Radio Propagation

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S/N ratio should be above a certain threshold (a function of modulation scheme). Signal power should be above the sensitivity threshold. Predict received signal strength when the transmitter and receiver have a clear line-of-sight path between them.

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Nội dung Text: Radio Propagation

Chapter 2: RADIO PROPAGATION Đặng Lê Khoa Email: dlkhoa@fetel.hcmuns.edu.vn Facuty of Electronics & Telecommunications, HCMUS 1
Understanding RF Propagation Goals 1. Estimate radio coverage area 2. Estimate link performance 3. Estimate network design parameters 1. Transmitters and their location 2. Transmit power 3. Antenna type
Conditions for correct reception Pt sensitivity threshold Noise C power A B  S/N ratio should be above a certain threshold (a function of modulation scheme)  Signal power should be above the sensitivity threshold What will happen if noise level increases? What will happen in presence of another transmitter?
Interesting Scenarios At which locations will correct reception take place?
Antenna Basics Pdirectional G= Pisotropic High gain Dipole Isotropic directional 0 dBi 2.2 dBi 14 dBi
Free Space Propagation Model PT Isotropic power 2 PDi = PR W / m density 4πd 2 d PT PT GT Power density along PD = the direction of 4πd 2 maximum radiation Power received by PR = PD Aeff Antenna λ2 PT GT Aeff = PR = Aeff 4π G 4πd 2 Predict received signal strength when the transmitter and receiver have a clear 2 Also known λ line-of-sight path between them PR = PT GT GR  as Friis free   4πd  space formula
Near Field and Far Field 2 λ PR PR = PT GT GR    4πd  Pt Good predictor when 2D 2 Largest physical df receiver is in far field df = dimension of antenna (the region beyond df) λ near field Example Find the near field distance for an antenna with max dimension of 1m And operating frequency of 900 Mhz Answer λ at 900 Mhz = 33 cm 2(1) 2 df = = 6m 0.33
Path Loss (relative measure) 2 λ PR PR = GT GR    4πd  PT Pt 0.57 *10 −3 f is in MHz PR = GT GR d is in Km (df ) 2 PT  PR    = (GT ) dB + (GR ) dB − (32.5 + 20 log10 d + 20 log10 f ) P   T  dB Path Loss represents signal attenuation (measured on dB) between the effective transmitted power and the receive power (excluding antenna gains)
Path Loss (Example) Assume that antennas are isotropic. PR Calculate receive power (in dBm) at free space distance of 100m from the antenna. What is PR at 10Km? Pt  PR  50 W   = (GT ) dB + (GR ) dB − (32.5 + 20 log10 d + 20 log10 f ) P  = 47 dBm  T  dB  PR  59   = 0 + 0 − (32.5 + 20 log10 0.1 + 20 log10 900) P   T  dB 20 (for d = 10) -20 (for d = 0.1)  PR   PR    = −111.5dB   = −71.5dB P  P   T  dB  T  dB ( PR ) dBm = 47 − 111.5 = −64.5dBm ( PR ) dBm = 47 − 71.5 = −24.5dBm
Path Loss (another example) Path Loss Vs. Distance 2.4 GHz 5 GHz 160 140 120 100 Path Loss (dB) 80 60 40 20 0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Distance (Km)
Path Loss (another example) Path Loss Vs. Distance 2.4 GHz 5 GHz 150 140 130 120 Path Loss (dB) 110 100 90 80 70 60 0.01 0.1 1 10 100 Distance (Km) Log Scale
Quiz questions A B C Pt 2d d 10d PA PB PC  PR    = (GT ) dB + (GR ) dB − (32.5 + 20 log10 d + 20 log10 f ) P   T  dB Path loss between point A & point B ?  Path loss between point A & point C ?  Path loss between point B & C ?  If I use a 10dBi gain antenna at the transmitter end, by how  much will the receive power increase at point A?
In real life How signal attenuates with distance ?
Radio propagation: path loss near field path loss in 2.4 Ghz band Pr r ≤ 8m r > 8m Pt far field near field ∝ ∝ r3.3 r2 r Pr path loss = 10 log (4π r2/λ ) r ≤ 8m = 58.3 + 10 log (r3.3 /8) r > 8m
Indoor Signal Measurement Signal Strength Channel 3 Channel 4 Channel 5 Channel 10 0 -10 1 61 121 181 241 301 361 421 -20 -30 RSSI -40 -50 -60 -70 -80 -90 Time
Outdoor P2P Link Signal Measurement Signal Strength -81 -83 -85 dBm -87 -89 -91 -93 -95 1 51 101 151 201 251 Packet number
BASICS OF RF PROPAGATION AND FADING Facuty of Electronics & Telecommunications, HCMUS 17
Basic Questions Tx What will happen if the transmitter Transmit power, data rate, - changes transmit power ? signal bandwidth, frequency - changes frequency ? tradeoff - operates at higher speed ? What will happen if we conduct this experiment in different types of environments? Channel efffects Desert Metro Street Indoor Rx What will happen if Effect of mobility the receiver moves?
Fundamental Design considerations Noise, interference spectrum power Recovered Data signal x(t) data signal C ratio must be I above some threshold for correct reception Channel Attenuation Distortion
Understanding the effect of attenuation Parabolic 24 dBi Yagi 14 dBi Patch 8 dBi Omni 20 dBm 0 dBi cable loss