Cellular Concepts
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Transcript of Cellular Concepts
Cellular NetworksConcepts and Fundamentals
ByWaheed ur Rehman
Agenda Cellular Concept Frequency Reuse Channel Assignment Strategies
Fixed Dynamic
Handover or Handoff Handover Strategies Prioritizing HO Practical HO consideration Okumura propagation model HO types and considerations
Cellular Concept
Coverage area is called a cell Breakthrough in solving of spectral
congestion and user capacity Single high power transmitter is replaced with
many lower power transmitters. Portion of total number of channels are
assigned to each cell
Cellular Concept (2)
Can be reused as many times as needed as long as co-channel interference is kept below acceptable level.
If demand for service increasenumber of BSs increases + decrease of transmission power.
Cellular Concept (3)
Coverage area of a cell depends upon Transmit power of BS Transmit power of the MS Height of the BS antenna The topology of the landscape (terrain)
Coverage can range from too few yards to tens of kilometers
Cellular Concept (4)
CDMA cell (“breathe”) don’t have interference problem
Under light load = large cell size, shrinks with load increases ( due to growing noise)
If more users are in a cell, the higher the noise, higher the path loss and higher the transmission error will be.
Mobile users far away from BS are dropped out.
Cellular : Advantages
Higher Capacity Less Transmission Power
Receiver away from BS require more power
Local Interference only Robust
Cellular : Disadvantages
Infrastructure Needed Handovers Needed Frequency Planning
To Avoid Interference
Question : Why don’t we use Square instead of Hexagon? Square
4 cells apart d 4 cells apart √2d
√(d2 + d2) = √2d
Hexagon Equidistant √3R
R = √(a2 + (R/2)2) Equidistance simplifies the
decision of when and which antenna to choose in case of HO
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Frequency Reuse
Adjacent cells have different channels The design process is called Frequency
Reuse or Frequency Planning Footprint : radio coverage area of the cell Hexagon are better than square of equilateral
triangle.
Frequency Reuse (2)
Center excited cell: BS in the middle. Corner Excited Cell: BS on the vertices Practical consideration usually do no allow
BS to be placed exactly as they appear in hexagon.
Most system design permit BS to be positioned upto one fourth of the cell radius away from ideal location.
Cellular system
Frequency Reuse (3)
S = total duplex channels k= channels in each cell k < S N= Total Number of cells S= kN The N cells which collectively use the complete set of
channels are called Clusters. If cluster is replicated M times then capacity C can be C = MkN = MS Capacity is directly proportional to cluster number N is the cluster size.
Cell structure with microcells
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Cluster
microcells
Reuse factor is 7
Increasing Cell Capacity
Adding New Channels Frequency Borrowing Cell Splitting Cell Sectoring Microcells Repeater for range extension
Increasing Cell Capacity (2)
Macrocell Microcell
Cell Radius 1 to 20 km 0.1 to 1 km
Transmission Power 1 to 10W 0.1 to 1W
Average Delay Spread 0.1 to 10micSec 10 to100 ns
Maximum bit Rate 0.3 Mbps 1Mbps
Channel Assignment Strategies Fixed Dynamic
Channel Assignment Strategies (2) Fixed Fix number of channels are assigned to the
cell Call can be blocked One variation is channel borrowing strategy. MSC supervises borrowing strategy. Fixed Channel Assignment is used by GSM
Channel Assignment Strategies (3) Dynamic Channels are not allocated permanently When a call is made, serving BS request a
channel from MSC The channel is allocated following the
algorithm that takes into account parameters like likelihood of future blocking, reuse distance of the channel etc.
Channel Assignment Strategies (4) Dynamic Require MSC to collect real-time collection of
data on channel occupancy, traffic distribution, radio signal strength indication(RSSI) on continuous basis.
Increases the load and storage. Dynamic Channel Assignment is used by
DECT
Interference
Co-Channel Interference Interference at same frequencies
Adjacent Channel Interference Interference with the neighboring frequency
HOW to Avoid ????
Ability of the subscriber to maintain a call while moving within the network
Handoff can be between Two frequencies (interference) Two sectors on the same BS Between BS Between BSC Between MSC belonging to the same operators Even between two different networks (normally not supported
because of billing reasons)
Handoff/Handover
Handoff/Handover (2)
Two basic reasons ( more than 40 identified by GSM standard)
Signal Strength or SNR
Load Balancing
Handoff : Reasons
Soft Handover Hard Handover Softer Handover
Horizontal HO Vertical HO
Upward and Downward HO
Handoff Types
Four approaches for handoff Network Controlled HO 1G
Network measure the signal strength In case of weak signal than HO to the near by cell
Mobile assisted HO ->2G Signal measurements sent by mobile station.
Network Assisted HO
Mobile Controlled HO
Handoff Approaches
Handoff Strategies
There should be some threshold value. Should be carefully selected to minimize the
ping pong effect. As infrequent as possible Δ = Pr handoff – Pr Min usable
Δ should not be too big or too small
Handoff Strategies (2)
In 1G MSC was responsible for HO Locator Receiver were used at BS for
measuring signal strength and reporting it to MSC.
NCHO
Handoff Strategies (3)
In 2G MAHO strategy was used. Less burden on MSC and improved HO Intersystem HO: from one MSC to another
MSC
HO should be given more priority over originating call.
HO should be as lossless as possible.
Prioritizing Handover
Guard channel capacity: some channels are reserved for HO.
Disadv: reducing the total carried traffic Efficient spectrum utilization in case of
dynamic channel allocation strategy. Queuing of HO request is another strategy.
Practical HO Considerations
User mobility considerations High speed vs low speed users Umbrella cell approach Cell dragging problem in microcells
HO thresholds and radio coverage parameters must be adjusted carefully.
Practical HO Considerations (2) 1G required 10 sec for HO Value of Δ was about 6dB to 12 dB 2G require 1 to 2 sec Value of Δ was about 0 to 6dB Newer cellular system consider more
matrices for HO decision making the process complex
Handover types and recent Considerations Hard handover
GSM Soft handover
IS-95 Softer handover
IS-95 MCHO and NAHO
Mobile Radio Propagation Models Okumura Model that is refined my Hata. Original details analysis of the Tokyo area For Urban environment, predicted path loss is
LdB = 69.55 + 26.16 log fc – 13.82 log ht – A(hr) + (44.9 – 6.55loght) log d
Mobile Radio Propagation Models (2)LdB = 69.55 + 26.16 log fc – 13.82 log ht – A(hr)
+ (44.9 – 6.55loght) log d
fc = Carrier frequency in MHz from 150 to 1500 MHz
ht = Height of transmitting antenna(BS) in m, from 30 t0 300 m
hr =Height of receiving antenna(MS) in m, from 1 t0 10 m d = Propagation distance between antennas in km, from 1 to
20 km. A(hr)= correction factor for mobile unit antenna height
Mobile Radio Propagation Models (3)For a small or medium size city, the correction factor is given by
A(hr) = (1.1 log fc – 0.7) hr – (1.56 log fc – 0.8) dBAnd for a larger city
A(hr) = 8.29[log(1.54hr)]2 -1.1 dB for fc<= 300MHzA(hr) = 3.2[log(11.75hr)]2 -4.97 dB for fc>= 300MHz
For suburban area
LdB (suburban) = LdB(urban) – 2[log (fc/28)]2 – 5.4
And the path loss in open areas is
LdB (open) = LdB(urban) – 4.78(log (fc)]2 – 18.733(log (fc) – 40.98
Discussion
Differentiate between co-channel interference and adjacent channel interference
What are the different techniques for improving coverage and capacity in cellular systems.
Considering duplex channels, what are the alternatives for implementation in wireless networks? What about typical wired networks?
FDD and TDD ?
What have you learnt now?
Cell and cellular concepts Frequency reuse, cluster, sectorization etc. Channel Assignment Strategies including
fixed and dynamic allocation Handover concepts and strategies like Mobile
and Network assisted etc. Practical HO considerations
References
“Wireless Communication”, Theodore S Rappaport, second Edition, chapter 3
“Mobile Communication”, Jochen H. Schiller, 2001. Chapter 2 ,4
“3G Wireless Networks” ,Clint Smith and Daniel Collins, McGraw Hill Telecom 2002 chapter 1,2,3