physical layer
DESCRIPTION
Physical layer. Taekyoung Kwon. signal. physical representation of data function of time and location signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift E.g., sinewave is expressed as s(t) = A t sin(2 f t t + t ). - PowerPoint PPT PresentationTRANSCRIPT
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Physical layer
Taekyoung Kwon
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signal
• physical representation of data
• function of time and location
• signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift – E.g., sinewave is expressed as
s(t) = At sin(2 ft t + t)
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Signal (Fourier representation)
)2cos()2sin(2
1)(
11
nftbnftactgn
nn
n
1
0
1
0
t t
ideal periodic signal real composition
Digital signals need
• infinite frequencies for perfect transmission (UWB?)
• modulation with a carrier frequency for transmission (analog signal!)
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signal• Different representations of signals
– amplitude (amplitude domain)– frequency spectrum (frequency domain)– phase state diagram (amplitude M and phase in polar
coordinates)
f [Hz]
A [V]
I= M cos
Q = M sin
A [V]
t[s]
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Radio frequency
직진성
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Radio channel type
* Ground wave = surface wave + space wave
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Radio channel type
-> Really? 802.16
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Radio channel type
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Why 60GHz?
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Why 60GHz? Frequency reuse
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Signal propagation ranges
• Transmission range– communication possible– low error rate
• Detection range– detection of the signal
possible– no communication
possible
• Interference range– signal may not be
detected – signal adds to the
background noise
distance
Xmission
detection
interference
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Radio propagation
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Attenuation in real world
• Exponent “a” can be up to 6, 7
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propagation
reflection scattering diffraction
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Signal propagation models• Slow fading (shadowing)
– Distance between Tx-Rx– Signal strength over distance
• fast fading– Fluctuations of the signal strength– Short distance– Short time duration– LOS vs. NLOS
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Slow fading vs. fast fading
short term fading
long termfading
t
power
• Slow fading = long-term fading• Fast fading = short-term fading
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shadowing
• Real world• Main propagation mechanism: reflections• Attenuation of signal strength due to power loss
along distance traveled: shadowing• Distribution of power loss in dBs: Log-Normal• Log-Normal shadowing model• Fluctuations around a slowly varying mean
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shadowing
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Fast fading
T-R separation distances are smallHeavily populated, urban areasMain propagation mechanism: scatteringMultiple copies of transmitted signal arriving at the transmitted via different paths and at different time-delays, add vector-like at the receiver: fadingDistribution of signal attenuation coefficient: Rayleigh, Ricean.Short-term fading modelRapid and severe signal fluctuations around a slowly varying mean
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Fast fading
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Fast fading
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Fast fading
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The final propagation model
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Real world example
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Modulation and demodulation
synchronizationdecision
digitaldataanalog
demodulation
radiocarrier
analogbasebandsignal
101101001 radio receiver
digitalmodulation
digitaldata analog
modulation
radiocarrier
analogbasebandsignal
101101001 radio transmitter
UWB: no carrier-> low cost, low power
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modulation
• Digital modulation– digital data is translated into an analog signal (baseband)– ASK, FSK, PSK– differences in spectral efficiency, power efficiency, robustness
• Analog modulation– shifts center frequency of baseband signal up to the radio carrier– Motivation
• smaller antennas (e.g., /4)• Frequency Division Multiplexing• medium characteristics
– Basic schemes• Amplitude Modulation (AM)• Frequency Modulation (FM)• Phase Modulation (PM)
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Digital modulation• Modulation of digital signals known as Shift Keying• Amplitude Shift Keying (ASK):
– very simple– low bandwidth requirements– very susceptible to interference
• Frequency Shift Keying (FSK):– needs larger bandwidth
• Phase Shift Keying (PSK):– more complex– robust against interference
1 0 1
t
1 0 1
t
1 0 1
t
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antenna• Radiation and reception of electromagnetic waves• Isotropic radiator: equal radiation in all directions (three
dimensional) - only a theoretical reference antenna• Real antennas always have directive effects (vertically
and/or horizontally)
zy
x
z
y x idealisotropicradiator
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antenna
• Isotropic
• Omni-directional– Radiation in every direction on
azimuth/horizontal plane
• Directional– Narrower beamwidth, higher gain
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Omni vs directional
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Antenna (directed or sectorized)• E.g. 3 sectors per BS in cellular networks
side view (xy-plane)
x
y
side view (yz-plane)
z
y
top view (xz-plane)
x
z
top view, 3 sector
x
z
top view, 6 sector
x
z
directedantenna
sectorizedantenna
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Switched vs. adaptive
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Switched vs. adaptive
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MIMO?
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Why directional antenna?
• Wireless channel is a shared one• Transmission along a single multi-
hop path inhibits a lot of nodes• Shorter hops help, but to a certain
degree• Gupta-Kumar capacity result:
– T = O( W / sqrt(nlogn) )
• Major culprit is “omnidirectionality”
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Why directional antenna?
• Less energy in wrong directions
• Higher spatial reuse– Higher throughput
• Longer ranges– Less e2e delay
• Better immunity to other transmission– Due to “nulling” capability
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Directional vs. networks
• One-hop wireless environments– Cellular, WLAN infrastructure mode- BS, AP: directional antenna- Mobile: omni-directional
• Ad hoc, sensor networking- Every node is directional
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Directional antenna types
• Switched: can select one from a set of predefined beams/antennas
• Adaptive (steerable): – can point in almost any direction– can combine signals received at different
antennas– requires more signal processing
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Antenna model2 Operation Modes: Omni and Directional
A node may operate in any one mode at any given time
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Antenna modelIn Omni Mode:• Nodes receive signals with gain Go
• While idle a node stays in omni mode
In Directional Mode:• Capable of beamforming in specified direction• Directional Gain Gd (Gd > Go)
Symmetry: Transmit gain = Receive gain
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Potential benefits
• Increase “range”, keeping transmit power constant
• Reduce transmit power, keeping range comparable with omni mode– Reduces interference, potentially
increasing spatial reuse
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neighbor
• Notion of a “neighbor” needs to be reconsidered
– Similarly, the notion of a “broadcast” must also be reconsidered
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Directional neighbor
B
A
• When C transmits directionally
•Node A sufficiently close to receive in omni mode
•Node C and A are Directional-Omni (DO) neighbors
•Nodes C and B are not DO neighbors
C
Transmit BeamReceive Beam
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Directional neighbor
AB C
•When C transmits directionally
• Node B receives packets from C only in directional mode
•C and B are Directional-Directional (DD) neighbors
Transmit BeamReceive Beam
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Directional antenna for MAC
• Less energy consumption– Within the boundary of omni-
directional Xmission range
• Same energy consumption
• DD neighbor is possible
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Directional antenna for routing
• same energy consumption
• One hop directional transmission across multi-hop omnidirectional transmission
• DO neighbor will be the norm
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D-MAC Protocol[Ko2000Infocom]
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DATA DATA
RTS RTS
CTS CTS
ACKACK
B C ED
Reserved area
AF
IEEE 802.11
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Directional MAC (D-MAC)
• Directional antenna can limit transmission to a smaller region (e.g., 90 degrees).
• Basic philosophy: MAC protocol similar to IEEE 802.11, but on a per-antenna basis
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D-MAC• IEEE802.11: Node X is blocked if node X has
received an RTS or CTS for on-going transfer between two other nodes
• D-MAC: Antenna T at node X is blocked if antenna T received an RTS or CTS for an on-going transmission
• Transfer allowed using unblocked antennas• If multiple transmissions are received on
different antennas, they are assumed to interfere
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D-MAC Protocols
• Based on location information of the receiver, sender selects an appropriate directional antenna
• Signature table
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D-MAC Scheme 1
• Uses directional antenna for sending RTS, DATA and ACK in a particular direction, whereas CTS sent omni-directionally
• Directional RTS (DRTS) andOmni-directional CTS (OCTS)
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DATA
DRTS(B)
OCTS(B,C) OCTS(B,C)
ACK
A B C ED
DRTS(D)
DATA
ACK
OCTS(D,E)
DRTS(B) - Directional RTS includinglocation information of node B
OCTS(B,C) – Omni-directional CTSincluding location informationof nodes B and C
D-MAC Scheme 1: DRTS/OCTS
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DATA
DRTS(B)
OCTS(B,C) OCTS(B,C)
ACK
A B C D
DRTS(A)
?
DRTS(A)
Drawback of Scheme 1
• Collision-free ACK transmission not guaranteed
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D-MAC Scheme 2
• Scheme 2 is similar to Scheme 1, except for using two types of RTS
• Directional RTS (DRTS) / Omni-directional RTS (ORTS) both used – If none of the sender’s directional antennas are
blocked, send ORTS– Otherwise, send DRTS when the desired antenna
is not blocked
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D-MAC Scheme 2
• Probability of ACK collision lower than scheme 1
• Possibilities for simultaneous transmission by neighboring nodes reduced compared to scheme 1