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WIRELESS TRANSMISSION SYSTEMS

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Basics of Wireless Networks

Harvey Lehpamer – Transmission Systems Design Handbook for Wireless NetworksChapter 2 – pages 5 to 45

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Basics of Wireless Networks

Historical Background – The Cellular Concept Wireless Local Loop (WLL) Cellular Systems

Analog (FDMA) – AMPS Digital:

TDMA: NA-TDMA (IS-136), GSM CDMA (IS-95)

1G, 2G, 2.5G, 3G,….. Satellite Networks Microwave Systems Bluetooth

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Historical Background 1946 – the first public mobile telephone service was introduced in

25 major American cities Transmitter tower that serves a metropolitan area (> 50 km) Mobile users Several channels available

Half-duplex mode 120 khz of bandwidth per channel (voice)

1960’s Channels

30 Khz of bandwidth per channel (voice) IMTS – Improved Mobile Telephone Service

Full duplex, auto-dial, auto-trunking phone systems Spectrally innefficient

1976 – New York – 12 channels that could serve only 543 paying customers

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Cellular Concept (proposed in 1968, implemented late 70’s) Metropolitan areas were divided into cells Small geographic area: cells Low power transmitters Frequency re-use Each cells operating on a set of frequencies that differed

from the frequencies of adjacent cells.

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Cellular Concept

Figure 3: Mobile Telephone System Using a Cellular Architecture

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Cellular Concept – Frequency reuse

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7 Cell clusterCluster Size N = 7

Frequency reuse factor = 1/7Each cell contains one-seventh of thetotal number of available channels

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Frequency reuse concept

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A cell cluster isreplicated over thecoverage area

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Cellular Network

Figure 8: Cellular System Components

Mobile Switching Center

Up Link

Down Link

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Handoffs

Figure 7: Handoff between Adjacent Cells

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Method of locating co-channel cells in a cellular system

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Number of cells per cluster NN can only have values thatSatisfy the equation:

N = i2 + i j + j2

To find the nearest co-channelneighbors of a particular cell: 1) move i cells along any chainof hexagons 2) turn 60 degrees counter-clockwise and move j cells

N = 7i = 2j = 1

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Example – To understand how frequency re-use improves capacity If a total of 5 MHz of bandwidth is allocated to a

particular FDD wireless system which uses 25 KHz simplex channels to provide full duplex voice and control channels, compute the number of channels available per metropolitan area.

25 KHz

5,000 KHz

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Solution Total bandwidth = 5 MHz Channel bandwidth = 25 KHz x 2 simplex channels = 50

KHz/duplex channel Total available channels = 5,000/50 = 100 channels

25 KHz

5,000 KHz

Reverse Channel Forward Channel

1 2 3 100 1 2 3 100

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Now assume:

Number of channels C =100 Traffic intensity generated by each user:

μ: call request rate: 2 calls/hour H: Holding time: 3 minutes/call Au = μ x H = 2 x (3/60) = 0.1 Erlangs

Grade of Service: 1% Blocked calls cleared (Erlang-B formula) How many users can this system support?

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How many users?

A = 84.1 Erlangs U = A / Au = 84.1 Erlangs / 0.1 Erlangs/user =

= 841 users

Number of channels C Capacity (Erlangs) for GOS=0.01

100 84.1

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Example – Cellular System If a total of 5 MHz of bandwidth is allocated to a

particular FDD cellular wireless system which uses 25 KHz simplex channels to provide full duplex voice and control channels, compute the number of channels available per cell if a system uses 4-cell reuse 7-cell reuse

25 KHz

5,000 KHz

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Solution Total bandwidth = 5 MHz Channel bandwidth = 25 KHz x 2 simplex channels = 50

KHz/duplex channel Total available channels = 5,000/50 = 100 channels

A) for N=4 Total number of channels per cell = 100/4 = 25 channels

B) for N=7 Total number of channels per cell = 100/7 ~14 channels

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For N=7:

Number of channels per cell C =14 Traffic intensity generated by each user:

μ: call request rate: 2 calls/hour H: Holding time: 3 minutes/call Au = μ x H = 2 x (3/60) = 0.1 Erlangs

Grade of Service: 1% Blocked calls cleared (Erlang-B formula) How many users per cell?

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For N=7: (cont.)

From the Erlang B chart:Total carried traffic A = 9 ErlangsNumber of users/cell

U = A/ Au = 9/0.1 = 90 users If there are 100 cells in a metropolitan area,

then the total number of subscribers is 9000.Exercise:

Do the calculations for N=4

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Conclusion

C = 100 channels

Number of subscribers

One antenna – no frequency re-use

Cellular, N=7

Cellular, N=4

841 users

90 users/cell

Homework: page 8, explain the numbers of second paragraph