ATTENUATION OF RADIO COMMUNICATION SIGNALS BY RAIN AT MILLIMETER WAVE BAND AT SOME LOCATIONS IN NIGERIA

Abstract

Millimeter Wave (MMW) radio systems operating at 30 to 300GHz band provides higher bandwidth, frequency reuse and communications security but suffers greatly from attenuation by rain. The design of radio communication equipment has been based on predicted rain rate and attenuation from the International Telecommunication Union-Radio (ITU-R) model. However, such equipment fails in the tropics because rainfall in this region is more intense with larger drop sizes than those in the temperate regions on which the ITU-R model was based. Thus, Rain Rates (RR) and attenuation information from the tropics are needed for the design of radio communication equipment for the region. This study was therefore aimed at estimating rain induced attenuation of radio communication signals at some locations across Nigeria. The RR data was obtained from 14 automatic weather stations across eco-climatic zones of Nigeria.The equipment measures rainfall at 1- min (2 locations) and 5-min (12 locations) integration time. Lavergnat and Gole model was used to convert the 5- min to 1-min RR, while logarithmic scale was used to convert the RR to exceedance time percentages (0.001 to 1%). These RR were compared with those predicted by the ITU-R model using the MatLab RR statistics. The specific rain attenuation   R  for Horizontal ( RHP  ) and Vertical ( R p  v ) Polarisation at MW Frequencies (MWFs) were estimated at 0.01% RR (R0.01) using the power law     0.01 R  kR relationship where k and α are regression coefficients. These estimated values were then compared with the ITU-R predicted values. The Path Attenuation (PA) at 20 km (at 0.01%, A0.01) Path Length (PL) was computed using A0.01   Rdeff dB to determine the Clear Signal Bands (CSBs) at MWFs (deff is the effective path length). The mean annual 1-minute RR (mm/hr) ranged from 22.78 in Mid Altitude Savanna (MAS) to 116.67 mm/hr in Southern Guinea Savanna (SGS). Throughout the country, the ITU-R predicted RR ranged from 87.10 mm/hr in the MAS to 91.60 mm/hr in the SGS. The highest value of R  for all 14 locations occurred at 120 and 150 GHz MWFs and RHP  > RVP  . The estimated maximum RH p  vs ITU-R values at 120 GHz were: 35.05, 29.85; 26.96, 17.90; 30.94, 18.01; 11.85, 28.86; 29.78, 30.56; and 35.91, 33.52 dB/km at SGS, Sudan Savanna (SS), Northern Guinea Savanna (NGS), MAS, Derived Savanna (DS) and Humid Forest (HF), respectively. The corresponding RVp  vs ITU-R values were: 34.64, 29.52; 26.68, 17.74; 30.60, 17.85; 11.77, 28.55; 29.46, 20.00; and 35.48,33.14 dB/km, respectively at the 0.01%. At 150 GHz, maximum RH p  vs ITU-R were: 34.81, 29.74; 26.93, 18.03; 30.81, 18.14; 12.05, 28.78; 29.68, 30.43 and 35.63,33.32 dB/km, respectively. The corresponding maximum RVp  vs ITU-R were: 34.50, 29.50; 26.72, 17.93; 30.55, 18.04; 12.00, 28.56; 29.44, 30.18 and 35.32, 33.03 dB/km, respectively. The CSBs at 20 km PL were estimated to be 40 and 45 GHz across the zones, while the predicted value by ITU-R included 150 GHz and overlaps with 45 GHz band. The estimated PA at 40 and 45 GHz ranged from 64.65-206.85 dB and 71.40-219.45 dB, respectively; the ITU-R predicted range of PA at 40, 45 and 150 GHz were 90.01-192.18 dB, 107.79-204.43 dB and 135-150 dB, respectively. vii The rain rates across the eco-climatic zones have been determined. The estimated specific, path attenuation and clear signal bands varies from the International Telecommunication Union-Radio predictions for Nigeria.