Cellullar Concept & Design Fundamentals Waheed
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Transcript of Cellullar Concept & Design Fundamentals Waheed
CELLULAR CONCEPTS&
SYSTEM DESIGN FUNDAMENTALS
Dr. Muhammad WaheedPhD, RRM in 4G & Beyond Networks
Certified Lte [email protected]
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