Transcript Wireless Communications and Networks

Wireless Communications
Engineering
Cellular Fundamentals
Definitions – Wireless
Communication

What is Wireless Communication?
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Ability to communicate via wireless links.
Mobile Communication =
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?
Wireless Communication
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Wireless Communication are of two
types:
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Fixed Wireless Communication
Mobile Wireless Communication.
Mobile Wireless Communication
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Mobile Wireless Communication
(Infrastructured Network)
Single Hop Wireless Link to reach a
mobile Terminal.
Mobile Communication =
+
?
Mobile Ad Hoc Networks
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Infrastructureless or Adhoc Network
Multihop Wireless path from source to
destination.
Mobile Radio Environment
Mobile Radio Environment
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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
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Transmitted signal attenuates over distance because
it is spread over larger and larger area
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This is known as free space loss and for isotropic antennas
Pt
(4d ) 2 (4fd ) 2


2
Pr

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
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For other antennas
Pt
(4d ) 2 (d ) 2


2
P r Gr Gt 
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
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Thermal noise is introduced due to thermal agitation
of electrons
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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
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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


2
Pr

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
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For other antennas
Pt
(4d ) 2 (d ) 2


2
P r Gr Gt 
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
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Thermal noise is introduced due to thermal agitation
of electrons
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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
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Bit error rate is a decreasing function of Eb/N0.
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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
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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
fd 
v

S
cos 
where
fd =change in frequency
X
due to Doppler’s shift
v = constant velocity of the
mobile receiver
λ = wavelength of the transmission
θ
Y
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
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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
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Channel is unreliable
Spectrum is scarce, and not all ranges
are suitable for mobile communication
Transmission power is often limited
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Battery
Interference to others
Advent of Cellular Systems
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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
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Cell
A cell is a small geographical area served by a
singlebase station or a cluster of base stations
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Areas divided into cells
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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
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Use multiple low-power transmitters
Areas divided into cells
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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
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Transmit frequencies are re-used across these
cells and the system becomes interference
rather than noise limited
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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
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Mobile Station
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Base Station (BS)
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users transceiver terminal (handset, mobile)
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)
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handles routing of calls in a service area
tracks user
connects to base stations and PSTN
Cellular Systems Terms (Cont’d)
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Two types of channels available between
mobile unit and BS
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Control channels – used to exchange information
for setting up and maintaining calls
Traffic channels – carry voice or data connection
between users
Handoff or handover
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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)
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Downlink or Forward Channel
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Uplink or Reverse Channel
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radio channel for transmission of information (e.g.speech)
from mobile station to base station
Paging
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radio channel for transmission of information (e.g.speech)
from base station to mobile station
a message broadcast over an entire service area, includes
use for mobile station alert (ringing)
Roaming
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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
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Mobile unit initialization
Mobile-originated call
Paging
Call accepted
Ongoing call
Handoff
Frequency Reuse
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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)
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If we limit the coverage area within the cell
by design of the antennas
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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
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the issue is to determine how many cells must intervene
between two cells using the same frequency
Radio Planning
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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
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tessellate a 2D space
represent contours of equal transmit power
Real cells are never regular in shape
Two-Dimensional Cell Clusters
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Regular geometric shapes tessellating a 2D
space: Square, triangle, and hexagon.
‘Tessellating Hexagon’ is often used to model
cells in wireless systems:
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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)
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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)
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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)
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Using this equation to locate co-channel cells, we start
from a reference cell and move i hexagons along the uaxis 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
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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
i
j
1
1
1
2
1
2
1
0
1
2
2
3
3
4
Number of Normalised
cells in rereuse
use pattern distance
N
SQRT(N)
1
1
3
1.732
7
2.646
12
3.464
13
3.606
19
4.359
21
4.583
since D = SQRT(N)
Co–channel Cell Location
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Method of locating co–channel cells
Example for N=19, i=3, j=2
Cell Planning Example
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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?
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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 cochannel interference
Remarks on Reuse Ratio
Co-channel Interference with
Omnidirectional Cell Site
Propagation model
Cochannel interference ratio
Worst-case scenario for cochannel interference
Worst-case scenario for cochannel interference
Reuse Factor and SIR
Remarks
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SIGNAL TO INTERFERENCE LEVEL IS
INDEPENDENT OF CELL RADIUS!
System performance (voice quality) only
depends on cluster size
What cell radius do we choose?
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Depends on traffic we wish to carry (smaller cell
means more compact reuse or higher capacity)
Limited by handoff
Adjacent channel interference
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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
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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)
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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
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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
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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
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Smaller cell means higher capacity (frequency reused
more).
However, smaller cell also results in higher handoff
probability, which also means higher overhead
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Moreover, cell splitting should not introduce too
much interference to users in other cells
Handoff (Handover) Process
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Handoff: Changing physical radio channels of
network connections involved in a call, while
maintaining the call
Basic reasons for a handoff
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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
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-
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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
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Intra-cell handoff
– narrow-band interference => change carrier frequency
– controlled by BSC
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Inter-cell, intra-BSC handoff
– typical handover scenario
– BSC performs the handover, assigns new radio channel in the
new cell, releases the old one
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Inter-BSC, intra-MSC handoff
– handoff between cells controlled by different BSCs
– controlled by the MSC
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Inter-MSC handoff
– handoff between cells belonging to different MSCs
– controlled by both MSCs
Handoff Types (cont’d)
Handoff Strategies
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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
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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
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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
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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
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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 midprogress 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
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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)
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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
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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