Microwave and Photonics Systems - Essay Example

1) What is Radio over Fibre technology Why is this technology adopted widely in cellular phone systems Ans) Radio-over-Fibre technology is a form of signal transmission which is characterized by the use of links comprising optical fibre to carry Radio Frequency signals from a centralized headend unit to Remote Antenna Units (RAUs). This is in sharp contrast to the scenario where, in Wireless Local Area Networks and narrowband communication systems, frequency step-up, modulation, and multiplexing of the signals are carried out at the Base Station or the Radio Access Point, before being fed into the antenna which is located at a geographically adjacent point. But the Radio-over-Fibre configuration makes it possible to have all the signal processing functions to be performed at one location called the headend. The signal is then transmitted over optical fibre to the Remote Antenna Unit. The fibre is such that it offers low signal loss of 0.3 dB/km for 1550 nm, and 0.5 dB/km for 1310 nm wavelengths(Ng'oma, 18-23). This results in significantly reduced complexity of the Remote Antenna Unit, as the major portion of the processing is avoided, at least at the regional unit level and only optoelectronic conversion and amplification of the converted signal is necessary. Radio over Fibre technology is credited with the following advantages which makes it a popular option for mobile communication: a) Low Attenuation Loss b) Large Bandwidth c) Immunity to Radio Frequency Interference d) Easy Installation and Maintenance e) Reduced Power Consumption f) Multi-Operator and Multi-Service Operation g) Dynamic Resource Allocation(Ng'oma, 18-23). 2) Estimate a difference in roundtrip time for data transmission between Tokyo and San Francisco (distance: 8270 km) as for the following two communication methods; radio-wave (wireless) communication using a geostationary satellite, and fiber-optic communication. Assume that a refractive index for the air is 1, and that of the optical fiber is 1.5. Distance between the earth and the satellite is 38,800 km. Ans) 3) What is impedance matching Why is it important for many Radio Frequency applications Ans) Impedance matching in a circuit comprised of linear devices is defined as the process of making the output impedance of the source equal to the input impedance of the load, in order to maximize the power transfer from source to load and thereby minimize reflections from the load end. This results in increased efficiency. Fig: Matched transmission line system (Liao 107) In the context of radio and fiber optics systems, where the wavelength of the transmitted signal is very much less in comparison to the length of the line, ie, where the changes in the signal are rapid compared to the time of propagation, the impedances at each end of the line must be matched to the characteristic impedance of the transmission line to prevent reflections of the signal at the ends of the line. Otherwise, echoes may occur and cause the formation of standing waves in the transmission line. In radio-frequency (RF) systems, a common type of RF load used is a quarter-wave ground plane antenna using a modified ground plane or a matching network. 4) Explain one example of representative antennas which are used in our radio wave(wireless) systems such as cellular phones, wireless LANs, etc, with respect to its type and operation. Ans) Consider a microstrip antenna as an example. A microstrip antenna gains its name from the fact that it is structurally a simple metallic strip of desired shape suspended over a ground plane. The strip may be flush mounted onto a dielectric or other surface. The feed line is placed behind the ground plane (Prasad 809). They are simple to fabricate and easy to modify and customize. The microstrip patch antenna is the most common type of microstrip antenna. It is a narrowband, wide-beam antenna with an inherent ability to have polarization diversity. These antennas can easily be designed to have Vertical, Horizontal, Right Hand Circular or Left Hand Circular Polarizations, using multiple feed points, or a single feedpoint with asymmetric patch structures(Bancroft chap 2-3). This offers application flexibility, ie, they can be used in many areas and types of communications links that may have varied requirements. The shape is also variant, where common shapes are square, rectangular, circular and elliptical, but any continuous shape is possible. These antennas are relatively inexpensive to manufacture and design because of the simple 2-dimensional physical geometry. The most common is rectangular in shape. This includes an approximately one-half wavelength long section of rectangular microstrip transmission line. When air is the antenna substrate, the length of the rectangular microstrip antenna is approximately one-half of a free-space wavelength. When the antenna is loaded with dielectric substrate, the length of the antenna decreases as the relative dielectric constant of the substrate increases. The resonant length of the antenna is slightly shorter than the actual length because of the extended electric "fringing fields" which increase what is known as the electrical length. An early model of the microstrip antenna is a section of microstrip transmission line with equivalent loads on either end to represent the radiation loss. A single patch antenna provides a maximum directive gain of around 6-9 dB. The radiation pattern and impedance bandwidth of the microstrip antenna is largely determined by the dielectric loading. The antenna bandwidth is inversely proportional to the dielectric constant of the substrate. This leads to an increase in the Q factor of the antenna and thus a decrease in the impedance bandwidth. This relationship did not immediately follow when using the transmission line model of the antenna, but is apparent when using the cavity model which was introduced in the late 1970s(Lo, Y.T. et al, 137-149). Fig: Two views of the Microstrip antenna (Prasad, 809) 5) Explain merits and demerits of millimeter waves and/or tetrahertz waves for applications to sensing, comparing with conventional microwaves and infrared light waves. Ans) Merits of Millimeter waves a) Highly directional, pencil-beam signal characteristics permit systems in these bands to be engineered in close proximity to one another without causing interference. b) Allows for smaller frequency reuse distances than lower frequencies. The small wavelength allows modest size antennas to have a small beam width, further increasing frequency reuse potential c) Satellite-based remote sensing near 60 GHz can determine temperature distributions in the upper atmosphere by measuring radiation emitted from oxygen molecules that is a function of temperature and pressure d) Shorter wavelengths permit the use of smaller antennas e) This high directivity, coupled with the high free space loss at these frequencies, brings the possibility of a more efficient use of the spectrum for point-to-multipoint applications. f) Since a greater number of high directive antennas can be placed than less directive antennas in a given area, the net result is higher reuse of the spectrum, and higher density of users, as compared to lower frequencies. g) More voice channels or broadband information can be placed in this higher frequency, so this spectrum could potentially be used as a replacement for or supplement to fiber optics Demerits of Millimeter waves a) Extremely prone to atmospheric attenuation b) Signals in the 57-64 GHz region are subject to a resonance of the oxygen molecule and are severely attenuated c) Rain fade occurs over short distances in case of unfavourable weather d) Humidity impacts propagation Works cited 1. Bancroft, R. Microstrip and Printed Antenna Design Noble Publishing 2004, Chapters 2-3 2. Liao, Samuel Y. Microwave Devices and Circuits, Pearson Education 2003, Second Impression 2006 3. Prasad, KD. Antenna and Wave Propagation, Satya Prakashan Tech India Publishers, Reprint edition 2007 4. Ng'oma, Anthony. Radio-over-Fibre Technology for Broadband Wireless Communication Systems Copyright 2005 by Anthony Ng'oma 5. Lo, Y.T., Solomon D. and Richards, W.F. "Theory and Experiment on Microstrip Antennas," IEEE Transactions on Antennas and Propagation, AP-27, 1979 pp. 137-149. Read More