Communication Link Analysis Pranesh Sthapit Chapter 5

5.4.1 Two E b /N 0 Values of interest E b /N 0 referrers to value of bit energy per noise power spectra density required to yield a specified error probability To facilitate calculating a margin or safety factor M, we need to differentiate between the required (E b /N 0 ) reqd and the received (E b /N 0 ) r Safety margin or link margin M insures that actually received signal would be somewhat larger than the required signal The difference in decibels between (E b /N 0 ) reqd and the received (E b /N 0 ) r yields the link margin

Link margin equation We have, and Substituting in, We get, G r is gain of receiving antenna and T ̊ is the effective system temperature. EIRP is the power associated with electromagnetic wave at the output of the transmitting antenna The link margin equation contains all the parameters contributing to the link’s error performance Each of these parameters and EIRP is defined with reference to a particular system location Link margin equation

5.4.2 Link Budgets are Typically calculated in Decibels Since link budget analysis is typically calculated in decibels, we can express link margin equation as All terms are in decibels (db) The numerical value of parameters constitute the link budget, a useful tool for allocating communication resources We might trade off any parameter with any other parameter to maintain good margin Link margin equation

5.4.3 How much Link Margin is Enough The margin needed depends on how much confidence one has about the system For system employing new technology or new operating frequency need more margin than for system that have been repeatedly built and tested Link budget provides an allowance for fades due to weather and rain degradation Designs using higher frequencies (e.g. 12/14GHz) generally requires larger margins because atmospheric losses increase with frequency and are highly variable attenuation due to atmospheric loss causes greater antenna noise small weather changes can result in increase of 40 to 50 k in antenna temperature

5.4.4 Link Availability Means the percentage of time the link is close or available on an average annual basis for a given geographical location –E.g. 98% means 98% of time the link is available (for given Margin) and 2% of time has problem –SNR degradation is a function of signal frequency –Link availability and required margin must be examined in the context of a particular transmission frequency

5.4.4 Link Availability Figure summarizes worldwide satellite link availability at 44Ghz Plot illustrates percentage of the earth visible (the link closes and a prescribed probability of error is met) as a function of margin for 3 geostationary satellite Figure shows a family of visibility curves with different required link availabilities In general, for fixed link margin, visibility is inversely proportional to required availability and for flexed availability, visibility increases monotonically with margin

5.4.4 Link Availability Earth coverage (unshaded) for 0.99 link availability for three geostationary satellites (1) Longer path results in reduce power density at receiver (2) Reduced satellite antenna gain (3) Propagation to the edge of earth traverse a thicker atmospheric layer

5.5.1 Noise Figure Noise figure, F, is ratio of SNR at the input to the SNR at the output F measures the SNR degradation caused by the network the amplifier has 10 db noise figure Noise figure express the noisiness of a device compare to the reference noise source at the input port

Noise Figure Noise figure is a parameter that expresses the noisiness of a device compared with a reference noise source at the input and given by above equation –S i is signal power at the amplifier input port –N i is noise power at the amplifier input port –N ai is amplifier noise referred to the input port –G is amplifier gain Noise figure expresses the noisiness of a network relative to an input source noise The noisiness of a system is measure in basis of N i IEEE standard define noise to measure at T ̊ o =290k, the value of noise spectral density N 0 is given as

5.5.2 Noise Temperature T ̊ o (290k) is the reference temperature of the source and T ̊ R is called the effective noise temperature of the receiver The noisiness of an amplifier can be modeled as if it were caused by an additional noise source operating at some effective temperature T ̊ R using

5.5.3 Line Loss Line loss is SNR degradation result from the signal being attenuated while the noise remains fixed We shall define power loss as Then the network gain G equal 1/L (less than 1 for a lossy line).

5.5.4 Composite Noise Figure and Composite Noise Temperature When two networks are connected in series, their composite noise figure can be written as G 1 is the gain associated with network 1 When n networks are connected in series F 1 should be low as possible Similarly composite effective noise temperature is given by

5.5.4 Comparison of Noise Figure and Noise Temperature Noise Figure and Noise Temperature characterize the noise performance of device Noise Figure and Noise Temperature has equal importance For terrestrial application, F is almost universally used For space application, T ̊ is the more common figure of merit In general, application involving very low noise device seem to favor the effective temperature measure over the noise figure

5.5.5 System Effective Temperature Receiving system: antenna, line, and the preamplifier play the primary role in SNR degradation –Amplifier injects additional noise into the link –Line attenuates the signal Other noise source –Natural: lightning, weather, thermal radiation –Man made: electrical machinery, radio transmission

5.5.5 System Effective Temperature Total noise contributed by all external sources can be characterized by k T ̊ a W where T ̊ a is known as the antenna temperature Noise depends on antenna temperature The antenna temperature is a measure of the effective temperature integrated over the entire antenna pattern System temperature T ̊ s by adding together all the system noise contributors T ̊ a is the antenna temperature and T ̊ comp is the composite temperature of the line and the preamplifier T ̊ a and T ̊ comp are the two primary sources of noise and interference T ̊ a represents degradation form the outside world T ̊ comp is thermal noise cause by the motion of electrons in all conductors