Cellular Wireless Networks Common issues for wireless solutions

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Cellular Wireless NetworksCommon issues for
wireless solutions
Kevin BoldingElectrical EngineeringSeattle
Pacific University
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Wireless Systems
Handsets Portable mobile devices
Base Station - Receiver
Network connects base stations
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Wireless (Voice) System Design Goals

• 1. Transmit the voice signal without errors

2. Minimize the handset power
3. Minimize the handset cost
4. Maximize the handset/base station ratio
5. Stay within the allocated band/power
1st Generation Analog (AMPS)
2nd Generation Digital (GSM, CDMA)
3rd Generation Better Digital (WCDMA, CDMA2000)
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Second Generation Cell Phones

• GSM Global System for Mobile Communications
• Open international standard
• 213 countries
• 2 billion subscribers (82)
• All of Europe, much of Asia, much of Americas
• Uses digital TDM / WDM

• CDMA Standard owned by QualComm
• 600 million subscribers (15)
• Largest in North America and Asia
• Uses digital CDMA

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Generic Cellular System
Base Station Controller
Mobile Switching Center
Base Station Controller
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Basic Issues for Mobile Communications
How do we manage handsets entering and leaving
communication?
How do we manage multiple handsets communicating
with one base?
How do we manage handsets moving from base
station to base station?
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Basic Issues for Mobile Communications
How do we manage multiple handsets communicating
with one base?

• Multiplexing (sharing the channel)
• Analog -
• Frequency-division

• Digital -
• Frequency-division and Time-division (GSM)
• Code-division (CDMA)

• We need at least
• 1 send channel for each mobile
• 1 receive channel for each mobile
• 1 control channel

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Basic Issues for Mobile Communications

• To initiate a call
• Mobile issues request on paging channel
• Receiving towers discuss who will answer

• To receive a call
• System must know where the mobile is
• Idle mobile periodically broadcasts on paging
  channel
• System broadcasts page signal on paging channel
  for all bases near mobile

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Basic Issues for Mobile Communications

• Mobile uses idle slots to monitor control
  channels of nearby bases
• Keeps sorted list of the most powerful ones

• If error rate increases, mobile can either
• Increase power on same channel, same base
• Switch to a new base

• Handoff from base to base managed at higher level

• May be a soft handoff

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Cells
Ideally, each base station serves a circular area

• Circles dont tile well

• Use Hexagons as approximations

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Cell Channel Assignment Patterns

• We need to organize patterns for assigning
  channels to cells
• Form a basic cell cluster that will be repeated
  (tiled) to cover the entire service area
• Each cell in a cluster is assigned a different
  set of channels
• More cells in cluster ? Fewer channels per cell

• Basic criterion adjacent cells never have the
  same channel group
• Keep cells with the same channel group as far
  apart as possible
• Use D/R ratio (Larger is better)
• Secondary criterion adjacent cells have
  channels at least two channel groups apart
• Channel 3 not adjacent to channels 2 or 4, etc.

R Cell radius
D Distance between co-channel neighbors
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Co-Channel Interference Model

• C/I is Carrier-to-Interference Ratio
• AMPS modulation characteristicsrequire ? 18 dB
  co-channel C/I over single interferer
• Between a pair of sites using same channel,
  three C/I regions exist
• Site A C/I better than 18 dB
• Neither site gives usable C/I
• Site B C/I better than 18 dB

• Need a D/R that provides 18dB C/I
• D/R gt 4 generally works

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Cell Arrangement as a Function of N

• N1 Lethal, works well in CDMA
• Awful C/I Every neighbor is co-channel
• Every neighbor cell is adjacent channel too!
• Center 1/3 of each cell OK, rest is lost in
  horrible interference

1
1
1
1
1
1
1

• N2 Better, but still lethal
• Each cell still has 2 co-channel neighbors
• Each cell has 4 adjacent channel neighbors

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Cell Arrangement as a Function of N

• N 3 Better, but still lethal
• Co-channel neighbors are now spaced at D/R of 3.0
  - better, but not 18 dB....
• Each cell has 6 adjacent channel neighbors - all
  neighbors are adjacent!!

D
R

• N 4 Better, but still lethal
• Co-channel neighbors are now spaced at D/R of
  3.464
• Each cell has 4 adjacent channel neighbors

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Cell Arrangement as a Function of N

• N 5 Better, but not good enough
• Co-channel neighbors farther away
• 2 at D/R of 3.0
• 4 at D/R of 4.58
• Some cells have 2 adjacent channel neighbors,
  some have 3

• N 6 Better, but not by much
• Co-channel neighbors farther away
• 2 at D/R of 3.464
• 2 at D/R of 4.58
• 2 at D/R of 6.0
• Some cells have 2 adjacent channel neighbors,
  some have 3

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Cell Arrangement as a Function of N

• N 7 First arrangement that works in most
  propagation environments, giving 18 dB C/I
• Co-channel neighbors farther away
• 6 at D/R of 4.58
• Each cell always has 2 adjacent channel neighbors

• N 8 Better, but not worthwhile
• Co-channel neighbors farther away
• 4 at D/R of 4.58
• 2 at D/R of 6.0
• 2 at D/R of 6.93
• Of the eight cells in the cluster, 2 have 2
  adjacent-channel neighbors and 4 have 1 adjacent
  channel neighbor

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Cell Arrangement as a Function of N

• N 9 Significant improvement
• Co-channel neighbors farther away
• 6 at D/R of 5.20
• Out of 9 cells in cluster, 4 have 1 adjacent
  channel neighbor and 3 have 2 such neighbors

• N 10 Not impressively better
• Co-channel neighbors farther away
• 2 at D/R of 4.58
• 2 at D/R of 6.0
• 2 at D/R of 6.06
• Out of 10 cells in cluster,
• 6 have 1 adjacent channel neighbor
• 3 have 2 adjacent-channel neighbors

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Cell Arrangement as a Function of N

• N 11 Slightly better
• Co-channel neighbors farther away
• 2 at D/R of 4.58
• 2 at D/R of 6.06
• 4 at D/R of 7.14
• Out of 11 cells in cluster, eight each have one
  adjacent channel neighbor

• N 12 Excellent but inefficient
• Co-channel neighbors farther away
• 6 at D/R of 6.0
• No adjacent-channel neighbors

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Frequency Reuse Implications of N

• N is number of cells in frequency reuse pattern
  and is critically important since it determines
• Capacity of an individual cell
• Channels per cell
• (total channels) / N
• As N goes up, capacity progressively decreases
• Interference
• As N goes up, interference becomes progressively
  less troublesome

Channels per Cell
Min D/R
N
395
1.732
1
198
1.732
2
132
3.000
3
99
3.464
4
79
3.000
5
66
3.464
6
56
4.583
7
49
4.583
8
44
5.200
9
40
4.583
10
36
4.583
11
12
33
6.000
Assumes use of 395 voice channels
including expanded spectrum
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Signal-to-Interference Ratios
Signal-to-Interference (Also known as
Carrier-to-Interference) Ratio
S/I Signal Power / Interference Power
Signal-to-NoiseInterference (Or just
Signal-to-Noise) Ratio
S/(IN) Signal Power / (Interference Power
Noise Power)
In a cellular system, the main source of
interference is Co-channel Inteference (CCI)
For any regular hex tiling pattern, there are 6
co-channel neighbors.
CCI (total) 6 x (CCI from individual interferer)
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AMPS Cellular Bands

• Cellular telephony provides full-duplex
  communications
• Two-way simultaneous conversation requires
  simultaneous voice paths in both directions
• 25 MHz band of frequencies used for mobile
  transmission (Uplink)
• 25 MHz band of frequencies used for cell site
  transmission (Downlink)
• Cellular bands divided equally between two
  competing operators
• A operator
• B operator

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800 MHz Channel Assignments by Band
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A Band N 7 Channel Sets
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B Band N 7 Channel Sets
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Sector Cell Cluster Frequency Reuse
120 degree sector
B1
B3
B2
G1
C1
G3
C3
G2
C2
A1
A3
A2
F1
D1
F3
D3
F2
E1
D2
E3
E2
Directional sector antennas reduce the required
D/R ratio
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Sectoring Reduces the Interference
Only 2 of the 6 co-channelsinterfere now
Reduces CCI to 1/3 the previous level (reduced by
4.77dB)
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Cell Splitting

• A basic N7 Frequency Reuse Plan with split cells

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3
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6
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7
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