 Welcome to the session. I am Mr. Praveen Yalapakumbar. Today we want to see design of downlinks. The learning outcome of this topic is at the end of the session student will be able to explain the concept of downlink design in satellite communication. The content of these topics are noise figure and noise temperature, GY2 ratio for R stations, design of downlinks and link buzzers. Noise figure and noise temperature. Noise figure is frequently used to specify the noise generated within a device. The operational noise figure is defined by the following formula. The noise figure is defined as ratio of signal to noise ratio at the input to the signal to noise ratio at the output. To convert this noise figure to noise temperature T because noise temperature is more useful in satellite communication system. The relationship between the noise figure and noise temperature is defined as T is equal to T0 in bracket NF minus 1 where NF is noise figure. T0 is the reference temperature used to calculate the standard noise figure and the value for T0 is 290 Kelvin. This is a table which comparison of noise temperature and noise figure. Now for noise temperature at 0 Kelvin the noise figure is 0 dB. For the different specific temperature here we calculate the noise figure in dB. GYT ratio for R stations. The link equation can be rewritten in terms of C by N at the R station is C by N is equal to PTGTGR divided by KTSBN into lambda by 4 power bracket square. Now this equation we want to simplify. We get as PTGT divided by KBN into lambda by 4 power bracket square into GR by TS where GR by TS is the G by T in decibels that is decibel per Kelvin which specify the quality of a receiving a station or a satellite receiving system. Now design of downlinks. The design of any satellite communication is mainly based on the two objectives. The first one is meeting a minimum C by N ratio for a specified percentage of time. Second one is carrying the maximum revenue earning traffic at minimum cost. Any satellite link can be designed very large antenna to achieve the high C by N ratio under all conditions. Now for the high C and ratio here we use the large antenna therefore obviously the cost for that will be the increases. The art of a good system design is depends on the best compromise of system parameters that meets the specification at the lowest cost. Now all the satellite communication links are affected by the rain attenuation. Now we are taking an example for the different bands which the rain affected at that satellite communication. Now for example at the 6 by 4 gigahertz band the rain attenuation is small. But now we compare that 14 by 11 gigahertz band that is also called as a KU band the rain attenuation is more as compared to 6 by 4 gigahertz. Now at the 30 by 20 gigahertz that is also called a KA band the rain attenuation is more as compared to the KU band and obviously it is also even more as compared to the 6 by 4 gigahertz band. C links here can be designed to achieve the 99.99 percentage because here rain attenuation we can consider that factor is one or two dB and for this achievement the time requirement is 0.01 percentage of the year that is equivalent to the 52 minutes. Now here rain attenuation statistics is not stable. Again how is the C links is designed the same procedure we use for the KL links can be designed. But here we cannot be achieved the 99.99 percentage because here rain attenuation is 10 or 20 dB and again here same time we use that is 0.01 percentage of the year that is 52 minutes. Now if you observe these two links we designed here the C links is used for the real time and K is used for the another purpose. Now for the telephone traffic needs real time channels that are maintained for the duration of a call therefore we use C or KU band. Now for the internet transmission is less affected by short outage therefore we use K band suited for internet access. Link budgets CN ratio calculation is simplified by the use of link budgets. A link budget is a tabular method for evaluating the received power and noise power in a radio link. Link budgets invariably use decibel units for all quantities so that signal and noise power can be calculated by addition and subtraction. The link budget calculated at an individual transponder repeated for the each of the individual links therefore in a two way satellite communication there are total repeated means four times therefore in a two way satellite communication link we use four separate links each require a calculation of a CN ratio. Now the overall CN ratio is calculated with the help of uplink as well as the downlink CN ratio therefore when a band pipe transponder is used the uplink and downlink CN ratio must be combined to give an overall CN. The calculation of C by N in a satellite link is based on the two equations. Now first equation is on the received signal power and second one is receiver noise power. Now the received carrier power in dB watts as now here pause the video and recall the received carrier power okay now the received carrier power is equal to EIRP plus GR minus LP minus LA minus LTA minus LA the unit of this received carrier power is dB watt where EIRP is equal to 10 log to the base 10 PT GT that's the unit of EIRP is dB watt. GR is equal to 10 log to the base 10 4 pi A divided by lambda square the unit of that GR is dB. Path loss LP is equal to 10 log base 10 4 pi R by lambda bracket square LA is equal to attenuation in atmosphere LTA is equal to losses associated with transmitting antenna LRA is equal to losses associated with receiving antenna. A noise power PN referred to the output terminal of the antenna where PN is equal to k into TS into BN that unit of that PN is watts where TS is a receiving terminal with a system noise temperature in Kelvin, BN is a noise bandwidth in hertz. The receiving system noise power is usually written in decibel unit as N is equal to k plus TS plus BN the unit of that noise power is dB watt where k is a Boltzmann constant TS is the system noise temperature in dB Kelvin, BN is the noise bandwidth of the receiver in dB hertz. Now the question is an earth station antenna has a diameter of 30 meter has an overall efficiency of 68 percentage and is used to receive a signal at 4150 megahertz at this frequency the system noise temperature is 79 Kelvin where the antenna points at the satellite at an elevation angle of 20 a degree what is the earth station G by T ratio under these conditions. Now basically in this question I have to calculate the G by T ratio. Now for the calculation of a G by T ratio first of all we calculate the gain and after that calculate this temperature and after the calculation of this gain and temperature we can calculate the G by T ratio. Now first of all we have to write the given quantity what is a given quantity at 4150 megahertz lambda is equal to 0.0723 meter then the formula for the gain is NA into in bracket pi by D by lambda bracket complete square. Now put the values of the in this equation we get this NA means efficiency is 0.68 that is a 68 percent is provided in this question. Therefore we write here 0.68 into pi into this D D means what diameter that is a 30 meter we have to write here 30 divided by lambda. So lambda at this megahertz the lambda is 0.0723 we have to put here and the bracket square and after this solving after the simplification of this one we get 1.16 into 10 raise to 6. Now convert this into the DB why because if we convert into this DB here we calculation of a G by T ratio for this calculation of a G by T ratio if we calculate in the DB simply we subtract that one and we get this ratio converting TS into the DBK because here it is a calculated in DB therefore the TS is also calculated we want into the DB. For the calculation of a TS into DB 10 log 79 that is gives the 19 DBK. Now the we have to calculate here G by T ratio. Now the G by T ratio in the decibel we calculate by simply the subtraction of T from G. Therefore G by T is equal to 60.6 minus 19 we get 41.6 DB per Kelvin that is a G by T ratio. The reference for this topic is thank you.