=Paper=
{{Paper
|id=Vol-1755/46-52
|storemode=property
|title=A Space-Diversity Technique for Mitigating Signal Fading in Radio Transmission
|pdfUrl=https://ceur-ws.org/Vol-1755/46-52.pdf
|volume=Vol-1755
|authors=Gerald Ijemaru,Olaitan Akinsanmi,Gbenga Obikoya
|dblpUrl=https://dblp.org/rec/conf/cori/IjemaruAO16
}}
==A Space-Diversity Technique for Mitigating Signal Fading in Radio Transmission==
https://ceur-ws.org/Vol-1755/46-52.pdf
A Space-Diversity Technique for Mitigating Signal Fading
in Radio Transmission
Ijemaru Gerald Kelechi Akinsanmi Olaitan Obikoya Gbenga Daniel
Federal University Oye-Ekiti Federal University Oye-Ekiti Federal University Oye-Ekiti
Department of Electrical Department of Electrical Department of Electrical
& Electronics Engineering & Electronics Engineering & Electronics Engineering
+2348035523341 +2348068108694 +2348060722200
gerald.ijemaru@fuoye.edu.ng olaitan.akinsanmi@fuoye.edu.ng gbenga.obikoya@fuoye.edu.ng
ABSTRACT No. 39 of 1956, the Nigerian Broadcasting Corporation, NBC was
Despite over seventy years of broadcasting, Radio Nigeria born [1, 2, 35]. NBC had MW/SW stations at Lagos, Ibadan,
(FRCN) network signals still have poor reception and sometimes Enugu and Kaduna. In 1978, NBC was reorganized to become the
total failure. Apart from the problems of attenuation, fading, Federal Radio Corporation of Nigeria, FRCN otherwise known as
distortion, interference and noise, which are age-long problems Radio Nigeria.
militating against the performance of a digital communication
system, radio communications engineers also face some new In the early days of Federal Radio Corporation of Nigeria
challenges arising from bandwidth limitation, equipment (FRCN), her network programmes (signals) were originated from
complexity, power limitation, noise limitation and cost. These Ikoyi studios and transmitted by 300KW short wave transmitters
problems are often encountered in the design of any located at Ikorodu, Lagos state. Other zonal stations on receiving
communication systems and arose due to the imperfect nature of the signal through an AM Yaesu receiver then retransmitted
the communication channel. Each attempt to solve the problems simultaneously for her zonal listeners. Poor reception and
creates additional cost, resulting from increased complexity in the inherent noise associated with AM transmission caused the
design or increased radio frequency (RF) power requirement. The organization to acquire NITEL dedicated telephone lines that
diversity technique presented in this paper is aimed at solving the linked the studios of the zonal stations to that of the network
problem of signal fading and poor reception of radio signals. The studio. Although, signal quality improved greatly, NITEL
results of the previous studies have presented space diversity microwave link failure and dead lines occasioned by weather
technique as a good tool for combating signal fading in a radio (rainfall) still made FRCN network signal to be unstable. Recently
broadcasting station, whose studio and transmitting stations are the organization acquired a channel in the NTA satellite earth
located at different geographical locations, by connecting multiple station located at Victoria Island Lagos to ensure that the sources
antennas at both the transmit and the receiver ends. Simulation of her network programmes were decentralized and reception
was conducted using MATLAB to verify the relationship between crystal clear around the world. This acquisition led to TVRO
antenna heights and coverage distance in the line-of-sight system used by FRCN Zonal and FM stations to relay her network
transmission. The results are in total compliance with the programmes. After all these transformations, FRCN network
attenuation square law, which states that signal strength is programmes still suffer intermittent break (fast fading) and
inversely proportional to distance. sometimes total failure (slow fading).
CCS Concepts The block diagram of Fig. 1 can symbolically represent any
• Hardware ➝Integrated circuit ➝Interconnect ➝Radio communication system. The information or message from the
frequency and wireless interconnect source is fed to the transmitter for modulation and amplification,
after which the modulated signal is transmitted to the receiver
Keywords through a channel or path. At the receiver, the information is
Space-Diversity Technique, Signal Fading, Antennas, Simulation, decoded or demodulated and the message signal is reproduced.
MATLAB, Attenuation, Interference, Noise. The central section of the communication system is the channel,
which electrically connects the transmitter and the receiver. A
channel may be a pair of wire, a coaxial cable, free space, power
1. INTRODUCTION line carrier (PLC), a radio wave or even a laser beam. Regardless
Radio broadcasting was introduced into Nigeria in 1933 by the of the type of medium used, the output of the channel is usually a
then colonial government to relay the overseas service of the smeared or distorted version of the input. This deterioration is as a
BBC. The service was called Radio Diffusion System, RDS [1, result of the non-ideal nature of the communication channel. This
35]. From the RDS emerged the Nigeria Broadcasting Service, causes certain unwanted and undesirable effects, which invariably
NBS, in April 1950. In April 1957 through an Act of Parliament corrupt the information-bearing signal [1]. As a result, errors are
introduced in the information being transmitted. Factors that
degrade system performance are attenuation, which is a
progressive decrease in signal power with increasing distance,
distortion and interference [1]. The problem of interference is very
CoRI’16, Sept 7–9, 2016, Ibadan, Nigeria.
common in broadcasting where a radio receiver may pick up two
or more signals at the same time.
46
Section 4 gives a detailed step-by-step approach in carrying out
the study, which includes research methodology, data collection
Transmitter and presentation. The relationship between antenna
heights/distances and electric field strength simulated using
MATLAB is presented in section 5. Also included in this section
Source Encoder Modulator is the analysis of signal strength when the number of receiving
antennas connected in the circuit is varied. This is followed by an
acknowledgement in section 6, while section 7 presents the
conclusion and recommendations. Finally, section 8 looks at the
future work to be carried out.
2. REVIEW OF RELEVANT WORKS IN
MIMO SYSTEMS
(Noise, Distortion, Since the invention of the radio telegraph by Marconi in 1895,
Interference, etc.) wireless communication has attracted great interest and is now
one of the most rapidly developing technologies [3]. From
narrow-band voice communications to broadband multimedia
communications, the data rate of the wireless communications has
been increased dramatically, from kilobits per second to megabits
per second [4]. According to the authors in [3], the last two
decades witnessed an explosion in the advancements of wireless
systems and hence future wireless networks face challenges of
Destinati Decoder De-Modulator supporting data rates higher than one gigabit per second.
on
Among numerous factors that limit the data rate of wireless
communications, multipath propagation plays an important role
Receiver [5], [16-18]. In wireless communications, the radio signals may
arrive at the receiver through multiple paths because of reflection,
diffraction, and scattering. This phenomenon is called multipath
Fig. 1: Block Diagram of a General Communication System propagation, which causes constructive and destructive effects due
to signal phase shifting. Channels with multipath fading fluctuate
Radio network programmes in Nigeria are often associated with
randomly, resulting in significant degradation of signal quality.
intermittent breaks (fast fading) and sometimes degenerate into
When the bandwidth of the signal is greater than the coherence
static noise and total failure (slow fading). This can be traced to
bandwidth of the fading channel, different frequency components
daily incidents of high rise of buildings and towers occasioned by
of the signals experience different fading. This frequency-
the topography of the country with hills, trees and valleys [2].
selective fading may further limit the data rate of wireless
Again, atmospheric conditions during most part of the year, like
communications.
rainfall, always pose a great challenge. For instance, during rainy
season, most of the newly established FM stations in Nigeria shut In order to mitigate multipath fading, Code Division Multiple
down at a slight sign of the rain because of the accompanying Access (CDMA) and Orthogonal Frequency-Division
lightning, which on many occasions have damaged the power Multiplexing (OFDM) were developed [4], [19] – [20]. While
modules of their transmitters and the analogue output cards of CDMA mitigates multipath fading by transmitting signals which
their digital console. occupy a wider bandwidth, OFDMA splits the channel into many
small bandwidth carriers, each of which occupies a narrowband
channel [4]. Though a small percentage of this wideband channel
The purpose of this work is to explore the applications of MIMO
may undergo deep fading, the overall channel could still be in
systems with focus on a Space-Diversity Technique for combating
good shape. The lost signals could be recovered with the help of a
signal fading, so that broadcasting media in Nigeria can tap from
Rake receiver and/or maximum-ratio combining [6]. Although
its gains to improve on the reception quality of their signals with
these two schemes are effective in mitigating multipath fading,
an increased coverage area. Again, the study also aims at
they have the limitation of providing a higher data rate compared
analyzing the relationship between antenna heights and maximum
to other techniques [4].
coverage distances in line-of-sight propagation and signal strength
of the received signal using simulation. The last decade witnessed the deployment of multi-antenna
systems, which are also referred to as multiple-input multiple-
output (MIMO) systems. These technologies are undoubtedly the
This study – “A Space-Diversity Technique for Mitigating Signal
most promising to achieve higher data rates. MIMO is a method
Fading in Radio Transmission” will focus mainly on radio
of transmitting multiple data beams on multiple transmitters to
broadcasting network signals in Nigeria. However, the result of
multiple receivers [7], [21-23].
the study will still be applicable to other communication systems.
The benefits of these arise from the use of extra spatial dimension,
This paper work is organized into eight sections. The introductory which involves the use of multiple antennas at the transmitter
part in section 1 deals with the general perspectives and objectives and/or receiver ends. With this technique, the rich scattering
of the work. Section 2 reviews relevant work in MIMO systems, channel is exploited to create a multiplicity of parallel links over
while section 3 presents the various types of diversity schemes, the same radio band, thus providing MIMO with several
combating signal fading with a space-diversity technique and advantages such as array gain, spatial diversity gain, and spatial
exploitation of diversity techniques in multi-path propagation. multiplexing gain [8].
47
The authors in [9], [24], and [28] present some of the performance 3.1 Frequency Diversity
improvements and potential applications of MIMO technology to
include: This involves sending of the same information using two or more
Array Gain – improves system robustness to the noise different frequencies of transmission or different transmitters set
thereby increasing coverage and quality of service to different frequencies. The arrangement is such that the
(QoS) transmitter (s) selects the frequency with better signal and uses
Diversity Gain – MIMO achieves spatial diversity by that as its preferred signal [10, 31]. This is however difficult to
providing the receiver with multiple identical copies of implement as the task of generating severally transmitted signals
the transmitted signal to mitigate multipath fading, and combining signals received at different frequencies is quite
increase coverage and to improve the quality and enormous. Again, the scheme requires commercial FM and TV
reliability of the reception. Diversity is maximized to stations to obtain as many licenses as the number of frequencies
mitigate channel fading and decrease the bit error rate they intend to use. This factor alone makes it unattractive to
(BER) broadcast stations, coupled with the fact that it also demands a
Multiplexing Gain – is achieved by transmitting large space to site the receiver and their antennas.
independent data signals from different antennas to 3.2 Time Diversity
increase throughput and spectral efficiency.
Co-channel interference mitigation – increases cellular This involves the transmission of multiple versions of the same
capacity. signal at different time intervals. Essentially, a redundant forward
error-correction code is added and the message is spread in time
For MIMO to be effective, the paths need to be de-correlated.
by means of bit-interleaving before it is transmitted. This is
That is, the signals travelling need to behave differently from each
predicated on the perception that the obstacles blocking signal
other, so that if any one part experiences fading, there is a high
paths are not stationary, hence the essence of transmitting the
probability that the other parts will not undergo fading and hence
same information at different time intervals [11, 28, & 32].
the signal can still get through [7], [25-27]. This can be possible
However, time is wasted while the receiver evaluates the received
with proper exploitation of techniques such as spatial separation
signals to determine the best to use. Moreover, while the scheme
(space-diversity) of the antennas or separation of the transmitted
may be better in data transmission, it is out of need in a radio and
waveforms via time separation, polarization separation, frequency
TV transmission where a live broadcast may be required. Also, in
separation, etc.
the event where the obstacles are permanent, transmitting the
RF signal transmission between two antennas commonly suffers same information even a couple of times will not yield any better
from power loss in the space. This power loss between transmitter reception.
and receiver is as a result of three different phenomena: distance-
dependent decrease in power density called path loss or free 3.3 Rake Receiver
space attenuation, absorption due to the molecules in the A rake receiver is a radio receiver designed to counter the effects
atmosphere of the earth and signal fading caused by terrain and of multipath fading [12], [27, and 33]. This is achieved by
weather conditions in the propagation path [2], [29] – [30]. Path utilizing several sub-receivers called fingers. Each finger
loss is the attenuation which occurs under free-line-of-sight independently decodes a single multi-path component, which is
conditions and which increases with the distance between base later combined to make the most use of the different transmission
station and mobile. Fading is an attenuation that varies in an characteristics of each transmission path. Rake receivers are
irregular way. Signals move through areas with obstacles of common in a wide variety of CDMA radio devices such as mobile
various sizes, surrounded by mountains, buildings and tunnels. phones and wireless LAN equipment [12].
Occasionally, these obstacles will shadow or completely cut off
the signal. Although the consequences of such shadowing effect 3.4 Space Diversity
depend on the size of an obstacle and the distance to it, the This is also known as antenna diversity. It is one of the diversity
received signal strength inevitably varies. This type of fading is schemes that utilize two or more antennas to improve the quality
referred to as shadow fading [26], [28]. Multi-path fading is a and reliability of a wireless link. In urban and indoor
completely different kind of fading involving irregular signal environments, there is usually no clear line-of-sight between the
strength variations as a result of several signals at the receiver. transmitter and the receiver; rather the signal is reflected along
The performance degradation can be solved by increasing the multiple paths before finally being received. Each of the bounces
transmitted power and resizing the antenna. However, this can introduce phase shifts, time delays, attenuations, and even
solution is not economically attractive, hence the need for special distortions that can destructively interfere with one another at the
reception techniques such as the space-diversity technique apertures of the receiving antenna [2, 24, 30-32]. Space diversity
(multiple receiver combining technique) [3-4], [28-29]. is effective at mitigating these multipath situations, because
In telecommunication, a diversity technique refers to a method for multiple antennas afford a receiver several copies of the same
improving the reliability of a message signal by utilizing two or signal. Each antenna experiences different interference
more communication channels with different characteristics. environment. Thus, if one is experiencing a deep fade, it is likely
Diversity is important especially in mitigating fading and co- that another has a sufficient signal. Signals received from the
channel interference [2], [28-30]. It is based on the fact that various antennas are then fed to a diversity combiner, which either
individual channels experience different levels of fading and selects the antenna with the best signal strength or adds the signals
interference. coherently.
3. TYPES OF DIVERSITY SCHEMES Application of space diversity to combat signal fading is not new
to mobile and fixed wireless telecommunication providers. In this
This section presents the various methods of reducing signal
research work where the scheme will be helpful in radio
fading and the types of diversity combiner.
broadcasting was explored. Radio Nigeria, like some older radio
48
and television stations whose studios and transmitting stations are effects, MATLAB was used to verify its effects to maximum
located at afar distance, could make use of this scheme. A coverage distances in line-of-sight transmission and field strength
microwave transmitter and receiver called studio-transmitter link of the received signal. Approximate value for the maximum
(STL) is used to couple the studio output to the main transmitter distance between transmitter and receiver over a reasonably level
through the line-of-sight propagation. The signal suffers a lot of terrain was calculated using:
degradation before reaching the microwave receiver at the main
transmitting station. √ ……………………………………. (i)
The usability of this scheme was tested at Federal Radio
Corporation of Nigeria (FRCN) transmitting station Shogunle Where d = maximum distance (km), ht = transmitting antenna
Lagos state, and it was found that multiple antennas spaced out height (m), and hr = receiving antenna height (m). The power
provided increased signal strength. The space diversity scheme density of the signal was calculated using the power density
tested in this research work is reception diversity. formula
3.4.1 Advantages of Space Diversity
……………… …………………………... (ii)
The following are some of the advantages of space diversity
technique as outlined by the authors in [13, 26, 32, & 34]:
Where, PD = Power Density of the signal, Pt = transmitted Power
Improvement in uplink performance for reception of
and r = maximum distance in meters.
both mobiles and portables
It is flexible as it can be applied either at the
transmitting station (transmit diversity) or at the The final calculation made in the simulation was to calculate the
receiver end (diversity reception). electric field strength (Signal Strength) using
It is very simple and can be used independently unlike
pattern and polarization diversity schemes. √ ………………………………………… (iii)
It is cheaper than frequency diversity that requires
several transmitters to generate the required frequencies. 4.1 Data Collection and Presentation
The disadvantage of space diversity is that it requires large
structures and thereby occupies space. The applicability of space diversity technique was tested at radio
Nigeria Metro FM station located in Lagos state. At the Ikoyi
3.4.2 Diversity Combiner broadcasting house, the output of the studio was fed to a 250w,
This is an electronic device used to combine the multiple received 450.15MHz STL transmitter located in the control room. The
signals of diversity reception device into a single improved signal. output of the studio transmission link (STL), through 50 ohms
Several diversity combining methods as shown in [14] include: coaxial cable is terminated in an antenna mounted on a 120ft
mast. At the Ikeja GRA, an STL receiver of the same frequency
Switching – the signal from one antenna is fed to the was installed. A receiving antenna mounted on 100ft mast through
receiver for as long as the quality of that signal remains line of sight propagation receives signal of 450.15MHz frequency.
above some prescribed threshold. As soon as the signal Through a 50 ohms coaxial cable, the received signal is fed to the
degrades, another antenna is switched in. Of the antenna STL receiver. The output of the receiver passes through audio
diversity processing techniques, switching is the easiest processors to a 20KW BE transmitter for terrestrial transmission.
and the least power consuming.
Selection – selection combining presents only one With the original connection of one receiving antenna at the Ikeja
antenna signal to the receiver at any given time. The GRA station, the received signal strength as displayed on the STL
antenna selected is the one with the best signal-to-noise receiver metering unit was 8dB. When the second antenna was
ratio (SNR) among the received signals. connected and the output paralleled, the STL receiver reading
Combining – here, all antennas maintain established increased to 12dB. A third antenna was introduced and the
connections at all times. The signals are then combined received signal strength further increased to 15dB. These antennas
and presented to the receiver. were spaced at 0.5m apart and were mounted facing different
4. RESEARCH PROCEDURES positions.
Space diversity technique was applied at reception in radio
broadcast station at FRCN Lagos operations main transmitting Table 1: Signal Strengths for various Number of Antennas.
station using three antennas. The strength of the received signal
was displayed on the STL receiver metering unit. After which, a
second antenna was mounted on the mast with a space of 0.5m S/N Number of Antennas Signal
from the first one. Since there was unavailability of an electronic Strength
combiner, an RF multi-switch connector was used to parallel the
outputs of the antennas. With the second antenna in circuit, a
change in the signal strength reading was observed and noted. 1. One 8dB
Finally, a third antenna was introduced and the new signal
strength noted. The STL receiver metering unit served as the 2. Two 12dB
spectrum analyzer for measuring signal strengths.
Again, because of the important roles antenna heights play in 3. Three 15dB
bypassing obstacles that cause fading and other interference
49
12
5. SIMULATION RESULTS AND 11.5
ANALYSIS
11
To verify the relationship between antenna heights and signal 10.5
strength, which is one of the objectives of this research, some
conversions were made from feet to meters in S.I units and 10
simulation was carried out. 9.5
Transmitting antenna height ht 9
8.5
Receiving antenna hr
8
In the simulated results shown, the transmitting antenna height ht 7.5
was kept constant while the receiving antenna height hr was 15 20 25 30 35 40
varied to take 15m, 20m, 30.48m, 35m, and 40m. The STL Figure: Graph of power density against antenna height.
transmitter rated power Pt was maintained at 250w. Equations (i), Fig 5.2: Graph of power density against Antenna height.
(ii), and (iii) were used to evaluate the maximum coverage
distances d, power density and electric field strength 5.3 Relationship between Power Density and
respectively. The following are the analysis of results from Maximum Distance
simulation:
5.1 Relationship between Distance and Fig 5.3 shows that power density decreases with increasing
maximum coverage distance. At the distance d = 42km, the power
Antenna Heights density of the signal Pd = 11.25nW/m2 approximately. As d
increases to 50km, Pd dropped below 8nW/m2, showing that
Fig 5.1 is a simulation result showing the effect of antenna heights maximum distance varies inversely with power density. To ensure
to coverage distances. It is observed that the receiving antenna that the signal level did not degenerate into a static noise, repeater
height is directly proportional to the maximum coverage distance or transceiver station can be sited at d = 46km.
in line of transmission. It was evident that the receiving antenna
height hr increased equally with the maximum coverage distance
d at which a signal transmitted through line-of-sight propagation. 12
52
11.5
11
50
10.5
48
10
46 9.5
9
44
8.5
42
8
40
15 20 25 30 35 40 7.5
40 42 44 46 48 50 52
Figure: A graph of distance d(Km) against antenna height hr(m)
Figure: Graph of power density against max. distance.
Fig 5.1: Graph of Maximum distance against Antenna
height Fig 5.3: Graph of Power Density against Maximum
Distance
5.2 Relationship between Power Density and 5.4 Relationship between Electric Field
Antenna Heights intensity and Antenna Height
Fig 5.2 shows that although the maximum coverage distance Fig 5.4 shows a graph of signal strength against receiving antenna
increases with increase in receiving antenna height, the power height. The signal steadily decreased as the receiving antenna
density of the received signal tends to decrease. This has placed a height was increased to obtain a new maximum distance. Between
limit to the increase of antenna heights to achieve wider coverage. 30.48m and 35m, constant signal strength erroneously displayed
For instance, beyond 40m high, the power density of the signal as a result of approximations done on the close values of the field
tends to zero. intensity using MATLAB programme. In general, it is observed
that the receiving antenna height cannot be increased without limit
50
to obtain an increased maximum distance, since the signal increasing the number of receiving antennas also increases the
strength will deteriorate. signal strength. Again, with multiple antennas spaced out, there
will be improvement in signal quality while drop-outs or signal
failure will be greatly minimized. Furthermore, it can be seen that
2.3 although increasing the antenna heights also increases the
maximum coverage distance in line-of-sight propagation, this
2.2
cannot be increased indefinitely since there will be corresponding
decrease in signal power resulting in fading.
2.1
We recommend that transmitters should not be sited far away
2 from the information source (i.e. studio). But where it becomes
inevitable, a repeater station or transceiver may be required to
1.9 maintain the signal quality. Again, frequency allocation should be
properly planned to prevent interference between RF channels.
1.8 Besides, broadcasting houses should be properly earthed to
prevent damage of equipment often caused by thunder and
1.7
lightning and to guarantee longer life span of digital broadcasting
equipment as well as safety of workers
1.6
15 20 25 30 35 40
Figure: Graph of Electric field intensity against antenna height 8. FUTURE WORK
Future studies may focus more on developing an affordable and
less power consuming combiner. This will help radio broadcasting
5.5 Relationship between Field intensity and stations and other small communication outfits to exploit the
Maximum Distance advantages of diversity schemes to improve the quality of their
signals.
Fig. 5.5 shows a graph of electric field intensity against distance.
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