Introduc)on to cellular mobile system

Prepared by: RUCHA PATEL

Objec)ve of early mobile system

Achieve large coverage area High power transmi=er

Limita6ons: Impossible to reuse the same frequencies Could not make spectrum alloca)ons

Introduc)on to Mobile Systems Cellular System Concept

Cellular concept Solving problem of spectral conges)on And user capacity Oers high capacity Many low power transmi=ers Interference between base sta)ons are minimized

Demand for service increases, the no. of base st. may be increased, implying addi)onal radio capacity

Frequency reuse A cell is the basic geographic unit of a cellular system. Each cellular base sta)on is allocated a group of radio

channels to be used within small geographic area called a cell. Same group of channels may be used to cover dierent cells

that are separated from one another by distances large enough to keep interference level in limits.

The design process of selec)ng and alloca)ng channel groups for all of the cellular base sta)ons within the system is called frequency reuse or frequency planning.

Cell Footprint

Coverage contour should be circular. However it is imprac)cal because it provides ambiguous areas with either mul)ple or no coverage.

Due to economic reasons, the hexagon has been chosen due to its maximum area coverage.

Hence, a conven)onal cellular layout is oLen dened by a uniform grid of regular hexagons.

Cell Footprint

Omni direc)onal antenna: base sta)on tr. At the centre of cell

Sectored antenna: B.S. on three of the six cell ver)ces

Consider a cellular system which has S duplex channels available for use Each cell has Group of k channels (k

If cluster is replicated M )mes, Total no. of duplex channels, C, can be used as a measure of capacity and given by

C=MkN = MS Capacity of a cellular system is directly propor)onal to the no. of )mes a cluster is replicated in a xed service area.

Factor N is called cluster size.

Large cluster size indicates the ra)o between cell radius and distance between co-channel cells is small and vice-versa.

N: is a func)on of how much interference a mobile or B.S. can tolerate while maintaining sucient quality of communica)ons.

Frequency reuse factor of cellular system is given by 1/N, since each cell within a cluster is only assigned 1/N of total available channels.

Frequency reuse factor

Can also verify that where Q is the co-channel reuse ra)o

To nd co-channel neighbors of a par)cular cell in a cellular system

N= i^2 + ij + j^2 I,j are non-nega)ve nos. (1)Move I cells along any chain of hexagon (2) turn 60 degree counter-clockwise and move

j cells.

AA

A

A

A

A

A

i

j

i=1, j=2 , N=1+2+4=7

Reuse distance calcula)on: closest distance D between the centers of two cells using the same frequency is given by

D= sqrt(3N) * R

System Expansion Techniques As demand for wireless services increases, the number of

channels assigned to a cell eventually becomes insucient to support the required number of users. More channels must therefore be made available per unit area. This can be accomplished by dividing each ini)al cell area into a

number of smaller cells, a technique known as cell-splieng. It can also be accomplished by having more channels per cell,

i.e. by having a smaller reuse factor. However, to have a smaller reuse factor, the co-channel interference must be reduced. This can be done by using antenna sectoriza)on.

System Expansion Techniques--Cell splieng

Cell splieng increases the number of BSs in order to

increase capacity. There will be a corresponding reduc)on in antenna height and transmi=er power.

Cell splieng accommodates a modular growth capability. This in turn leads to capacity increase essen)ally via a system re-scaling of the cellular geometry without any changes in frequency planning.

Small cells lead to more cells/area which in turn leads to increased trac capacity.

System Expansion Techniques--Cell splieng

Channel assignment strategies: The objec)ve of increasing capacity & minimizing interference Fixed channel assignment: Each cell is allocated Predetermined set of voice channels Call blocked Borrowing strategy

Dynamic channel assignment Voice channels not allocated to each cell permanently Requires the MSC to collect real-)me data on channel

occupancy, trac distribu)on and RSSI of all channels on con)nuous basis.

Alloca)ng channels on demand Increases storage and computa)onal load on system Advantage of increased channel u)liza)on and decreased probability of blocked call

Hando strategies - Processing hando

Dwell 6me: The )me over which the call is maintained within a cell, without hando is called as dwell )me.

Hando decision: NCHO( network control hando) MCHO (mobile control hando) MAHO ( mobile assisted hando)

Intersystem hando: during the course of a call, if a mobile moves from one cellular system to a dierent cellular system controlled by dierent MSC.

Compa)bility between two MSCs must be determined .

To improve quality of services, various methods Priori6zing handos: -guard channel concept - when dynamic channel strategy is used, it minimize the no. of guard channels

Queuing of hando: - Decrease the probability of forced termina)on of call due lack of available channels

- Possible due to presence of nite )me interval between the )me the received signals drops below the hando threshold and the )me the call is terminated due to insucient signal level.

Prac6cal hando considera6ons: For minimizing hando interven)on from MSC. Umbrella approach

Cell dragging: results from the pedestrian users

Types of Handos

Hard hando: break before make connec)on Intra and inter-cell handos

Hard Hando between the MS and BSs

Types of Handos

SoL hando: make-before-break connec=on. Mobile directed hando.

Mul)ways and soLer handos

SoL Hando between MS and BSTs

In types of hando, there is break before make i.e. hard hando & make before break that is soL hando. So, which one of the two type is be=er and under which

circumstances for each scenario? If possible, explain with example.

SoL hando is advantageous over hard hando because the mobile does not lose contact with the system during hando execu)on, also unnecessary call termina)ons doesnt occur. No one likes unnecessary call termina)ons.

Hard hando is advantageous when system performance is not eected even if the mobile system has to reconnect to the BST.

Hybrid channel assignment Each cell has a sta)c channel set as well as dynamic channel-

borrowing capability. Or combina)on of xed and dynamic channel assignment A part of total frequency channels will use xed channel

assignment and the rest will use dynamic channel assignment. The rela)ve propor)on of xed and dynamic channels mainly

depends on trac characteris)cs. Two methods used in assigning dynamic channels with a

centralised control with up-to-date trac-pa=ern informa)on: (a) schedule (b)predic)ve

(a)Schedule dynamic channel assignment: a priori es)mates about varia)on in trac in terms

of loca)on and )me needed to schedule dynamic channels at predetermined peaks of trac change.

(b) Predic)ve dynamic channel assignment: the trac intensity and blocking probability is

monitored in each cell at all the )me.

Advantages of hybrid channel assignment Low overhead of channel management Lower run-)me overhead Reduc)on in hot-spot Improved channel-load balancing Limita)on: If channel assignment is not coordinated properly, the benets of sta)c and dynamic strategies are lost.

under low-to-moderate trac loads, dynamic channel assignment performs be=er

Trac load increase up to 50%, hybrid channel assignment perform be=er than xed channel assignment

Heavy trac load condi)on, xed channel assignment performs be=er

Interference and system capacity

Sources: another mobile in the same cell, A call progress in a

neighboring cell, any non-cellular system which inadvertently leaks energy into the cellular frequency band.

Interference on voice channels causes cross-talk And on control channels , leads to missed or blocked calls More in urban area due to RF noise oor and many base

sta)ons. Two types: co-channel interference

adjacent channel interference

Co-channel interference Frequency reuse implies that in a given coverage area there

are several cells that uses the same frequency. Interference between signals from co-channel cells To reduce co-channel interference, D must be minimum to

provide sucient isola)on due to propaga)on. Q= D/R= 3N Q called co-channel reuse ra)o A small value of Q provides larger capacity since N is small Large Q improves transmission quality, due to a smaller level-

of co-channel interference.

Smaller N is greater capacity

If Di is the distance of the ith interferer from mobile, the received power at a given mobile due to ith

interfering cell will be propor)onal to

There are always six co-channel cells in rst )er.

Test-1: To measure co-channel interference at the mobile unit, which is moving in its serving cell.

not so accurate for the study of co-channel interference problem at the cell-site.

Test-2: A mobile unit is transmieng as well as 6 mobile units are transmieng in co-channel cells simultaneously

Co-channel cells for 7-cell reuse

Co-channel Interference reduc)on methods: i) Increasing D but results into reduc)on of system capacity in

terms of no. of channels available per cell. i) Lowering antenna heights at the cell-site such as on

high-hill or in valley. limita)on in forested area. i) Using direc)onal antennas at cell-site: use of 120

degree direc)onal antenna can reduce the co-channel interference in the system by elimina)on the radia)on to the rest of its 240 degree sector.

Adjacent channel interference

Interference resul)ng from signals which are adjacent in frequency to the desired signal is called adjacent channel interference.

Results from imperfect receiver lters which allow nearby frequencies to leak into the pass-band.

Near-far eect: when a mobile close to a base sta)on transmits on a channel close to one being used by a weak mobile.

Can be minimized through careful ltering and channel assignments.

Tight base sta)on lters are needed when close-in and distant users share the same cell.

Improving coverage and capacity in cellular systems

Cellular Techniques needed to provide more channels per unit coverage area.

Cell splieng Sectoring Coverage zone approaches

Cells are split to add channels with no new spectrum usage

Dening new smaller cells having smaller radius than original layer cells

Smaller cells c/a microcells. System user capacity increases due to Availability of addi)onal no. of channels per unit service area

No. of )mes that frequency channels are reused Two most important key parameters: cluster size K and cell radius R

Techniques of cell splieng: Permanent cell spliKng: the installa)on of every new split cells have to be planned in advance.

The actual change over of service with split cells should take place during lowest trac period

Dynamic cell spliKng: based on u)lizing the allocated spectrum eciency in real )me.

Should be implemented to prevent dropped calls at heavy trac hours.

size of splieng cells is also dependent on the radio aspect and switching processor.

Eects of cell splieng Reduc)on in coverage area of a split cell Reduc)on in the cell-site transmi=er power of a split cell

Increase in trac load aLer cell-splieng Changing in frequency re-use plan Changing the channel assignment

Original larger cell is split repeatedly n no. of )mes , and every )me the radius of the split cell is one-half of its immediate previous cell,

i.e. radius of nth split cell Rn=Ro/n If R2= Ro/2, The area covered by such a circle is 4 )mes as large as the area covered by a circle with radius Ro/2.

Not all the cells are split at the same )me. Also the hand o issues must be addressed. By decreasing the cell radius and keeping co-channel reuse ra6o D/R unchanged, cell spliKng increases the no. of channels per unit area.

Cell Splieng increases capacity

Sectoring improves S/I

Sectoring increases SIR so that cluster size may be reduced.

The SIR is improved using direc)onal antenna , then capacity improvement is achieved by reducing the no. of cells in a cluster, thus increasing the frequency re-use.

Replace a single omni-direc)onal antenna at the B.S. by several direc)onal antennas.

The technique for decreasing co-channel interference and thus increasing system performance by using direc)onal antennas is called as sectoring.

A cell is normally par))oned into six 60 degree sectors or three 120 degree sectors.

Draw-back Increased no. of antennas at each base sta)on and decrease in trunking eciency due to channel sectoring at b.s.

Sectoring improves S/I

Repeaters Coverage for hard-to-reach areas, such as within buildings, or in valleys or tunnels where coverage region has been tradi)onally weak.

Radio retransmi=ers, k/a repeaters Bidirec=onal in nature Installed any where Amplies and re-radiates the b.s. signals to the specic coverage region.

DAS (distributed antenna system)

In-building deployment is the next great growth phase

The Zone Cell Concept

The increased no. of handos required when sectoring is employed results in an increased load on the switching and control link elements of mobile system.

Zone sites are connected to single b.s. and share the same radio equipment.

Mul)ple zones and a single b.s. make up a cell. Useful along highways or along urban trac corridors.

Zone Cell Concept

Power control for reducing interference

Power level transmi=ed by every subscriber unit are under constant control by serving BS.

Each mobile should transmit the smallest power necessary on reverse channel.

Helps prolong ba=ery life for subscriber unit Reduces reverse channel S/I in the system.

Power control techniques: OPEN LOOP power control: The mobile adjusts its transmission power based on its received signal power from BS.

CLOSED LOOP power control: Based on the measurement of link quality, the BS sends a power control command instruc)ng the mobile to increase or decrease its transmission power level.

Trunking and Grade of service Trunking: to accommodate a large no. of users in a limited radio spectrum

One erlang represents the amount of trac intensity carried by a channel that is completely occupied.

GOS is a measure of the ability of a user to access a trunked system during busiest hour.

GOS is used to dene the system performance of a par)cular trunked system.

Trunking: a xed no. of channels or circuits may accommodate a large, random user community.

Telephone company uses the trunking theory

The trac intensity oered by each user is equal to the call request rate mul)plied by the holding )me. That is, each user generates a trac intensity of Au Erlangs given by

Au = H where H is the average dura=on of a call and is the

average number of call requests per unit )me for each user.

For a system containing U users and an unspecied number of channels, the total oered trac intensity A, is given as

A =UAu

In a C channel trunked system, if the trac is equally distributed among the channels,then the trac intensity per channel, Ac, is given as

Ac = UAu C

Two types of trunked systems: Blocked calls Cleared Blocked calls Delayed

Blocked calls cleared: No queuing for call requests

Assume no setup )me and user is given immediate access to a channel

If no channels are available , the reques)ng user is blocked without access and is free to try again.

Erlang B formula also called as blocked calls cleared formula .

It determines the probability that a call is blocked and is a measure of GOS for a trunked system which provides no queuing for blocked calls.

It provides a conserva)ve es)mate of the GOS.

Erlang B formula

Where C is the no. of trunked channels oered by a trunked radio system And A is the total oered trac.

Blocked calls delayed:

Queue is provided to hold calls

Calls are blocked

If channel is not available immediately , the call request may be delayed un)l channel

becomes available.

Its measure of GOS is dened as the probability that a call is blocked aLer wai)ng a specic length of )me in the queue.

The likelihood of a call not having immediate access to a channel is determined by Erlang C formula.

The probability that the delayed call is forced to wait more than t seconds is given by the probability that a call is delayed, mul)plied by the condi)onal probability that the delay is greater than t seconds

The average delay D for all calls in a queued system is given by

Erlang B Trunking GOS

Erlang B

Erlang C

ANTENNAS FOR THE BASE STATION Two types of antennas used in wireless industry: omni-direc=onal or bidirec=onal antennas Antenna elements comprise the collinear array antenna will be

Longer for the antennas used for lower frequencies Shorter for he antennas used for higher frequencies As lower frequencies emits longer radio wavelengths and higher frequencies emit shorter wavelengths.

Recently, 0dB or 3dB gain antennas are used in urban areas.

Direc)onal antennas are eec)vely onmiantennas that use a reec)ng element, which direct the RF signal(energy) over a specied beamwidth and produces more gain than Omni base sta)on antenna produces.

Most used beam-width is 120-degree beam-width. At cell sites with very high tower and a high gain antenna, coverage shadows may be created near the tower.

To compensate coverage shadows, electrical down)lt antennas and mechanical down)lt

Down)lted antennas is an antenna whose radia)on pa=ern is electrically or mechanically )lted at a specied no. of degree downward.

Down)l)ng of antennas decreases distance coverage horizontally but increases signal coverage closer to the cell-site.

Omniantennas can only be Down)lted electrically, by adjus)ng the phasing of RF signal that is fed to the collinear antenna elements.

Electrical down)lt antennas are manufactured to down)lt to a preset amount of degrees.

Direc)onal antennas can be Down)lted either electrically or mechanically.

Mechanical Down)l)ng is by manipula)ng the antennas so that they )lt toward the ground.

But it distorts the side lobes of the radia)on pa=ern and interfere with adjacent sectors.

Down)lt antennas are commonly install at cell-site on very tall tower or a hill, or near a large body of water.

Down )lt antennas reduces the impact of far-eld eect. far-eld eect can occur due to: I. RF power level is too high at BS II. Down)lt antennas are not being used at BS. III. BS transmieng antenna is too high on tower IV. Antenna gain is too high at BS, exceeding its

intended coverage area.

Types of BS antennas chosen depends on many factors:

I. Size of he area to be covered II. Neighboring cell sites congura)on III. Antenna is Omni or direc)onal IV. Antennas beamwidth in case of direc)onal

antenna V. Allo=ed RF spectrum the antenna can u)lize.

Spectrum eciency of cellular system Each BS can carry more than 1 telephone call in its cell let Wchannel= total bandwidth for the cellular net Wsignal = occupied bandwidth per channel Wchannel = kNWsignal Spectrum eciency SE is dened as the carried trac per cell

Ac, divided by the bandwidth of total system Wchannel and divided by the area of a cell Su.

SE = Ac/ (kNWsignal Su) expressed in erlang/MHz/km^2 . Ac is mostly computed from Erlang B formula

AWributes of CDMA in cellular systems Frequency reuse is not required because the users are dieren)ated on the basis of their codes

Users are transmieng at same frequency. vSoY hando vSoY capacity or graceful degrada6on vMul6path tolerance vNo-channel equaliza6on needed vPrivacy

vSoY hando Every cell uses the same radio frequency band Only dierence between the user channels is the spreading code sequence.

No jump from one frequency to another when a user moves between cells

Two BS can simultaneously transmit to the same user terminal

Mobile receiver can resolve the two signals sepeartely and combine them. This feature is called soY hando.

vSoY capacity or graceful degrada6on In FDMA and TDMA, N channels can be used virtually without interference from users in same cell.

Capacity in TDMA and FDMA is xed at N users. In CDMA addi)onal users may be added by sacricing some link quality with eect that voice quality is slightly degraded.

vMul6path tolerance Spread spectrum techniques combat the frequency selec)ve fading that occurs in mul)path channels.

Frequency selec)ve fade will corrupt only a small por)on of the signals power spectrum, and passing remaining por)on undamaged.

Upon despreading, it is probable that the signal recovered correctly.

But for unspread signal whose spectral density is misplaced in deep fade, unrecoverable signal is assured at receiver.

To op)mally combine signals received over various delayed paths , a rake receiver is used.

vNo-channel equaliza6on needed In FDMA &TDMA , an equalizer is needed for reducing ISI caused by )me delay spread when transmieng rate >>10kbps

In CDMA , a correlator is needed at the minimum.

vPrivacy For spreading signals, pseudo random is assured. Despreading the signals requires knowledge of the users code.

Dicult for an authorized a=acker to tap into or transmit on another users channel.

In cellular systems, the codes are fully described in publicly available standards.

In digital systems, security is obtained through encryp)on.

ASSIGNMENT 1. List and describe some methods to reduce co-

channel interference in cellular communica)on. 2. Write dierence between xed and dynamic

channel assignment strategies. 3. A system has 394 cells with 19 channels each. Find

the no. of users that can be supported at 2% blocking if each user averages two calls per hour at an average call dura)on of three minutes.

4. 5.