Wireless; Cellular

49048 Session 4:

Cellular SystemsK.Sandrasegaran

Contents

• Cellular Concept • Frequency Reuse• Cell Geometry• Signal to Interference Ratio• Increasing capacity of cellular systems• Operation of Cellular Systems• Traffic Engineering • Generations of Cellular Systems

Before Cellular Era … (Ref : Kupper)

What is the Cellular Concept?

• Developed for mobile radio-telephone– Replace single high power transmitter/receiver system with more lower power,

shorter range transmitters that provide coverage to a subset of service area.

• Geographical coverage area divided into cells– Each cell with own antenna or set of antennas and served by a Base Station

containing Transmitter, Receiver, Control Unit, etc – Each cell with own group of frequencies (portion of the bandwidth).– Adjacent cells on different frequencies to avoid interference (adjacent ch

interference)– Non adjacent cells may use same frequency (frequency reuse) resulting in co-

channel interference which has to be minimized. – Handover mechanisms necessary as users move from one cell to another.– System needs to know the cell location of a user for mobile terminating calls.

Why cellular works?

• Power level decreases with distance• Power level tradeoff

– High power => better performance within cell– Low power => less interference with cells on same

frequency (co-channel)

Distance

Cell size Acceptable interference level

Propagation in the Mobile Environment

Summary of Cellular Concept (Kupper)

What is the definition of a Cell?

• What is a cell?– A coverage defined area with a number of radio frequencies

(channels) to carry voice or data traffic & control signalling– Each cells has a beacon channel to communicate with all idle

mobiles • To broadcast System Information to mobiles• To find & update the location of each mobile

– Each cell has traffic channels to carry user traffic after a call is established (details given later)

What factors determine cell size?

• Cell sizes from some 100 m in cities to, e.g., 35 km in the country side (GSM) - even less for higher frequencies

• Maximum size determined by– propagation loss and propagation delay (digital only)

• Actual size is determined by – number of customers or traffic quantity– In-building coverage requirements

• Cell sizes can change in CDMA-based systems (cell breathing)

What are Macrocells and Microcells?

Cell Geometry

(Ideal) Geometric Representation

• Cells are commonly represented by hexagons.

• Why hexagon? • How about circle?• How about square, or triangle?

What are possible shape of cells?

• Square– Width d cell has four neighbours at distance d and four at distance d– Better if all adjacent cell’s antennas are equidistant

• Simplifies choosing and switching to new cell

• Hexagon– Provides equidistant cell’s antennas– Radius defined as radius of circum-circle

• Distance from centre to vertex equals length of side– Distance between centres of cells radius R is R– Not usually precise hexagons

• Topographical limitations• Local signal propagation conditions• Location of antennas

Cellular Geometries

Realistic Cells

Source: School of Info. Tech., The Uni. of Sydney.

Summary of cell shapes (Kupper)

Frequency Resuse

• Refers to the fact that the same transmission frequencies can be reused in non-adjacent cells – Reused frequency becomes interference to the original cell resulting in co-channel

interference– Transmission power controlled to limit power at that frequency escaping to

adjacent cells– Measured as Carrier to Interference (C/I) Ratio or SIR

• Why use Frequency Reuse? To increase capacity • Frequency Reuse Factor or Cluster Size (K)

– It refers to a minimum group of cells that use all the available frequencies – K is cluster size or frequency re-use factor – F is total number of frequencies available to be used in system– Each cell uses F/K – Eg. Advanced Mobile Phone Service (AMPS)(1st Generation)

• F=395, K=7 giving 57 frequencies per cell on average• Frequency Reuse Distance (D)

– Is distance between cells that use the same frequency

What is Frequency Reuse? Why is it used?

Cell Geometry

• D = minimum distance between centres of cells that use the same group of frequencies (called co-channels)

• R = radius of a cell• d = distance between centres of adjacent cells • K = number of cells in repetitious pattern

– Reuse factor or Cluster Size – Each cell in cluster uses unique group of frequencies

• It can be proved (see Appendix) that for a hexagonal cellpattern, possible values of K are – K = I2 + J2 + (I x J), I, J = 0, 1, 2, 3, …– Possible values of K are 1, 3, 4, 7, 9, 12, 13, 16, 19, 21, …

• D/R=• D/d =

K3K Use (i,j) to denote a particular cell.

Example: Cell A is represented by (2,1).

Reuse Distance Formula

RjijiD

)(3 22

22 jijiK ++=

Note: i and j are integers

Reuse factor

K= 3; D/R= 3

K=4; D/R = ?

K = 7; D/R = ?

Frequency reuse K = 7

• Using 7 sets of frequencies, can ensure no set is reused within 2 cell radius

K = 19

Summary

Frequency re-use in real life

• In practice, there are physical factors which influence cell re-use– Actual placement of base stations– Geographical features– Sectorised versus Omni-directional cells– Coverage for distance (roads) versus area (city)– Capacity through micro and macro overlay

What are two types of frequency reuse strategies?

• 1. Fixed frequency assignment:– certain frequencies are assigned to a certain cell– problem: different traffic load in different cells

• 2. Dynamic frequency assignment:– base station chooses frequencies depending on the frequencies

already used in neighbour cells– more capacity in cells with more traffic– assignment can also be based on interference measurements

Compare omni-directional versus sectored cells

Omni site• Base Station at centre• one cell/Base Station• one link/cell• 360 degree coverage

3-sector site• Base Station at edges of cells• 3 cells/Base Station• one link/3 cells• 120 degree sectors• better interference rejection

Omni-directional BTS

f1,f2, f3

3-sector BTS

2-sector BTS

f1, f2

f5, f6

f3, f4

BTSBTS

BTS BTS

BTS configurations

Highway coverage

• Use of highly directional cells

Source: Dr. A. Kupper

Summary of Cellular terms (Kupper)

Carrier to Interference (C/I) Ratio

or SIR

Derive an expression for co-channel interference

Determination of Cluster Size (Kupper)

Increasing Capacity of Cellular Network

What approaches have been used to increase the capacity of a cellular

network?1. Adding new channels2. Frequency borrowing – frequencies are taken from adjacent cells by

congested cells3. Cell splitting – cells in areas of high usage can be split into smaller

cells4. Cell sectoring – cells are divided into a number of wedge-shaped

sectors, each with their own set of channels5. Microcells – antennas move to buildings and lamp posts

2. Frequency Borrowing (Kupper)

2. Dynamic and Hybrid Channel Allocation (Kupper)

3. Cell Splitting

3. Cell splitting

• To meet capacity in busy areas, split large cells into smaller cells in some areas

3. Cell Splitting (Summary from Kupper)

4. Cell Sectoring (Kupper)

Frequency reuse with 3-sector cells

• Uses directional antennas– insensitive to signals from rear of antenna– better able to down-tilt antenna

• Able to reuse frequencies more closely than with omni sites

• Example: omni using K=7 plan can be replaced by K=4/12 plan: 3 sectors with each frequency used every 4th cell.

• Reuse every 4th cell, Directional antennas limit Interference

4/12 reuse for 3-sector cells

1C 2C 1C 2C 1C

1C 2C 1C 2C

4B 3B 4B 3B 4B4A

1C 2C 1C 2C 1C4B 3B 4B 3B 4B

Frequency reuse patterns (3/9)

1 234 5

•1 234 5

• •

5. Microcells

• Microcells– Move antennas from tops of hills and large buildings to tops of

small buildings and sides of large buildings• Even lamp posts

– Microcells can be islands of coverage– Reduced power– Good for city streets, along roads and inside large buildings

5. Macro-Micro Cell Overlay

• Overlay two networks with different diameter cells• Macro cells to support vehicular users• Micro cells typically 100-500m

– Reduced antenna height or inside public areas– Reduced power– Target high concentration areas

• Railway stations• Shopping centres

How is in-building coverage provided in a cellular network?

• Pico-cells– small, low power– wall or ceiling mounted

• Leaky feeder– strung along roof cavity– also suitable for tunnel coverage

• Radio repeater

The pros and cons of a cellular structure

• Advantages of cell structures:– higher capacity, higher number of users– less transmission power needed– more robust, decentralized– base station deals with interference, coverage area etc. locally

• Problems:– fixed network needed for the base stations– handover (changing from one cell to another) necessary– interference with other cells

What are the advantages and disadvantages of cellular systems?

Operation of a Cellular Network

How does a cellular subscriber communicate with other users and other networks?

Nodes in a Cellular Network

• Base Station (BS) – includes an antenna, a controller, and a number of receivers

• Mobile telecommunications switching office (MTSO) – connects calls between mobile units (GSM PLMN: MSC, BSC)

• Two types of channels available between mobile unit and BS– Control channels – used to exchange information having to do with setting up

and maintaining calls– Traffic channels – carry voice or data connection between users

Operation of Cellular Systems

• Base station (BS) at centre of each cell– Antenna/s, controller, Transceivers– BS connected to mobile telecommunications switching office

(MTSO)• MTSO:

– One MTSO serves multiple BS– MTSO to BS link by wire or wireless fixed link– Connects calls between mobile units and from mobile to fixed

telecommunications network– Assigns voice channel when requested – Performs handoffs– Monitors calls and performs billing

• Fully automated

Stages involved in setting up a call

• Mobile unit initialization• Paging• Call accepted• Ongoing call• Handoff

1. Mobile unit initialization

• On switch on , MS scans and selects strongest control channel– Usually but not always nearest

(propagation anomalies)• Handshake to identify user,

register location and authenticate user.

• Scan repeated to allow for movement– Change of cell

• Mobile unit monitors for paging messages

Mobile originated call

• Request a control channel form BS

• Send number on control channel

Paging

• MTSO checks if called number is free and attempts to connect to called mobile unit

• Paging message sent to BSs depending on called mobile number

• Paging signal transmitted on control channel

Call accepted

• Called Mobile unit recognizes number on control channel

• Responds to BS which sends response to MTSO

• MTSO sets up circuit between calling and called BSs using available traffic channel within cells

• BSs notify mobile unit of channel for communication

Ongoing call

• Voice/data exchanged through respective BSsand MTSO

Handoff

• Mobile unit moves out of range of original cell into range of another cell

• Traffic channel changes to one assigned to new BS– Without interruption of

service to user

MTSO Controlled Call, Additional Functions

• Call blocking• Call termination• Call drop• Calls to/from fixed and remote mobile subscriber

Traffic Engineering

What is Traffic Engineering? Distinuish between blocking and non-blocking system.

• If number of channels = number of subscribers active at any one time then all calls will be served by system.

• In practice due to cost reasons, not feasible to have capacity handle all possible calls

• For N simultaneous user capacity and L subscribers– L < N – non-blocking system– L > N – blocking system

• Given the Blocking Probabilty, it is possible to determine the traffic that can be handled by the network.

What is Traffic Intensity?

• Traffic is measured in Erlangs• Load presented to a system:

• λ = mean rate of calls attempted per unit time• h = mean holding time per successful call• A = average number of calls arriving during average holding period

hA λ=

Erlang B (Infinite Source LCC)

Erlang B Table (Given in Exam)

Cellular Systems Generations

1G, 2G, 2.5G, 3G, 4G ..

• First-generation (1G): Analogue cellular systems (450-900 MHz)– Frequency shift keying for signalling– FDMA for spectrum sharing– Examples: NMT (Europe), AMPS (US)

• Second-generation (2G): Digital cellular systems (900, 1800 MHz)– TDMA/CDMA for spectrum sharing– Circuit Switched traffic– GSM (Europe), IS-136 (US), PDC (Japan)

• 2.5G: Packet switching extensions to 2G– Digital: GSM to GPRS– Analog: AMPS to CDPD

• 3G: UMTS & cdma2000– High speed, data and Internet services– IMT-2000 compatible

Briefly explain the operation of AMPS.

• Advanced Mobile Phone Service (AMPS)– In North America, two 25-MHz bands were allocated to AMPS

• One for transmission from base to mobile unit• One for transmission from mobile unit to base

– Each band was split in two to encourage competition– Frequency reuse exploited

• Typical call involves – Subscriber initiates call by keying in phone number and presses send key– MTSO verifies number and authorizes user– MTSO issues message to user’s cell phone indicating send and receive traffic

channels– MTSO sends ringing signal to called party– Party answers; MTSO establishes circuit and initiates billing information– Either party hangs up; MTSO releases circuit, frees channels, completes

billing

AMPS Parameters

What are the differences between 1G and 2G?

• Digital traffic channels – first-generation systems are almost purely analog; second-generation systems are digital

• Encryption – all second generation systems provide encryption to prevent eavesdropping

• Error detection and correction – second-generation digital traffic allows for detection and correction, giving clear voice reception

• Channel access – second-generation systems allow channels to be dynamically shared by a number of users

GSM, IS-136, IS-95

Appendix

Source: Dr. A. KupperSource: Dr. A. Kupper

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