Multiple Access Schemes Explained

Multiple Access Schemes
Example of different multiple access schemes: a) TDMA; b) FDMA; c) OFDMA;
d) CDMA/SDMA; e) possible NOMA
4/3/2023

Table of Contents
FDMA (Frequency Divison Multiplex Aceessing)
TDMA (Time Divison Multiplex Aceessing)
CDMA ( Divison Multiplex Aceessing)
SDMA (Space Divison Multiplex Aceessing)
OFDMA (Optical Fibre Divison Multiplex Aceessing)
NOMA (Non-Orthogonal Multiple Access )
ALOHA (Advacates of Linux Open Source Hawaii
Association)
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Dr KRB Prof ECE SCET India WCN

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 Multiple access schemes are used
To allow many mobile users to share simultaneously a
finite amount of radio spectrum. or
 The sharing of spectrum is required to achieve high
capacity by simultaneously allocating the available
bandwidth (or the available amount of channels)
to multiple users.
• For high quality communications, this must be
done without severe degradation in the performance of
the system.
Multiple Access Schemes

Access methods /schemes are multiplexing techniques
that provide Communications services to multiple users
in a single-bandwidth wired or wireless medium.
 Communications channels, whether they’re wireless
spectrum segments or cable connections, are expensive.
Communications services providers must engage
multiple paid users over limited resources to make a
profit.
 Access methods allow many users to share these limited
channels to provide the economy of scale necessary for
a successful communications business.
 There are five basic access or multiplexing methods
which are (FDMA), (TDMA), (CDMA), (OFDMA),
(SDMA).
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The Multiplexing methods for FDMA, TDMA, CDMA,
OFDMA, and SDMA are all generally similar.
 In each case, a plurality of communication channels is
multiplexed together to form a single communication
channel.
 The communication channels are then divided into a
plurality of sub-channels, each of which is assigned to a
different user.
Each user is then allocated a time slot in which to transmit
data.
 In FDMA, the time slots are assigned to the users in a
sequential fashion. In TDMA, the time slots are assigned
to the users in a random fashion.
 In CDMA, the time slots are assigned to the users based
on their code sequences.
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 In OFDMA, the time slots are assigned to the users
based on their frequency offsets.
 In SDMA, the time slots are assigned to the users
based on their location.
 Users can then transmit data in their assigned time
slots.
The data is then demultiplexed at the receive end and sent
to the appropriate user.
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what is the defference between simplex,duplex,
Multiple Access ...?
Tx ----> Rx Simplex
Tx----->Rx Half Duplex
Rx------>Tx Half Duplex
Tx -------->
<------- Rx Duplex

Full Duplex can be Implemented in 3 diffenrent ways
FDD(Freq Division Mux)
TDD(Time Division Mux)
TDD+FDD Mixer
FDD:
Tx ----->
<---- Rx
Tx: used for +ve Amplitudes
Rx: used for -ve Amplitudes
TDD
Tx,Rx signal in same time.

FDMA(Frequency Divison Multiplex Aceessing)
FDMA is the process of dividing one channel or
bandwidth into multiple individual bands, each for use by
a single user (Fig.)
Each individual band or channel is wide enough to
accommodate the signal spectra of the transmissions to
be propagated. The data to be transmitted is modulated
on to each subcarrier, and all of them are linearly mixed
together.
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Example :
Cable television system.
The medium is a single coax cable that is used to
broadcast hundreds of channels of video/audio
programming to homes.
The coax cable has a useful bandwidth from about
4 MHz to 1 GHz.
This bandwidth is divided up into 6-MHz wide
channels.
Initially, one TV station or channel used a single 6-MHz
band.
But with digital techniques, multiple TV channels may
share a single band today thanks to compression and
multiplexing techniques used in each channel
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This technique is also used in fiber optic
Communications systems.
A single fiber optic cable has enormous bandwidth that
can be subdivided to provide FDMA.
Different data or information sources are each assigned a
different light frequency for transmission.
Light generally isn’t referred to by frequency but by its
wavelength (λ).
As a result, fiber optic FDMA is called wavelength
division multiple access (WDMA) or
just wavelength division multiplexing (WDM).
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One of the older FDMA systems is the original analog
telephone system, which used a hierarchy of frequency
multiplex techniques to put multiple telephone calls on
single line.
The analog 300-Hz to 3400-Hz voice signals were used
to modulate subcarriers in 12 channels from 60 kHz to
108 kHz. Modulator/mixers created single sideband
(SSB) signals, both upper and lower sidebands.
These subcarriers were then further frequency
multiplexed on subcarriers in the 312-kHz to 552-kHz
range using the same modulation methods.
At the receiving end of the system, the signals were
sorted out and recovered with filters and demodulators.
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Each one of these takes advantage of multiplexing
methods....
dividing the bandwidth of the signal into different sub-
bands, which are then assigned to different users in order
to allow multiple users to share a single channel.
Multiplexing is a communications technique that
multiplexes, multiple signals into a single signal.
The reverse process is called demultiplexing.
example:
the voice signals of many telephone callers can be
multiplexed over a single data link by using time-division
multiplexing (TDM).
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Original aerospace telemetry systems used an FDMA
system to accommodate multiple sensor data on a single
radio channel. Early satellite systems shared individual
36-MHz bandwidth transponders in the 4-GHz to 6-GHz
range with multiple voice, video, or data signals via FDMA.
Today, all of these applications use TDMA digital
techniques.
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Advantages of FDMA:
• If channel is not in use, it sits idle.
• channel bandwidth is relatively narrow (30kHz).
• Simple algorithmically and lower complexity.
• Fairly efficient when the traffic is uniformly constant.
Capacity increase can be obtained by reducing the
information bit rate and using efficient digital code
• No need for network timing – fewer bits required for
synchronization and framing
• No restriction regarding the type of baseband or type of
modulation.

Disadvantages to using FDM
The presence of guard bands
• It increases cost as duplexer is compulsory
• Maximum bit rate per channel is fixed
• Many channels use same antenna, causes
nonlinear effect &
signal will spread in frequency domain (Inter
modulation frequencyIM ).
• Requires right RF filtering to minimize adjacent
Channel Interface.

TDMA(Time Divison Multiplex Aceessing)
• TDMA is a digital technique that divides a single channel
or band into time slots.
• Each time slot is used to transmit one byte or another
digital segment of each signal in sequential serial data
format.
• This technique works well with slow voice data signals,
but it’s also useful for compressed video and other high-
speed data.
example : the widely used T1 transmission system, which
has been used for years in the telecom industry. T1 lines
carry up to 24 individual voice telephone calls on a single
line (Fig. 2). Each voice signal usually covers 300 Hz to 3000
Hz and is digitized at an 8-kHz rate, which is just a bit more
than the minimal Nyquist rate of two times the highest-
frequency component needed to retain all the analog
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fig 2. This T1 digital telephony frame illustrates TDM and TDMA. Each time slot is
allocated to one user. The high data rate makes the user unaware of the lack of
simultaneity.
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The digitized voice appears as individual serial bytes that
occur at a 64-kHz rate, and 24 of these bytes are
interleaved, producing one T1 frame of data.
The frame occurs at a 1.536-MHz rate (24 by 64 kHz) for a
total of 192 bits.
A single synchronizing bit is added for timing purposes
for an overall data rate of 1.544 Mbits/s.
At the receiving end, the individual voice bytes are
recovered at the 64-kHz rate and passed through a
digital-to-analog converter (DAC) that reproduces the
analog voice.
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Advantages of TDMA:
• Flexible bit rate
• No frequency guard band required
• No need for precise narrowband filters
• Easy for mobile or base stations to initiate and
execute hands off
• Extended battery life &• BW can be supplied on demand
• Dis Advantages
Requires network-wide timing synchronization
• Requires signal processing for matched
filtering and correlation detection
• Multipath distortion

CDMA(Code Division Multiple Access)
CDMA is another pure digital technique. It is also known
as spread spectrum because it takes the digitized
version of an analog signal and spreads it out over a
wider bandwidth at a lower power level.
This method is called direct sequence spread spectrum
(DSSS) as well (Fig. 4).
The digitized and compressed voice signal in serial data
form is spread by processing it in an XOR circuit along
with a chipping signal at a much higher frequency.
In the cdma IS-95 standard, a 1.2288-Mbit/s chipping
signal spreads the digitized compressed voice at 13
kbits/s.
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Spread spectrum is the technique of CDMA. The compressed and digitized voice signal is
processed in an XOR logic circuit along with a higher-frequency coded chipping signal. The
result is that the digital voice is spread over a much wider bandwidth that can be shared
with other users using different codes.
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The chipping signal is derived from a pseudorandom code
generator that assigns a unique code to each user of the
channel. This code spreads the voice signal over a
bandwidth of 1.25 MHz. The resulting signal is at a low
power level and appears more like noise. Many such signals
can occupy the same channel simultaneously. For example,
using 64 unique chipping codes allows up to 64 users to
occupy the same 1.25-MHz channel at the same time. At the
receiver, a correlating circuit finds and identifies a specific
caller’s code and recovers it.
The third generation (3G) cell-phone technology called
wideband CDMA (WCDMA) uses a similar method with
compressed voice and 3.84-Mbit/s chipping codes in a 5-
MHz channel to allow multiple users to share the same
band.
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Advantages of CDMA
High spectral capacity.
• Reusing the same frequency in every cell
• eliminates the need for frequency planning.
• Rejection of narrow band interference (i.e. FDMA)
Usage of the soft hand off, which is undetectable
and provides a more reliable and higher quality
signal.
• Call dropout occurrence is very rare.
• Covers more area than GSM.
• Dis Advantages
Lack of international roaming capabilities compared to GSM
• Limited variety of the handset
• The ability to upgrade or change to another
handset is not easy

4/3/2023 Dr KRB Prof ECE SCET India WCN
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SDMA(Space Division Multiple Access)
• used for allocating a separated space to users in wireless
networks.
• A typical application involves assigning a optimal base station to
mobile phone user.
• The mobile phone may receive several base stations with
different quality.
• A MAC algorithm could now decide which base station is best,
taking into account which frequencies (FSM), time slots (TDM)
or code (CDM) are still available.
• Typically, SDMA is never used in isolation but always in
combination with one or other schemes.
• The basis for the SDMA algorithm is formed by cells and
sectorized antennas which constitute the infrastructure
implementing space division multiplexing (SDM).

OFDMA(Optical Fibre Division Mutiplexing Access)
• OFDMA is the access technique used in Long-Term
Evolution (LTE) cellular systems to accommodate
multiple users in a given bandwidth.
• Orthogonal frequency division multiplexing (OFDM) is a
modulation method that divides a channel into multiple
narrow orthogonal bands that are spaced so they don’t
interfere with one another.
• Each band is divided into hundreds or even thousands of
15-kHz wide subcarriers.
• The data to be transmitted is divided into many lower-
speed bit streams and modulated onto the subcarriers.
4/3/2023 27

• Time slots within each subchannel data stream are
used to package the data to be transmitted (Fig. ).
• This technique is very spectrally efficient, so it
provides very high data rates. It also is less affected
by multipath propagation effects.

. OFDMA assigns a group of subcarriers to each user. The subcarriers are part of the
large number of subcarriers used to implement OFDM for LTE. The data may be voice,
video, or something else, and it’s assembled into time segments that are then
transmitted over some of the assigned subcarriers.
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The basic GSM (Global System of Mobile Communications)
cellular phone system is TDMA-based. It divides up the radio
spectrum into 200-kHz bands and then uses time division
techniques to put eight voice calls into one channel.
Fig shows one frame of a GSM TDMA signal. The eight time
slots can be voice signals or data such as texts or e-mails. The
frame is transmitted at a 270-kbit/s rate using Gaussian
minimum shift keying (GMSK), which is a form of frequency
shift keying (FSK) modulation.
fig 3. This GSM digital cellular method shows how up to eight users can share a 200-
kHz channel in different time slots within a frame of 1248 bits
4/3/2023 30

SDMA(Space Division Multiple Access)
• SDMA uses physical separation methods that permit the
sharing of wireless channels.
• For instance, a single channel may be used
simultaneously if the users are spaced far enough from
one another to avoid interference. Known as frequency
reuse, the method is widely used in cellular radio
systems.
• Cell sites are spaced from one another to minimize
interference.
• In addition to spacing, directional antennas are used to
avoid interference.
• Most cell sites use three antennas to create 120° sectors
that allow frequency sharing (Fig. 6a).

• New technologies like smart antennas or adaptive
arrays use dynamic beamforming to shrink signals
into narrow beams that can be focused on specific
users, excluding all others

SDMA separates users on shared frequencies by isolating them with directional
antennas. Most cell sites have three antenna arrays to separate their coverage into
isolated 120° sectors (a). Adaptive arrays use beamforming to pinpoint desired users
while ignoring any others on the same frequency (b).

4/3/2023 Dr KRB Prof ECE SCET India WCN
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• One unique variation of SDMA, polarization division
multiple access (PDMA), separates signals by using
different polarizations of the antennas.
• Two different signals then can use the same frequency,
one transmitting a vertically polarized signal and the
other transmitting a horizontally polarized signal.
• The signals won’t interfere with one another even if
they’re on the same frequency because they’re
orthogonal and the antennas won’t respond to the
oppositely polarized signal.
• Separate vertical and horizontal receiver antennas are
used to recover the two orthogonal signals. This
technique is widely used in satellite systems.

Other Methods
• A unique and widely used method of multiple access is
carrier sense multiple access with collision detection
(CSMA-CD). This is the classical access method used in
Ethernet local-area networks (LANs).
• It allows multiple users of the network to access the
single cable for transmission. All network nodes listen
continuously, When they want to send data, they listen
first and then transmit if no other signals are on the
line.
• For instance, the transmission will be one packet or
frame. Then the process repeats. If two or more
transmissions occur simultaneously, a collision occurs.
The network interface circuitry can detect a collision,
and then the nodes will wait a random time before
retransmitting.

A variation of this method is called carrier sense multiple access with collision
avoidance (CSMA-CA). This method is similar to CSMA-CD. However, a special
scheduling algorithm is used to determine the appropriate time to transmit over the
shared channel. While the CSMA-CD technique is most used in wired networks,
CSMA-CA is the preferred method in wireless networks.
Conclusion
So, what is the future of communications access technologies? There is no one-
size-fits-all answer to this question, as the future of each communications access
technology will depend on the specific requirements of the applications and
networks that they are being used for.
However, it is generally agreed that FDMA, TDMA, CDMA, OFDMA, and SDMA will
all continue to play important roles in communications systems in the future.
They will continue to provide higher bandwidths and capacities to meet the needs of
businesses and consumers.
The trend toward more wireless and mobile communications will continue as well,
as these technologies offer more flexibility and increased throughput for future
generations.

What is non-orthogonal multiple access Noma for 5G?
NOMA is fundamentally different than these multiple access
schemes which provide orthogonal access to the users
either in time, frequency, code or space.
In NOMA, each user operates in the same band and at the
same time where they are distinguished by their power
levels.
What is OMA and NOMA?
Non-orthogonal multiple access (NOMA) is a better
multiple access technique than orthogonal multiple access
(OMA),
precisely orthogonal frequency division multiple access
(OFDMA) scheme, at the conceptual level for fifth-
generation (5G) networks and beyond 5G (B5G) networks

Nonorthogonal : inner product != 0,magnetude is not equal to 1
orthogonal: inner product =0,magnetude is not equal to 1
orthonormal:inner product =0,magnetude is equal to 1
]
4
3
2
1
[
a=
b =
Type equation here.
[-1
1
4
3
]
a.b= 1+3+12+12= 28

ALOHA in it's full form stands for:
Advocates of Linux Open-source Hawaii Association.
It is a computer protocol that was designed at the University of Hawaii in the 70's for use
with satellite communication systems in the Pacific.
The protocol itself is quite simple. When a frame* arrives for transmission, the user
transmits the frame, and awaits an acknowledgement. If this acknowledgement arrives,
the process is complete, and we know that the transmission of the frame was successful.
If not, process is completed again.

Problems
If two users transmit at the same time, and they overlap, the transmission will
be corrupted, and neither device will receive a acknowledgement.
*In this instance, a frame is defined as a packet of data

4/3/2023 41

Multiple Access Schemes Explained

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