Adaptive Code Modulation for Rainfall Fade Mitigation in Ethiopia

Abstract

The massive demand for efficient, and reliable wireless communication systems has motivated researchers, and network designers to study communication systems that operate at microwave and millimetric wave bands. This is due to congestion in the lower frequency spectrum and increasing demand for large bandwidth and high channel capacity. However, the reliability of radio communication systems at the higher operation frequency spectrum can be affected by various atmospheric elements. Of all atmospheric constituent, rainfall is the major cause of impairment at higher frequency band bringing about scattering, attenuation and depolarization of signals at the receiver. Rain attenuation, is considerably noticed above 7 GHz and 10 GHz in tropical equatorial and temperate climates, respectively. It causes attenuation in the transmitted signal and reduction of the link availability. In order to satisfy the Quality of Service QoS specifications and to achieve high levels of link availability, rain fade counter measures are required. Adaptive coding and modulation technique (ACM) is one of the several Fade Mitigation techniques employed to mitigate the effects of time-varying channel conditions imposed by fading, interference, and noise on wireless communications. The International Telecommunication Union -Recommendation ITU-R, through Recommendation P 530-16 and P 618-13, provides basic Line-of-Sight (LOS) link design assumptions based on propagation prediction methods which are not suitable for tropical regions and at high rainfall rate since average radius of raindrop in tropical region is greater than that in non-tropical and data for ITU model is based on data collected from temperate regions. Thus, ITU-R recommends to use locally measured rain data to predict the rain attenuation for this reason. Unfortunately, a rain fade mitigation technique based on local rain data has not been adequately studied. This situation is more prevalent when it comes to African equatorial and tropical countries. In addition to this, since the condition of the wireless channel is varying with time, an intelligent adaptive technique, which is good in decision making, is required. In other words, due to complexity, uncertainty and adaptive nature of the wireless channel, the conventional nonintelligent systems cannot cope with an adaptive environment. Soft computing techniques such as fuzzy logic, neural networks, and neuro-fuzzy systems are preferred over the adaptive and fixed coding and modulation techniques in decision-making. In this thesis, a one-minute rain rate data collected using a measuring device installed at Jimma University,Ethiopia is used to determine the rain attenuation. Then, based on this calculated rain attenuation Neuro xiv Fuzzy based Adaptive Coding and Modulation technique is employed to mitigate rain fade in a particular microwave link between Jimma and Muja.SNR,BER,modulation orde and coding rate are the input parameters that are used to enhance ACM using Neuro-Fussy based decision making system. Furthermore, the performance of this Neuro-Fuzzy based adaptive coding and modulation scheme is compared with non-adaptive technique, and fuzzy-based adaptive modulation and coding technique.The rain data analysis depicts that the signal-to-noise ratio at clear sky is 32.5dB for this particular microwave link.Where as ,as the rain rate is above 130 mm/hr ,signal-to-noise ratio drops to 0dB and network outage will occurred.Thus,lower order modulation scheme with lower coding rate,such as 4QAM-1/3,8QAM-1/4,16QAM-1/4, is better in maintaining link availability.However,when the channel is not affected by rain spectral efficiency is improved by utilizing larger constellation size modulation scheme such as 256QAM,512QAM and 1024 QAM with higher coding rate like 3/4. .In addition to this, MATLAB simulation result showed that adaptation of channel condition using Neuro-Fuzzy based adaptive coding and modulation is better than fuzzy logic based adaptation and threshold adaptive coding and modulation techniques.