Course: Microwave Communication System Design (EE-874), Fall 2012 Book: Radio System Design for Telecommunication, 3rd Edition, by Roger Freeman Class: MS(EE)
Lecture-9: LINE OF SIGHT (LOS) MICROWAVE RADIO LINKS (CONT’D) Instructor: Dr. Munir A. Tarar
[email protected] Department of Electrical Engineering, NUST NU ST-SEECS -SEECS Islamabad
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
Path Analysis • The path analysis or link power budget task
provides the designer with the necessary equipment parameters to prepare a block diagram of the terminal or repeater configuration and to specify equipment requirements both quantitatively and qualitatively.(analog radio links in this chapter) • Being analog LOS links, Noise and S/N only are the task outputs in the standard voice channel or video channel • We also assume here that the modulation waveform is conventional FM.
Path Analysis Objective and Scope • This section will provide us with the tools to derive
antenna aperture, receiver front end characteristics, 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 to measure link noise performance will be described. • The analysis described in this chapter is valid for radio-links in the 1-10-GHz band.
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.
Unfaded Signal Level at the Receiver
Unfaded Carrier to Noise Ratio for a Receiver • Objective and Basic Calculation • To calculate the unfaded carrier-to-noise ratio C/N
• where Pt is the receiver thermal noise threshold.
Note that RSL and Pt must be in the same units, conventionally in dBm or dBW.
Receiver Thermal Noise Threshold • Law of Boltzmann and Maxwell states that the
available power per unit bandwidth of a thermal noise source is:
• For a bandwidth of BW Hz
• Or
Receiver Thermal Noise Threshold
• Example 8 and 9
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
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 Carson’s 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.
Pre-emphesis / De-emphesis After demodulation in a FM system, thermal noise power in some texts called ‘‘ idle noise’’ is minimum for a given signal at the lowest demodulated baseband 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
Pre-emphesis / De-emphesis 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. This results in, at the output of the Rx, a constant S/N ratio.
Calculation of Antenna Gain • From book • EXAMPLES 13 AND 14
Fading in LOS links • Fading: “The variation with time of
the intensity or relative phase, or both, of any of the frequency components of a received radio signal due to changes in the characteristics of the propagation path with time.” • Estimation of Fade Margin, (book)
Mitigation of Fading Effects • Frequency
Diversity • S PACE D IVERSITY
Sources of Noise in a Radiolink
FM Improvement Threshold • FM is wasteful of
bandwidth when compared to AM-SSB • Benefit: bandwidth is compensated for by an improvement in thermal noise power • Benefit: Thermal noise is spread at broader bandwidth
IM noise Up to this point we have dealt only with thermal noise in a radiolink. In an operational analog radiolink a second type of noise can be equally important. This is intermodulation IM noise.
Calculation of Thermal Noise S N
RSL ( 2ktbF )(
f
f c
)
2
• S is the demodulated signal • f the channel test tone peak
deviation (Table 2.4)
Frequency Assignment, Compatibility, Frequency Plan • ITU REGULATIONS • FAB REGULATIONS • EMC/EMI LIMITATIONS
