CELLULAR CONCEPTS&

SYSTEM DESIGN FUNDAMENTALS

Dr. Muhammad WaheedPhD, RRM in 4G & Beyond Networks

Certified Lte RNPOWaheed.Muhammad@ptcl.net.pk

OUTLINE Introduction Frequency Reuse/Planning Channel Assignment Strategies Handoff Strategies Interference & System Capacity Trunking & GOS Improving Coverage & Capacity in

Cellular Systems

INTRODUCTION Early Mobile Radio System

Design objective Large coverage

Limitations No Frequency reuse Interference limitations

Challenges Increased service demands non-availability of spectrum allocations in proportion to demands

Requirement Radio system restructuring Cellular Systems, to achieve:

high capacity With limited spectrum While having large coverage

FREQUENCY REUSE/PLANNING Cell

Small geographic area Base station (BS) in each cell is allocated group of radio

channels Adjacent Cells BS……different group of channels.

Cellular Base Stations Antennae designed for limiting coverage within cell. Avoiding interference to cells using same group of channels

Frequency reuse/planning In cellular system design Channel grouping Group allocation to cellular BSs

FREQUENCY REUSE/PLANNING ……… Cluster

Group of cells Frequency reuse factor

If cluster size =N Total number of duplex channels =S If each cell has uniform same group of channels= k Freq, reuse factor =1/N Total no. of available radio channels =S=kN

System Capacity If no. of clusters using same S radio channels=M Capacity = C=MkN=MS

FREQUENCY REUSE/PLANNING ……… Hexagon geometry

To connect cells without gaps b/w adjacent cells Geometry of hexagon No. of cells per cluster, N can have value satisfying:

Nearest co-channel cell:1. Move i cells along any chain2. Turn 60 degree counter-clockwise and move j cells

If i =2, j = 2, ……..gives…… N=12

FREQUENCY REUSE/PLANNING ……… Example:

FDD System Total Bandwidth 10MHz Paired(10+10) Channel bandwidth= 100k Total available chls =10,000K/100k=100

1. No. of Chls per cell, for N = 7 = 100/7=142. No. of Chls per cell, for N = 12 = 100/12=83. No. of Chls per cell, for N = 19 = 100/19=5

CHANNEL ASSIGNMENT STRATEGIES Classification

Fixed Dynamic

Fixed Channel Assignment Predetermined set of channels for voice/data Service subject to unused channel availability Call blocking Channel borrowing (MSC supervise)

Dynamic Channel Assignment Chls not allocated permanently Call request…BS request MSC for chl allocation MSC allocates chls ….avoiding …blocking….interference MSC collects Radio Signal Strength Indicator (RSSI) Increase computational load of system

Increased chl utilization Decreased blocking probability

HANDOFF STRATEGIES Handoff

Active state When mobile moves into a different cell while

conversation/data session is in progress MSC automatically transfers the call/session to new channel

belonging to new BS. Voice/Data , Control chls are allocated are that of new BS

Idle State. When mobile moves into a different cell while its in idle

state.

Handoff strategies. Priority

Prioritize Handoffs over new initiations requests Allocate un-allocated chls in a cell site for H/O

HANDOFF STRATEGIES .. …….. Initiation thresholds

Need optimum signal levels at which H/O can be initiated Minimum usable signal level at MS/BS For BS is normally between -90dBm and -100dBm And slightly lower on BS side* Initiation thresholds

System/Operator Policies Different systems, operators have different policies

For managing Handoffs Treating HO as new calls HO failure equals blocking probability

* Network EIRP threshold is lower & antenna gain is higher than MS

HANDOFF STRATEGIES ……….. Prioritizing Handoffs

Guard Channel concept Fraction of total available chls in cell are reserved for handoffs Static , reduces cell traffic/throughput. Dynamic , efficient spectrum utilization

Queuing Handoff requests To decrease probability of forced termination of call

Due to lack of chls available for HO Time between the…….. time the received signal level drops

handoff threshold & Time the call is terminated due to insufficient signal level

Doesn’t guarantee zero probability of forced termination Since large delays cause signal drop below the level required to

maintain communication

HANDOFF STRATEGIES ……….. Practical Consideration

Mobility/Speed User travelling Cars/high speed Pedestrian/Slow speed. Dynamic , efficient spectrum utilization

Cell Size Micro ….capacity Macro ….Coverage Umbrella…Wide coverage…Highways

LoS/Low penetration losses Cell dragging

HO doesn’t occur, …user move into other cells Traffic management issues

Different technologies Hard handoff

Different Chls after HO…Chanellized wireless systems…GSM Soft Handoff

Different BS after HO….Spread spectrum systems….CDMA

INTERFERENCE & SYSTEM CAPACITY Interference

Major factor limiting….cellular system performance

Sources Other BS/Transceivers, MS, Calls, Sessions in the same cell,

energy in the band Types

Co-channel Adjacent channel

Competitors transmitters are often significant source of out of band interference

INTERFERENCE & SYSTEM CAPACITY ……….. Co-Channel Interference & System Capacity

Co-channel Cells Frequency re-use systems Cells using same set of frequencies/chls Co-channel cells

Co-Channel Interference Interference between signals form Co-channel cells Thermal noise can be overcome …improving SINR Co-channel interference cannot be combated by increasing carrier power of

transmitter Increasing carrier tx power…increases interference in neighboring co-

channel cells To reduce co-channel interference…..physical separation/sufficient isolation Co-channel reuse ratio…related to cluster size:

Where D is distance b/w centers of co-channel cells of radius R Small Q …large capacity…cluster size “N” is small

INTERFERENCE & SYSTEM CAPACITY ……….. Channel Planning for wireless Systems

1G/2G/GSM Frequency planning is difficult Sectoring is used to improve SINR

Leads to smaller cell size Coverage region and interference levels are well defined

CDMA Cluster size N =1 Has dynamic , time varying coverage region Cell Breathing:

Coverage region varies depending upon instantaneous no. of users

Difficult decisions for power assignment to control /voice channels

INTERFERENCE & SYSTEM CAPACITY ……….. Adjacent Channel Interference & System

Capacity Interference form signals which are adjacent in

frequency to desired signal. Result from improper receive filters which allow

nearby frequencies to leak into pass band. Serious if adjacent chl user is transmitting very

close………near-far –effect.

Adjacent Chl interference can be minimized Through careful filtering & Channel assignments

INTERFERENCE & SYSTEM CAPACITY ……….. Power Control

For reducing interference BS controls MS transmit power

To ensue MS transmits at minimum power level required for quality common reverse link

Improve battery life of MS Reduces reverse channel S/I in the system

CDMA Power is important to allow every user in every cell

to share same radio channel

TRUNKING AND GRADE OF SERVICE (GOS) Trunking allows a large number of users to share

relatively small no. of channels Pool of chls….each user is assigned chl on per call

basis. Chl returned to pool upon call termination

Grade of Service (GoS) GoS is measure of ability of a user to access a

trunked system during busiest hour GoS is a benchmark used to define the desired

performance of a particular trunked system by specify a desired likelihood of a user obtaining chl

access out of available in trunked system.

THANKS