Semi-Automated Microwave Radio Link Planning Tool

Transcription

1 Semi-Automated Microwave Radio Link Planning Tool W.M.D.R. Gunathilaka, H.G.C.P. Dinesh, K.M.M.W.N.B. Narampanawe Abstract Link Budget is a main estimate in telecommunication microwave link planning for accurate and reliable link. This software program is a comprehensive path design tool and it can be used for radio links operating in the microwave frequency range. When a link budget is prepared, many parameters are considered for calculations. Therefore this software program is very essential tool in the telecommunication industry. This paper presents a software tool that can be used to prepare link budgets by providing essential parameters. Moreover this software can be used for applications such as frequency selection according to ITU (International Telecommunication Union) standards, graphical representation of the radio path profile between any two geographical locations on the loaded terrain and generates the Fresnel zone according to the frequency. Furthermore this software tool can be interfaced with a GPS (Global Positioning System) receiver to gather geographical locations and based on this information, microwave frequency band is suggested according to regulatory body guidelines. Index Terms Frequency planning, Link Budget, Link Planning, Microwave Radios, Software Tools I. INTRODUCTION A communication system that utilizes the radio frequency band as electromagnetic waves between 3GHz and 30 GHz are called microwaves as their wavelength is smaller. Microwave frequencies are used in telecommunication link designing because they are less prone to accidental damage, links between mountains, over reservoirs and over jungles are more economically and feasible, single point security, single point installation and maintenance and they can quickly be deployed Microwave link design is a metrological, systematical and lengthy process that includes Loss/attenuation calculations, fading and fades margin calculations, frequency planning and interference calculations and quality (Bit Error Rate-BER) and availability calculations. As shown in Fig. 1 these four processes are very important in microwave link planning. W.M.D.R. Gunathilaka is with the Department of Electrical and Electronic Engineering, University of Peradeniya, Peradeniya, Sri Lanka ( dinusharg@gmail.com). H.G.C.P. Dinesh is with the Department of Electrical and Electronic Engineering, University of Peradeniya, Peradeniya, Sri Lanka ( karunaa33@yahoo.com). K.M.M.W.N.B. Narampanawe is with the Department of Electrical and Electronic Engineering, University of Peradeniya, Peradeniya, Sri Lanka ( narampanawe@ee.pdn.ac.lk). Interference analysis Propagation losses Branching losses Other losses Rain attenuation Frequency Planning Link Budget Fading Prediction Multipath Propagation Fig. 1 Steps of link planning Quality and Availability Calculation Diffraction-refraction losses According to quality and availability calculations, it is required to re-calculate link budget to improve reliability and quality of the link. This is the reason for using software to calculate and design Microwave links in telecommunication industry [1] [2]. This developed software is for telecommunication companies and regulators deal with many microwave radio links. This tool can be used store data of each microwave links through user friendly GUI (Graphical User Interface). Further it has tools for frequency planning, calculating link budget, fading prediction and graphically implementation of terrain profile. This software allows any user to perform step by step analysis of all important propagation related phenomena that needed to generate a planning report containing all the data necessary for accurate and reliable implementation of microwave radio links and maintain database about them. There are some microwave link planning software (ex: Pathloss, Atoll Microwave, Global Mapper) available commercially for link budget and graphically implementations but they are very expensive for some commercial and educational usage. Further they require computer platforms with high-end resources. This developed software has many tools for frequency planning, link budget calculation, fading prediction and graphical implementation of path profile. In addition to that software tool can be used to implement the whole microwave links in a network including link data, tower information of a particular client company /11/$ IEEE 294

2 II. LINK DATABASE SYSTEM One of the key functions of this software is Link Database System. It contains the data of microwave links such as longitude, latitude, elevation of site, antenna height, polarization, transmit/receive frequencies, transmit power, cable losses, antenna gains, receiver sensitivity, received signal strength, free space losses, fade margin and reliability. According to longitude and latitude, software graphically presents the actual location on terrain map. As shown in Fig. 2, it shows every uploaded microwave radio links which is an inheritable function compared to other commercial software. Detailed information of each microwave link can easily be read by clicking mouse on microwave link or tower position. As shown in Fig. 3, path profile and Fresnel zone of particular microwave radio link can be obtained by just a mouse click. III. MICROWAVE RADIO LINK PLANNING There are two scenario of planning a new Microwave Radio Link with this software. 1.) Two station Planning. 2.) Plan one station respect to database system. When two stations planning are considered, user has to enter data of both stations. In the second method reference station data are automatically entered to the link budget. There are several steps involved in planning a new Microwave Radio Link such as Path Distance and Elevation Calculation, Frequency Planning, Link Budget calculation, Path Profile and Fresnel Zone Analysis. A. Path Distance and Elevation Calculation First step of planning a microwave link is map study and path profile preparation. Preliminary map studies help to determine the actual topography of the terrain, the height, and obstacles along the desired path for line of sight clearance. Desired stations longitude and latitude are collected for path profile preparation. Using these longitude and latitude values, path distance is calculated and it is used to select the transmit frequency and other parameters. There is a unique feature in this software to collect the longitude and latitude data from a GPS receiver module as shown in Fig 4. By connecting the GPS receiver to a computer running this software in link planning mode at proposed station location, longitude and latitude data are automatically uploaded to the link budget calculation as shown in Fig 5 [3]. After entering the longitude and latitude values of two sites to this software, the path distance and elevations of two points are calculated automatically [4] [5]. B. Frequency Planning Fig.3 Path profile of a specific radio link The second step of the link planning is Frequency Planning. The goal of frequency planning is to allocate microwave frequencies to a radio link as a few frequencies possible so that the availability and the quality of the radio link are less affected by the interference. Path length, site location, terrain topography and atmospheric effects are the basic considerations involved in the assignment of radio frequency when determining a frequency band that is suitable for the specific link [6]. Assigning a microwave frequency channels are authorization given by an authority for an operator to use a microwave frequency channels. It is created in accordance with the series-f recommendation given by the ITU-R (International Telecommunication Union-Radio communications and wireless) [7]. The available frequency band is subdivided in to two Fig. 2 Microwave Links in Terrain map Fig.4 GPS Receiver and Antenna 295

3 To calculate the link budget user has to enter parameters such as Transmit power of antenna, cable Losses, Transmit Gain, Receiver Sensitivity, terrain factor and Climate factor. 1. Transmit Power The transmit power is the RF (Radio Frequency) power coming out of a transmitter. It is measured in dbm and does not include the signal loss of the coaxial cable or the gain of the antenna. 2. Cable Losses Usually cable losses are called as branching losses that come from the hardware used to deliver the transmitter/receiver output to/from the antenna. Fig. 5 GPS Data halves, a lower/go, and an upper/return duplex half. The space between two duplex halves should sufficiently be large such that the radio transceiver can function under minimum interference full duplex operation. The bandwidth of a channel based on the capacity of the radio link and the modulation scheme used. The most important goal of frequency planning is to allocate available channels to a different link in the network without exceeding the quality and availability objectives of the individual links because of the radio frequency interferences [8]. There are two possible methods for frequency selection in this software, namely they are Manual and ITU data as shown in Fig. 6. In Manual frequency selection mode user can assign frequencies to two stations and the software will calculate the link budget for the given frequencies. In ITU data mode, the software indicates frequency sub bands / channels as shown in Fig. 6. After selecting the suitable frequency band, user can choose the site either as high or low frequency. There is a special feature in this software; it suggests the frequency band according to path distance which is issued by the spectrum governing body of a country as shown in Table 1. This automatically selects the suitable frequency band using a standard frequency vs. distance data guidelines suggested by regulatory body. This special function is useful to choose suitable frequency for a desired link which will be approved by the regulator. 3. Transmit Antenna Gain Transmit gain is the quantity that an antenna boosters the RF signal over a specified direction. Antenna achieves Gain simply by focusing RF energy. 4. Receiver Sensitivity The minimum input signal level required to produce a specified signal-to-noise ratio at the output of the receiver to recover the transmitted signal properly. 5. Terrain Factor The terrain factor is used to calculate the availability of a link., it can either be calculated from terrain roughness and humidity information or be included directly. C. Link Budget The link budget is a mathematical calculation involving loss and gain factors related with the transmission lines, antennas and propagation channel, to find the maximum operating distance between a transmitter and a receiver can operate successfully [10]. According to Fig. 7 it can be seen the output power is varying in each step and finally it reduces. The final outcome is fade margin of the link. Fig. 6 Frequency selection interface 296

4 Transmitter Output Power (Tx) Splitter Branching Losses Antenna Gain Propagation Losses Antenna Gain Branching Losses Splitter Receiver Received Power (Rx) TABLE I DISTANCE/ FREQUENCY GUIDELINE Frequency Band/GHz Distance/km 4 > ~ ~ ~ ~ < < Very smooth terrain, including over water Average terrain, with some roughness. 0.25(1/4) - Mountains, very rough, or very dry areas. 6. Climate Factor The climate factor is used to calculate the availability of a link; it can either be computed from average annual temperature information or be entered directly (1/2) - gulf coast or similar hot, humid areas (1/4) - normal interior temperate or northern areas (1/8) - mountainous or very dry areas. According to those parameters link budget is calculated including Receiver Signal Level, EIRP (Effective Isotropic Radiate Power), and Free Space Loss, Fade Margin, Outage probability and Reliability. Free Space Loss (FSL) Free Space loss is the theoretical attenuation of a radio signal when it propagates away from the transmitting antenna. When a radio signal radiates from the antenna with the distance it spreads out more and more. As the area covered increases, the amount of power per unit area (the power density) decreases. This effectively weakens the radio signals [11]. FSL is calculated using following equation (1) [12]. Fade Margin Receiver threshold Value Fig 7 Variation of power between transmitter and receiver FSL = log( f ) + 20 log( d ) f = frequency, MHz d = distance between two stations, Km (1) 7. Effective Isotropic Radiated Power (EIRP) EIRP is the radio frequency power measured at the main focal point of the antenna. It is equal to the sum of the transmit power in the antenna (in dbm) added to the gain (dbi) of the antenna. Since it is a power level that measured in dbm [9], EIRP is calculated using following equation (2). EIRP = P out + G t C t Pout = Output power of transmitter Ct = Cable loss in transmitter Gt = Antenna gain of transmitter (2) 8. Received (Rx) Signal Level Received signal level is the actual received signal (dbm) presented to the radio receiver at far station. It is calculated using equation (3) [10]. Received Signal Level = EIRP - FSL Gr = Antenna gain of receiver Cr = Cable loss in receiver + G r C r - (3) 9. Fade Margin The difference between received signal level and the receiver sensitivity threshold is called fade margin, each link must have sufficient fade margin to protect against path padding s, that weakness the microwave radio signal. Fade margin is the insurance against unexpected system outage [13]. Fade margin can be calculated by using equation (4). Fade margin = Rx signal Level - Receiver Sensitivit y (4) To determine the feasibility of a link, the calculated receive signal level is compared with the Receiver sensitivity threshold. The link is theoretically feasible if, Received signal Level Receiver sensitivity. If the receiver sensitivity threshold is smaller than or equal to the received signal level then the link might be feasible 297

5 implement, because the received signal must be strong enough to be interpreted by the receiver demodulator. 10. Reliability The reliability of a microwave link depends on the calculated fade margin. The reliability is calculated using following equation (5). F 6 3 Reliability = a b f D a = terrain factor b = climate factor f = frequency, GHz D = path length, miles F = fade margin, db D. Path profile and Fresnel zone - (5) 1. Path profile When the link budget is calculated, it can be feasible theoretically but practically it may not be feasible because the link may not be in line-of sight. Microwave radio communication strictly required line-of-sight condition and the clear path of first Fresnel zone. Therefore path profile is important in link planning. Graphical representation of the path traveled by the radio waves between the two radio stations of a link is the path profile. The path profile determines the height of the antennas and the locations at each end of the link, and it insures that the link is out of obstacles. According to input longitude and latitude the software graphically represent the terrain variation between two stations using height database and the Fresnel zone graph. Fig. 8 shows graphical representation of terrain variation for a given data set. The SRTM (Shuttle Radar Topography Mission) data base is used to obtain path profile. This database indicates the height for an accuracy of 90m. This data base is in UTM (Universal Transverse Mercator) coordinate system with WGS84 datum. Fig.8 Path Profile and Fresnel zone graphically represent. Radius of the first Fresnel zone is calculated using equation (6). x( d x) R = f d (6) R = Radius of first Fresnel zone, m x = distance from one point to radius point, Km d = distance between antenna, Km f =frequency, GHz The software represents a terrain map and Fresnel zone graph together as shown in Fig. 8, when the path is not clear for the Fresnel zone, height of the antenna can be increased graphically using software tool. A new antenna height automatically adds to the link budget report. Final report includes inputs and calculated data as shown in Fig. 10 that can be taken as the final soft/ hard print. IV. CONCLUSION To design this software it was required to write some additional software for handling databases of frequency bands and mean sea levels (elevations). It was compared with existing reports of several links to check whether the accuracy of this software. The results were acceptable with those existing reports which conclude that this software works properly according to the standards. 2. Fresnel zone Radio frequency waves travel along a straight line. When they get away from the transmitting antenna, they spread out the farther. Fresnel zone is the area that the microwave signal spreads out. When there is an obstacle in the Fresnel zone, part of the microwave radio signal will be diffracted away from the straight-line path. The practical effect is that on a microwave radio link, is reduce the amount of energy that reaching the receive antenna [14]. The radius of the Fresnel zone depends on the frequency of the signal, when smaller the frequency, higher the Fresnel zone [9]. Fig. 9 shows the radius of Fresnel zone. There is special tool in software to calculate the Fresnel zone and x R d Fig.9 Radius of Fresnel Zone 298

6 Fig. 10 Final Report Sometimes calculated values may vary with actual values because of many immeasurable factors and natural phenomena s. This software is calculating the link budget and path profile as like in other commercially available software s but time taking to calculations and plotting to path profile is less than other software. Accuracy of calculations is under standard of ITU. Further this tool requires only low cost minimal hardware that is a computer and a GPS receiver. In further this software will be developed to select suitable frequency according to climate and geographical factors. In addition to that 3D terrain and 3D Fresnel views will developed for planning and optimize microwave communication network. V. REFERENCES [1] William C.Y.Lee, Wireless & Cellular Telecommunications, Third Edition, pp [2] S.Ramabhadran, Telecommunications Principles Circuits Systems and Experiments, pp [3] Mini USB & Bluetooth Interface GPS Demo Board User s Guide. [4] Digital Microwave Communication Principals, Huawei Technologies Co,Ltd,4/4/2007, pp [5] Steven Dutch, Converting UTM to Latitude and Longitude, National and applied Science, University of Wisconsin-Green Bay, November 16, 2009 [online] Available: [6] A.Michael Noll, Introduction to Telephones & Telephone Systems, Second Edition, pp [7] ITU-R F-Seris Recommendation [8] Udo W.Pocch, Dennis Machel, John McCahn, Telecommunications and Networking, pp [9] Bhupesh Batra, Microvawe link-fudamentals, February 10, 2010 [online] Available: [10] TCIL Bhawn, Microwave Link Design, presentation, December 10,2009 [online] Available: [11] An Introduction to microwave Radio Link Desing,SAF tehnikaa/s 202, December 3,2009 [online] Available : [12] Nihal Kularathna, Dileeka Dias, Essentials of Modern Telecommunications System, pp 156. [13] Michael F.Young, Planning Microwave Radio Link, November 20, 2009 [online] Available: [14] Fresnel Zone Calculator, November 20,2009 [online] Available: 299