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) = At sin(2 ft t + t)

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!)

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]

Radio frequency

직진성

Radio channel type

* Ground wave = surface wave + space wave

Radio channel type

-> Really? 802.16

Radio channel type

Why 60GHz?

Why 60GHz? Frequency reuse

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

Radio propagation

Attenuation in real world

• Exponent “a” can be up to 6, 7

propagation

reflection scattering diffraction

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

Slow fading vs. fast fading

short term fading

long termfading

t

power

• Slow fading = long-term fading• Fast fading = short-term fading

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

shadowing

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

Fast fading

Fast fading

Fast fading

The final propagation model

Real world example

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

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)

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

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

antenna

• Isotropic

• Omni-directional– Radiation in every direction on

azimuth/horizontal plane

• Directional– Narrower beamwidth, higher gain

Omni vs directional

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

Switched vs. adaptive

Switched vs. adaptive

MIMO?

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”

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

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

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

Antenna model2 Operation Modes: Omni and Directional

A node may operate in any one mode at any given time

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

Potential benefits

• Increase “range”, keeping transmit power constant

• Reduce transmit power, keeping range comparable with omni mode– Reduces interference, potentially

increasing spatial reuse

neighbor

• Notion of a “neighbor” needs to be reconsidered

– Similarly, the notion of a “broadcast” must also be reconsidered

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

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

Directional antenna for MAC

• Less energy consumption– Within the boundary of omni-

directional Xmission range

• Same energy consumption

• DD neighbor is possible

Directional antenna for routing

• same energy consumption

• One hop directional transmission across multi-hop omnidirectional transmission

• DO neighbor will be the norm

D-MAC Protocol[Ko2000Infocom]

DATA DATA

RTS RTS

CTS CTS

ACKACK

B C ED

Reserved area

AF

IEEE 802.11

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

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

D-MAC Protocols

• Based on location information of the receiver, sender selects an appropriate directional antenna

• Signature table

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)

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

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

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

D-MAC Scheme 2

• Probability of ACK collision lower than scheme 1

• Possibilities for simultaneous transmission by neighboring nodes reduced compared to scheme 1