UNIT 4

Multiplexing and
Multi-user Access

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 Key Topics
• Multiplexing and multiple access
• FDM, TDM, CDM, and SDM
• Fixed allocation access: FDMA, TDMA, CDMA, and SDMA
• OFDMA multicarrier scheme
• SCFDMA, IDMA Schemes and Hybrid Method of Multiple
Access Schemes, RAKE Receiver;
• Random access: ALOHA, Slotted ALOHA, CSMA/CD,
ISMA, and DAMA
• Reservation-based access: PRMA, polling, token passing

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9.1 Multiplexing and Multiple Access
When the multiple user informations are to be transmitted
together, such special techniques are required.
Multiplexing Schemes
Multiplexing schemes are used when mainly four schemes for
signals from multiple users are to be multiplexing based on the four
combined possibilities:
and sent on a single channel as a single (a) Frequency division multiplexing
input stream. Multiplexing is used (FDM)
to enable several users share a medium (b) Time division multiplexing
with minimum or no interference. (TDM)
(c) Code division multiplexing
(CDM)
For wireless communication, (d) Space division multiplexing
(SDM)
multiplexing can be carried out in
four dimensions: frequency, time,
code, and space.

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Frequency Division Multiplexing

In FDM, individual users

are provided individual
channels, which will in
combination make the
whole transmission
bandwidth.

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Time Division Multiplexing of two
Channels

In TDM, each individual user is pre-assigned a time slot in which

he or she can send the information, and once that slot is over,
the slot for the next user will start. The scenario for two users is
given in Figure.
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 Code Division Multiplexing

In CDM, separation is achieved by assigning each user channel

its own code. Guard spaces are realized by using codes with the
necessary distance in code space. Good protection against
unauthorized reception is the main advantage of CDM.

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Space Division Multiplexing

In SDM, signals can be

transmitted by different
directional antennas, or the
signals received by a
multidimensional antenna can
be combined to get all of them
back. Three directional
antennas along with their lobes
are shown in Figure.

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 Multiple Access Schemes
Multiple access schemes allow many simultaneous users to share the same
available channel bandwidth or radio spectrum on an individual basis.

Multiple access may be achieved by four different

ways as follows:
(a) Fixed assignment of resources in terms of carrier
allotment, time slot allocation, code allocation, or
area allocation to specific users--Frequency
division multiple access (FDMA), time division
multiple access (TDMA), code division multiple
access (CDMA), and space division multiple
access (SDMA)
(b) Demand assignment
(c) Random access, that is, a dynamic assignment of
spectrum resources in time or bandwidth to the
users on the basis of demand
(d) Reservation-based access, where prior
reservations intimate other users about the request
of a particular user User access over shared channel

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Classification of Multiple Access
Schemes

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• Depending on how the available bandwidth is allocated to the
users, the access techniques (FDMA, TDMA, CDMA) are grouped
into narrowband and wideband systems.
Narrowband systems
• Relates the bandwidth of a single channel to the expected
coherence bandwidth of the channel.
• The available radio spectrum is divided into a large number of
narrowband channels.
• Narrowband FDMA, a user is assigned a particular channel which
is not shared by other users in the vicinity. With FDD, its called
FDMA/FDD.
• Narrowband TDMA, allows user to share the same channel but
allocates a unique time slot to each user in a cyclical fashion on
the channel. Both TDMA/FDD or TDMA/TDD access systems are
possible.

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Wideband systems
• The transmission bandwidth of a single channel is much larger
than the coherence bandwidth of the channel.
• Multipath fading doesn’t greatly affect the received signal within
a wideband channel & frequency selective fades occur in only a
small fraction of the signal bandwidth.
• The users are allowed to transmit in a large part of the spectrum.
• A large number of transmitters are also allowed to transmit on
the same channel.
• TDMA & CDMA systems may use either FDD or TDD multiplexing
techniques.

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 9.2 Frequency Division Multiple
Access
In FDMA, it is not possible for each user to use the entire bandwidth
and only limited bandwidth is allocated.
FDMA supports transmission of direct analog or digital data such as
amplitude modulation (AM), frequency modulation (FM), and
frequency shift keying (FSK) because FDMA requires no buffering.

A full-duplex or frequency division duplex (FDD) FDMA

has been used since first-generation analog systems.

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Features of FDMA:
• The FDMA channel carries only one phone circuit at time.
• If an FDMA is not in use, then it sits idle and cannot be used by
other users to increase or share capacity.
• The bandwidths of FDMA channels are relatively narrow as each
channel supports only one circuit per carrier (narrowband
systems).
• FDMA requires tight RF filtering to minimize adjacent channel
interference.
• Fewer bits are needed for overhead purposes.
• The symbol time is large as compared to the average delay
spread.

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 Limitations of FDMA
• Carrier synchronization is required in FDMA
• It also requires expensive filters to reduce adjacent channel
interference.
• Intermodulation (IM) is another problem faced.
• Non-linearity in power amplifiers causes signal spreading in
the frequency domain.
• There is undefined RF radiation that leaks into other
channels.
• There is generation of undesirable harmonics that cause
interference to other users in the mobile system or other
systems in adjacent spectrum bands.

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 Capacity of FDMA
• Suppose the number of channels is Nc , total
bandwidth is Wchannel , and guard band is
Wguard , and each channel has a bandwidth
Wsignal , then the FDMA capacity is given by

where Nu is the number of users supported and

Ncch is the number of control channels.

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 9.3 Time Division Multiple Access
In TDMA, the user can use the total bandwidth but for only a limited
slot duration. It is a bursty communication method.

Capacity of TDMA (with FDMA)

TDMA Frame Generation

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• FDMA systems which accommodate analog FM. Digital data and
digital modulation must be used with TDMA.
• In TDMA frame, the preamble contains the address and
synchronization information that both the base station and the
subscribers use to identify each other.
Feature s of TDMA:
• TDMA shares a single carrier frequency with several users, each
user makes use of nonoverlapping time slots.
• Data transmission for users of a TDMA is not continuous , but
occurs in bursts. This results in low battery consumption
• TDMA uses different time slots for transmission and reception. So
duplexers are not required.
• Adaptive equalization is necessary in TDMA systems, since
transmission rates are very high.

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Advantages and Disadvantages of
TDMA

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TDMA-FDMA Combined

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 Spectral Efficiency
For FDMA
Spectral efficiency is
measured by the ratio of
the total time–frequency
domain dedicated for
voice or data transmission
to the total time
frequency domain
available to the system.

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 9.4 Spread Spectrum Multiple
Access
• There are mainly two multiple access schemes that are very
popular with SSM techniques:
1. Direct sequence SSM : It enables Code Division Multiple access
(CDMA) with sufficient fading rejection and security.
2. Frequency hopping SSM : It enables frequency hopped multiple
access (FHMA).
In CDMA, the total available bandwidth can be shared by all
the users at a time.

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 CDMA
• Code division multiple access is used along with SSM,
which uses neither frequency channels nor time slots. In
SSM, spreading is achieved through the pseudo-noise
(PN) code. If a unique code is assigned to each individual
user, demodulation will be possible only if the code
matches at the receiver end. This enables multiple
access.
The following are some of the properties that have made CDMA useful:
• Signal hiding and non-interference with the existing systems
• Anti-jam and interference rejection
• Information security
• Accurate ranging
• Multipath tolerance

CDMA suffers from near-far problem, which is solved by adjusting the

transmitter power using a closed loop system.
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 Capacity of CDMA
• If all N users received power Pr and there are N − 1 interferers
and no channel noise, the signal-to-interference ratio (SIR) is
calculated as

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 Increase the Capacity by

• Cell sectorisation
• Voice activity factor
The multiuser interference then becomes

Where = Number of users per sector,

then

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Frequency Hopped Multiple Access
Frequency hopping spread spectrum (FHSS) is a method of
transmitting radio signals by rapidly switching a carrier among
many frequency channels using a pseudo-random sequence known
to both the transmitter and the receiver. Such spread spectrum
signals are difficult to intercept.

Two types of systems:

slow hopping and fast
hopping .

Hopping patterns for an FHMA scheme

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• The digital data is broken into uniform sized bursts which are
transmitted on different carrier frequencies.
• The instantaneous bandwidth of any one transmission burst is
much smaller than the total spread bandwidth.
• In the FH receiver, a locally generated PN code is used to
synchronize the receivers instantaneous frequency with that
of transmitter.
• A frequency hopped system provides a level of security, when
a large number of channels are used.
• The FH signal is immune to fading, since error control coding
and interleaving can be used to protect the frequency hopped
signal against deep fades which may occasionally occur during
the hopping sequence.

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9.5 Space Division Multiple Access
• Space division multiple access is geographical or cellular. The idea
behind the concept is that if two transmitter–receiver pairs are
far enough to interfere, they can operate on the same frequency
(by reusing the carrier) without interfering with each other.

An SDMA system uses a directional antenna for

splitting the coverage It is also a satellite
communication mode that optimizes the use of
radio spectrum and minimizes system cost by
taking advantage of the directional properties
of dish antennas.

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Frequency Reuse Concept in SDMA
for a Satellite System

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Various Important Aspects Related
to SDMA for Cellular Systems
• Antenna Technologies
– 1. Beam switching
– 2. Adaptive antenna

• Combining Signals from Different Antenna

Branches
– Best diversity scheme is maximum ratio combining

SDMA is very much dependent upon division of zones, frequency

reuse, and antenna technologies.
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9.6 Orthogonal Frequency Division
Multiple Access
It is a hybrid multiple access or
multiplexing technique with
multicarrier modulation, which
divides the available spectrum
into many carriers, each one
being modulated by a low-rate
data stream.
Though OFDMA is similar to FDMA, it uses the
spectrum much more efficiently by spacing the
channels much closer together. This is achieved
by making all the carriers orthogonal to one
another, thereby preventing interference
between the closely spaced carriers. OFDMA is
better in comparison to FDMA and TDMA.

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 9.7 Hybrid Methods of Multiple
Access
• FDMA–CDMA The spectrum is divided into channels and each channel is a
narrowband CDMA system with processing gain lower than the original CDMA system.

• DSSS–FHSS The direct sequence modulates the signal and hops centre frequency
using a pseudo-random hopping pattern. The method avoids near–far effect.

• TDMA–CDMA Different spreading codes are assigned to different cells. One user per
cell is allotted a particular time slot. Only one CDMA user transmits in each cell at any
given time. The method avoids near–far effect.

• TDMA–FHSS It involves a hop to a new frequency at the start of a new TDMA frame.
The method avoids severe fades on the channel. Hopping sequences are predefined
and unique per cell. It avoids co-channel interference if other base stations transmit
on different frequencies at different times.

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9.8 Multiple Access for Packet Radio
Systems
• Data link control layer is subdivided into two layers:
The lower sublayer is the medium access control (MAC) layer.
The upper sublayer is the logical link control (LLC) layer.

• Standard MAC schemes from wired networks often fail in the wireless domain, so
wireless networks require special MACs. In contrast to a wired network, wireless
networks face serious problems such as hidden and exposed terminals and near
and far terminals.

• Three types of methods are discussed here.

1. Pure ALOHA and slotted ALOHA are blind access schemes
2. carrier sense multiple access (CSMA) and CSMA with collision detection
(CSMA/CD) and collision-avoidance protocols are carrier sensing based random
access schemes.
3. collision-free methods, called contention-free protocols.
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 Pure ALOHA
According to ALOHA, terminals can transmit
their data regardless of the activity of other
terminals. A terminal is allowed to transmit
without considering whether the channel is
idle or busy. If a packet is received correctly,
the base station transmits an
acknowledgement. If no acknowledgement
is received by the mobile computer, the
following takes place:
The transmitter assumes the packet to be
lost.
It retransmits the packet after waiting for a
random time.
ALOHA needs some adaptive control
of the retransmission scheme to make it stable.
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 Slotted ALOHA
In slotted ALOHA, the time axis is divided into
slots. All terminals are assumed to know the
times at which a new slot begins; that is, time
synchronization is needed in the network.
Packets may be transmitted only at the beginning
of a new slot. Slotted ALOHA has significantly
better throughput than unslotted ALOHA.
One can study GSM Call Set-up: Example of
Slotted ALOHA

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 Carrier Sense Multiple Access
• In this scheme carrier sensing is used to identify whether the channel is busy
before transmission.

• There are two versions of CSMA.

– CSMA with collision detection CSMA/CD
– CSMA with collision avoidance CSMA/CA

• CSMA/CD enhances the throughput compared to a system that acknowledges

reception only after the transmission of the full message or packet.

• CSMA/CA by default uses the carrier sensing mechanism with exponential

back-off.

• Two main problem in wireless CSMA—Hidden and exposed terminal problems

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 Hidden Terminal Problem
The following observations can be made:
A sends to B ;
C cannot receive A as it is out of range.
C wants to send to B ;
C senses a free medium (carrier sensing fails).
Collision occurs at B ;
A cannot receive the collision (collision detection fails).
A is hidden for C .

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 Exposed Terminal Problem
B sends to A ;
C wants to send to another terminal (not A or B ).
C has to wait; carrier sensing signals a medium in use but A is
outside the radio range of C, and therefore, waiting is not
necessary.
C is exposed to B .

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 Analysing States of CSMA/ISMA

In inhibit sense multiple access (ISMA), the base station transmits

a busy signal on an outbound channel to inhibit all other mobile
terminals from transmitting as soon as an inbound packet is being
received.
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Versions of Carrier and Inhibit Sense
Multiple Access
• Non-persistent 1-persistent
For non-persistent CSMA
and ISMA, rescheduling
(with random back-off
time) always
occurs if the channel is
busy at the instant of
sensing. Hence, if a packet
arrives at a nonpersistent
terminal when the base
station transmits a busy
signal, the attempt is
considered to have failed.

p-persistent
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Throughput Comparison of Random
Access Schemes

The offered or attempted traffic,

denoted as G

Throughput, denoted as S

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 9.9 Reservation-Based Multiple
Access Schemes
Packet Reservation Multiple Access (PRMA)
• A station needs to make a reservation before sending data. Time
is divided into intervals.
• In each interval, a reservation frame precedes the data frames
sent in that interval.
• If there are N stations in the system, there are exactly N
reservation minislots in the reservation frame. Each minislot
belongs to a station.
• When a station needs to send a data frame, it makes a
reservation in its own minislot. The stations that have made
reservations can send their data frames after the reservation
frame.
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Polling
• Polling works with topologies in which one device is
designated as a primary station and the other devices are
secondary stations.
• All data exchanges must be made through the primary device
even when the ultimate destination is a secondary device.
• The primary device controls the link; the secondary devices
follow its instructions.
• It is up to the primary device to determine which device is
allowed to use the channel at a given time.
• If the primary wants to receive data, it asks the secondaries. if
they have anything to send; this is called poll function. If the
primary wants to send data, it tells the secondary to get ready
to receive; this is called select function.

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Token Passing
• The stations in a network are organized in a logical ring. In other words,
for each station, there is a predecessor and a successor.
• The predecessor is the station which is logically before the station in
the ring; the successor is the station which is after the station in the
ring.
• The current station is the one that is accessing the channel now. The
right to this access has been passed from the predecessor to the
current station.
• The right will be passed to the successor when the current station has
no more data to send.
• In this method, a special packet called a token circulates through the
ring. The possession of the token gives the station the right to access
the channel and send its data.
• When a station has some data to send, it waits until it receives the
token from its predecessor.
• Token management is needed for this access method. Stations must be
limited in the time they can have possession of the token.
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 DAMA
• Demand assigned multiple access (DAMA) or random access
schemes dynamically assign radio resources to users as per
availability of the resources. This scheme is suitable in a mobile
scenario and in a satellite system.

• The request for communication from the user will be transferred

to the central authority that can assign the available or free carrier
or time slot to the user on the basis of the current situation.

• The SPADE system for satellite communication and GSM are

examples of systems using such a method of multiple access in
support.

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