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1
Communications Systems(EN0566) – Lecture
Dr. MICHAEL ELSDON
Room: Ellison Building E404
E-mail: [email protected]
Application of Multiple Access in Mobile Phones
2
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
4
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
6
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
7
“SOME of the bandwidth ALL of the time”
Frequency Divisional Multiple Access (FDMA)
Individual users are assigned different FREQUENCY slots
8
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
9
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)
10
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
11
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
12
MTS Bandwidth Example
freq
1
40MHz
M
460 MHz
MTS only has BW from 40-460MHz
Could only have 32 channels – we need more channels
2 3 32
13
Using Cells to achieve more channels?
14
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 ?
15
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
16
Strong
SignalWeaker Signal
No
Signal
Attenuation is a GOOD thing sometimes ?
17
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
18
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
19
Both antennas have same performance
Different Designs - Same Performance
Old Technology New Technology
20
21
We have a REGION
Each Region is divided into cells
Base Station at each intersection
Mobile Phone inside each cell
Cells
Base Station
Region
22
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
23
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
24
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
7
Cells – Practical Application
26
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
27
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)
29
2G - TDMA
You can do lots of things with a Digital signals
Time Divisional Multiple Access
30
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”
31
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
32
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
35
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
38
To understand Coded Divisional Multiple Access we need to understand logic gates
39
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
40
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
41
The EX OR gates is very important in CDMA
42
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
45
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
47
4G - Orthogonal Frequency Divisional Multiple Access
(OFDM)
484G Phones - OFDM
49OFDMA
OFDM used in 4G (also used in Digital Audio Broadcast)
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