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Communications Systems(EN0566) – Lecture

Dr. MICHAEL ELSDON

Room: Ellison Building E404

E-mail: michael.elsdon@northumbria.ac.uk

Application of Multiple Access in Mobile Phones

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Don’t take any notes

33 possible Solutions - How can users share a link

0G / 1G - Frequency divisional Multiple Access

2G - Time Divisional Multiple Access

3G -Coded Divisional Multiple Access

4G – Orthogonal Frequency Divisional

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Frequency divisional Multiple Access

5Frequency Divisional Multiple Access

Divide the link into different frequencies – each user has a different frequency

Transmission Channel

1 link

User A

User B

User C

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Spectrum of FDMA Signal

freqBW1

BW2 BWn

User A User B User C

Tx BW

Assume we have a transmission medium (e.g. microstrip, coaxial) with a WIDE bandwidth.We can transmit a number of signals over this medium.

M

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“SOME of the bandwidth ALL of the time”

Frequency Divisional Multiple Access (FDMA)

Individual users are assigned different FREQUENCY slots

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freqBW1 BW2 BWn

m1 m2mn

Tx BW

M

Guardbands are inserted to prevent interference between adjacent signals.

These are un-used portions of the frequency spectrum

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FDMA in mobile phones - OG

First used in 1946 – Mobile Telephone System (MTS)

Operator makes connection

1964 – Improved Mobile Telephone System (IMTS)

(Automatic - No operator)

Typical Mobile (Car) Phone (1950s)

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FDMA in mobile phones

First ‘real’ Mobile Phone (1973)

1 kg

£2000 (¥20000)

30 mins

Too many problems with mobile phones in early days:

Price, Capacity, Technology

Mobile phones developed rapidly in mid 1990s

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Compare 1st mobile phone with iPhone

iPhone (2012)

First ‘real’ Mobile Phone (1973)

1 kg

£2000 (¥20000)

30 mins

0.11 kg

£100 (contract) (¥1000)

hrs

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MTS Bandwidth Example

freq

1

40MHz

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460 MHz

MTS only has BW from 40-460MHz

Could only have 32 channels – we need more channels

2 3 32

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Using Cells to achieve more channels?

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Only 32 channels ?

How can we fit more channels into this bandwidth ?

Make use of ‘Attenuation’

Signal strength reduces with distance

What happens when we have more mobile phones ?

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Received Power versus distance Pr = Pt Gt Gr λ/(4πd)2

dGt

Pt

Pr α 1 / d2

Pr

Pr PrPr

PrPr

Pr

Received Power reduces with distance - ATTENUATION

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Strong

SignalWeaker Signal

No

Signal

Attenuation is a GOOD thing sometimes ?

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Powerful transmitter located at the highest spot in an area would broadcast over a wide radius (up to 50 km)

Early Mobile Phone Systems

Good coverage – but had limitations

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Divide the region into smaller areas

Use LESS POWERFUL transmitters at different locations

Directional antenna – focus beam in 3 directions – TRI SECTOR

New Approach – use Cells

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Both antennas have same performance

Different Designs - Same Performance

Old Technology New Technology

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We have a REGION

Each Region is divided into cells

Base Station at each intersection

Mobile Phone inside each cell

Cells

Base Station

Region

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Each Cell has a BAND of frequencies

Each BAND signal within the Cell is sub-divided into different channels

Each ADJACENT CELL must use a different BAND of frequencies

Cells

Base Station

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Base Station at each intersection

Each colour represents a different frequency

Signal from one cell is weak by the time it reaches distant cell - ATTENUATION

You can re-use the same frequency as long as cells with same frequency are not adjacent to each other

Cells – 1G

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Base Station at each intersection

Each colour represents a different frequency

Signal from one cell is weak by the time it reaches distant cell - ATTENUATION

You can re-use the same frequency as long as cells with same frequency are not adjacent to each other

No adjacent cells have the same colour

Cells – 1G

1

2

3

4

5

6

7 1

2

3

4

5

6

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Cells – Practical Application

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1G – 2G The first 1G system - 1978 (AMPS) – Trials. Can only handle

small number of users

1990s – too many users – need to move to 2G system. – System bursting at the seam.

Moving from analog to digital system - allows big increase in number of users.

IF you code in digital form you can use TDMA

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Time Divisional Multiple Access

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Mobile phone signal were transmitted in the original ANALOG form

Only used for calls

Analog signal converted to digital form

Signals transmitted in DIGITAL form

Text

1 1 10 0 0

Early 1990s – ANALOG (1G)

MID 1990s – DIGITAL (2G)

(2002)

(1990)

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2G - TDMA

You can do lots of things with a Digital signals

Time Divisional Multiple Access

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Time Divisional Multiple Access (TDMA)

Each person uses the same frequency and take turns to

transmit the data in a ‘round-robin’ fashion.

Multiple signals can be carried on a single transmission medium by interleaving portions of each signal in time.

Only possible if:

data rate of tx medium > data rate of signals to be transmitted

“ALL of the bandwidth SOME of the time”

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GuardTimes

time

Time Slot 1

Guard-TimesM

To prevent this GUARD-TIMES are inserted between signals

Time Slot 2

Time Slot n

GUARD-TIMES are a safety measure to prevent interference

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TDMA combined with FDMA

Used in 2G phones

Time

Frequency

33TDMA / FDMA

A

B

C

3 users all on different frequency bands

How can we fit more users in this system ?

34TDMA / FDMA

Time

Frequency

A1

B1

C1

We can separate them in TIME

Time Division on top of Frequency Division

(T / FDMA)

A3

B3

C3

A2

B2

C2

A1

B1

C1

A2

B2

C2

A3

B3

C3

(Only possible with DIGITAL signals)

GSM

(1982 - Europe)

3x capacity

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3G - Coded Divisional Multiple Access

36Coded Divisional Multiple Access

(CDMA)

First used in USA for 2G as an alternative to T / FDMA

3G solely based on CDMA

37CDMA

3 users transmit all of the time, across full frequency range.

Each user has a different CODE

Time

Frequency

Code

C

B

A

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To understand Coded Divisional Multiple Access we need to understand logic gates

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3 sets of users. Each user is assigned a CODE.CDMA example

A1

B2

C2

B1

1. Each signal (A1, B1, C1) is ENCODED (encrypted / scrambled) by modulating it a UNIQUE CODE.

2. The ENCODED signal is transmitted across the network

3. To DECODE the signal the receiver must know the UNIQUE CODE.

C1 A2

Transmitters Receivers

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Modulator

Spreading Code

Transmitter

Input Data

Input Data

Receiver

Spreading Code

Encoded Signal

Important: To recover the original signal accurately, the

carrier in the demodulator must have the same UNIQUE CODE

CDMA example

Encoded Signal

DeModulator

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The EX OR gates is very important in CDMA

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Modulator

Spreading Code

Transmitter

Input Data

Input Data

Receiver

Spreading Code

Encoded Signal

CDMA uses Logic Gates to Encode / Decode

DeModulator

Encoded SignalX-OR

X-OR

43CDMA – Encoding 1 signal

XOR

Code

i/p o/p

Input Signal

Spreading Code

Encoded Signal

0

1 1

0

0

1 1

0

1

0 0

1

0

1 1

0

0

1 t

t

t

0

1 1

0

0

1 1

0

1

0 0

1

0

1 1

0

0

1

t

t

t

44CDMA – Decoding 1 signal

XOR

Code

i/p o/p

Original Input Signal

Spreading Code

Encoded Signal

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Encoder Decoder

0

1 1

0

0

1 1

0

1

0 0

1

0

1 1

0

0

1 t

t

t

0

1 1

0

0

1 1

0

1

0 0

1

0

1 1

0

0

1

t

t

t

46CDMA example

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4G - Orthogonal Frequency Divisional Multiple Access

(OFDM)

484G Phones - OFDM

49OFDMA

OFDM used in 4G (also used in Digital Audio Broadcast)

Information is split up into different bits of information