1. Mobile Communication Systems
Part 7- Multiplexing
Professor Z Ghassemlooy
Faculty of Engineering and
Environment
University of Northumbria
U.K.
Z. Ghassemlooy

2. Contents Multiple Access Multiplexing Wideband Schemes Duplex Method
SDM
FDM
TDM
CDM
Wideband Schemes
Duplex Method
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3. Multiple Access
In today’s data communications systems there is a need for several users to share a common channel resource at the same time.
The resource could be:
high speed optical fibre links between continents
frequency spectrum in a cellular telephone system
twisted pair ‘ethernet’ cable in the office
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4. Multiple Access
For multiple users to be able to share a common resource in a managed and effective way, it requires:
Some form of access protocol
Defines how or when the sharing is to take place and the means for identifying individual messages. Process is known as multiplexing in wired networks and multiple access in wireless digital communications.
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5. Multiplexing/Multiple Access
There four possible ways to divide the frequency spectrum among many channels:
Space-division multiplexing (SDM)
Frequency-division multiplexing (FDM) / Frequency Division Multiple Access (FDMA)
Time-division multiplexing (TDM) / Time Division Multiple Access (TDMA)
Code-division multiplexing (CDM) / Code Division Multiple Access (CDMA)
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6. Space-division Multiplexing (SDM)f t c k2 k3 k4 k5 k6 k1
channels ki
The spatial dimension is used for
multiplexing
Data stream are transmitted over,
non-overlapping transmission
channels
Uses spot beam antennas
Base station tracks user when moving
Cover areas with same frequency as TDMA, CDMA, and FDMA
Can be achieved using:
Beam forming
Sectorization
But, needs perfect adaptive antenna system: infinitely large antenna needed
Compromise needed
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7. Frequency Multiplexing I
Dividing the entire frequency spectrum into smaller bands
A frequency band (carrier) is allocated per channel for the entire transmission time
FDM, used in 1st generation systems (wastes spectrum), GSM and UMTS FDD Mode
Two bands of frequencies for every user
Advantages:
lower channel bit rate (than TDM)
means less susceptible to multi
path ISI
requires coordination
works also for analog signals
in contrast to TDM, each
stream can continuously
transmit within its sub-
band k2 k3 k4 k5 k6 k1 f t
code
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8. Frequency Multiplexing II
OFDM
Is an efficient FDMs, which offers minimum spacing of the sub-bands without ISI
Frequency Division Multiple Access
In multiple access schemes, where different data streams belong to different users
Disadvantages
In-efficient use of bandwidth if the traffic is distributed unevenly
Requires guard band between channels
Cannot readily support variable user data rates, fixed channel width means fixed bit rate
Number of channels in a FDMA system
N … number of channels
Bt … total spectrum allocation
Bguard … guard band
Bc … channel bandwidth
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9. Time multiplexing I Disadvantages: code f t
Entire spectrum is allocated for a channel some of the time. multiple users share a single radio channel
For 2nd generation
Uses time for forward and reverse link
Advantages:
Only one carrier in the medium at any given time
High throughput even for many users
Common TX component design,
only one power amplifier
Disadvantages:
precise synchronization necessary
requires terminal to support a
much higher data rate than the user information rate f t
code k2 k3 k4 k5 k6 k1
Time slots
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10. Example TDMA System GSM is a good example of a TDMA system
GSM ( MHz) handsets transmit data at a rate of 270 kbit/s in a 200 kHz channel using GMSK modulation.
Each frequency channel is assigned 8 users, each having a basic data rate of around 13 kbit/s
Signal is divided by time using the fundamental unit of a burst period. This burst period = 15/26 ms and is grouped together by 8 bursts into a frame.
A single traffic channel is defined by grouping 26 frames together; giving a total timeframe of 120 ms. These traffic channels are used to transfer speech and data.
Number of channels
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11. Switching point between uplink and downlink
TDMA Frame
TDMA used for the 3G air interface
A frame length: ms and it consist of
64 1/64 time slots of length 72 usec
16 1/16 time slots of length 288 usec
Downlink
Uplink
72ms
288ms
Switching point between uplink and downlink
Efficiency: It is a measure of the percentage of transmitted data that contains information as opposed to providing overhead for the access scheme
f: Frame efficiency
bOH: Number of overhead bits per frame
bT: Total number of bits per frame
f = (1-bOH/bT)*100%
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12. Time and Frequency Multiplexing I
Combination of both methods
A certain frequency band for a given amount of time is allocated per channel
Example: GSM
Advantages:
Improved protection against tapping
and frequency selective interference
Higher data rates compared to code
multiplex
Disadvantages:
Requires precise
coordination f t
code k2 k3 k4 k5 k6 k1
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13. Code Division Multiplexing
Each channel has a unique spreading code.
All channels use the same
spectrum at the same time.
Spreading codes should as far as possible
be orthogonal to each other to reduce interference
The codes, one/zero sequences, used to
differentiate signals, are designed and
generated at a much higher rate than the
baseband information.  
This rate is referred to
as a chip rate
rather than a bit rate. k2 k3 k4 k5 k6 k1 f t
coding
Spreading factor = Chip rate/Data rate

14. Code Division Multiplexing
Advantages:
bandwidth efficient and good power control
no need for coordination and synchronization
good protection against interference and
tapping
Disadvantages:
lower user data rates
more complex signal regeneration
Implemented using spread spectrum technology

15. CDMA Classification CDMA : direct sequence (DS)
CDMA : frequency hopping (FH)
Carrier frequency changes periodically, after T secs
Hopping pattern determined by spread code
CDMA : time hopping (TH)
Data transmitted in rapid bursts
Time intervals determined by code
Direct sequence
Frequency hopping
Time hopping
Time
Frequency
First I will introduce you to 3G communication systems then I will mention my research objectives.
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16. Direct Sequence CDMA
Directly modulated, discrete time, discrete valued code signal
Analogue or Digital
Code bits are ‘chips’ (1)
Rate of Code >> Rate of Data
PSK, BPSK, D-BPSK, QPSK or MPSK
Spreading
modulation
Data
modulator
DS-SS Transmitter
Code
generator
Carrier
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17. DS-SS Transmitter & ReceiverX
Wideband
modulator
Binary
Data
Code
generator
Carrier
Despreading
Data
demodulator
Binary
Code
generator
Carrier
Synchronisation/tracking
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18. CDMA Evolution Early Stages Narrowband Wideband 1949
John Pierce : time hopping spread spectrum
Claude Shannon and Robert Pierce : basic ideas of CDMA
1950
De Rosa-
Rogoff : direct sequence spread spectrum
1956
Price and Green :
antimultipath “RAKE” patent
1961
Magnuski : near-far problem
1970s
Several developments for military field and navigation systems
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19. Wideband-CDMA Tc = chip time = 1 / 3.84 ms Framing structure
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20. High Speed Wireless Access
Mobile communication system
Up to 30 Mbps
Using the SHF and other band (3-60 GHz)
Used for mobile video telephone conversations
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21. Ultra High Speed Wireless LAN
Up to 156 Mbps
Using the millimeter wave radio band
( GHz)
Used for high quality TV conferences.
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22. 5GHz Band Mobile Access Two types Using 5GHz band
ATM type Wireless Access
Ethernet type Wireless LAN
Using 5GHz band
Each system can transmit at up to 20-25Mbps
Used for multimedia information
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23. High Data Rate Wireless LAN Evolution
ATM
Gigabit Ethernet
（1G bit/s)
Fast Ethernet
（100M bit/s)
Ethernet
（10M bit/s)
Ethernet (10M bit/s)
Conventional 2.4GHz
Ethernet Wireless LAN
5GHz
(IEEE802.11)
Future 5GHz
ATM Wireless LAN
25M bit/s
IMT 2000
384kbit/s〜2Mbit/s
36Mbit/s
2M bit/s
ARIB, Japan, 1999
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24. Wireless Home-Link Wireless Home-Link Up to 100Mbps
Using the SHF and other band(3-60GHz)
Between PCs and Audio Visual equipments
Multimedia information.
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25. Home Link Concept Satellite Tuner CATV DVD VTR Telephone line 5 GHz
Personal Computer
Display
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26. Duplex Methods
Separating the send and receive signals (remember full duplex). Two approaches:
Frequency Division Duplex (FDD)
Uses a pair of frequency bands – one for uplink and another for downlink
used in all second generation cellular systems
requires good frequency separation filters - diplexer
Time Division Duplex (TDD)
Uses a single frequency band for both uplink and downlink – sharing the transmission time
propagation delay limits cell size
very efficient for asymmetric traffic, e.g. internet download
used in cordless systems (DECT) and wireless LANs
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27. What is Universal Mobile Telecommunication System ?
European name for third generation (3G) radio system
(1G = analog, 2G = digital voice and low speed data (GSM))
Key features with respect to 2G:
Integration of fixed and mobile networks
Expanded range of services (Packet, Internet, Multimedia)
Bit rates:
Rural outdoor: 144 kb/s, 500 km/h
Suburban outdoor: 384 kb/s, 120 km/h
Indoor, low range outdoor: 2Mb/s, 10 km/h
Flexibility:
Variable bit rates
Circuit switched and packet oriented bearers
Negotiation of bearer service attributes (bearer type, bit rate, delay BER, up/down symmetry, protection)
Adaptability to quality, traffic, network load & radio conditions
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28. Summary Multiple Access - sharing resources Duplex Methods
Frequency Division Multiple Access - FDMA
Time Division Multiple Access - TDMA
[Code Division Multiple Access – CDMA]
Duplex Methods
Frequency Division Duplex - FDD
Time Division Duplex - TDD
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29. Questions and Answers Tell me what you think about this lecture
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