Diversity in Radio Communications
1. Introduction:
In the context of radio communications, diversity refers to the transmission and reception of various versions of the message signal to combat signal fading and improve the message reliability at relatively low cost. Diversity plays an important role in reducing the effects of fade and possibly others degradations. The rationale behind diversity is that the probability of receiving several replicas of the same message signal, where each transmission is impacted statistically independent of the others, is quite low. In fact, diversity techniques can be effective, even if there may exist some correlation among the received signals of the same message signal.
Diversity is now a critical part of wireless communication. In mobile communications, diversity allows the transmit power on the reverse link, which is limited by the battery capacity and is required to protect the link during the short intervals of deep fading, to be reduced. Also, low-signal outage improves voice quality and handoff performance. In addition, in mobile cellular communications which is mostly interference limited, diversity helps reduce the variability of carrier-to-interference ratio, thus improving the frequency-reuse factor and system capacity.
Large-scale fading is caused by shadowing, which is due to variations in the terrain profile and the nature of the surroundings. In deeply shadowed conditions, the received signal strength at a mobile can be much less than that of free space. Large-scale fading is a log-normal distribution. To combat large-scale fading, that is, to significantly improve the average signal-to-noise ratio (SNR), macro-diversity can be employed. Since shadowing is almost independent of transmit frequency and polarization, frequency and polarization diversity are not effective. The simplest method for macro-diversity is the use of on-frequency repeaters that receive the signal and re-transmit an amplified version of it, but at the cost of additional delay. Another viable method is simulcast, by which the same signal is transmitted simultaneously from different sites, but at the cost of further synchronization.
Small-scale fading is characterized by deep and rapid amplitude fluctuations that occur as the mobile moves over distances of just a few wavelengths. Small-scale fading has a Rayleigh distribution . To combat small-scale fading (i.e., to prevent deep fades from occurring) micro-diversity can be employed. For instance, with two antennas, one may receive a null while the other receives a strong signal.
There are many ways to achieve diversity, including time diversity, space diversity, site diversity, frequency diversity, polarization diversity, angle diversity, and path diversity, or some combination of these techniques. Most of these techniques are suitable for micro-diversity, except site diversity, which is best suited for macro-diversity. The overall diversity performance is due to the combination of different diversity techniques and how signals are combined.
2. Types of Diversity Techniques
Diversity techniques are classified based on the dimension in which multiple signal paths are exploited: time, frequency, space, site, polarization, angle,and path diversity.
2.1 Time Diversity
In mobile communications, time selective fading of the signal with Rayleigh fading statistics for the signal envelope occurs. Time or temporal diversity is when the message signal is transmitted multiple times at different time instants separated by intervals longer than the underlying fade duration, so the transmitted copies of the message signal are then likely subjected to independent fading. Time diversity is effective when the time separation between transmissions exceeds the coherence time of the channel (i.e., it is greater than the time between peaks or valleys in the fading signal). The coherence time depends on the Doppler space of the signal, which in turn is a function of the mobile speed and the carrier frequency. If the channel is static, for instance, when neither the transmit end nor the receive end is on the move, time diversity is not very effective, as the coherence can be quite long. Time diversity is not suitable for delay-sensitive applications, such as voice communications. Time diversity requires storage, additional processing, such as interleaving, additional bandwidth for FEC coding, and additional power for repeated transmissions using ARQ techniques.
2.2 Frequency Diversity
Frequency diversity allows the transmission of the same message signal at different carrier frequencies. In order for the received signals to be statistically independent or at least uncorrelated, the carrier frequencies must have a separation that is greater than the ccoherence bandwidth of the radio channel. The coherence bandwidth depends on the multi-path delay spread of the channel. One limitation of this technique is the ability of the receiver to pick up all these signals (i.e., the need for multiple receivers to tune to these frequencies). It is not common to actually repeat the same message signal at two different frequencies, as this would greatly decrease spectral efficiency. Instead, the signal is spread over a large bandwidth, so parts of the signal are conveyed by different frequency components. This spreading can be done by different ways, including multi-carrier modulation using an inverse DFT and frequency hopping using widely-separated frequencies from burst to burst.
2.3 Space Diversity (Antenna Diversity)
Space diversity, due to its ease of implementation, is widely used in mobile-radio base stations. Space diversity takes advantage of the random nature of propagation in different directions. Space or spatial diversity exploits propagation environment characteristics by employing multiple antennas at the transmitter and/or receiver to create spatial channels, for it is not very likely that all the channels will fade simultaneously. Depending on the physical separation between the two antennas, also known as coherence distance, in a site, signals arriving at the two antennas may be independent or at least uncorrelated. The required spacing depends on the degree of multi-path angle spread. In principle, the smaller the angle of arrival between the two signals, the farther the distance between the two antennas must be. For instance, in the cellular telephony, to achieve effective diversity for decorrelation of received signals, a separation on the order of 10 to 20 times the wavelength is required at the base station, whereas the separation on the order of 1 to 3 times the quarter of the wavelength is needed at the mobile.
2.4 Site Diversity
Site diversity, as an effective macro-diversity, is a form of space diversity, where different transmit or receive antennas are located in geographically-dispersed sites. Site diversity can thus by definition be applied to base stations in cellular network or gateway (hub) stations in satellite networks. In satellite networks, site diversity is employed in broadcast satellite systems, as the feeder-link (the link between the hub and the satellite) availability, which is susceptible to rain fade attenuation, is of paramount importance.
2.5 Polarization Diversity
In mobile radio communications, signals transmitted on orthogonal polarization experience uncorrelated fading or low fade correlation, as the reflection and diffraction processes depend on polarization. Since the fading of signals with different polarization is statistically independent, receiving both polarization using a dual-polarized antenna and processing the received signals independently, known as polarization diversity, offers potential for diversity combining. The cross-polarized antennas do not need large physical separation as needed in space diversity antennas. The only limitation of this technique is the inability to generate more than two diverse signals. There are two ways to achieve polarization diversity. One is that the transmit antenna transmits on both polarization, thus power for each polarization will be 3 dB lower than if single polarization is used. The other one is due to the fact that the scattering medium can depolarize the polarized transmitted signal. The depolarized signal received at the antenna can be split into two polarization, producing two independently fading signals. In other words, the transmission is a single polarization and reception is on both polarization; however, the average received signal strength in the two diversity branches is not identical. In mobile communications, polarization diversity cannot be effective for mobile devices, but can be effective for base stations. As the mobile phone is held at random orientations during a call, the resulting depolarization of the transmitted signal on the reverse link and use of cross-polarized antennas at the base station can make polarization diversity attractive.
2.6 Angle Diversity
Angle or angular diversity, also known as pattern diversity, makes use of directional antennas with different patterns that can be pointed in different directions to provide uncorrelated replicas of the transmitted signal, even when mounted close to each other at the receiver site. Angular diversity is usually used in conjunction with space diversity. Antennas with different patterns can see differently-weighted multi-paths, even if they are identical antennas and mounted close to each other. Although space diversity is implemented at the base station, angle diversity may be implemented at the base station or at the mobile unit. The different patterns are more pronounced when the antennas are located on different parts of the casing.
2.7 Path Diversity
Multi-path signals are all time shifted with respect to one another, as they travel different paths. Although multi-path is usually detrimental, it can be sometimes beneficial. In path or multi-path diversity, the multi-path components when they are resolvable and non overlapping can be effectively utilized. This implicit diversity is available if the signal bandwidth is much larger than the channel coherence bandwidth. When the time duration of the pulses is very small, such as more than one chip period in CDMA systems or more than one symbol period in high-burst-rate TDMA systems, the multi-path signals can be non overlapping and uncorrelated.
3. Diversity Combining Techniques
To utilize the benefits of diversity, signals received from multiple paths or antennas must be combined effectively. Several combining methods exist, each with different complexity and performance trade-offs.
Maximum Ratio Combining (MRC)
MRC weights each received signal proportionally to its signal-to-noise ratio (SNR) and coherently sums them. This method maximizes the output SNR and achieves the best performance among combining techniques. However, it requires knowledge of the channel conditions for each branch.
Equal Gain Combining (EGC)
align signals coherently. It offers performance close to MRC but with less complexity since it does not require amplitude weighting.
Selection Combining (SC)
SC selects the signal from the antenna or branch with the highest SNR for processing, ignoring others. It is the simplest technique EGC combines signals from multiple antennas with equal weights but adjusts the phases to e but less effective than MRC or EGC because it discards potentially useful signal copies.
4. Applications of Diversity in Radio Communications
Diversity techniques are essential in many wireless systems to enhance signal reliability and quality:
- Mobile Communications: Cellular base stations use antenna diversity to reduce dropouts and improve call quality. Macro-diversity is used by coordinating antennas at different sites.
- Wi-Fi and Wireless Networks: Access points employ antenna diversity to mitigate multi-path fading and interference indoors.
- Wireless Microphones and Audio Equipment: Diversity receivers switch between antennas to avoid dropouts caused by movement or obstacles.
- Broadcasting: Diversity schemes help maintain signal quality over large areas and varying propagation conditions.
- Emerging Technologies: Cooperative diversity and multi-user diversity are integral to advanced wireless standards like 5G, enhancing throughput and reliability.
5. Importance of Diversity Beyond Technical Aspects
Diversity in radio communications also refers to the inclusion of diverse voices and content in broadcasting. According to UNESCO and WACC, radio is a powerful medium for celebrating human diversity and democratic discourse. Ensuring diversity in radio ownership, newsrooms, and programming supports representation of different communities and viewpoints, especially in public and community broadcasting.
6. Conclusion
Diversity in radio communications stands as a foundational strategy to combat the unpredictable and often harsh nature of wireless channels. The wireless medium is inherently susceptible to fading, interference, shadowing, and multi-path propagation—phenomena that can significantly degrade signal quality and system performance. To address these challenges, diversity techniques have emerged as essential tools in ensuring reliable, consistent, and high-quality communication across a broad spectrum of applications.
By leveraging variations in time, frequency, space, polarization, angle, and even user cooperation, diversity schemes introduce redundancy and statistical independence into signal transmission and reception. This redundancy ensures that even if one path or method fails due to fading or interference, others are likely to succeed, thereby preserving the integrity and clarity of the transmitted information. Whether it’s frequency diversity using multiple channels, time diversity through re-transmissions, space diversity via multiple antennas, or polarization diversity exploiting electromagnetic orientation, each technique contributes uniquely to the robustness of wireless systems.
Moreover, these diversity methods are not isolated; they are often combined with advanced signal processing and combining techniques such as maximal ratio combining (MRC), selection combining (SC), or equal gain combining (EGC) to further enhance their effectiveness. The synergy between diversity and signal processing enables systems to adapt dynamically to changing channel conditions, improving not only the bit error rate (BER) but also the overall system capacity and coverage.
As modern communication demands continue to grow—with the rise of 5G, IoT (Internet of Things), satellite systems, and mission-critical applications—the importance of diversity becomes even more pronounced. These systems require exceptionally high reliability and low latency, which can only be achieved by integrating robust diversity schemes at both the physical and protocol layers. From mobile telephony and public broadcasting to remote sensing and emergency response networks, diversity plays a pivotal role in ensuring that communication remains uninterrupted and effective, even in the most challenging scenarios.
In conclusion, diversity is more than just a technical feature; it is a strategic necessity in the architecture of modern wireless communication systems. Its ability to exploit the fundamental properties of radio wave propagation transforms variability into reliability, turning potential weaknesses into strengths. As research and technology continue to evolve, diversity will remain a central pillar in the ongoing pursuit of universal, dependable, and high-performance wireless communication.
7. References
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