Dept. of EE, NDHU 1 Chapter Five Communication Link Analysis

Dept. of EE, NDHU 2 Introduction Communication link encompasses the path from the information source to the information sink –Through all the encoding, modulation, the transmitter, the channel and the receiver with all the signal processing Link budget –The result of the communication link –Describes the apportionment of transmission and reception resources, noise sources, signal attenuation –Help one learns if the system will meet many of its requirements Link analysis is to determine the actually system operating point in the BER curve

Dept. of EE, NDHU 3 The Channel The free space –Free of all hindrances of RF propagation, such as absorption, reflection, refraction or diffraction –RF energy arriving at the receiver is assumed to be a function only of distance from the transmitter Error performance degradation –SNR degrades through the decrease of the desired signal power or through the increase of noise power –Not considered ISI in the link budget

Dept. of EE, NDHU 4 Sources of Signal Loss and Noise Transmitting terminal –Bandlimitting loss –Modulation loss –Antenna efficiency Channel –Pointing loss –Atmospheric loss and noise Receiving terminal –Antenna efficiency –Receiver loss

Dept. of EE, NDHU 5 Satellite Transmitter-to-Receiver Link with Loss and Noise

Dept. of EE, NDHU 6 Received Signal Power The range equation –Relate power received to the distance between the transmitter and the receiver –Transmitted power density –Power extracted at a receiving antenna Effective radiated power to an isotropic radiator (EIRP) –The product of the transmitted power and the gain of the transmitting antenna gain –

Dept. of EE, NDHU 7 Isotropic Radiator

Dept. of EE, NDHU 8 Antenna Gain

Dept. of EE, NDHU 9 EIRP

Dept. of EE, NDHU 10 EIRP with Range Equation The relationship between antenna gain and antenna effective area A e Power extracted at a receiving antenna Received power with EIRP representation Received signal power is as a function of frequency –

Dept. of EE, NDHU 11 Received Power as a Function of Frequency

Dept. of EE, NDHU 12 Example for Antenna Design for Measuring Path Loss

Dept. of EE, NDHU 13 Thermal Noise Power Thermal noise is modeled as an AWGN process in communication systems Physical model for thermal noise maximum available thermal noise power

Dept. of EE, NDHU 14 Link Budget Analysis The quantity of greatest interest is the SNR for the receiving-system SNR is sometimes called carrier-to-noise ratio (C/N) Pr/N representation Two Eb/N0 values of interest –Required Eb/N0 –Received Eb/N0 –Link margin M

Dept. of EE, NDHU 15 Link Margin Design The margin needed depends on how much confidence one has in each of the link budget entries Sometimes the link budget provides an allowance for fades due to weather directly Examples of the link margin –Satellite communication at C-band (uplink at 6 GHz, downlink at 4 GHz)  1 dB link margin –Satellite telephone system (INTELSAT system)  4 to 5 dB –Designs using higher frequency (14/12 GHz) generally call for larger margins The margin will be positive if “the link can be closed”

Dept. of EE, NDHU 16 Earth Coverage Versus Link Margin

Dept. of EE, NDHU 17 Noise Figure Noise figure, F, denotes the degradation caused by the network Example for an amplifier

Dept. of EE, NDHU 18 Noise Treatment in Amplifiers The noise figure, F, can be rewritten as

Dept. of EE, NDHU 19 Noise Temperature The noise power is relative to the noise temperature The effective noise temperature of the receiver Output noise of an amplifier

Dept. of EE, NDHU 20 Line Loss SNR degradation due to the signal attenuation

Dept. of EE, NDHU 21 Composite Noise Figure For the two-stages network For the n-stages network

Dept. of EE, NDHU 22 Composite Noise Temperature For the n-stages network The lossy line is in series with the amplifier

Dept. of EE, NDHU 23 System Effective Temperature

Dept. of EE, NDHU 24 Improve a Receiver Front-end

Dept. of EE, NDHU 25 Key Parameters of a Link Analysis