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

7

1

4

6

5

2

3

d

R

d

d d

R

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

7

1

4

6

5

2

3

7

1

4

6

5

2

3

7

1

4

6

5

2

3

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