Array Antennas For Mobile Computer Science Essay

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Abstract - This paper discusses about the usage of phased array antennas for mobile earth stations. With the development of modern-day mobile satellite communications and the increased demands hereof, phased array antennas are now of interest for such applications, also from a commercial perspective. Generally, omni-antennas are well suited for applications where the requirement of the earth station antenna gain is sufficiently low. However, in applications where the gain requirements are larger, the omni-antennas are not feasible choices. Therefore, phased array antennas are the feasible choices. In phased arrays all the antenna elements are excited simultaneously and the distinct main beam of the array is electronically steered by applying a progressive phase shift across the array aperture to achieve higher gain.

INTRODUCTION

Since 1945, with the introduction by Arthur C. Clarke of the concept of an artificial satellite, antennas have played a significant role in the development of satellite communications [1-3]. The simplistic role of antennas in this case can be viewed as follows. The signal is beamed into space by an uplink antenna, electronically processed onboard the satellite, sent back to earth using a downlink antenna, received by the earth station antenna and processed by the electronic receiver [4]. The types of antennas that are used in this system depend on a number of factors. These, to a large extent, are related to the distance between the satellite and the earth.

A variety of configurations involving satellites for wireless communications exists. A geostationary satellite may provide a large coverage similar to the large and fixed coverage of a mobile radio communications system. The communications satellites tend to complement the terrestrial network. GEOS's have a number of drawbacks when it comes to global voice communication. The communication system requires high-power transmitters with large antennas to overcome the propagation loss suffered by the signal due to a large distance. Such conditions are not practical for mobile systems. To overcome these problems, a number of low-orbit satellites have been proposed for mobile satellite communications systems such as LEOs.

ANTENNA TYPES FOR MOBILE EARTH STATIONS

The various satellite communications systems set very different requirements for the earth stations. Depending on the distance to the satellite, data rate, and the satellite repeater gain, the earth stations must comply with certain requirements for the Effective Isotropic Radiated Power (EIRP) and receiver G/T, (ratio of antenna gain to system noise temperature) in the transmit and receive cases, respectively. Hence, a range of different antenna types are employed in earth stations in order to comply with these requirements such as omni-antennas and phased array antennas.

For applications where the requirement of the earth station antenna gain is sufficiently low, omni-antennas are well suited. In particular for LEO constellations, such as employed by Iridium, the path loss is relatively small. Since the LEO satellites are not geostationary, the omni-antennas are even more advantageous because there is no need to direct an antenna main beam towards the satellite. Omni-antennas are also used with GEO constellations, an example being the low data rate Inmarsat mini-C service [9]. Examples are quadrifilar helix antennas [10], drooping dipole antenna [11], and small omnidirectional arrays [12]. In order to increase the directivity at low elevation angles, some omni-antennas are designed with a null in the radiation pattern in the antenna bore-sight direction. An example of such an antenna is the JAST WorldSpace Boat Antenna [13].

In applications where the gain requirements are larger, the omni-antennas are not feasible choices. In such cases the antenna must have a distinct main beam, directed towards the satellite either by mechanical or electronic means.

In phased arrays all the antenna elements are excited simultaneously and the main beam of the array is steered by applying a progressive phase shift across the array aperture. Thereby higher gain can be achieved. Some of the first applications to satellite communications were developed for land-vehicular use under the MSAT-X programme in the 1980s. Two different 19-element phased arrays were constructed, a cavity-baked crossed-slot array by Teledyne Ryan Electronics [14, 15] and a microstrip patch array by Ball Aerospace [16], both in co-operation with Jet Propulsion Laboratory (JPL). In the late 1980s, planar [17] and trigonal shaped [18] phased arrays were developed in Japan, consisting of microstrip and crossed-slot elements. Both were aimed at the Inmarsat aeronautical services which received much interest in the wake of the EMSS project. For the maritime Inmarsat-M service, a 12-element spiral antenna array was developed in [19] and more recently a 19-element patch array [20]. A further example is the 12-element stacked-patch array for use with the Optus MobileSat service, developed in [21].

Typically, satellite applications employ highly directive antennas, i.e. parabolic dishes that are not suitable for mobile applications, where low profile antennas are required. This requirement involves the need of implementing a phased array antenna with beam scanning capability. Furthermore, these antennas need to be setup precisely because of the high angular reception. When pointed directly to a satellite, a signal is received which can be used for television or communication. The setup is fixed and can therefore not be moved; in a mobile situation such high angular reception could be performed with the use of a phased array antenna. To keep up the satellite link on the move, continuous elevation coverage shall be guaranteed, moreover, in order to maintain low profile characteristics, elevation beam steering shall be assured electronically, and a suitable phase shifters network is included in the feeding network in order to provide the right phase relation among the radiating elements feeding signals [36].

High rate downlink communication has been traditionally accomplished via a system employing a high gain reflector antenna. An electronically steered phased array antenna system offers several advantages over reflector antennas in this application. A phased array antenna does not require mechanisms for either deployment or beam steering. Electronic beam steering can be extremely agile, allowing virtually instantaneous beam re-pointing from one target to another. The generation of multiple independently steered beams from a single aperture is also possible. A final advantage of a phased array antenna is that an aperture consisting of multiple, distributed power amplifiers is inherently tolerant of individual amplifier failures, avoiding catastrophic single point failures, without requiring complex component redundancy [37].

OVERVIEW OF PHASED ARRAY ANTENNA TECHNOLOGY

Nowadays, the phased array antenna technology has been developed to meet the emerging communications need. It remains the most promising type of sharply directional antennas [22]. By assembling a number of antenna elements to form a phased array, the direction of the main beam can be controlled. This is accomplished through the adjustment of the signal amplitude and phase of each antenna element in the array [5]. Accurate pointing of the beam in the desired direction minimizes radiation in the unwanted direction, and improves the signal-to-noise ratio (SNR) and the overall efficiency of the system [32].

Electronically steerable antennas, also known as phased arrays, have been a research topic during more than 60 years with the analysis of linear arrays by Schelkunoff in 1943 [23] being one of the pioneering works. Following the 2nd World War, the interest in phased arrays received a boost as the necessity of improving the existing radar technology became apparent [24]. Thus it was realized that the phased array antennas enable very fast scanning compared to the bulky mechanical radars, prevalent in the air defense systems of that time. In the 1960s and 1970s the increasing armament with ballistic missiles spurred the development of more advanced air defense systems, including ground-based missile tracking radars [25] as well as radars for ships and aircraft [26, 27].

It is often desirable to electronically scan the beam of an antenna. This can be accomplished by changing the phases of the signals at the antenna elements. If only the phases are changed, with the amplitude weights remained fixed as the beam is steered; the array is commonly known as a phased array [5]. As shown in Figure 1, a phased array consists simply of antenna elements, each of which is connected to a phase shifter, and a power combiner for adding the signals together from the antenna elements. The phase shifters control either the phase of the excitation current or the phase of the received signals. When all the signals are combined, a beam is formed in the desired direction. That is, on transmit side, a beam is formed in space, and on receiver side, the signals from the antenna elements add coherently if the signals are received from the correct region of space. A beam forming network is used to either distribute the signal from the transmitter to the elements or combine the signals from the elements to form a single signal path to the receiver. The network may also be used to provide the required aperture distribution for beam shaping and side lobe control [33].

Figure 1: Linear Phased Array Antenna [5]

CHALLENGES OF PHASED ARRAY ANTENNA DESIGN

A number of challenges must be overcome in order to arrive at a well working phased array design. These concern the actual performance of the array as well as simulations and numerical models necessary for predicting the array performance. The height of the antenna array is of some importance. This is particularly the case for aeronautical applications where the antenna should not compromise the aerodynamic shape of the aircraft. However, the height is also important for land-mobile applications. In this case the visual impression of the antenna is very important from a commercial perspective.

A critical parameter of the antenna performance is the G/T which is the ratio between the gain of the antenna and the system noise temperature. This number is also known as the Figure of Merit [34] and it is of interest because it is directly proportional to the ratio of the received carrier power to the noise power spectral density. The antenna loss plays an important role in the G/T in that it diminishes the gain as well as increases the system noise temperature. Another important antenna property is the EIRP which relates to the transmit mode of the antenna and is the product of the gain and input power. Typically, the requirements for the G/T are the most difficult to fulfill since both the antenna gain and loss must be considered. For MSS applications where geostationary satellites are employed, e.g., the Inmarsat system, it may be necessary to scan the beam to low elevation angles, especially if the earth station is positioned in the northern or southern regions of the Earth. Therefore, sufficient G/T must be obtained for a wide range of scan angles. In order to ensure this, the elements must have wide element patterns. They must be located close to each other to avoid the occurrence of grating lobes, and generally the antenna loss and system noise temperature must be kept as small as possible.

Since the mutual coupling is generally larger for small element separations than for large, wide element separations may seem preferable. However, it is also well known that a small element separation has the effect of reducing the impedance variation with scan angle [35]. From the above it is clear that a number of challenges exist and that the successful design of a phased array antenna for wide-angle scanning applications is not a trivial task.

From a simple investigation of a single isolated element it is difficult, if not impossible, to ascertain the overall array performance. While application of simple array theory, i.e., the "array factor", may provide an approximate assessment of radiation properties, such as main beam directivity, side lobes, and grating lobes, it basically disregards all the challenging issues [35]. It is therefore important to be able to model the array with sufficient accuracy, that is, to predict the mutual coupling, the active element patterns, the resulting array scan loss, and ultimately the array G/T.

In many cases, a numerical simulation of an entire array is difficult to conduct and requires large computational resources. In order to avoid this problem it may be preferable to model the elements of the array individually or in pairs of two to assess the mutual coupling. Alternatively, the array may be modeled as an infinite array. In the case of a large or moderate-sized array, both approximate models may imply significant reductions of the computational complexity. However, these approximations may yield unacceptably inaccurate results. A further aspect of the array performance is how the radiation is affected by surrounding structures such as a finite ground plane which vary in shapes, curvatures, etc.

MAIN TYPES OF APPLICATIONS

While the early applications of phased arrays were almost exclusively military, the technology slowly permeated into civilian applications during the late 1980s and early 1990s. It remained a very expensive technology, employed mostly for specialized applications such as air traffic control in airports [25, 28], as well as various space-borne applications [29]. During the 1980s and 1990s early experiments with mobile satellite communications systems were conducted. In the Mobile Satellite Experiment (MSAT-X) in the USA [30] and the Experimental Mobile Satellite System (EMSS) in Japan [31], a number of phased array antennas were developed for land-vehicular and aeronautical applications [14, 16-18]. With the continued breakthroughs and cost reductions of electronic components, phased array technology is no longer prohibitively expensive. Furthermore, the development of modern-day mobile satellite communications and the increased demands hereof, imply that phased arrays are now of interest for such applications, also from a commercial perspective.

The Phased Array Antenna is the heart of a communication on the move terminal, that can be widely used for military vehicles and general ground applications where communication on the move and space saving are mandatory requirements [36]. An array of antennas may be mounted on a mobile (vehicle) to communicate directly with a satellite, along with its control circuitry, to steer a beam toward the satellite. As the direction of the satellite with respect to the mobile is changing constantly due to vehicle movement, it requires constant tracking of the satellite and adjusting of the direction of the beam such that it points toward the satellite. Apart from this, the structure of the land-mobile antenna also needs to take into account the aesthetic aspects, which is not the case for base-station antennas [38].

Nowadays achievements on mobile satellite systems (MSS) communication give a great concern for research in global wireless communication fields. Mobile satellite communications systems, or Mobile Satellite Services (MSSs), enable people to communicate from almost anywhere on the surface of the Earth without the need for fixed communications infrastructure, e.g., traditional land-line and cellular phone services. Important areas of application are ships (maritime), aircraft (aeronautical), and ground use (land-mobile).

Present-day MSSs primarily operate with satellite constellations in the Geostationary Earth Orbit (GEO) and Low Earth Orbit (LEO). The LEO is located at an altitude of about 700-1500 km, while the GEO is at about 35600 km, above the Equator [6]. The early MSSs, including the globally operating Inmarsat, were all based on GEO constellations. Advantages of the GEO are that the entire Earth, except the polar regions, can be covered with 3 satellites and that the satellite positions remain fixed relative to the Earth. Disadvantages are the high altitude which implies a large signal path loss, lack of coverage at the polar regions, and the very costly process of inserting satellites in the high-altitude GEO [7].

Recent years have witnessed an introduction of a large number of mobile satellite (MSS) communications systems. These include a global GEO MSS system, Inmarsat, and domestic GEO MSS systems such as North-American AMSC, Japan's N-Star and the Australian Mobilesat [2]. At present, two types of antennas can be used to access GEO MSS services. They include (1) fixed position antennas, which are used with portable transceivers, and (2) fully mobile antennas such as those installed on a land vehicle. The fixed position antennas are relatively easy to design, as they have to meet a moderate (approximately 7%) operational bandwidth and a medium (about I0dBi) gain. There are a number of complete portable systems in the commercial market that include both a transceiver and an antenna [8]. One inconvenience of portable systems is that they require the user be stationary with respect to the ground. This inconvenience can be overcome with a mobile antenna system.

In the late 1980s it was concluded that LEO constellations were no longer infeasible for commercial MSSs [6] and some of the attention shifted from GEO to LEO. Within a relatively short time a number of LEO services sprang to life. The developments of several Low Earth Orbit (LEO) satellite systems have promised worldwide connectivity with low delay real-time voice communications. Furthermore, the path loss is much smaller and thus the requirements for the antenna gain of the user terminals are not so severe [7]. Since the LEO satellite systems revolving around the Earth overlay mobile terminals (MT) or Earth stations over several minutes only, a sophisticated LEO satellite tracking must be introduced. Phased array antenna is seen to be the most promising solution [5]. There have been increasing demands for phased-array antenna systems that can improve system performance as well as expand its functions. A lot of architectures for phased-array antennas have been investigated for use of radar and communication systems because of the attractiveness of beam steering without mechanical elements [4].

CONCLUSION

As a conclusion, phased array antenna consists of a number of antenna elements assembled together in order to control the direction of the main beam. All the antenna elements in the phased array are excited simultaneously and the distinct main beam of the array is electronically steered by applying a progressive phase shift across the array aperture to achieve higher gain. Phased array antennas are the feasible choices for mobile earth station since they provide larger gain requirements compared to omni-antennas. With the development of modern-day mobile satellite communications and the increased demands hereof, phased array antennas are now of interest for such applications, also from a commercial perspective.

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