved, it became possible to use microwave frequencies and several radar systems were allocated to use S band or 3 GHz band.

As a result of this higher gain antennas could be used with dimensions, similar to lower gain antennas at VHF. Higher gain antennas resulted in a narrower beam widths was attainable. Therefore, by rotating the antenna to potential targets, the angle of objects or targets could be discovered within a beam width of the antenna.

The rotation was mechanical whereby an electric motor would rotate the antenna structure. The antenna structure could also be rotated in elevation thus allowing a two-dimensional resolution of the target to be found. Signal processing would allow the range to be found.

At the present time Radar systems, are not rotated mechanically they have being replaced by phased array antenna, where the beam direction is controlled electronically.

And depending on the array configuration, if a planar or spherical array is used it enables a targets to be located in both azimuth and elevation.

The control of the beam in azimuth and elevation allows targets to be

detected and identified, the Radar system can then switch to tracking mode

to follow the movement of the target. By using multiple beam-formers, many targets can

be detected and tracked at the same time.

4.2 Satellite communications

Present and future satellite systems, will require multiple beamed antennas or phased array antennas to provide better gain, beam shaping and frequency reuse facilities depending on the requirements of the communication system.

Multiple spot beams can achieve higher EIRP and G/T than what is achievable by

an omni- directional antenna system; while at the same time employing frequency reuse techniques in order to increase the overall system throughput. By the use of frequency reuse systems, the same spectrum can then be further reused

The frequency reuse system is limited by the beam to beam level of isolation

and also the interference rejection.

MEO and LEO satellite constellations require frequency scanning capabilities, with a view up to 60°, by using frequency reuse schemes; the same spectrum can be reused

multiple times over the coverage area as shown in figure 4.1. However, in

practice, frequency reuse is limited by the achievable beam to beam isolation

and interference rejection. Therefore, a solution with adaptive arrays may be

considered in this case.

Figure 4.1. Satellite with frequency re-use enabled.

The possibility of a higher number of radiating elements in the GPS transmitting array is being considered to provide either high EIRP spot beams over the coverage area or adaptive nulling capabilities.

The efficiency of future satellite communications and navigation systems can

be significantly improved by the use of adaptive antenna arrays with intelligent

beam-forming and beam-steering. Degradation of the system performance caused

by multipath propagation or interferers can be reduced by limited bandwidth.

Side lobe reduction and nulling in the direction of the interferer.

Conclusion

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