Evaluation of Time Diversity Technique for Satellite Mitigation in Tropical Climate

Time diversity is one of the mitigation techniques for satellite communications (SatCom) systems that aims to combat deep fades cause by propagation impairments. This study presents the performance of time diversity technique for rain attenuation at Ku- and Ka-band SatCom systems in Johor Bahru, Malaysia. The Synthetic-Storm Technique (SST) has been utilized to obtain rain attenuation time series based on rain rate data measured by local rain gauge. The time diversity has been quantified through diversity gain as a function of retransmission time delay, in which shows the effectiveness of time diversity technique to improve the Quality of Service (QoS) and link availability. The results demonstrated it is feasible to use time diversity as mitigation technique at higher frequencies in a mission to support future SatCom operation in tropical region.

Atmospheric Impairments and Mitigation Techniques for High-Frequency Earth-Space Communication System in Heavy Rain Region: A Brief Review

This work surveys the atmospheric impairments that affect a satellite link operating in a high-frequency band, such as Ka and Q/V bands, particularly in heavy rain regions. The impacts of hydrometeors and cloud attenuation are emphasised and discussed along with the contribution of gases and scintillation to signal fade. Also, propagation impairment mitigation techniques are reviewed from the perspective of satellite operators in heavy rain areas

Rain fade dynamics for Ka-band satellite communication mitigation technique in equatorial Malaysia

Modern satellite communication system in higher frequency (Ka-band and above) is very much impaired by rain attenuation particularly in tropical and equatorial region. The desired Quality of Service (QoS) and system availability can be guaranteed only by resorting to smart strategies, named Propagation Impairment Mitigation Techniques (PIMTs) such as power control, adaptive modulation schemes and link diversity. These requires knowledge of the first- and second-order statistics of rain attenuation. Hence, this work concentrates on those aspects in equatorial Johor Bahru, Malaysia, based on one year Ka-band propagation measurement campaign, utilizing the equipment of Beacon Receiver and 2D-Video-Disdrometer (2DVD). Study begins by investigation the rain fade behaviour such as rain attenuation, fade duration, inter-fade duration and fade slope as well as their seasonal and diurnal variations. It is observed that rain attenuation experienced by the Ka-band link requires fade margin of 26.8 dB for 99.9% link availability with the convective events mostly like to occur during the afternoon hour (12:00 pm to 6:00 pm) at high intensity, shorter duration and relatively high rate of change of attenuation particularly during Northeast Moonsoon. Then, the Stratiform Convective-Synthetic Storm Technique (SC-SST) is proposed to estimate the dynamic characteristics of rain attenuation in equatorial region. The SC-SST is found 11% better than SST and 51% better than ITU-R P.1623-1 model in average value of fade dynamics prediction. Finally, a time diversity technique is recommended to mitigate strong signal fades in equatorial region. The results depicted that 10-minute outage tolerance will significantly lower the fade margin requirement to 15 dB for 99.9% of link availability. Afterwards, the generation of time diversity statistics is modelled which can be best represents by gamma-law in this area. The results can provide system engineers with critical information in the design and implementation of PIMTs, and it is expected that the probability of system outages will be greatly reduced

Interfade Duration Statistics at Ku-band for Satellite Earth Links System in Equatorial Malaysia: Modeling Distribution

 Fade dynamics is one of more important parameters when implementing Fade Mitigation Techniques (FMTs) to counteract an excessive attenuation that affect satellite communication systems operating above 10 GHz. The statistics of probable duration between two rain fade namely interfade duration enables system operator to estimate how long the system will need to recover before the next outage and assist in designing the FMTs. In this paper, interfade duration statistics have been derived from one year of slant path attenuation measurements data collected in Equatorial Johor Bahru at 12.2 GHz with elevation angle of 75.61o. The result had shown the dependency of number of events with attenuation thresholds. Empirical interfade duration statistics are also obtained and suitable model distribution are proposed.

Evaluation of time diversity technique for satellite mitigation in tropical climate

Time diversity is one of the mitigation techniques for satellite communications (SatCom) systems that aims to combat deep fades cause by propagation impairments. This study presents the performance of time diversity technique for rain attenuation at Ku- and Ka-band SatCom systems in Johor Bahru, Malaysia. The Synthetic-Storm Technique (SST) has been utilized to obtain rain attenuation time series based on rain rate data measured by local rain gauge. The time diversity has been quantified through diversity gain as a function of retransmission time delay, in which shows the effectiveness of time diversity technique to improve the Quality of Service (QoS) and link availability. The results demonstrated it is feasible to use time diversity as mitigation technique at higher frequencies in a mission to support future SatCom operation in tropical region

Rain attenuation statistics for mobile satellite communications estimated from radar measurements in Malaysia

Mobile satellite communications will play a significant role in the next 5th generation mobile services. The use of high-frequency bands will be the enabler of this advancement. However, at high frequencies, excess rain attenuation causes severe signal losses and presents a major threat for the system availability, especially in the tropical region. To that end, this study presents the rain attenuation impact on mobile satellite communications estimated using long-term radar measurements in Malaysia, by exploiting the horizontal structure of rain from the radar database and simulating inner-city and highway mobile terminals scenarios. Additionally, a scaling factor was presented to scale available fixed satellite terminals measurements to mobile terminals operating at the same locality under similar conditions. In comparison to the available link measurements, the radar database was reliable enough to provide highly accurate estimates. In all simulation scenarios, the mobile terminal will depart the rainy area soon enough and experience lower attenuation statistics in comparison with the fixed terminal. The provided results will help determine the overall future system performance, especially in tropical regions

Rainfall attenuation prediction model for dynamic rain fade mitigation technique considering millimeter wave communication link.

Doctoral Degree. University of KwaZulu-Natal, Durban.To deliver modern day broadband services to both fixed and mobile devices, ultra-high speed wireless networks are required. Innovative services such as the Internet-of-Things (IoT) can be facilitated by the deployment of next generation telecommunication networks such as 5G technologies. The deployment of 5G technologies is envisioned as a catalyst in the alleviation of spectrum congestion experienced by current technologies. With their improved network speed, capacity and reduced communication latency, 5G technologies are expected to enhance telecommunication networks for next generation services. These technologies, in addition to using current Long Term Evolution (LTE) frequency range (600 MHz to 6 GHz), will also utilize millimetre wave bands in the range 24-86 GHz. However, these high frequencies are susceptible to signal loss under rain storms. At such high frequencies, the size of the rain drop is comparable to the wavelength of the operating signal frequency, resulting in energy loss in the form of absorption and scattering by water droplets. This study investigates the effect of intense rain storms on link performance to accurately determine and apply dynamic rain fade mitigation techniques such as the use of a combination of modulation schemes to maintain link connectivity during a rain event. The backpropagation neural network (BPNN) model is employed in this study to predict the state of the link for decision making in employment of dynamic rain fade mitigation. This prediction model was tested on all rainfall regimes including intense rain storms and initial results are encouraging. Further on, the prediction model has been tested on a rainfall event rainfall data collected over Butare (2.6078° S, 29.7368° E), Rwanda, and the results demonstrate the portability of the proposed prediction model to other regions. The evolution of R0.01 (rain rate exceeded for 0.01% of the time in an average year) parameter due to intense rain storms over the region of study is examined and detailed analysis shows that this parameter is double the proposed ITU-R value of 60 mm/h. Moreover, an investigation on the largest rain drop size present in each rain storm is carried out for different storm magnitudes. The study goes further to examine the frequency of occurrence of rain storms using the Markov chain approach. Results of this approach show that rain spikes with maximum rain rates from 150 mm/h and above (intense storms) are experienced in the region of study with probability of occurrence of 11.42%. Additionally, rain spike service times for various rain storm magnitudes are analyzed using the queueing theory technique. From this approach, a model is developed for estimation of rain cell diameter that can be useful for site diversity as a dynamic rain fade mitigation strategy. Finally, the study further investigates second-order rain fade statistics at different attenuation thresholds

Semi-empirical modelling of subtropical rain attenuation on earth-satellite microwave links.

Doctoral Degree. University of KwaZulu-Natal, Durban.The exponential rise in demand for high fidelity content on multiple platforms has in recent years made increased use of the higher echelons of radio communication frequency inevitable. At these high frequencies, wavelength becomes small enough to compare with the size of rain drops and in some cases smaller than drop size. This implies that the impairment due to rain, which already usually forms the most severe form of impairment at higher radio frequency bands, will become even more acute and require rigorous parameterization. This thesis investigates both by rigorous measurements and by theoretical approaches, the attenuation effect of rainfall in a subtropical climate (Durban, South Africa) on a microwave earth-satellite link operating at 12.6 GHz. The link was set up and the received signal level monitored via spectrum analyser sweeps conducted every minute. A Joss-Waldvogel impact disdrometer was installed such that its diaphragm is located a few meters away from the link’s receive antenna. From such a location, all precipitation recorded by the disdrometer are assumed to have some effect on the link. The monthly variation in the received signal during clear air was investigated by taking into consideration the average monthly values of temperature, relative humidity and atmospheric pressure. By employing multiple regression, a linear expression was obtained that can be used to predict the change in received signal level in clear air over the link given the values of these three atmospheric parameters. The attenuation due to the rain events was extracted from the data by carrying out an even-by-event matching of rain rate spikes with the corresponding drop observed in the received signal level at and around the time of the precipitation. The average monthly received signal level during clear air was extracted from the spectrum analyser data and used as the base channel power to which the received signal during rain in the particular month is compared. The difference between the two is stored as the attenuation due to rain in that instant of measurement time. The attenuation data thus accumulated were entered into a computer algorithm and a regression fitting procedure carried out to deduce an empirical set of logarithmic and power law models that relate the total path attenuation to rain rate. The models were then validated by a largely favourable comparison with four existing models, one of which is the in-force ITU-recommended model for slant path attenuation estimations. Random number properties of rain attenuation statistics obtained from the measurement model were exploited to develop a Markov chain approach by which seasonal and annual slant path rain attenuation time series can be generated. By investigating the nature of the probability distributions of the seasonal and annual measured path attenuation statistics, which was found to be lognormal, the state probability matrix necessary for implementing a Markov chain prediction model for future patterns of rain attenuation on a similar link was obtained as the lognormal probability density function. The state transition probability vector for each time period was developed by extracting the fade slope statistics of the measured attenuation. The discrete-time Gaussian distributed fade slope PDF forms the basis for the state transition probability matrix. With these, Markov-generated time series of seasonal and annual slant path attenuation for up to five iterations were obtained. The results make useful data that can be used for long-term planning for rain fade mitigation in a subtropical climate easier to generate without the expense of measurements. The theoretical approach called the Synthetic Storm Technique was also applied to investigate the nature of slant path rain attenuation in Durban. Based on the rainfall pattern captured by the disdrometer, SST approximations for the four seasons of the subtropical year and for years of rain data collection were carried out. The results were compared with the values generated from the measurement model. It reveals that the two models exhibit significant agreement because in a majority of the cases, the A0.01 values obtained are very close. Comparison of the performance of SST as a theoretical model with that of the ITU-recommended method also reveals that the ITU performs slightly better as an alternative to measurement than the SST model. It was observed that during certain precipitation events, the satellite link registers significant attenuation levels several minutes before the disdrometer records any precipitation on the ground. This anomaly was investigated in this work and a few conclusions drawn. By proceeding on the assumption that the observed delay was due to the migrating rain cell interacting with the satellite beam several minutes before reaching the receive antenna, it was demonstrated that the time of delay between precipitation and attenuation is related to the rain height during that particular rain event. A simple mathematical analysis is presented that enables the rain height to be estimated from the delay time. The results obtained range between 1.4 km to 6.7 km which is similarity to rain height values obtained by the ITU model which range from 1.36 km to 6.36 km

Interfade duration statistics at Ku-band for satellite earth links system in equatorial Malaysia: Modeling distribution

Fade dynamics is one of more important parameters when implementing Fade Mitigation Techniques (FMTs) to counteract an excessive attenuation that affect satellite communication systems operating above 10 GHz. The statistics of probable duration between two rain fade namely interfade duration enables system operator to estimate how long the system will need to recover before the next outage and assist in designing the FMTs. In this paper, interfade duration statistics have been derived from one year of slant path attenuation measurements data collected in Equatorial Johor Bahru at 12.2 GHz with elevation angle of 75.61o. The result had shown the dependency of number of events with attenuation thresholds. Empirical interfade duration statistics are also obtained and suitable model distribution are proposed

Microwave and millimetre radio wave propagation modelling for terrestrial line-of-sight links in Central Africa.

Doctoral Degree. University of KwaZulu-Natal, Durban.The rapid expansion of the global telecommunication has led to an exponential growth in the demand of wireless services. This has led to the migration to higher frequency bands in the microwave and millimeter wave spectrum. Research has shown that rainfall is the most dominant factor affecting the provision of network services in these bands. Rainfall attenuation is among the major factors often considered in the design of wireless networks operating at higher bands within microwave and millimeter wave spectrum. At tropical and equatorial locations, not only is the occurrence frequency of rainfall events of serious concern to terrestrial and satellite communication systems, but also the high intensity of rain rates and drop size distribution result in extreme fading of line of sight (LOS) system during such events. In this work, daily rainfall measurements from the Rwanda Meteorology Agency (Meteo Rwanda) are obtained for 60 locations within equatorial Rwanda (between latitudes of 1o2'S and 2o45'S and longitudes of 280 45'E and 30052'E), in Central Africa, to develop rain rate and rain attenuation maps for wireless radio links. From these long term annual rainfall measurements spanning a minimum of 10 years at these locations, rainfall rate statistics and drop size distribution result in extreme fading of line of sight (LOS) system during such events. In this work, daily rainfall measurements from the Rwanda Meteorology Agency (Meteo Rwanda) are obtained for 60 locations within equatorial Rwanda (between latitudes of 1o2'S and 2o45'S and longitudes of 280 45'E and 30052'E), in Central Africa, to develop rain rate and rain attenuation maps for wireless radio links. From these long term annual rainfall measurements spanning a minimum of 10 years at these locations, rainfall rate statistics estimated from appropriate models are applied to determine fade margin for radio link availabilities between 99% and 99.999%. Furthermore, specific attenuation estimates due to rainfall are proposed from International Telecommunication Union (ITU) recommendations at selected frequencies of the microwave and millimeter bands, for the design of wireless networks. Results obtained from this approach incorporating both rainfall rate zones and specific attenuation over Rwanda are presented as spatial contour maps representations for different ranges of link availability. Further, disdrometer data collected in Butare, Rwanda (20 35' 53.88” S and 290 44' 31.5” E) for a period of 32 months between 2012 and 2015 have been use to develop a suitable model on drop size distribution in the region. Rainfall data was classified into four different regimes, namely, drizzle, widespread, shower and thunderstorm. Different raindrop size distribution (DSD) models such as Lognormal, Gamma, Marshall-Palmer and Weibull distributions are selected and the method of moment technique is applied for estimating input DSD fit-parameters for those DSD models. From the results, it is observed that different models have varying performances as the rainfall regime varies from drizzle to widespread, shower and later as thunderstorm, except the Marshall- Palmer model which shows the inadequacy for the region. It is found that neither the Lognormal nor other models match perfectly wel I with the measured DSD, particularly at high rainfall rates. Therefore, a new rainfall DSD model or Central Africa is developed and found to be an improvement over the existing models. The Mie Scattering technique (spherical method) is employed to derive the scattering parameters. Therefore, the derived scattering parameters with DSD models are used for the estimation of rainfall attenuation in the region of Central Africa. Finally, the synthetic storm techniques (SST) is applied for comparison with other rainfall attenuation models