microwave communication system design

7/30/2019 Microwave communication system design

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Lecture-9:LINE OF SIGHT (LOS)

MICROWAVE RADIO LINKS (CONTD)

Instructor: Dr. Munir A. [email protected]

Department of Electrical Engineering, NUST-SEECS

Islamabad

Course: Microwave CommunicationSystem Design (EE-874),

Fall 2012Book: Radio System Design for

Telecommunication, 3rd Edition, by Roger FreemanClass: MS(EE)


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OUTLINE

Path Analysis

Pre-emphesis / De-emphesis

FM Improvement Threshold

Fading

Estimation of Fade Margin

Mitigation of Fading Effects

Analysis of Noise on a FM Radio-link

Frequency Assignment, Compatibility,Frequency Plan


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Path Analysis

The path analysis or link power budget taskprovides the designer with the necessaryequipment parameters to prepare a blockdiagram of the terminal or repeater

configuration and to specify equipmentrequirements both quantitatively andqualitatively.(analog radio links in this chapter)

Being analog LOS links, Noise and S/N only are

the task outputs in the standard voice channelor video channel

We also assume here that the modulationwaveform is conventional FM.


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Path AnalysisObjective and Scope

This section will provide us with the tools to deriveantenna aperture, receiver front endcharacteristics, FM deviation, IF/RF bandwidth,transmitter output power, diversity arrangements if

any , and link availability due to propagation.

The use of NPR (noise power ratio) as a tool tomeasure link noise performance will be described.

The analysis described in this chapter is valid for

radio-links in the 1-10-GHz band.


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Unfaded Signal Level at the Receiver

Simplified model, radiolink path analysis.

Lt and Lr are the transmission line

losses; Gt and Gr are the antenna gains.


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Unfaded Signal Level at the Receiver


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Receiver Thermal Noise Threshold

Law of Boltzmann and Maxwell states that theavailable power per unit bandwidth of a thermalnoise source is:

For a bandwidth of BW Hz

Or


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Receiver Thermal Noise Threshold

Example 8 and 9


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Practical Applications

Simple but Mixer Noise Figure is 8-10dB

In practice LNA (NF of 0.5 tot 2dB) is used in front to

reduce the overall noise figure of the receiving system


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Calculation of IF Bandwidth

The IF of a FM receiver must accommodate the RF

bandwidth, which consists of the total peak deviation

spread and a number of generated sidebands. The IF

bandwidth can be estimated from Carsons rule,

where BIF is the peak frequency deviation and Fm is

the highest modulating frequency given in the middle

column of Table 2.5 of the text book.


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After demodulation in a FMsystem, thermal noise powerin

some texts called idle noise

is minimum for a given signal

at the lowest demodulatedbaseband frequency and

increases at about 6 dB per

octave as the baseband

frequency increases. This

effect is shown in Figure, which

compares thermal noise in an

AM system with that in a FM

system. Example 12


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FM baseband signal at the Tx is amplified gradually

towards the outer edges of the band.

When received, the demodulated signal including noise is

attenuated towards the outer edges of the band to revert

the corresponding amplification done at the Tx. Thisresults in, at the output of the Rx, a constant S/N ratio.


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Calculation of Antenna Gain

From book

EXAMPLES 13 AND 14


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Fading in LOS links

Fading: The variation with time of

the intensity or relative phase, orboth, of any of the frequency

components of a received radiosignal due to changes in thecharacteristics of the propagation

path with time.

Estimation of Fade Margin, (book)


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Mitigation of Fading Effects

FrequencyDiversity

SPACEDIVERSITY


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Sources of Noise in aRadiolink


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FM Improvement Threshold

FM is wasteful ofbandwidth whencompared to AM-SSB

Benefit: bandwidth iscompensated for byan improvement inthermal noise power

Benefit: Thermal

noise is spread atbroader bandwidth


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IM noise

Up to this point we have dealt only withthermal

noise in a radiolink. In an operational

analog radiolink a second type of noisecan be equally important. This isintermodulation IM noise.


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Calculation of Thermal Noise

S is the demodulated signal the channel test tone peak

deviation (Table 2.4)

2))(2(cffktbF

RSL

N

S

f


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Frequency Assignment,Compatibility, Frequency Plan

ITU REGULATIONS

FAB REGULATIONS

EMC/EMI LIMITATIONS

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