Multiplexing and Multiple Access

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Multiplexing and Multiple Access (MA)
Multiplexing:
 Multiplexing technique combine signals from several sources
 Thus allows one channel to be used by multiple sources to send multiple messages without
interfering each other
 It works on the physical layer (L1) of OSI model
Multiple Access (MA):
Decides on - Who will transmit? Whom to transmit? When to transmit? How to transmit?
 MA techniques are channel access methods based on some principles including multiplexing
 Allocates channels to different users and also handles the situation when there are more
message sources than available channels
 It works on the data link layer (L2) of OSI model
 Multiple users shares the same channel Multiple users under a single base station

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Multiplexing Techniques
Various types:
 Time division multiplexing (TDM)
 Frequency division multiplexing (FDM)
 Wavelength division multiplexing (WDM)
 Code division multiplexing (CDM)
 Space division multiplexing (SDM)
 Orthogonal frequency division multiplexing (OFDM): a variety of FDM
 Polarization division multiplexing (PDM)
 Multiplexing techniques allow sharing a channel by keeping the transmitted
signals separate from various sources so that they do not interfere with one another
 This separation is accomplished by making the signals orthogonal to one another
in the dimensions of frequency, time, code, space, etc.

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FDM
P
t
f
Sub-channel NSub-channel 2Sub-channel 1
Channel
User 1 User 2 User N
 Available bandwidth of the common channel is divided into bands
 Signals are orthogonal (separated) in frequency domain
 Requires guard bands to avoid adjacent-channel interference
 Requires filtering to minimize adjacent channel interference: costly
3D view
2D view

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FDM (2)
Block Diagram of an
FDM System

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TDM (1)
 A digital transmission technology
 Transmission time is divided into time-slots and unique time slot(s) are allocated to
each user
 Different users can transmit or receive messages, one after the next in the same
bandwidth but in different time slots: Orthogonal in time-domain

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TDM (2)
Block Diagram of a TDM System
 Increases the transmission efficiency (i.e., better resource utilization)
 Permits the utilization of all the advantages of digital techniques: digital speech
interpolation, source coding, channel coding, error correction, bit interleaving, etc.
 Suitable for asymmetric (i.e., unequal uplink and downlink data rate) data rate
 Equipment is becoming increasingly cheaper
 Requires a significant amount of signal processing for synchronization as the
transmission of all users must be exactly synchronized
 Requires guard times between time slots to compensate clock instabilities and
transmission time delay

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TDM Frame: Four Signals
1 1 1
2 2 2
3 3 3
4
4
4
Time
TDM Frame TDM Frame TDM Frame

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TDM Frame
Typical TDMA frame formation
Slot 1 Slot 2 Slot 3 … Slot N
Preamble Information Message Trail Bits
One TDMA Frame
Trail Bits Sync. Bits Information Data Guard Bits

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School of Electrical and Information Engineering
Following figure shows synchronous TDM with a data stream for each input
and one data stream for the output. The unit of data is 1 bit. Find (a) the input
bit duration, (b) the output bit duration, (c) the output bit rate, and (d) the
output frame rate.
Example 1
a. The input bit duration is the inverse of the bit rate: 1/1 Mbps = 1 μs
b. The output bit duration is one-fourth of the input bit duration, or 1/4 μs
c. The output bit rate is the inverse of the output bit duration, i.e., 4 Mbps
d. The frame rate is always the same as any input rate. So the frame rate is 1,000,000
frames per second

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Example 2
We have four sources, each creating 250 8-bit characters per
second. If the interleaved unit is a character and 1
synchronizing bit is added to each frame, find –
(a) the data rate of each source
(b) the duration of each character in each source
(c) the frame rate
(d) the duration of each frame
(e) the number of bits in each frame
(f) the data rate of the link
Solution
a. The data rate of each source is 250 × 8 = 2000 bps = 2 kbps

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Example 2
b. Each source sends 250 characters per second. Therefore,
the duration of a character is 1/250 s, or 4 ms.
c. Each frame has one character from each source, which
means the link needs to send 250 frames per second to
keep the transmission rate of each source.
d. The duration of each frame is 1/250 s, or 4 ms. Note that the
duration of each frame is the same as the duration of each
character coming from each source.
e. Each frame carries 4 characters and 1 extra synchronizing
bit. This means that each frame is 4 × 8 + 1 = 33 bits
f. 33 bits are transmitted in 4 ms. Hence the data rate = 33 x
1000 /4 = 8250 bps

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Digital Carrier Systems using TDM
 Two main systems:
1. T-carrier
 Developer: Bell Labs, USA
 Used in North America, Japan and South Korea
 US system based on DS-1 signaling format
 ITU-T use a similar (but different) system
 Formats: T-1, T-2, T-3, T-4
2. E-Carrier
 Developer: European Conference of Postal and
Telecommunications Administrations (CEPT)
 With some revisions, ITU-T has accepted it
 Used throughout Europe and most of the rest of the world
* DS = Digital Signal, ** ITU-T = ITU Telecommunication Standardization Sector

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T-Carrier (1)
T-1 Lines for Multiplexing
Telephone Lines
 24 channels per frame
 1 bit per frame (The first bit of a frame) is framing bit used for
synchronization
 8 kHz sampling rate and 8 bits/sample = 64 kbps per channel
 Uses μ-law with μ = 255

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T-Carrier (2)
T-1 Frame Structure
(Frame duration: 125 µs)
(Bit duration: 0.6477 µs)

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T-Carrier (3)
 Can also interleave DS-1 channels:
 For example, DS-2 is four DS-1 giving 6.312 Mbps

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E-Carrier (1)
 E-Carrier system multiplexes 32 DS-0 channels (time slots
each carrying 8 bits) together to form an E-1 circuit
 Time slot 0 is devoted to transmission management and time
slot 16 for signaling
 The rest slots are assigned for voice/data transport
 Data rate: 32*8*8 kbps = 2.048 Mbps
 Uses A-law
** DS = Digital Signal

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Joint TDM and FDM
For certain applications, such as synchronous optical network (SONET) or synchronous
digital hierarchy (SDH), both TDM and FDM can be employed simultaneously

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WDM
Block Diagram of an
WDM System
 Conceptually same as FDM, except that multiplexing and demultiplexing involves light signals
transmitted through fibre-optic channels
 Combines different frequency signals (same as FDM). However, the frequencies are very high.
 WDM is designed to utilize the high data rate capability of fibre optic cable

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Multiple Access (MA) Techniques
Decides on - who will transmit? whom to transmit? when to transmit? How to transmit?
Random access (contention methods): No station is superior to another station and none is
assigned the control over another. No station permits, or does not permit, another station to
send.
Controlled access: The stations consult one another to find which station has the right to send.
A station cannot send unless it has been authorized by other stations.
Channelization techniques: The available bandwidth of a link is shared in time, frequency, or
through code, between different stations. Usually, it is controlled by a system administrator.

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Multiple Access (MA) Techniques
Various forms of channelization techniques:
 Frequency division multiple access (FDMA): e.g., 1G cellular system
 Time division multiple access (TDMA) : e.g., 2G GSM system
 Wavelength division multiple access (WDMA)
 Code division multiple access (CDMA): e.g., 2G CDMA, 3G UMTS system
 Orthogonal frequency division multiple access (OFDMA): e.g., LTE, WiMAX
 Space division multiple access (SDMA)
These techniques can be used in combination

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Case Study: GSM TDMA Frames
Frame
Multiframe
Superframe
Hyperframe
Bursts

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CDMA
 A spread spectrum (SS) multiple access technique, which allows multiple signals occupying
the same bandwidth to be transmitted simultaneously without interfering with one another
 In a CDMA system, each user is assigned a particular code, named as pseudo-noise (PN)
code, which are ideally supposed to be unique for each user
 This unique code enables the desired message to be extracted at the receiver
 The transmissions from other users looks like interference
What is a spread spectrum (SS) system?
 Spreads a narrowband communication signal over a
wide range of frequencies
 Signal spreading is done before transmission by using
a spreading sequence
 De-spreads it into the original data bandwidth at the
receiver
 Same sequence is used at the receiver to retrieve the
signal
Frequency
Power
Spread Spectrum
(Low Peak Power)
Narrowband
(High Peak Power)

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CDMA: Principle (1)
Two types:
 Direct sequence CDMA (DS-CDMA)
 Frequency hoping CDMA (FH-CDMA)
DS-CDMA System:
Processing gain, G = No. of chips per bit = Tb/Tc
Frequency
Power
b(t)a(t)
Narrowband
b(t)
Spread Spectrum
1 0 1
Data
b(t)
Symbol Duration TS
Time
Chip Duration TC
PN
Sequence
a(t)
b(t)a(t)
Bit duration Tb
Data
d(t)
PN
sequence
c(t)
d(t)c(t)

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CDMA System
Modulator
PN code of
User 1
Spreaded signal for
user 1, bS1
Data of
user 1, b1
PN 1
Transmitted signal
of user 1, TX1
Modulator
PN code of
User 2
PN 2
Transmitted signal
of user 2, TX2
Data of
user 2, b2
Modulator
PN code of
User K
PN N
Transmitted signal
of user K, TXK
Data of
user 2, bK
Demodulator
PN code of
User 1
Output of
Receiver 1, b1'
bS2
bSK
Despreading
Input Signal of
Receiver 1 before
Despreading, bS1'
Receiver
Transmitter
PN 1

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CDMA: Principle (3)
PN code

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CDMA: Principle (5)
Detection by receiver (station) 2:

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CDMA System with Multi-User (1)
Modulator
PN code of
User 1
Spreaded signal for
user 1, bS1
Data of
user 1, b1
PN 1
Transmitted signal
of user 1, TX1
Modulator
PN code of
User 2
PN 2
Transmitted signal
of user 2, TX2
Data of
user 2, b2
Modulator
PN code of
User K
PN N
Transmitted signal
of user K, TXK
Data of
user 2, bK
Demodulator
PN code of
User 1
Output of
Receiver 1, b1'
bS2
bSK
Despreading
Input Signal of
Receiver 1 before
Despreading, bS1'
Receiver
Transmitter
PN 1

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CDMA System with Multi-User (2)
Input signal of
receiver 1 before
despreading, bS1'
f
0 fC
- fC
User K
User 3
User 2
User 1
Output of
Receiver 1, b1
'
f0 fC
- fC
User 1
User 3
User 2
User K
fs- fs
Data of User 1, b1
f
0 fs- fs- 2fs
2fs
Spreaded Signal for
User 1, bS1
f0 fC
- fC
Spreading
Total
Despreading

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CDMA with Narrowband Interference
PN
Code
PN
Code
Channel
Narrowband /
Wideband
Interference
PNt PNr
Input Data,
bt
(t)
Output Data,
br
(t)
TXb
RXb
DespreadingSpreading
f
|Br(f)|
f0 fs- fs
|Bt(f)|
0 fs- fs- 2fs
2fs
|RXb
(f)|
f0 fC
- fC
fc- fc
Data Signal
Narrowband
Interference
DS-CDMA Signal
(spread)
DS-CDMA Signal
(despread)
Whitened
Interference
Spreading
Despreading

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PN Sequence Generation
 M-sequence
 Gold sequence
 Walsh code
 Kasami sequence
Gold sequence generator

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FH Spread Spectrum: Invention
George Antheil (1900-1959)
Composer, pianist, author, and
inventor
Invention (1941): For controlling radio-controlled torpedoes
US patent: “Secret Communication System”, August 1942
First implementation (modified form): For the sake of national defense, government did not allow
publication of its details. First implemented by US Defense during ‘Cuban Missile Crisis’ in 1962.
Award: Pioneer Awards, Electronic Frontier Foundation, 1997
Actress and inventor
(1914-2000)

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CDMA: Advantages
Some of the advantages:
 Hard to intercept: secure communications
 Difficult to jam
 Improved interference rejection and suppression
 No guard-band like FDMA or guard-time like TDMA
 Easy addition of more users
 Can accommodate more users than TDMA and FDMA
 Improved multi-path effect mitigation
 Graceful degradation of performance as the number of simultaneous
users increases
 Less susceptible to effects induced from a changing environment

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CDMA: Drawbacks
 Requires high bandwidth
 Self-jamming problem due to spreading sequences not being exactly
orthogonal
 Power control necessary for mitigating near-far problem
 Inappropriate for ultra high rate wireless access because
 Tremendous width of BW necessary
 Hardware complexity
 Synchronization problem

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Duplexing
Duplexing refers to the technique of separating the transmitting and receiving
channels
FDD TDD
Frequency-division duplexing (FDD):
Transmitter and receiver operate at
different carrier frequencies
Time-division duplexing (FDD):
Transmitter and receiver operate at
same carrier frequencies, but through
different time-slots
Communication Systems: Simplex, Half-duplex, Full-duplex

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MA and Duplexing Schemes in Use
System Multiple Access
Advanced Mobile Phone System (AMPS) FDMA/FDD
2G Global System for Mobile (GSM) TDMA/FDD
US Digital Cellular (USDC) TDMA/FDD
Digital European Cordless Telephone (DECT) FDMA/TDD
US Narrowband Spread Spectrum (IS-95) CDMA/FDD
Satellite Communication TDMA, FDMA, CDMA
3G WCDMA/FDD
LTE OFDMA/FDD or TDD
WiMax OFDMA/FDD or TDD
(Don’t need to memorize all of these)

Multiplexing and Multiple Access

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