Diversity Tenchnique In Radio Communication
Times have changed, and many new technologies have been discovered, with almost every field now having its own technology suited to its function. This includes telecommunications, where communication now utilizes waves specifically electromagnetic waves that facilitate the exchange of information. In the past, communication relied on electrical signals transmitted through wired media, which limited distance and involved expensive infrastructure. Today, communication has evolved significantly; the use of electromagnetic waves allows for faster communication that can reach remote areas more affordably. Electromagnetic waves cover a broad spectrum, including radio waves, microwaves, infrared, visible light, ultraviolet, gamma rays, and X-rays. The wave currently being developed for telecommunications is the radio wave, known for its long-range capabilities and high data transmission speed.
Microwave radio communication operates at high frequencies, typically between 1 GHz and 300 GHz. With such high frequencies, data transmission speeds are impressive, ranging from 300 Mbps to 1 Gbps. Microwave radio communication enables users to make video calls, live stream, transfer data quickly, download large files, and more. Its range can extend to several miles or kilometers.
Although microwave radio communication has advanced rapidly and offers many advantages, it still faces challenges namely, maintaining availability. Availability is the ability of a system to operate continuously without interruptions and to provide services that meet desired standards. This is where the role of diversity techniques in radio communication systems becomes relevant. These techniques are applied to enhance system reliability against propagation disturbances, such as multipath fading and rain attenuation.
This article will discuss the concept of diversity, causes of interference in wireless communication systems, diversity techniques, and how their application can enhance system performance and availability.
Definition of Availability and Diversity Techniques
Let us first understand what availability is. Availability is a measure of system performance and evaluates the combined effects of reliability, maintenance, and logistics support on the operational effectiveness of a system. In simpler terms, availability refers to the ability of a microwave radio communication system to provide reliable and expected services without significant issues, such that technicians are not required to resolve problems frequently.
One major obstacle to availability is fading a phenomenon where the signal transmitted from the transmitter side becomes scattered or experiences interference, causing the signal received to vary significantly. Fading is caused by the effects of electromagnetic wave propagation, such as refraction, reflection, scattering, and attenuation. It results in fluctuations or instability in the magnitude, phase, and angle of arrival of the received signal in wireless communication systems due to varying signal attenuation along different paths.
Fading can be mitigated using diversity techniques. These techniques ensure that even if the transmitted signal experiences fading, the received signal remains strong or readable. Diversity techniques can improve the availability of a radio communication system and ensure data can still be received even if one path fails. Various types of diversity techniques include Frequency Diversity, Time Diversity, Space Diversity, Angle Diversity, and Polarized Diversity. Below is an explanation of each type.
1. Frequency Diversity
In frequency diversity, the same data is transmitted on different carrier frequencies. Because fading typically affects only specific frequencies at certain times, transmitting signals on multiple frequencies reduces the likelihood that all frequencies will be affected simultaneously. As a result, the received signal can still be accurately interpreted.
The same concept applies to Frequency Division Multiplexing (FDM) and Orthogonal FDM (OFDM), where data is divided across N different frequency channels. Error correction coding is used to recover any lost data due to interference. This means that even if some channels suffer from severe fading and data loss, the overall information can still be reconstructed during decoding.
However, frequency channels in FDM or OFDM are often highly correlated, especially adjacent ones. To avoid the need for additional equalizers, each channel should ideally experience flat fading. This correlation implies that fading on one channel might also affect nearby channels.
Another example of frequency diversity is Frequency Hopping Spread Spectrum (FH-SS). Although FH-SS can experience deep fading or interference on certain center frequencies within its hopping set, the system can still recover most of the data due to error correction codes. Additionally, FH-SS rarely loses all data because it hops randomly between channels.
The main advantage of frequency diversity is that it requires only one antenna and one RF chain. However, its downside lies in power efficiency: some transmission power is wasted on channels undergoing severe fading, where ideally all power would be focused on a reliable channel.
2. Time Diversity
Wireless communication channels vary over time. Time diversity is a technique that transmits the same signal two or more times at different time intervals through the same channel to reduce the impact of fading. These intervals must be spaced longer than the typical fading duration to ensure that the transmissions are independently affected. Since signal interference is usually temporary, retransmitting data at different times increases the chances of successful reception.However, this approach is inefficient in practical systems because it uses communication resources less optimally.
Figure 1 interleaving
Nevertheless, time diversity is widely used in commercial systems through a method known as "interleaving." Interleaving takes the incoming coded bitstream and spreads the bits across the transmitted packet in a known pattern. At the receiver, de-interleaving reverses the process. This rearrangement causes burst bit errors introduced by the channel to be distributed throughout the data packet. Such distribution enhances the effectiveness of error correction codes, which generally perform better when errors are spread out rather than clustered.
For example, block coding schemes typically assume only one error per block of six or seven coded bits. Most coding techniques are designed to correct a limited number of errors in each group of received bits not multiple, but concentrated errors.
The main disadvantage of interleaving is latency. The system must wait until the entire block of coded bits is received before it can be reordered, decoded, and potentially converted into usable output, such as audio signals. The acceptable delay depends on the application. For instance, voice communication is highly sensitive to latency, yet even mobile voice data still uses interleaving to improve reliability.
Another limitation is that temporal correlation in wireless channels can remain high over time, particularly in real-world scenarios like vehicular communication, which may reduce interleaving's effectiveness in breaking error patterns.
3. Space Diveristy
Space diversity is considered one of the most effective and straightforward diversity techniques. This method involves the use of multiple antennas, which can be positioned at the base station, mobile device, or both ends of a wireless communication link. The main idea is to receive several independent versions of the same signal through these antennas. To ensure the technique's effectiveness, the antennas must be physically spaced ideally by at least half a wavelength so that each received signal experiences independent fading.
Unlike frequency diversity and time diversity, space diversity does not require additional signal processing on the receiver side, making it more practical in certain scenarios. However, a major limitation of this technique is the physical space requirement, which may make it difficult to implement in compact devices such as smartphones or small IoT units. Despite this, space diversity is widely used to mitigate frequency-selective and time-selective (fast) fading.
Additionally, space diversity can be extended to the transmitter side by installing multiple antennas that send duplicate versions of the same signal. When multiple antennas are used at both the transmitter and receiver ends, the system is referred to as a Multiple-Input Multiple-Output (MIMO) system. MIMO technology significantly enhances the performance of wireless communication systems, offering better reliability and higher capacity. However, like other technologies, MIMO also comes with certain challenges and limitations.
One of the advantages of space diversity is that no additional signal needs to be transmitted, and no extra bandwidth is required. Space diversity can also be implemented at the transmitter by switching the transmitting antenna until the received SNR is sufficiently high. However, this approach requires several closed-loop controls and is thus less commonly used.
4. Polarized Diversity
Polarization diversity is a technique that uses two antennas with different polarizations. This is feasible because the reflection coefficients for horizontally and vertically polarized signal components differ. Furthermore, the amplitude and phase of signals that undergo scattering and diffraction also vary depending on their polarization direction. As a result, a signal with one type of polarization (which is a combination of various reflections and scattering) will have very low correlation with the signal of the opposite polarization. Diversity combining techniques are applied at the receiver side.
The main advantage of polarization diversity is that the two antennas do not need to be separated by half a wavelength, making it suitable for implementation in space-constrained mobile devices. This technique can also be combined with space diversity to further reduce the correlation between signals received by the two antennas. Similar to space diversity, polarization diversity does not require additional bandwidth or extra signal transmission from the transmitter side.
The main drawback of polarization diversity lies in the limited channel types—typically only vertical and horizontal polarization, though right- and left-handed circular polarization can also be used as alternatives. To fully implement polarization diversity, the system usually requires two RF chains at the receiver. However, if methods like scanning combiners are used, the system can alternatively select the best signal from the two paths without needing two RF chains operating simultaneously.
5. Angle Diversity
Angle diversity is an advanced wireless communication technique that leverages the directional nature of signal propagation to enhance system reliability and performance. This method uses multiple antenna beams or receiving elements, each oriented at different angles, to capture signals arriving from various directions. To further improve reception, optical or electromagnetic concentrators such as hemispherical lenses and compound parabolic concentrators (CPCs) are used to focus and direct incoming signals to their respective receivers. Angle diversity is particularly beneficial in environments with high multipath interference or non-line-of-sight (NLOS) conditions, such as infrared communication channels, as it significantly reduces path loss, ambient noise, and multipath distortion.
One of the key advantages of angle diversity is its ability to maintain strong signal quality in complex or dynamic propagation scenarios, delivering improved performance without the need for extra bandwidth or time redundancy. Angle diversity also allows for spatial filtering, which helps suppress interference from unwanted directions. However, a notable disadvantage is the complexity of the hardware and spatial requirements. Implementing multiple directional elements with associated concentrators demands more physical space and precise alignment, which may not be practical for compact or mobile devices. Additionally, the system's complexity can lead to higher costs and design overhead, especially in multi-user or rapidly changing environments. Despite these challenges, angle diversity remains a valuable approach in scenarios where directional signal reception can be effectively utilized.
Diversity Combining
After applying diversity techniques (such as space diversity or polarization diversity), multiple independent signals are obtained. In order for these signals to be interpreted or decoded correctly, they must be combined using diversity combining techniques. Diversity combining is divided into several methods, namely Selective Combining, Equal Gain Combining, and Maximal Ratio Combining, as explained below:
1. Selective Combiner
The selective combiner method selects the signal from multiple channels that has the highest signal-to-noise ratio (SNR). Only the signal from that channel is passed to the demodulator and used for data interpretation. This is the simplest method, but it is not fully optimal because it utilizes only one signal from one channel, leaving potentially useful information from other channels unused.
2. Equal Gain Combiner
Equal Gain Combining (EGC) combines all received signals with equal amplitude weights, while adjusting the phase of each signal so they can reinforce each other. EGC provides better performance than selective combining and maintains a moderate level of computational complexity.
3. Maximal Ratio Combiner
Maximal Ratio Combining (MRC) assigns different weights to each signal based on their individual SNR levels and aligns their phases so that the signals can be combined in the most optimal way. This method produces the highest total SNR and offers the best signal quality among the three techniques. However, this advantage comes at the cost of high computational complexity and the need for more sophisticated hardware for implementation.
Conclusion
Microwave radio communication is a form of wireless communication that utilizes electromagnetic waves to transmit data quickly and over long distances. One of the key aspects of this system is availability, which refers to the system's ability to provide a stable connection without significant disruptions. A major challenge to availability is multipath fading, a condition in which the received signal experiences changes in amplitude, phase, and power, resulting in weakened or distorted information. To address this issue, diversity techniques are employed. There are five main types of diversity techniques: space, frequency, time, polarization, and angle diversity.
· Space Diversity is a technique that involves using two or more antennas placed at a distance of several wavelengths apart to capture signals from different propagation paths. Since signals experience fading differently at each location, the system can select or combine the signal with the best quality. This technique is highly effective in mitigating spatial interference and is widely used in transmitting stations, MIMO systems, and wireless network devices such as Wi-Fi.
· Frequency Diversity works by transmitting the same information over two or more different carrier frequencies. Because frequency fading typically does not affect all frequencies simultaneously, the system can still receive data through the unaffected frequencies. This technique is particularly useful in dealing with frequency-selective fading and is commonly implemented in technologies such as OFDM, FHSS, and modern digital communication systems.
· Time Diversity leverages the retransmission of data at different time intervals as a safeguard against temporary disruptions caused by rapid channel changes. This approach includes the use of ARQ, HARQ, and interleaving, which spread data bits over different time instances to prevent consecutive data loss. This technique is highly effective in dynamic environments such as mobile communication systems and digital broadcasting.
· Polarization Diversity employs pairs of antennas with different polarization orientations, such as vertical and horizontal. Since each polarization responds differently to fading, the system can choose or combine the most stable signal. Its main advantage is that it does not require physical separation between antennas, making it suitable for portable devices, satellite communication, and MIMO systems.
· Angle Diversity involves using antennas directed at various reception angles to capture signals from different directions. This technique helps minimize the effects of multipath and directional interference. Angle diversity is often used in free-space optical (FSO) communication, indoor systems, and antenna sectorization at transmitting stations.
BIODATA
