5G New Radio: Introduction to the Physical

Layer

WHITE PAPER

5G New Radio:
Introduction to the Physical Layer

CONTENTS

Introduction

PHY Design

Considerations

Waveforms for 5G NR

Flexible Subcarrier Spacing and Symbol

Lengths NR Reference Signals

MIMO

mmWave for

5G Bandwidth

Parts

Conclusion: Comparing LTE and 5G NR

PHY Glossary

ni.com/
5g/nr
2 5G New Radio: Introduction to the Physical
Layer

Introduction
To understand the three broad use cases that 5G wireless technology seeks to
transform (Figure 1), consider a typical morning office commute in a 5G-connected
car just a few years down the road. The vehicle is constantly exchanging position,
behavior, and system status information with nearby vehicles, the surrounding
highway infrastructure, and traffic control centers. Doing so in a fast and reliable
manner augments the car’s awareness of its surroundings and allows the driver to
turn the steering, accelerating, and braking functions over to the car’s
semiautonomous driving system. He can now focus on the morning’s first
conference call.

The driver’s team is trying to find the root cause of a turbine malfunction. He puts
on his augmented reality (AR) set, and a wireless 4K video feed of an airplane
turbine overlaid with sensor data and gauge readings fills his screen.
Collaborating in real time with a group of engineers in three different countries,
the team guides a technician to isolate one of the components and recommends
a troubleshooting procedure.

A few minutes later, when his intelligent-highway exit comes up, the driver takes
back control of the car, switches over to a low-bandwidth voice-only connection,
and drives into work. The car guides him to the closest available parking spot with
an electric charging station. The parking sensor at that spot detects his car and
updates the parking availability information on the network. When he plugs in the
car to charge, the charging terminal establishes a low- data-rate connection to
verify his account and process payment.
10,000X More Traffic

20 Gbps DL Peak Data 100 Mbps, High Mobility

Rates ENHANCED
MOBILE
BROADBAND

5G
MASSIVE ULTRA-RELIABLE
MACHINE TYPE MACHINE
COMMUNICATION TYPE
1 M devices per 1 ms Latency
COMMUNICATION
km2

10-Year Battery Life 99.9999% Reliability

ni.com/
5g/nr
3 5G New Radio: Introduction to the Physical
Layer

Figure 1. 5G Use Cases and Targeted Key Performance Indicators

ni.com/
5g/nr
4 5G New Radio: Introduction to the Physical
Layer

Enhanced Mobile Broadband (eMBB) seeks to significantly improve the data rate,
latency, user density, capacity, and coverage of mobile broadband access to
allow the live streaming of AR/VR applications, even in more crowded
environments, such as the intelligent highway the driver uses. Ultra-Reliable Low-
Latency Communication (URLLC) enables users and devices to communicate
bidirectionally with other devices while generating minimal latency and
guaranteeing high network availability. Finally, Massive Machine-Type
Communication (mMTC) makes it possible for many low-cost, low-power, long-life
devices to support applications such as embedded highway sensors, parking
sensors, and smart utility meters.

The requirements of these distinct use cases pose complex technical trade-offs,
which involve delicate design decisions. To guarantee interoperability and global
access to 5G, the International Telecommunication Union (ITU), an international
union of telecommunication industry players, national and regional standards
development organizations, regulators, network operators, universities, and
research institutions, must approve the standards for 5G technologies. The ITU-R
(the radiocommunication sector), the ITU-T (the standardization sector), and the
3GPP (3rd Generation Partnership Project between groups of telecommunications
associations that standardizes cellular wireless access) are working concurrently
toward a unified 5G New Radio standard. The 3GPP expects to complete Phase 1
by June 2018 with Release 15, which will focus on the eMBB and URLLC use cases.
Around the end of 2019, Phase 2 will add functionality to 5G to support more
services, large IoT deployments, and much higher frequency bands beyond 52.6
GHz.

For now, the standards bodies have reached fundamental decisions with the
Release 14 study phase, including the current focus on non-stand-alone
operation of NR to support only the eMBB and URLLC use cases. NR does this by
relying on existing 4G infrastructure, the EPC (enhanced packet core) and the
eNodeB acting as the Master cell, and the 5G gNodeB (gNB) providing secondary
access (based on the dual connectivity principle, as Figure 2 shows).

ni.com/
5g/nr
5 5G New Radio: Introduction to the Physical
Layer

Evolved Packet Core (EPC)

Evolved Packet Core (EPC)
NON–STANDALONE (DEC 2017) STANDALONE (JUN, 2018)

Figure 2. Non-Stand-Alone Versus Stand-Alone Operation

ni.com/
5g/nr
6 5G New Radio: Introduction to the Physical
Layer

Furthermore, the IEEE has a 5G track that oversees the direction of future
developments that existing and new IEEE technologies will need to support 5G
targets. These include 802.11ax and 802.11ay (WLAN), 802.15 (short range
technologies), and 802.22 (fixed wireless broadband).

A key point to keep in mind is that Release 15 breaks compatibility with 4G

standards. This is similar to how 4G standards (LTE) departed from 3G wireless
standards (UMTS). Yet, the designers of the NR standard planned for future 5G
releases to maintain compatibility with the initial 5G NR specification. Because
designers, industry experts, and the market haven’t defined all the possible new
uses for 5G technology, the 5G physical layer needs to be flexible. This paper
presents an introductory tutorial to the 5G physical layer and its implementation
to support the key 5G target applications of eMBB and URLLC.

PHY Design Considerations

Many researchers from industry and academia are actively working on addressing
the requirements for a robust and reliable 5G implementation. The following key
features have played a defining role in the 5G NR physical layer:
■■
Supporting a wide range of operation bands, a variety of channel bandwidths
within those bands, and multiple deployment options.
■■
Serving applications with very low latency, which requires short subframes
and puncturing and bursty interference from mission-critical transmissions.
■■
Sharing the spectrum dynamically to provision uplink (UL), downlink (DL),
sidelink, and backhaul links.
■■
Implementing multiantenna technology (multiple input, multiple output or
MIMO) for larger spectral efficiency.
■■
Maintaining tight time operation and more efficient frequency use for better
time division duplex (TDD) and frequency division duplex (FDD) deployments.
■■
Having symmetrical DL and UL requirements to enable operation at millimeter
wave (mmWave) frequencies of compact, low-cost base stations.

Waveforms for 5G NR
CP-OFDM: Downlink and Uplink
Recently, researchers have been investigating many different multicarrier
waveforms and proposing them for 5G radio access. However, because
orthogonal frequency division multiplexing (OFDM) implementations lend
themselves well to TDD operation and delay- sensitive applications, and because
they have demonstrated successful commercial implementation by efficiently
processing ever-larger bandwidth signals, cyclic prefix (CP) OFDM became the
preferred choice for NR. The strong benefits of CP-OFDM that make it a great fit
for 5G implementation are:

High Spectral Efficiency—This essential feature of OFDM access helps meet

the extreme data rate needs, especially for backhaul links. Also, in future cases
like vehicular communication in dense urban environments, high spectral
efficiency will help address capacity constraints when many users broadcast
periodically and asynchronously.

MIMO Compatibility—Both base stations and mobile devices will take

ni.com/
5g/nr
7 5G New Radio: Introduction to the Physical
Layer advantage of MIMO technology to implement spatial and frequency multiplexing
with Single-User MIMO and

ni.com/
5g/nr
8 5G New Radio: Introduction to the Physical
Layer

Multiuser MIMO (MU-MIMO). MIMO deployments also overcome high

propagation losses and extend coverage range with beamforming.

Phase Noise Resistance—As the frequency of operation (and with it the

oscillator phase noise) increases, an OFDM system can minimize intersymbol
interference due to phase noise by applying larger OFDM subcarrier spacing
(SCS).

Transceiver Simplicity—OFDM transceivers offer lower implementation

complexity compared with other waveforms that designers considered for 5G
deployments. Having worked with OFDM designs for several years, the wireless
industry knows that their well- understood operation and wide commercial
deployment can enable 5G devices with powerful OFDM baseband processing
at lower prices.

Channel Time- and Frequency-Selectivity Resistance—With the right selection

of SCS and frequency of operation, an OFDM system can finish a transmission
between devices in an interval shorter than the channel coherence time and
enable high-mobility (high-speed) and high-data-rate scenarios while minimizing
the effects of time selectivity. Also, as Figure 3 shows, with channel estimation
and equalization techniques, OFDM waveforms demonstrate great resiliency
against frequency-selective channels.

CHANNEL

f f f
GENERATED
RECEIVED EQUALIZED
OFDM
OFDM OFDM
WAVEFORM
WAVEFORM WAVEFORM

Figure 3. Representation of an OFDM Waveform’s Frequency-Selectivity Resistance

Timing Error and Intersymbol Interference Resistance—Because of the CP, a

receiver can better tolerate synchronization errors and prevent the previous
OFDM symbol from smearing into the currently received OFDM symbol. Figure
4 shows two subsequent OFDM symbols, each with a dedicated CP. The CP at
the beginning of each OFDM symbol contains a copy of the end of the OFDM
symbol. When the receiver demodulates the signal, it operates on the symbol
after the CP (FFT window). This mechanism prevents intersymbol interference
between adjacent OFDM symbols

ni.com/
5g/nr
9 5G New Radio: Introduction to the Physical
Layer

OFDM SYMBOL 1 OFDM SYMBOL 2

CP FFT WINDOW CP FFT WINDOW

CYCLIC PREFIX CYCLIC PREFIX

Figure 4. A Cyclic Prefix Separates OFDM Symbols

With CP-OFDM, user equipment (UE) supports the following modulation schemes:
■■
QPSK
■■
16-QAM
■■
64-QAM
■■
256-QAM

DFT-S-OFDM: Higher Efficiency Uplink

One of the main drawbacks of OFDM waveforms is their high peak-to-average
power ratio (PAPR). As a result, RF output power amplifiers on transmitters lose
efficiency and can’t minimize high-order, nonlinear effects well. For UE such as
smartphones, preserving battery life and being energy efficient is important. The
RF power amplifier that transmits the signal to the base station consumes the
most power within the mobile device, so system designers need a type of
waveform that promotes high-efficiency amplifier operation while meeting the
spectral demands of 5G applications. Although single-carrier waveforms have
very low PAPR and more efficient power amplifier operation, they don’t offer high
spectral efficiency and dynamic spectrum utilization, their compatibility with
MIMO systems is lower, and they are susceptible to frequency-selective
channels.

For uplink, NR allows UEs to use CP-OFDM or a hybrid format waveform called
discrete Fourier transform spread orthogonal frequency division multiplex (DFT-S-
OFDM). In DFT-S-OFDM, the transmitter modulates all subcarriers with the same
data. The right side of Figure 5 shows that the first group of subcarriers (all red)
takes the same amount of bandwidth as the OFDM symbol on the left. The DFT-S-
OFDM modulator maps the same data to all subcarriers but for a shorter duration.
It then maps the next data symbol (green) to all subcarriers for another short
interval. By the end of the equivalent OFDM symbol time, the transmitter sends
the same amount of data as it sends with an OFDM waveform by mapping the
data symbols to all subcarriers simultaneously but with shorter transmission
intervals. This DFT-S-OFDM waveform combines a lower PAPR with the multipath
interference resilience and flexible subcarrier frequency allocation that OFDM
provides.

ni.com/
5g/nr
1 5G New Radio: Introduction to the Physical
Layer

PAPR: 11–13 dB HIGHER EFFICIENCY WITH

QPSK Modulating Data LOWER PAPR: 6–9 dB
Symbols

-1,1 1,1 Transmit QPSK Data Symbols Sequence

1,1 -1,-1 -1,1 1,-1

-1,-1 1,-1
Data Symbols Occupy N x Subcarrier

Time Time

OFDM Symbol OFDM Symbol Duration

Duration Frequency Frequency
Subcarrier
Symbol Width, e.g.: 60
Spacing e.g.:
kHz
15 kHz

Downlink: CP-OFDM Uplink: CP-OFDM and DFT-S-OFDM

Figure 5. OFDM Versus DFT-S-OFDM

With DFT-S-OFDM, UE supports the following modulation schemes:

■■
Pi/2-BPSK—note that this is a new modulation scheme in NR, and it
requires new IP for implementation
■■
16-QAM
■■
64-QAM
■■
256-QAM

Flexible Subcarrier Spacing and Symbol Lengths

The 3GPP intends for NR to operate in multiple frequency bands ranging from
existing cellular bands (below 1 GHz) to wider bands between 3 GHz and 5 GHz
and up to the mmWave region of the spectrum. Figure 6 illustrates the current
bands defined for NR operation above
26.25
6 GHz.
29.5 40

3GPP: ALREADY DEFINED 24.25 27.5 37

29.5

KORE 26.5
29.5 40 43.5

A 24.25 37 40.5
27.5 42.5

JAPA 24.75 37
28.35 38.6 40

N 27.5 37 38.6
24.25 43.5

CHIN 27.5 40.5

ni.com/
5g/nr
1 5G New Radio: Introduction to the Physical
Layer US FCC

5G

EUROP

FREQUENCY (GHz)
Figure 6. 3GPP-Defined and Locally Adopted Bands for NR in the Millimeter Wave Portion of the Spectrum

ni.com/
5g/nr
1 5G New Radio: Introduction to the Physical
Layer

As the carrier frequency increases, so does the system phase noise. For example,
in the carrier phase noise plot of Figure 7, the difference in phase noise between a
carrier at 1 GHz and 28 GHz is about 20 dB. This phase noise increase makes it
difficult for a receiver to demodulate OFDM waveforms with the narrow, fixed SCS
and symbol duration of LTE at mmWave frequencies.
CARRIER PHASE NOISE SUBCARRIER WITH PHASE NOISE EVM

0
-20
-40
Phase Noise Power

-60
-80
-100
(dBc/Hz)

-120

-140
10 1001 000100001000001000000
Offset Frequency

Figure 7. Effect of Phase Noise on Error Vector Magnitude

Additionally, the Doppler shift increases with car0r.ier frequency, as shown in

Figure 8. For example, UE traveling at a speed of 60 km/h using a carrier
frequency of 28 GHz sees a Doppler shift of close to 1500 Hz, or 10 percent of a
15 kHz SCS. Because the channel coherence time, or the time when the system
can assume that the radio channel remains constant, is approximately inversely
proportional to the Doppler shift, it decreases as mobility increases. Therefore, at
higher carrier frequencies and higher speeds, the system has less time to
measure the channel and finish a single slot transmission.
DOPPLER SHIFT CHANNEL COHERENCE TIME VS. SPEED

3500.00 0.06

3000 .00
0.05
Doppler Frequency

2500.00
0.04
2000 .00
0.03
1500.00
0 0.02
10 0.00
(Hz)

0.01
500.00

0.00 0

10 20 30 40 50 60 70 80 90 100 110 120 10 20 30 40 50 60 70 80 90 100 110 120

130 130
Speed (km/h) Speed (km/h)

Doppler 1 GHz
Doppler 4 Doppler 28 Doppler 1 Doppler 4 Doppler 28 GHz
GHz GHz GHz GHz

Figure 8. Doppler Shift and Channel Coherence

Also, phase noise and Doppler shift define the requirements for SCS to meet
specific error vector magnitude (EVM) criteria. That means using narrow SCS
causes higher EVM because of phase noise unless system designers implement
the design with a high-quality local oscillator at a high cost. Also, when SCS is
small, the system performance can suffer because of Doppler shift in high-
mobility scenarios. On the other hand, selecting a large SCS results in excessive
channel bandwidth. Furthermore, given that SCS is inversely proportional to the
OFDM symbol duration, the OFDM symbol and CP length shortens as SCS

ni.com/
5g/nr
1 5G New Radio: Introduction to the Physical
Layer increases and makes the system

ni.com/
5g/nr
1 5G New Radio: Introduction to the Physical
Layer

more susceptible to delay spread. Therefore, SCS should be as small as possible

while providing enough performance in the presence of phase noise and Doppler
for a desired channel bandwidth.

In the context of cellular standards, “numerology” refers to the physical layer SCS
and symbol length. The 3GPP standardized on a flexible numerology that scales
the space between orthogonal subcarriers, starting with the 15 kHz SCS of LTE.
One of the fundamental reasons for leveraging the exhaustive work already
completed for LTE numerology was the ability of NR deployments to coexist and
be time-aligned with LTE networks during the first phases of deployment. This
gives LTE users a gradual path to adoption of the new technology.

The NR numerology scales according to the following formula1,2:

15 kHz SCS: 12 Subcarrier RB –>180 kHz

30 kHz SCS: 12 Subcarrier RB –>360 kHz

60 kHz SCS: 12 Subcarrier RB –>720 kHz

120 kHz SCS: 12 Subcarrier RB –>1440 kHz

*The NR specification also includes 240 kHz SCS.

Figure 9. Flexible SCS

The standard specifies that the smallest allocatable frequency unit consists of 12
subcarriers, designated as a physical resource block (PRB). Consequently, the
smaller the SCS, the narrower the PRB, as shown in Figure 9.

Figure 10 illustrates the NR channel PRBs and guard bands.

ni.com/
5g/nr
1 5G New Radio: Introduction to the Physical
Layer

Channel Bandwidth (MHz)

Transmission Bandwidth Configuration (Number of RB)

Transmission Bandwidth (RB)

Edge
Channel
Channel

Block
Resource
Edge

Active
Resource
Blocks

Guardband, Can Be Asymmetric

Figure 10. NR Channel Divided Into Resource Blocks

A More Scalable and Flexible Frame Structure

Along with flexible SCS, 5G NR implements a flexible frame structure that
ensures 5G forward compatibility. It also minimizes design trade-offs for
supporting key features like low latency, coexistence with LTE, variable length
transmissions, and TDD and FDD operation in licensed and unlicensed spectra.

Slot Configurations Scale With SCS

NR slots have 14 OFDM symbols. A special case for 60 kHz SCS can have an
extended CP and 12 OFDM symbols. Since OFDM symbol duration has an
inversely proportional relationship with SCS, the duration of the slots scales down
as SCS increases. Figure 11 shows the standard NR slot configurations.

ni.com/
5g/nr
1 5G New Radio: Introduction to the Physical
Layer

SLOT
14 OFDM
15 kHz
Symbols
1 ms

SLOT
14 Symbols 30 kHz

500 µs

SLOT
14 Symbols 60 kHz

250 µs

SLOT
14 120 kHz
Symbols
125 µs

Figure 11. TDD Slot-Based Scheduling

The frame structure numbers the slots and groups them into subframes of 1 ms
duration. Ten 1 ms subframes form a complete NR frame. The number of slots
within a frame also varies with the choice of numerology, for example:
■■
Using 15 kHz of SCS results in a single 1 ms slot within the subframe,
amounting to 10 slots per frame
■■
Using 30 kHz of SCS results in a subframe with two 500 µs slots within the
subframe, amounting to 20 slots per frame

OFDM
1ms
Symb
CP Symbol 0 C Symbol 1 15 kHz SCS
ol P
CP Symbol
13

Slot 02 13 26 27 30 kHz SCS

14 OFDM Symbols 01 23 45 55 65 75 2 3 4 5 60 kHz SCS

081 2 34 5 67
11 1 1 1 1
01 2 3 4 5
120 kHz SCS
9

Subframe

1ms

0
1 2 3 4 5 6 7 8 9
Frame

10 Subframes - 10 ms
Figure 12. NR Frame
Structure

ni.com/
5g/nr
1 5G New Radio: Introduction to the Physical
Layer

The time and frequency resource structure defines the NR resource grid in Figure
13. Depending on SCS, the resource grid changes as the number of available
subcarriers and OFDM symbols change. That is, for each numerology and carrier,
NR specifies a resource grid with a width given by the maximum number of
resource blocks per SCS multiplied by the number of subcarriers per resource
block, and a length given by the number of OFDM symbols per subframe.
T Slot

Slot

N OFDM Symbols

Resource Block
Subcarrie
Channel

Resource Element
Width

rs

Figure 13. NR Resource Grid

To support agile and efficient use of TDD resources, NR also implements a flexible
slot structure. The system can allocate a slot as all DL, all UL, or a mix of DL and
UL to service asymmetric traffic, as Figure 14 illustrates. DL control takes place at
the beginning of the slot and UL control at the end. The system can either
configure the mixed DL/UL slot statically, as in an LTE DL/UL TDD configuration, or
change the allocation of the DL/UL mix dynamically for better efficiency and
scheduling based on traffic needs.

To accomplish this, the NR standard includes the slot format indicator (SFI), a
field that informs a user whether an OFDM symbol contains DL, UL, or flexible
(either DL or UL) slots. The SFI indicates the link direction for one or many slots
by indexing a row of the UE’s preconfigured table of possible link direction
assignments.

ni.com/
5g/nr
1 5G New Radio: Introduction to the Physical
Layer

DL DL DL DL DL DL DL DL DL DL DL DL
UL DL–HEAVY TRANSMISSION
WITH UL PART
SLOT

DL
UL UL UL UL UL UL UL UL UL UL UL
UL UL–HEAVY
TRANSMISSION
WITH DL
SLO
T Control

Figure 14. Flexible Slot Structure for Managing TDD Resources Dynamically

Furthermore, when the system needs to work with large payloads that don’t
demand the most immediate attention, the standard allows for slot aggregation.
In the case of eMBB, for example, having aggregated slots and longer
transmission times meets application requirements while reducing TDD switching
and signaling overhead.

Minislots Enable Even Further Dynamic Scheduling

The NR standard is also considering the use of “minislots” to support bursty,
asynchronous transmissions with variable start positions and durations shorter
than the typical, 14-symbol slot. A minislot represents the smallest possible
scheduling unit, and it can last for 7, 4, or 2 OFDM symbols. Minislots are
especially important for enabling low-latency transmissions.

Imagine the future case of a mission-critical system (URLLC) that needs to

communicate its information with minimal latency, so the standard 10 ms frame
is too long. The NR numerology allows minislots to “puncture” an existing frame
without waiting for the system to schedule it. To avoid collisions, the network
detects a minislot burst and manages the URLLC device with the highest priority.
Additionally, the network can schedule minislots ahead of time, which becomes
increasingly relevant in a mmWave operation where the transmission of a few
OFDM symbols mapped across large bandwidths might be enough
to carry smaller payloads. As of December 2017, the 3GPP has not yet fully
specified this feature, and it will not be part of Release 15.3

NR Reference Signals
To increase protocol efficiency, and to keep transmissions contained within a slot
or beam without having to depend on other slots and beams, NR introduces the
following four main reference signals. Unlike the LTE standard, which is
constantly exchanging reference signals to manage the link, an NR transmitter
sends these reference signals only when necessary.4
1. Demodulation Reference Signal (DMRS)—The DMRS is specific for
specific UE, and a system uses this signal to estimate the radio channel.
The system can beamform the
DMRS, keep it within a scheduled resource, and transmit it only when
necessary in either DL or UL. Additionally, multiple orthogonal DMRSs can be
allocated to support MIMO transmission. The network presents users with
DMRS information early on for the initial decoding requirement that low-
latency applications need, but it only occasionally presents this information for

ni.com/
5g/nr
1 5G New Radio: Introduction to the Physical
Layer low-speed scenarios in which the channel shows little change.
Alternatively, tracking fast changes in high-mobility scenarios might increase
the rate of transmission (called “additional DMRS”).
2. Phase Tracking Reference Signal (PTRS)—As mentioned before, the phase
noise of the transmitters increases as the frequency of operation increases.
The PTRS plays a crucial role especially at mmWave frequencies to minimize
the effect of the oscillator phase noise
on system performance. One of the main problems that phase noise introduces into an

ni.com/
5g/nr
2 5G New Radio: Introduction to the Physical
Layer

OFDM signal appears as a common phase rotation of all the subcarriers, known
as common phase error (CPE). The NR system typically maps the PTRS
information to a few subcarriers per symbol because the phase rotation affects
all subcarriers within an OFDM symbol equally but shows low correlation from
symbol to symbol. The system configures the PTRS depending on the quality of
the oscillators, carrier frequency, SCS, and modulation and coding schemes
that the transmission uses.
3. Sounding Reference Signal (SRS)—As a UL-only signal, the SRS is transmitted
by the UE to help the system obtain the channel state information (CSI) for
each user. This information describes how the NR signal propagates from the
transmitter to the receiver and represents the combined effect of scattering,
fading, and power decay with distance, for example. The system uses the SRS
for resource scheduling, link adaptation, Massive MIMO, and beam
management.
4. Channel State Information Reference Signal (CSI-RS)—As a DL-only signal,
the CSI-RS the UE receives is used to estimate the channel and report
channel quality information back to the gNB. During MIMO operations, NR
uses different antenna approaches based on the carrier frequency. At lower
frequencies, the system uses a modest number of active antennas for MU-
MIMO and adds FDD operations. In this case, the UE needs the CSI-RS to
MIM calculate the CSI and report it back in the UL direction.
O

With the goal of using the spectrum more efficiently and serving more users, NR
plans to take full advantage of MU-MIMO technology. MU-MIMO adds multiple
access (multiuser) capabilities to MIMO by exploiting the distributed and
uncorrelated spatial location of those multiple users. In this configuration, the
gNB sends the CSI-RS to UE in the coverage area, and based on the SRS
response of each UE device, the gNB computes the spatial location of each
receiver. The streams of data destined for each receiver go through a precoding
matrix (W-Matrix), where the data symbols get combined into signals streaming
to each of the elements of the gNB’s antenna array5 (see Figure 15.)

h00
h10

X0 r0
UE
S0 TX ESTIMATES S0

h01
h11
W

X1 r1 UE
S
1 ESTIMATES
1
S

gNB COMPUTERS
PRECODING W-
MATRIX

ni.com/
5g/nr
2 5G New Radio: Introduction to the Physical
Layer Figure 15. Representation of MU-MIMO on the DL

ni.com/
5g/nr
2 5G New Radio: Introduction to the Physical
Layer

The multiple data streams have their own independent and appropriate
weightings that apply different phase offsets to each stream so that the
waveforms interfere constructively and arrive in phase at each receiver. This
maximizes the signal strength at each user’s location while presenting minimum
signal strength (a null) in the directions of the other receivers,
as Figure 16 shows.

gNB MU-MIMO ANTENNA ARRAY

Signal
Signal Null

Null Maximum Maximum Directivity

and Signal Strength
Directivity
and Signal Strength

CÁC THIẾT BỊ UE ĐA TRUY CẬP KHÔNG GIAN (SPATIALLY MULTIPLEXED)

Figure 16. MIMO Beamforming for Spatial Multiplexing

Consequently, the gNB talks to multiple UE devices independently and

simultaneously, effectively multiplexing them in space. As an additional benefit,
in this MU-MIMO implementation, the UE devices don’t need any knowledge of
the channel or additional processing to get their data streams.

MU-MIMO on the DL boosts the NR system’s capacity. It scales with the minimum
of the number of gNB antennas and the sum of the number of UE devices
multiplied by the number of antennas per UE device. In other words, MU-MIMO
can achieve MIMO capacity gains with gNB antenna arrays and much simpler
single-antenna UE devices.

Massive MIMO for 5G

Taking the MIMO approach a step further, a Massive MIMO configuration is
implemented when a system has many times more gNB antennas than the
number of UE devices per signaling resource. The large number of gNB antennas
relative to the number of UE devices can yield huge gains in spectral efficiency.
Such conditions enable the system to serve many more devices simultaneously
within the same frequency band compared with today’s 4G systems (see Figure
17). NI, along with industry leaders such as Samsung, continues to demonstrate
the viability of Massive MIMO systems through its platform of software defined
radio and flexible software for rapid wireless prototyping.

ni.com/
5g/nr
2 5G New Radio: Introduction to the Physical
Layer

Figure 17. Multiantenna Array for Massive MIMO

Currently, the strongest case for Massive MIMO operation is at frequencies below
6 GHz. Spectrum is scarce and valuable in this region. In these bands, Massive
MIMO systems can achieve significant spectral efficiency by spatially multiplexing
many terminals. The system can also achieve superior energy efficiency by
exploiting large antenna array gains to lower the amount of power that each front
end must handle.

In Massive MIMO systems, each antenna has its own RF and digital baseband
chain. The gNB maintains tight phase control and processes the signals from all
antennas. The system can gain a fuller picture of the channel response on the UL
and respond quickly to changes in the channel using digital processing. Massive
MIMO operates mainly in TDD, which permits the assumption of channel
reciprocity. That enables the system to estimate DL channels from UL pilots and
eliminates the need for prior knowledge of the channel.

Another advantage of future Massive MIMO systems is that they’ll provide better
and more consistent service to all UE in a coverage area. Because of an improved
link budget and the ability to place target UE precisely within the radiated beam
while nulling nontarget UE (spatial resolution), power control algorithms can
achieve greater fairness among the UE.

User mobility can limit how well Massive MIMO solutions scale up in
performance. For proper channel estimation, the system needs to send UL pilots
and payload in the UL direction. The faster UE moves, the shorter the channel
coherence time. For example, in large coverage areas with fast UE, such as a car
traveling at 120 km/h on a highway, the channel’s coherence time at 2 GHz
carrier frequency drops to around 1 ms. This requires the system to recalculate
the channel 1000 times per second to track the UE as it moves and limit the
multiplexing gain to a smaller number of terminals. Figure 18 shows how
coherence time scales with both carrier frequency and UE speed. Conversely, in
more controlled environments with little or no mobility such as fixed wireless
access, the system can accommodate hundreds of terminals through spatial
multiplexing using narrow beams.

ni.com/
5g/nr
 2 5G New Radio: Introduction to the Physical
Layer

KHOẢNG THỜI GIAN LIÊN KẾT KÊNH

Uplink Pilots Matrix Downlink Symbols

Computation
Frequency 3 km/h 30 km/h 120 500
km/h km/h
2 GHz 45 ms 4.5 ms 1.125 ms 27 µs
Limited Mobility
28 GHz 3.2 ms 320 µs 80 µs 19 µs

Figure 18. Effect of Mobility on Channel Coherence Time

mmWave for 5G
Industry and academic researchers consider available mmWave bands as the
next frontier to serve the data-hungry wireless applications of the future. New
5G systems operating at 28 GHz and above offer more available spectrum for
larger channels, which work well for multi-Gbps links. Although these
frequencies see less spectral crowding than those below 6 GHz, they
experience different propagation effects such as higher free-space path loss and
atmospheric attenuation, weak indoor penetration, and poor diffraction around
objects.

To overcome these undesired effects, mmWave antenna arrays can focus their
beams and take advantage of the antenna array gain. Fortunately, the size of
these antenna arrays decreases as the frequency of operation increases, allowing
a mmWave antenna array with many elements to take up the same area that a
single sub-6 GHz element occupies (Figure 19).

SINGLE-ELEMENT PATCH ANTENNA 64-ELEMENT ANTENNA ARRAY (30 GHz)

Omnidirection
Highly Directive
al Radiation
Radiation Pattern
Pattern
mm

mm
60

60

60 mm 60 mm

Figure 19. Comparison of mmWave and Sub-6 GHz Antenna Arrays

ni.com/
5g/nr
2 5G New Radio: Introduction to the Physical
Layer

Analog Beam Steering to Manage Complexity

As presented above, Massive MU-MIMO systems require far more transmit RF
chains than UE devices for proper spatial multiplexing. This differs from a system
comprising just one RF chain that feeds many antennas, the phase of which is
controlled analogically to focus and steer the radiation pattern (see Figure 20).
For MU-MIMO purposes, such a system can be categorized as a single-antenna
terminal that happens to have a directive, steerable antenna.

ANALOG PHASE CONTROL

Baseband Chain
RF Chain

NA

PHASE SHIFTER

Figure 20. Single RF Chain With Analog Beam Steering

One of the main drawbacks of Massive MIMO systems is the high complexity
and cost of integrating and deploying a massive number of RF chains,
especially at mmWave frequencies. Researchers have proposed several
hybrid (digital and analog) beamforming alternatives6 to allow 5G gNBs to
maintain a high number of antennas while keeping the
MU-MIMO implementation costs down. Figure 21 illustrates a hybrid system with
a common baseband processing stage that feeds multiple data streams to their
corresponding RF chains. These streams undergo digital beamforming signal
processing before moving on to the analog stage. At this last stage, the system
applies beam steering with analog phase shifters, which focus the beam toward
a specific direction. This results in spatially multiplexed RF streams contained
within a directional beam.

ni.com/
5g/nr
2 5G New Radio: Introduction to the Physical
Layer

ANALOG PHASE CONTROL

RF Chain

NA

Baseband Processing
W-Matrix

ND
RF Chain

NA

PHASE SHIFTER

Figure 21. Hybrid Digital and Analog Beamforming

Finally, recall that the channel coherence time decreases significantly at

mmWave frequencies, which places tough restrictions on mobility applications.
Researchers continue to investigate new ways to improve UE mobility at
mmWave frequencies, but most likely the first 5G mmWave deployments will
serve fixed wireless access applications such as backhaul and sidelink.

Gaining Access and Managing Beams

Managing the large signal propagation loss at frequencies above 20 GHz is one
of the main technical challenges of operating in mmWave bands. In practical
terms, this loss reduces the possible cell coverage area and range. To
compensate for it, the standard designers settled on beamforming technology
with antenna arrays as a way of focusing the RF energy toward individual users
and boosting the signal gain. However, UE can no longer rely on the mmWave
gNBs to broadcast omnidirectional signals to establish the first connection.

The NR standard implements a new procedure for UE to gain initial access to the
gNB. Upon arrival to a new cell coverage area, UE is blind to the location of the
beam, ignoring the direction in which the gNB is currently transmitting to begin
the network access procedure.

ni.com/
5g/nr
2 5G New Radio: Introduction to the Physical
Layer

The NR initial access procedure presents an elegant solution for UE to establish

communication with the gNB. It solves the problem of finding the gNB in the dark
not only for mmWave operation but also for sub-6 GHz omnidirectional operation.
This means that the initial access procedure must work in single-beam and
multibeam scenarios. It also must support NR and LTE coexistence.

The procedure follows the steps depicted in Figure 22.

gNB UE

Synchronization Signals

Beam-
Sweeping Basic System Information for All UEs
Transmission

Random Access Channel

Beam-
Sweeping
ReceptionRandom Access Response and System Information
UE-Specific
Selected
Beam

UE-Specific Data and Control Channels

Beamforming

Figure 22. Initial Access Procedure

1. Beam-sweeping transmission—The gNB transmits the physical broadcast

channel (PBCH) in groups of four OFDM symbols called synchronization signal
blocks (SS blocks) sequentially in multiple directions, as depicted by the blue,
green, and yellow beams, and maps each one to a different spatial direction.
Using the concept of beam sweeping, the gNB transmits both the
synchronization signals and the system configuration information that the UE
needs to access the network.
2. Beam-sweeping reception—The UE detects the best SS block (strongest
detected beam) by listening until it matches the beam direction of the
transmitter. This allows the UE to decode the best SS block and extract its time
index. Knowing when the gNB will use that beam direction again, the UE
transmits to the gNB on the physical random access channel (PRACH) at the
right time. The gNB now knows in which direction and at what time the UE will
transmit UL information.
3. UE-specific selected beam—Once the UE and gNB establish communication
on the best beam, the gNB sends the rest of the system information that the
UE needs to set up a connection with the gNB.
4. UE-specific beamforming—At this point, the system can switch from general,
wider beam coverage to UE-specific coverage with a narrower beam using
beam-refining procedures.

ni.com/
5g/nr
2 5G New Radio: Introduction to the Physical
Layer

Bandwidth Parts
In future 5G applications, a large range of devices and equipment will have to
operate successfully across many different bands with varied spectrum
availability. An example is UE with limited RF bandwidth needing to operate
alongside a more powerful device that can fill a whole channel using carrier
aggregation and a third device that can cover the whole channel with a single RF
chain7 (Figure 23).

Wideband UE

Carrier Aggregated UE

Narrowband UE

Resource Block
NR Carrier From the Network Perspective

Figure 23. Parts to Manage the Spectrum More Efficiently

Though wide bandwidth operation has a direct effect on the data rates that
users can experience, it comes at a cost. When UE doesn’t need high data rates,
wide bandwidth leads to inefficient use of RF and baseband processing
resources.

To address this, the 3GPP developed the new concept of bandwidth parts (BWPs):
the network configures certain UE with one wideband carrier and separately
configures other UE with a set of intraband contiguous component carriers using
carrier aggregation. This allows for a greater diversity of devices with varying
capabilities to share the same wideband carrier. This kind of flexible network
operation that adjusts to UE’s differing RF capability does not exist in LTE.

A BWP consists of a group of contiguous PRBs. Each BWP has an associated SCS
and CP (numerology). As a result, the system can use the BWP to reconfigure UE
with a certain numerology. UE starts out with a default active BWP during the
initial access until the system configures the UE’s BWPs explicitly during or after
connection establishment. Figure 25 shows that the network is allocating two
BWPs (BWP 1 and 2) to one UE device while reserving a third, full-channel,
overlapping BWP (BWP 3) for potential use by another higher bandwidth
UE device or application.

ni.com/
5g/nr
2 5G New Radio: Introduction to the Physical
Layer

BWP 3

BWP 1 BWP 2

Numerology 1 Numerology

2 Figure 24. Bandwidth Parts

The system can configure DL and UL BWPs for each serving cell separately and
independently. In Release 15, only one BWP in DL and one in UL are active at any
point in time, but the UE can have up to four configured BWPs.

To summarize, NR will have the flexibility to serve many different use cases
effectively by using BWPs, for example:

PRB

Supporting UE that
has narrow RF
capabilities and
reducing energy BWP
consumption when a
device doesn’t
require full
bandwidth operation

BWP 3

Supporting multiple
numerologies and
BWP 1 BWP 2
allowing operation in
noncontiguous
spectrum

Numerology 1 Numerology 2

Enabling forward
compatibility with
devices and TBD Future Application
BWP
applications that the
market will define in
the future

Figure 25. NR Serving Many Use Cases With BWPs

ni.com/
5g/nr
3 5G New Radio: Introduction to the Physical
Layer

Conclusion: Comparing LTE and 5G NR PHY

Now compare the fundamental technical features of 5G NR with those of current
LTE implementations.

Better Spectrum Utilization

Wideband 5G carriers are planned to occupy up to 98 percent of the channel,
avoiding large guard bands between carriers. This helps reduce channel overhead
and allows for faster load balancing than LTE aggregated carriers can implement.
For example, Figure 27 compares five 20 MHz aggregated LTE carriers versus a
proposed single 98 MHz 5G NR carrier.

LTE 5x20 MHz

100 MHz

18 MHz 18 MHz 18 MHz 18 MHz 18 MHz

5G 100 MHz

100 MHz

Up to 98 MHz

Figure 26. Improved Channel Utilization With Wideband 5G Carriers

Flexible Numerology and Frame Structure

LTE uses fixed 15 kHz SCS with a maximum of 1200 subcarriers in a 20 MHz
channel. In contrast, NR allows for greater spectrum utilization with channels of
various sizes, variable SCS and slot length, and a maximum of 3300 subcarriers
per channel.

Enhanced Efficiency With Leaner Signaling

Unlike LTE, which transmits cell-specific reference signals four times per
millisecond, synchronizes every 5 ms, and broadcasts every 10 ms, 5G has
no cell-specific reference signals and synchronizes and broadcasts every 20
ms. This enables greater base station power savings.

ni.com/
5g/nr
3 5G New Radio: Introduction to the Physical
Layer

LTE

Synchronization Broadcast LTE Reference Signals

20 ms
5G NR

Figure 27. Signaling Efficiency in LTE Versus 5G NR

Manage TDD Resources Dynamically

LTE has a fixed, static TDD structure that allocates slots to either DL, UL, or
synchronization and control signals. That is, within a radio frame, LTE TDD
switches multiple times between DL and UL transmission and vice versa. On the
other hand, within a slot, 5G can change dynamically between DL and UL to
handle traffic demands in either direction.

Download Guard Uplink LTE

Slot 0
Slot 1 Slot 2

Download UL DL DL UL 5G

Slot 0
Slot 1 Slot 2

Figure 28. NR Manages TDD Resources More Dynamically

Operation at mmWave Frequencies With Wider Channels

Today’s licensed LTE networks are limited to operating at a maximum frequency
of around 3800 MHz. The 5G NR networks will take advantage of both existing
cellular bands and wide channels in newly licensed spectrum around 30 and 40
GHz.

The specification of higher bandwidth channels and multiple numerology options

will enable NR systems to operate in sub-6 GHz bands and mmWave bands with
appropriate handling of multipath delay spread, channel coherence time, and
phase noise. Furthermore, NR will support existing and new services with even
higher data rates and address different latency and mobility requirements by
changing the transmission turnaround time using variable SCS and by allocating

ni.com/
5g/nr
3 5G New Radio: Introduction to the Physical
Layer wide channels around and above 28 GHz. NR will use the latest

ni.com/
5g/nr
3 5G New Radio: Introduction to the Physical
Layer

developments in Massive MIMO and beamforming technology to maximize

spectral efficiency and guarantee better service for a larger number of users.

Additionally, considering the commercial practicalities of deploying different UE

with different RF capabilities, the new BWP concept in NR will lead to more
energy-efficient UE operation and superior spectrum management.

In conclusion, 5G wireless technology promises to deliver an abundance of

reliable, data-rich, and highly connected applications for more of the world’s
population. Although deploying an infrastructure that can support this and
creating the next generation of 5G devices present significant design and test
challenges, NI’s platform-based approach to designing, prototyping, and testing
wireless technologies will be key in making 5G a reality within the next decade.
Glossar
y
3GPP Third Generation Partnership

Project AR augmented reality

BCH broadcast channel

BPSK binary phase-shift keying

BS base station

BWP bandwidth parts

CP cyclic prefix

CP-OFDM cyclic prefix orthogonal frequency division

multiplexing CSI-RS Channel State Information Reference

Signal

DFT discrete Fourier transform

DFT-SOFDM discrete Fourier transform spread orthogonal frequency division

multiplexing DL downlink

DMRS demodulation reference

signal eMBB Enhanced Mobile

Broadband

EPC Evolved Packet Core

EVM error vector magnitude

FDD frequency division duplex

FDMA frequency division multiple

access FFT fast Fourier transform

ni.com/
5g/nr
 3 5G New Radio: Introduction to the Physical
GP Layer
guard
period

ni.com/
5g/nr
3 5G New Radio: Introduction to the Physical
Layer

gNB g node b, a 5G base station

mMTC Massive Machine-Type

Communication mmWave millimeter

wave

MIMO multiple input, multiple

output MU-MIMO Multiuser MIMO

NR New Radio

OFDM orthogonal frequency division

multiplexing PA power amplifier

PAPR peak-to-average power ratio

PBCH physical broadcast channel

PRACH Physical Random Access

Channel PRB physical resource block

PTRS phase-tracking reference signal

QAM quadrature amplitude modulation

QPSK quadrature (quaternary) phase-shift

keying RAN radio access network

SCS subcarrier spacing

SRS Sounding Reference Signal

TDD time division duplex(ing)

TDM time division multiplexing

UE user equipment

UL uplink (reverse link)

URLLC Ultra-Reliable Low-Latency

Communication VR virtual reality

ni.com/
5g/nr
3 5G New Radio: Introduction to the Physical
Layer

Endnotes
1
3GPP TS 38.101-1 V15.0.0 (2017-12) Technical Specification Radio Access Network; NR; User Equipment (UE)
radio transmission and reception.
2
3GPP TS 38.211 V15.0.0 (2017-12) Technical Specification Radio Access Network; NR; Physical channels and modulation.
3
3GPP TR 38.912 V14.1.0 Technical Specification Group Radio Access Network; Study on New Radio (NR)
access technology (Release 14.)
4
3GPP TS 38.214 V15.0.0 (2017-12) Technical Specification Radio Access Network; NR; Physical layer procedures for data.
5
Q. H. Spencer, C. B. Peel, A. L. Swindlehurst, and M. Haardt, “An introduction to the multi-user MIMO
downlink,” in IEEE Communications Magazine, vol. 42, no. 10, pp. 60–-67, Oct. 2004.
6
S. Han, C. l. I, Z. Xu, and C. Rowell, “Large-scale antenna systems with hybrid analog and digital beamforming for
millimeter wave 5G,” in IEEE Communications Magazine, vol. 53, no. 1, pp. 186–194, January 2015.
7
3GPP TS 38.213 V15.0.0 (2017-12) Technical Specification Radio Access Network; NR; Physical layer procedures for control.

©2018 National Instruments. All rights reserved. National Instruments, NI, and ni.com are trademarks of National Instruments. Other product and company
names listed are trademarks or trade names of their respective companies. 32178

ni.com/
5g/nr