Chapter 6

Chapter 6
Multiplexing and
Multiple Access
DEPARTMENT OF COMMUNICATION ENGINEERING

FACULTY OF ELECTRICAL AND ELECTRONICS ENGINEERING
Allocation of Communication Resource
There are 3 ways to increase the throughput (total data rate) of a
communication resource (CR) as follows:
1. Increase the transmitter’s effective isotropic radiated power (EIRP) or

to reduce the system losses so that the received Eb/N0 is increased.
2. Provide more channel bandwidth
3. To make the allocation of CR more efficient
(multiplexing/multiple access).
Multiplexing and Multiple Access
 Similarities:
- multiplexing/multiple access refers to the sharing of a CR
 Differences:
- With multiplexing, users’ requirements for CR sharing are fixed,
or at most slowly changing. The sharing is usually a process that
takes place within the confines of a local site (e.g., a circuit board)
- Multiple access always involves the remote sharing of a resource

such in the case of satellite communications
The basic ways of distributing the CR using multiplexing/multiple access
are:
 Frequency Division (FD) : Use different frequencies
 Time Division (TD) : Use same frequency, different times
 Code Division (CD) : Use same frequency, same time, different codes
 Space Division (SD) : Use same frequency, different space

Frequency Division Multiplexing/Multiple Access
A major development in the early 1900s, FDM telephony, made it possible to

transmit several telephone signals simultaneously on a single wire, and
thereby transformed the methods of telephone transmission.
 Frequency is divided between users. All users can transmit signals

simultaneously and they are separated one another by their
frequency of operation.
 Simplest, best suited for analog links (widely used in radio and TV
broadcast)
 The assignment of a signal or user to a frequency band (channel) is
long term or permanent.
 The available bandwidth can simultaneously contain several spectrally
separate signal bands (channels). The first frequency band contains
signals that operate between frequencies f0 and f1, the second
between f2 and f3, and so on.
 Guard band- buffer zone to reduce interference between adjacent
frequency channels
 Lower channel bit rate than TDMA means less susceptible to

multipath ISI
 Cannot readily support variable user data rates, fixed channel
width means fixed bit rate
 Used in
 Advanced Mobile Phone System (AMPS) in USA
 CT2 in UK
 Digital European Cordless Telephone (DECT) in Europe
Advantages:
 No dynamic coordination
 Simple planning of the radio network
 Works well for analogue signals
 Predictable performance
Disadvantages:
 Inflexible & inefficient if channel load is dynamic & uneven
 Not bandwidth efficient if the traffic is distributed unevenly
 Limited flexibility
 Guard spaces at each band edge limit spectral efficiency
 Fading can be a problem
FDMA Example
 Fig. 11.15 – simple FDM example with three translated voice channels. In
channel 1,2 and 3, the 300-3400 Hz voice signals are mixed with the 20KHz,
16KHz and 12KHz oscillator carrier signals respectively.
 Only the lower sidebands are retained; the result of the mixing and filtering (to
remove the upper sidebands) of double-sideband (DSB) signals yields the
frequency-shifting voice channel.
 The total output waveform is just the sum of 3 signals, having total bandwidth
in the range 8.6 to 19.7 KHz.
Time Division Multiplexing/Multiple Access
 Users share the same frequency by using alternating time

slots (need time synchronization)
 “All of the bandwidth some of the time”
 Each channel gets entire spectrum for a certain time period
 Guard time can be as small as the synchronization of the network
permits
 All users must be synchronized with base station to within a fraction of
guard time
 Guard time of 30-50 microsecond common in TDMA
 Cannot readily support variable user data rates, fixed channel
width means fixed bit rate
 Used in:
 Global System for Mobile (GSM) in Europe
 North American TDMA (NA-TDMA)
 Pacific Digital Cellular (PDC) in Japan
 Advantages:
 Can assign more time to senders with heavier loads
 3 times the capacity of FDMA
 1/3 of power consumption
 many users on one block of frequencies
 Fewer guard bands
 Less precise frequency accuracy needed
 Disadvantages:
 Requires precise synchronization
 Must allow for propagation delay
Fixed-Assignment TDMA
 Simplest TDMA scheme – M time slot for each data
frame are pre-assigned to signal sources, long term.
 The multiplexing operation consists of providing each
source with an opportunity to occupy one or more
slots.
 The demultiplexing operation consists of deslotting the
information and delivering the data to the intended
sink.
 The two commutating switches must be synchronized
for signal-to-channel assignment.
 A message contains preamble and data portions. The
preamble contains information on synchronization,
addressing and error control sequence.
 Unsuitable for bursty or sporadic traffic (time slot are
not always filled). Wasted time slot when no data to
send at a particular frame. Use dynamic assignment of
time slot instead.
Space Division Multiple Access (SDMA)
Many cells can share

the same frequencies
if they are separated
in space
Pattern can be replicated

over the entire earth
200 frequencies in one cell

SDMA (continued…)
 Widely used in the Global System for Mobile (GSM)
 Transmitter send power in all horizontal directions (omni-
directional antenna)
 Cell has a “usable” range
 Once the base station transmission power has fallen below a
certain level you can re-use the same cell design
 that distance is significantly greater than the cell size
 controlled by signal to interference requirements
Code Division Multiplexing/Multiple Access
 Hybrid combination of FDMA and TDMA.
 “All of the bandwidth all of the time”
 An application of spread spectrum (SS) techniques.
 Can be divided into Frequency Hopping CDMA (FH-CDMA) and
Direct Sequence CDMA (DS-CDMA)
 Data is broken into uniquely identified packets & transmitted

over multiple frequencies
 Uses unique codes to identify connections & reassemble
messages
 Every user uses the entire spectrum all of the time
 Allows most efficient usage of frequency spectrum
 All channels use same spectrum at same time but with
orthogonal codes
Advantages:
 Bandwidth efficient (code space is huge)
 No coordination or synchronization
between different channels
 Resists interference
 3 times the capacity of TDMA
 1/25 power consumption
Disadvantages:
 More complex signal regeneration
 Need power control to avoid capture
Generation of CDMA Signal
Antenna
Input Speech
speech coder
Spreading Carrier
code frequency
Transmitter – the spreading or coding process

Reception of CDMA Signal
Antenna
Speech Output
decoder speech
Carrier Spreading
frequency code
Receiver – the despreading process

Spreading Codes in CDMA
 Each user signal is spread by a particular
spreading code
 Spreading codes should be orthogonal so that
they do not generate interference
 Two codes are said to be orthogonal to each other
if, when multiplied together over the length of
the spreading codes, the total summed is zero.
CDMA Transmission With One User
User data from

speech coder
1 0
Spreading
code
1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1
Transmit Orthogonal
waveform, data =
10 1 -1 1 -1 1 -1 1 -1 -1 1 -1 1 -1 1 -1 1
Receiver
code
1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1
Multiplication
1 1 1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1 Σ=0
Comparison of FDMA, TDMA and CDMA
Advantages Disadvantages System

FDMA No dynamic Inefficient use of Analog cellular
coordination. spectrum. systems - 1G.
Simplicity.
TDMA More efficient Requires precise Digital cellular
than FDMA. synchronization. system - 2G.
Less expensive.
CDMA The highest The highest WCDMA
spectrum complexity. (UMTS) - 3G.
efficiency.
Multiple Access Algorithms
 A multiple access protocol or multiple access algorithm (MAA) is that
rule by which a user knows how to use time, frequency, and code
functions to communicate through a satellite to other users.
 The general goal of a multiple access system is to provide

communication service in a timely, orderly, and efficient way.
 Fixed assignment MAA give the station periodic access to the channel
independent of its actual need.
 Dynamic assignment MAA give the station access too the channel only
when it requests access.
 Examples : ALOHA, Slotted ALOHA

ALOHA
 ALOHA is developed in the 1970s at the University of Hawaii.
 The system concept is simple:
Transmission mode: Users transmit at any time they desire, encoding
their transmissions with an error detection code.
Listening mode: After a message transmission, a user listens for an
acknowledgement (ACK) from the receiver. Collision might occur due
to transmission overlapping in time. In such cases, the errors are
detected, and the users receive a negative acknowledgement (NAK).
Retransmission mode: When a NAK is received the messages are
retransmitted.The users retransmit after a random delay.
Timeout mode: If, after a transmission, the user does not receive either
a ACK or NAK within a specified time, the user retransmits the
message.
A Sketch of Frame Generation
 In Pure Aloha, frames are transmitted at completely arbitrary times.

 Note that all packets have the same length because the throughput
of ALOHA systems is maximized by having a uniform packet size
ALOHA
 ALOHA is developed in the 1970s at the University of Hawaii.
 The system concept is simple:
Transmission mode: Users transmit at any time they desire, encoding
their transmissions with an error detection code.
Listening mode: After a message transmission, a user listens for an
acknowledgement (ACK) from the receiver. Collision might occur due
to transmission overlapping in time. In such cases, the errors are
detected, and the users receive a negative acknowledgement (NAK).
Retransmission mode: When a NAK is received the messages are
retransmitted.The users retransmit after a random delay.
Timeout mode: If, after a transmission, the user does not receive either
a ACK or NAK within a specified time, the user retransmits the
message.
Slotted-ALOHA(S-ALOHA)
 S-ALOHA improves pure ALOHA by requiring a small amount of
coordination among the stations.
 A sequence of synchronization pulses is broadcast to all stations.
 As with pure ALOHA, packet lengths are constant.
 Messages are required to be sent in the slot time between
synchronization pulses, and can be started only at the beginning of a
time slot.
 This simple change reduces the rate the rate of collisions by half, since
only messages transmitted in the same slot can interfere with one
another
Slotted ALOHA
 Transmission of frames are synchronized slot by slot.

 Channel feedback about whether packet is received or not
Slotted ALOHA (continued)
 Slotted ALOHA cuts the vulnerable period for packets
from 2t to t.
 Time is slotted. Packets must be transmitted within a
slot.
 Procedure
1. If a host has a packet to transmit, it waits until the
beginning of the next slot before sending
2. Listen to the broadcast and check if the packet was
destroyed
3. If there was a collision, wait a random number of slots
and try to send again
Performance of ALOHA
 Throughput versus offered traffic for ALOHA systems

 The main reason for poor channel utilization of ALOHA (pure
or slotted) is that all stations can transmit at will, without
paying attention to what the other stations are doing

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DEPARTMENT OF COMMUNICATION ENGINEERING

1. Increase the transmitter’s effective isotropic radiated power (EIRP) or

- Multiple access always involves the remote sharing of a resource

 Frequency Division (FD) : Use different frequencies

 Time Division (TD) : Use same frequency, different times

 Space Division (SD) : Use same frequency, different space

A major development in the early 1900s, FDM telephony, made it possible to

 Frequency is divided between users. All users can transmit signals

 Lower channel bit rate than TDMA means less susceptible to

 Users share the same frequency by using alternating time

Many cells can share

Pattern can be replicated

200 frequencies in one cell

 Data is broken into uniquely identified packets & transmitted

Transmitter – the spreading or coding process

Receiver – the despreading process

User data from

Advantages Disadvantages System

 The general goal of a multiple access system is to provide

 Examples : ALOHA, Slotted ALOHA

 In Pure Aloha, frames are transmitted at completely arbitrary times.

 Transmission of frames are synchronized slot by slot.

 Throughput versus offered traffic for ALOHA systems

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