Overcoming the Rain Fade Obstacle – Understanding Singapore’s Rain Dynamics and Feasible Countermeasures Against Rain Fading ABSTRACT Rain fade has always been perceived as the major impediment to satellite communications operating at higher frequencies, particularly in tropical zones like Singapore. Researchers have proposed various rain models to predict the extent of rain fading in different regions globally. However, there still exists an element of uncertainty of their applicability to our climate. As such, it is of paramount importance to establish a suitable high fidelity rain attenuation model based on local weather information. By means of a more accurate estimation of rain fade, and in tandem with the expected benefits of fade mitigation techniques, these key technologies will enable the realisation of a reliable satellite communications system.
Dr Lee Yee Hui Koh Wee Sain Lee Yuen Sin Michelle Ho Xiu Mei
• Scattering – this is a physical process, caused An ongoing research collaboration between by either refraction or diffraction, in which the DSTA and Nanyang Technological University direction of the radio wave deviates from its (NTU) is looking into the establishment of a original path as it passes through a medium suitable high fidelity rain attenuation model, containing raindrops. This disperses the energy based on weather information from of the signal from its initial travel direction. Singapore’s Meteorological Services Division of the National Environment Agency. The accumulation of these different reactions ultimately leads to a decrease in the level of The principal objective of a rain attenuation received signal, thus resulting in rain model tailored for Singapore’s environment is attenuation. to achieve an accurate estimation of the attenuation level incurred by the signal due Rain Attenuation Model to rain which is key to realising a reliable SATCOM system. To date, several rain attenuation models such as the Global Crane (Crane, 1980) and ITU-R This rain attenuation model utilises a P.618-8 (ITU Recommendation, 2007) have been methodology similar to ITU-R P.618-8, with the created to compute the rain fade figure for derivation of rainfall parameters such as the earth-space telecommunication systems. Rain specific attenuation and path reduction factor models predict attenuation for a given being optimised to local data and findings. frequency from site-related parameters such Together with the site-related parameters, the as rain intensity statistics, rain height and path- four key characteristics that influence the level elevation angle. Most of the models available of rain attenuation experienced are as today are developed based on the data explained in Table 1. collected from temperate regions (Mandeep, Allnutt, 2007).
Parameters Description Definition
Specific This is a fundamental quantity Specific attenuation defines Attenuation to model rain attenuation. It is the rain attenuation per unit a function of rain temperature, distance. velocity of rainfall and most importantly, raindrop size distribution.
Slant Path Rain fade is not experienced at The slant path length is defined Length a specific collection point but as the length of the satellite to extended throughout the entire ground path that is affected by propagation path. rain.
Path Reduction The rain rate along a propagation The path reduction is incorporated Factor path is not constant, especially for to account for the spatial large distances as in the case of variability of a rain event space-to-ground transmissions. and ensure higher accuracy.
Rainfall Rate Rainfall data is essential for Rain rate data is presented in units Statistics determining the degree of rain of mm/hr over an average year as 7 Factor attenuation in a SATCOM system. a function of the cumulative probability of occurrence.
Table 1. Description and definition of key parameters that affect rain attenuation 8
station sited in Singapore points to ST-1, the SINGAPORE’S RAIN slant path length through rain is calculated to ENVIRONMENT be 4.75km.
The following sections describe the main Path Reduction Factor contributing factors and characteristics that shape the investigation and development of The Meteorological Doppler Weather Radar a high fidelity rain attenuation model for system is developed to intercept returning higher frequencies of K band in Singapore. radio energy, which will aid in identifying the intensity of a rain event, through measuring Specific Attenuation the radiated energy that is being backscattered and reflected to its receiver. This system Raindrop size distribution (DSD) is a primary processes weather information detected by contributing factor to express the specific the Doppler radar to provide a representation attenuation and the severity of signal of the motion and intensity of rain known as attenuation is a result of different raindrop the reflectivity plot. sizes. Preliminary DSD collection results attained with a distrometer for varying rain rates in As the path reduction factor is introduced to Singapore showed that the critical drop compensate for the inhomogeneity of a rain diameter is between 0.771mm and 5.3mm. event, an understanding of the rain distribution A negligible number of drop diameters were over the slant path is required. Figure 2 shows out of this range and hence rendered as having a reflectivity plot of both a stratiform and an insignificant effect on the overall rain rate convective rain event captured on 15 and 19 and specific attenuation. June 1998 respectively.
Slant Path Length It can be seen that for a convective event, there is a larger variation in the rainfall rate over For earth-space propagation, the slant path the propagation slant path as compared to a length from the satellite to the ground station stratiform event. There are typically more that transits through the rain cell is dependent occurrences of convective rain events in the on rain height and elevation angle. While ITU- equatorial region. Thus, the path reduction R P.839-3 (ITU Recommendation, 1999) defines factor will not be a static formula but a the rain height in Singapore to be 4.5km, the summation of values due to the fast varying elevation angle varies with different satellites rain rate. and earth station locations. Assuming an earth
Figure 1. Slant path length through rain Figure 2. Reflectivity plot of a stratiform and convective rain event
Rainfall Statistics These figures reflect that for 99.99% of the time, the statistical rainfall rate falls below the The location of the earth station is one other indicated value for the respective models. In consideration that will vastly affect the rain other words, the rainfall rate exceeds that attenuation. Due to the lack of comprehensive value for 0.01% of the time. actual rain data, prediction models like ITU P-837 (ITU Recommendation, 2003) and CCIR Using measured rainfall statistics obtained Rep. 563-4 (CCIR, 1990) have been developed from the local meteorological stations through to determine the rainfall intensity in various a five-year period (2000-2004) over five regions. The rainfall rates for Singapore at distributed sites, Singapore’s rainfall rate is 0.01% exceedance worked out from the found to be 149mm/hr at 0.01% exceedance different prediction models are depicted in (Leong, Foo, 2007). This empirical figure reveals Table 2. that both CCIR Rep. 563-4 and Crane Global models are currently good reference models to be adopted.
Location ITU P-837 CCIR Rep. 563-4 Crane Global
Singapore 120mm/hr 145mm/hr 147mm/hr
Table 2. Predicted rain rate at 0.01% exceedance in Singapore 9 10
Figure 3. Locations of the rain measurement sites
East Asia. In one such study, measurements of RAIN FADE AT HIGHER rain attenuation in Indonesia collected over a FREQUENCY period of two years have indicated that the TRANSMISSIONS measured rain attenuation is relatively close to the DAH1 and ITU Model (Suryana, 2005). IN SINGAPORE In a separate study done by NTU, the results in Singapore pointed to a great discrepancy One of the most common methodologies for between the measured and various models, as gauging rain fade is the utilisation of rain shown in Table 3 (Ong, Zhu, Lee, 1997). attenuation models. They predict the long- term rain attenuation statistics from point These conflicting results potentially suggest rainfall rate given the site-related parameters. that these rain models might not provide an Due to the differences in rain characteristics accurate prediction of rain attenuation for in both temperate and tropical regions, tropical areas. Continued developing efforts academics have been relentlessly attempting are thus required to build a high fidelity rain to determine the accuracy of these models in attenuation model that is tailored to our various local environments. climate.
Several studies have been conducted to validate the different rain attenuation models in South
Annual Rain Attenuation Availability (%) CCIR 564-3 CCIR 564-4 ITU-R P.618.3 ITU-R P.618.8 Measure Mean
99.77% 6.91 3.85 4.41 5.72 8.9
99.90% 10.27 5.72 6.56 8.69 14.56
Table 3. Comparison of measured and modelled rain attenuation for Ku band in Singapore • Closed loop power control – Similar to the FADE MITIGATION open loop power control architecture, the TECHNIQUES ground station utilises a loopback communication signal instead. Its The occurrence of rain outage does not transmitted beacon signal is analysed to necessarily suggest that there will be a estimate and counteract for the rain complete total disruption to the attenuation i.e. the received signal comprises communications link. To overcome the both uplink and downlink rain effect dynamics of outages, FMT is undeniably a degradation. critical element in the design of SATCOM networks as the effects of fading can be • Feedback loop power control – A central combated by exploiting the correct mitigation station monitors the signal levels of all the measure(s) to improve the communications received carriers and analyses the power availability and reliability of satellite links. In adjustment needed for the affected carriers. other words, the incorporation of FMT This control information and command are throughout the chain from the user terminal, subsequently routed to the transmitting the spaceborne payload to the overall network ground station for the corrective measure(s) design, can potentially offer a reduction in to be effected. outage time. Though uplink power control is affordable and Uplink Power Control simple, it provides marginal benefit as it cannot continuously amplify its margin. Most amplifiers The simplest way to compensate for the rain exhibit a non-linear behaviour and the output fade effect is to increase the transmission power power will be limited despite an increase at the terminal end. This method is known as in power. uplink power control. However, constant transmission of high power may result in Adaptive Coding and interference among users during clear sky Modulation / Variable Coding conditions. The underlying technology and Modulation therefore resides in the dynamism of the system that is capable of adjusting the power, in In recent years, open standards such as the response to the fading variations, to the Digital Video Broadcast-Satellite Second required signal level essential for higher Generation (DVB-S2) have emerged. One of frequency operations. Three types of uplink its features, Adaptive Coding and Modulation power control algorithms can be executed to / Variable Coding and Modulation (ACM / VCM), maintain carrier power or signal quality during lies in its dynamic assignment of coding and rainy periods. modulation as a key enabler for dynamic bandwidth allocation and optimisation. • Open loop power control – The satellite Examples of DVB-S2 compliant satellite modems generates a beacon signal to the receiving that are equipped with ACM / VCM capability ground station which is used to ascertain are Viasat’s Linkstar DVB-S2 Broadband Very the level of downlink attenuation. The Small Aperture Terminal System and the power controller at the ground station then iDirect Evolution Series of Modems. estimates the uplink fade required by The ACM / VCM technique leverages coding applying the frequency scaling ratios for 11 cloud, gaseous and tropospheric scintillation and modulation adjustments to achieve attenuation. acceptable throughput over fading link 12
channels. In each DVB-S2 frame that is the limited size and spatial extent of a high- transmitted, the header contains information rain-intensity cell to yield an improved network on how that frame will be modulated and throughput. coded. An ACM / VCM-enabled demodulator will read this information to demodulate and In other words, the concept of this technique decode the frame, wherein each header allows is to have two or more interconnected earth for the varying of the modulation and coding stations situated at a sufficient distance apart scheme for every transmitted frame. and to evaluate the signal fading at each individual remote terminal before choosing a The differentiating component between ACM site with better propagation conditions. The and VCM is the return channel from each architecture of this system encompasses the receiving site to the transmit site which is only linkage of several stations receiving the same available in ACM. In the case of ACM, the signal and a control centre that will perform return channel supports a dynamic modification the computations and switchings. A diversity of the coding and modulation rate of each system that switches between signals obtained frame in accordance with the measured channel at various receivers can benefit from the local conditions. The application of this feature will spatial variation and movements of the rain improve the system capacity, peak data rate cell, thereby enhancing the overall link quality. and coverage reliability as the signal transmitted to and from a particular user is The performance of the site diversity technique, modified to account for the signal quality carried out in Singapore, was evaluated with variation. This will exploit good transmission Ku band signals from INTELSAT. These signals conditions to obtain higher throughput, while were monitored at two earth stations, namely reducing the demands on the channel when NTU and Bukit Timah (BKT) which are separated the conditions are temporarily bad. by 12.3km (Isaiah, Ong, Choo, 2000). The cumulative distribution of attenuation Site Diversity measured at NTU and BKT is illustrated in Figure 4. The downlink transmissions of a satellite cover a very large geographical area, with different It can be seen that when the site diversity areas experiencing different weather or rain technique is employed, the overall level of intensities. Site diversity takes advantage of attenuation will decrease. For instance, the
Figure 4. Cumulative distribution of attenuation exceedance percentage is reduced from 0.25% to 0.035% for a typical link margin of 7dB CONCLUSION which translates to a decrease of 18.8 hours in signal outage in a year. It is undeniable that the next generation SATCOM system necessitates the employment Frequency Diversity of a methodical approach in its planning and designing, in view of the challenge to the The strategy is to draw on the lower frequency availability of frequency spectrum at lower C band payload during the occurrence of fading band frequencies as well as the rising demand at higher frequencies via a ground control for a wide array of mobile and broadband station. The affected earth stations will be applications. Rain fade poses a persistent authorised by the control station to tap the challenge which cannot be totally eradicated, lower frequency resources when the climatic especially for SATCOM links above C band. condition reaches a certain threshold while a Fortunately, rain fade usually does not last processor onboard the satellite ensures the long and normal communications return once interconnection among the stations operating a heavy shower has passed. over two different bands. To better anticipate the impact of rain fade, This method is suitable for satellites operating it is essential to develop an applicable rain in two frequency bands, typically Ka band and attenuation model customised for the local C or Ku band, and usually achieves low levels environment. This comprehensive model of outage probability especially during severe presents an opportunity to shed more light on rain fade. Furthermore, it can serve as a the rain dynamics experienced in tropical countermeasure to mitigate any interference regions and will form the foundation for encountered. The downside is that it is not a yielding a consistent process in the overall cost-effective solution to be implemented designing of the SATCOM network. onboard commercial satellites as it makes greater demands on both the space and ground Further to that, FMTs, either used singularly segment resources. or in combination, offer enormous potential in overcoming the adverse effects of rain fade. Satellite Diversity Demonstration initiatives and pre-commercial activities have already been launched and Another means of FMT that can be adopted is commenced to examine the use of FMT on satellite diversity. This concept allows the earth mobile DVB-S2/RCS-based broadband station to select one of the available GEO interactive satellite systems on the railway. satellites that will provide the most favourable With the commercialisation of mitigation link under the existing propagation conditions. techniques and proper system planning, hopes Satellite diversity trials involving three earth- of a more effective combating of rain fade space propagation paths between Osaka, Japan effects in the tropical region may be raised. and three satellites, namely JCSAT, BS and N- Star, have reflected an attenuation reduction at Ka band of at least 3dB for a time percentage of 0.01% (Maekawa, 2002).
Besides enhancing the link performance and availability, a vital advantage of satellite 13 diversity is its ability to mitigate the effects of network operational issues in the event of a satellite failure. 14
Leong, S.C., Dr Foo, Y.C. (2007). Singapore Rain REFERENCES Rate Distributions.
CCIR. (1990). Radiometeorological Data. Vol.V, LyngSat. (2009). Satellite Launches Asia. Rep 563-4. Retrieved on 1 April 2009 from http://www.lyngsat.com/launches/asia.html Crane, R.K. (1980). IEEE Transactions on Communications, Vol. COM-28, No. 9, Mandeep, J.S., Allnutt, J.E. (2007). Rain September 1980; 0090-6778/80/0900-1717. Attenuation Predictions at Ku band in South East Asia Countries. Freer, J.R. (1996). Computer Communications and Networks, 2nd Edition. Maekawa, Y. (2002). A study on the effects of satellite diversity on Ka band attenuation in Futron Corporation. (2003). Global Analysis of three earth-space paths. Satellite Transponder Usage and Coverage. Retrieved on 25 June 2008 from Ong, J.T., Zhu, C.N., Lee, Y.K. (1997). Ku band http://www.satelliteonthenet.co.uk/white/ Satellite Beacon attenuation and Rain Rate futron3.html Measurement in Singapore - Comparison with ITU-R Models. Hartshorn, D. (2007). OpEd: Global Threat to C band Satellite Services. Retrieved on 29 Suryana, J., Utoro S, Tanaka, K., Igarashi, K., August 2008 from http://www.space.com/ Iida, M. (2005). Study of Prediction Models spacenews/archive07/hartshornoped_0226.html Compared with the Measurement Results of Rainfall Rate and Ku band Rain Attenuation Isaiah K.T., Ong, J.T., Choo, E.B.L. (2000). at Indonesian Tropical Cities. Performance of the Site Diversity Technique in Singapore: Preliminary Results. ENDNOTES ITU Recommendation ITU-R P.618-8. (2007). Propagation data and prediction methods 1DAH (Dissanayake, Allnutt, Haidara Model) is required for the design of Earth-space one of the rain models used for rain telecommunication systems. attenuation predictions.
ITU Recommendation ITU-R P.837-4. (2003). Characteristics of precipitation for propagation modelling.
ITU Recommendation ITU-R P.839-3. (1999). Rain Height model for prediction methods. BIOGRAPHY
Dr Lee Yee Hui is an Assistant Professor at the School of Electrical and Electronic Engineering, Nanyang Technological University (NTU). She is currently researching on channel characterisation as well as electromagnetic wave propogation and the effects of weather on it. Her research interests lie in antenna design, computational electromagnetics and propagation through electromagnetic band gap structures. She received her Bachelor and Master degrees from NTU, and holds a PhD from University of York, England.
Koh Wee Sain is a Senior Engineer and Project Manager (Communications and Tactical Command and Control) who is responsible for assessing emerging beyond-line-of-sight wireless technologies for the Singapore Armed Forces (SAF). Wee Sain was part of the pioneer team which set up the SAF Centre for Military Experimentation (SCME), the one-stop centre for all SAF experiments. He received his Bachelor degree in Electrical and Electronic Engineering (Honours) from NTU in 1999.
Lee Yuen Sin is a Senior Engineer (Communications and Tactical Command and Control) with responsibilities in assessing emerging non-terrestrial wireless communications technologies and systems. Previously a senior consultant in SCME, Yuen Sin was a key player in designing, conducting and analysing C4I experiments that explore future operational concepts enabled by cutting-edge technology. She received her Bachelor degree in Electrical and Electronic Engineering from NTU in 2000.
Michelle Ho Xiu Mei is an Engineer (Communications and Tactical Command and Control). She is actively involved in the conceptualisation and design of an accurate high fidelity rain model for Singapore, and is also responsible for the assessment and implementation of advanced Communications ground segment technologies and systems. Michelle obtained her Bachelor degree in Electrical and Electronic Engineering (Honours) from NTU in 2008.
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