Wireless

1
Wireless Mobile CommunicationsChapter 2
Wireless Transmission

• Frequencies
• Signals
• Antennas
• Signal propagation
• Multiplexing
• Spread spectrum
• Modulation
• Cellular systems

2
Spectrum Allocation
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
VLF
LF
MF
HF
VHF
UHF
SHF
EHF
infrared
UV
visible light

• 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
• Relationship between frequency f and wave
  length ?
• ? c/f
• where c is the speed of light ? 3x108m/s

3
Frequencies Allocated for Mobile Communication

• VHF UHF ranges for mobile radio
• allows for simple, small antennas for cars
• deterministic propagation characteristics
• less subject to weather conditions gt more
  reliable connections
• SHF and higher for directed radio links,
  satellite communication
• small antennas with directed transmission
• large bandwidths 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
• weather dependent fading, signal loss caused by
  heavy rainfall, etc.

4
Allocated Frequencies

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

5
Signals I

• 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 f1/T, amplitude A, phase shift ?
• sine wave as special periodic signal for a
  carrier s(t) At sin(2 ? ft t ?t)

6
Fourier Representation of Periodic Signals
1
1
0
0
t
t
ideal periodic signal
real composition (based on harmonics)
7
Signals II

• Different representations of signals
• amplitude (amplitude domain)
• frequency spectrum (frequency domain)
• phase state diagram (amplitude M and phase ? in
  polar coordinates)
• Composite signals mapped into frequency domain
  using Fourier transformation
• Digital signals need
• infinite frequencies for perfect representation
• modulation with a carrier frequency for
  transmission (-gtanalog signal!)

Q M sin ?
A V
A V
ts
?
I M cos ?
?
f Hz
8
Antennas

• Antennas are used to radiate and receive EM waves
  (energy)
• Antennas link this energy between the ether and a
  device such as a transmission line (e.g., coaxial
  cable)
• Antennas consist of one or several radiating
  elements through which an electric current
  circulates
• Types of antennas
• omnidirectional
• directional
• phased arrays
• adaptive
• optimal
• Principal characteristics used to characterize an
  antenna are
• radiation pattern
• directivity
• gain
• efficiency

9
Isotropic Antennas

• Isotropic radiator equal radiation in all
  directions (three dimensional) - only a
  theoretical reference antenna
• Real antennas always have directive effects
  (vertical and/or horizontal)
• Radiation pattern measurement of radiation
  around an antenna

ideal isotropic radiator
10
Omnidirectional Antennas simple dipoles

• Real antennas are not isotropic radiators but,
  e.g., dipoles with lengths ?/4, or Hertzian
  dipole ?/2 (2 dipoles)? shape/size of antenna
  proportional to wavelength
• Example Radiation pattern of a simple Hertzian
  dipole
• Gain ratio of the maximum power in the direction
  of the main lobe to the power of an isotropic
  radiator (with the same average power)

?/4
y
y
z
simple dipole
x
z
x
side view (xy-plane)
side view (yz-plane)
top view (xz-plane)
11
Directional Antennas

• Often used for microwave connections (directed
  point to point transmission) or base stations for
  mobile phones (e.g., radio coverage of a valley
  or sectors for frequency reuse)

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
12
Array Antennas

• Grouping of 2 or more antennas to obtain
  radiating characteristics that cannot be obtained
  from a single element
• Antenna diversity
• switched diversity, selection diversity
• receiver chooses antenna with largest output
• diversity combining
• combine output power to produce gain
• cophasing needed to avoid cancellation

13
Signal Propagation Ranges

• Transmission range
• communication possible
• low error rate
• Detection range
• detection of the signal possible
• no communication possible, high error rate
• Interference range
• signal may not be detected
• signal adds to the background noise

sender
transmission
distance
detection
interference
14
Signal Propagation I

• Radio wave propagation is affected by the
  following mechanisms
• reflection at large obstacles
• scattering at small obstacles
• diffraction at edges

15
Signal Propagation II

• The signal is also subject to degradation
  resulting from propagation in the mobile radio
  environment. The principal phenomena are
• pathloss due to distance covered by radio signal
  (frequency dependent, less at low frequencies)
• fading (frequency dependent, related to multipath
  propagation)
• shadowing induced by obstacles in the path
  between the transmitted and the receiver

16
Signal Propagation III

• Interference from other sources and noise will
  also impact signal behavior
• co-channel (mobile users in adjacent cells using
  same frequency) and adjacent (mobile users using
  frequencies adjacent to transmission/reception
  frequency) channel interference
• ambient noise from the radio transmitter
  components or other electronic devices,
• Propagation characteristics differ with the
  environment through and over which radio waves
  travel. Several types of environments can be
  identified (dense urban, urban, suburban and
  rural) and are classified according to the
  following parameters
• terrain morphology
• vegetation density
• buildings density and height
• open areas
• water surfaces

17
Pathloss I

• Free-space pathloss
• To define free-space propagation, consider an
  isotropic source consisting of a transmitter with
  a power Pt W. At a distance d from this source,
  the power transmitted is spread uniformly on the
  surface of a sphere of radius d. The power
  density at the distance d is then as follows
• Sr Pt/4pd2
• The power received by an antenna at a distance
  d from the transmitter is then equal to
• Pr PtAe/4pd2
• where Ae is the effective area of the antenna.

18
Pathloss II

• Noting that Ae Gr/(4p/l2)
• where Gr is the gain of the receiver
• And if we replace the isotropic source by a
  transmitting antenna with a gain Gt the power
  received at a distance d of the transmitter by
  a receiving antenna of gain Gr becomes
• Pr PtGrGt/4p(d/l)2
• In decibels the propagation pathloss (PL) is
  given by
• PL(db) -10log10(Pr/Pt) -10log10(GrGt/4p(d/l)
  2)
• This is for the ideal case and can only be
  applied sensibly to satellite systems and short
  range LOS propagation.

19
Multipath Propagation I

• Signal can take many different paths between
  sender and receiver due to reflection,
  scattering, diffraction
• Positive effects of multipath
• enables communication even when transmitter and
  receiver are not in LOS conditions - allows radio
  waves effectively to go through obstacles by
  getting around them thereby increasing the radio
  coverage area

signal at sender
signal at receiver
20
Multipath Propagation II

• Negative effects of multipath
• Time dispersion or delay spread signal is
  dispersed over time due signals coming over
  different paths of different lengths
• ? Causes interference with neighboring
  symbols, this is referred to as Inter Symbol
  Interference (ISI)
• multipath spread (in secs) (longest1
  shortest2)/c
• For a 5ms symbol duration a 1ms delay spread
  means about a 20 intersymbol overlap.
• The signal reaches a receiver directly and phase
  shifted (due to reflections)
• ? Distorted signal depending on the phases of
  the different parts, this is referred to as
  Rayleigh fading, due to the distribution of the
  fades. It creates fast fluctuations of the
  received signal (fast fading).
• Random frequency modulation due to Doppler
  shifts on the different paths. Doppler shift is
  caused by the relative velocity of the receiver
  to the transmitter, leads to a frequency
  variation of the received signal.

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)

long term fading
power
t
short term fading
22
Multiplexing Techniques

• Multiplexing techniques are used to allow many
  users to share a common transmission resource. In
  our case the users are mobile and the
  transmission resource is the radio spectrum.
  Sharing a common resource requires an access
  mechanism that will control the multiplexing
  mechanism.
• As in wireline systems, it is desirable to allow
  the simultaneous transmission of information
  between two users engaged in a connection. This
  is called duplexing.
• Two types of duplexing exist
• Frequency division duplexing (FDD), whereby two
  frequency channels are assigned to a connection,
  one channel for each direction of transmission.
• Time division duplexing (TDD), whereby two time
  slots (closely placed in time for duplex effect)
  are assigned to a connection, one slot for each
  direction of transmission.

23
Multiplexing
channels ki

• Multiplexing in 3 dimensions
• time (t) (TDM)
• frequency (f) (FDM)
• code (c) (CDM)
• Goal multiple use of a shared medium

k2
k3
k4
k5
k6
k1
c
t
c
s1
t
s2
f
f
c
t
s3
f
24
Narrowband versus Wideband

• These multiple access schemes can be grouped into
  two categories
• Narrowband systems - the total spectrum is
  divided into a large number of narrow radio bands
  that are shared.
• Wideband systems - the total spectrum is used by
  each mobile unit for both directions of
  transmission. Only applicable for TDM and CDM.

25
Frequency Division Multiplexing (FDM)

• Separation of the whole spectrum into smaller
  frequency bands
• A channel gets a certain band of the spectrum for
  the whole time orthogonal system
• Advantages
• no dynamic coordination necessary, i.e., sync.
  and
• framing
• works also for analog signals
• low bit rates cheaper,
• delay spread
• Disadvantages
• waste of bandwidth if the traffic is
  distributed unevenly
• inflexible
• guard bands
• narrow filters

k2
k3
k4
k5
k6
k1
c
f
t
26
Time Division Multiplexing (TDM)

• A channel gets the whole spectrum for a certain
  amount of time orthogonal system
• Advantages
• only one carrier in themedium at any time
• throughput high - supports bursts
• flexible multiple slots
• no guard bands ?!
• Disadvantages
• Framing and precise synchronization necessary
• high bit rates
• at each
• Tx/Rx

k2
k3
k4
k5
k6
k1
c
f
t
27
Hybrid TDM/FDM

• Combination of both methods
• A channel gets a certain frequency band for a
  certain amount of time (slot).
• Example GSM, hops from one band to another each
  time slot
• Advantages
• better protection against tapping (hopping among
• frequencies)
• protection against frequency selective
  interference
• Disadvantages
• Framing and
• sync. required

k2
k3
k4
k5
k6
k1
c
f
t
28
Code Division Multiplexing (CDM)

• Each channel has a unique code
• (not necessarily orthogonal)
• 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 due to high gains required
  to reduce interference
• more complex signal regeneration

k2
k3
k4
k5
k6
k1
c
f
t
2.19.1
29
Issues with CDM

• CDM has a soft capacity. The more users the more
  codes that are used. However as more codes are
  used the signal to interference (S/I) ratio will
  drop and the bit error rate (BER) will go up for
  all users.
• CDM requires tight power control as it suffers
  from far-near effect. In other words, a user
  close to the base station transmitting with the
  same power as a user farther away will drown the
  latters signal. All signals must have more or
  less equal power at the receiver.
• Rake receivers can be used to improve signal
  reception. Time delayed versions (a chip or more
  delayed) of the signal (multipath signals) can be
  collected and used to make bit level decisions.
• Soft handoffs can be used. Mobiles can switch
  base stations without switching carriers. Two
  base stations receive the mobile signal and the
  mobile is receiving from two base stations (one
  of the rake receivers is used to listen to other
  signals).
• Burst transmission - reduces interference

30
Types of CDM I

• Two types exist
• Direct Sequence CDM (DS-CDM)
• spreads the narrowband user signal (Rbps) over
  the full spectrum by multiplying it by a very
  wide bandwidth signal (W). This is done by taking
  every bit in the user stream and replacing it
  with a pseudonoise (PN) code (a long bit sequence
  called the chip rate). The codes are orthogonal
  (or approx.. orthogonal).
• This results in a processing gain G W/R
  (chips/bit). The higher G the better the system
  performance as the lower the interference. G2
  indicates the number of possible codes. Not all
  of the codes are orthogonal.

31
Types of CDM II

• Frequency hopping CDM (FH-CDM)
• FH-CDM is based on a narrowband FDM system in
  which an individual users transmission is spread
  out over a number of channels over time (the
  channel choice is varied in a pseudorandom
  fashion). If the carrier is changed every symbol
  then it is referred to as a fast FH system, if it
  is changed every few symbols it is a slow FH
  system.

32
Orthogonality and Codes

• An m-bit PN generator generates N2m - 1
  different codes.
• Out of these codes only m codes are orthogonal
  -gt zero cross correlation.
• For example a 3 bit shift register circuit shown
  below generates N7 codes.

33
Orthogonal Codes

• A pair of codes is said to be orthogonal if the
  cross correlation is zero Rxy(0) 0 .
• For two m-bit codes x1,x2,x3,...,xm and
  y1,y2,y3,...,ym
• For example x 0011 and y 0110. Replace 0
  with -1, 1 stays as is. Then
• x -1 -1 1 1
• y -1 1 1 -1
• -----------------
• Rxy(0) 1 -1 1 -1 0

34
Example of an Orthogonal Code Walsh Codes

• In 1923 J.L. Walsh introduced a complete set of
  orthogonal codes. To generate a Walsh code the
  following two steps must be followed
• Step 1 represent a NxN matrix as four quadrants
  (start off with 2x2)
• Step 2 make the first, second and third
  quadrants indentical and invert the fourth

35
Modulation

• Digital modulation
• digital data is translated into an analog signal
  (baseband)
• ASK, FSK, PSK - main focus in this chapter
• 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)

36
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
37
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
38
Advanced Frequency Shift Keying

• bandwidth needed for FSK depends on the distance
  between the carrier frequencies
• special pre-computation avoids sudden phase
  shifts ? MSK (Minimum Shift Keying)
• bit separated into even and odd bits, the
  duration of each bit is doubled
• depending on the bit values (even, odd) the
  higher or lower frequency, original or inverted
  is chosen
• the frequency of one carrier is twice the
  frequency of the other
• even higher bandwidth efficiency using a Gaussian
  low-pass filter ? GMSK (Gaussian MSK), used in
  GSM

39
Example of MSK
1
1
1
1
0
0
0
data
bit
even 0 1 0 1
even bits
odd 0 0 1 1
signal h n n hvalue - -
odd bits
low frequency
h high frequency n low frequency original
signal - inverted signal
highfrequency
MSK signal
t
No phase shifts!
40
Advanced Phase Shift Keying

• BPSK (Binary Phase Shift Keying)
• bit value 0 sine wave
• bit value 1 inverted sine wave
• very simple PSK
• low spectral efficiency
• robust, used e.g. in satellite systems
• QPSK (Quadrature Phase Shift Keying)
• 2 bits coded as one symbol
• symbol determines shift of sine wave
• needs less bandwidth compared to BPSK
• more complex
• Often also transmission of relative, not absolute
  phase shift DQPSK - Differential QPSK (IS-136,
  PACS, PHS)

A
t
11
10
00
01
41
Quadrature Amplitude Modulation

• Quadrature Amplitude Modulation (QAM) combines
  amplitude and phase modulation
• it is possible to code n bits using one symbol
• 2n discrete levels, n2 identical to QPSK
• bit error rate increases with n, but less errors
  compared to comparable PSK schemes
• Example 16-QAM (4 bits 1 symbol)
• Symbols 0011 and 0001 have the same
  phase, but different amplitude. 0000 and 1000
  have different phase, but same amplitude.
• ? used in standard 9600 bit/s modems

Q
0010
0001
0011
0000
I
1000
42
Spread spectrum technology CDM

• 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
• protection against narrow band interference
• protection against narrowband interference
• Side effects
• coexistence of several signals without dynamic
  coordination
• tap-proof
• Alternatives Direct Sequence, Frequency Hopping

signal
interference
spread signal
power
power
spread interference
detection at receiver
f
f
43
Effects of spreading and interference
P
P
user signal broadband interference narrowband
interference
i)
ii)
f
f
sender
P
P
P
iii)
iv)
v)
f
f
f
receiver
2.28.1
44
Spreading and frequency selective fading
channelquality
2
1
5
6
narrowband channels
3
4
frequency
narrow bandsignal
guard space
spread spectrum channels
2.29.1
45
DSSS (Direct Sequence Spread Spectrum) I

• 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
• in cellular networks
• base stations can use the same frequency range
• several base stations can detect and recover the
  signal
• soft handover
• Disadvantages
• precise power control necessary

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
2.30.1
46
DSSS (Direct Sequence Spread Spectrum) II
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
2.31.1
47
FHSS (Frequency Hopping Spread Spectrum) I

• 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

2.32.1
48
FHSS (Frequency Hopping Spread Spectrum) II
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
2.33.1
49
FHSS (Frequency Hopping Spread Spectrum) III
spread transmit signal
narrowband signal
user data
modulator
modulator
hopping sequence
frequency synthesizer
transmitter
narrowband signal
received signal
data
demodulator
demodulator
hopping sequence
frequency synthesizer
receiver
2.34.1
50
Concept of Cellular Communications

• In the late 60s it was proposed to alleviate the
  problem of spectrum congestion by restructuring
  the coverage area of mobile radio systems.
• The cellular concept does not use broadcasting
  over large areas. Instead smaller areas called
  cells are handled by less powerful base stations
  that use less power for transmission. Now the
  available spectrum can be re-used from one cell
  to another thereby increasing the capacity of the
  system.
• However this did give rise to a new problem, as a
  mobile unit moved it could potentially leave the
  coverage area (cell) of a base station in which
  it established the call. This required complex
  controls that enabled the handing over of a
  connection (called handoff) to the new cell that
  the mobile unit moved into.
• In summary, the essential elements of a cellular
  system are
• Low power transmitter and small coverage areas
  called cells
• Spectrum (frequency) re-use
• Handoff

51
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

2.35.1
52
Cellular Network
53
Some Definitions

• Forward path or down link - from base station
  down to the mobile
• Reverse path or up link - from the mobile up to
  the base station
• The mobile unit - a portable voice and/or data
  comm. transceiver. It has a 10 digit telephone
  number that is represented by a 34 bit mobile
  identification number -gt [phone] is
  divided into two parts MIN1 215 translated into
  10bits and MIN2 684-3201 translated into 24bits.
  In addition each mobile unit is also permanently
  programmed at the factory with a 32 bit
  electronic serial number (ESN) which guards
  against tampering.
• The cell - a geographical area covered by Radio
  Frequency (RF) signals. It is essentially a radio
  communication center comprising radios, antennas
  and supporting equipment to enable mobile to land
  and land to mobile communication. Its shape and
  size depend on the location, height , gain and
  directivity of the antenna, the power of the
  transmitter, the terrain, obstacles such as
  foliage, buildings, propagation paths, etc. It is
  a highly irregular shape, its boundaries defined
  by received signal strength! But for traffic
  engineering purposes and system planning and
  design a hexagonal shape is used.

54
More definitions

• The base station (BS) - a transmitter and
  receiver that relays signals (control and
  information (voice or data)) from the mobile unit
  to the MSC and vice versa.
• The mobile switching center (MSC) - a switching
  center that controls a cluster of cells. Base
  stations are connected to the MSC via wireline
  links. The MSC is directly connected to the PSTN
  and is responsible for all calls related to
  mobiles located within its domain. MSCs
  intercommunicate using a link protocol specified
  by IS (International Standard) 41. This enables
  roaming of mobile units (i.e. obtaining service
  outside of the home base). The MSC is also
  responsible for billing, it keeps track of air
  time, errors, delays, blocking, call dropping
  (due to handoff failure), etc. It is also
  responsible for the handoff process, it keeps
  track of signal strengths and will initiate a
  handoff when deemed necessary (note to handoff or
  not to handoff is not a trivial issue!)

55
The Basic Cellular Communication Protocol I

• Every mobile unit whether at home or roaming, has
  to register with the MSC controlling the area it
  is in. If it does not register then the MSC does
  not know of its existence and will not be able to
  process any of its calls.
• The home location register (HLR) is used to keep
  information regarding a mobile unit/user, it is a
  database for storing and managing subscriber
  information. When roaming, a mobile unit
  registers with a foreign MSC and data from its
  HRL is relayed to the visitor location register
  (VLR). The VLR is a dynamic database used to
  store roaming mobile subscriber information. The
  HLR and VLR communicate via the MSCs using IS 41.
• The cellular system uses out of band signalling.
  Most of the control information is sent over
  different channels from the user information
  (voice or data) channels. Inband signalling is
  used for control during the connection
  (disconnect, handoff, etc.)

56
The Basic Cellular Communication Protocol II

• A mobile unit when enabled (power on) scans the
  control channels and tunes to the one with the
  strongest signal. The control channels are known
  and carry signals pertaining to the cell sites,
  e.g. transmission power to be used by the mobile
  unit in a particular cell. This process is called
  initialization.
• If the mobile wants to initiate a call, it sends
  in a service request on the reverse path control
  link. The service request contains the
  destination phone number and identification
  information (MIN1, MIN2, and ESN) of the source
  mobile unit to verify the originator.
• When the base station receives the request, it
  relays it to the MSC. The MSC then checks to see
  it is it a number of another mobile or of a
  fixed user. If the latter the call is forwarded
  to the PSTN. If the former, it checks to see if
  the destination mobile unit is a subscriber
  (local or visitor/roamer). If not it relays the
  call to the PSTN to forward to the appropriate
  MSC.

57
The Basic Cellular Communication Protocol III

• If the destination is within its cluster it sends
  out a paging message to all the base stations.
  Every base station then relays this message by
  broadcasting it on its control channel. If the
  destination mobile unit is enabled (power on) it
  will detect this message and respond to the base
  station.
• The base station relays this response to the MSC.
  The MSC then allocates channels to both the
  source mobile unit and the destination mobile
  unit. The corresponding base stations pass this
  information on to the respective mobile units.
  The mobile units then tune to the correct
  channels and the communication link is
  established.

58
Spectrum and Capacity Issues

• Spectrum is limited

59
Frequency Re-use I

• To be able to increase the capacity of the
  system, frequencies must be re-used in the
  cellular layout (unless we are using spread
  spectrum techniques).
• Frequencies cannot be re-used in adjacent cells
  because of co-channel interference. The cells
  using the same frequencies must be dispersed
  across the cellular layout. The closer the
  spacing the more efficient the scheme!

60
Frequency Re-use II

• For an omni-directional antenna, with constant
  signal power, each cell site coverage area would
  be circular (barring any terrain irregularities
  or obstacles).
• To achieve full coverage without dead spots, a
  series of regular polygons for cell sites are
  required.
• The hexagonal was chosen as it comes the closest
  to the shape of a circle, and a hexagonal layout
  requires fewer cells (when compared to triangles
  or rectangles, it has the largest surface area
  given the same radius R) -gt less cells.
• Goal is to find the minimum distance between
  cells using same frequencies.

61
Frequency re-use distance I
62
Frequency re-use distance II

• For two adjacent cells D31/2R
• The closest we can place the same frequencies is
  called the first tier around the center cell
  (minimal re-use distance -gt lower -gt more
  capacity!).
• For simplicity we only take the first tier of
  cells into account for co-channel interference
  (i.e., we ignore 2nd, 3rd, etc. tiers, cause much
  less interference, negligible!).

63
Frequency re-use distance III
64
Frequency re-use distance III

• Radius dist. between two co-channel cells
  (3R2i2j2ij)1/2 D
• Since the area of a hexagon is proportional to
  the square of the distance between its center and
  a vertex (i.e., its radius), the area of the
  large hexagon is
• Alarge kRadius2 k3R2i2j2ij
• where k is a constant.
• Similarly the area of each cell (i.e., small
  hexagon) is
• Asmall kR2
• Comparing these expressions we find that
• Alarge/Asmall 3i2j2ij D2/R2

65
Frequency re-use distance IV

• From symmetry we can see that the large hexagon
  encloses the center cluster of N cells plus 1/3
  the number of the cells associated with 6 other
  peripheral hexagons. Thus the total number of
  cells enclosed by the first tier is
• N6(1/3N) 3N
• Since the area of a hexagon is proportional to
  the number of cells contained within it
• Alarge/Asmall 3N/1 3N
• Substituting we get
• 3N 3i2j2ij D2/R2
• Or
• D/R q (3N)1/2
• q is referred to as the reuse ratio!

66
Co-channel Interference I

• The co-channel interference ratio S/I is given
  as
• S desired signal power in a cell (note that
  many texts use C instead of S), Ik
  interference signal power from the kth cell, Ni
  number of interfering cells.
• If we only assume the first tier of interfering
  cells, then Ni6,and all cells interfere equally
  (they are all equidistant!).
• The signal power at any point is inversely
  proportional to the inverse of the distance from
  the source raised to the g power. (2ltglt5)

67
Co-channel Interference II

• Ik is proportional to Dg , and S is proportional
  to Rg , where g is the propagation path loss
  and is dependent upon terrain environment. For
  cellular systems it is often taken as 4.
• Therefore
• The relationship between SNR (signal to noise
  ratio - Eb/No) and S/I for cellular systems with
  Rayleigh fading channels SNR S/I(db) 9db.

68
For a given S/I how to get N

• Recall that D/R q (3N)1/2
• An S/I 18db (decibels10logS/I) 63.1, gives
  an acceptable voice quality.
• Therefore q 6x63.11/4 4.41 when g 4
• Substituting for N we get N (4.41)2/3 equals
  approx. 7
• This means that if we have 49 frequency channels
  available, each cell gets 49/7 7 frequency
  channels.
• If we have 82 available then 82/7 11.714 -gt
  which means that 5 cells will have 12 and 2 cells
  will have 11!
• How does that translate to i and j for a cell
  layout?
• N i2j2ij, find i,j that satisfy the
  equation!

69
Calculating i, j, and D from N
70
Frequency planning

• Frequency reuse only with a certain distance
  between the base stations
• Standard model using 7 frequencies
• Fixed frequency assignment
• certain frequencies are assigned to a certain
  cell
• problem different traffic load in different
  cells
• Dynamic frequency assignment
• base station chooses frequencies depending on the
  frequencies already used in neighbor cells
• more capacity in cells with more traffic
• assignment can also be based on interference
  measurements

2.36.1
71
Increasing Capacity

• We can see that by reducing the area of a cell we
  can increase capacity as we will have more cells
  each with its own set of frequencies.
• What is drawback of shrinking the size of the
  cells (cell splitting)? Increase in the number of
  handoffs -gt increased load on the system! Also
  need more infrastrucutre -gt base stations (each
  cell needs a BS).
• An easier solution exists, sectorization. It does
  not reduce handoffs, its advantage it does not
  require more infrastructure.

72
Sectorization I

• We can also increase the capacity by using
  sectors in cells.
• Directional antennas instead of being
  omnidirectional, will only beam over a certain
  angle.

3 cell cluster
3 cell cluster with 3 sectors
73
Sectorization II

• What does that mean?
• We can now assign frequency sets to sectors and
  decrease the re-use distance or improve S/I ratio
  (i.e. signal quality).
• Question By how much? Depends on number of
  sectors (i.e., 60 or 120).

74
Other Capacity or Signal Improvement Tech.

• Dynamic channel allocation (DCA) allows cells to
  borrow frequencies from other cells within the
  cluster if not used by them. Can be used to
  alleviate hotspots. Another implementation
  basically has all channels available to all
  cells, they get allocated based upon demand.
• Power control by reducing the transmitted power,
  the battery life of a mobile can be extended. It
  also helps in reducing -channel and adjacent
  channel interference.