PLANAR ANTENNA SOLUTIONS FOR INTER-SATELLITE LINK BASED FUTURE COMMUNICATION SYSTEMS

Abstract

Prehistoric times of civilization in ancient history, are defined by the materials harnessed by the human being during that period such as, the Stone age, the Bronze age, and the Iron age. Similarly, in present we live in the the information age, space age, and the nuclear age. Nowadays, a mobile phone has brought an evolution in the information age, in the context of sharing the information by us, which has changed our lives. Present era of mobile communication is turning into the smart cell phone communication, which extends the core use of mobile phones from communication between Human to Human (H2H), to, in between human and device (H2D), as well as in between one device and another device (D2D) [1, 2]. As per the current status, more than 50% of cell phone owners now own smart phones, and the number of smart phone owners globally, is expected to reach up to 2.87 billion till 2020 [3]. It means nearly every living human being over the age of 20 years, would have some sort of cellphones and it means that in a year or two, almost everyone over the age of 20 is going to have a smart phone, and non-smart phones are going to be things of the past. It leads to the situation, where almost everyone is going to have portal to the web in their pocket, and hence human dependency on the Internet just to function, would be increasing, dramatically. Thus Internet would be ubiquitous, and its usage would be growing drastically, and hence a reduction in time delay is critical requirement for the future wireless networks [4, 5]. Moreover, faster speed and more functionality has been delivered to our cellphones by every generation of wireless networks. For example; the very first cellphones were brought to us by 1G; for the very first time, text was used by us through 2G; video calling and Internet was brought to us by 3G; and the speed that we enjoy today is delivered by 4G. However, demand of more and more data for the smart phones and devices has been growing by the users, as congestion of Internet users has been increasing dramatically. Thus the limits of current generation mobile networks are already pushing the developers to provide the next generation high-speed communication network. 5G is the next generation mobile network which would replace the current generation network 4G. 5G network is expected to be largely operated by using the clouds system which means that it will have better capacity and hence faster data rates than the current 4G networks [6]. It is expected that thousand times more traffic than today’s 4G Long Term Evolution (LTE) networks, would be handled by 5G which would provide approximately 10 times faster speed. For example; to download a 2 hours High definition (HD) movie on 4G, it will take about 8 minutes, while on 5G it will only take about 5 seconds. 5G networks aims to reduce the time delay or response time of Internet connectivity from one point to another, throughout the globe. 5G will also become the main network to link the Internet to robots, industrial machines, medical devices, agriculture machinery and a lot more. i This kind of technology would be ideal for data-heavy applications, such as autonomous driving, virtual-reality, home-automation, Internet-of-Things (IoT), live-video streaming, and online-gaming, since these applications require the continuous connectivity of data for better and safer use. The new technology will definitely improve the Internet experience when it comes to business, entertainment, and personal use. Beyond mobile networks, 5G is also expected to be widely utilized for applications in private networks, such as critical mobile broadband solutions, serving both users and IoT enabled devices; Group- Radio on demand such as Push-to-Talk; location and dispatch services to professional and commercial users; and enterprise networking [4, 7, 8]. For 5G, mobile network providers need to have a brand new infrastructures to provide a cooperative communication network coverage, globally. But when 5G does establish itself and fulfill its supposed potential, it could even change the usage of Internet in our daily lifestyle, as the current system of phone lines and cables would be replace by high-speed ubiquitous wireless network. 5G is expected to provide a much more personalized web experience in shorter millimeter (mm) wave range, by using network slicing. Network slicing is a way of deploying separate cellular networks on the cloud at frequency above 6 GHz, allowing users to create their own bespoke network [9–11]. Currently, Radio Frequency (RF) spectrum, typically below 6 GHz, is being utilized by the present smart phones and other electronics devices. However, these frequencies bands are very congested commercially as well as academically, and on the same amount of frequency spectrum, data can only be squeezed by the carriers, as more devices come online, speed of the Internet service start reducing and more connections are dropped. The solution of this problem is to open up some new real estate of frequency bands, and hence researchers are experimenting with broadcasting on shorter millimeter waves or sub-millimeter waves. These frequency bands lie between 6 GHz to 300 GHz RF spectrum and these bands can offer high frequency bandwidth and hence high data rates required for future mobile communications. This section of RF spectrum has never been considered earlier, particularly for cellular communication [7]. However, there is a problem with mmWaves as buildings and other obstacles can not be well penetrated by mmWaves and also they tend to be absorbed by trees and rain. The optimum solution of this problem is to set-up small-cell networks based on network slicing, which can provide high-speed short-range communication [9, 10]. Presently, large high-powered cell towers are used to broadcast the signal over the long-distances. However,mmWaves have a harder time traveling through obstacles, which means a user behind the any of the obstacle can not receive the signal properly. By using thousands of low-powered mini base stations, small-cell networks based on network slicing can be configured, that would solve the absorption problem with the mmWaves [9, 10]. A ii short-range communication network of a relay team would be configured by placing these mini base stations closer to each other as compared to the traditional towers [4, 5]. This would be especially useful in cities where a mobile user is surrounded by a number of obstacles, in this situation user’s smart phone would automatically switch to a new base station in better range of user’s device, to maintain a continuous network connection. Such networks are expected to utilize massive Multi-Input-Multi-Output (MIMO) antennas with ultra-wide bandwidth in sub-mmWave range [12, 13]. In the current mobile generation technology, all cellular traffic is handled by dozen of ports for antennas placed at the base stations but in the case of massive MIMO, about a hundred ports can be supported by base stations, to feed compact broadband antennas. This way the capacity of today’s networks could be increased by a factor of 22 or more. Information is broadcasted in every direction at once by the present cellular antennas and it could cause serious interference which leads to requirement of smart beamforming antennas. Instead of broadcasting in every direction, beamforming would allow a base station to send a focused data-stream to a particular user and hence it would work as a traffic signaling system for future mobile signals. This phenomena require high precision and hence interference can be prevented. It means, more incoming and outgoing data streams at once could be handled by the base stations in the more efficient way [4, 5, 14]. The timing and the direction of arrival (DOA) of various data signals individually, would be tracked continuously by a massive MIMO base station, and hence very complicated signal processing algorithms would be used to triangulate the DOA of each signal. Then the best transmission route back through the air to each phone is assigned. Sometimes individual packets of data in different directions would be bounced to keep the signals from interfering with each other. Hence, a coherent data stream would be sent only to a specific user in a least response time which would further increase the bandwidth utilization in the spectrum [9, 13, 15]. Moreover, another prime requirement of next generation mobile technology is global coverage. Covering the entire earth using small cell networks is not possible due to connectivity constraints of some remote geographical locations e.g., regions of Antarctica, the Sahara, Himalayas and Oceans. Therefore, there are some regions on the earth where coverage of cellular network is not possible. By integrating the satellite communication, along with the small-cell wireless communication networks, it is possible to extend the coverage of the mobile network all over the world [16]. A satellite is an artificial body placed in an orbit around the earth or another planet. The communication satellites relay around the world telephone, fax messages, television programs, navigation signals, radio signals and so on. Process of satellite communication begin at base station or earth station. Earth station is an installation designed to transmit and receive signals from a satellite in iii an orbit around the earth. Earth station sends information in the form of high-powered high-frequency signals to satellites that receive and retransmit the signals back to the earth. These retransmitted signals can be received by other earth stations in the coverage area of the satellite, that area is known as satellite footprint. To increase the coverage area of satellites on the earth, it is needed to place more and more satellites in an orbit, so that a user present anywhere on the earth, may always remain in direct contact with at least one satellite [17, 18]. The most feasible solution to accomplish the objective of providing the global coverage to the next generation mobile networks, is to establish an Inter-Satellite communication Link (ISL). In ISL, group of satellites are inter-linked together, form a network which is always connected with the ground station on the earth. ISL is extremely useful, when cellphone users are either placed at a substantial distance or present opposite side to each other of the globe. Communication link among the satellites placed in the same orbit is known as intra-orbit ISL, while the communication among the satellites placed in the different orbits is known as inter-orbit ISL. Low Earth Orbit (LEO) and GEO-stationary Earth Orbit (GEO), are two prime orbits to establish a global mobile networks using LEOLEO intra-orbit ISL and LEO-GEO inter-orbit ISL. Antenna is most crucial element for the any wireless communication systems. The research work in this thesis is fully oriented to find the possible antenna solutions for ISL applications, which may also benefit to the next generation high-speed mobile networks [16, 19]. Requirement of ISL, objective and organization of the thesis is discussed in details in Chapter-1. Presently, very bulky non-planar directional antennas are being used for satellite communications. Least payload is the prime requirement for any satellite mission, which plays most important role in deciding the cost of the mission. Payload of the spacecraft can be reduced dramatically, by replacing bulky non-planar antennas with the low-profile planar antennas and antenna arrays. According to another aspect, requirements of high data rates can be fulfilled by using broadband antennas, while achieving the wide bandwidth from the non-planar antennas, is very complicated and costly [13, 18]. However, planar antennas and antenna arrays can exhibit broadband behavior with far less complexity and cost on the fabrication [20]. In Chapter-2, detailed investigation about ISL and its design constraints is carried out, at the end of this chapter historical review of the previous ISL missions, and various proposed antennas particularly for ISL, are discussed. As per the available literature a few articles are reported, dedicated to ISL application, hence currently, this research area is open for space engineers and antenna research scholars. This thesis is mainly divided into two parts; Part (A) Investigation of broadband antennas for LEO-LEO intra-orbit satellite communication link, which composed of Chapter-3, 4, and 5 ; and Part (B) Investigation of compact tunable antennas for LEO-GEO inter-orbit iv satellite communication link which is discussed in Chapter-6. Compact planar antipodal end-fire broadband antennas based on novel windmill-like shape are proposed for future mobile applications in the Chapter-3. Asymmetric structure of the proposed antennas makes these antennas to radiate in boresight direction while antipodal structure provides wideband behavior. In the 1st phase, an UWB antenna is designed using FR4 substrate with a size of 34 mm 30 mm, for the frequency range from 4 GHz to 10 GHz and peak gain of 5 dBi. This antenna is then upgraded to Antenna- II, to shift its frequency band to more than 10 GHz and up to >150 GHz. To improve the performance of Antenna-II, a low loss substrate Rogers RO4232 is used. The final optimized antenna achieves at least 175% fractional bandwidth in the frequency range from 10 GHz to >150 GHz and exhibits a peak gain of 8.7 dBi while maintaining overall average gain of 6.4 dBi in entire frequency range. Design as well as performance analysis of this antenna is also investigated for LEO to LEO inter-satellite link applications with its power analysis at the end of this chapter. A broadband, unidirectional, non-linear phased array is proposed in the Chapter-4. Single element antenna of the prototyped array is the broadband windmill-like shaped antenna, investigated in Chapter-3. 4-elements of this antenna are fed by Wilkinson power divider, and are configured in a novel semicircular-shaped, angular-phased array. Phase difference among the antenna elements was introduced by providing angular path difference in the array, and it is adjusted so that array may always radiate in its bore-sight direction. Total 21.5% of size reduction is achieved by introducing semicircular antenna array as compare to the conventional linear phased array. The semicircular antenna array of a radius of 65 mm, is designed for the operating frequency range from 9.35 GHz to 42.89 GHz, using Rogers RO4232 substrate with a thickness of 1.52 mm, and dielectric constant of 3.2. Proposed array configuration can maintain the average gain of 8.34 dBi in entire frequency range while exhibiting peak gain of 12.17 dBi. By using antenna theory, it is also mathematically proved that directivity and gain of single element antenna have been improved remarkably, by organizing such elements in the form of the semicircular array. Presented array structure exhibits highly directional behavior in the end-fire direction ( = 90o), with the 3 dB angular beamwidth in the range from 76:7o to < 16:5o, in the operating frequency band. Detailed investigation of its abilities, to operate in sub-mmWave range and exhibiting wideband highly directional behavior, has been carried out for its Inter-satellite link based indoor and outdoor applications for future mobile communications. A novel low-profile broadband slot antenna is proposed in the Chapter-5. Instead of traditional slots, fractal-geometry based Defected Ground Structure (DGS) is configured for the very first time in the conventional slot antenna, to achieve the broadband radiation v properties in 15.76-93 GHz frequency range. Proposed structure is printed on 1.52 mm thick, low loss substrate ROGERS 4232, with a relative permittivity of 3.2. The overall dimension of the proposed structure is 25 mm 35 mm which can achieve at least 142% fractional bandwidth in its operating frequency range. Predicted, average gain of 4.3 dBi and the peak gain of 7 dBi is maintained in the entire frequency range by the proposed structure. Laboratory prototype of the antennawas fabricated and experimentally measured and measured S11 results are found to be in good agreement with the simulation results. Detailed investigation of radiation mechanism of the proposed antenna in such a wide frequency range is carried out by analyzing its surface current distribution at some intermediate frequencies. The proposed antenna can find its applications in short-range communication network and LEO-LEO ISL for future mobile technologies like 5G . A novel frequency-tunable antenna is proposed in the Chapter-6 which is configured by two Composite Right/Left-Handed (CRLH)-Substrate Integrated Waveguide (SIW) based identical antennas, Antenna-I and Antenna-II. CRLH configuration in the design is composed by configuring two etched inter-digital capacitance slots on the patch. Both the Antenna-I and Antenna-II are designed on the 1.52 mm thick Rogers Uranium 2000 substrate, having a relative permittivity of 2.5, and loss tangent of 0.0019. Antenna- I is optimized to radiate at 25.3 GHz, with a peak gain of 8.2 dBi while Antenna-II exhibits the minimum reflection coefficient at 27.25 GHz, with a peak gain of 8.5 dBi. Frequency reconfigurability in the antenna configuration is achieved by using an SPDT switch between the feedline of Antenna-I and Antenna-II. Two different types of SPDT switch configurations are investigated to achieve the reconfigurability. Simulation results for reflection coefficient, gain, and radiation patterns are also presented in the chapter. Finally, Chapter-7 concludes the achievements and contributions of the research work done in the thesis and at the end of this chapter, future scope of the present research work is outlined. In summary, this thesis contributes towards the detailed study, design and development of the compact planar antennas and antenna array for intra-orbit and inter-orbit, intersatellite communication links.