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 powerful transmitters

Advantages: Very high capacity Limited spectrum usage Mobile sets can be manufactured with same

sets of 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 Def’n: The process of allocating channel

groups to each BS in the system

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

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

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

Frequency Reuse Illustration

AC

D

BG

FE

AC

D

BG

FE

AC

D

BG

FE

Cluster Size Tradeoff If a cluster is replicated M times,

capacity is:C=MkN=MS

If the cluster size (N) is reduced (while cell size remains constant), more clusters will be required; hence, capacity will increase; but interference will also increase

We want to minimize N such that a certain SIR ratio can be maintained

Frequency Reuse Factor The Frequency Reuse factor of a

cellular system is defined as:

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

Channel Assignment Fixed Assignment

Predetermined fixed set of channels are assigned to each cell

If all channels are busy, calls are blocked

Borrowing Strategy Borrows a channel

from neighboring cells

MSC supervises the process

Dynamic Assignment MSC assigns a

channel to the BS as per some algorithm

Advantages: Increases capacity Increases channel

utilization Disadvantages:

Increased computational load

Handoff Def’n: 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 Def’n: Optimal signal level at which to initiate

a handoff Handoff Threshold (Pht)is usually set at a value

slightly higher than the minimum usable 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 because

there will be insufficient time to complete handoff

Handoff deception Fading can result in the signal level

dropping below Pht

Running average signal level (over a time period) must be used to counter this deception

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

signal statistics

Dwell Time Def’n: 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 to

measure 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 the results to BS

Handoff is initiated when Power received from 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 Def’n: The ability to select

between RSSI from various BS In IS-95, CDMA spread spectrum

systems, MU’s share the same channel in 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 Umbrella cells solve this problem

New Cell sites Zoning laws & barriers restricts new cell’s 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 physically

separated to provide isolation It is a function of cell radius (R) and

distance to 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 separation between co-channel cells; however, it also increase N thereby decreasing capacity

Tradeoff must be made between Q and N

NR

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-channel interfering cells and n is the propagation exponent

00

11

/

1i

i

ni

i

i

i RDI

SSIR

SIR Approximation If we consider only the first layer of

interfering cells, then the SIR will be:

Note that SIR N! For AMPS, Given SIR=18dB, then N=7

00

3/

i

N

i

RDSIR

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 contiguous channels

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

Power Control Power level transmitted by MU’s are

constantly controlled by BS’s PC ensures that each MU transmits at

the smallest power level necessary This process reduces SIR, increases

capacity and increases battery life It is especially important in CDMA where

all users 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 numbered Channel 1

FCC licensed out the channels to two competitors 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 or

about 57 channels (channels are at least 7 channels away from each other)

For example, one group will contain channels 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 in a cell according to statistics

How many channels do I need to accommodate 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 a

maximum capacity level for a set of channels in 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

Traffic Intensity carried by a channel that is completely occupied

The Traffic Intensity per user is:Au=µH

where µ is the average number of call requests per unit time and H is the average call 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 evenly amongst C Channels, then the total Traffic 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

demand and 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 number of 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 a channel 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

are queued 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 channel trunked 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-use

increased. 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 1/16 or 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 channel groups

Antenna downtilting is used to limit the coverage 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 a decrease 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

the cell 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

7/3 or 2.33 over a conventional 7-cell omni system

ComparisonCell Splitting

Sectoring Microcell Zone

No. of BS Increase Same Same

Co-Channel Interference

Same Decrease Decrease

Trunking Efficiency

Same Decrease Same

Handoffs Same Increase Same