Wireless and cellular concepts - rev1

8/7/2019 Wireless and cellular concepts - rev1

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WIRELESS AND

CELLULAR 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 communication withmobile stations within its coverage region. It consists of severaltransmitters and receivers which simultaneously handle full duplexcommunications and generally has a tower which supports severaltransmitting and receiving antennas.

Mobile station: a radio terminal intended for use while in motion.

It contains a transceiver, an antenna, and control circuitry, and maybe hand-held units (portables) or mounted in vehicles (mobiles). Forward channel: radio channel used for transmission of

information from the base station to the mobile Reverse channel: radio channel used for transmission of

information from the mobile to the base station Control channel: radio channel used for transmission of call setup,call request, call initiation, and other beacon or control purposes


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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 Division

Duplex (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

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,which allow multiple users share full spectrum of theavailable bandwidth simultaneously without interferingsignificantly with each other. Frequency-hopped multiple access (FHMA): each user has a

different hopping pattern, which is determined by its own distinct PNcode.

Code-division multiple access (CDMA): each user has its owndistinct PN sequence. All active users transmit their signals on thesame bandwidth and overlap in time. Signal separation is achieved atthe receiver by correlation with the proper PN sequence. Therefore, inCDMA each SS signal represents a low interference signal to theothers.


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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 usersover a large geographic area within a limited radio

spectrum?

The solution is the use of cellular structure whichallows frequency reuse.


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The large geographic area is divided into smaller areas cells. Each cell has its own base station providing coverage only

for that cell. Each base station is allocated a portion of the total number of

channels available to the entire system.

Neighboring base stations are assigned different groups ofchannels to minimize interference.

The same group of channels can be reused by another basestation located sufficiently far away to keep co-channelinterference levels within tolerable limits.


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Cellular System Basics


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

All base stations are connected to MSC.

A base station serves as a bridge between all mobile users inits cell and connects simultaneous mobile calls to MSC.

MSC coordinates the activities of all base stations andconnects the entire cellular system to the public switchedtelephone network (PSTN).

MSC is sometimes referred to as mobile telephone switching

office (MTSO), since it is responsible for connecting allmobiles in a cellular system to the PSTN.


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Communication between the base station and the mobiles isdefined by a standard common air interface (CAI) thatspecifies four different channels. Forward voice channel (FVC): for voice transmission from the

base station to mobiles

Reverse voice channel (RVC): for voice transmission frommobiles 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 unused

voice channel. Control channels transmit and receive data messages that carry

call initiation and service requests, and are monitored by mobileswhen they do not have a call in progress.

Forward control channels also serve as beacons whichcontinually 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 is

not noticeable to the user. MIN: mobile identification number, which is the

subscribers 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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Handover

MSC


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Cellular Network


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Cellular Network

A cellular system provides coverage for a particular territory, called

coverage 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 coverage region.

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

systems. Authentication center (AuC): matches the MIN and ESN of every active mobile 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 itmoves through different coverage regions. Such a service is referred to asroaming


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Radio Signal Attenuation

Average received signal power decreases with distance

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

rP d


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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 disjoint channel

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 channels iscalled 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 howmuch interference a mobile or base station cantolerate 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 thefrequency 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 to providefull-duplex voice and controlchannels, compute the number of channels available per cell ifthe system uses (a) 4-cell reuse, (b) 7-cell reuse, and (c) 12-cellreuse. If 1 MHz of the allocated spectrum is dedicated to control

channels, determine an equitable distribution of control channelsand voice channels in each 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 channelTotal 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 = 20control channels 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 91voice channels 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 53voice channels each, and 4 cells would have 54 voice channels each. (640-5312=4)


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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 base stationrequests a channel from the MSC.

MSC allocates a channel provided that the particular channel isnot presently in use in the requesting cell or any other cell which

falls within the minimum restricted distance of frequency reuseto avoid co-channel interference.

MSC needs to collect real-time data on channel occupancy,traffic distribution, and radio signal strength of all channels.

Dynamic channel assignment strategy increases the trunkingefficiency since all the available channels of a system isaccessible to all cells, at a price of increased storage andcomputational load on the MSC.


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Hexagonal Geometry

In order to tessellate clusters of hexagon cells, the clustersize Kcan only have values which satisfy the followingequation

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 cando the following:(1) move i cells along any chain of hexagons and then(2) turn 60 degrees counter-clockwise (or clockwise) and move j cells.

* The roles of i and j can be reversed.


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Examples


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Co-channel Interference For hexagonal geometry, the co-channel reuse ratio Q, defined as the

ratio of D to R, is related to the cluster size by

Smaller Q, larger capacity

Larger Q, higher transmission quality

The signal-to-interference ratio (S/I or SIR) for a mobile receiver isgiven by

where S is the received signal power from the desired base station and Ii is thereceived interference power from the ith co-channel cell base station, and L isthe number of co-channel interfering cells.

3Q D R K = =

1

/L

i

i

SS I

I=

=


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Co-channel Interference As signal power attenuates proportionally to the power of the signals traveling

distance, only the first tier of co-channel cells (the closest co-channel cells)needs to be considered and co-channel cells that are farther away can beignored (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 approximation for theS/I of

a mobile at cell boundary (worst case) is given by

The S/I may be further weakened by adjacent channel interference andmultipath fading.

( ) ( )

1

3

/6 6 6

nnn

L n

i

i

KD RS RS I

DI

=

= = =


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Adjacent Channel Interference

Interference resulting from signals which are

adjacent 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 channel

in a given cell


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Example


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Power Control

In practice, the power levels transmitted by every

mobile are under constant control by the servingbase stations.

To ensure that each mobile transmits the smallest powernecessary to maintain a good quality link on the reverse

channel

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 a relatively

small number of channels by providing access to each user, on

demand, from the pool of available channels. Trunking exploits the statistical behavior of users so that a fixed

number of channels may accommodate a large, random usercommunity.

The grade of service (GOS) is a measure of the ability of a user

to access a trunked system during the busiest hour. 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(holding time) 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 channel

available at the arrival of the request, i.e., the system offersno queueing for call requests. This is referred as blockedcalls cleared.

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 probability of

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 request maybe delayed until a channel becomes available. The GOS is definedas the probability that a call is blocked after waiting a specificlength of time in the queue. The Erlang C formula is used todetermine the GOS.

0

!

!

N

B Nl

l

NGOS P

l

=

= =

uU U h = =

uh =


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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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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 5trunked channels can supportonly 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 unitarea.

Coexistence of different cell sizes make channel assignments morecomplicated.

Need for handoffs increases.


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Sectoring

Use directional antennas to decrease co-channel interference and

increase capacity S/I increase K decrease Capacity increase

A cell is normally partitioned into three 120sectors or six60sectors.

Channels assigned to a cell must be partitioned between thesectors. Requires intra-cell handoff Reduces trunking efficiency of a cell


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Sectoring

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Wireless and cellular concepts - rev1