1. Wirelss and Cellular Conceptspdf

7/28/2019 1. Wirelss and Cellular Conceptspdf

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WIRELESS ANDCELLULAR CONCEPTS


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Introduction

Enable communication to and from mobile users by

using radio transmission


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Definitions

Base station: a fixed station used for radio communicationwith mobile stations within its coverage region. It consists ofseveral transmitters and receivers which simultaneouslyhandle full duplex communications and generally has a towerwhich supports several transmitting and receiving antennas.

Mobile station: a radio terminal intended for use while inmotion. It contains a transceiver, an antenna, and controlcircuitry, and may be hand-held units (portables) or mountedin vehicles (mobiles).

Forward channel: radio channel used for transmission ofinformation from the base station to the mobile

Reserved channel: radio channel used for transmission ofinformation from the mobile to the base station

Control channel: radio channel used for transmission of callsetup, call request, call initiation, and other beacon or controlpurposes


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Definitions

Simplex

Half-duplex

Full-duplex

The 2 channels can be separated in frequency Frequency Division Duplex (FDD)

The 2 channels can be separated in time toshare a single physical channel Time DivisionDuplex (TDD)


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FDD vs TDD


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Multiple Access


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Multiple Access

Multiple access

FDMA (Frequency Division Multiple Access)

TDMA (Time Division Multiple Access)

SDMA (Space Division Multiple Access) SSMA (Spread Spectrum Multiple Access)

FHMA (Frequency Hopped Multiple Access)

CDMA (Code Division Multiple Access)


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Multiple Access


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Multiple Access

Spread-spectrum multiple access (SSMA): SSMA usessignals which have a transmission bandwidth that is severalorders of magnitude greater than the minimum required RFbandwidth. Each user is assigned a distinct pseudo-noise (PN)code. The userscodes are approximately orthogonal, whichallow multiple users share full spectrum of the availablebandwidth simultaneously without interfering significantly witheach other. Frequency-hopped multiple access (FHMA): each user has a different

hopping pattern, which is determined by its own distinct PN code.

Code-division multiple access (CDMA): each user has its own distinctPN sequence. All active users transmit their signals on the same

bandwidth and overlap in time. Signal separation is achieved at thereceiver by correlation with the proper PN sequence. Therefore, in CDMAeach SS signal represents a low interference signal to the others.


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Multiple Access


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Multiple Access


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The Cellular Concept


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Why cellular?

Radio spectrum is a finite resource.

How to accommodate a large number of users

over a large geographic area within a limited radiospectrum?

The solution is the use of cellular structure which

allows frequency reuse.


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The large geographic area is divided into smallerareas cells.

Each cell has its own base station providing coverageonly for that cell.

Each base station is allocated a portion of the totalnumber of channels available to the entire system.

Neighboring base stations are assigned differentgroups of channels to minimize interference.

The same group of channels can be reused byanother base station located sufficiently far away tokeep co-channel interference levels within tolerablelimits.


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Radio Signal Attenuation

Average received signal power decreases withdistance

where d = distance from transmitter to receivern = path loss exponent

Typical values of n:n = 2: free spacen = 2.7 ~ 5: urban cellular radion = 3: open country

n = 1.6 ~ 1.8: indoor line-of-sight

Larger values of n preferred, leading to less interference

rP

n

rd


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Frequency Reuse

There is a total of G full-duplex channels available for use.

G channels are divided among K cells into unique and disjointchannel groups which each has g channels.

Total number of available channels can then be expressed as

G = gK

K cells which collectively use the complete set of available channelsis called a cluster. K is the cluster size.

If a cluster is replicated M times, the total number of channels as ameasure of capacity is given by

C = M g K = MG

For a given area, if K is reduced while the cell size is kept constant,more clusters are required to cover the area, and hence more

capacity. However, a smaller cluster size indicates that co-channel cells are

much closer, leading to stronger co-channel interference.


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Frequency Reuse

The smallest possible value of K is desirable for

maximizing capacity. This value depends on how

much interference a mobile or base station can

tolerate while maintaining a sufficient quality of

communication.

Since each cell within a cluster is only assigned 1/K

of the total available channels, 1/K is defined as the

frequency reuse factor.


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Example

A total of 33 MHz of bandwidth is allocated to a cellulartelephone system which uses two 25 kHz channels toprovide full-duplex voice and controlchannels, compute the number of channels available percell if the system uses (a) 4-cell reuse, (b) 7-cell reuse,

and (c) 12-cell reuse. If 1 MHz of the allocated spectrumis dedicated to control channels, determine an equitabledistribution of control channels and voice channels ineach cell for each of the three systems.


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Example - Solutions

Total bandwidth = 33 MHz

Channel bandwidth = 25 kHz 2 = 50 kHz / duplex channels

Total available channels = 33,000/50 = 660 channels

(a) For K = 4, total number of channels per cell = 660/4 = 165

(b) For K = 7, total number of channels per cell = 660/7 95

(c) For K = 12, total number of channels per cell = 660/12 = 55

A 1 MHz spectrum for control channels implies that there are 1000/50 = 20 controlchannels out of the 660 channels available.

(a) For K = 4, we can have 5 control channels and 160 voice channels per cell.However, in practice each cell only needs a single control channel. Thus, 1control channel and 160 voice channels would be assigned to each cell.

(b) For K = 7, each cell would have 1 control channel, 4 cells would have 91 voicechannels each, and 3 cells would have 92 voice channels each. (640-917=3)

(c) For K = 12, each cell would have 1 control channel, 8 cells would have 53 voicechannels each, and 4 cells would have 54 voice channels each. (640-5312=4)


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Cell Geometry

Actual radio coverage of a cell is known as footprint and isdetermined by environmental conditions.

Although a real footprint is amorphous in nature, a regular cellshape is needed for systematic system design and analysis.

From the signal attenuation model, it seems natural to choosea circle to represent the coverage area of a base station.

However, circles cannot be tessellated, i.e., be overlaidwithout leaving gaps or overlap.

There are three possible choices: a square, an equilateraltriangle, and a hexagon. Hexagon is used as it is the mostcircular.


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Hexagonal Geometry

In order to tessellate clusters of hexagon cells, the

cluster size Kcan only have values which satisfy the

following equation

K = i2+ ij + j2

where i and j are non-negative integers. Hence K = 3, 4, 7, 9,12, etc.

To find the nearest co-channel neighbors of a cell,one can do the following:(1) move i cells along any chain of hexagons and then(2) turn 60 degrees counter-clockwise (or clockwise) and move jcells.* The roles of i and j can be reversed.


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Hexagonal Geometry


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Examples


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Examples


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Co-channel Cells

For hexagonal geometry, the co-channel reuse ratio Q,defined as the ratio of D to R, is related to the clustersize by

Smaller Q, larger capacity Larger Q, higher transmission quality

3Q D R K


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Co-channel Interference

The signal-to-interference ratio (S/I or SIR) for a mobilereceiver is given by

where S is the received signal power from the desired base stationand Ii is the received interference power from the i

th co-channel cellbase station, and L is the number of co-channel interfering cells.

1

/L

ii

SS I

I


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Co-channel Interference

As signal power attenuates proportionally to the power of thesignals traveling distance, only the first tier of co-channel cells (theclosest co-channel cells) needs to be considered and co-channelcells that are farther away can be ignored (in cellular environment,typical value of the path loss exponent n = 4).

For hexagonal geometry, there are 6 co-channel cells in the first tier,i.e., L= 6.

For equal power transmission from base stations, an approximationfor theS/I of a mobile at cell boundary (worst case) is given by

The S/I may be further weakened by adjacent channel interferenceand multipath fading.

1

3

/6 6 6

nnn

L n

i

i

D RS RS I

DI


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Interference - Example

K


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Adjacent Channel Interference

Interference resulting from signals which areadjacent in frequency to the desired signal

It is due to imperfect receiver filtering which allownearby frequencies leak into the passband.

It is the cause of near-far effect.

To minimize adjacent channel interference: Use high Q filters

Maximize the frequency separation between each channelin a given cell


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Frequency Planning


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Example


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Channel Assignment

Strategies

The choice of channel assignment strategies impactsthe system performance, particularly as to how calls aremanaged when a mobile user is handed off from onecell to another.

There are two different strategies

Fixed Each cell is allocated a predetermined set of voice channels.

Call attempted within the cell can only be served by the unusedchannels in that particular cell.

Call is Blocked if all channels in the cell are occupied.

A variation of fixed assignment strategy borrowing strategy: A cell is allowed to borrow channels from a neighboring cell if it runs

out of its own channels. The MSC supervises the borrowing procedures.


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Channel Assignment

Strategies

Dynamic Channels are not allocated to different cells permanently.

Each time a call request is being made, the serving basestation requests a channel from the MSC.

MSC allocates a channel provided that the particularchannel is not presently in use in the requesting cell or any

other cell which falls within the minimum restricteddistance of frequency reuse to avoid co-channelinterference.

MSC needs to collect real-time data on channeloccupancy, traffic distribution, and radio signal strength ofall channels.

Dynamic channel assignment strategy increases thetrunking efficiency since all the available channels of asystem is accessible to all cells, at a price of increasedstorage and computational load on the MSC.


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Power Control

In practice, the power levels transmitted by every

mobile are under constant control by the serving

base stations.

To ensure that each mobile transmits the smallest power

necessary to maintain a good quality link on the reversechannel

To help prolong battery life

To increase dramatically the reverse channel S/I


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Trunking and GOS

Trunking allows a large number of users to share arelatively small number of channels by providing accessto each user, on demand, from the pool of availablechannels.

Trunking exploits the statistical behavior of users so that

a fixed number of channels may accommodate a large,random user community.

The grade of service (GOS) is a measure of the ability ofa user to access a trunked system during the busiesthour.

GOS is typically given as the likelihood that a call is

blocked.


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GOS

Consider the following trunked system model: It has a total of U users and a pool of N channels.

For each user, the average number of call requests per unit time(call request rate) is l, and the average duration of a call (holdingtime) is h.

The following assumptions are made: Memoryless arrivals of call requests Exponentially distributed call duration

A call request is rejected (blocked) if there is no channelavailable at the arrival of the request, i.e., the system offers noqueueing for call requests. This is referred as blocked callscleared.

The above trunked system is an M/M/N/N queue.


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GOS

The probability that a call is blocked, i.e., the blocking probabilityof the M/M/N/N queue, which is the GOS for a trunked systemdescribed above, is given by the Erlang B formula

where the traffic intensity and is the traffic intensitygenerated by each user.

Another type of trunked system is called blocked calls delayed,which provides a queue to hold blocked calls. A call requestmay be delayed until a channel becomes available. The GOS isdefined as the probability that a call is blocked after waiting aspecific length of time in the queue. The Erlang C formula isused to determine the GOS.

0

!

!

N

B Nl

l

NGOS P

l

u

U U h u h


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Example


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Example


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Erlang B Table


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Erlang B Table


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Erlang B Table


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Erlang B Table


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Erlang B Table


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Trunking Efficiency

The number of users can be supported by a channel

in a trunked system for a given GOS.

For the same GOS, the larger the pool, the higher

the trunking efficiency.

Example: 10 trunked channels at a GOS of 0.01 cansupport 4.46 Erlangs of traffic, whereas 2 groups of 5

trunked channels can support

only 2 1.36 Erlangs, or 2.72 Erlangs of traffic.


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Cell Splitting

Subdivides a congested cell into smaller cells, each with

its own base station and a corresponding reduction in

transmitter power.

Increases capacity due to the additional number of

channels per unit area. Coexistence of different cell sizes make channel

assignments more complicated.

Need for handoffs increases.


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Cell Splitting

Cell Distribution following the

splitting of the cell label AOriginal Cell Distribution


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Cell Splitting


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Sectoring

Use directional antennas to decrease co-channelinterference and increase capacity S/I increase K decrease Capacity increase

A cell is normally partitioned into three 120sectors or

six 60sectors.

Channels assigned to a cell must be partitionedbetween the sectors. Requires intra-cell handoff

Reduces trunking efficiency of a cell


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Sectoring


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Sectoring


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Sectoring


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Sectoring


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Cellular System Basics


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Cellular system consists of mobile stations, basestations, and mobile-services switching center(MSC)

All base stations are connected to MSC. A base station serves as a bridge between all mobile users in

its cell and connects simultaneous mobile calls to MSC. MSC coordinates the activities of all base stations

and connects the entire cellular system to the publicswitched telephone network (PSTN). MSC is sometimes referred to as mobile telephone switching

office (MTSO), since it is responsible for connecting all

mobiles in a cellular system to the PSTN.


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Communication between the base station and themobiles is defined by a standard common airinterface (CAI) that specifies four different channels. Forward voice channel (FVC): for voice transmission from the

base station to mobiles

Reverse voice channel (RVC): for voice transmission from

mobiles to the base station Forward control channel (FCC) & reverse control channel

(RCC): for initiating mobile calls. Control channels are often called setup channels because they

are only involved in setting up a call and moving it to an unusedvoice channel.

Control channels transmit and receive data messages that carrycall initiation and service requests, and are monitored by mobiles

when they do not have a call in progress. Forward control channels also serve as beacons which

continually broadcast all of the traffic requests for all mobiles inthe system.


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Call Setup


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Call Setup


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Call Setup

Call setup is completed within a few seconds and isnot noticeable to the user.

MIN: mobile identification number, which is thesubscribers telephone number

ESN: electronic serial number Station class mark (SCM): indicates what the

maximum transmitter power level is for the mobile


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Handoff

When the mobile moves from one cell to another

during a call, the MSC changes the channel of

mobile unit and base stations to maintain

uninterrupted connection. Special control signaling

is applied to the voice channels so that the mobilemay be controlled by the base station and the MSC

while a call is in progress.


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Handover


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Handover decision

receive level

BTSold

receive level

BTSold

MS MS

HO_MARGIN

BTSold BTSnew


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Cellular Network


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Cellular Network

A cellular system provides coverage for a particular territory,calledcoverage region or market

The MSC relies on the following information databases Home location register (HLR): a list of all users (along with their MIN and

ESN) who originally subscribed to the cellular system in the coverageregion.

Visitor location register (VLR): a time-varying list of visiting users (calledromers) in the coverage region who originally subscribed to other cellularsystems.

Authentication center (AuC): matches the MIN and ESN of every activemobile in the system with the data stored in the HLR to prevent fraud.

Interconnection of cellular systems forms a cellular network MSCs are connected via dedicated signaling channels for exchange of

location, validation, and call signaling information. Cellular network is able to provide service to a mobile

subscriber as it moves through different coverage regions.Such a service is referred to as roaming

R i


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Roaming

This allows subscribers to operate in service areas otherthan the one from which service is subscribed.

Every several minutes, the MSC issues a globalcommand over each FCC in the system, asking for allmobiles which are previously unregistered to report theirMIN and ESN over the RCC.

By comparing the MIN of a mobile with the MINscontained in its HLR, the MSC is able to quickly identifyroamers.

Once a roamer is identified, the MSC sends aregistration request over the landline signaling networkto the mobiles home MSC.

The home MSC validates that the particular mobile hasroaming authorization and returns a customer profile tothe visited MSC which indicates the availability offeatures (call waiting, call forwarding, etc.) for the mobile.


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1. Wirelss and Cellular Conceptspdf

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