Diversity Techniques in Wireless Communication Systems
Nowadays, especially in this digital era, wireless communication plays an important role in human life in the field of telecommunications. However, there are many challenges when using wireless communication, such as Line of Sight, Diffraction, Scattering, Fading, Refraction, and Reflection. But one of the main challenges faced by wireless communication is Fading. Fading can significantly degrade signal quality and strength which impacts the data rate and overall performance of the telecommunication system. So, to overcome fading and improve the reliability of wireless links, use diversity techniques.
Type of Fading
There are two types of fading: small-scale and large-scale. Small-scale fading refers to rapid and significant changes in signal strength that occur when a mobile receiver moves over distances as short as a few wavelengths. Large-scale fading occurs due to shadowing effects and can be addressed by using suitable diversity techniques. In this case, the distances considered are typically similar to the spacing between two base stations.
What is Diversity?
Diversity
is an efficient communication receiving system that enhances wireless
connectivity at minimal effort. In wireless communication systems, diversity
techniques are primarily used to significantly improve performance when signals
experience fading in radio channels.
Type of Diversity
Persamaan penting untuk menghitung kehilangan jalur adalah. Formula ini esensial dalam analisis sistem nirkabel.
There
are two basic types of diversity are: microscopic diversity and macroscopic
diversity.
Microscopic
diversity techniques are used to reduce the effect of small-scale fading, which
happens when the received signal experiences deep drops in strength over short
distances. By using two antennas with a small separation, it is possible to
minimize these sudden changes in signal strength.
The method works by selecting the signal with the highest strength at any given time, allowing the receiver to lessen the impact of fading. As shown in the graph, small-scale fading causes rapid fluctuations in signal power, while large-scale
fading results in more gradual changes, especially in indoor settings.
Macroscopic
diversity techniques are applied to address large-scale fading, which is mainly
caused by shadowing effects that decrease signal strength. By selecting a base
station that experiences less shadowing compared to others, a mobile device can
achieve a higher signal-to-noise ratio (SNR) for its communication link. The
method of reducing the impact of large-scale fading by choosing the most favorable
base station is known as macroscopic diversity. It is especially effective at
the receiver side of the base station.
Type of Diversity Techniques
There
are basically five diversity techniques
- Frequency Diversity (Multipath)
- Time Diversity (Temporal)
- Space Diversity
- Polarization Diversity
- Angle Diversity
Frequency Diversity
In
frequency diversity, the same information is sent over several different
carrier frequencies. The main principle is that if these frequencies are spaced
farther apart than the channel’s coherence bandwidth, the signals on each
frequency will be uncorrelated. As a result, they are unlikely to experience
fading at the same time. In such uncorrelated channels, the overall probability
of all signals experiencing fading at once is the product of the individual
probabilities for each frequency.
The
use of this diversity technique requires two RF stages at both the receiver and
the transmitter. An improvement factor can be calculated for this type of
diversity method.
Time Diversity
In
this technique, the same information is transmitted multiple times, with each
transmission separated by a time interval longer than the channel’s coherence
time. This repeated transmission ensures that the signal copies are affected by
different fading conditions. As a result, sending the same data in various time
slots allows the receiver to create diversity branches from these repeated
signals.
Space Diversity
This
technique is also known as antenna diversity. In space diversity, the receiver
setup is straightforward, and multiple diversity branches can be selected. To
achieve effective diversity at each cell site, several receiving antennas are
used at the base station. It is important to ensure that these antennas are
placed far enough apart, typically several wavelengths, to reduce correlation
between the received signals. This separation helps because most of the signal
scattering happens near the mobile device, close to the ground. Space diversity
can be implemented at the base station, the mobile unit, or both.
As
a result of using the space diversity technique, it is necessary to have two or
more receiving antennas and two or more RF stages at the receiver.
Improvement Factor value on space diversity:
\[ ls = \frac{1.2 \cdot 10^{-3} \cdot s^{-2} \cdot f \cdot 10^{\frac{F - V}{10}}}{d} \]
Figure 3 Schematic Space Diversity
Figure 4 Simple block diagram of space
diversity technique
If
the distance between antennas is greater than half a wavelength, it is enough
to reduce the correlation of fading between different diversity branches. At a
base station, antenna separations of 50 to 100 wavelengths are recommended to
ensure effective diversity.
The
block diagram above illustrates a typical space diversity system. In this
setup, multiple branches are used, each with its own antenna and gain value
(such as G1, G2, and so on). These signals are then processed by a switching
logic unit or a demodulator, which combines the inputs to produce the final
output.
Polarization Diversity
Polarization
diversity uses both horizontal and vertical polarizations. When a signal is
transmitted with a pair of polarized antennas and received by another pair, the
receiver gets two signals that are uncorrelated in their fading. This happens
because horizontal and vertical polarizations experience different fading
effects and reflect differently off tall buildings.
Studies
have shown that the signal paths for vertical and horizontal polarization
between the base station and the mobile device are uncorrelated. This lack of
correlation is mainly due to the multiple reflections that occur in the radio
channel. However, the type of polarization received still depends to some
extent on the type of polarization that was transmitted.
Angle Diversity
This
type of diversity system requires several directional antennas, each of which
responds independently to waves coming from different directions. When an
antenna receives a signal arriving at a specific angle, the faded signal it
receives is uncorrelated with signals picked up by other antennas.
To
achieve angle diversity, antennas with narrow beam widths are placed in
different sectors of the system. This setup allows the receiver to distinguish
and combine signals arriving from various directions. As a result, angle
diversity not only provides diversity benefits but also increases antenna gain
and reduces interference by offering better angular discrimination.
Diversity Combining Techniques
It
is essential to combine the faded signals from different diversity branches,
especially when these signals are uncorrelated, to fully achieve the benefits
of diversity. The combining process should be designed to enhance the
performance of the communication system, such as by improving the signal-to-noise
ratio (SNR) or increasing the received signal power. Although combining is
usually performed at the receiver, it can also be applied at the transmitter.
There are several methods for diversity combining, with maximum ratio combining
(MRC), equal gain combining (EGC), and selection combining (SC) being the most
commonly used.
Selection Combining
Maximum
Ratio Combining (MRC) and Equal Gain Combining (EGC) techniques are not
well-suited for very high frequency (VHF), ultra high frequency (UHF), or
mobile radio applications. This is because building a precise and stable
co-phasing circuit is difficult in environments where the signal changes often
due to multipath fading and random phase shifts. In contrast, Selection
Combining (SC) is easier to implement and more practical for mobile radio use.
In Selection Combining, the branch with the strongest signal is chosen. The main idea is to monitor all the diversity branches and select the one with the highest signal-to-noise ratio (SNR). This process ensures that the best available signal is always used at the receiver. Although it can be difficult to measure SNR quickly, choosing the branch with the highest SNR is almost the same as picking the branch with the highest received power when the noise level is similar on each branch. Thus, in practice, the branch with the largest signal, least noise, and minimum interference is selected.
Feedback Diversity
This
technique is also known as scanning diversity, works by sequentially checking
multiple received signals to find one that exceeds a predetermined threshold
value. In this approach, the signals from several antennas are monitored, and
the receiver selects the first signal that is stronger than the set threshold.
A
scanning process is performed on all incoming signals, but a limitation of this
technique is that it reduces the fading level less effectively than other
diversity methods. For each received signal, its strength is compared with the
preset threshold, and the best available signal is chosen.
Figure
6 Feedback Diversity
Maximum Ratio Combining
The
selection combining method does not make use of all the received power from the
diversity branches, as it only selects the strongest signal. In contrast, the
maximum ratio combining (MRC) technique combines signals from all branches,
weighting each one based on its signal-to-noise ratio (SNR). This approach
produces an average SNR that equals the sum of the average SNRs from each
branch. In MRC, the signals from all channels are first aligned in phase and
then weighted to achieve the highest possible SNR at the receiver. The gain
assigned to each channel is proportional to its root mean square (RMS) signal
level and inversely proportional to its mean square value. By adding the
voltage signals coherently from each branch, MRC can effectively restore the
original shape of the received signal.
Equal
Gain Combiner
In equal gain combining, the signals from all diversity branches are aligned in phase and then combined using the same weighting factor for each branch. This method ensures that each signal contributes equally to the final output. Compared to maximal ratio combining, equal gain combining has a simpler setup. This simplicity makes it easier for the receiver to reconstruct the original signals.
Switched Combining
Switched
Combining (SWC) is used because it is not practical to monitor all diversity branches
at the same time in selection combining. If continuous monitoring is needed,
the system would require the same number of receivers as branches, which is not
efficient. SWC solves this by switching from one branch to another only when
the signal strength falls below a certain threshold.
The
threshold value is set for a small area, but it may not be ideal for the entire
coverage area. Therefore, the threshold must be adjusted frequently, especially
when the receiver, such as a moving vehicle, changes its position. Choosing the
right threshold is very important: if it is set too high, the system will
switch too often, causing unwanted interruptions. If it is too low, the
diversity gain will be small.
In some systems, especially those using frequency hopping, switching can be done at regular intervals.
Periodic Switching Method
This method involves sequentially selecting diversity branches using a standard free-running oscillator. It is particularly effective in frequency modulation systems with large deviations and low speeds, where phase transients caused by switching can be minimized. The switching rate, the sole adjustable parameter, is typically set to twice the signal's bit rate. This ensures that the better branch is selected within each signaling period.
However, performance may degrade in adjacent-channel scenarios due to spectral folding, where the spectrum of the adjacent channel overlaps into the desired band during periodic switching at the radio frequency stage (see Figure 10). To mitigate this issue, increasing the receiver's adjacent-channel selectivity is recommended.
References
Raut,
P. W., & Badjate, S. L. (2013). Diversity Techniques for Wireless
Communication. International Journal of Advanced Research in Engineering and
Technology (IJARET), 4(2), 144-160.
Jasmine, K., Kavitha, K., & Dhivyapraba, R. (2020). Performance Analysis of Diversity Techniques for Wireless Communication.
Sharma,
S., & Chowdhary, P. (2018). Diversity Techniques in Wireless Communication.
IRE Journals, 1(10), 96-100. ISSN: 2456-8880.
Hudiono, dkk. (2021). Sistem komunikasi radio dan
laboratorium: diploma 3 politeknik.
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