1. Wireless Environment

2. Frequencies for communication
VLF = Very Low Frequency UHF = Ultra High Frequency
LF = Low Frequency SHF = Super High Frequency
MF = Medium Frequency EHF = Extra High Frequency
HF = High Frequency UV = Ultraviolet Light
VHF = Very High Frequency
Frequency and wave length:
λ = c/f
wave length λ, speed of light c ≅ 3x108m/s, frequency f

3. Frequencies for mobile communication
VHF-/UHF-ranges for mobile radio
simple, small antenna for cars
deterministic propagation characteristics, reliable connections
SHF and higher for directed radio links, satellite communication
small antenna, focusing
large bandwidth available
Wireless LANs use frequencies in UHF to SHF spectrum
some systems planned up to EHF
limitations due to absorption by water and oxygen molecules
(resonance frequencies)

4. Antennas: isotropic radiator
Radiation and reception of electromagnetic waves, coupling of wires to space for radio transmission
Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna
Real antennas always have directive effects (vertically and/or horizontally)
Radiation pattern: measurement of radiation around an antenna

5. Antennas: directed and sectorized
Often used for microwave connections or base stations for mobile phones (e.g., radio coverage of a valley)

6. Antennas: directed and sectorized Cell Sizes
Fig. 2.11

7. Signals physical representation of data
function of time and location
signal parameters: parameters representing the value of data
classification
continuous time/discrete time
continuous values/discrete values
analog signal = continuous time and continuous values
digital signal = discrete time and discrete values
Signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift ϕ
sine wave as special periodic signal for a carrier:
s(t) = At sin(2 π ft t + ϕt)

8. Signal Important quantities
Important quantities to measure the strength of the signal to the receiver , noise , interference e.g.
SNR . Signal to Interference Ratio in dB
SIR =received power of reference user in dBm/received power of all interferers in dBm
C/I . Carrier over Interference in dB
Carrier Power (dBm) / received power of all interferers in dBm

9. Signal Important quantities
Eb/No . Signal Energy per bit to noise Power Density per hertz.
-Eb/No = Signal energy (per bit ) dBm / noise Power dBm .Measures how strong the signal is .
-Different forms of modulation BPSK, QPSK, QAM, etc. have different curves of theoretical bit error rates versus Eb/No.
Eb/No

10. 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

11. Signal propagation
Propagation in free space always like light (straight line)
Receiving power proportional to 1/d²(d = distance between sender and receiver)
Receiving power additionally influenced by
fading (frequency dependent)
shadowing
reflection at large obstacles
refraction depending on the density of a medium
scattering at small obstacles
diffraction at edges

12. Real world example

13. Multipath propagation
Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction
Time dispersion: signal is dispersed over time
interference with “neighbor” symbols, Inter Symbol Interference (ISI)
The signal reaches a receiver directly and phase shifted
distorted signal depending on the phases of the different parts

14. Typical large-scale path loss
Fig. 2.15

15. Measured large-scale path loss
Fig. 2.16

16. Partition losses
Fig. 2.16

17. Measured indoor path loss
Fig. 2.16

18. Measured indoor path loss
Fig. 2.16

19. Fig. 2.8
Measured received power levels over a 605 m 38 GHz fixed wireless link in clear sky, rain, and hail [from [Xu00], ©IEEE].

20. Fig. 2.9
Measured received power during rain storm at 38 GHz [from [Xu00], ©IEEE].

21. Effects of mobility
Channel characteristics change over time and location
signal paths change
different delay variations of different signal parts
different phases of signal parts
quick changes in the power received (short term fading)
Additional changes in
distance to sender
obstacles further away
slow changes in the average power received (long term fading)

22. Multiple Access protocols
single shared broadcast channel
two or more simultaneous transmissions by nodes: interference
only one node can send successfully at a time
multiple access protocol
distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit
communication about channel sharing must use channel itself!
what to look for in multiple access protocols:

23. Ideal Multiple Access Protocol
Broadcast channel of rate R bps
1. When one node wants to transmit, it can send at rate R.
2. When M nodes want to transmit, each can send at average rate R/M
3. Fully decentralized:
no special node to coordinate transmissions
no synchronization of clocks, slots
4. Simple

24. MAC Protocols: a taxonomy
Three broad classes:
Channel Partitioning
divide channel into smaller “pieces” (time slots, frequency, code)
allocate piece to node for exclusive use
Random Access
channel not divided, allow collisions
“recover” from collisions
“Taking turns”
tightly coordinate shared access to avoid collisions

25. Multiplexing Multiplexing in 4 dimensions
space (si)
time (t)
frequency (f)
code (c)
Goal: multiple use of a shared medium
Important: guard spaces needed!

26. Time multiplex
A channel gets the whole spectrum for a certain amount of time
Advantages:
only one carrier in themedium at any time
throughput high even for many users
Disadvantages:
Precise
synchronization
necessary

27. Channel Partitioning MAC protocols: TDMA
TDMA: time division multiple access
access to channel in "rounds"
each station gets fixed length slot (length = pkt trans time) in each round
unused slots go idle
example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle
TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load.
FDM (Frequency Division Multiplexing): frequency subdivided.

28. Frequency multiplex
Separation of the whole spectrum into smaller frequency bands
A channel gets a certain band of the spectrum for the whole time
Advantages:
no dynamic coordination necessary
works also for analog signals
Disadvantages:
waste of bandwidth if the traffic is distributed unevenly
inflexible
guard spaces

29. Channel Partitioning MAC protocols: FDMA
FDMA: frequency division multiple access
channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle
time
frequency bands

30. Time and frequency multiplex
Combination of both methods
A channel gets a certain frequency band for a certain amount of time
Example: GSM
Advantages:
better protection against tapping
protection against frequency selective interference
higher data rates compared to code multiplex
but: precise coordination required

31. Code multiplex Each channel has a unique code
All channels use the same spectrum at the same time
Advantages:
bandwidth efficient
no coordination and synchronization necessary
good protection against interference and tapping
Disadvantages:
lower user data rates
more complex signal regeneration
Implemented using spread spectrum technology

32. Channel Partitioning (CDMA)
CDMA (Code Division Multiple Access)
unique “code” assigned to each user; i.e., code set partitioning
used mostly in wireless broadcast channels (cellular, satellite, etc)
all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding: inner-product of encoded signal and chipping sequence
allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)

33. CDMA Encode/Decode

34. CDMA: two-sender interference

35. space division multiplex
Cell structure
Implements space division multiplex: base station covers a certain transmission area (cell)
Mobile stations communicate only via the base station
Advantages of cell structures:
higher capacity, higher number of users
less transmission power needed
more robust, decentralized
base station deals with interference, transmission area etc. locally
Problems:
fixed network needed for the base stations
handover (changing from one cell to another) necessary
interference with other cells
Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies

36. Medium access control Motivation for a specialized MAC in wireless
– Consider carrier sense –CS- multiple access with collision detection-CD- (CSMA/CD) – wired nets:
A sender senses the medium to see if it is free.
If the medium is busy, the sender waits until it is free.
If the medium is free, the sender starts transmitting data and continues to listen the medium.
If the sender detects a collision while sending, it stops at once.

37. CSMA collisions collisions can still occur: collision: note:
spatial layout of nodes
collisions can still occur:
propagation delay means
two nodes may not hear
each other’s transmission
collision:
entire packet transmission
time wasted
note:
role of distance & propagation delay in determining collision probability

38. CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA
collisions detected within short time
colliding transmissions aborted, reducing channel wastage
collision detection:
easy in wired LANs: measure signal strengths, compare transmitted, received signals
difficult in wireless LANs: receiver shut off while transmitting

39. CSMA/CD collision detection

40. Ethernet uses CSMA/CD No slots
adapter doesn’t transmit if it senses that some other adapter is transmitting, that is, carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting, that is, collision detection
Before attempting a retransmission, adapter waits a random time, that is, random access

41. Wireless MAC CSMA makes sense: Problems in wireless networks
get all the bandwidth if you’re the only one transmitting
shouldn’t cause a collision if you sense another transmission
Problems in wireless networks
signal strength decreases proportional to the square of the distance
the sender would apply CS and CD, but the collisions happen at the receiver due to a second sender
it might be the case that a sender cannot “hear” the collision, i.e., CD does not work
furthermore, CS might not work if, e.g., a terminal is “hidden”

42. Wireless MAC (a) The hidden station problem.
(b) The exposed station problem.

43. Wireless MAC Hidden terminal problem A sends to B, C cannot receive A
C wants to send to B, C senses a “free” medium (CS fails)
C also stars sending causing a collision at B
A cannot receive the collision (CD fails) and continues with its transmission
A is “hidden” for C

44. Medium access control Exposed terminals B sends to A
C wants to send to another terminal (not A or B)
C has to wait, CS signals a medium in use
but A is outside the radio range of C, therefore waiting is not necessary
C is “exposed” to B

45. Medium access control Solution:
CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)
sensing the carrier is combined with a back-off scheme in case of a busy medium to achieve some fairness among the competing stations
uses short signaling packets for collision avoidance
RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet
CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive
Signaling packets contain
sender address
receiver address
packet length (the length of the future transmission)

46. IEEE 802.11 MAC Protocol: CSMA/CA
CSMA: sender
- if sense channel idle for DISF sec.
then transmit entire frame (no collision detection)
-if sense channel busy then binary backoff
CSMA receiver
- if received OK
return ACK after SIFS
(ACK is needed due to hidden terminal problem)

47. Collision Avoidance: RTS-CTS exchange
sender transmits short RTS (request to send) packet: indicates duration of transmission
receiver replies with short CTS (clear to send) packet
notifying (possibly hidden) nodes
hidden nodes will not transmit for specified duration: NAV

48. Collision Avoidance: RTS-CTS exchange
RTS and CTS short:
collisions less likely, of shorter duration
end result similar to collision detection
IEEE allows:
CSMA
CSMA/CA: reservations
polling from AP

49. IEEE 802.11 MAC Protocol: CSMA/CA
CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)
avoids the problem of hidden terminals
– A and C want to send to B
– A sends RTS first
– C waits after receiving CTS from B

50. IEEE 802.11 MAC Protocol: CSMA/CA
CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)
avoids the problem of exposed terminals
– B wants to send to A
– C to another terminal
– now C does not have to wait for it cannot receive CTS from A

51. Modulation Digital modulation Analog modulation Motivation
digital data is translated into an analog signal (base band)
ASK, FSK, PSK - main focus in this chapter
differences in spectral efficiency, power efficiency, robustness
Analog modulation
shifts center frequency of base band signal up to the radio carrier
Motivation
smaller antennas (e.g., λ/4)
Frequency Division Multiplexing
medium characteristics
Basic schemes
Amplitude Modulation (AM)
Frequency Modulation (FM)
Phase Modulation (PM)

52. Modulation and demodulation

53. Channel allocation methods and systems for a common channel.
Summary
Channel allocation methods and systems for a common channel.

54. IEEE MAC Protocol
Fig. 2.10
Overview of the IEEE Wireless LAN standard.

55. IEEE MAC Protocol
Fig. 2.12
Channelization scheme for IEEE b throughout the world.