physical layer (continued)€¦ · 04/10/2019 · physical layer cse 461 university of washington...
TRANSCRIPT
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Physical Layer (continued)10/4/2019
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Topics
1. Coding and Modulation schemes• Representing bits, noise
2. Properties of media• Wires, fiber optics, wireless, propagation• Bandwidth, attenuation, noise
3. Fundamental limits• Nyquist, Shannon
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Wednesday’s Class
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Where we are
● Working our way up the stack starting with the Physical layer
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Physical
Link
Network
Transport
Application Made of up physical things- wires- fiber- electromagnetic waves/light
Warning! Brief Review!
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Philosophical Takeaways
●Everything is analog, even digital signals
● Digital information is a discrete concept represented in an analog physical medium○ A printed book (analog) vs.○ Words conveyed in the book (digital)
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Warning! Brief Review!
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Types of Media
● Media propagate analog signals that carry bits of digital information
●We looked at some common types...??? What are some examples???
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Warning! Brief Review!
○Copper Wires (twisted pair | coax)○Fiber (fiber optic cables)○Wireless
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What is the difference between light, radio waves, and gamma radiation?
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Well… only one makes the Hulk
Photo credit Marvel via topmovieclips.com
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They are all the same thing (electromagnetic radiation) at different frequencies…
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Different frequencies have different properties!Not all frequencies are created equal...
wikimedia commons
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WiFi
WiFi
Warning! Brief Review!
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Theoretical Limits“Information Theory”
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Real World Limits
●How rapidly can we send information over a link?○Nyquist limit (~1924)○Shannon capacity (1948)
●Practical systems (I.E. your cellphone) approach these limits! Pretty cool : )
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Analog Vocabulary Again
● Often easier to think about signals in frequency
13
Warning! Brief EE Moment!
Attribution: Pbchem at en.wikipedia
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Important Analog Vocabulary (2)
●Every analog signal has a given bandwidth
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Warning! Brief EE Moment!
Attribution: Henrikb4 at en.wikipedia
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Key Channel Properties
●The bandwidth (B), signal power (S), and noise power (N)○B (in hertz) limits the rate of transitions○S and N (in watts) limit how many signal levels we can
distinguish
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Bandwidth B Signal S,Noise N
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What is the bandwidth of a square wave?
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Warning! Brief EE Moment!
Attribution: Henrikb4 at en.wikipedia
Time
Amplitude
Infinite! No true square wave exists in the real world
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Warning! Brief EE Moment!
Attribution: captainprog at dsp.stackexchange.com
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Brief Activity...
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Nyquist Limit
● The maximum symbol rate is 2*Bandwidth
● Thus if there are V signal levels, ignoring noise, the maximum bit rate is:
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R = 2B log2V bits/sec
1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
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Claude Shannon (1916-2001)
•Father of information theory•“A Mathematical Theory of
Communication”, 1948•Fundamental contributions
to digital computers, security, and communications
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Credit: Courtesy MIT Museum
Electromechanical mouse that “solves” mazes!
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Shannon Capacity
•How many levels we can distinguish depends on S/N•Or SNR, the Signal-to-Noise Ratio•Note noise is random, hence some errors
•SNR given on a log-scale in deciBels:
•SNRdB
= 10log10
(S/N)
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0
1
2
3
N
S+N
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Shannon Capacity (2)
• Shannon limit is for capacity (C), the maximum lossless information carrying rate of the channel:
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C = B log2(1 + S/N) bits/sec
• Deriving this is outside the scope of this course, but it is an elegant result with incredible implications...
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Shannon Capacity Takeaways
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C = B log2(1 + S/N) bits/sec
• There is some rate at which we can transmit data without loss over a random channel
• Assuming noise fixed, increasing the signal power yields diminishing returns : (
• Assuming signal is fixed, increasing bandwidth increases capacity linearly!
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No matter what fancy code you use, you can’t beat Shannon (in AWGN)
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NRZ signal of bits
Amplitude shift keying
Frequency shift keying
Phase shift keying
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Wired/Wireless Perspective
•Wires, and Fiber• Engineer link to have requisite SNR and B
→ Can fix data rate
•Wireless• Given B, but SNR varies greatly, e.g., up to 60 dB!
→ Can’t design for worst case, must adapt data rate
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Engineer SNR for data rate
Adapt data rate to SNR
??? Which is better ???
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5G...
● To increase the data rate, you need either more spectrum or more power
● Both are limited by physics… how can we work around it???
There is no magic
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“Spatial Reuse”
medium.com/@artiedarrell
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Make the cells smaller… so we can have more of them!
govtech.com
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Phy Layer Innovation Still Happening!
● Backscatter “zero power” wireless
● mm wave 30GHz+ radio equipment
● Free space optical (FSO)
● Cooperative interference management
● Massive MIMO and beamforming
● Powerline Networking
● 100 GbE in datacetners, etc.
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All distilled to a simple link model
• Rate (or bandwidth, capacity, speed) in bits/second• Delay in seconds, related to length
• Other important properties:• Whether the channel is broadcast, its error rate, and its stability
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Delay D, Rate R
Message