a glimps at cellular mobile radio communications · –microcell zone concept (if interested...
TRANSCRIPT
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A Glimps at Cellular
Mobile Radio
Communications
By: Assoc. Prof. Dr. Erhan A. İnce
Electrical and Electronic Engineering Dept.
FALL 2016
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CELLULAR Cellular refers to communications systems that divide a
geographic region into sections, called cells.
Hexagonal cell shape is perfect over square , circular or
triangular cell shapes because it covers an entire area
without overlapping i.e. they can cover the entire
geographical region without any gaps.
The purpose of this division is to make the most use out of
a limited number of transmission frequencies.
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Each connection, or conversation, requires its own dedicated
frequency
Suppose,
total Bandwidth = 33 MHz
channel Bandwidth = 50 KHz
total available channel = 33000/50
= 660 channels
If N is the number of cells in a cluster
Now, for N = 4, channel/cell = 660/4 = 165 channels
for N = 7, channel/cell = 660/7 = 94.286 94 channels
for N = 12, channel/cell = 660/12 = 55 channels
Hence, from above, we see that the number of channel per
cell (capacity) increases as N decreases and vice versa.
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MOBILE Cellular phones allow a person to make or receive a call from
almost anywhere. Likewise, a person is allowed to continue
the phone conversation while on the move. Hence MOBILE.
Cellular communications is supported by an infrastructure
called a cellular network, which integrates cellular phones into
the public switched telephone network.
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Cellular Concepts
• developed by Bell Labs 1960’s-70’s
• areas divided into cells
• Each cell is served by a base station with a low power transmitter
• Each cell gets portion of total number of channels
• Neighboring cells assigned different groups of channels, to minimize interference
• The available channels can be reused as many times as necessary as long as the
interference between co-channel stations is kept below acceptable levels
• Cells using the same channels should be spaced enough to reduce co-channel
interference
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HSCSD: High-Speed Circuit-Switched Data
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First-generation: Analog cellular systems (450-900 MHz)
Frequency shift keying for signaling
FDMA for spectrum sharing
NMT (Europe), AMPS (US)
NMT: Nordic Mobile Telephone, AMPS: Advanced Mobile Phone System
Second-generation: Digital cellular systems (900, 1800 MHz)
TDMA/CDMA for spectrum sharing
Circuit switching
GSM (Europe), IS-136 (US), PDC (Japan)
GSM: Global System for Mobile Communications
PDC: Personal Digital Cellular
2.5G: Packet switching extensions
Digital: GSM to GPRS (General Packet Radio Service)
Analog: AMPS to CDPD (Cellular Digital Packet Data )
3G:
High speed, data and Internet services
IMT-2000 (International Mobile Telecommunications for the year 2000 )
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Cellular Structure
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In a cellular system the hand-sets carried by the users are called
Mobile Stations (MS).
The Mobile Stations communicate to the Base Stations (BS)
through a pair of frequency channels, one for up-link and another
for down-link.
All the base stations of a Cellular systems are controlled by a
central switching station called Mobile Switching Center (MSC) or
Mobile Telephone Switching Office (MTSO).
The MSC is responsible for all kinds of network management
functions such as channel allocations, Handoffs, billing, power
control etc.
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A cell = basic geographical unit of a cellular network; is the area around an antenna where a specific frequency range is used.
when a subscriber moves to another cell, the antenna of the new cell takes over the signal transmission
a cluster is a group of adjacent cells, usually 7 cells; no frequency reuse is done within a cluster
the frequency spectrum is divided into sub-bands and each sub-band is used within one cell of the cluster
in heavy traffic zones cells are smaller, while in isolated zones cells are larger
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CAPACITY IN A CELLULAR SYSTEM
Capacity in a cell is basically the number of users the
operator can give service to.
High demand for cellular service, especially in large urban
markets, has created a need to serve a greater number of
users in a limited amount of frequency space.
Cellular system operators are looking for new ways to fit
more users into their increasingly crowded network
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Cellular Systems-Basic Concepts Cellular system solves the problem of spectral congestion.
High capacity is achieved by limiting the coverage area of
each BS to a small geographical area called cell.
Replaces high powered transmitter with several low power
transmitters.
Each BS is allocated a portion of total channels and nearby
cells are allocated completely different channels.
All available channels are allocated to small no of neighboring
BS.
Interference between neighboring BSs is minimized by
allocating different channels.
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Cellular Systems-Basic Concepts Same frequencies are reused by spatially
separated BSs.
Interference between co-channels stations is
kept below acceptable level.
Additional radio capacity is achieved.
Frequency Reuse-Fix no of channels serve
an arbitrarily large no of subscribers
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Frequency Reuse used by service providers to improve the efficiency of a
cellular network and to serve millions of subscribers using a
limited radio spectrum
After covering a certain distance a radio wave gets attenuated
and the signal falls below a point where it can no longer be
used or cause any interference
A transmitter transmitting in a specific frequency range will
have only a limited coverage area
Beyond this coverage area, that frequency can be reused by
another transmitter.
The entire network coverage area is divided into cells based on the principle of frequency reuse
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Frequency Reuse
The design process of selecting and
allocating channel groups for all of the
cellular base stations within a system is
called frequency reuse or frequency planning.
Cell labeled with same letter use the same
set of frequencies.
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Frequency Reuse
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Cluster Size
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Cluster Size vs Capacity A cellular system has ‘S’ duplex channels,
each cell is allocated ‘k’ channels(k<S)
For N cells ,the total no of available channels
is S=kN.
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Cluster Size vs Capacity N cells collectively using all the channels is called a cluster, is
a group of adjacent cells.
If cluster if repeated M times, the capacity C of system is
given as
C=MkN=MS
Capacity of system is directly proportional to the no of times
cluster is repeated. Reducing the cluster size N while keeping the cell size
constant, more clusters are required to cover the given area
and hence more capacity.
Co-channel interference is dependent on cluster size, large
cluster size less interference and vice versa.
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Frequency Reuse The Frequency Reuse factor is given as 1/N, each cell is
assigned 1/N of total channels.
Lines joining a cell and each of its neighbor are separated by
multiple of 600, only certain cluster sizes and cell layouts are
possible
Geometery of hexagon is such that no of cells per cluster i.e
N, can only have values which satisfy the equation
N=i2+ij+j2
N, the cluster size is typically 4, 7 or 12.
In GSM normally N =7 is used.
i and j are integers, for i=3 and j=2 N=19.
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Locating co-channel Cell
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To find the nearest co-channel neighbors of a particular cell, one must do the following: 1. Move i cells along any chain of hexagons. 2. Turn 60 degrees counter-clockwise and move j cells. Figure below illustrates this process with i = 3, j = 2, and N = 19.
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Improving Capacity & Coverage in
Cellular Systems
–Power Control to reduce interference
–Cell Splitting
–Sectorizing
–Microcell Zone Concept (if interested
research)
–Repeaters for Range Extension
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Power Control to Reduce Interference
•In practical systems, power level of every subscriber is
under constant control by the serving BS
•Power control not only reduces interference levels but also
prolongs battery life
•In CDMA spread spectrum systems, power control is a key
feature to ensure maximal utilization of system capacity
•Reduced Interference leads to high capacity
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Cell Splitting •Cell splitting is the process of subdividing a congested
cell into smaller cells with
–their own BS
–a corresponding reduction in antenna height
–a corresponding reduction in transmit power
•Splitting the cell reduces the cell size and thus more
number of cells have to be used
•For the new cells to be smaller in size the transmit
power of these cells must be reduced.
•More number of cells ,more number of clusters ,more
channels ,high capacity.
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Cell Splitting Cells are split to add channels with no new spectrum usage
Cells are split to add channels with no new spectrum usage
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Picocells : 50mW to 1W
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Cell Splitting-Power Issues
Suppose the cell radius (R) of cells is reduced by half,
then what is the required transmit power for these new cells?
Pt2= Pt1/2n
where Pt1and Pt2are the transmit powers of the larger and
smaller cell base stations respectively,
and
n is the path loss exponent.
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n: path loss exponent
For n =3
PT2 = PT1/23 = PT1/8
9dB lower transmit power
Interference will be mitigated (lowered)
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Cell Splitting Suppose a congested service area is originally covered by 5
cells
where each cell has 80 channels.
If cell splitting is used how much more capacity do we get ?
Capacity_original = 5 x 80 = 400
After cell splitting Rnew = R/2
We now have 24 new cells
Capacity_new = 24 x 80 = 19200
For n=4,
Transmit Power of new BS is 12 dB lower
No change in frequency use!
Cost is due to more base stations and more handoffs .
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Sectoring
•As opposed to cell splitting, where D/R is kept constant
while decreasing R, sectoring keeps R untouched and
reduces the D/R
•Capacity improvement is achieved by reducing the
number of cells per cluster, thus increasing frequency
reuse
•In this approach first SIR is improved using directional
antennas
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Sectoring •The CCI may be decreased by replacing the single omni-
directional antenna by several directional antennas, each
radiating within a specified sector
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Sectoring A directional antenna transmits to and receives from only a
fraction of total of the co-channel cells. Thus CCI is
reduced
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Problems with Sectoring
•Increases the number of antennas at each BS
•Decrease in trunking efficiency due to sectoring (dividing
the bigger pool of channels into smaller groups)
•Increase number of handoffs(sector-to sector)
•Good news: many modern BS support sectoring and
related handoff without help of MSC
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Effects of Interference
Interference in voice channels causes
Crosstalk
Noise in background
Interference in control channels causes
Error in digital signaling, which causes
Missed calls
Blocked calls
Dropped calls
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Interference
Two major types of Interferences
Co-channel Interference (CCI)
Adjacent channel Interference (ACI)
CCI is caused due to the cells that reuse the same frequency
set. These cells using the same frequency set are called Co-
channel cells
ACI is caused due to the signals that are adjacent in
frequency
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Co-channel Interference
Q = D / R : co-channel reuse ratio
hexagonal cells → Q = D/R = 3𝑁
Smaller value of Q provides larger capacity, but higher CCI
Hence there is tradeoff between capacity and interference.
small Q → small cluster size → more frequency reuse → larger
system capacity
small Q → small cell separation → increased CCI
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Signal to Interference ratio S/I
The Signal-to-Interference (S/I) for a mobile is
S is desired signal power ,Ii : interference power from ith co-channel cell
The average received power at distance d is
Pr=Po (d/do)-n
The RSS decays as a power law of the distance of separation
between transmitter and receiver
Where Po is received power at reference distance do and n is the
path loss exponent and ranges between 2-4
If Di is the distance of ith interferer, the received power is
proportional to (Di)-n
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Signal to Interference ratio S/I
The S/I for mobile is given by
With only the first tier(layer of) equidistant interferers.
For a hexagonal cluster size, which always have 6 CC cell in
first tier
41
66
)3(
6
)/(
)(
4
6
1
0
QNRD
D
R
I
S nn
i
i
n
i
n
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Signal to Interference ratio S/I The MS is at cell
boundary
The aproximate S/I is
given by, both in terms
of R and D, along with
channel reuse ratio Q
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Example for cluster size and S/I
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TDMA can tolerate S/I down to 15 dB
What is the optimal value of N for omni-directional antennas?
Path loss = 4.
Co-channel Interference
cluster size N =7 (choices 4,7,12)
path loss exponent (means) n=4
co-channel reuse ratio Q=sqrt(3N)=4.582576
Ratio of distance to radius Q=D/R=4.582576
number of neighboring cells io=6 # of sides of hexagon
signal to interference ratio S/I= (D/R)^n / io =73.5
convert to dB, S/I=10log(S/I) =18.66287dB
S/I is greater than required, it will work.
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Example S/I cluster size N=4 (choices 4,7,12)
path loss exponent (means) n=4
co-channel reuse ratio Q= sqrt(3N)=3.464102
Ratio of distance to radius Q=D/R=3.464102
number of neighboring cells io=6 ,# of sides of hexagon
signal to interference ratio S/I= (D/R)^n / io =24
convert to dB, S/I= 10log(S/I)=13.80211dB
S/I is less than required, it will not work!
cluster size N=7
path loss exponent n=3
Q=sqrt(3N)=4.582576
number of neighboring cells io= 6, # of sides of hexagon
signal to interference ratio=S/I= (D/R)^n / io =16.03901
convert to dB, S/I=10log(S/I)=12.05178dB
S/I is less than required, it will not work!
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Trunking /Traffic Theory
Concept of Trunking
Key definitions in Trunking /Traffic Theory
Erlang-(unit of traffic)
Grade of Service
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Trunking & Grade of Service Cellular radio systems rely on trunking to accommodate a
large number of users in a limited radio spectrum.
Trunking allows a large no of users to share a relatively small
number of channels in a cell by providing access to each
user, on demand, from a pool of available channels.
In a trunked radio system (TRS) each user is allocated a
channel on a per call basis, upon termination of the call, the
previously occupied channel is immediately returned to the
pool of available channels.
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Key Definitions Setup Time: Time required to allocate a radio channel to a
requesting user
Blocked Call: Call which cannot be completed at the time of
request, due to congestion(lost call)
Holding Time: Average duration of a typical call. Denoted by
H(in seconds)
Request Rate: The average number of calls requests per unit
time( λ)
Traffic Intensity: Measure of channel time utilization or the
average channel occupancy measured in Erlangs.
Dimensionless quantity. Denoted by A
Load: Traffic intensity across the entire TRS (Erlangs)
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Erlang-a unit of traffic The fundamentals of trunking theory were developed by
Erlang, a Danish mathematician, the unit bears his name.
Erlang represents the continuous use of one voice path.
It is used to describe the total traffic volume of one hour
A channel kept busy for one hour is defined as having a load
of one Erlang
For example, a radio channel that is occupied for thirty
minutes during an hour carries 0.5 Erlangs of traffic
For 1 channel
Min load=0 Erlang (0% time utilization)
Max load=1 Erlang (100% time utilization)
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Erlang-a unit of traffic For example, if a group of 100 users made 30 calls in one
hour, and each call had an average call duration(holding time)
of 5 minutes, then the number of Erlangs this represents is
worked out as follows:
Minutes of traffic in the hour = number of calls x duration
Minutes of traffic in the hour = 30 x 5 = 150
Hours of traffic in the hour = 150 / 60 = 2.5
Traffic Intensity= 2.5 Erlangs
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Traffic Concepts Traffic Intensity offered by each user(Au): Equals average call
arrival rate multiplied by the holding time(service time)
Au=λH(Erlangs)
Total Offered Traffic Intensity for a system of U users (A):
A =U*Au(Erlangs)
Traffic Intensity per channel, in a C channel trunked system
Ac=U*Au/C(Erlangs)
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Trunking & Grade of Service In a TRS, when a particular user requests service and all the
available radio channels are already in use , the user is
blocked or denied access to the system. In some systems a
queue may be used to hold the requesting users until a
channel becomes available.
Trunking systems must be designed carefully in order to
ensure that there is a low likelihood that a user will be blocked
or denied access.
The likelihood that a call is blocked, or the likelihood that a
call experiences a delay greater than a certain queuing time is
called “Grade of Service” (GOS)’’.
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Trunking & Grade of Service Grade of Service (GOS): Measure of ability of a user to
access a trunked system during the busiest hour.
Measure of the congestion which is specified as a
probability.
Blocked calls cleared(BCC) system or
Lost Call Cleared(LCC) or
Erlang B systems
The probability of a call being delayed beyond a certain
amount of time before being granted access
Blocked call delayed or Lost Call Delayed(LCD) or Erlang
C systems
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Blocked Call Cleared Systems When a user requests service, there is a minimal call set-up
time and the user is given immediate access to a channel if
one is available
If channels are already in use and no new channels are
available, call is blocked without access to the system
The user does not receive service, but is free to try again later
All blocked calls are instantly returned to the user pool
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Modeling of BCC Systems
Erlang B formula is given by
where C is the number of trunked channels offered by a
trunked radio system and A is the total offered traffic.
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Pr[blocking]= (AC/C ! )
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Erlang B
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Example How many users can be supported for 0.5% blocking
probability for the following number of trunked channels in a
BCC system? (a) 5, (b) 10. Assumed that each user
generates 0.1 Erlangs of traffic.
Solution
Given C=5, GOS=0.005, Au=0.1,
From graph using C=5 and GOS=0.005,A=1.13
Total Number of users U=A/Au=1.13/0.1=11 users
Given C=10, GOS=0.005, Au=0.1,
From graph/Table using C=5 and GOS=0.005,A=3.96
Total Number of users U=A/Au=3.96/0.1=39 users
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Erlang B Trunking GOS
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Blocked Call Delayed(BCD) Systems
Queues are used to hold call requests that are initially blocked
When a user attempts a call and a channel is not immediately
available, the call request may be delayed until a channel
becomes available
Mathematical modeling of such systems is done by Erlang C
formula
The Erlang C model is based on following assumptions :
Additionally, if offered call cannot be assigned a channel, it is
placed in a queue of infinite length
Each call is then serviced in the order of its arrival
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Blocked Call Delayed Systems
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Erlang C formula which gives likelihood of a call not having
immediate access to a channel (all channels are already in
use)
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Erlang C
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Repeaters for Range Extension
•Useful for hard to reach areas
–Buildings
–Tunnels
–Valleys
•Radio transmitters called Repeaters can be used to
provide coverage in these area
•Repeaters are bi-directional
–Rx signals from BS
–Amplify the signals
–Reradiate the signals
•Received noise and interference is also reradiated.
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OMNIDIRECTIONAL Antennas
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DIRECTIONAL Antennas
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Fixed BS
Mobile BS
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Antenna Patterns
Omnidirectional Antenna Directional Antenna
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Thank You
for
Listening