1. Wireless and Cellular Concepts - Rev3

8/11/2019 1. Wireless and Cellular Concepts - Rev3

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WIRELESS ANDCELLULAR CONCEPTS

Introduction

Enable communication to and from mobile users byusing radio transmission

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DefinitionsBase 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 mobileReserved channel: radio channel used for transmission ofinformation from the mobile to the base stationControl channel: radio channel used for transmission of callsetup, call request, call initiation, and other beacon or controlpurposes 3

Definitions

SimplexHalf-duplexFull-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 accessFDMA (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 differenthopping pattern, which is determined by i ts own distinct PN code.Code-division multiple access (CDMA): each user has its own distinctPN sequence. All active users transmit their signals on the samebandwidth 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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Radio Telephone System

Radio telephonesystem should bestructured toachieve highcapacity withlimited radio

spectrum while atthe same timecovering verylarge areas

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Design a radio telephone system: A total of 20 MHz of bandwidth is allocated to a typicalanalog telephone system. Each channel needs tohave a bandwidth of around 25 KHz to enablesufficient audio quality to be carried, as well asallowing for a guard band between adjacent signals toensure there are no undue level of interference (SIR >18 dB).The system needs to cover a area of 25 km 2 which

has 30,000 users; and supports at least 1,000simultaneous calls.Each user make a 2-minute call per hour,Blocking probability of the system is 1% as rule. 13


One BS can coverthe whole area?

Number ofsimultaneous calls?SIR > 18 dB?

S Channels = S simultaneous UsersCapacity = S 14


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M small cellsCapacity = M S SIR > 18 dB?

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Radio Signal Attenuation

Free space power received by a receiver antennawhich is separated from a radiating transmittingantenna by a distance d (Friis free space equation):

P t is the transmitted power P r (d) is the received power G t , G R is the transmitter and receiver antenna giand is the T-R separation distance in meters

L is the system loss factor not related to propagation ( L 1) is the wavelength in meters

2

2 2

. . .( )

( 4 ) . .t t r

r

P G G P d

d L


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Radio Signal Attenuation Average received signal power decreases withdistance

where d = distance from transmitter to receiver n = 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

r P

nr P d

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Radio Signal Attenuation


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The Cellular Concept

Why cellular?Radio spectrum is a finite resource.How to accommodate a large number of usersover a large geographic area within a limited radiospectrum?The solution is the use of cellular structure whichallows frequency reuse.

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The largegeographicarea is dividedinto smallerareas cells .Each cell hasits own basestationprovidingcoverage onlyfor that cell.

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Each base station is allocateda portion of the total numberof channels available to theentire system.Neighboring base stationsare assigned different groupsof channels to minimizeinterference.The same group of channelscan be reused by anotherbase station locatedsufficiently far away to keepco-channel interferencelevels within tolerable limits.

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Frequency Reuse

A cellular system which has a total of S duplexchannels.S channels are divided among K cells, witheach cell uses unique and disjoint channels.If each cell is allocated a group of n channels,then

S = n K

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Frequency Reuse

Cluster size : The K cellswhich collectively use thecomplete set of availablefrequency is called the clustersize.Co-channel cell : The set ofcells using the same set offrequencies as the target cell.Interference tier : A set of co-channel cells at the same

distance from the reference cellis called an interference tier.The set of closest co-channelcells is call the first tier.

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Frequency Reuse

If a cluster is replicated M times, the totalnumber of channels as a measure of capacity isgiven by

C = M n K = M SThe capacity is directly proportional to the number ofreplication M .For a given area, if K is reduced while the cell size iskept constant, more clusters are required to cover thearea, 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 formaximizing capacity. This value depends on howmuch interference a mobile or base station cantolerate while maintaining a sufficient quality ofcommunication.Since each cell within a cluster is only assigned 1/K of the total available channels , 1/K is defined asthe 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 spectrum is dedicated to controlchannels, determine an equitable distribution of controlchannels and voice channels in each cell for each of thethree systems.

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Example - SolutionsTotal bandwidth = 33 MHzChannel bandwidth = 25 kHz 2 = 50 kHz / duplex channelsTotal 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 andis determined by environmental conditions.

Although a real footprint is amorphous in nature, aregular cell shape is needed for systematic systemdesign and analysis.From the signal attenuation model, it seems natural tochoose a circle to represent the coverage area of a basestation.

However, circles cannot be tessellated, i.e., be overlaidwithout leaving gaps or overlap.There are three possible choices: a square, anequilateral triangle, and a hexagon. Hexagon is used asit is the most circular.

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Cell Geometry

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Hexagonal Geometry

In order to tessellate clusters of hexagon cells, thecluster size K can only have values which satisfy thefollowing equation

K = i 2 + ij + j 2

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

whereS is the received signal power from the desired base station

I i is the received interference power from the i th co-channel cellbase station, and L is the number of co-channel interferingcells.

1

/ Li

i

S S I

I

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Interference - Example

K 38


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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 the S/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

K D RS RS I

D I

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Adjacent Channel Interference

Interferenceresulting fromsignals which areadjacent infrequency to thedesired signalIt is due to imperfect

receiver filteringwhich allow nearbyfrequencies leakinto the pass-band.It is the cause ofnear-far effect.

desired signal

receiving filterresponse

desired signalinterference

interference

signal on adjacent channelsignal on adjacent channel

FILTER

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Adjacent Channel Interference

To minimize adjacent channel interference:Use high Q filtersMaximize the frequency separation between eachchannel in a given cell

Adjacent channel interference can be minimizedthrough careful filtering and channel assignment.Keep the frequency separation between eachchannel in a given cell as large as possible

A channel separation greater than six is neededto bring the adjacent channel interference to anacceptable level .

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Frequency Planning

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Example

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Channel AssignmentStrategies

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

FixedEach 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 runsout of its own channels.The MSC supervises the borrowing procedures.

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Channel AssignmentStrategies

DynamicChannels 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 anyother 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 everymobile 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 reversechannelTo help prolong battery lifeTo increase dramatically the reverse channel S/I

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Trunking and GOSTrunking 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 thata 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 isblocked.

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GOSConsider 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 , and the average duration of a call(holding time) is h.

The following assumptions are made :Memoryless arrivals of call requestsExponentially 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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GOSThe 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 N l

l

N GOS P

l

uU 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 channelin a trunked system for a given GOS.For the same GOS, the larger the pool, the higherthe 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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Improving Capacity

As the demand for wireless serviceincreases, the number of channels assignedto a cell eventually becomes insufficient tosupport the required number of usersThe techniques

Cell splittingSectoringCoverage zone approaches

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Cell Splitting

Subdivides a congested cellinto smaller cells, each with itsown base station and acorresponding reduction intransmitter power.Increases capacity due to theadditional number of channelsper unit area.Coexistence of different cell

sizes make channelassignments more complicated.Need for handoffs increases.

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Cell Splitting

Cell Distribution following thesplitting of the cell label A

Original Cell Distribution60


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Cell Splitting

The new small cells are re-assigned newfrequencies that do not cause co-channelinterference with adjacent cellsThe power transmitted in the small cells isreduced compared to the power transmitted inthe large cells as:

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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 orsix 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 Basics

Radio Access Network

Base Station

Core Network

SwitchSubscriber Information

BillingRecords

Network Operationsand Maintenance

To other Networks

Mobile Device

Radio Link

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Cellular System BasicsCellular 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 inits cell and connects simultaneous mobile calls to MSC.

MSC coordinates the activities of all base stationsand connects the entire cellular system to the publicswitched telephone network (PSTN).

MSC is sometimes referred to as mobile telephone switchingoffice (MTSO), since it is responsible for connecting allmobiles in a cellular system to the PSTN.

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Cellular System BasicsCommunication 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 thebase station to mobilesReverse voice channel (RVC): for voice transmission frommobiles to the base stationForward control channel (FCC) & reverse control channel(RCC): for initiating mobile calls.

Control channels are often called setup channels because theyare 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 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

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 themaximum transmitter power level is for the mobile

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Handoff

When the mobile moves from one cell to anotherduring a call, the MSC changes the channel ofmobile unit and base stations to maintainuninterrupted connection. Special control signalingis applied to the voice channels so that the mobilemay be controlled by the base station and the MSCwhile a call is in progress.

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Handover

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Handover decision

receive levelBTS old

receive levelBTS old

MS MS

HO_MARGIN

BTS old BTS new

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RoamingThis 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 identify

roamers.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. 76


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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 andESN) 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 oflocation, validation, and call signaling information.

Cellular network is able to provide service to a mobilesubscriber as it moves through different coverage regions.Such a service is referred to as roaming

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Documents

1. Wireless and Cellular Concepts - Rev3