EN 253 205 (2/2562)Mobile Communication

Cellular System, Multiuser Capacity & Interference

Management

Asst. Prof. Nararat RuangchaijatuponElectrical EngineeringKhon Kaen University

Office: EN04325A, Email: nararat@kku.ac.th

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Recall: Multiple Access & System Capacity

• FDMA/FDD in AMPS (Advanced Mobile Phone System) => 1G

• Capacity = No. of Users per Cell• Capacity = ?

869-894MHz

Source: https://en.wikipedia.org/wiki/Advanced_Mobile_Phone_System

3

Recall: D-AMPS (or TDMA)

• IS-54, IS-136 => 2G

• Each AMPS ch. => 3 timeslots => full-rate ch.

• Each timeslot is divide into 2 => half-rate ch.• Capacity = ?

3 timeslotsor 6 timeslots

Source: https://commons.wikimedia.org/

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Recall: GSM

• 8-slot TDMA scheme used in GSM• 124 carriers (FDMA/TDMA/FDD) in each direction• Capacity = ?

Source: https://www.scmp.com/news/world/article/1519439/whats-old-new-again-vintage-cellphones-take

The Cellular Concept – Cell Shape

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The Cellular Concept

• Cluster of cell - to minimize interference

• Co-channel interference– Co-channel cells

• Adjacent channel interference– Each cluster utilize entire available radio

spectrum

– Different clusters Co-channel interference

– Within cluster Adjacent channel interference

The Cellular Concept: Cluster

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The Cellular Concept: Cluster (cont.)

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The Cellular Concept (cont.)

W = Spectrum width

B = Bandwidth needed per user

N = frequency reuse factorm = no. of cells/clusters required for the coverage area

n = no. of simultaneous users

N

m(W/B) n

Cellular Hierarchy• Femtocells

– connect personal equipment

• Picocells– small cell inside a

building e.g. WLANs

• Microcells– street, urban

• Macrocells– metropolitan

• Megacells– nationwide/satellite

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Cell Fundamentals• Cochannel cells must be placed as far

apart as possible

DL = Distance between cochannel cells

RL = Cell radius

Hexagonal Cell Shape

Cochannel Reuse Ratio =>

N= i2 + ij + j2 , i,j Zinteger

N3L

L

R

D

Source: https://www.researchgate.net/figure/Co-channel-reuse-ratio-Generally-for-we-can-find-the-nearest-co-channel-neighbours_fig1_323172290

Signal-to-interference Calculation

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N=7,Js=6

sJ

nn

r

t

tr

iierference

desiredr

d

dS

d

d

dKP

dKPS

P

PS

1

0

1

2

2

1

,int

24

4

4

2

3

6

1N

R

D

DJ

RS L

Lsr

NR

DS L

r log2076.1log4078.7

Clustering in AMPS

13Source: https://www.slideserve.com/wenda/lecture-10

Clustering in GSM

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Source: https://www.slideshare.net/sanjida2222/presentationj

System Comparison

15Source: https://www.slideserve.com/moana/medium-access-control-for-wireless-links

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Capacity Expansion Techniques

• Why do we need to expand capacity?

• Basically 4 methods– Obtain additional spectrum

• Not good

– Change the cellular architecture

– Change the frequency allocation• Redistribute the frequency bands

– Change the modem and access technology• Expensive

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Architectural Methods

• Cell splitting

• Using directional antennas for cell splitting

• Lee’s microcell method

• Using overlaid cells– Split-band analog systems

– Reuse partitioning

• Using smart antennas

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Cell Splitting

• Effects1. Capacity of cell A

decreases forcing the cell splitting until all cells in the coverage are split

2. Increase complexity of a BS in cell A

Cell splitting for capacityexpansion

• Focusing radio propagation in the required direction

• Reducing cochannelinterference

• Increase SIR at the terminal longer battery life

• Less expensive + no planning effort (compared to cell splitting technique)

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Cell Sectoring using Directional Antennas

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Lee’s Microcell Method

• Sectoring just creates a new cell with a different shape handoff load

• Employ directional antennas at the cell’s corners Zone-sites

• BS decides which zone-site, uses it for downlink transmission

Each zone-site 135º

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Using Overlaid Cells• Channels are divided among

a macrocell and a microcell

• The same BS serves both the macro- and microcells

– Split-band analog systems:• Each ch. in overlay cell gets

half BW (FM)

• Require 4xSIR

• Co-ch. cells are far apart

– Reuse partitioning• Channels are divided into 2

groups

• Microcell -> 3-4 reuse ratio• Macrocell -> 7-12 reuse ratio

Overlaid Cells: AMPS’ Band Splitting

• AMPS: 30 kHz, Sr = 18 dB

• Overlay System: 15 kHz, Sr=18+6=24 dB– In FM, If BW is reduced by half, Sr requires 4

times increasing

• Fix D1=D2, N=7– both overlay and underlay networks employ

the same frequency reuse factor

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Overlaid Cells: AMPS’ Band Splitting (cont.)

• Hence, R2=0.7079R1

• A2=0.5A1

• The overlay cell is responsible for users within a small hexagonal

• The underlay cell is responsible for users between the boundary of the overlay cell and the boundary of the underlay cell

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dBRD

RD6

/

/log10 4

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422

Overlaid Cells: AMPS’ Band Splitting (cont.)

• The no. of channels in the overlay and underlay cells are equal (M)

• Original AMPS: 12.5 MHz, 395 channels, 30 kHz

• M = 263

• Total no. of channels per cell = 2xM = 526

• Capacity increases by 34 percent

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303953015 M

Overlaid Cells: Reuse Partitioning

• AMPS network requires 18 dB

• D1/R1 = D2/R2 = 4.6

• R2 < R1 D2 < D1

• The improvement in cochannel reuse ratio comes from the fact that the microcells in the overlay are not contiguous to one another

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Using Smart Antennas

• Users in the same cell can use the same physical communication channel– As long as they are not located in the same

angular region with respect to a BS

• Space Division Multiple Access (SDMA)

• BS directs an antenna beam toward a mobile communicating with it

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Migration to Digital System

• To increase capacity

• AMPS Digital AMPS– Increase capacity to 833x3 = 2,499 users

• IS-136

- Sr = 12 dB, N= 4

• GSM- Sr = 9 dB, N= 3

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Capacity of CDMA

• Processing gain N• Information bandwidth

R• Transmission

bandwidth W• M simultaneous users• Received power from a

terminal P (equal, perfect power control)

• Received SNR for the target receiver Sr

R

WNRNW ,

11

M

N

PM

NPSr

rr SR

W

SR

WM

11

1

Received power from the target user after processing at the receiver is NP

1rS

NM

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Practical Consideration

• Sectorization gain factor GA

– Usually equal to the number of sectors in the cell

• Voice activity factor GV

– Ratio of total connection time to active talk time

• Interference increase factor H0

– Interference from users in other cells

• Hence

0

11

1

H

GG

SR

W

SR

WM VA

rr

Performance Improvement

Factor

Capacity of 4G LTE• Choices of Channel Bandwidth

– 1.4 / 3 / 5 / 10 / 15 / 20 MHz

– Subcarriers spacing is 15 kHz

– More bandwidth, more data rate, less users

• Choices of Modulation– Downlink: OFDM

• MS can receive 2048 subcarriers

• BS can transmit 72 subcarriers

• MS is able to talk to any BSs

• Choice of DL Modulation: QPSK, 16QAM, 64QAM

– Uplink: Single Carrier Frequency Division Multiple Access (SC-FDMA)

30Source: https://www.electronics-notes.com/articles/connectivity/4g-lte-long-term-evolution/ofdm-ofdma-scfdma-modulation.php

Capacity of 4G (LTE, LTE-A)• 3G: 60-100 users/BS, 4G: 300-400 users/BS

Source: https://electronics.howstuffworks.com/4g3.htm

• Coordinated Multipoint– Intrasite CoMP / Intersite CoMP

– Joint Processing/Coordinated Scheduling

• LTE-A: Carrier aggregation, 4x4 MIMO, 256QAM

• Good Watch– https://www.youtube.com/watch?v=MBcGO3EC3Qs (7.57

min)

– https://www.youtube.com/watch?v=CV2-CSeHzSE (5.40 min) (Please Watch!)

– https://www.youtube.com/watch?v=uIPtLr8R1-U (6.27 min) (Please Watch!)

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Question & Discussion

AssignmentIn AMPS system, the required SIR is 18 dB. If 120̊ directional antennas are used in the AMPS system with frequency reuse factor N =7, calculate the new SIR.