1. IT351: Mobile & Wireless Computing
Wireless Radio Communications
Objectives:
To study the wireless radio communication medium, spectrum and signals.
To study antennas and their role in wireless communications.
To study the process of wireless signal propagation.
To introduce basic issues in signal processing and signal modulation.
To study signal modulation techniques including ASK, FSK and PSK.
To detail the concept of spread spectrum and study its techniques; FHSS, DSSS.
To study issues in radio resource management and detail the cellular concept of channel allocation.

2. Outline The radio spectrum Signals Antennas
Signal propagation problems
Multiplexing
Modulation
Spread spectrum
Radio Management

3. Wireless communications
The physical media – Radio Spectrum
There is one finite range of frequencies over which radio waves can exist – this is the Radio Spectrum
Spectrum is divided into bands for use in different systems, so Wi-Fi uses a different band to GSM, etc.
Spectrum is (mostly) regulated to ensure fair access

4. Frequencies for communication
Universität Karlsruhe
Institut für Telematik
Frequencies for communication
Mobilkommunikation
SS 1998
VLF = Very Low Frequency UHF = Ultra High Frequency (DAB, dig-TV, mobile phone, GSM)
LF = Low Frequency (submarine) SHF = Super High Frequency (satellite)
MF = Medium Frequency (radio AM) EHF = Extremely High Frequency (direct link)
HF = High Frequency (radio FM & SW) UV = Ultraviolet Light
VHF = Very High Frequency (analog TV broadcast)
Frequency and wave length
 = c/f
wave length , speed of light c  3x108m/s, frequency f
twisted pair
coax cable
optical transmission
1 Mm
300 Hz
10 km
30 kHz
100 m
3 MHz
1 m
300 MHz
10 mm
30 GHz
100 m
3 THz
1 m
300 THz
VLF LF MF HF
VHF
UHF
SHF
EHF
infrared
visible light UV
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

5. Frequencies for mobile communication
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
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, beam forming
large bandwidth available
Wireless LANs use frequencies in UHF to SHF range
some systems planned up to EHF
limitations due to absorption by water and oxygen molecules (resonance frequencies)
weather dependent fading, signal loss caused by heavy rainfall etc.
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

6. Frequencies and regulations
Universität Karlsruhe
Institut für Telematik
Frequencies and regulations
Mobilkommunikation
SS 1998
ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences)
Examples
Europe
USA
Japan
Cellular phones
GSM , , ,
UMTS ,
AMPS, TDMA, CDMA, GSM ,
TDMA, CDMA, GSM, UMTS ,
PDC, FOMA ,
PDC ,
FOMA ,
Cordless phones
CT , CT
DECT
PACS ,
PACS-UB
PHS
JCT
Wireless LANs
802.11b/g
802.11b/g
802.11b
802.11g
Other RF systems
27, 128, 418, 433, 868
315, 915
426, 868
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

7. Wireless communications
Signals
Physical representation of data is the signal
Signals are function of time and location
In wireless sine waves are used as the basic signal:
Amplitude: strength of the signal
Frequency: no of waves generated per second
Phase shift: where the wave starts and stops
These factors are transformed into the exactly required signal by Fourier transforms (complicated equations that parameterise the sine wave)

8. Signals Sine wave representation t
signal parameters: parameters representing the value of data
signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift 
sine wave as special periodic signal for a carrier: s(t) = At sin(2  ft t + t)
Amplitude
Frequency
Phase Shift t

9. Fourier representation of periodic signals
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Fourier representation of periodic signals 1 1 t t
ideal periodic signal
real composition
(based on harmonics)
It is easy to isolate/ separate signals with different frequencies using filters
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

10. Antennas Sending and receiving signals is performed via antennas
Role: Radiation and reception of electromagnetic waves, coupling of wires to space and vice versa for radio transmission
Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna z y x
ideal
isotropic
radiator

11. Universität Karlsruhe Institut für Telematik
Mobilkommunikation
SS 1998
Antennas
Real antennas do not produce radiate signals in equal power in all directions. They always have directive effects (vertically and/or horizontally)
Radiation pattern: measurement of
radiation around an antenna
Most basic antenna is the dipole
Two antennas both of length /4
(/2 in total)
Small gap between the two antennas
Produces an omni-directional signal in
one plane of the three dimensions
Source: Wikipedia
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

12. Antennas
Omni-Directional Antennas are wasteful in areas where obstacles occur (e.g. valleys)
Directional antennas reshape the signal to point towards a target, e.g. an open street
Placing directional antennas together can be used to form cellular reuse patterns
Antennas arrays can be used to increase reliability (strongest one will be received)
Smart antennas use signal processing software to adapt to conditions – e.g. following a moving receiver (known as beam forming), these are some way off commercially

13. Antennas simple dipole directed antenna /4 /2 y y z x z x
side view (xy-plane)
side view (yz-plane)
top view (xz-plane) y y z
directed
antenna x z x
side view (xy-plane)
side view (yz-plane)
top view (xz-plane)

14. Signal propagation
In perfect conditions (a vacuum) wireless signals will weaken predictably
Transmission range: receivers can understand enough of the signal (i.e. low error) for data
Detection range: receivers hear the signal but cannot recover the data (i.e. high error)
Interference range: there is a signal but it is indistinguishable from other noise
Wireless is less predictable since it has to travel in unpredictable substances – air, dust, rain, bricks
distance
sender
transmission
detection
interference

15. Signal propagation: Path loss (attenuation)
In free space signals propagate as light in a straight line (independently of their frequency).
If a straight line exists between a sender and a receiver it is called line-of-sight (LOS)
Receiving power proportional to 1/d² in vacuum (free space loss) – much more in real environments (d = distance between sender and receiver)
Situation becomes worse if there is any matter between sender and receiver especially for long distances
Atmosphere heavily influences satellite transmission
Mobile phone systems are influenced by weather condition as heavy rain which can absorb much of the radiated energy

16. Signal propagation
Radio waves can penetrate objects depending on frequency. The lower the frequency, the better the penetration
Low frequencies perform better in denser materials
High frequencies can get blocked by, e.g. Trees
Radio waves can exhibit three fundamental propagation behaviours depending on their frequencies:
Ground wave (<2 MHz): follow the earth surface and can propagate long distances – submarine communication
Sky wave (2-30 MHz): These short waves are reflected at the ionosphere. Waves can bounce back and forth between the earth surface and the ionosphere, travelling around the world – International broadcast and amateur radio
Line-of-sight (>30 MHz): These waves follow a straight line of sight – mobile phone systems, satellite systems

17. Additional signal propagation effects
Receiving power additionally influenced by
fading (frequency dependent): signals can change as the receiver moves
Blocking/ Shadowing: large objects may block signals (building,..etc)
Reflection: waves can bounce off dense objects
Refraction: waves can bend through objects depending on the density of a medium
scattering : small objects may reflect multiple weaker signals
diffraction at edges
shadowing
reflection
refraction
scattering
diffraction

18. Multipath propagation
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Multipath propagation
Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction,…
Different signals use different length paths
The difference is called delay spread
Systems must compensate for the delay spread
Interference with “neighbor” symbols, Inter Symbol Interference (ISI)
Symbols may cancel each other out
Increasing frequencies suffer worse ISI
multipath
pulses
LOS pulses
signal at sender
signal at receiver
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

19. Universität Karlsruhe Institut für Telematik
Mobilkommunikation
SS 1998
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)
long term
fading
power t
short term fading
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

20. Multiplexing
Multiplexing describes how several users can share a medium with minimum or no interference
It is concerned with sharing the frequency range amongst the users
Bands are split into channels
Four main ways of assigning channels
Space Division Multiplexing (SDM) : allocate according to location
Time Division Multiplexing (TDM): allocate according to units of time
Frequency Division Multiplexing (FDM): allocate according to the frequencies
Code Division Multiplexing (CDM) : allocate according to access codes
Guard Space: gaps between allocations

21. Universität Karlsruhe Institut für Telematik
Mobilkommunikation
SS 1998
Multiplexing s2 s3 s1 f t c k2 k3 k4 k5 k6 k1
channels ki
Multiplexing in 4 dimensions
space (si)
time (t)
frequency (f)
code (c)
Goal: multiple use of a shared medium
Important: guard spaces needed!
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

22. Space Division Multiplexing (SDM)f t c k2 k3 k4 k5 k6 k1
channels ki
Space Division
This is the basis of frequency reuse
Each physical space is assigned channels
Spaces that don’t overlap can have the same channels assigned to them
Example: FM radio stations in different countries

23. Frequency Division Multiplexing (FDM)
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Frequency Division Multiplexing (FDM)
Separation of the whole spectrum into smaller non overlapping frequency bands (guard spaces are needed)
A channel gets a certain band of the spectrum for the whole time – receiver has to tune to the sender frequency
Advantages
no dynamic coordination necessary
works also for analog signals
Disadvantages
waste of bandwidth if the traffic is distributed unevenly
inflexible k1 k2 k3 k4 k5 k6 c f t
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

24. Time Division Multiplexing (TDM)
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Time Division Multiplexing (TDM)
A channel gets the whole spectrum for a certain amount of time
Guard spaces (time gaps) are needed
Advantages
only one carrier in the medium at any time
throughput high even for many users
Disadvantages
precise clock synchronization necessary k1 k2 k3 k4 k5 k6 c f t
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

25. Time and frequency multiplexing
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Time and frequency multiplexing
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
but: precise coordination required k1 k2 k3 k4 k5 k6 c f t
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

26. Code Division Multiplexing (CDM)
Instead of splitting the channel, the receiver is told which channel to access according to a pseudo-random code that is synchronised with the sender
The code changes frequently
Security: unless you know the code it is (almost) impossible to lock onto the signals
Interference: reduced as the code space is huge
Complexity: very high

27. Universität Karlsruhe Institut für Telematik
Mobilkommunikation
SS 1998
Code multiplexing
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
precise power control required
more complex signal regeneration
Implemented using spread spectrum technology k1 k2 k3 k4 k5 k6 c f t
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

28. Modulation
Definition: transforming the information to be transmitted into a format suitable for the used medium
The signals are transmitted as a sign wave which has three parameters: amplitude, frequency and phase shift.
These parameters can be varied in accordance with data or another modulating signal
Two types of modulation
Digital modulation: digital data (0, 1) is translated into an analog signal (baseband signal)
Analog modulation: the center frequency of the baseband signal generated by digital modulation is shifted up to the radio carrier

29. Why we need digital modulation?
Digital modulation is required if digital data has to be transmitted over a medium that only allows analog transmission (modems in wired networks).
Digital signals, i.e. 0/1, can be sent over wires using voltages
Wireless must use analogue sine waves
This translation is performed by digital modulation
digital data is translated into an analog signal (baseband)
Shift Keying is the translation process
Amplitude, Freq., Phase Shift Keying (ASK/FSK/PSK)
differences in:
spectral efficiency: how efficiently the modulation scheme utilizes the available frequency spectrum
power efficiency: how much power is needed to transfer bits
Robustness: how much protection against noise, interference and multi-path propagation

30. Why we need analogue modulation ?
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Why we need analogue modulation ?
Analogue modulation then moves the signal into the right part of the channel
Motivation
smaller antennas (e.g., /4)
Frequency Division Multiplexing
medium characteristics – path loss, penetration of objects, reflection,..etc
Basic schemes
Amplitude Modulation (AM)
Frequency Modulation (FM)
Phase Modulation (PM)
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

31. Modulation and demodulation
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
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
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

32. Digital Modulation - Amplitude Shift Keying (ASK)
0 and 1 represented by different amplitudes
i.e. a basic sine wave
Problem: susceptible to interference
Constant amplitude is hard to achieve
ASK is used for optical transmissions such as infra-red and fibre (simple + high performance)
In optical  light on = 1 light off = 0

33. Digital Modulation - Frequency Shift Keying (FSK)
0 and 1 represented by different frequencies
Switch between two oscillators accordingly
Twice the bandwidth but more resilient to error

34. Digital Modulation - Phase Shift Keying (PSK)
0 and 1 represented by different (longer) phases
Flip the sine wave 180 to switch between 0/1
Better still than FSK but more complex
Other modulation schemes are mostly complex variants of ASK, FSK, or PSK…

35. Digital modulation - summary
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Digital modulation - summary
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
more error resilience than AM
Phase Shift Keying (PSK):
more complex
robust against interference 1 1 t 1 1 t 1 1 t
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

36. Analog modulation
Definition: Impress an information-bearing analog waveform onto a carrier waveform for transmission

37. Spread spectrum technology
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Spread spectrum technology
Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference
Solution: spread the narrow band signal into a broad band signal using a special code
Advantage: protection against narrow band interference
Side effects:
coexistence of several signals without dynamic coordination
tap-proof
signal
power
interference
spread signal
power
spread
interference
detection at
receiver f f
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

38. Spread spectrum Basic idea
Spread the bandwidth needed to transmit data
Lower signal power, more bandwidth, same energy
Resistant to narrowband interference
Steps
Apply spreading (convert narrow band to broadband)
Send low power spread signal
Signal picks up interference
Receiver can de-spread signal
Signal is more powerful than remaining interference
Signal is therefore able to be interpreted

39. Effects of spreading and interference
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Effects of spreading and interference
dP/df
dP/df
user signal
broadband interference
narrowband interference i)
ii) f f
sender
dP/df
dP/df
dP/df
iii)
iv) v) f f f
receiver
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

40. Spreading and frequency selective fading
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
Spreading and frequency selective fading
channel quality 2 1 5 6
narrowband channels 3 4
frequency
narrow band signal
guard space 2
frequency
channel quality 1
spread spectrum
spread spectrum channels
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

41. Spread spectrum problems
Increased complexity of receivers
Raising background noise
Spread spectrum can be achieved in two different ways:
Direct Sequence
Frequency Hopping

42. Spread Spectrum – Direct Sequence Spread Spectrum (DSSS)
Each bit in original signal is represented by multiple bits in the transmitted signal
Spreading code spreads signal across a wider frequency band
XOR of the signal with pseudo-random number (chipping sequence)
many chips per bit (e.g., 128) result in higher bandwidth of the signal
Advantages
reduces frequency selective fading

43. DSSS Chipping sequence appears like noise, to others
Spreading factor S = tb /tc
If the original signal needs a bandwidth w, the resulting signal needs s*w
The exact codes are optimised for wireless
E.g. for Wi-Fi (Barker code)
For civil application spreading code between 10 and 100
For military application the spreading code is up to 10,000 tb
user data 1
XOR tc
chipping
sequence 1 1 1 1 1 1 1 1 =
resulting
signal 1 1 1 1 1 1 1
tb: bit period
tc: chip period

44. DSSS New modulation process:
Sender: Chipping  Digital Mod.  Analog Mod.
Receiver: Demod.  Chipping  Integrator  Decision?
At the receiver after the XOR operation (despreading), an integrator adds all these products, then a decision is taken for each bit period
Even if some of the chips of the spreading code are affected by noise, the receiver may recognize the sequence and take a correct decision regarding the received message bit.

45. DSSS (Direct Sequence Spread Spectrum)
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
DSSS (Direct Sequence Spread Spectrum)
spread
spectrum
signal
transmit
signal
user data X
modulator
chipping
sequence
radio
carrier
transmitter
correlator
lowpass
filtered
signal
sampled
sums
products
received
signal
data
demodulator X
integrator
decision
radio
carrier
chipping
sequence
receiver
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

46. Spread Spectrum – Frequency Hopping Spread Spectrum (FHSS)
Uses entire bandwidth for signals
Signal is broadcast over seemingly random series of radio frequencies
A number of channels allocated for the FH signal
Width of each channel corresponds to bandwidth of input signal
Signal hops from frequency to frequency at fixed intervals
Transmitter operates in one channel at a time
At each successive interval, a new carrier frequency is selected. Pattern of hopping is the hopping sequence
Time on each frequency is the dwell time
Fast hopping = many hops per bit
Slow hopping = many bits per hop
Fast hopping is more robust but more complex
FHSS is used in Bluetooth hops/s, 79 channels

47. Frequency Hopping Spread Spectrum (FHSS)
Process 1 - Spreading code modulation
The frequency of the carrier is periodically modified (hopped) following a specific sequence of frequencies.
In FHSS systems, the spreading code is this list of frequencies to be used for the carrier signal, the “hopping sequence”
The amount of time spent on each hop is known as dwell time and is typically in the range of 100 ms.
Process 2 - Message modulation
The message modulates the (hopping) carrier, thus generating a narrow band signal for the duration of each dwell, but generating a wide band signal if the process is regarded over periods of time in the range of seconds.

48. Universität Karlsruhe Institut für Telematik
Mobilkommunikation
SS 1998
FHSS
Discrete changes of carrier frequency
sequence of frequency changes determined via pseudo random number sequence
Two versions
Fast Hopping: several frequencies per user bit
Slow Hopping: several user bits per frequency
Advantages
frequency selective fading and interference limited to short period
simple implementation
uses only small portion of spectrum at any time
Disadvantages
not as robust as DSSS
simpler to detect
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

49. Universität Karlsruhe Institut für Telematik
Mobilkommunikation
SS 1998
FHSS tb
user data 1 1 1 t f td f3
slow
hopping
(3 bits/hop) f2 f1 t td f f3
fast
hopping
(3 hops/bit) f2 f1 t
tb: bit period td: dwell time
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

50. FHSS (Frequency Hopping Spread Spectrum)
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
FHSS (Frequency Hopping Spread Spectrum)
narrowband
signal
spread
transmit
signal
user data
modulator
modulator
frequency
synthesizer
hopping
sequence
transmitter
narrowband
signal
received
signal
data
demodulator
demodulator
hopping
sequence
frequency
synthesizer
receiver
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

51. Resource Management Radio Resource Management Power Management
Channel Access
Channel Assignment
Power Management
Mobility Management
Location Management
Handoff/Handover: the term handover or handoff refers to the process of transferring an ongoing call or data session from one channel to another
Example: The cellular System

52. The cellular system: cell structure
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
The cellular system: cell structure
Channel allocation: Implements space division multiplexing (SDM)
base station covers a certain transmission area (cell)
Cellular concept: channel reuse across the network prevents interference, improves the likelihood of a good signal in each 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
Expensive
fixed network needed for the base stations
handover (changing from one cell to another) necessary
interference with other cells f4 f5 f1 f3 f2 f6 f7
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

53. Universität Karlsruhe Institut für Telematik
Mobilkommunikation
SS 1998
Frequency planning
Frequency reuse only with a certain distance between the base stations
Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies
Cells are combined in clusters
All cells within a cluster use disjointed sets of frequencies
The transmission power of a sender has to
be limited to avoid interference
Standard model using 7 frequencies
To reduce interference further, sectorized antennas can be used especially for larger cell radii f4 f5 f1 f3 f2 f6 f7
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

54. Universität Karlsruhe Institut für Telematik
Mobilkommunikation
SS 1998
Frequency planning f1 f2 f3 f2 f3 f7
3 cell cluster f5 f2 f4 f6 f5 f1 f4 f3 f7 f1 f2 f3 f6 f5 f2
7 cell cluster f1 f2 f3 h1 h2 h3 g1 g2 g3
3 cell cluster
with 3 sector antennas
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller

55. Radio resource management
Channel Allocation
Channel Allocation is required to optimise frequency reuse
Fixed Channel Allocation
Dynamic Channel Allocation
Hybrid Channel Allocation 3 6 6 1 1 5 7
Frequency Reuse 7 2 2 4 3

56. Radio resource management : Channel allocation
Fixed Channel Allocation (FCA)
Permanent or semi-permanent allocation
Certain frequencies are assigned to a certain cell
Problem: different traffic load in different cells
Methods:
Simple: all cells have same number of channels
Non-uniform: optimise usage
according to expected traffic
Borrowing: channels can be
reassigned if underused
(BCA) 3 6 6 1 1 5 7
Frequency Reuse 7 2 2 4 3

57. Radio resource management
Dynamic Channel Allocation (DCA)
Gives control to base stations / switches to adapt
Channels are assigned as needed, not in advance
Base station chooses frequencies depending on the frequency already used in neighbour cells
Channels are returned when user has finished
More capacity in cells with more traffic
Assignment can also be based on interference measurements
Affecting factors include:
Blocking probability
Usage patterns and reuse distance
Current channel measurement

58. Radio resource management
Hybrid Channel Allocation (HCA)
Fixed schemes are not flexible enough
Dynamic schemes are too complex / difficult
Hybrid Schemes:
Split resources into pools of fixed and dynamic channels
Assign core of fixed channels then allocate rest dynamically
Altering the ratio may optimise the system
E.g. produce the lowest blocking rate

59. Radio resource management
Overlapping Cells
Cells are naturally overlap (ideal shape is circular)
System may push some users into adjacent cells
Cost: increased handoff rate
Handoff
Two types of channel assignment: new calls, handoff
New calls have lower priority than handoff calls
QoS Channel Access Control should favour handoff over new

60. Radio resource management
Macrocell/Microcell Overlay
Smaller cells increases frequency of handoff
Overlaying large cells on top of small ones:
Fast moving terminals are assigned channels in Macrocells
Slow moving terminals can use microcells
Overlap can be used to handoff during congestion
Increases the capacity (area)
But, increases the complexity

61. CDM cellular systems: Cell breathing
Universität Karlsruhe
Institut für Telematik
Mobilkommunikation
SS 1998
CDM cellular systems: Cell breathing
CDM instead of FDM. Do not need elaborate channel allocation schemes and complex frequency planning.
Cell size depends on current load: cell breathe
Additional traffic appears as noise to other users
If the noise level is too high users drop out of cells
Prof. Dr. Dr. h.c. G. Krüger
E. Dorner / Dr. J. Schiller