Chapter 3.1-3.4 the Cellular Concept

8/4/2019 Chapter 3.1-3.4 the Cellular Concept

1/55

January, 2004M. Junaid Mughal

Wireless CommunicationsPrinciples and Practice

2nd

EditionPrentice-Hall

By Theodore S. Rappaport

UMAIR HASHMI Spring 2011


2/55


The Cellular Concept-System Design Fundamentals

Chapter 3



3/55


3.1 Introduction





4/55


3.1 Introduction

UMAIR HASHMI Spring 2011UMAIR HASHMI Spring 2011UMAIR HASHMI Spring 2011


5/55


3.2 Frequency Reuse



6/55


Linear Cells as an Exampleof Frequency Reuse

For acceptable voice quality Signal to Interference ratio

P/I > 50 (17dB)

f1 f2 f3 f1 f2 f3

P I

Cell 1 Cell 2 Cell 3 Cell 1 Cell 2 Cell 3

Region 1 Region 2

Total Band width (BW) is divided into three adjacent bands f1, f2 and f3

Such that BW = f1+f2+f3



7/55


3.1 Introduction (Contd.)

Different Type of Possible Cell shapes

For the same Cell Radius (R)

(distance from center toCell boundary)

Area of Hexagon is the largest

R

R

R



8/55


Cluster Size

12

312

3

12

3

12

3 4

12

3 4

1

2

3 4

3 Cell Cluster 4 Cell Cluster



9/55


10/55


Hexagonal Cells for Area Coverage



11/55


v1

v2

60o

In area coverage using Hexagonal Cells the distance

between any two Cells can be expressed as a linear

combination of two Basis Vectors v1 and v2

A

B

C



12/55


v1

v2

60o

R30o

|v1| = 2 x R cos(30o) = 3 R

|v2| = 2 x R cos(30o) = 3 R

Magnitude of the Basis Vectors



13/55


v1

v2

60o

Area of the Parallelogram = v1 x v2 = (3R)(3R)sin(60)

= 3R2sin(60).

A Parallelogram can be defined by Basis

Vectors v1 and v2



14/55


The Parallelograms defined by v1 and v2 have the same

transitional periodicity as the hexagonal Cells

Therefore, if M hexagonal cells are required to cover an area thenwe will have M Parallelograms covering the same area

Hence the area of the Parallelogram is equal to the area of the Hexagon



15/55


Area of the Parallelogram = v1 x v2

v1

v2

Area of Parallelogram is equal to the area of the Hexagon ??



16/55


Cellular Concept



17/55


Capacity



18/55


Capacity



19/55


12

3 4

5

67

v1

v2

1

2

3 4

5

67

12

3 4

5

67

U1

U2

60o

The region of Frequency reuse

can be composed of any integernumber N of contiguous (adjacent)

cells

All other regions are obtained by

translation of the defining region

through a linear combination of

the frequency reuse vectors U1 and

U2

In the Figure N = 7.

In the Figure same colour

represents a frequency reuse

region

Frequency Reuse Vectors U1 and U2



20/55


The displacement between

any two cells using the same

frequency can also be

expressed as a linear

combination of the two reuse

vectors.

e.g.,Displacement AB = U1+U2

Displacement AC = 2U2


U1

U2

A

B

C



21/55


The Parallelogram defined by U1

and U2 has the same transitional

periodicity as the frequency reuse

region

Hence the area of the frequency

reuse region is equal to the area of

the Parallelogram

Area = |U1 x U2|

= N time area of a single cell

|U1 x U2| = N.|v1 x v2|

Parallelogram defined by Frequency

Reuse Vectors U1 and U2

U1

U2

A



22/55


12

3 4

5

67

v1

v2

1

2

3 4

5

67

12

3 4

5

67

U1

U2

60o

The Frequency reuse

vectors/displacement vectors canbe expressed in terms of the

Basis Vectors (v1 & v2) as

U1 = k1v1 + m1v2

U2 = k2v1 + m2v2Where the constants k1, k2, m1and m2 are integers.


e.g., in the present case U1 = 2v1 + v2

U2 = -v1 + 3v2



23/55


12

3 4

5

67

v1

v2

1

2

3 4

5

67

12

3 4

5

67

U1

U2

60o

|U1xU2| = |k1m2 k2m1||v1x v2|

Area of Parallelogram defined by

Vectors U1 and U2

As discussed earlier

Area = |U1 x U2|

= N time area of a single cell|U1 x U2| = N.|v1 x v2|

N = |k1m

2 k

2m

1|



24/55


12

3 4

5

67

v1

v2

1

2

3 4

5

67

12

3 4

5

67

U1

U2

60o

For symmetric reuse patterns

only certain values of N are

allowed.

To find these values k2 and m2

are expressed in terms of k1

and m1 under the condition that

1. Magnitude ofU2 and U1 is

same

2. U2 is rotated 60o counter

clockwise w.r.t U1.

Calculation for the Value of N for

symmetrically located co-channel cells



25/55


-v1

v1

v2 U1

U2

60o

v2-v1

-v1

If we represent U1 as alinear combination ofv1and v2 as,

U1

= k1v

1+ m

1v

2

Similarly, we can

represent U2 as a

linear combination of

v2 and (v2- v1) as,

U2 = k1v2 + m1(v2-v1)

Calculation for the Value of N for symmetrically

located co-channel cells



26/55


Calculation for the Value of N for

symmetrically located co-channel cells

U2 = k1v2 + m1(v2-v1)

Previously it was defined as

U2

= k2v

1+ m

2v

2-------------------(2)

Comparing (1) and (2) we get

k2 = -m1and

m2 = k1+ m1Therefore,

rearrangingU2 = -m1v1 +(k1+ m1)v2 ------------(1)

N = |k1m2 k2m1| = m12 + m1k1+ k1

2



27/55


Locating Co-Channel Cells



28/55


Locating Co-Channel Cells



29/55


3.3 Channel Assignment Strategies

Channel Assignment Strategies are used for Efficient

Utilization of Radio Spectrum with the main Objectives

of:

Increasing Capacity

Minimizing Interference

Classification of Channel Assignment Strategies

Fixed channel assignment strategy

Dynamic channel assignment strategy



30/55


3.3 Channel Assignment Strategies (Contd.)



31/55



Fixed Channel Assignment Strategy

Each cell allocated a predetermined set of voice channels

If all channels are occupied then calls are blocked

* Several variations of the fixed assignment strategy exist

such as Borrowing strategyto tackle call blockage

* MSC supervises this borrowing ensuring that borrowing

of channels does not disrupt or interfere with any other

calls Or Reserves some channels for handoff.



32/55



Dynamic Channel Assignment Strategy

voice channels are not allocated to different cells

permanently

at a call request the serving base station requests the

MSC for a channel

MSC allocates the channel following an algorithm that

takes into account,

* Likelihood of future blocking with in the cell

* Frequency of use of the candidate channel

* Reuse distance, etc.



33/55





34/55



Dynamic Channel Assignment Strategy

Advantages

Reduces the likelihood of blocking, which increasesthe trunking capacity of the system

Disadvantages

MSC has to collect real-time data on channel

occupancy and radio signal indications (RSSI),hence, increases storage and computational load



35/55





36/55


3.4 Handoff Strategies

Handoff (HO)

When a mobile moves into a different cell while aconversation is in progress, the MSC automaticallytransfers the call to a new channel belonging to thenew base station

The HO operation not only involves identifying a

new base station, but also requires that the voice

and control signals be allocated to channelsassociated with the new base station



37/55





38/55





39/55


3.4 Handoff Strategies (Contd..)



40/55



Many HO strategies prioritizeHO requests over call initiation

requests when allocating unused channels HO must be performed

Successfully, infrequently and should be imperceptible to the users

Threshold signal level

= Pr handoff Pr minimum usable

Pr handoff specifies the optimum signal level at which to initiate handoffPr minimum usableminimum usable signal for acceptable voice quality at the

base station receiverIf to large: unnecessary HOs

If to small: calls may be lost due to insufficient time for HO



41/55





42/55





43/55





44/55



In 1G analog cellular systems,

signal strengths measurements are made by BS

and supervised by MSC. BS measures signal

strength of all Reverse Voice Channels RVCs

In each BS a spare receiveror locator receiver

measures signal strengths of channels in

neighboring cells

Based on the information from all the locator

receivers the MSC decides if HO is necessary or

not



45/55



In 2G cellular systems that uses digital TDMA

HO decisions are mobile assisted; mobile assisted handoff(MAHO)

In MAHO every mobile measures the received power from

the surrounding BSs and continually reports the results to the

serving BS

MAHO allows much faster HOs, as the HO measurements

are made by each mobile and the MSC no longer constantly

monitors the signal strength MAHO is particularly suited for microcellular environments

where HOs are more frequent



46/55



Intersystem Handoff

During the course of a call if a mobile moves from one cellularsystem to a different cellular system controlled by a different

MSC and intersystem handoff becomes necessary

Conditions for Intersystem HO

When a mobile signal becomes weak in a given cell and the MSC

cannot find another cell within its system to which it can transfer the

call in progress

Issues to be addressed when Implementing Intersystem HO

A local call may become long distance call (billing issue)

Compatibility between the two MSCs



47/55





48/55


3.4.1 Prioritizing Handoffs

Different systems have different policies

and methods for managing handoff

requests. Some systems give priority to

HO over call initiation others deal them at

same priority



49/55



Guard channel concept

Disadvantage reduces total carried traffic

Advantage efficient spectrum utilization with dynamic

channel assignment

Queuing of Handoff requests Possible because a finite time interval between the time the

signal drops below the HO threshold and the time the call is

terminated due to insufficient signal level

* There is a tradeoff between the decrease in probability

of forced termination and total carried traffic



50/55




3 4 2 Practical Handoff Considerations


51/55


3.4.2 Practical Handoff Considerations

Umbrella Cells




52/55






53/55



Cell Dragging



54/55


Cont .




55/55



IS-95 CDMA

Soft Handoff

This technique exploits macroscopicspace diversity provided by the different

physical locations of the base stations andallows the MSC to make soft decision asto which version of the users signal to

pass along to the PSTN at any instance.


Documents

Chapter 3.1-3.4 the Cellular Concept