Chapter Five

1
Chapter Five

• Communication Link Analysis

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

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

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

5
Satellite Transmitter-to-Receiver Link with Loss
and Noise
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

7
Isotropic Radiator
8
Antenna Gain
9
EIRP
10
EIRP with Range Equation

• The relationship between antenna gain and antenna
  effective area Ae
• Power extracted at a receiving antenna
• Received power with EIRP representation
• Received signal power is as a function of
  frequency

11
Received Power as a Function of Frequency
12
Example for Antenna Design for Measuring Path
Loss
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

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

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

16
Earth Coverage Versus Link Margin
17
Noise Figure

• Noise figure, F, denotes the degradation caused
  by the network
• Example for an amplifier

18
Noise Treatment in Amplifiers

• The noise figure, F, can be rewritten as

19
Noise Temperature

• The noise power is relative to the noise
  temperature
• The effective noise temperature of the receiver
• Output noise of an amplifier

20
Line Loss

• SNR degradation due to the signal attenuation

21
Composite Noise Figure

• For the two-stages network
• For the n-stages network

22
Composite Noise Temperature

• For the n-stages network
• The lossy line is in series with the amplifier

23
System Effective Temperature

24
Improve a Receiver Front-end
25
Key Parameters of a Link Analysis