1. Physical layer
Taekyoung Kwon

2. 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)

3. Signal (Fourier representation)1 1 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!)

4. signal Different representations of signals
amplitude (amplitude domain)
frequency spectrum (frequency domain)
phase state diagram (amplitude M and phase  in polar coordinates)
Q = M sin 
A [V]
A [V]
t[s] 
I= M cos  
f [Hz]

5. Radio frequency
직진성

6. * Ground wave = surface wave + space wave
Radio channel type
* Ground wave = surface wave + space wave

7. Radio channel type
-> Really?

8. Radio channel type

9. Why 60GHz?

10. Why 60GHz? Frequency reuse

11. 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
Xmission
distance
detection
interference

12. Radio propagation

13. Attenuation in real world
Exponent “a” can be up to 6, 7

14. propagation
reflection
scattering
diffraction

15. 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

16. Slow fading vs. fast fading
Slow fading = long-term fading
Fast fading = short-term fading
long term
fading
power t
short term fading

17. 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

18. shadowing

19. Fast fading T-R separation distances are small
Heavily populated, urban areas
Main propagation mechanism: scattering
Multiple copies of transmitted signal arriving at the transmitted via different paths and at different time-delays, add vector-like at the receiver: fading
Distribution of signal attenuation
coefficient: Rayleigh, Ricean.
Short-term fading model
Rapid and severe signal fluctuations around a slowly varying mean

20. Fast fading

21. Fast fading

22. Fast fading

23. The final propagation model

24. Real world example

25. Modulation and demodulation
analog
baseband
signal
digital
data
digital
modulation
analog
modulation
radio transmitter
radio
carrier
analog
baseband
signal
digital
data
analog
demodulation
synchronization
decision
radio receiver
radio
carrier
UWB: no carrier
-> low cost, low power

26. modulation Digital modulation Analog 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)

27. 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 1 t 1 1 t 1 1 t

28. 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) z y z
ideal
isotropic
radiator y x x

29. antenna Isotropic Omni-directional Directional
Radiation in every direction on azimuth/horizontal plane
Directional
Narrower beamwidth, higher gain

30. Omni vs directional

31. Antenna (directed or sectorized)
E.g. 3 sectors per BS in cellular networks y y z
directed
antenna x z x
side view (xy-plane)
side view (yz-plane)
top view (xz-plane) z z
sectorized
antenna x x
top view, 3 sector
top view, 6 sector

32. Switched vs. adaptive

33. Switched vs. adaptive

34. MIMO?

35. 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”

36. 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

37. 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

38. 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

39. Antenna model 2 Operation Modes: Omni and Directional
A node may operate in any one mode at any given time

40. Antenna model In 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

41. Potential benefits Increase “range”, keeping transmit power constant
Reduce transmit power, keeping range comparable with omni mode
Reduces interference, potentially increasing spatial reuse

42. neighbor Notion of a “neighbor” needs to be reconsidered
Similarly, the notion of a “broadcast” must also be reconsidered

43. Directional neighbor When C transmits directionally Receive Beam
Transmit Beam B A C
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

44. Directional neighbor When C transmits directionally
Receive Beam
Transmit Beam A C B
When C transmits directionally
Node B receives packets from C only in directional mode
C and B are Directional-Directional (DD) neighbors

45. Directional antenna for MAC
Less energy consumption
Within the boundary of omni-directional Xmission range
Same energy consumption
DD neighbor is possible

46. Directional antenna for routing
same energy consumption
One hop directional transmission across multi-hop omnidirectional transmission
DO neighbor will be the norm

47. D-MAC Protocol [Ko2000Infocom]

48. IEEE 802.11 Reserved area F A B C D E RTS RTS CTS CTS DATA DATA ACK

49. Directional MAC (D-MAC)
Directional antenna can limit transmission to a smaller region (e.g., 90 degrees).
Basic philosophy: MAC protocol similar to IEEE , but on a per-antenna basis

50. 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

51. D-MAC Protocols
Based on location information of the receiver, sender selects an appropriate directional antenna
Signature table

52. 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) and Omni-directional CTS (OCTS)

53. D-MAC Scheme 1: DRTS/OCTSB C D E
DRTS(B)
DRTS(B) - Directional RTS including
location information of node B
OCTS(B,C)
OCTS(B,C)
DRTS(D)
OCTS(D,E)
DATA
OCTS(B,C) – Omni-directional CTS
including location information
of nodes B and C
DATA
ACK
ACK

54. Drawback of Scheme 1 Collision-free ACK transmission not guaranteed ?
DRTS(B)
OCTS(B,C)
OCTS(B,C)
DRTS(A)
DATA
DRTS(A)
ACK

55. 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

56. D-MAC Scheme 2 Probability of ACK collision lower than scheme 1
Possibilities for simultaneous transmission by neighboring nodes reduced compared to scheme 1