Rain Dependant Site Diversity Outage Improvements Statistics for Systems Working in Millimeter-wave Band

The signal propagation at the frequencies above 10 GHz suffers from significant attenuation due to rain, which in case of point-to-multipoint systems is unlikely to be compensated for by available fade margin alone. For this reason and especially thanks to the non-uniform spatial distribution of the rain rate, it is suitable to utilize site diversity concept during planning of point-to-multipoint wireless systems. A database of radar data for numerous types of rain events has been established to enable system testing under different conditions. Outage improvement was studied using space-time simulations of point-to-multipoint systems in a specific area under a specific rain conditions. System outage improvement probability was derived as a straightforward function of the rain fade margin, angle separation and distances of main and diversity hubs. Some of the simulation results at 42 GHz and corresponding outage improvement probability using the site diversity are presented as well. INTRODUCTION The increasing demand for broadband communications applications such as mobile internet, videoconferencing, and video on demand, has led to an increased pressure to make use of the higher frequencies currently unused in the radio spectrum. Wireless systems such as Fixed Wireless Access (FWA) [1], Local Multipoint Distribution Systems (LMDS), Multimedia Wireless Systems (MWS) etc. are working or be expecting to be deployed in millimeter wave bands throughout the World. However, the frequencies available above 10 GHz suffer from significant attenuation due to rain. Therefore, it is necessary to investigate alternate methods of compensating for the fades that can cause loss of signal. An important characteristic of rain fields is their spatial and temporal non-uniform distribution. Intense rain cells that cause large amounts of attenuation on radio paths often have horizontal dimensions of no more than a few kilometers. The site diversity concept employing two or more base stations receiving the same signal with an angle and/or distance separation usually improves system performance considerably. This can be achieved since the attenuation statistics for a single path and two diversity paths are very likely different as it was observed for two links in [2]. The motivation to our work was to offer simpler way to evaluate possibilities of the site diversity employment for specific scenarios instead of exhausting power balance calculations of every potential diversity connection. Using space-time simulations of a point-to-multipoint system in a specific area under a specific rain conditions we derived an outage improvement probability function for a given scenario. Acknowledgment This work has been partially supported by the Czech Ministry of Education, Youth and Sports in the frame of the project MSM 6840770014 “Research in the Area of the Prospective Information and Navigation Technologies” and by the GAČR grant No. 102/04/2153 “Time-space Simulations of MWS Systems in 42 GHz Band.” RAINFALL RADAR DATA Radar based rain data [3] have been utilized as an input for system simulations. There are several methods how to obtain the rain rate and attenuation from radar data given in reflectivity (dBZ). The Marshal-Palmer relationship (1) was used to conversion of reflectivity Z (dBZ) to rain rate values R (mm/h). ( ) 5 , 1 300 log 10 R Z ⋅ = (1) Large rain event database containing over thousand events for Czech Republic (situated in ITU rain region H) was build for the simulations. Each rain event is represented by a sequence of radar images for 50 x 50 km area with 1 km grid resolution in 1-minute step. The analysis of the statistical variations in spatial organization of precipitation is of great importance. Therefore, the statistical investigation is being performed to obtain description for the occurrence of particular rain structures. SITE DIVERSITY OUTAGE IMPROVEMENTS STATISTICS Site diversity improvement is highly dependent on the actual spatial distribution of the rain rate. Different results can be expected for different rain patterns. The rain rate distribution in both time and space is given by the rain type and geographical region. At first, we concentrate on the site diversity improvement probability function definition using the simulation tool. This tool was developed to evaluate various aspects of the system performance under different conditions (more about simulation tool can be seen in [4]). During simulations for each rainfall radar scan the position of the hypothetical terminal station was changed in 1 km step grid throughout whole area. Position of the main hub was changed in a circle around the terminal station with fixed radius. The receiving signal strength was calculated for every azimuth of the hypothetical main link. If the signal strength dropped below the chosen threshold, the diversity links for angular separation from 1 to 359 degrees and diversity link length ranging from 2 to 6 kilometers were investigated. This way the outage improvement statistics dependence on link length and angular separation was obtained. The result for particular rain rate distributions Fig. 1a for the rain fade margin 17 dB is demonstrated in Fig. 1b. Fig. 1. Rain no.1 a) rain intensity [mm/h]; b) calculated outage improvement probability for rain fade margin 17 dB using (2) As it was mentioned, the outage improvement probability determines the ratio between the successful trials to establish the diversity link and total number of all main link outages. Based on our simulations we derived an empirical formula (2) for the outage improvement probability as a function of rain fade margin, angle separation θ and a ratio of the main and diverse link lengths (dmain/ddiv):