PHY FUNDAMENTALS II
Wireless Signal Propagation
1. Antenna
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2. Reflection, Diffraction, Scattering
3. Fading, Shadowing, Multipath
4. Inter-symbol Interference
5. Path loss model (Frii’s, 2-ray)
6. MIMO (Diversity, Multiplexing, Beamforming)
7. Orthogonal Frequency Division Multiplexing (OFDM)
8. Orthogonal Frequency Division Multiple Access (OFDMA)
9. Effect of Frequency
q Transmitter converts electrical energy to electromagnetic waves q Receiver converts electromagnetic waves to electrical energy
q Same antenna is used for transmission and reception
q Omni-Directional: Power radiated in all directions
q Directional: Most power in the desired direction
q Isotropic antenna: Radiates in all directions equally
q Antenna Gain = Power at particular point/Power with Isotropic Expressed in dBi (“decibel relative to isotropic”)
Omni-Directional Directional Isotropic
Question: How much stronger a 17 dBi antenna effectively receives (transmits) the signal compared to the isotropic antenna?
Power of isotropic antenna = Piso Power of 17 dBi antenna = P We have
17 = 10log10(P/Piso)
Thus P/Piso = 101.7 = 50.12, i.e., the 17 dBi antenna will effectively receive (transmit) the signal 50.12 times stronger than the isotropic antenna albeit using the same actual transmit power.
Relationship between antenna size and frequency
q Antennas are designed to transmit or receive a specific frequency band
Ø Cannot use a TV antenna for wireless router, or vice-versa (why?)
q End-to-end antenna length = 1⁄2 wavelength
Ø So that electrons can travel back and forth the antenna in
q If dipole (two rods), each rod is 1⁄4 wavelength
Reflection, Diffraction, Scattering
Reflection, Diffraction
and Scattering (Cont)
q Reflection: Surface large relative to wavelength (l) of signal
Ø May have phase shift from original
Ø May cancel out original signal or increase/strengthen it at receiver
q Diffraction: Edge of impenetrable body; large relative to l
Ø Receiver may receive signal even if no line-of-sight (LOS) to transmitter
q Scattering
Ø Obstacle size on order of wavelength. Lamp posts etc.
Ø Reflection/diffraction are more directional; scattering in many directions
q If LOS, diffracted and scattered signals not significant (LOS dominates) Ø Reflected signals may be significant
q If no LOS, diffraction and scattering are primary means of reception
q Received power (PR) at a particular location is only a fraction of the total power used by the transmitter to transmit the signal (PT)
q Pathloss=PT –PR
Ø Depends on distance/separation between Tx-Rx
q Need to estimate path loss to design wireless links q How to estimate path loss?
q There are well-known path loss models
Ø Frii’s model: designed for free-space (no reflections); frequency dependent
Ø 2-Ray model: reflections considered, but frequency-independent (antenna heights are important)
Free Space Path Loss (
PR=PT( c )2 4πdf
q A factor of 10 increase in distanceà20dB more path loss (20dB/decade) Ø 2.4 GHz path loss at 1 meter = 40.04dB, 10 meter = 60.04dB
q The higher the frequency, the greater the path loss at fixed distance Ø At 10 meter, 2.4GHz = 60.04dB, 5GHz = 67.25dB
space path loss (
Question: If 50W power is applied to a 900 MHz frequency at a transmitter, find the receive power at a distance of 100 meter from the transmitter (assume free space path loss with unit antenna gains).
Unit antenna gain means: GT=GR=1.
We have d=100m, f=900×106 Hz, PT=50W, c=3×108 m/sec, and 𝜋 = 3.14
𝑃=𝑃 𝑐 2=3.5𝜇𝑊 𝑅 𝑇4𝜋𝑓𝑑
Receiver Sensitivity
q The received power (received signal strength or RSS) has to be greater than a threshold for the receiver to decode information correctly (with low error probability)
Ø Toachieveaminimumsignal-to-noiseratio(SNR)
q Different hardware/standard/equipment specify different values
Ø Dependsonchannelbandwidthandreceivernoisefigure(andtemperature)
Ø Larger bandwidth à larger minimum power (and vice versa)
Ø Larger receiver noiseàlarger minimum power (and vice versa)
q Examples (for room temperature) Ø LTE:-52dBm[roughly]
Ø Bluetooth:-70dBm[roughly]
q To increase the coverage with low transmit power, a manufacturer produced Bluetooth chipsets with a receiver sensitivity of -80 dBm. What is the maximum communication range that could be achieved for this chipset for a transmit power of 1 mW? Assume Free Space Path Loss with unit antenna gains.
Bluetooth frequency f = 2.4 GHz, PT= 1 mW, PR=-80 dBm=10-8 mW Wehave PR=PT( c )2
4πdf d=c PT/PR
= 99.5 meter ©2020
q Multiple copies of the signal received (LoS+reflected NLoS)
q The LoS signal reaches the receiver first followed by the NLoS copy (NLoS has
longer path length compared to the LoS path)
q RSS(LoS) > RSS(NLoS). NLoS signal travels further and hence attenuates more compared to the LoS.
Symbol Interference
q Multipath effect: receiver continues to receive the signal (its reflections) even after the transmitter completes symbol transmission
q As a result, symbols become wider; two consecutively transmitted symbols overlap and interfere with each other
q Longer bit intervals or symbol lengths are required to avoid ISI
q Limits the number of bits/s (data rate inverse of symbol length) ©2020
q How long to wait to avoid ISI?
q RSS of late arrivals fluctuate, but consistently diminish on average
q Delay Spread = Time between the first (LoS) and the last copy of NLoS
q RSS(last copy of NLoS) < RSSthreshold (so next symbol can still be decoded) ©2020
ray path loss model
Path loss exponent
q Empty/free space (no reflector)àd-2 law (Frii’s)
Ø PL(dB) = 10log10(d2) + C = 10x2xlog10(d) + C (a straight line with slope = 2)
q 2-ray modelàd-4 law
Ø PL(dB) = 10log10(d4) + C = 10x4xlog10(d) + C (a straight line with slope = 4)
q Measurements in real environmentsàd-n (n = 1.5 to 5.5, typically 4)
Ø PL(dB) = 10log10(dn) + C = 10nlog10(d) + C (a straight line with slope = n)
C is a constant; frequency related (Free-space) or antenna height related (2-ray)
q Multipath has phase change (due to reflection and different path to travel)
q Fading: the signal amplitude can change significantly by moving a few centimeters (called small scale fading: fluctuates in small time scale): half-wavelength path distance can cause 180 degree phase shift!
q Constructive: increased amplitude due to alignment of phase q Destructive: reduced amplitude due to misaligned phase
Small Scale
Constructive
Destructive
q Shadowing gives rise to large scale fading
Ø Mobile may be in the shadow of a building (fading) for several meters Ø RSS drops when in shadow
(large scale fading)
Received Power
Total Path Loss
q Traditionally, single antennas were used
q Multiple antennas are increasingly being used to boost
quality/reliability and capacity of wireless communications Ø E.g., most recent WiFi routers have multiple antennas
q Multiple input (multiple antennas at the transmitter) q Multiple output (multiple antennas at the receiver) q MIMO – multiple input multiple output
MIMO Antenna Configurations
q SISO – single input (1 Tx antenna) single output (1 Rx antenna)
q SIMO – single input (1 Tx antenna) multiple output (>1 Rx antenna) q MISO – multiple input (>1 Tx antenna) single output (1 Rx antenna)
q MIMO – multiple input (>1 Tx antenna) multiple output (>1 Rx antenna)
Ø 2×2 MIMO – 2 input & 2 output (2 Tx antennas & 2 Rx antennas)
Ø 4×2 MIMO – 4 input & 2 output (4 Tx antennas & 2 Rx antennas)
Ø 1000×2 MIMO – 1000 input & 2 output (1000 Tx antennas & 2 Rx antennas)
Why Multiple Antennas Can Improve Performance?
q Multi-path scenario: if antennas are spaced >λ/2 apart, multipath signals for different antennas can be uncorrelated
Ø multiple (spatial) channels using the same frequency!
Ø More channels means opportunity to improve signal quality
and data rate
q Line-of-Sight (LOS) scenario: multiple antennas at the transmitter can be used to realize virtual directional antennas (beamforming)
Ø Increase coverage and signal strength at a particular direction of choice
Spatial Channels
MIMO Techniques
q Spatial Diversity (a.k.a. Diversity)
Ø Improve reliability by exploiting spatial channels
q Spatial Multiplexing (a.k.a. Multiplexing)
Ø Improve data rate by exploiting spatial channels
q Beamforming
Ø Increase coverage and signal strength by exploiting multiple Tx antennas to focus the beam at a narrow angle
q Total # of independent paths = NT x NR
Ø NT = # of transmit antenna, NR = # of receive antenna
q Send same data (copied) over NT x NR redundant paths
q Increases reliability – probability that all paths will suffer bad
fading at the same time is low
Ø SNR at receiver can be improved (diversity gain)
q A base station is equipped with an antenna array consisting of 100 elements. What is the maximum number of spatial channels that could be created from this base station to an ordinary mobile device equipped with a single antenna?
Answer: It is a 100×1 MIMO. 100 spatial channels are possible.
Multiplexing
q Send different bits of the data on different channels
q The combined data rate is increased due to multiplexing
q Overall multiplexing gain is limited by degrees of freedom q Degrees of freedom = min(NT, NR)
10 1 Diversity Multiplexing
q What is the degrees of freedom for an 802.11ac WiFi system with the access point having 8 antennas and communicating to a laptop equipped with 2 antennas?
q Answer: degrees of freedom = min(8,2) = 2
Beamforming
q Phased Antenna Arrays:
Transmit the same signal using multiple antennas
q By phase-shifting various signals Þ Focus on a narrow directional beam (increased SNR and long-distance coverage)
q Receiver does the same, i.e., focus its reception from a particular BS q Used when LOS
q Orthogonal Frequency Division Multiplexing
q Ten 100 kHz channels are better than one 1 MHz Channel
q Frequency band is divided into 256 or more sub-bands. q Orthogonal: Peak of one at null of others
q Each carrier is modulated indecently with a BPSK, QPSK, 16-QAM, 64- QAM etc. depending on the fading in the channel (frequency selective fading means different channel has different fading and requires different modulation and coding)
q Used in newer generation of WiFi and 4G/5G ©2020
Advantages of OFDM
q Robustness against frequency selective burst errors
q Allows adaptive modulation and coding of subcarriers
q Robust against narrowband interference (affecting only some subcarriers) q Allows pilot subcarriers for channel estimation
OFDM: Design considerations
q Subcarrier spacing = Frequency bandwidth/Number of subcarriers
q Large number of carriers Þ Smaller data rate per carrier
Þ Larger symbol duration Þ Less inter-symbol interference
q Reduced subcarrier spacing Þ Increased inter-carrier interference due to Doppler spread in mobile applications
Small # of carriers
Shorter symbol durations Higher data rates per carrier
Large # of carriers
Longer symbol durations Lower data rates per carrier
q Orthogonal Frequency Division Multiple Access
q Each user has a subset of subcarriers for a few time slots q OFDM systems use TDMA (e.g., in WiFi)
q OFDMA allows Time+Freq DMA Þ 2D Scheduling
OFDM Freq.
OFDMA Freq.
q With a subcarrier spacing of 10 kHz, how many subcarriers will be used in an OFDM system with 20 MHz channel bandwidth?
Number of subcarriers = channel bandwidth/subcarrier spacing = 20×106/10×103 = 2000
Effect of Frequency
q Higher Frequencies have higher attenuation, e.g., 18 GHz has 20 dB/m more than 1.8 GHz
q Higher frequencies need smaller antenna Antenna > Wavelength/2, 800 MHz Þ 6”
q Higher frequencies are affected more by weather Higher than 10 GHz affected by rainfall
60 GHz affected by absorption of oxygen molecules
q Higher frequencies have more bandwidth and higher data rate
q Higher frequencies allow more frequency reuse
They attenuate close to cell boundaries. Low frequencies propagate far.
Effect of Frequency (Cont)
q Lower frequencies have longer reach Þ Longer Cell Radius
Þ Good for rural areas
Þ Smaller number of towers
Þ Longer battery life
q Lower frequencies require larger antenna and antenna spacing
Þ MIMO difficult particularly on mobile devices q Lower frequencies Þ Smaller channel width
Þ Need aggressive MCS, e.g., 256-QAM
q Doppler shift = vf/c = Velocity ×Frequency/(speed of light)
Þ Lower Doppler spread at lower frequencies q Mobility Þ Below 10 GHz
1. Path loss increases at a power of 2 to 5.5 with distance.
2. Fading = Changes in power with changes in position
3. Multiple Antennas: Receive diversity, transmit diversity, multiplexing, and beamforming
4. OFDM splits a band into many orthogonal subcarriers. OFDMA = FDMA + TDMA
q BPSK q BS
q DMA q DSP
q DVB-H q FDMA q FFT
q IDFT q IFFT q ISI
q kHz q LoS
Binary Phase-Shift Keying
Base Station
Deci Bels Intrinsic
DeciBels milliwatt
Discrete Fourier Transform
Direct Memory Access
Digital Signal Processing
Digital Video Broadcast handheld Frequency Division Multiple Access Fast Fourier Transform
Inverse Discrete Fourier Transform Inverse Fast Fourier Transform Inter-symbol interference
Line of Sight
q MIMO q MS
q OFDM q OFDMA q QAM
Acronyms (Cont)
Mega Hertz
Multiple Input Multiple Output
Mobile Station
Orthogonal Frequency Division Multiplexing Orthogonal Frequency Division Multiple Access Quadrature Amplitude Modulation
Quadrature Phase-Shift Keying
Radio Frequency
Signal to Noise Ratio
Subscriber Station
Space Time Block Codes
Time Division Multiple Access
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