Wireless Radio Communications

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

• 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

coax cable
twisted pair
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
visible light
VLF
LF
MF
HF
VHF
UHF
SHF
EHF
infrared
UV
5
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.

6
Frequencies and regulations

• ITU-R holds auctions for new frequencies, manages
  frequency bands worldwide (WRC, World Radio
  Conferences)

Examples Europe USA Japan
Cellular phones GSM 880-915, 925-960, 1710-1785, 1805-1880 UMTS 1920-1980, 2110-2170 AMPS, TDMA, CDMA, GSM 824-849, 869-894 TDMA, CDMA, GSM, UMTS 1850-1910, 1930-1990 PDC, FOMA 810-888, 893-958 PDC 1429-1453, 1477-1501 FOMA 1920-1980, 2110-2170
Cordless phones CT1 885-887, 930-932 CT2 864-868 DECT 1880-1900 PACS 1850-1910, 1930-1990 PACS-UB 1910-1930 PHS 1895-1918 JCT 245-380
Wireless LANs 802.11b/g 2412-2472 802.11b/g 2412-2462 802.11b 2412-2484 802.11g 2412-2472
Other RF systems 27, 128, 418, 433, 868 315, 915 426, 868
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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
• signal parameters parameters representing the
  value of data
• signal parameters of periodic signals period T,
  frequency f1/T, amplitude A, phase shift ?
• sine wave as special periodic signal for a
  carrier s(t) At sin(2 ? ft t ?t)

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Fourier representation of periodic signals
1
1
0
0
t
t
ideal periodic signal
real composition (based on harmonics)

• It is easy to isolate/ separate signals with
  different frequencies using filters

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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

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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
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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

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Antennas
?/4
?/2
y
y
z
simple dipole
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)
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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

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

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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 (lt2 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 (gt30 MHz) These waves follow a
  straight line of sight mobile phone systems,
  satellite systems

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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

refraction
reflection
scattering
diffraction
shadowing
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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
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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 averagepower received
  (long term fading)

long term fading
power
t
short term fading
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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

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Multiplexing

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

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Space Division Multiplexing (SDM)

• Space Division
• This is the basis of frequency reuse
• Each physical space is assigned channels
• Spaces that dont overlap can have the same
  channels assigned to them
• Example FM radio stations in different countries

23
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

k2
k3
k4
k5
k6
k1
c
f
t
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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 themedium at any time
• throughput high even for many users
• Disadvantages
• precise clocksynchronization necessary

k2
k3
k4
k5
k6
k1
c
f
t
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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 coordinationrequired

k2
k3
k4
k5
k6
k1
c
f
t
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Code Division Multiplexing (CDM)

• Code Division
• 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

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Code multiplexing

• Each channel has a unique code
• All channels use the same spectrum at the same
  time
• Advantages
• bandwidth efficient
• no coordination and synchronizationnecessary
• good protection against interferenceand tapping
• Disadvantages
• precise power control required
• more complex signal regeneration
• Implemented using spread spectrum technology

k2
k3
k4
k5
k6
k1
c
f
t
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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

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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 ?

• 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)

31
Modulation and demodulation
analog baseband signal
digital data
digital modulation
analog modulation
radio transmitter
101101001
radio carrier
analog baseband signal
digital data
synchronization decision
analog demodulation
radio receiver
101101001
radio carrier
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Digital Modulation - Amplitude Shift Keying (ASK)

• 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)

• 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)

• 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

• 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
0
1
t
1
0
1
t
1
0
1
t
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Analog modulation

• Definition Impress an information-bearing analog
  waveform onto a carrier waveform for transmission

37
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
interference
power
power
spread signal
spread interference
detection at receiver
f
f
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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
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
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Spreading and frequency selective fading
channelquality
2
1
5
6
narrowband channels
3
4
frequency
narrow bandsignal
guard space
spread spectrum channels
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Spread spectrum problems

• 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 sw
• The exact codes are optimised for wireless
• E.g. for Wi-Fi 10110111000 (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
0
1
XOR
tc
chipping sequence
0
1
1
0
1
0
1
0
1
0
0
1
1
1

resulting signal
0
1
1
0
0
1
0
1
1
0
1
0
0
1
tb bit period tc chip period
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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)
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
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 - 1600 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
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

49
FHSS
tb
user data
0
1
0
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
50
FHSS (Frequency Hopping Spread Spectrum)
narrowband signal
spread transmit signal
user data
modulator
modulator
hopping sequence
frequency synthesizer
transmitter
narrowband signal
received signal
data
demodulator
demodulator
hopping sequence
frequency synthesizer
receiver
51
Resource Management

• Radio Resource 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

• 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

53
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

54
Frequency planning
f3
f7
f2
f5
f2
3 cell cluster
f4
f6
f5
f1
f4
f3
f7
f1
f2
f3
f6
f2
f5
7 cell cluster
3 cell cluster with 3 sector antennas
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
1
6
1
5
7
2
7
Frequency Reuse
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
1
6
1
5
7
2
7
Frequency Reuse
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

• 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