Orthogonal Multiple-Access Systems [Autosaved].pptx

Orthogonal Multiple-Access
Systems
By
Vishnu S

Syllabus:
Module III : 5G Radio-Access Technologies
Access design principles for multi-user communications, Orthogonal multiple-access systems, Spread spectrum
multiple access systems, Capacity limits of multiple-access methods, SCMA, IDMA, Radio access for dense
deployments, OFDM numerology for small-cell deployments, Radio access for V2X communication, Radio
access for massive MIMO.

Access design principles for multi-user
communications
MULTI-USER
COMMUNICATION
Massive MIMO
(Multiple-Input
Multiple-Output)
Network Slicing
Edge Computing
Integration
Device-to-Device
(D2D)
Communication
Non-Orthogonal
Multiple Access
(NOMA)
Millimeter-Wave
(mmWave)
Communications
Ultra-Dense
Networks (UDN)
Beamforming
advanced antenna
arrays and beamforming
physical network into
multiple virtual slices
multiple users to share the same
time, frequency, and code
resources
Uses power-domain
multiplexing
nearby devices without
routing through the BS
Deploys a high
density of small
Reduces latency by processing data
closer to the user at the network edge
Enables spatial
multiplexing
Directs
transmission
towards specific
users

Orthogonal multiple-access (OMA)
• Orthogonal multiple-access (OMA) systems are traditional multi-user
communication techniques where different users are allocated non-
overlapping resources in the time, frequency, or code domain to avoid
interference.
• These systems ensure reliable communication but may suffer from
spectral inefficiency, especially in scenarios with high user density.

Types of OMA Systems
Type of OMA System Description Where It Is Used
Time Division
Multiple Access
(TDMA)
Assigns separate frequency bands
to each user.
Analog cellular systems (AMPS),
Satellite Communications
Frequency Division
Multiple Access
(FDMA)
Allocates different time slots to
users on the same frequency.
GSM (2G), Digital Enhanced
Cordless Telecommunications
(DECT), TETRA (Terrestrial Trunked
Radio)
Code Division
Multiple Access
(CDMA)
Uses unique spreading codes for
each user to share the same
bandwidth.
3G Networks (CDMA2000,
WCDMA), GPS
Orthogonal
Frequency Division
Multiple Access
(OFDMA)
Divides bandwidth into multiple
orthogonal subcarriers assigned to
different users.
4G LTE, 5G NR, WiMAX, Wi-Fi (IEEE
802.11ax)

Orthogonal Multiple-Access Systems [Autosaved].pptx

Problem:
A 4G LTE system uses OFDM with a total available bandwidth of 20 MHz.
The system employs 2048 subcarriers, but only 1200 subcarriers are used
for data transmission, while the rest are reserved for guard bands and pilots.
The cyclic prefix (CP) occupies 7% of the total OFDM symbol duration.
Find:
1.The subcarrier spacing.
2.The useful symbol duration (excluding the cyclic prefix).
3.The total OFDM symbol duration (including the cyclic prefix).

Calculate Subcarrier Spacing
𝑘 𝐾

Compute Useful Symbol Duration

=

Compute Total OFDM Symbol Duration

OFDM Variants Without Guard Bands
• Zero Guard Band OFDM (ZGB-OFDM)
No guard bands, leading to higher spectral efficiency. Requires
advanced filtering to reduce adjacent-channel interference.
• Generalized Frequency Division Multiplexing (GFDM)
Uses non-orthogonal subcarriers to allow flexible time-
frequency allocation. Reduces or removes guard bands by using
circular pulse shaping.

OFDM Variants with Subcarriers and Guard
Bands
Types of OFDM use subcarriers for data and control while reserving
guard bands to reduce adjacent channel interference:
• Conventional OFDM (COFDM)
• Filter Bank Multi-Carrier (FBMC)
• Discrete Fourier Transform Spread OFDM (DFT-S-
OFDM)

Conventional OFDM (COFDM)
• Uses multiple orthogonal subcarriers for transmission.
• Includes guard bands to prevent inter-symbol and inter-carrier
interference.
• Used in DVB-T, Wi-Fi (IEEE 802.11a/g/n/ac), and LTE downlink.

Filter Bank Multi-Carrier (FBMC)
• Uses subcarriers, but replaces cyclic prefix (CP) with filter banks.
• Minimizes the need for guard bands by reducing out-of-band
emissions.
• Used in 5G waveform research and cognitive radio systems.

Discrete Fourier Transform Spread OFDM
(DFT-S-OFDM)
• Used in LTE uplink (SC-FDMA).
• Reduces Peak-to-Average Power Ratio (PAPR) by spreading data
across subcarriers.
• Uses subcarriers and guard bands but behaves like a single-carrier
system.

OFDM Variants Without Subcarriers
Some waveform designs remove the concept of individual
subcarriers, using different techniques for multiplexing.
Single-Carrier Frequency Division Multiple Access (SC-FDMA):
Used in LTE uplink (called DFT-S-OFDM).
Behaves like a single-carrier system, reducing PAPR.
Uses contiguous frequency bands instead of independent
subcarriers.

Access Protocol used in 5G
The access protocol used in 5G networks is based on the NR (New
Radio) air interface defined by 3GPP (3rd Generation Partnership
Project). It employs Orthogonal Frequency Division Multiple Access
(OFDMA) for the downlink and a combination of OFDMA and SC-
FDMA (Single-Carrier Frequency Division Multiple Access) for the
uplink.

Access Protocol used in 5G
2. Single Carrier Frequency Division Multiple Access (SC-FDMA)
3. Time Division Duplex (TDD) & Frequency Division Duplex (FDD)
4. Non-Orthogonal Multiple Access (NOMA)
5. Massive MIMO (Multiple Input Multiple Output)
6. Random Access Protocol (RACH)

SC-FDMA
SC-FDMA is a multiple-access technique that combines the advantages of single-
carrier transmission with frequency domain equalization. It is used primarily in
uplink transmissions of 4G LTE due to its lower Peak-to-Average Power Ratio
(PAPR) compared to OFDMA.

SC-FDMA
• Modulation: Data symbols are modulated (e.g., QPSK, 16-QAM).
• DFT Pre-coding: A Discrete Fourier Transform (DFT) converts the
time-domain signal into frequency components.
• OFDM Modulation: An Inverse Fast Fourier Transform (IFFT) is
applied, making it similar to OFDM.
• Transmission: The cyclic prefix (CP) is added, and the signal is
transmitted.

Blocks
Serial-to-Parallel (S-to-P) Conversion
• Converts a serial data stream into parallel symbols for efficient
processing.
N-point DFT (Discrete Fourier Transform)
• Transforms the input data from the time domain to the frequency
domain.
• This step helps in DFT-spread OFDM, reducing Peak-to-Average
Power Ratio (PAPR).

N-point DFT (Discrete Fourier
Transform)
8-point DFT

Blocks
Subcarrier Mapping
•The transformed frequency-domain symbols are mapped onto specific
subcarriers.
•Different users are assigned different subcarriers in an LTE uplink
scenario.
M-point IDFT (Inverse Discrete Fourier Transform)
•Converts the mapped frequency-domain symbols back to the time
domain.
•This operation is similar to OFDM modulation but maintains a single-
carrier structure.

B𝑆𝐹=
Occupied Bandwidth (after spreading)
Original Symbol Bandwidth

Carrier Characteristics in OFDM and SC-
FDMA
• OFDM: Multi-carrier system where data is transmitted over multiple
orthogonal subcarriers.
• SC-FDMA: Single-carrier system (after DFT spreading), though it
uses multiple subcarriers in the frequency domain.

Types of Subcarrier Allocation:
1.Fixed Allocation: Each user is assigned a fixed set of subcarriers.
2.Dynamic Allocation: The system dynamically assigns subcarriers
based on channel quality, user demand, and interference
conditions (e.g., LTE’s scheduler).
3.Full Bandwidth Usage: Not all subcarriers are always used;
subcarriers can be assigned flexibly based on network conditions.
LTE downlink (OFDM) assigns subcarriers to users in
Resource Blocks (RBs) (12 subcarriers per RB).
(Resource Block) - RB

5G NR
5G NR (5G New Radio) is a radio access technology (RAT) developed
by the 3rd Generation Partnership Project (3GPP) for the 5G (fifth
generation) mobile network. It was designed to be the global standard
for the air interface of 5G networks. It is based on
orthogonal frequency-division multiplexing (OFDM), as is the 4G
(fourth generation) long-term evolution (LTE) standard.
• The 3GPP specification 38 series
provides the technical details behind 5G NR, the successor of LTE.

Fig: a) TDMA; b) FDMA; c) OFDMA; d) CDMA/SDMA; e)
possible NOMA solution.

Peak-to-Average Power Ratio (PAPR)
PAPR (Peak-to-Average Power Ratio) occurs due to variations in
signal amplitude :
1. Multi-Carrier Nature (OFDM) (Increases peak powers)
2. Random Phase of Subcarriers (Leads to constructive interference, causing high peaks)
3. Large Signal Variations in Modulation Techniques - High-order
modulation ( Increases amplitude variations)
4. Inverse Fast Fourier Transform (IFFT) Processing (Creates signal peaks by
overlapping subcarriers)
5. Cyclic Prefix Addition (Helps in ISI but does not reduce PAPR)

Multiple Access
Multiple access techniques are methods that allow multiple users or
terminals to share a common communication channel or medium.
To enable simultaneous communication between multiple users over a
shared resource.
TYPES:
• Frequency Division Multiple Access (FDMA): Different users are assigned
different frequency bands.
• Time Division Multiple Access (TDMA): Users are assigned different time
slots.
• Code Division Multiple Access (CDMA): Users transmit signals using
unique codes that allow them to share the same frequency band.

Multiplexing and Multiple Access
• Multiple access techniques often rely on multiplexing to
enable efficient sharing of the communication channel.
• For instance, FDMA uses frequency division multiplexing to
allow multiple users to share a frequency band, and TDMA
uses time division multiplexing to allow multiple users to
share a time slot.

Conceptual Diagram of Code-Division
Multiplexing

Access Protocols in 5G
1. Orthogonal Frequency Division Multiple Access (OFDMA):
• Used in both downlink and uplink (mainly downlink). Divides
the available bandwidth into multiple orthogonal subcarriers.
• Each user is allocated a subset of subcarriers, enabling
simultaneous communication without interference.
• Provides high spectral efficiency and is suitable for massive
data transmission.

OFDM - Orthogonal frequency-division
multiplexing
In telecommunications, orthogonal frequency-division
multiplexing (OFDM) is a type of digital transmission used in digital
modulation for encoding digital (binary) data on multiple carrier
frequencies. OFDM has developed into a popular scheme for wideband
digital communication, used in applications such as digital television
and audio broadcasting, DSL internet access, wireless networks,
power line networks, and 4G/5G mobile communications.

Principle of OFDM
The core idea of OFDM is to divide a high-speed data stream into
multiple lower-speed sub-streams, which are then transmitted
simultaneously over orthogonal subcarriers. These subcarriers are
spaced precisely to maintain orthogonality, preventing mutual
interference despite their close proximity.

How Orthogonality is achieved in OFDM
• Two signals s1(t) and s2(t) are said to be orthogonal if their inner product over a time
period T is zero:
In OFDM, the subcarriers are chosen so that their frequencies are spaced apart by an
integer multiple of the inverse of the symbol duration T:
Where;
fn= Frequency of the nth
subcarrier
f0= Base frequency
N = Number of subcarriers
T = OFDM symbol duration

OFDM Signal Representation:
where;
s(t) = Transmitted OFDM signal in the time domain
𝑋
𝑛 = Complex modulation symbol on the 𝑛𝑡ℎ
subcarrier (e.g., QAM or PSK)
𝑁 = Total number of subcarriers
Δ𝑓= Frequency spacing between adjacent subcarriers (Δ =1
𝑓 𝑇𝑠)
𝑇𝑠= OFDM symbol duration (including cyclic prefix if present)
𝑡 = Time index within the symbol duration = Imaginary unit ( =−1, j= −1
)
𝑗 𝑗

OFDM in digital television and audio
broadcasting
OFDM is similar to the broadcasting technique known as frequency
division multiplexing (also known as FDM), which uses a multitude of
transmitters and receivers to send information on different
frequencies over a single wire, such as an electrical power cable.

Discrete Fourier Transform (DFT)
The Discrete Fourier Transform (DFT) is a mathematical technique
that transforms a finite-length discrete-time signal from the time domain
to the frequency domain.
 
 
X DTFT x n x n e
x n IDTFT X X e d
j n
n
j n
( ) ( ) ( )
( ) ( ) ( )



 




 
 

 





1
2
x n
( )
n
N  1 
X ( )

discrete time
continuous frequency

 
…..

 
 
X DTFT x n x n e
x n IDTFT X X e d
j n
n
j n
( ) ( ) ( )
( ) ( ) ( )



 




 
 

 





1
2
x n
( )
n
N  1 
X ( )

discrete time
continuous frequency

 
…..
…..

DFT - FFT
The Fast Fourier Transform (FFT) is an algorithm that efficiently
computes the Discrete Fourier Transform (DFT), which transforms a
signal from the time domain to the frequency domain, but does so much
faster than a naive DFT calculation.

Digital television (DTV)
Digital television (DTV) transmits TV signals using digital encoding, unlike
older analog systems, offering advantages like higher resolution, better
sound, and the ability to transmit multiple channels within the same
bandwidth.
Digital television uses digital compression to convert digital signals into
digital packets of data, which are then transmitted over the air or via cable
or satellite.
The digital signal can be received by a digital TV antenna, cable box or
satellite receiver depending on the type of service being used. The digital
signal is decoded and converted back into an analog form before it is
displayed on the viewer's screen.

Applications of OFDM
• Wireless Communication: Wi-Fi (802.11a/g/n/ac/ax), LTE, and 5G
use OFDM for high-speed wireless data transfer.
• Broadcast Systems: Digital TV (DVB-T) and digital radio (DAB)
employ OFDM for reliable broadcasting.
• Optical Networks: OFDM is also used in coherent optical
communications for long-distance data transmission.

Massive MIMO (Multiple Input Multiple
Output)
Massive MIMO is an advanced wireless technology that significantly
improves spectral and energy efficiency by deploying a large number
of antennas at the base station (BS).
It is widely used in 5G and beyond for enhanced data rates, reliability,
and coverage.

Advantages:
• Higher capacity: Supports more users with the same
bandwidth.
• Improved reliability: Better signal quality and fewer
dropouts.
• Energy-efficient: Reduces transmit power per antenna while
maintaining performance.
• Interference reduction: Smart beamforming minimizes
cross-user interference.

Random Access Protocol (RACH) in
LTE/5G
Random Access Channel (RACH) is a protocol used in LTE and 5G
for initial access, handovers, and uplink synchronization when
a device wants to communicate with a base station.
The Random Access Channel (RACH) Protocol is a critical
mechanism in LTE and 5G networks that allows User Equipment
(UE) to initiate communication with the network.
Types of RACH:
1.Contention-Based RACH (Common for most users)
2. Contention-Free RACH (Dedicated for handovers)

Random Access Protocol (RACH) in
LTE/5G
System Information Block
2
(Access Class
Barring)
•If r≤p, the UE proceeds with the random access process.
•If r>p, the UE is barred and must wait before retrying.
The UE generates a random number r between 0 and
1.

TDD (Time Division Duplex)
TDD uses the same frequency band for both uplink and
downlink but transmits them in different time slots.
It have a Guard time to Prevent collision.
Prone to interference: If many users transmit
simultaneously, performance drops.

Advantage
• Dynamic allocation
• Efficient spectrum use
• Better suited for asymmetric traffic (e.g., internet browsing, video
streaming).

FDD (Frequency Division Duplex)
FDD uses separate frequency bands for uplink and downlink,
allowing simultaneous transmission.
A frequency gap called Duplex Spacing prevents interference.
Requires more spectrum: Need separate frequency bands for UL and
DL.
Fixed allocation: Cannot dynamically adjust UL/DL ratio like TDD.

Advantage
• Less interference: Since UL and DL operate on different frequencies.
• No guard time required: More spectrum efficiency compared to
TDD.
• Lower latency: Since UL and DL are simultaneous.
• Eg:
•LTE FDD (used in most countries for cellular networks)
•5G NR (on licensed spectrum)
•2G (GSM) & 3G (UMTS)
•Satellite communication
•Traditional telephone networks (PSTN, VoIP, etc.)

Basic questions
1. Why is SC-FDMA preferred over OFDMA in LTE uplink? Can it be
used in the downlink?
2. Why is FDD more suitable for high-mobility scenarios than TDD?
3. In NOMA, what happens if two users have very similar channel
conditions?

Summary
Technique Resource Sharing Used In Pros Cons
FDMA Frequency 2G (GSM)
Simple, low
interference
Inefficient
spectrum use
TDMA Time 2G (GSM)
Efficient for
bursty traffic
Synchronization
complexity
CDMA Code
3G (UMTS,
CDMA2000)
High capacity,
interference-
resistant
High processing
power
OFDMA
Frequency
(Subcarriers)
4G LTE, 5G
High spectral
efficiency
Synchronization
issues
SC-FDMA
Frequency (DFT-
based)
LTE Uplink
Low PAPR, energy
efficient
Higher
complexity

Cont…
• Orthogonal Multiple-Access (OMA) systems allocate distinct resources
(frequency, time, or code) to users, preventing interference.
• FDMA assigns separate frequency bands, TDMA uses time slots, and CDMA
differentiates users via unique codes.
• OFDMA, used in 4G/5G, divides frequency into orthogonal subcarriers,
improving spectral efficiency.
• SC-FDMA, used in LTE uplink, reduces power consumption with DFT-based
pre-coding.
• OMA ensures reliable communication, but Non-Orthogonal Multiple Access
(NOMA) is emerging for better spectrum utilization in future networks.

Reference:
• Wikipedia - https://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiple_access
• https://www.3gpp.org/dynareport?code=38-series.htm
• https://www.itu.int/
• https://www.sciencedirect.com/topics/computer-science/ort
hogonal-frequency-division-multiplexing
• http://ieeexplore.ieee.org/document/4454682/

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