08 - Diversity Techniques in Radio Communication

Microwave radio communication is one of the main pillars of modern telecommunication network infrastructure. This system is widely used in backbone networks, metropolitan networks, as well as long-distance links between transmitters that cannot be reached by optical fiber or copper cables. By utilizing high-frequency electromagnetic waves, typically ranging from 1 GHz to 30 GHz, this system offers high data capacity, high speed, and low latency.

However, one of the main challenges in microwave communication systems is maintaining availability the system’s ability to operate continuously without significant interruptions. This is where the important 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 provide an in-depth discussion on the concept of availability, the causes of disturbances in microwave systems, diversity techniques, and how their implementation can improve the performance and availability of the system.

Definition of Availability

Availability in radio communication refers to a quantitative measure of how often a communication system can be accessed and used reliably over a specific period of time, typically within one year. It is expressed as the percentage of total operational time during which the system functions without significant interruptions (downtime).

Usually, availability is presented as the percentage of time during which the network operates properly. This is where the term “five-nines” originates. The term refers to 99.999%, a figure long used as a marketing standard and regarded as an ideal target for availability, especially at the core network level. This availability metric is crucial for vital services such as banking, military networks, and emergency communications.

Propagation Disturbances in Microwave Systems

Despite their advantages, microwave systems are vulnerable to propagation disturbances that directly affect their availability. Microwave signals can be disrupted by various environmental and technical factors. Major causes of reduced availability in microwave systems include:

a. Multipath Fading

Multipath fading is a form of RF signal disturbance or interference that occurs when a signal travels along more than one path, causing differences in amplitude, phase, and delay at the receiver. The interference between these paths leads to large fluctuations in the received signal strength (deep fade), which often results in communication interruptions. Multipath fading is commonly encountered in long-distance communication over water bodies.

b. Rain Attenuation

Rain attenuation becomes highly significant at frequencies above 10 GHz. When RF electromagnetic waves pass through rainfall, the water droplets absorb and scatter the wave energy. The absorbed energy is converted into heat, while the scattered energy changes direction, leading to substantial signal weakening. This effect is particularly pronounced in tropical regions with high rainfall, such as Indonesia.

c. Refraction and Ducting

Refraction occurs when a signal bends from its straight-line path due to changes in atmospheric density. Under certain conditions, such as atmospheric ducting (a phenomenon where a layer of the atmosphere traps the signal), the signal may deviate from its intended path or suffer interference. This can result in unexpected losses or gains in signal strength, impacting communication reliability.

d. Interference

Signals from other systems can interfere with microwave transmissions. This interference may be temporary or persistent, depending on the frequency and surrounding environment. Interference occurs when signals of the same or nearby frequencies overlap, producing resultant waves with amplitudes or power levels that differ from the original, thereby disrupting system performance.

Diversity Techniques to Improve Availability

Diversity is a strategy used to enhance the reliability of communication systems by providing multiple alternative paths for a signal to reach the receiver. This can be achieved by installing two or more systems or subsystems simultaneously, offering additional paths for signal transmission so that issues like multipath fading can be mitigated. There are several types of diversity, including space diversity, frequency diversity, time diversity, angle diversity, and polarization diversity. Below is an explanation of the various diversity techniques:

Types of Diversity Techniques

a. Space Diversity

 

Figure 1: Space Diversity

Space diversity is a technique to improve the reliability of radio communication systems using the same frequency by employing two or more antennas that are physically separated on the receiver side. These antennas are typically arranged either vertically (on the same tower at different heights) or horizontally (at different locations but within line-of-sight), with separations of several meters.

Because the signal arriving at each antenna travels through a different propagation path, the probability of all signals experiencing deep fade simultaneously is extremely low. Space diversity offers these alternative paths without the need to increase transmitter power or bandwidth.

To achieve optimal operation, the distance between the main antenna and the diversity antenna should be carefully configured. Generally, the separation between the main antenna and the space diversity antenna should be between 70λ to 200λ (where λ is the wavelength of the signal). A commonly used formula for determining the optimal distance is:

Δh = h1 - h2 = ρ . λ

where ρ is the diversity coefficient ranging from 100 to 200, and λ is the wavelength. To calculate the value of the wavelength, the following formula is used:

λ=c/f

with

λ = wavelength (m)

c = speed of light (3 × 10⁸ m/s)

f = antenna frequency (Hz)

By using the space diversity technique to mitigate fading, the improvement factor is obtained with:

I_SD=1,2×〖10〗^(-3)×(f/D)×S^2×v^2×〖10〗^(A/10)

With

I_SD= space diversity improvement factor (dB)

f = frequency (GHz)

D = path length (km)

s = antenna spacing (m)

v = RSL difference (mV); between main antenna and space diversity antenna

A = effective fade margin (dB)

In signal reception, there are several methods within space diversity to ensure that only the best signal is read or used. These methods include:

Selection Diversity

This method selects the signal with the highest Signal-to-Noise Ratio (SNR) from multiple antennas installed in a space diversity setup. The signal from that antenna is then sampled and sent for demodulation. This technique is relatively easy to implement because it only requires monitoring signals from several antennas and using a switch to select the best one.

Although simple, selection diversity is not entirely optimal because it utilizes only one piece of information from a single antenna and does not take advantage of all the available data from the other antennas. 

Feedback Diversity

N signals are scanned at the receiver side until a signal exceeding a predetermined threshold is found. That signal is accepted until it drops below the threshold, at which point the scanning process starts again. So, when the received signal exceeds the defined threshold, it will be accepted until it falls below that threshold, and the scanning process is repeated until all the information in the signal is obtained. This technique does not require the system to process all signals simultaneously, but the scanning time may lead to data loss if the signal quality suddenly degrades.

Maximal Ratio Combining (MRC)

Signals from all antennas are weighted according to their SNR and then summed. The signals must be co-phased (adjusted in phase). The output SNR is the sum of the individual SNRs. In theory, this is the best method to use because it utilizes all SNR values and takes advantage of all the energy from the received signals. However, this technique is complex because it requires knowing the SNR of each antenna and demands phase synchronization.

Equal Gain Combining (EGC)

Signals from all antennas are combined with equal (unity) weighting, but their phases are still adjusted. Unlike MRC, this technique does not weight the signals based on SNR but simply aligns the phases and combines them with equal weight. The performance of this method is slightly lower than MRC but better than selection diversity. It is simpler than MRC because it does not require SNR weighting but still utilizes all received signals.

b. Frequency Diversity

Frequency diversity is a technique of transmitting the same information using two microwave frequencies on a single antenna at both the transmitter and receiver. The information is simultaneously transmitted by two transmitters operating at different frequencies and then sent through one transmitting antenna. At the receiving antenna, the information is collected and separated into two signals. In frequency diversity, only one antenna is needed.

At the receiving antenna, the information is gathered and separated into two signals. The frequency gap (Δf) between the two signals only needs to be approximately 2%, although a 6% difference is recommended to better minimize the risk of major interference..

Since fading (signal disturbance or loss) usually occurs only at certain frequencies at specific times, transmitting the signal across multiple frequencies reduces the likelihood that all frequencies will be affected simultaneously. As a result, the received signal can still be accurately reconstructed.

 

Figure 2 Frequency Diversity

When using frequency diversity, an improvement factor can be obtained as shown in the formula: 

I_FD=10log⁡∆f-20log⁡f-10log⁡D+FM^0.9

Where:

I_FD = frequency diversity improvement factor

∆f = frequency difference used in the system

F = frequency (GHz)

D = path lenght (km)

Frequency diversity involves duplicating equipment at the microwave transmitter to send the same information at different frequencies.

c. Polarization Diversity

 

Figure 3: Polarization Diversity

Polarization Diversity requires two transmitting antennas and two receiving antennas with different polarizations. Transmission waves with two different polarizations form two different paths. This provides two distinct diversity branches.

Polarization diversity splits the transmission power between two differently polarized antennas, thus using only half the power per antenna. It utilizes the difference between horizontal and vertical wave polarization. Since the signal propagation path varies depending on polarization, interference affecting one polarization may not affect the other. Fading usually occurs differently for each polarization direction, so using polarization diversity makes it very unlikely that both will be disrupted at the same time.

d. Time Diversity

Time diversity is conceptually similar to frequency diversity—it transmits the same information more than once, but at different time intervals to overcome temporary fading (temporal fading) in the communication channel.

Because disturbances are often temporary, by repeating the transmission at different times, the system has a greater chance of successfully receiving the data at least once. The retransmission interval must be shorter than the coherence time. Time diversity also does not require an increase in transmission power.

In time diversity, the fading of the channel is averaged over time using channel coding and interleaving, so that each part of the codeword is affected by different fading over time. If deep fading occurs, only a portion of the codeword will be lost, not the entire message. To explain this, let’s look at an example:

 

Figure 4: Time Diversity

e. Angle Diversity

Angle Diversity is a diversity technique in wireless communication that uses multiple antennas oriented at different reception angles to reduce the effects of multipath fading and interference. On the receiver side, signals can arrive from various paths—these are known as multipaths—which can differ in length, angle, and arrival time. As a result, many versions of the same signal may be received.

With angle diversity, selection diversity can be used—choosing the signal with the highest Signal-to-Noise Ratio (SNR) among the antennas—or maximal ratio combining, where signals from all antennas are weighted based on their SNR and then combined. Either method can be used in a system to take advantage of the fact that signals from different directions do not experience fading simultaneously, thereby improving signal availability.

Conclusion

Microwave radio communication is a form of wireless communication that utilizes electromagnetic waves. Thanks to these waves, data transmission becomes faster and can cover long distances. One of the key concepts in microwave radio systems is availability, the system's ability to provide continuous service without significant interruption.

A major challenge to availability is multipath fading, a condition where the received signal experiences changes in amplitude, phase, and power, resulting in poor or distorted information compared to what was transmitted. To overcome this challenge, diversity techniques are employed. There are five types of diversity techniques: frequency diversity, time diversity, space diversity, angle diversity, and polarization diversity.

Space Diversity uses two or more physically separated antennas (several wavelengths apart). Each antenna receives signals through different propagation paths. Since fading doesn’t usually occur at the same time in different locations, the signal with the best quality can be selected or combined. This technique is effective for spatial fading and is commonly applied in base stations, MIMO systems, and Wi-Fi devices.

Frequency Diversity employs two or more different carrier frequencies to transmit the same information. Since fading does not usually affect all frequencies simultaneously, the system can still receive the signal from an unaffected frequency. This technique is particularly effective for combating frequency-selective fading and is widely used in OFDM systems, Frequency Hopping Spread Spectrum (FHSS), and other modern digital communications.

Time Diversity relies on repeating the transmission of data at different times to protect the signal from temporary disruptions or temporal fading. Its implementations include Automatic Repeat Request (ARQ), Hybrid ARQ (HARQ), and interleaving, which spreads data over time. This technique is highly effective in rapidly changing environments, such as mobile systems and digital broadcasting.

Polarization Diversity uses antennas with different polarization directions, such as vertical and horizontal. Since signals with different polarizations experience different fading, the system can choose or combine the signal with the best quality. A major advantage is spatial efficiency—no physical separation between antennas is needed—making it suitable for MIMO systems, satellite communication, and portable devices.

Angle Diversity uses two or more antennas oriented at different reception angles to receive signals from various directions. This technique helps reduce multipath fading and directional interference and is widely used in Free Space Optical Communication (FSO), indoor systems, and antenna sectoring at base stations.

Reference

geeksforgeeks, "Diversity and its Types"www.geeksforgeeks.org. Diakses pada senin 9 Juni 2025. https://www.geeksforgeeks.org/diversity-and-its-types/

Pradana, Z.H., Wahyudin, A. ANALISIS OPTIMASI SPACE DIVERSITY PADA LINK MICROWAVE MENGGUNAKAN ITU MODELS, 2017. https://download.garuda.kemdikbud.go.id/article.php

tidjma, "angle diversity"www.tidjma.tn. Diakses pada senin 9 Juni 2025. https://www.tidjma.tn/en/electrical/angle-diversity-/