CS-435 spring semester 2016 Network Technology & Programming Laboratory University of Crete Computer Science Department Stefanos Papadakis & Manolis Spanakis
CS-435 Lecture preview Wireless Networking Radio Communications Explored
Radio transmission: two endpoints Tx Rx
Radio transmission: two endpoints Signal wave Propagation path Tx Rx Propagation medium Signal transformations due to natural phenomenon; attenuation, external noise, fading, reflection, diffraction, refraction, and interference
Interference (!) Interference: anything which alters, modifies, or disrupts a signal during transmission over a wireless channel. Superposition of unwanted signals to a useful signal. Examples : Electromagnetic interference (EMI): disturbance of an electrical circuit due to electromagnetic induction or electromagnetic radiation emitted from an external source Co-channel interference (CCI): different radio transmitters using the same frequency Adjacent-channel interference (ACI) (filter interference) Inter-symbol interference (ISI): distortion of a signal in which one symbol interferes with subsequent symbols Inter-carrier interference (ICI), caused by Doppler shift in OFDM modulation. Conducted interference (noise interference) Inter/Intra-flow interference refers to the interference between source sharing the same busy channel of path.
Interference (!) Everything on same channel sum all powers On different channels inter-channel power quotient(proportion)
Thermal Noise Thermal noise due to agitation of electrons Present in all electronic devices and transmission media Cannot be eliminated Function of temperature Particularly significant for satellite communication
Noise Terminology Intermodulation noise occurs if signals with different frequencies share the same medium Interference caused by a signal produced at a frequency that is the sum or difference of original frequencies Crosstalk unwanted coupling between signal paths Impulse noise irregular pulses or noise spikes Short duration and of relatively high amplitude Caused by external electromagnetic disturbances, or faults and flaws in the communications system
Other Impairments Atmospheric absorption water vapor and oxygen contribute to attenuation Multipath obstacles reflect signals so that multiple copies with varying delays are received Refraction bending of radio waves as they propagate through the atmosphere
SNR / SIR / SINR What is interference? What is noise? noise floor: noise factor / noise figure: SNR / SINR / SIR:
Sensitivity SINR is not the only criterion for reception! The Received Signal power must be over a threshold Vendors usually provide only RSS thresholds, not SINR
Sensitivity
Rates vs. Sensitivity/SINR Different modulation schemes have different constellations Denser constellations carry more bits/point higher rate increased BER needs larger SINR
802.11a/g OFDM
Multipath Propagation
Multipath Propagation Reflection - occurs when signal encounters a surface that is large relative to the wavelength of the signal Diffraction - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave Scattering occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less
Classical two-ray (ground model)
The Effects of Multipath Propagation Multiple copies of a signal may arrive at different phases If phases add destructively, the signal level relative to noise declines, making detection more difficult Intersymbol interference (ISI) One or more delayed copies of a pulse may arrive at the same time as the primary pulse for a subsequent bit
Types of Fading Fast fading Slow fading Flat fading Selective fading Rayleigh fading Rician fading
Fading in a mobile environment The term fading refers to the time variation of received signal power caused by changes in the transmission medium or paths. Atmospheric condition, such as rainfall The relative location of various obstacles changes over time
The fading channel Additive White Gaussian Noise (AWGN) channel thermal noise as well as electronics at the transmitter and receiver Rayleigh fading there are multiple indirect paths between transmitter and receiver and no distinct dominant path, such as an LOS path Rician fading there is a direct LOS path in additional to a number of indirect multipath signals
Fading: Small and Large scale
Path Loss Free Space propagation model: Two Ray (Ground Reflection) model: Log Distance model
Measured indoor path loss
Measured large-scale path loss
Path Loss Exponent for Different Environments
Signal Propagation Reflection Diffraction Scattering MultiPath Fading Shadow
Radio Propagation Model An empirical mathematical formulation for the: characterization of radio wave propagation as a function of : frequency, distance and other conditions A single model developed to predict the behavior of propagation for similar links under similar constraints formalize the way radio waves are propagated from one place to another Goal : predict the path loss along a link or the effective coverage area of a transmitter.
Propagation Modes Ground-wave propagation Sky-wave propagation Line-of-sight propagation
Ground Wave Propagation Follows contour of the earth Can Propagate considerable distances Frequencies up to 2 MHz Example : AM radio
Sky Wave Propagation Signal reflected from ionized layer of atmosphere back down to earth Signal can travel a number of hops, back and forth between ionosphere and earth s surface Reflection effect caused by refraction Examples Amateur radio CB radio
Line-of-Sight Propagation
Line-of-Sight Propagation Transmitting and receiving antennas must be within line of sight Satellite communication signal above 30 MHz not reflected by ionosphere Ground communication antennas within effective line of sight of each other due to refraction Refraction bending of microwaves by the atmosphere Velocity of electromagnetic wave is a function of the density of the medium When wave changes medium, speed changes Wave bends at the boundary between mediums
Fresnel Zone The area around the visual line-of-sight that radio waves spread out into after they leave the antenna. This area must be clear or else signal strength will weaken.
Fre s nell Zone (silent s ) We know that : Each wave-front point creates new circular waves Microwave beams widen, and Waves of one frequency can interfere with each other Fresnel zone theory: looks at a line from T to R, and at the space around that line that contributes to what is arriving at point R. Some waves travel directly from T to R, while Others travel on paths off axis. their path is longer, introducing a phase shift between the direct and indirect beam Whenever a phase shift is one full wavelength, you get constructive interference: the signals add up optimally Taking this approach and calculating accordingly, you find that: there are ring zones around the direct line T to R which contribute to the signal arriving at point T.
Fre s nell Zone (silent s ) There are many possible Fresnel zones, but we are concerned with zone 1. If this area were blocked by an obstruction, e.g. a tree or a building, the signal arriving at the far end would be diminished. We need to make sure that these zones be kept free of obstructions usually we check that 60 percent of the first Fresnel zone is kept free. A formula for calculating the radius of the first Fresnel zone: r 1 2 N d d 1 2 r N is the radius of the zone in meters N is the zone to calculate (i.e. N=1) d 1 and d 2 are distances from the obstructing screen at height h λ is the wavelength h<<d1,d2 and h>>λ N d d
At the Receiver Signal of Interest Account path loss + delayed reflections Interference Transmissions in the same or neighboring channels/frequencies Noise Thermal + System noise
Antennas The antenna provides three fundamental properties Gain Direction Polarization Gain: (pos/neg) increase in power Direction: transmission shape/pattern Polarization: electric field oscillation axis orientation
Antennas Near field Far field / Fraunhofer region
Antennas
Near/Far Field
Omni-directional Antenna Patterns
Directional Antennas Patterns
Received Power Effective Isotropic Radiated Power
Link Budget Predict the wireless link Estimate the Received Power => Rate Use db (additions & subtractions)
Link Budget
Link Budget Example We want to estimate the feasibility of a 5km link, with one access point and one client radio. The access point is connected to an omnidirectional antenna with 10dBi gain, while the client is connected to a sectorial antenna with 14dBi gain. The transmitting power of the AP is 100mW (or 20dBm) and its sensitivity is -89dBm. Cable losses for both the Rx and the Tx are the same at 2 dbm The transmitting power of the client is 30mW (or 15dBm) and its sensitivity is -82dBm.
Link Budget Example (cont.) Adding up all the gains and subtracting all the losses for the AP to client link gives: 20 dbm (TX Power Radio 1) + 10 dbi (Antenna Gain Radio 1) + 14 dbi (Antenna Gain Radio 2) - 2 dbm (Cable loses Rx) - 2 dbm (Cable loses Tx) -------------------------------------------------- 40 db = Total Gain The path loss for a 5km link, considering free space loss is: Path Loss = 40 + 20log(5000) = 113 db Subtracting the path loss from the total gain 40 db - 113 db = -73 db Since -73dB is greater than the minimum receive sensitivity of the client radio (-82dBm), the signal level is just enough for the client radio to be able to hear the access point. There is 9dB of margin (82dB -73dB)
Link Budget Example (cont.) Next we calculate the link from the client back to the access point: 15 dbm (TX Power Radio 2) + 14 dbi (Antenna Gain Radio 2) + 10 dbi (Antenna Gain Radio 1) - 2 dbm (Cable loses Rx) - 2 dbm (Cable loses Tx) -------------------------------------------------- 35 dbm = Total Gain Obviously, the path loss is the same on the return trip. So our received signal level on the access point side is: 35 db - 113 db = -78 db The receive sensitivity of the AP is -89dBm, this leaves us 11dB of margin (89dB -78dB) For the case of 802.11b (2,4GHz) E.I.R.P is 20dBm IS EVERYTHING OK? ANY PROBLEMS?. (think about it)
Link Budget (homework) Exercise 1: 802.11g, 54Mbps => -73dBm sens. Tx Power 20dBm EIRP 30dBm distance covered? Exercise 2: 802.11g 2km distance EIRP 20dBm achievable rate?
References (images/material) Wireless Communications - Principles and Practice (Second Edition), by Theodore S. Rappaport
APPENDIX
When using Watt: multiply, divide When using db/dbm: add, subtract Algebra The decibel (db) is a logarithmic unit that indicates the ratio of a physical quantity (usually power or intensity) relative to a specified or implied reference level Decibel suffix: dbm: indicates that the reference quantity is one milliwatt dbi : db(isotropic) the forward gain of an antenna compared with the hypothetical isotropic antenna, which uniformly distributes energy in all directions.
Decibel Relative measurement unit: Examples Rule of thumb: +10dB <=> x10
Decibel Rule of thumb: +3dB <=> x2 10 mw + 3 db = 20 mw 100 mw - 3dB = 50 mw 10 mw + 10 db = 100 mw 300 mw - 10 db = 30 mw
Decibel From db to units: -3dB = half the power in mw +3dB = double the power in mw -10dB = one tenth the power in mw +10dB = ten times the power in mw
more algebra
Basic Encoding Techniques Digital data to analog signal Amplitude-shift keying (ASK) Amplitude difference of carrier frequency Frequency-shift keying (FSK) Frequency difference near carrier frequency Phase-shift keying (PSK) Phase of carrier signal shifted
Amplitude modulation
Basic Encoding Techniques
Amplitude-Shift Keying One binary digit represented by presence of carrier, at constant amplitude Other binary digit represented by absence of carrier where the carrier signal is Acos(2πf c t)
Binary Frequency-Shift Keying (BFSK) Two binary digits represented by two different frequencies near the carrier frequency where f 1 and f 2 are offset from carrier frequency f c by equal but opposite amounts
Multiple Frequency-Shift Keying (MFSK) More than two frequencies are used More bandwidth efficient but more susceptible to error f i = f c + (2i 1 M)f d f c = the carrier frequency f d = the difference frequency M = number of different signal elements = 2 L L = number of bits per signal element
Phase-Shift Keying (PSK) Two-level PSK (BPSK) Uses two phases to represent binary digits
Phase-Shift Keying (PSK) Differential PSK (DPSK) Phase shift with reference to previous bit Binary 0 signal burst of same phase as previous signal burst Binary 1 signal burst of opposite phase to previous signal burst
Phase-Shift Keying (PSK) Four-level PSK (QPSK) Each element represents more than one bit
Phase-Shift Keying (PSK) Multilevel PSK Using multiple phase angles with each angle having more than one amplitude, multiple signals elements can be achieved D = modulation rate, baud R = data rate, bps M = number of different signal elements = 2 L L = number of bits per signal element
Performance Bandwidth of modulated signal (B T ) ASK, PSK FSK B T =(1+r)R B T =2DF+(1+r)R R = bit rate 0 < r < 1; related to how signal is filtered DF = f 2 -f c =f c -f 1
Performance Bandwidth of modulated signal (B T ) MPSK MFSK l = number of bits encoded per signal element M = number of different signal elements
Performance Bandwidth efficiency The ratio of data rate to transmission bandwidth (R/B T ) For MFSK, with the increase of M, the bandwidth efficiency is decreased. For MPSK, with the increase of M, the bandwidth efficiency is increased.
Performance
Performance
Performance Tradeoff between bandwidth efficiency and error performances: an increase in bandwidth efficiency results in an increase in error probability.