Cellular Concept- Part 02 Udhay Prakash Jntuh

7/30/2019 Cellular Concept- Part 02 Udhay Prakash Jntuh

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Cellular Concept-System Design FundamentalsPart 2

Udhay Prakash

Lecturer, ECE Dept., [email protected]


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Content for discussion Interference and System capacity

Co-channel and adjacent channel interference

Power control to reduce interference Trunking and Grade of Service (gos)

Techniques to improve capacityin cellular

systems Cell splitting, sectoring and microcell concept

15-07-2013 & 22-07-2013 2M.Tech(DT-SSP) JNTUH CEH


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Approaches to Cope with Increasing Capacity Cellular concept Frequency Reuse

Adding new channels

Frequency borrowing frequencies are taken from adjacent

cells by congested cells. Trunked radio system- on demand access to pool of resources;

follows statistical approach in resource allocation.

Cell splitting cells in areas of high usage can be split intosmaller cells

Cell sectoring cells are divided into a number of wedge-shaped sectors, each with their own set of channels

Microcellsantennas move to buildings, hills, and lamp posts



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Signal propagation ranges

distance

sender

transmission

detection

interference

Transmission range communication possible low error rate

Detection range

detection of the signalpossible no communication

possible

Interference range

signal may not bedetected signal adds to the

background noise



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Frequency Reuse & interference Interference can be caused by a variety of factors.

Higher interference, lower the call quality.

Frequency re-use distance -distance between two

identical frequencies in a re-use pattern.

Lower the frequency reuse distance, more the

available network capacity.



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Interference & System Capacity Interference limits the maximum value of

frequency reuse and the traffic handlingcapacity.

Sources of interferences: another mobile in the same cell,

a call in progress in a neighbouring cell,

Other base stations operating in the same frequency

band, any non-cellular system which inadvertently leaks

energy into the cellular frequency band,

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Interference Interference in

voice channels leads to cross talk.

control channels leads to missed/blocked calls.

Cellular system interferences Co-channel &Adjacent channel interferences

15-07-2013 & 22-07-2013 M.Tech(DT-SSP) JNTUH CEH 7


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Interference Co-channel Interference (C/I)

Cellular networks are more often limited by problemscaused by interference rather than by signal strength

problems. Co-channel interference is caused by the use of a

frequency close to the exact same frequency.

The former will interfere with the latter, leading to the

terms interfering frequency (I) and carrier frequency(C).



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Co-channel Interference Interference among cells using same set of

frequencies, in a cellular system.



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Co-channel Interference It cant be combated by increasing power

level, as it further increases the interference.

Solution: physically separating co-channel

cells by a minimum distance to provide

sufficient isolation due to propagation.



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Co-channel interference & capacity When all the cell sizes and transmitted powers are identical, then

co-channel interference becomes a function of radius of the cell

(R) and the distance between centres of the nearest co-channel

cells (D).

For hexagonal geometry, co-channel reuse ratio is

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Smaller N is greater capacity



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Co-channel cells for 7-cell reuse



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Adjacent channel interference (C/A) Adjacent frequencies (A), that is frequencies shifted

200kHz from the carrier frequency (C), must beavoided in the same cell and preferably inneighboring cells also.

Although adjacent frequencies are at differentfrequencies to the carrier frequency they can stillcause interference and quality problems.

Adjacent channel interference is caused due to: Interfering channels from the neighboring cells.

Interfering channels from the same cell.


E l id dj h l


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Example to avoid adjacent channelinterference- AMPS Duopoly Channels



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Channel Capacity



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Channel Capacity



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Radio Trunking Trunking stands for sharing. Whenever resources are in short supply, they will have

to be shared.

Allow a large number (n) of users to share the relatively

small number of channels in a cell (or a sector) byproviding access to each user, on demand, from a pool ofavailable channels.

Exploit the statistical behaviour of users.

A fixed number of telephone lines and radio channelsare installed in base and are shared by large number ofremote/ mobile subscribers.



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Trunking

Each user is allocated a channel on a per call basis, and upontermination of the call, the previously occupied channel is immediatelyreturned to the pool of available channels.

First trunked radio standard introduced was analog MPT 1327 fromROHDE & SCHWARZ.

An example of digital trunking system is the TETRA (TErrestrial

Trunked RAdio). Advantage of Digital Trunking System over Analog Trunking System:

More reliable Flexible Have connectivity with internet Low noise

Disadvantages of Digital Trunking System over Analog TrunkingSystem: lower coverage range as compared with Analog Trunking System



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Trunking- terminology



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Trunking- terminology

Setup time: Time required to allocate a trunked radiochannel to a requested user. Blocked Call: Call which cannot be completed at time of

request, due to congestion. The blocked call may be denied or access to resources, and

thereby , called lost call or delayed call system, respectively. Grade of Service (gos):

Measure of congestion which is specified as the probability ofa call being blocked.

Measure of ability of a user to access a trunked system,during the busiest hour. gos of 2% means 2 out of 100 calls will be blocked due to

channel occupancy during the busiest hour.



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Erlang B formula



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Kendall Notation- A/S/s/c/p/D To describe characteristics of the queuing systems. First letter (A)-arrival process description

M stands for Markovian inter-arrivals, or memoryless arrival process

GI (or G) for general (any distribution) independent arrivals

MAP for arrivals driven by a Markovian Arrival Process)

Second letter (S)- Service time distribution M stands for Markovian service,

G for general (any distribution) service,

PH for phase-type service

Third letter (s)- No.of servers in the system ; can beany integer 1.



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Kendall Notation- A/S/s/c/p/D Fourth letter(c) - Queue capacity (c0).

If this argument is missing, then, by default, the queue capacity isinfinity.

Fifth letter (p)- System population, i.e., themaximum no.of jobs(calls) that can arrive in thequeue. If this argument is missing then, by default, the system population is

infinity.

Sixth Letter(D)- Queuing discipline

FIFO (first come first serve), LIFO (last come first serve), or any otherqueuing discipline. If this argument is missing, then, by default, the queuing discipline is

FIFO.



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M/M/m/m system



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Channel capacity calculation& Erlang B formula



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Erlang B Trunking GOS



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Example 1



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Example 1a



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Example 1b



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Erlang C The second type of trunking system is the one

which employs a queue, to hold calls whichare blocked.Blocked calls delayed system

This trunking system uses Erlang C



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Erlang C



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Cell Splitting Process of subdividing a congested cell into smaller

cells, each with its own base station and acorresponding reduction in antenna height and

transmitter power. Frequency reuse large cellsmall cells

Cell splitting Small cellmicro cells

Coexists with frequency reuse scheme, maintainingreuse distance for required S/I.

Enhances capacity byincreasing spectral efficiency



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Cell Splitting


Cells are split to add channels with no new spectrum usage.Thereby, results an increase in capacity.


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Cell Splitting In the cell splitting process,

Let each new cell carry the same maximum traffic load of theold cell, then

Also, the power levels of new microcells must be reduced.



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Cell Splitting-Types Permanent Splitting

Preplanned cell splitting Preplanned resource division During splitting, some call are lost. So service cut is done in

lowest traffic hours (eg: mid-night).

Dynamic Splitting Real-time cell splitting Channels are divided into two groups, with umbrellas cell

concept. Initially, Less channels allocated to microcells. Later, as traffic

increases, more channels of old cell are assigned to micro cells. More complex algorithms required for splitting during peak

traffic hours. Antenna Down-tilting to reduce coverage area of base stations



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Cell Sectoringo Another method of improving

spectral efficiency.o Dividing the cell into parts,

called sectors.o Deserves no new base station

installation.o Involves antenna downtilting,

reduction in beam focussingand reducing the powertransmissions.

o Intracell Handoff takes placeamong the directionalantennas of same cell.



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Sectoring improves S/I



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Ways of Cell SectoringOriginal cell site is not used Original cell site is used


In building deployment is the next


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In-building deployment is the nextgreat growth phase



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The Zone Cell Concept



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Zone Cell Concept



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Conclusions Cellular concept and frequency reuse are used to increase spectral

efficiency.

Cellular design techniques (cell splitting, sectoring and coverage zoneapproach) are used to further enhance the system capacity.

Cell splitting

allows orderly growth of cellular system.

Increases number of base stations.

Sectoring

uses directional antennas to control interference and frequency of channels.

Increases the number of base station antennas. Suffers the trunking inefficiencies, and increases computational load on MSC

Zone microcell concept- distributes cell coverage and extends cellboundary .

These three techniques are coexisting to increase the system capacity.



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References Theodore S. Rappaport, Wirelesss Communications-

Principles and Practice, 2e, PHI, 2002.

William C.Y. Lee, Wireless & Cellular

Telecommunications, 3e, ISBN:0-07-143686-3, TataMcGraw Hills.

Kendall notation,http://www.cs.wm.edu/~riska/PhD-thesis-

html/node6.html.

Cellular Concept- Part 02 Udhay Prakash Jntuh

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