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Z. Ghassemlooy
Mobile Communication Systems
Professor Z Ghassemlooy
Faculty of Engineering and
Environment
University of Northumbria
U.K.
http://soe.ac.uk/ocr
Professor Z Ghassemlooy
Faculty of Engineering and
Environment
University of Northumbria
U.K.
http://soe.ac.uk/ocr
Part 7- Multiplexing
Z. Ghassemlooy
Contents
Multiple Access
Multiplexing
– SDM
– FDM
– TDM
– CDM
Wideband Schemes
Duplex Method
Z. Ghassemlooy
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
Z. Ghassemlooy
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.
Z. Ghassemlooy
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)
Z. Ghassemlooy
Space-division Multiplexing (SDM)
s2
s3
s1f
t
c
k2 k3 k4 k5 k6k1
f
t
c
f
t
c
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
Z. Ghassemlooy
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 k6k1
f
t
code
Frequency Multiplexing I
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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 systemN … number of channels
Bt … total spectrum allocation
Bguard … guard band
Bc … channel bandwidth
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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
Time multiplexing I
f
t
code
k2 k3 k4 k5 k6k1
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Example TDMA System
GSM is a good example of a TDMA system– GSM (850-900 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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TDMA Frame
TDMA used for the 3G air interface
A frame length: 4.615 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 = (1-bOH/bT)*100%f: Frame efficiency
bOH: Number of overhead bits per frame
bT: Total number of bits per frame
Z. Ghassemlooy
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 k6k1
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 k6k1
f
t
coding
Spreading factor = Chip rate/Data rate
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
Z. Ghassemlooy
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
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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
modulatorData
DS-SS Transmitter
Code
generator
Carrier
generator
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DS-SS Transmitter & Receiver
XWideband
modulatorBinary
Data
Code
generator
Carrier
generator
DespreadingData
demodulatorBinary
Data
Code
generator
Carrier
generator
Code
Synchronisation/trac
king
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CDMA Evolution
Early Stages
Narrowband
Wideband
1978 Cooper and Nettleton : cellular application of spread spectrum
1980s Investigation of narrowband CDMA techniques for cellular applications
1986 Formulation of optimum multiuser detection by Verdu
1993 IS-95 standard
1995 - Europe : FRAMES FMA2
Japan : Core-A
USA : cdma2000
Korea : TTA I, TTA II
2000s Commercialization of wideband CDMA systems
WCDMA
1949 John Pierce : time hopping spread spectrum
1949 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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Wideband-CDMA
frame #i frame #i+1
timeslot #0 timeslot #1 timeslot #2 timeslot #13 timeslot #14
Radio Frame (10ms)
Time Slot (2560*Tc)
Tc = chip time = 1 / 3.84 ms
Framing structure
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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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Ultra High Speed Wireless LAN
Wireless LAN
Up to 156 Mbps
Using the millimeter wave radio band
(30-300 GHz)
Used for high quality TV conferences.
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5GHz Band Mobile Access
Two types
– 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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High Data Rate Wireless LAN Evolution
ATMATM
Gigabit Ethernet
(1G bit/s)
Gigabit Ethernet
(1G bit/s)
Fast Ethernet
(100M bit/s)
Fast Ethernet
(100M bit/s)
Ethernet
(10M bit/s)
Ethernet
(10M bit/s)
Ethernet (10M bit/s)Ethernet (10M bit/s)
Conventional 2.4GHz
Ethernet Wireless LAN
Conventional 2.4GHz
Ethernet Wireless LAN
5GHz
Ethernet Wireless LAN
(IEEE802.11)
5GHz
Ethernet Wireless LAN
(IEEE802.11)
Future 5GHz
ATM Wireless LAN
Future 5GHz
ATM Wireless LAN
25M bit/s25M bit/s
IMT 2000
384kbit/s〜2Mbit/s
IMT 2000
384kbit/s〜2Mbit/s
36Mbit/s36Mbit/s
2M bit/s2M bit/s
ARIB, Japan, 1999
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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.
Z. Ghassemlooy
Home Link Concept
SatelliteTuner
CATV DVDVTR
Telephone line
5 GHz
5 GHz
Personal Computer DisplayPersonal Computer
Display
DisplayPersonal Computer
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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
Z. Ghassemlooy
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
Z. Ghassemlooy
Summary
Multiple Access - sharing resources
– 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
Z. Ghassemlooy
Questions and Answers
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