A long while ago I wrote a piece about surface multipath interference over the sea and recently, I had to explain that theory to someone. It was regarding their C band microwave access system which was not performing to their expectations. This system had one end station on the land and the other in the form of a tracking antenna mounted on a ship. Being a modern system, the signal level data could be recalled and graphed. In front of me I observed a great example of the fading phenomenon, so I dug out the old Excel model, tweaked the parameters for the atmospheric state of the day et voila:
The sea was a little unsettled on the day in question and the boat had several metres of heave. The multipath effects are clearly visible including impact of the heave that can be observed in the little oscillations in the last measured lobe. Beyond that, the signal diffracts due to the sea state causing the horizon to fall short of the smooth sea prediction.
A few years ago I had a number of projects in the broadcast world, where I was looking a national digital television and radio plans. I needed to look at what assignments were coordinated internationally to reconcile this with national data and look at neighbouring use. I then looked at opportunities to use coordinated assignments in some way, as some of the assignment plans were created before there was a good vision of national requirements.
Usually this kind of process involves using the ITU BRIFIC which is not for the faint hearted! Having much work to perform, I set about creating a tool to gather the relevant ITU data and import data from national databases, so I could compare, then modify and use in a coverage planning tool. The software needed good filtering, sorting, editing and comparison tools as well as a nice GUI with a map interface. These needs gave birth to the Broadcast data pump!
The tool can read BRIFIC databases with aplomb and has a number of file import filters for many types of ITU eNotice and the CEPT formats from Wiesbaden 95 and Chester 97. When it comes to export it has good support for exporting to Excel, Google Earth and planning tools such as ATDI ICS Telecom along with some limited support for LS Telcom CHIRplus_BC. The tool even has support for various legacy tools for Band II FM like ITU GE84PLN and even for EBU LEGBAC (requires a compiled version of the FORTRAN source).
The tool is available for download here as a free version with some of the specialist database connectors and obscure tools removed. Beyond this I may still develop this software occasionally to correct or add minor new features on request and some ad-hoc support, but just don’t ask for a manual!
GPS gives precise and reasonably accurate fixes of position. However, as with all complex systems its limitations need to be understood so that it can be used appropriately.
In radio system planning getting a reasonable accurate three dimensional coordinate for the location of an antenna or the base of a mast can be useful for predicting signal propagation to help assess coverage or potential interference between systems.
Nowadays, with GPS selective availability turned off, we can typically expect accuracies of the order 20 metres or an order of magnitude better with secondary corrections from Wide Area Augmentation System (WAAS) or other ancillary systems.
The accuracy is a function of many things, including number of satellites in view, their geometry relative the receiver and radio propagation effects along the paths. The error is generally expressed with dilution of precision ratios in space and time, which when all combined result in positions being expressed as confidence within a circular area in the horizontal plane or within a sphere in three dimensions.
The first simple observation is that we have different metrics for error in the horizontal and combined horizontal/vertical planes. Conditions that lead to low error in the horizontal plane may not coincide with the vertical. For example, if all the satellites in view are on the horizon and none overhead then we may get a low horizontal error, but a high vertical error. Typically the vertical error is several times worse than the horizontal.
The size of error in each plane may have disproportionate effects when applied to propagation prediction. This is particularly true when considering diffraction geometry to terrain or clutter obstacles. In the horizontal plane, a 20 metre position error is often insignificant when we are looking at systems with separation distances of the order hundreds of metres or kilometres, especially when using general purpose gridded terrain models to model terrain obstacles surrounding the location of the fix. However, the same kind of error in the vertical plane can lead to gross changes in diffraction geometry, especially when considering antenna heights that are low in comparison to the terrain or local clutter.
The vertical height reported by default in most GPS receivers is the height above the WGS84 ellipse. This ellipse is a hypothetical construct that is an oblate sphere which is a coarse approximation to the Earth’s surface. In fact the Earth has a more complex shape than an oblate sphere, and a better datum for measuring height the is that of a geoid, which is defined as the line where gravity pulls equally.
Many mistake GPS heights for height above mean sea level, which can lead to gross errors. To give an example of the height difference between the GPS ellipse and a geoid model, taking EGM96, which is a global geoid model, then using the handy UNAVCO geoid height calculator page, the geoid height at say Newlyn harbour is 53.5 metres above WGS84. If the raw GPS height is used without correction the vertical height error will be at least 53.5 m along with the other sources mentioned. Coupled with a terrain elevation database for terrestrial propagation prediction and the results will be grossly errored, as most terrain data will be referenced a local datum which will be closer to the geoid. In the case of the Great Britain, the heights are most often quoted against the average tidal height datum measured in Newlyn from 1912-1921. Note that there are usually better local geoid models than the global EGM96, such as the OSGM02 which is generally applicable in Great Britain.
Are GPS heights too good to be true? Well not if used appropriately!
An activity in radio network planning that is all too often left till last, is that of assessing the radiated contribution spilling over international borders to see if the level exceeds the threshold requiring formal coordination. This can be a problem if not tackled up front, because coordination activities can be lengthy.
Within CEPT this situation is anticipated, and there are various criteria agreed to allow radiation either side of a border without extraordinary activity. Typically these agreements permit an administration to implement a station that radiates towards a border, as long as the field strength incident at and beyond the border do not exceed defined limits for not more than 10% of the time. The criteria include common methods for predicting propagation which include long term statistics to satisfy the time criterion.
To expedite network roll-out, it is often interesting to engineer the network design to meet the limits. This can be done by limiting sites close to the border, omitting sectors pointing towards the border, using terrain to screen radiation towards the border, or optimisation of power delivered to the antenna and sector pointing. Often these measures need to be used in combination. For example, a site on a hill close to a border will probably require more than antenna down-tilting to be satisfactory. Of course if a configuration causing these limits to be exceeded is essential, classic coordination activities are still possible, but plenty of time must be allowed, hence it is best to identify sites requiring coordination early on in network planning.
In respect of LTE, there are several recommendations for the various harmonised bands in CEPT including the ECC recommendations (08)02 for 900/1800 MHz, (11)04 for 800 MHz, (11)05 for 2600 MHz and (14)04 for 2300 MHz. The attached CEPT LTE cross border coordination spreadsheet gives an example of how to assess if a site requires coordination for the 800/900/1800 MHz bands, but could be easily extended to cover others.
At a receiving antenna system we generally find a series of signal vectors that arrive and aggregate in positive and negative ways. Often we have a significant wanted signal and multiple unwanted ones from other co-channel users that lead to the planned service being limited by interference rather than noise. The question arises, how to aggregate these signals so that we can compute a wanted to unwanted ratio such that we can ultimately determine the availability of the desired service as we vary location? This post examines some of the approaches and gives a practical spreadsheet example.
Simple summing of the means of these signals does not suffice, because generally they will fade independently due to the diversity of the interference paths, so a meaningful prediction will require a method that takes account of their stochastic nature. To simplify matters these signal are generally characterised by a mean and variance assuming a log-normal distribution, and they will also have that distribution in aggregate. A straight-forward approach is to use Monte-Carlo method to sum many random samples to compute the aggregate mean and variance. With modern computing power this can be done with a high degree of confidence, however, this may require a significant number of samples. If pixel plotting techniques are used to predict the service at a huge number of locations, then the simulation times can be significantly long, this can be limiting when network plans are being evaluated.
However, there are many alternative techniques using numerical approximations to help estimate the sum of log-normal variates that require substantially less computing power. This includes the seminal method by L F Fenton which is often known as Fenton-Wilkinson method. Fenton’s approach has the benefit of being simple, but suffers from significant accuracy issues which are quite limiting when dealing with a large number of different vectors. The Schwartz-Yeh method is an improvement under most circumstances and is especially useful for summing uncorrelated sources, but it is more complex. Other techniques improve upon these methods and may offer better fidelity in dealing with correlation and larger numbers of signals, but there is no perfect method, and Schwartz-Yeh stands as a good general purpose method for a small number of uncorrelated signals.
The interference summing methods compared spreadsheet implements a number of functions that can be called to compare Monte-Carlo, Fenton-Wilkinson and Schwartz-Yeh methods. The spreadsheet contains an example with 20 components that are assumed to be uncorrelated. These components are simply passed as arrays to user defined VBA functions making the worksheet compact and easy to experiment with. Aside from the helper functions, there are only a few core functions whose code is easily modified for re-use in other programming environments.
The disappearance of the MH370 from air traffic control displays was possibly deliberate, or perhaps just the unfortunate result of a series of failures. Time may reveal. Perhaps more worrying is the disappearance of many planes over the well monitored skies over Europe. Earlier this month, there were reports from air traffic control centres over Austria, Germany, Slovakia and the Czech republic, that a number of aircraft disappeared from radar displays, presumably arising from the failure of secondary surveillance radar. Classic symptoms of interference were in evidence, with these aircraft being lost intermittently over a few minutes. Given that multiple radars and multiple planes were subject to the same issue, it seems like a case of unintentional interference, with some rogue system blocking the transponders on the aircraft. Initially a NATO electronic warfare exercise was blamed in Hungary. The defence ministry in Hungary denied the issues citing that the electronic warfare devices only had a range of 4000 metres. Whilst it is credible that these devices are only used for intentional interference over ranges of 4 km, it is rather odd to think that the denial implies that radio waves stop their interference potential at 4 km! However, more convincing is the fact that these military exercises only coincided on one of the occasions where the interference was reported. The investigation continues to try and identify the source, but until then I wonder if we may see some more unscientific comment, including perhaps the odd conspiracy theory or two!
