Cellular Design Fundamentals

8/3/2019 Cellular Design Fundamentals

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Cellular Design

Concept & Fundamentals



Design Objectives Large Coverage Area

Tall antenna/ high power

High Capacity

Frequency reuse

Old Systems: A single antenna had a capacity of

only 12 users in an area of 1000 sq. mi.!Design Goals: High capacity/large coverage

area at optimal radio spectrum efficiency



Cellular Solution In the 1970s, Bell Labs developed a solution

(AMPS): Instead of using one large powerful

transmitter, lets use many small less powerfultransmitters

Advantages:

Very high capacity

Limited spectrum usage

Mobile sets can be manufactured with same setsof frequencies



The Cellular Concept Divide coverage area into smaller

regions(cells of radius 2-50km) with one base

station at the center Divide spectrum into groups of non-

contiguous RF channels

Allocate one frequency group to each BS;nearby cells use a different group

If demand increases, increase no. of cells



Some Design Parameters Cell size

Cell location RF channel allocation



Why Hexagonal Cells? Radio coverage of a BS is modeled as a

hexagon because:

It permits easy analysis

It resembles a circle (no overlaps & gaps)

It requires the fewest cells to cover an

area (compared to other shapes) It approximates a circular radiation pattern

for an omni-directional antenna



Frequency Reuse or Planning Defn: The process of allocating channel

groups to each BS in the system

Given a set of S duplex channels, divide theminto N cells with k channels/cell, I.e.

S=k N These N cells form a cluster (of size N)

Typical cluster size is N =4,7, or 12



Frequency Reuse Illustration

A

C

D

B

G

F

E

A

C

D

B

G

F

E

A

C

D

B

G

F

E



Cluster Size Tradeoff If a cluster is replicated M times, capacity is:

C=Mk N =MS If the cluster size (N) is reduced (while cell

size remains constant), more clusters will berequired; hence, capacity will increase; but

interference will also increase We want to minimize N such that a certainSIR ratio can be maintained



Frequency Reuse Factor The Frequency Reuse factor of a

cellular system is defined as:

1/N

because each cell uses only 1/N th of the available channels



Channel Assignment Fixed Assignment

Predetermined fixed set of channels are assignedto each cell

If all channels are busy,calls are blocked

Borrowing Strategy

Borrows a channel fromneighboring cells

MSC supervises theprocess

Dynamic Assignment MSC assigns a

channel to the BS asper some algorithm

Advantages: Increases capacity

Increases channel

utilization Disadvantages:

Increasedcomputational load



Handoff Defn: The transfer of a call from one BS to

another while a MU moves in the area

It involves: Identification of a new BS

New voice and control channel assignment

It must be performed: successfully, infrequently and imperceptibly to the

user



Handoff Threshold D efn: Optimal signal level at which to

initiate a handoff

Handoff Threshold (Pht )is usually set at avalue slightly higher than the minimumusable power level(Pmin) received at the BS

The margin, ( = Pht Pmin, is a system

parameter, which has to be set carefully If ( is too high, unnecessary handoffs occur

If ( is too low, the call will be lost becausethere will be insufficient time to complete

handoff



Handoff deception Fading can result in the signal level

dropping below Pht

Running average signal level (over atime period) must be used to counterthis deception

Speed of MU alters running average Speed can be computed at BS from

signal statistics



Dwell Time Defn: The time over which a call may be

maintained within a cell without handoff

Dwell time is determined by: Propagation

Interference

Distance

Time-varying effects (speed?) Dwell time statistics are needed to design

handoff algorithms



1G Handoff Strategy RSSI( Received Signal Strength Indicator) of

all MUs is measured by the BS

A locator receiver (in each BS) is used tomeasure RSSI of MUs in neighboring cells

Based on this information, the MSC decides if

handoff is necessary or not Typical Handoff time is about 10 sec,

requiring ( to be about 6-12 dB



2G Handoff Strategy MAHO (Mobile Assisted Handoff) used

Each MU measures the received power from

surrounding BS and continually reports theresults to BS

Handoff is initiated when Power receivedfrom neighboring BS is higher for a certain

period of time MAHO is much faster (about 1-2 sec); suited

for micro-cellular environments



Soft Handoff Defn: The ability to select between

RSSI from various BS

In IS-95, CDMA spread spectrumsystems, MUs share the same channelin each cell. Hence, handoff does not

require new channel assignment MSC decides which version of the signal

to send to the PSTN



Prioritizing Handoffs Many Handoff techniques prioritize

Handoff over call initiation by using: Guard Channels

Some channels are reserved for handoff.

Capacity decreases

With dynamic channel assignment, spectrum

utilization efficiency increases Queuing

Handoff requests are put in a queue



Practical Handoff Issues MU Speed

Vehicles need more handoffs than pedestrians

Umbr ella cells solve this problem

New Cell sites Zoning laws & barriers restricts new cells to be

formed

Cell Dragging

MU travels to next cell yet its RSSI is still good

Handoff Thresholds must to be adjusted carefully



Interference Major limiting factor

Sources are: Another mobile in the same cell A call in progress in a neighboring cell

Other BS operating in the same freq. band

other systems which inadvertently leak energy

into the cellular frequency band Voice channel cross talk

Control channel missed/blocked calls



Co-Channel Interference Interference from cells using the same

frequency group in a cluster

Cannot simply increase SNR to combat it

Co-channel cells have to be physicallyseparated to provide isolation

It is a function of cell radius (R) and distanceto the center of the nearest cell (D)



Co-Channel Reuse Ratio The Co-channel Reuse Ratio, Q, is defined as:

Increasing Q increases the spatial separationbetween co-channel cells; however, it alsoincrease N thereby decreasing capacity

Tradeoff must be made between Q and N

N R

DQ 3!!



Signal-to-Interference Ratio If the transmit power of each BS is equal,

then the Signal-to-Interference Ratio (SIR) is:

where S is the desired signal power, Ii , is the

interference power caused by the ith co-channel, i0 is the number of co-channelinterfering cells and n is the propagationexponent

§§!!

!!00

11

/

1

i

i

n

i

i

i

iR D I

S SIR



SIR Approximation If we consider only the first layer of

interfering cells, then the SIR will be:

Note that SIR E N!

For AMPS, Given SIR=18dB, then N =7

00

3/

i

N

i

R DSIR

nn

!!



Adjacent Channel Interference Interference from signals adjacent in

frequency

It is caused by: Imperfect receiver filters

Near-far effect

High & low power transmitted in contiguouschannels

It can be minimized by careful filtering, use of guard bands and channel assignment



Power Control Power level transmitted by MUs are

constantly controlled by BS s

PC ensures that each MU transmits at thesmallest power level necessary

This process reduces SIR, increases capacityand increases battery life

It is especially important in CDMA where allusers in the cell share one channel



AMPS Channel Allocation 832(666+166) channels allocated by FCC

The forward channel (870.030MHz) and

reverse channel (825.030MHz) is numberedChannel 1

FCC licensed out the channels to twocompetitors and divided the channels into

Block A & Block B Out of the 416 channels, 395 are voice

channels and 21 are control



AMPS (example 2.3) The 395 channels are divided into 21

groups of about 19 channels each

For N=7, each cell uses 3 groups orabout 57 channels (channels are at least 7 channels away from each other)

For example, one group will containchannels 1,8,15,22,29,309,670,1017(see table 2.2)



Trunking Theory It allows a large number of users to

share the limited number of channels ina cell according to statistics

How many channels do I need toaccommodate x numbers of users?

Tradeoff b/w number of channels, C,and Outage percentage



Grade of Service GOS is a measure of congestion in system,

I.e. it is the ability of a user to access a

trunked system during its busiest hour It is a benchmark

Design Issue: Given a GOS, estimate amaximum capacity level for a set of channelsin the wireless network

In AMPS, GOS is 2% blocking



Traffic Intensity It is a measure of channel utilization time, or

the average channel occupancy

One Erlang represents the amount of TrafficIntensity carried by a channel that iscompletely occupied

The Traffic Intensity per user is:

Au=µH where µ is the average number of callrequests per unit time and H is the averagecall duration



Total Offered Traffic Intensity If the system has U users, then the

total offered traffic Intensity is:

A= UAu

If the total Traffic is distributed evenlyamongst C Channels, then the totalTraffic Intensity per channel is:

A= Uau /C



Blocked Calls Cleared This trunked system offers no queuing for call

requests

User is given access to a channel on demandand blocked if no channel is available

Assumptions are: Poisson call arrivals/exponential channel

occupation

Infinite number of users/finite number of channels



Erlang B Formula Blocked Calls Cleared truncked system aka

M/M/m queue and leads to the Erlang B

formula It determines blocking probability and is a

measure of the GOS

It provides a conservative estimate of GOS

because in actual life there are finite numberof users

See Fig. 2.6 page 49 of text



Capacity At any given time, capacity of a system

is limited to the number of channels, C.

Using Trunking/Queuing theory,Capacity can be increased

Capacity increases with C and with GOS

(outage percentage)



Blocked Calls Delayed This trunked system provides a queue

to hold calls which are blocked

Call requests are delayed until achannel is available

GOS is the Probability that a call is

blocked after waiting t sec in a queue



Erlang C Formula It is the probability that a call is initially

denied access? I.e. Pr[delay>0].

It is a function of the Traffic Intensity, A, and the number of channels, C.

See Fig. 2.7 on page 50.



GOS of BCD Trunked system GOS is given by:

Pr[delay >t]=Pr[delay>0]Pr[delay >t |delay >0]

=Pr[delay>0]exp(-(C-A)t/H ) The average delay, D, for all calls is:

D=Pr[delay>0]H /(C-A)

The average delay for those calls that arequeued is:

Dq=H /(C-A)



Trunking Efficiency It is a measure of the number of users which

can be offered a particular GOS using fixed

number of channels 10 channel trunked system has higher

Trunking efficiency than two 5 channeltrunked systems because it can support 60%

more traffic [See table 2.4 on pg. 47]

Be careful when you allocate channels!



Capacity Improvements Increase in Demand warrants Capacity

enhancements

Three practical techniques are:

Cell splitting

Sectoring

Coverage zone



Cell Splitting It is the process of subdividing a congested

cell into smaller cells

Capacity increased because freq. re-useincreased. I.e. no. of channels increased

Channel allocation scheme remains intact

Antenna Power and height are subsequently

reduced If microcells have half the radius, and with

n=4, trasmit power must be reduced by 1/16or 12 dB for the same SIR



Cell Splitting 2 In practice, not all cells are split at the

same time. I.e. different cell sizes exist

simultaneously In such cases, channels in the old cell

must be broken into two channelgroups

Antenna downtilting is used to limit thecoverage of microcells



Sectoring Replace single omni-directional antenna

with several directional antenna,

thereby sectoring the cell Reduces the co-channel interference

Normally, three 120o sectors or six 60o

sectors are formed Channels are also broken into sectored

groups



Sectoring 2 For a 7-cell reuse, interference is reduced

from 6 to 2, resulting in a SIR of 24dB (up

from 17 dB) Antenna downtilting improves SIR further

Sectoring reduces interference by a factor of 12/7 or 1.7; this allows us to decrease N

Drawback is increased no. of Antennas and adecrease in trunking efficiency

Handoff s increase from one sector to another



Microcell Zone Divide the cell into zones and connect them

to the same BS and MSC

Antennas are placed at outer edges of thecell and channels are assigned to the BS

Handoff not required between zones; BS merely switches the channel to a different

zone Each channel is active in only one zone;

hence interference is reduced



Microcell Zone 2 Especially useful along highways

Co-channel interference is reduced Capacity is increased yet trunking

efficiency is not degraded

Capacity is increased by a factor of 7/3or 2.33 over a conventional 7-cell omnisystem



ComparisonCell Splitting Sectoring Microcell

Zone

No. of BS Increase Same Same

Co-ChannelInterference

Same Decrease Decrease

TrunkingEfficiency Same Decrease Same

Handoffs Same Increase Same

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