WHY LINK BUDGET IN BROADCASTING

This is the new text page Why Link Budget.Why Link Budget The first step in designing a satellite network is performance of a satellite link budget analysis. The link budget will determine what size antennae to use, SSPA or TWTA PA power requirements, link availability and bit error rate, and in general, the overall customer satisfaction with your work. What exactly is a link budget? Like most RF work, it has long been considered by the uninitiated as one of the " Black Arts " involving the dreaded invocation of mathematics, uttering of formula, and skillful estimation of the unknown. The secrets of the successful link budget have long been treated as if they are Masonic in Origin; satellite service providers tend to guard the Holy SFD and Righteous IO Backoff Value to the death…. and beyond. What a link budget actually involves is a relatively simple addition and subtraction of gains and losses within an RF link. When these gains and losses of various components are determined and summed, the result is an estimation of end-to-end system performance in the real world. To arrive at an accurate answer, factors such as the uplink power amplifier gain and noise factors, transmit antenna gain, slant angles and corresponding atmospheric loss over distance, satellite transponder noise levels and power gains, receive antenna and amplifier gains and noise factors, cable losses, adjacent satellite interference levels, and climatic attenuation factors must be taken into account. Fortunately in this age of computers and spreadsheet programs, the link budget does not have to be all that difficult to compute. Several companies now market quite sophisticated link budget calculation programs that contain large databases of information regarding satellite performance parameters, ground station antenna performance data, and other information vital to calculation. With one of these programs, all the user must do is fill in the blanks regarding earth station location, planned satellite(s) to use, required link availability, and "quick as a wink" the program generates a very good estimation of link performance. A very good example of this type of program is the link budget program included in the SatFinder CD Rom available (for both PC and Macintosh) from Design Publishers. If your liquid assets of knowledge exceed those in the bank, you can design a spreadsheet to obtain similar results, assuming you have to time to drag out the IEEE Engineering Handbook and input all of the formulae. Prepare to spend some time interpolating terrestrial microwave data into your satellite application. For those without either a lot of time or money, I have decided to risk the Holy Inquisition, and make my own Excel Version 7.0 Spreadsheet available. (It's also much easier to write an article covering this piece of work than to re-invent the wheel telling you, gentle reader, how to input all the math!) Since "The Half of Knowledge is to Know Where to Find Knowledge" CLICK HERE to download the program, and then read on. When you run the sheet using Microsoft Excel 7.0 for Windows 95, you will notice that some fields are highlighted in BOLD type. These fields are important, as they must contain data relevant to your network. You must change the data from my defaults in order to obtain an accurate calculation of your network's performance. Fields that are italicized indicate end results. As a former college botany professor of mine used to say: " Here's what you will need to know to pass the test. " The saturated EIRP and saturated flux density of the transponder. The satellite G/T figure appropriate to your planned uplink location. Satellite transponder bandwidth. Satellite transponder output backoff or attenuation. Satellite transponder input backoff or attenuation. The above information can generally be obtained from the satellite operator, or from a good satellite database such as SATNEWS.COM. Other sources of this data include printed media, such as the International Satellite Directory also available at SATNEWS. You will also need the following information that you and your customer can supply: Latitude and longitude of the uplink and downlink earth stations. Planned data or information rate. Modulation type (BPSK or QPSK) Forward error correction rate (1/2 or 3/4) Spread Factor - if any (use only for spread spectrum systems) Uplink and Downlink frequencies. Uplink and Downlink antenna sizes. Uplink and Downlink antenna efficiency. Uplink and Downlink transmit and receive gains at frequency. Minimum digital signal strength (EB/No) for desired Bit Error Rate (BER) performance. Once the above information is available, it is a simple matter to plug the data into the spreadsheet and calculate the link budget. In Ku band networks, it is a good rule of thumb to allow 7 or 8 dB of margin above threshold at the receive site with clear sky conditions. This will generally provide a link availability in excess of 99.5%. C band networks require much less margin, typically about 3 dB, for the same performance expectation, since there is less atmospheric attenuation with the C band. Most satellite space providers prefer to sell carrier power levels that are proportional to bandwidth used -- or balanced power/bandwidth service. If your link budget indicates that significantly more power is required than bandwidth indicates, you will want to use a larger receive antennae. This "free gain" will lower the power requirement from the satellite, and therefore, the continuing costs of network operation. " Bandwidth Limited " service, or the use of less power than would be used in balanced services, is sometimes available for even less cost. If you can oversize the network's receive antennas a bit, the difference in hardware costs will more than be paid for in lower bandwidth-limited spacetime fees. Always approach satellite space segment as you would a sexually transmitted disease. Once you've gotten it, you're stuck with it. Anything that can be done to minimize the recurring costs of satellite access should be done before you sign the provider's agreement! Most satellite operators limit satellite received EIRP to a specific maximum level of 6dBW/4kHz, or about minus140 dBW per square meter on the ground. If spectral density exceeds these limits, you should use better LNB's or larger receive antennas to lower the power requirements. You can also spread the signal over greater bandwidth; either by changing FEC rates, changing modulation formats from QPSK to BPSK, or by using some form of additional signal spreading. With today's newer satellites operating at 4 times traditional satellite power levels, the established limits can become a problem with even balanced power / bandwidth carriers. The sample data included in the Excel spreadsheet provided is valid for NSN Network Services' transponder 13 on the new GE-1 satellite, at the locations specified. Performance has been field verified and is accurate to within 1 dB EB/No. As you begin to manipulate the variables, you will quickly "get a grip" on how the various system parameters effect overall network performance. Once you understand what is going on, you can plug in the variables required in your own network applications. So what is a link budget? A link budget is actually not as complicated as it sounds. Put simply, it is the sum of all the losses between your transmitter and the satellite and back down again to a receiver. These losses are reduced by any gain you have at the transmitter, satellite or receiver. So in order to see if your signal is still going to be big enough to use after it has been sent to a receiver via satellite, the gains and losses are effectively added together and the result will be the net gain or loss. A loss means your signal has got smaller, and a gain means it has got bigger. This is a very simplified explanation, but it gives you an idea of what the link budget is trying to calculate. The following more in depth explanations talk you through each major parameter. The maths behind all of this, and there is a lot of it, is not looked at here. Words are much easier to understand than equations i think. These parameters may seem scary or alien to you. Fear not, learning what they are is all that is important at this stage. See also our satcom terms page. The transmitter: The transmit frequency is the RF radio frequency at which this carrier wave is transmitted. The EIRP is a measure of the power which is required to transmit the carrier wave so that it reaches the satellite. The G/T is a measure of the performance of the transmitter and is based on the gain of the transmitter (the amplifier, other parts of the uplink equipment chain and the antenna) and the noise of the equipment in the uplink chain. Just as in an audio system, a noisy amplifier is not as good as a quiet amplifier. The uplink chain is the series of stages the signal goes through before leaving the antenna on its way to the satellite. The Lat and Long is the location of the transmit terminal on the earth. Measured in degrees the latitude and longitude is a global position reference used by the GPS system amongst others. The elevation is the angle up from horizontal (0 degrees) that the antenna must point at to see the satellite in conjunction with azimuth. The azimuth is the compass angle from true north that the antenna must be pointed at to see the satellite in conjunction with elevation. Path loss is the attenuation of the signal due to the inverse square law and the earths atmosphere which reduces the size of your signal on its way to the satellite. Inverse square law is explained here. Losses are the attenuating factors within the transmitter system such as RF radio frequency cable connectors and different types of RF cable. Lastly the margin is used to allow for extra atmospheric attenuation due to localised rain or snow at the transmitter location. The Satellite: Starting with the translation frequency, this is used by the satellite to convert the transmitted signal to a new frequency so that the satellite doesn't retransmit the signal at the same frequency as the transmitter on earth. If it did the two signals would interfere with each other and the result would be unusable. Instead, the signal is moved, usually down in frequency to a 'Receive Band'. The translation frequency is the amount by which the transmitted signal is moved in MHz. The EIRP is again the same for the satellite as it is for the transmit terminal. The G/T is also the same for the satellite as it is for the transmit terminal. The C/No Sat is the carrier signal level to noise level ratio of the transmitted signal when it reaches the satellite. This is a measure of the signal reaching the satellite after travelling through the atmosphere. The Long is the longitude of the satellite. The satellite is located above the equator at a latitude of 0 degrees. Thus, only the longitude is required to identify the satellites location. Ant Gain is the gain of the receive antenna on the satellite. Transp Gain is the gain of the transponder on the satellite, this is in effect one channel of many that are arranged in bands of frequency on the satellite. They can be independantly controlled to increase or decrease the gain. They are sometimes even organised so that they cover different areas of the earth through the use of different antenna systems. Transp Gain is the gain of the satellite transponder. Req'd EIRP is the amount of power the satellite has to use to get your signal back to earth. % EIRP is the percentage of total satellite power available for all of the signals using it, that it must devote to your signal. Pwr @ Sat is the actual power of your signal transmitted from the satellite. The Modem: Data Rate is described at the beginning of this article and is the amount of data you wish to transmit per second. This is measured in bits. Eb/No Req'd is the energy per bit to noise level ratio that is required to provide error free data. This is usually specified by the modem manufacturer. Status is an indication of whether the receivers modem would be happy with your link budget. If it locks, the chances are that the link would be succesful and error free. Link margin is the overall amount of attenuation in any part of the satellite link that can be tolerated by the modems before they loose lock. Losing lock means losing the signal and thus the satellite link is lost, no data received. The Receiver: The Rx Freq is the receive frequency determined by the satellite translation freqency. the receiver must be tuned to this frequency to pick up the signal from the satellite. C/No is the carrier power to noise level ratio which is a measure of how much signal is visible above the noise. In audio terms this would be exactly the same as music and hiss. Less hiss more music. G/T is the same as the transmitter G/T except it is now for the receive terminal. Lat is the same as the transmitter Latitude except it is now for the receive terminal. Long is the same as the transmitter Longitude except it is now for the receive terminal. Elevation is the same as the transmitter Elevation except it is now for the receive terminal. Azimuth is the same as the transmitter Azimuth except it is now for the receive terminal. Path Loss is the same as the transmitter Path Loss except it is now for the receive terminal. Losses is the same as the transmitter Losses except it is now for the receive terminal. Margin is the same as the transmitter Margin except it is now for the receive terminal taking into account any weather at the location of the receive terminal. So Calculate: With all this data and a lot of maths, the calculations can be made and the Link Margin obtained, if the Link Margin is too small, extra losses may occur due to atmospheric conditions which cause the link to fail. Balancing the budget is to end up with either 0 dB if no extra margin is required, or about 3 dB if some protection is needed. The maths is all taken care of behind the scenes on our Online link budget calculator, all you need to know are the parameters for the satellite link.