Wireless Communications

Engineering

Cellular Fundamentals
 Definitions – Wireless
Communication
 What is Wireless Communication?

 Ability to communicate via wireless links.

Mobile Communication = + ?
Wireless Communication
 Wireless Communication are of two
types:
 Fixed Wireless Communication
 Mobile Wireless Communication.
Mobile Wireless Communication
 Mobile Wireless Communication
(Infrastructured Network)
Single Hop Wireless Link to reach a
mobile Terminal.

Mobile Communication = + ?
Mobile Ad Hoc Networks
 Infrastructureless or Adhoc Network
Multihop Wireless path from source to
destination.
Mobile Radio Environment
Mobile Radio Environment
 The transmissions over the wireless link are in
general very difficult to characterize.
 EM signals often encounter obstacles, causing
reflection, diffraction, and scattering.
 Mobility introduces further complexity.
 We have focused on simple models to help gain basic
insight and understanding of the wireless radio
medium.
 Three main components: Path Loss, Shadow fading,
Multipath fading (or fast fading).
Free Space loss
 Transmitted signal attenuates over distance because
it is spread over larger and larger area
 This is known as free space loss and for isotropic antennas

Pt (4d ) 2 (4fd ) 2
 
Pr 2
c2
Pt = power at the transmitting antenna
Pr = power at the receiving antenna
λ = carrier wavelength
d = propagation distance between the antennas
c = speed of light
Free Space loss
 For other antennas
Pt (4d ) 2 (d ) 2
 
P r Gr Gt  2
Ar At
Gt = Gain of transmitting antenna
Gr = Gain of receiving antenna
At = effective area of transmitting antenna
Ar = effective area of receiving antenna
Thermal Noise
 Thermal noise is introduced due to thermal agitation
of electrons
 Present in all transmission media and all electronic devices
 a function of temperature
 uniformly distributed across the frequency spectrum and
hence is often referred to as white noise
 amount of noise found in a bandwidth of 1 Hz is
N0 = k T
N0 = noise power density in watts per 1 Hz of bandwidth
k = Boltzman’s constant = 1.3803 x 10-23 J/K
T = temperature, in Kelvins
N = thermal noise in watts present in a bandwidth of B
= kTB where
Free Space loss
 Transmitted signal attenuates over distance because
it is spread over larger and larger area
 This is known as free space loss and for isotropic antennas

Pt (4d ) 2 (4fd ) 2
 
Pr 2
c2
Pt = power at the transmitting antenna
Pr = power at the receiving antenna
λ = carrier wavelength
d = propagation distance between the antennas
c = speed of light
Free Space loss
 For other antennas
Pt (4d ) 2 (d ) 2
 
P r Gr Gt  2
Ar At
Gt = Gain of transmitting antenna
Gr = Gain of receiving antenna
At = effective area of transmitting antenna
Ar = effective area of receiving antenna
Thermal Noise
 Thermal noise is introduced due to thermal agitation
of electrons
 Present in all transmission media and all electronic devices
 a function of temperature
 uniformly distributed across the frequency spectrum and
hence is often referred to as white noise
 amount of noise found in a bandwidth of 1 Hz is
N0 = k T
N0 = noise power density in watts per 1 Hz of bandwidth
k = Boltzman’s constant = 1.3803 x 10-23 J/K
T = temperature, in Kelvins
N = thermal noise in watts present in a bandwidth of B
= kTB where
 Data rate and error rate
 Bit error rate is a decreasing function of Eb/N0.

 If bit rate R is to increase, then to keep bit error rate (or Eb/N0)
same, the transmitted signal power must increase, relative to
noise

 Eb/N0 is related to SNR as follows

Eb S B

N0 N R

B = signal bandwidth
(since N = N0 B)
Doppler’s Shift
 When a client is mobile, the frequency of received
signal could be less or more than that of the
transmitted signal due to Doppler’s effect

 If the mobile is moving towards the direction of

arrival of the wave, the Doppler’s shift is positive

 If the mobile is moving away from the direction of

arrival of the wave, the Doppler’s shift is negative
Doppler’s Shift S

v
fd  cos 


where
fd =change in frequency θ
X
due to Doppler’s shift Y
v = constant velocity of the
mobile receiver
λ = wavelength of the transmission
Doppler’s shift

f = fc + fd
where
f = the received carrier frequency
fc = carrier frequency being transmitted
fd = Doppler’s shift as per the formula in the previous
slide.
Multipath Propagation
 Wireless signal can arrive at the receiver through
different paths
 LOS

 Reflections from objects

 Diffraction

 Occurs at the edge of an impenetrable body

that is large compared to the wavelength of the

signal
Multipath Propagation (source: Stallings)
Mobile Radio Channel: Fading
Limitations of Wireless
 Channel is unreliable
 Spectrum is scarce, and not all ranges
are suitable for mobile communication
 Transmission power is often limited
 Battery
 Interference to others
Advent of Cellular Systems
 Noting from the channel model, we know
signal will attenuated with distance and have
no interference to far users.
 In the late 1960s and early 1970s, work
began on the first cellular telephone systems.
 The term cellular refers to dividing the service
area into many small regions (cells) each
served by a low-power transmitter with
moderate antenna height.
Cell Concept
 Cell
A cell is a small geographical area served by a
singlebase station or a cluster of base stations

 Areas divided into cells

 Each served by its own antenna
 Served by base station consisting of transmitter,
receiver, and control unit
 Band of frequencies allocated
 Cells set up such that antennas of all neighbors
are equidistant
Cellular Networks
 Cellular Network Organization
 Use multiple low-power transmitters
 Areas divided into cells
 Each served by its own antenna
 Served by base station consisting of transmitter,
receiver, and control unit
 Band of frequencies allocated
 Cells set up such that antennas of all neighbors are
equidistant
Consequences
 Transmit frequencies are re-used across these
cells and the system becomes interference
rather than noise limited
 the need for careful radio frequency planning –
colouring in hexagons!
 a mechanism for handling the call as the user
crosses the cell boundary - call handoff (or
handover)
 increased network complexity to route the call and
track the users as they move around
 But one significant benefit: very much
increased traffic capacity, the ability to service
many users
Cellular System Architecture
Cellular Systems Terms
 Mobile Station
 users transceiver terminal (handset, mobile)
 Base Station (BS)
 fixed transmitter usually at centre of cell
 includes an antenna, a controller, and a number of
receivers
 Mobile Telecommunications Switching Office
(MTSO) /Mobile Switch Center (MSC)
 handles routing of calls in a service area
 tracks user
 connects to base stations and PSTN
Cellular Systems Terms (Cont’d)
 Two types of channels available between
mobile unit and BS
 Control channels – used to exchange information
for setting up and maintaining calls
 Traffic channels – carry voice or data connection
between users
 Handoff or handover
 process of transferring mobile station from one
base station to another, may also apply to change
of radio channel within a cell
Cellular Systems Terms (Cont’d)
 Downlink or Forward Channel
 radio channel for transmission of information (e.g.speech)
from base station to mobile station
 Uplink or Reverse Channel
 radio channel for transmission of information (e.g.speech)
from mobile station to base station
 Paging
 a message broadcast over an entire service area, includes
use for mobile station alert (ringing)
 Roaming
 a mobile station operating in a service area other than the
one to which it subscribes
Steps in an MTSO Controlled
Call between Mobile Users

 Mobile unit initialization

 Mobile-originated call
 Paging
 Call accepted
 Ongoing call
 Handoff
Frequency Reuse
 Cellular relies on the intelligent allocation and
re–use of radio channels throughout a
coverage area.
 Each base station is allocated a group of
radio channels to be used within the small
geographic area of its cell
 Neighbouring base stations are given
different channel allocation from each other
Frequency Reuse (Cont’d)
 If we limit the coverage area within the cell
by design of the antennas
 we can re-use that same group of frequencies to
cover another cell separated by a large enough
distance
 transmission power controlled to limit power at that
frequency to keep interference levels within
tolerable limits
 the issue is to determine how many cells must intervene
between two cells using the same frequency
 Radio Planning
 Design process of selecting and allocating
channel frequencies for all cellular base
stations within a system is known as
frequency re-use or frequency planning.
 Cell planning is carried out to find a
geometric shape to
 tessellate a 2D space
 represent contours of equal transmit power
 Real cells are never regular in shape
Two-Dimensional Cell Clusters
 Regular geometric shapes tessellating a 2D
space: Square, triangle, and hexagon.
 ‘Tessellating Hexagon’ is often used to model
cells in wireless systems:
 Good approximation to a circle (useful when
antennas radiate uniformly in the x-y directions).
 Also offer a wide variety of reuse pattern
 Simple geometric properties help gain basic
understanding and develop useful models.
Coverage Patterns
Cellular Coverage Representation
Geometry of Hexagons

Hexagonal cell geometry and axes

Geometry of Hexagons (Cont’d)
 D = minimum distance between centers of
cells that use the same band of frequencies
(called co-channels)
 R = radius of a cell
d = distance between centers of adjacent
cells (d = R√3)
 N = number of cells in repetitious pattern
(Cluster) Reuse factor
 Each cell in pattern uses unique band of
frequencies
Geometry of Hexagons (Cont’d)
 The distance between the nearest cochannel cells in
a hexagonal area can be calculated from the previous
figure
 The distance between the two adjacent co-channel
cells is D=√3R.
 (D/d)2 = j2 cos2(30) + (i+ jsin30)2
= i2 + j2 +ij = N
 D=Dnorm x √3 R =(√3N)R
 In general a candidate cell is surrounded by 6k cells
in tier k.
 Geometry of Hexagons (Cont’d)
 Using this equation to locate co-channel cells, we start
from a reference cell and move i hexagons along the u-
axis then j hexagons along the v-axis. Hence the
distance between co–channel cells in adjacent clusters is
given by:
 D = (i2 + ij + j2)1/2
 where D is the distance between co–channel cells in
adjacent clusters (called frequency reuse distance).
 and the number of cells in a cluster, N is given by D2
 N = i2 + ij + j2
Hexagon Reuse Clusters
3-cell reuse pattern (i=1,j=1)
4-cell reuse pattern (i=2,j=0)
7-cell reuse pattern (i=2,j=1)
12-cell reuse pattern (i=2,j=2)
19-cell reuse pattern (i=3,j=2)
Relationship between Q and N
Proof
Cell Clusters
Reuse coordinates Number of Normalised
cells in re- reuse
use pattern distance
i j N SQRT(N)
1 0 1 1
1 1 3 1.732
1 2 7 2.646
2 2 12 3.464
1 3 13 3.606
2 3 19 4.359
1 4 21 4.583

since D = SQRT(N)
Co–channel Cell Location

 Method of locating co–channel cells

 Example for N=19, i=3, j=2
 Cell Planning Example
 Suppose you have 33 MHz bandwidth available,
an FM system using 25 kHz channels, how many
channels per cell for 4,7,12 cell re-use?
 total channels = 33,000/25 = 1320
 N=4 channels per cell = 1320/4 = 330
 N=7 channels per cell = 1320/7 = 188
 N=12 channels per cell = 1320/12 = 110
 Smaller clusters can carry more traffic
 However, smaller clusters result in larger co-
channel interference
Remarks on Reuse Ratio
Co-channel Interference with
Omnidirectional Cell Site
Propagation model
Cochannel interference ratio
Worst-case scenario for co-
channel interference
Worst-case scenario for co-
channel interference
Reuse Factor and SIR
Remarks
 SIGNAL TO INTERFERENCE LEVEL IS
INDEPENDENT OF CELL RADIUS!
 System performance (voice quality) only
depends on cluster size
 What cell radius do we choose?
 Depends on traffic we wish to carry (smaller cell
means more compact reuse or higher capacity)
 Limited by handoff
Adjacent channel interference
 So far, we assume adjacent channels to be
orthogonal (i.e., they do not interfere with
each other).
 Unfortunately, this is not true in practice, so
users may also experience adjacent channel
interference besides co-channel interference.
 This is especially serious when the near-far
effect (in uplinks) is significant
 Desired mobile user is far from BS

 Many mobile users exist in the cell

Near-Far Effect
Near-Far Effect (Cont’d)
Reduce Adjacent channel
interference
 Use modulation schemes which have small
out-of-band radiation (e.g., MSK is better
than QPSK)
 Carefully design the receiver BPF
 Use proper channel interleaving by assigning
adjacent channels to different cells, e.g., for
N=7
Reduce Adjacent channel
interference (Cont’d)
 Furthermore, do not use adjacent channels in
adjacent cells, which is possible only when N
is very large. For example, if N =7, adjacent
channels must be used in adjacent cells

 Use FDD or TDD to separate the forward link

and reverse link.
 Improving Capacity in Cellular
Systems
 Adding new channels – often expensive or
impossible
 Frequency borrowing (or DCA)– frequencies are
taken from adjacent cells by congested cells
 Cell splitting – cells in areas of high usage can be
split into smaller cells (microcells with antennas
moved to buildings, hills, and lamp posts)
 Cell sectoring – cells are divided into a number of
wedge-shaped sectors, each with their own set of
channels
Sectoring
 Co-channel interference reduction with
the use of directional antennas
(sectorization)
 Each cell is divided into sectors and
uses directional antennas at the base
station.
 Each sector is assigned a set of
channels (frequencies).
Site Configurations
120 Sectorized Cell Sites
Worst case scenario
Sectorizd Cell Sites
60
Worst case scenario
Illustration of cell splitting 1
Illustration of cell splitting 2
Illustration of cell splitting 3
Cell Splitting
Design Tradeoff
 Smaller cell means higher capacity (frequency reused
more).

 However, smaller cell also results in higher handoff

probability, which also means higher overhead

 Moreover, cell splitting should not introduce too

much interference to users in other cells
Handoff (Handover) Process
 Handoff: Changing physical radio channels of
network connections involved in a call, while
maintaining the call
 Basic reasons for a handoff
 MS moves out of the range of a BTS (signal level
becomes too low or error rate becomes too high)
 Load balancing (traffic in one cell is too high, and
shift some MSs to other cells with a lower load)
 GSM standard identifies about 40 reasons for a
handoff!
Phases of Handoff
 MONITORING PHASE
- measurement of the quality of the current and possible
candidate radio links
- initiation of a handover when necessary
 HANDOVER HANDLING PHASE
- determination of a new point of attachment
- setting up of new links, release of old links
- initiation of a possible re-routing procedure
Handoff Types
 Intra-cell handoff
– narrow-band interference => change carrier frequency
– controlled by BSC
 Inter-cell, intra-BSC handoff
– typical handover scenario
– BSC performs the handover, assigns new radio channel in the
new cell, releases the old one
 Inter-BSC, intra-MSC handoff
– handoff between cells controlled by different BSCs
– controlled by the MSC
 Inter-MSC handoff
– handoff between cells belonging to different MSCs
– controlled by both MSCs
Handoff Types (cont’d)
Handoff Strategies
 Relative signal strength
 Relative signal strength with threshold
 Relative signal strength with hysteresis
 Relative signal strength with hysteresis and
threshold
 Prediction techniques
Intra-MSC Handoff (Mobile
Assisted)
Handover Scenario at Cell
Boundary
Handoff Based on Receive Level

How to avoid ping-pong problem?

Handoff – 1G (Analog) systems

 Signal strength measurements made by the

BSs and supervised by the MSC
 BS constantly monitors the signal strengths of
all the voice channels
 Locator receiver measures signal strength of
MSs in neighboring cells
 MSC decides if a handover is necessary
Handoff – 2G (Digital) TDMA
 Handoff decisions are mobile assisted
 Every MS measures the received power from
surrounding BSs and sends reportsto its own
BS
 Handoff is initiated when the power received
from a neighbor BS begins to exceed the
power from the current BS (by a certain level
and/or for a certain period)
Handoff – 2G (Digital) CDMA

 CDMA uses code to differentiate users.

 Soft handoff: a user keeps records of several

neighboring BSs.

 Soft handoff may decrease the handoff

blocking probability and handoff delay
Avoiding handoff: Umbrella cells
Mixed Cell Architecture
Handoff Prioritization
 The idea of reserving channels for handoff
calls was introduced in the mid 1980s as a
way of reducing the handoff call blocking
probability
 Motivation: users find calls blocked in mid-
progress a far greater irritant than
unsuccessful call attempts.
 The basic idea is to reserve a certain portion
of the total channel pool in a cell for handoff
users only.
Performance Metrics
 Call blocking probability – probability of a
new call being blocked
 Call dropping probability – probability that
a call is terminated due to a handoff
 Call completion probability – probability
that an admitted call is not dropped before it
terminates
 Handoff blocking probability – probability
that a handoff cannot be successfully
completed
Performance Metrics (Cont’d)
 Handoff probability – probability that a
handoff occurs before call termination
 Rate of handoff – number of handoffs per
unit time
 Interruption duration – duration of time
during a handoff in which a mobile is not
connected to either base station
 Handoff delay – distance the mobile moves
from the point at which the handoff should
occur to the point at which it does occur
Summary
 cellular mobile uses many small cells
 hexagonal planning, clusters of cells
 cell repeat patterns 3,7,12 etc...
 re-uses frequencies to obtain capacity
 is interference not noise (kTB) limited
 S/I is independent of cell radius
 choose cell radius to meet traffic demand
 N=7 is a good compromise between S/I and
capacity.
 handoff