I. Telecommunications and Radio
In the field of telecommunications, signal transmission—especially data— is a very crucial aspect. The term telecommunications itself comes from two words, namely tele (from Greek) which means "far", and communicare (from Latin) which means "to share" or "to communicate". In general, this process can be done through two main methods, namely wired transmission (such as using telephone lines, fiber optics, or coaxial cables) and wireless transmission (via antennas and other supporting devices). The transmission media can be further divided into communication channels that allow multiplexing, a technique for sending several communication sessions simultaneously through the same physical media. Longdistance communication technology that developed in the 20th and 21st centuries generally use electrical energy, and includes various innovations such as telegraph, telephone, television, and radio.
Ellingson (2016:42) states that radio refers to the utilization of electromagnetic fields that propagate without a physical medium (unguided propagation) within the frequency range of 3 kHz to 300 GHz to transmit information. Electromagnetic fields propagating within this frequency range are more commonly known as radio waves. In a radio communication system, the transmitter converts information in the form of analog signals (such as voice) or digital signals (i.e., data) into radio waves. These radio waves then propagate toward the receiver, which subsequently converts the signal represented by the radio waves back into its original form. Radio systems engineering encompasses a variety of topics related to the analysis and design of radio communication systems.
Although radio is one of the primary technologies for transmitting information via unguided electromagnetic radiation, it is not the only one. The electromagnetic spectrum also includes various other types of waves, such as infrared (IR), visible light (optical), ultraviolet (UV), X-rays, and gamma rays (Ξ³rays). Physically, the fundamental distinction among these types of waves lies in their wavelength (Ξ»), which is mathematically related to frequency (π) through the following equation:
ππ = π
Where π is the speed of light, approximately 3.0 × 10⁸ meters per second in free space (vacuum).
In practice, different forms of electromagnetic radiation exhibit significantly distinct characteristics. The behavior of electromagnetic waves is strongly influenced by their wavelength (Ξ»), particularly when compared to the size of structures in the environment through which they propagate, as well as the type of medium they traverse.
Radio waves are capable of propagating efficiently through air and most building materials. When encountering objects larger than their wavelength, these waves tend to scatter rather than be absorbed or dissipated as heat. This behavior contrasts with that of infrared (IR), visible light (optical), and ultraviolet (UV) waves, which are more easily absorbed or attenuated as they travel through air or building materials.
Meanwhile, X-rays and gamma rays (Ξ³-rays) possess high penetration capabilities and are not significantly hindered by media such as air or buildings. However, these waves are difficult to generate and detect, and they pose potential health risks to humans. Therefore, although wireless communication can theoretically be conducted using electromagnetic radiation across various frequency ranges, radio waves remain the most practical choice due to their ease of use, favorable propagation characteristics, and relative safety.
However, radio waves also come with limitations, particularly in terms of bandwidth. These limitations stem from two main factors. First, the available frequency range is limited. Although the radio spectrum extends up to 300 GHz, most of the favorable propagation characteristics mentioned earlier are only effective at lower frequencies. As a result, the majority of radio systems operate below 15 GHz.
By comparison, the frequency range of the optical spectrum reaches approximately 2 PHz (2 × 10¹⁵ Hz), and a single optical communication channel typically has a bandwidth of 15 GHz or more. This limitation of the radio spectrum drives the need for more complex modulation schemes to enhance spectral efficiency—that is, the amount of information that can be effectively transmitted within a given bandwidth. Implementing these more sophisticated schemes requires high-performance hardware, which in turn imposes stringent technical requirements in radio system design. On the other hand, communication systems operating at higher frequencies, such as optical systems, do not face this limitation due to the far greater availability of spectrum.
II. Radio Spectrum
The radio spectrum spans an extremely wide range of frequencies— approximately eight orders of magnitude—each with distinct physical characteristics and technical applications. To facilitate analysis and technical communication, this spectrum is conventionally divided into several subbands or frequency bands. One of the most widely adopted classification systems is that established by the International Telecommunication Union (ITU), as outlined in the table. Terms such as VLF (Very Low Frequency), LF (Low Frequency), and MF (Medium Frequency) are acronyms that refer to specific frequency categories. It is important to note that these divisions are not absolute and may vary slightly across different references or standards. Nevertheless, the ITU classification remains the primary reference that simplifies the articulation of ideas and specifications in the field of telecommunications engineering. Abbreviation Frequency Band Name Frequency Range Common Applications VLF Very Low Frequency 3 – 30 kHz Submarine communications (military), navigation, time signals LF Low Frequency 30 – 300 kHz Maritime navigation (LORAN), long-range radio broadcasting MF Medium Frequency 300 kHz – 3 MHz AM radio broadcasting, maritime communications HF High Frequency 3 – 30 MHz Amateur radio, shortwave broadcasting, long-distance aviation VHF Very High Frequency 30 – 300 MHz FM radio, analog TV, airport and marine communication UHF Ultra High Frequency 300 MHz – 3 GHz Digital TV, WiFi, mobile phones, GPS, two-way radios SHF Super High Frequency 3 – 30 GHz Radar, satellites, 5 GHz WiFi, point-to-point communication EHF Extremely High Frequency 30 – 300 GHz High-capacity satellite links, military systems, 5G mmWave
The use of the radio spectrum is governed by both technical and legal considerations. Although this book primarily focuses on the technical aspects, understanding the legal framework that regulates radio systems remains essential. The regulatory structure for radio spectrum usage has been established through a system of international agreements developed under the auspices of the International Telecommunication Union (ITU). This international regulatory framework is crucial for at least two reasons. First, certain forms of radio communication cross national borders; two primary examples are HF band broadcasting and satellite communication. Second, the existence of international standards for the technical characteristics of radio systems serves a common interest, enabling these systems to function globally—such as in the case of personal mobile services and air traffic control systems. Within this international framework, individual national governments establish and enforce additional regulations to further define permitted spectrum use. In the United States, federal spectrum usage is regulated by the National Telecommunications and Information Administration (NTIA), while non-federal use (such as commercial, amateur, and passive scientific applications) falls under the jurisdiction of the Federal Communications Commission (FCC). In Indonesia, the management and regulation of the radio frequency spectrum is overseen by the Ministry of Communication and Digital Affairs (Kemen Komdigi), specifically through the Directorate General of Digital Infrastructure.
III. Microwave Usage
Microwaves are a portion of the electromagnetic wave spectrum with wavelengths ranging from 1 meter to 1 millimeter, which corresponds to frequencies between 300 MHz and 300 GHz. Due to their line-of-sight propagation characteristics, the planning of microwave-based communication systems must carefully consider topographic and locational factors to minimize the risk of signal interference. Consequently, transmitters and receivers in such systems are typically installed in elevated locations—such as rooftops, towers, or even mountain peaks— to achieve optimal transmission paths. Despite their limited range caused by the nature of their propagation, microwaves are well known for supporting high data transfer rates, making them widely used in various modern communication applications.
Gambar 3.1 Sistem komunikasi gelombang mikro berbasis jalur pandang langsung (line-of-sight) Line-of-sight microwave communication systems played a pivotal role in telecommunications throughout the last two decades of the 20th century. These systems emerged as strong competitors to copper wire and coaxial cable technologies, particularly within national and international trunk networks. Their applications ranged widely—from supporting a small number of telephone channels to accommodating thousands of voice channels and several television broadcasts channels—spanning distances of thousands of kilometers while maintaining high standards of performance and reliability. Initial research on the feasibility of multichannel radio systems using this mode of propagation dates back to before World War II, primarily in the Very High Frequency (VHF) band. Significant advancements in these techniques were made simultaneously in Europe and the United States. Interest in utilizing frequencies above 300 MHz for commercial purposes began to grow in the early 1930s. One of the earliest microwave communication experiments took place in 1931, linking Dover (UK) and Calais (France) using a 1.700 MHz radio signal with just about one watt of transmission power. At the time, this frequency was considered extremely high and marked a significant technological breakthrough, shedding light on the vast potential of the Ultra High Frequency (UHF) band, which spans from 300 to 3.000 MHz. Advantages of Microwave Transmission: • Capable of transmitting data at high speeds with rapid propagation rates. • More cost-effective in terms of installation compared to cable-based systems. • Installation and implementation processes are relatively simple and flexible. Disadvantages of Microwave Transmission: • Susceptible to interference from other nearby radio waves. • Can only propagate in a straight line (line-of-sight) and requires a clear path free of physical obstructions such as buildings or hills. In a microwave-based radio communication system, there are several key components, each serving a specific role on both the transmitter and receiver sides. When the system is used to transmit digital data, the incoming binary signal is first converted into a high-frequency sinusoidal signal through a process known as modulation. The block diagram shown in Figure 3.2 illustrates the arrangement and function of each main element in this microwave radio system. Figure 3.2 Block Diagram Microwave Radio Communication System Since line-of-sight (LOS) microwave communication systems require transmission paths to be free from physical obstructions—such as tall buildings or geographical features like hills—the presence of obstacles, whether permanent (e.g., skyscrapers) or temporary due to force majeure (e.g., heavy rain or natural disasters), can significantly impact signal propagation quality. This makes microwave systems highly susceptible to environmental changes, which in turn may cause fluctuations in the received signal power—a phenomenon known as fading.
IV. Fading
Fading refers to the time-varying changes in the characteristics of the received signal, including its phase, polarization direction, and signal strength. These changes are generally associated with various radio wave propagation mechanisms such as refraction, reflection, diffraction, scattering, attenuation, and ducting. These mechanisms significantly influence signal parameters such as amplitude, phase, and polarization, as well as frequency- and location-dependent fading effects. By understanding how these mechanisms operate, various technical strategies can be designed to prevent or mitigate the adverse impacts they may cause. Fading phenomena generally occur as a result of a combination of surface features of the Earth—such as mountains, valleys, or tall buildings—and weather conditions such as heavy rainfall or changes in air temperature. All radio transmission systems operating within the frequency range of 0.3 to 300 GHz are susceptible to fading, including satellite communication systems that operate at low elevation angles or under adverse atmospheric conditions. There are various types of fading, classified based on different criteria, by scale (e.g., Rayleigh Fading, and Path Loss), and selective factor (e.g., Frequency-Selective and Flat Fading). Several methods can be employed to ensure the availability of a communication system, including: • System redundancy, which functions to enhance equipment reliability and ensure the continuous availability of hardware components. • Provision of fading margin, which involves allocating additional signal power reserves to maintain link stability against non-selective fading disturbances. • Implementation of adaptive equalizers, either in software or hardware form, aimed at mitigating selective fading and maintaining optimal link quality. One commonly applied approach to achieve both system redundancy and fading margin is the use of diversity techniques, which allow the system to remain operational even if disruptions occur in one of the transmission paths or system elements.
V. Diversity
The principle of diversity works by providing multiple distinct paths for transmitting redundant information, so that if one path experiences disruptions such as fading, the other paths can still successfully deliver the data. These path differences can be based on location (spatial), frequency range, or transmission time. The effectiveness of diversity techniques in mitigating the effects of fading depends heavily on how uncorrelated the signals from the various paths are. The greater the decorrelation, the more robust the system becomes against interference. If the correlation coefficient between signals is below 0.6, the benefits of diversity can be maximized. Two of the most commonly implemented diversity methods for reducing multipath fading effects are frequency diversity and space diversity. Meanwhile, in communication systems that use troposcatter or diffraction, a combination of space and frequency diversity is typically applied to achieve greater resistance to signal degradation caused by fading. Other diversity types include time diversity, angle diversity, and polarization diversity. This article will focus on explaining frequency diversity and space diversity.
1. Frequency Diversity
Figure 1. Frequency Diversity Block Diagram This technique employs an approach that differentiates the carrier frequencies during the transmission of information signals. In practice, the transmitter sends the same information signal across multiple distinct carrier frequencies. On the receiver side, these signals are received and then combined to obtain a more accurate estimation of the originally transmitted data. To prevent mutual interference between signals, the frequency separation between the carriers must be greater than the channel bandwidth, typically ranging between 2% to 5%. This ensures that each channel operates independently. However, the implementation of frequency diversity entails the need for additional hardware, specifically two RF stages (for both transmitter and receiver sides) for each carrier frequency used. The improvement factor for this technique is: πΌπ ≈ 0,8∆π 10πΉ/10 π 2π With: ∆π = Frequency separation (GHz) F = Fade depth (dB) π = Carrier Frequency (GHz) --> 2 ≤ π ≤ 11 π = Hop length (km) --> 30 ≤ π ≤ 70
2. Space Diversity
Figure 2. Space Diversity Block Diagram
This technique is also known as antenna diversity, as its main principle involves the use of more than one antenna at the receiving end (far end), typically installed with a separation distance of around 10 times the wavelength. With multiple antennas at the receiver, if one antenna receives a signal with low power due to propagation conditions, the other antenna still has a chance to receive the signal with better strength. Thus, the system can still optimally recover the information signal, even when fading occurs on one of the paths. However, implementing space diversity requires two or more receiving antennas and two or more RF stages at the receiver to process the incoming signals separately. The improvement factor for this technique is: πΌπ = 1,2 π₯ 10−3 π 2 π10(πΉ−π)/10 π With: π = vertical space in-between two antennas (m) = 80/π(πΊπ»π§), di kisaran 5 π ≤ π ≤ 15 π π = Carrier Frequency (GHz) F = Fading Margin (dB) V = Gain difference between two antennas π = Line length (km) --> 30 ≤ π ≤ 70
VI. Conclusion
Microwave-based systems gave a crucial role in today’s communication infrastructure, mainly because of their advantages like high data transmission speeds and cost-effective installation. Even so, the inherent characteristic of microwave signals that can only propagate in straight lines (line-of-sight) makes them susceptible to interference caused by physical obstructions—such as tall buildings or hills—as well as environmental changes like extreme weather condition changes. This susceptibility can generate the occurrence of fading, which refers to dynamic fluctuations in the amplitude, phase, or polarization of the received signal over time. To maintain service quality and system availability, various technical approaches are prompted, such as applicating system redundancy, providing fading margins, and using adaptive equalizers. One of the solutions to mitigate the effects of fading is the application of diversity techniques. These techniques work by transmitting information through alternative paths—whether in the frequency, spatial, or temporal domains—under the assumption that if one path experiences degradation, another path may still successfully deliver the signal. In frequency diversity, the same information is transmitted over different carrier frequencies to avoid disruptions that are specific to certain channels. Meanwhile, space diversity—or antenna diversity—employs two or more receiving antennas installed at certain distances apart to reduce the effects of multipath fading. Although these techniques require additional hardware, such as duplicate RF stages, their effectiveness in improving communication system reliability has been well established. In addition to the two mentioned techniques, there are other forms of diversity such as time diversity, angle diversity, and polarization diversity. There are also various methods for combining signals in diversity systems that are beyond the scope of this article. Overall, diversity techniques greatly facilitate the implementation of information mitigation strategies by providing a deeper understanding of wave propagation characteristics—particularly for microwave signals—and the factors that cause fading.
REFERENCES
Freeman, Roger L. (2007). Radio System Design for Telecommunications (edisi ke3). John Wiley & Sons Inc., USA. Hudiono, dkk. (2017). Buku Ajar Sistem Komunikasi Radio dan Laboratorium. Polinema Press, Indonesia. Ellingson, Steven W. (2016). Radio Systems Engineering. Cambridge University Press, United Kingdom. Panter, Philip F. (1972). Communication Systems Design: Line-of-sight and Troposcatter Systems. McGraw-Hill Inc., USA.
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