EG4217/7217 Advanced Communications

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EG4217/7217 Advanced CommunicationsCoursework 3Ray TracingIntroductionFor this part of the coursework, you will use a simple ray tracing programme written in Matlab. Thesoftware takes reflections into account, but not transmission through walls, scattering, diffraction orrefraction. The software allows a single transmitter, any number of receivers, as long as they are in aline, and any number of reflecting surfaces. The first two exercises require you to compare its outputto the physical models in Coursework1 for simple geometrical arrangements. The third exercise usesa more complex arrangement, representing Wifi propagation in an office and uses an enhancedversion of the programme that also produces a power-delay profile.How to use the softwareThe program is called rays.m, and can be run by simply typing ‘rays’ in the command-line with noarguments. The program will prompt the user to input the name of an input file, containing the threedimensional positions of the transmitter, receivers, and planes, as well as other information requiredto perform a ray-trace. A sample input file, ‘geom1.ray’ is supplied with the program. You can editthis file and rename it to create the right geometry for the tasks. The input file must have the suffix‘.ray’. All measurements are referred to an arbitrary origin and are in metres. Co-ordinates may beseparated by spaces or commas. Comment lines must start with a ‘%’ symbol.
txpos 10 0 0totpow 1.0rxpos1 0,40,40rxpos2 100,40,40rxsteps 20rxradius 1
This is the transmitter position (x, y, z)The transmitter power in watts.The position of the first receiver (x, y, z)The position of the last receiver (x, y, z)The total number of receiver positionsThe distance from a receiver within which aray must pass in order to be received.The ratio of the electric field of a ray afterreflection to before reflection.(Must bebetween 0 and 1)The wavelength of radiation in metresThis is a comment lineEach line represents a numbered point (x,y,z).‘inf’ is useful for ground planes.
reflratio 0.8
wavelength 0.3%the ground plane1 inf inf 02 -inf inf 0
3 -inf -inf 04 inf -inf 0
Each line represents the corner of arectangular plane.
1 join 1 2 3 4 The keyword ‘join’ joins 4 points into a planeThis line joins points 1,2,3 and 4 into plane 1%A vertical plane5 100 22 06 100 22 1007 0 22 1008 0 22 02 join 5 6 7 8 This line joins points 5,6,7 and 8 into plane 2…and so on, as many planes as required.end The file must be terminated with the word ‘end’Run the software using the data file geom1.ray as it stands. You should first see a representation ofthe geometry. The software should allow you to zoom in and alter the viewing angle of this 3Ddiagram, using the controls that appear at the top-right of the figure window when you place thecursor over the figure. If you have altered the input file, this gives you a chance to check that thegeometry is as you intended.After this, press any key to continue and you will see in the Matlab main window, the number of raystransmitted. Each iteration of the ray-tracing algorithm doubles the number of rays transmitted, from12 to about 6000. After this, the programme shows you the output so far and asks you whether tocontinue iterating. Type ‘c’ and press return to continue.The programme will show a 3D graph of the power level at each receiver as a function of each iterationof the program. Each time the programme halts to ask you whether to continue, you should be ableto zoom in and alter the viewing angle of this graph. When each red line on the graph is approximatelylevel, this means that the received power at each receiver has not changed significantly in the lastiteration of the programme, and the output has converged. Each iteration will take longer than thelast, so you should use your judgement to decide when to quit the programme by typing ‘q’ andpressing return. The time taken will depend on the processing power of the computer used, but youare not expected to wait more than a few minutes for each iteration.When you do this, the program will output the received power values (red crosses), with a smoothcurve joining them, as shown in Figure 1. The curve may not pass through the points, but is intendedto give approximate values in the absence of many sampling points.Another graph will output the received power as a function of the distance of each receiver from thetransmitter. This will be ethe most useful plot.Figure 1 Output from file geom1.rayTask 1Vary the number of rays traced, and relate this to the time taken for the program to run. Commenton how the final output graph changes.Edit the file to change each of the following in turn. Save it and run the program, each time,commenting on the change:Change the number of receivers.Change the receiver radius.Task 2Free space: Edit the file by removing planes 2 and 3. Move the ground plane to vertical position–inf. This will not actually be at infinity, but at a suitably large distance from the origin. Run theprogram with the new file and put your result in your electronic lab-book. Compare the output to theoutput of freespace.m, that you used in Coursework1. Try 50 or 100 receivers and a receiver radiusof 3m, going out to a distance of about a kilometre. Run the model till it has traced about a hundredthousand rays to get a good plot. Ensure that the transmitter is not directly in-line with the line ofreceivers, as this will give poor results.1 1.5 2 2.5 3 3.5 4 4.5 5position along rx line-130-120-110-100-90-80-70-60Received power in dBW
f
itted curve
Task 3Two-ray ground model: Edit the geometry file again. Bring the ground plane closer to the transmitterand vary the number and positions of the receivers and the wavelength to find the interference peaksseen in planereflec2.mTry f = 400MHz, 100 receiver positions: d = 100 – 300 m, ht = hr = 10m and rxradius = 3m and run for100 000 rays.Task 4Indoor environment: Use the programme rays2.m, and the input file, indoor.ray to observe thepower profile and delay profile for WiFi propagation in an office. By viewing the geometry, you willsee that the office has four external walls and a ceiling and floor. It also has an internal wall with adoor and three desks and a cupboard in it. There are four receivers, situated to on the desks tosimulate the reception at desktop computers. As well as the output you’ve seen before, theprogramme shows the power in dBW vs delay in nanoseconds for the four receiver positions.Run rays2 with the file indoor1.rays reflection ratio to 0.7 and put the output in your lab-book. Nowchange the reflection ratio to 0.9, run it again and put the output in your lab-book. Comment on thegeneral changes you see to the received power.For each receiver and for the two reflection ratios, estimate the time delay between the strongestcomponent of multipath and the last component that is no more than 20 dB below the strongestcomponent. Each panel in the multi-panel graph may be zoomed in separately.

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