International Journal of Scientific & Engineering Research, Volume 4, Issue 7, July2013 367
ISSN 22295518
IMPROVEMENTS OF SIGNAL GAIN FOR MEASAT2 AND MEASAT3 USING ORBITAL DIVERSITY UNDER RAIN ATTENUATION: A SIMULATION APPROACH
Harlisya Harun, Halid Agil, Ruhaida Abdul Rashid
Abstract—The effect of rain attenuation becomes significant for satellites operating at 10 GHz and above. This has become a matter of concern, especially in tropical regions where relatively heavy rainfall occurs throughout the year. Orbital diversity (OD) is seen to be a viable method to mitigate rain attenuation. It employs multiple satellites transmitting identical signal streams toward a mutual ground station. Although OD has been studied with great interest in regions such as Europe, there is little information of OD research in tropical regions, particularly in Malaysia. Therefore, this paper proposed an analytical approach towards the study of OD in Malaysian climate using MEASAT satellites. The performance of OD is dependent upon the operating frequency and the satellite’s elevation angle. From the simulation, the rain attenuation increases exponentially with the increasing frequency. Therefore, the signal gain decreases in inverse exponential manner. The simulation also shows that MEASAT3, having an elevation angle of 77.695°, experiences higher signal attenuation than MEASAT2 (elevation angle 34.324°). Using signal combination, an OD signal experiences signal boost of up to 2.3 times the individual signal gain. W ith this significant finding, the OD is proposed to mitigate rain attenuation in Malaysia.
Index Terms—Maximal Ratio Combining, Orbital Diversity, Rain Attenuation, Signal Strength.
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ain attenuation is the main contributor towards signal degradation for satellite systems, and the effect of rain attenuation is significant for systems operating at 10 GHz
and above [1]. Malaysia experiences heavy rainfall which causes significant attenuation effects toward satellite systems operating at 10 GHz and above, particularly MEASAT satellite system resulting to the instability of signal strength. The prob lem is clearly demonstrated by the reception problem of Astro that utilizes Kuband [2] during heavy rain.
Orbital diversity (OD) is seen as a viable method to mitigate rain attenuation effects. It is considered to be adaptive in nature [2]. To achieve this, multiple transmitting satellites are required, and they are spatially separated so that independent slant paths can be created. When rainfall occurs, the OD system selects the least attenuated slant path and adopts a reroute strategy [3].
The concept of OD is still relatively new in Malaysia. As far
as the research is concerned, although OD has been a subject of
interest among industries and academics for more than 20 years
[4], there are relatively sparse experiments or measurements
that have been performed to know its performance in the South
East Asian region, especially in Malaysia. Thus, this research
serves to propose OD as well as to provide a better understand
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• Harlisya Harun is a senior lecturer in Aerospace Engineering Department in University Putra Malaysia (UPM), Malaysia.
Email: harlisya@eng.upm.edu.my
• Halid Agil has earned a master degree in Aerospace Engineering from
Aerospace Engineering Department in University Putra Malaysia (UPM),
Malaysia.
Email: fatamorgana_2121@yahoo.com
• Ruhaida Abdul Rashid is a lecturer in Avionics Department in UniKL
MIAT, Malaysia.
Email: ruhaida@gmail.com
ing of its importance as fade mitigation technique (FMT) in Ma laysian climate
This research utilizes MATLAB simulation to predict the performance of OD under rain attenuation. The simulation has taken into account two transmitting satellites toward a mutual ground station, as discussed in [3] to [6]. The transmitting sat ellites are MEASAT2 and MEASAT3, while the selected ground station is MEASAT Satellite Control Centre (MSCC) situated in PulauLangkawi, Kedah.
OD refers to the usage of two or more appropriately sepa rated satellites to provide two or more separate converging paths to a common ground terminal. By utilizing the link with the lowest path attenuation, diversity gain is achieved. The main advantage of OD is that the satellites involved can be used in a resourcesharing scheme, meaning that they can be used with many ground sites.
With the advent of satellite television, there is a need for higher performance of satellite signal strength. But, the quali ty of the satellite TV signal reception can be affected by ad verse weather. Heavy rains for example can attenuate signal enough to result in noticeable degradation of image quality. For radio systems which operate above 10 GHz, excess attenu ation due to rainfall and losses due to atmospheric gases are significant. The impairment caused by rain increases with frequency and varies with regional locations [7].
The poor performance of the satellite TV signal reception is due to reliable communication which depends on the strength of a single signal path [8]. This can be mitigated by using OD. Independent signal paths have a low probability of experienc
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International Journal of Scientific & Engineering Research, Volume 4, Issue 7, July2013 368
ISSN 22295518
ing deep fades simultaneously [9]. With diversity, the same data will be sent over independent fading paths. These inde pendent paths are combined so that the fading of the resultant signal is reduced.
Consider a system with two antennas at either the transmit ter or receiver that experience independent fading, as shown in Fig. 1. Both antennas will not experience deep fades at the same time if the antennas are spaced sufficiently far apart. With selection combining, the antenna with the strongest sig nal will be selected and thus, a much better signal can be ob tained compared to one antenna.
pendent on θ.
D2
MEASAT3
D1
φ
MEASAT2
TX: Transmitting antenna θ2
RA: Receiving antenna
θ1
MEASAT Satellite Control Centre, Gunung Raya
Fig. 1. Illustration of diversity reception using two receiving antennas [10].
High rainfall rate in Malaysia degrade the performance of MEASAT satellite systems. Thus, the rain attenuation model is needed to improve the performance of the MEASAT satellite system. In this research, the ITUR model [11] has been used to analyze the performance of the MEASAT satellite system with OD.
The configuration of the OD simulation model is illustrated as in Fig. 2. The OD model takes the 2:1 configuration; that is two transmitting satellites to one receiving earth station. Ear lier works utilizing 2:1 configuration has been performed in [36]. The main reason to use only two transmitting satellites is to simplify the analysis of rain attenuation statistics over a limited amount of time.
It is clearly shown in Fig. 2 that there are two resulting earthspace paths, D1 and D2 , which connect MSCC with MEASAT2 and MEASAT3, respectively. Elevation angle (θ) is the angle of the earthspace path with respect to the earth's horizon. The angular position of MEASAT2 relative to MEA SAT3 is called the separation angle (φ).
Fig. 3 shows the schematic details of an earthspace path derived from ITUR recommendation (2009). The earthspace path (D) propagates through two regions in the atmosphere: frozen precipitation region (A) and liquid precipitation region (C). The rain height (hR , in km) is the transition height in the atmosphere at which beyond it the freezing phenomenon would occur. The rain height is region specific. The height position of the earth station (hS , in km) is measured above mean sea level. The slant path (LS , in km) exists along D from hS up to hR . The horizontal projection (LG , in km) is the pro jection of LS on the earth's horizon. Both LS and LG are de
Fig. 2. OD simulation model configuration.
Fig. 3. Schematic representation of an earthspace path [11].
The ITUR model has been chosen for this simulation pro cess. The relevant parameters are based on the ITUR recom mendation (2007) [12]. The ITUR model is based on the prob ability of rainfall rate in the percentage of time. It provides the rain attenuation at 0.01 percent of the time rain rate is exceed ed, as follows:
(1)
where R0.01 and r0.01 are the rain rate and reduction factor at 0.01 percent of the time respectively. The latter is expressed as:
(2)
and
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International Journal of Scientific & Engineering Research, Volume 4, Issue 7, July2013 369
ISSN 22295518
SNR
− A' / cos φ
SNR
− A' / cos φ
35e−0.015R , R0.01 < 100mm / hr d0
35e−1.5R , R0.01 < 100mm / hr
(3)
SNR
MRC
= 10 log10
cs,1 1
10
1
+ 10
cs,2 2 2
10
A0.01 is chosen to represent the requirements of obtaining near continuous transmission (99.99 percent of signal availability). The procedures to obtain A0.01 are based on ITUR Recommenda tion (2007) [12] prediction of attenuation statistics for an average year.
where
SNRMRC : total signal gain.
(7)
Maximal Ratio Combining (MRC) has been used to join the re
ceived signals from both MEASAT2 and MEASAT3. The joint
signals will indicate the signal strength of the OD scheme com
pared with the individual signals. Orbital diversity gain, GOD, is defined as the difference between the attenuation exceeded on a
single link and the attenuation jointly exceeded on the total links involved in the orbital diversity scheme [13].
SNRcs,1,2 : signaltonoise ratio transmitted during clear sky conditions for MEASAT2 and MEASAT3, respectively.
A1′,2: attenuation of projected earthspace links for MEA SAT2 and MEASAT3, respectively.
The joint signal gain is therefore used to compare with the in dividual signal gains for both MEASAT2 and MEASAT3. The difference between these gains is known as the diversity gain.
SNRi = SNRcs,i − A0.01,i
where
i = 1,
2 (dB)
(4)
The diversity gain is the figure of merit of this research. An or bital diversity gain (ODG) is defined as the difference between the attenuation exceeded on a single link and attenuation jointly exceeded on both links in an OD scheme for a fixed percentage of
SNRi: signaltonoise ratio for satellite i.
SNRcs,i: signaltonoise ratio transmitted during clear sky condi tions for satellite i.
A0.01,i: attenuation exceeded for 0.01% of an average year for satel lite i.
time [13]. It is obtained from:
GODi = Ai − AD ,
where
i = 1,2 dB
(8)
To obtain the joint signals for both satellites, SNR1 and SNR2 are combined using MRC. MRC receives individual signals from all involved channels, and combines them in linear scale to obtain the total gain:
GODi : orbital diversity gain for link i. Ai : attenuation exceeded on link i.
AD : attenuation threshold jointly exceeded for fixed percent age of time.
where
SNRMRC = SNR1 + SNR2
(5)
As attenuation reduces signal strength, Equation (8) can be ex pressed in terms of transmitted signal gain. Equations (4) and (7) are combined into Equation (8) to obtain:
SNRMRC: total signal gain.
SNR1: signal gain for MEASAT2. SNR2: signal gain for MEASAT3.
GODi
where
= SNRMRC − SNRi ,
i = 1,2 dB
(9)
Equation (5) is expressed in dB as:
SNRMRC = 10 log(10SNR1 /10 + 10SNR2 /10 ) dB (6)
An extension to Equation (6) has been proposed by Crane [15], by taking into account the description of the vertical vari ation of rainfall structure and combines it with Equation (5) to yield:
GODi : orbital diversity gain for link i. SNRMRC : total signal gain.
SNRi : signal gain for link.
To analyze the potential of the OD in improving the perfor mance of satellite communication, a series of simulation is per formed using MATLAB (as can be found in Section 4). It aims to highlight the impact of OD from the aspect of rain attenuation exceeded at 0.01 percent and signal strength as compared to the frequency.
The simulation is performed in MATLAB programming
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International Journal of Scientific & Engineering Research, Volume 4, Issue 7, July2013 370
ISSN 22295518
language. It is written and saved into three different script files: Script 1 for rain attenuation simulation, Script 2 for joint attenuation simulation and Script 3 for diversity gain simula tion.
Script 1 includes the identified input parameters into a se ries of calculations based on the ITUR recommendation (2009) [11]. The parameters are sorted into three groups. Each group represents the satellites’ parameters, the ground station’s pa rameters and climate specified parameters. The required pa rameters for the rain attenuation simulation can be found in Table 1.
TABLE 1
Input parameters for Script 1 rain attenuation simulation
obtain the diversity gain, in which the signals from both MEA SAT2 and MEASAT3 were combined together. The joint sig nals will indicate the signal strength of the orbital diversity (OD) scheme compared with the individual signals.
Rain attenuation level depicts the severity of rain fade on a particular frequency of a signal. As such, lower attenuation level is preferred over higher ones. Fig. 4 shows the rain attenuation exceeded at 0.01 percent for an average year for both MEASAT2 and MEASAT3. It is also shown from Fig. 4 that the level of rain attenuation is severe for tropical regions such as in Malaysia, with the minimum average is slightly above 20 dB for MEASAT2 and
60 dB for MEASAT3 at 10 GHz. As the operating frequency increases, the rain attenuation gain increases exponentially. The level of rain attenuation is also dependent upon elevation angle such that for MEASAT2 (θ = 34.324°) experiences lower rain attenuation than MEASAT3 (θ = 77.695°). Hence, higher elevation angle results to a higher rate of attenuation per frequency.
250
Attenuation Exceeded for 0.01% of an Average Year Vs. Frequency
M2 34.324° M3 77.695°
200
150
100
50
0
10 11 12 13 14 15 16 17 18 19 20
Frequency (GHz)
Script 1 simulation is designed to calculate and simulate the attenuation exceeded for 0.01 percent of an average year, A0.01 , for both MEASAT2 and MEASAT3 links. Thus, equations (1) until (3) were included in Script 1 simulation. The results are displayed in MATLAB Command Window as numerical fig ures. These numbers are then used to create a graph using MATLAB plot functions.
Script 2 includes the numerical results in Script 1 and con verts them into signal gain. The signal gains are then com bined using MRC to obtain the total signal gain. MRC is used in this research due to its relatively low selective complexity and it is one of the common methods to be used when combin ing diversified signals alongside selection combining. Sa karellos et al. [14] states that MRC performance is influenced by the satellite angular separation and climate conditions of the region in which the user’s ground segment is located. Equations (4) until (7) were included in Script 2 simulation.
Equations (8) and (9) can be found in Script 3 simulations to
Fig. 4. Graphical representations for rain attenuation simulation results.
Using exponential regression, an estimation of the curves for both A0.01,M2 and A0.01,M3 can be determined. The curves follow a general exponential pattern:
y = menx + c (10) (10)
where
m and n: numerical constants.
c: intersection point along y axis when x = 0.
The calculated exponential regressions for A0.011,M2 and A0.01,M3
are given below:
A0.01,M 2 = 7.2759e0.1183f + 12.30(1916)
A0.01,M 3 = 715.8774e0.1384f + 42.(0127)84
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where
A0.01,M2 : attenuation exceeded for 0.01 percent of an average year for MEASAT2.
A0.01,M3: attenuation exceeded for 0.01 percent of an average year for MEASAT3.
f: operating frequency.
jointly received. Comparing the joint signal with the individual signals, the decrement is nearly linear in pattern for the joint sig nal, while the strength for both the individual signals decrease in exponential pattern.
Signal Strength Vs. Frequency
100
50
From the simulation data, it is discovered that the residual sum of squares (RSS) i.e. error becomes more apparent when the elevation angle is increased. Thus, the estimation can be more erroneous for MEASAT3 with the total RSS of 83.7642 dB2 as opposed to MEASAT2 with only 44.4139 dB2. Fig. 5 and 6 show the error between the calculated A0.01 and the actual A0.01 for MEASAT2 and MEASAT3, respectively.
0
50
100
150
M2 34.324° M3 77.695° OD
100

10 11 12 13 14 15 16 17 18 19 20
200
80
60
40
20
0
10 12 14 16 18 20
Actual A0.01M2 (dB)
Calculated A0.01M2 (dB)
Frequency (GHz)
Fig. 7. Signal strength comparison between MEASAT2, MEASAT3 and com bined (OD) signals.
Looking at the lower end of the frequency range, at 10 GHz, the values of MEASAT2, MEASAT3, and the combined signal strengths are 69.083 dB, 29.847 dB and 87.563 dB, respectively. The diversity gain for the OD system at 10 GHz is therefore around 1.3 to 2.3 times higher than the signal strength for MEA SAT2 and MEASAT3, respectively. This is shown in Fig. 8. The
Fig. 5. Error (RSS) in A0.01 for MEASAT2.
value of the diversity gain varies with operating frequency.
300
250
200
150
100
50
0
10 12 14 16 18 20
Actual A0.01M3 (dB)
Calculated A0.01M3 (dB)
250
200
150
100
50
0
1011121314151617181920
OD Gain, M2
OD Gain, M3
Fig. 6. Error (RSS) in A0.01 for MEASAT3.
To conclude, the level of rain attenuation is severe for tropical regions such as in Malaysia, with the minimum average is slight ly above 20 dB for MEASAT2 and 60 dB for MEASAT3 at 10
GHz. As the operating frequency increases, the rain attenuation gain increases exponentially. Fig. 4 also shows that the level of rain attenuation is also dependent upon elevation angle, θ. MEA SAT2 (θ = 34.324°) experiences lower rain attenuation than
Fig. 8. Advantage of OD signal strength over the individual signals.
Using exponential regression, an estimation of the SNROD curve can be determined. It follows the exponential pattern given in Equation (10). The calculated exponential regression formula for SNROD is given as:
MEASAT3 (θ = 77.695°). In effect, higher elevation angle results
to a higher rate of attenuation per frequency.
A simulation model has been constructed to join the expected
received signals from both MEASAT2 and MEASAT3. The joint
signals will indicate the signal strength of the OD scheme com
pared with the individual signals. From Fig. 7, a significant im
SNROD = 97.7390e0.0103f  0.5758
where
SNROD: OD gain.
f: operating frequency.
(13)
provement in signal strength can be noticed when the signals are
The OD simulation shows that the signal strength of the diver
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sity (joint) system is higher than the individual signal strengths. It is important to have an increased signal gain within the same bandwidth to ensure that the signal is more robust, and therefore the quality of service could be maintained.
Rain attenuation remains to be a problem since the intro duction of the satellite television. Many research papers have been published in recent years, and this research area is still in a growing stage, particularly in Malaysia. This paper presents the detail study of the orbital diversity on MEASAT system to mitigate the rain attenuation problem.
OD is seen as one of the promising methods in which the ef fects of rain attenuation can be mitigated. The simulations of OD system based on MEASAT satellites under rain attenuation ef fects show that the minimum limit of signal strength can be sig nificantly increase of up to 2.3 times the strength of individual satellites at 10 GHz. The simulations also show that the perfor mance of an OD system is dependent upon the transmitting fre quency, the elevation angle and the separation angle.
Possible studies in the future can be conducted to further ex ploring the possibilities of implementing OD in Malaysia. For example, the performance of the OD system is not limited for signal strength, but also by measuring the outage probability. Several factors other than rain attenuation such as gaseous, cloud and fog attenuation should also be considered in assessing the OD performance. Utilizing multiple satellites with intersatellite links could also increase the efficiency of an OD system.
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