Uplink Grant-Free Random Access Solutions
for URLLC services in 5G New Radio
Nurul Huda Mahmood
1
, Renato Abreu
2
, Ronald B
¨
ohnke
3
, Martin Schubert
3
,
Gilberto Berardinelli
2
and Thomas H. Jacobsen
4
1
Center for Wireless Communication, University of Oulu, Finland.
2
Department of Electronic Systems, Aalborg
University, Denmark.
3
Huawei Technologies Duesseldorf GmbH, Munich Research Center, Germany.
4
Nokia Bell Labs, Aalborg, Denmark.
Emails: nurulhuda.mahmood@oulu.fi, {rba, gb}@es.aau.dk, {first name.last name}@huawei.com,
thomas.jacobsen@nokia.com
Abstract
The newly introduced ultra-reliable low latency communication service class in 5G New Radio depends on
innovative low latency radio resource management solutions that can guarantee high reliability. Grant-free random
access, where channel resources are accessed without undergoing assignment through a handshake process, is
proposed in 5G New Radio as an important latency reducing solution. However, this comes at an increased
likelihood of collisions resulting from uncontrolled channel access, when the same resources are preallocated
to a group of users. Novel reliability enhancement techniques are therefore needed. This article provides an
overview of grant-free random access in 5G New Radio focusing on the ultra-reliable low latency communication
service class, and presents two reliability-enhancing solutions. The first proposes retransmissions over shared
resources, whereas the second proposal incorporates grant-free transmission with non-orthogonal multiple access
with overlapping transmissions being resolved through the use of advanced receivers. Both proposed solutions
result in significant performance gains, in terms of reliability as well as resource efficiency. For example, the
proposed non-orthogonal multiple access scheme can support a normalized load of more than 1.5 users/slot at
packet loss rates of ∼ 10
−5
− a significant improvement over the maximum supported load with conventional
grant-free schemes like slotted-ALOHA.
Index Terms
URLLC, Grant-free random access, 5G NR, NOMA.
I. INTRODUCTION
Ultra-reliable low latency communication (URLLC) is a new service class introduced in Fifth-generation
New Radio (5G NR) cellular standard [1]. The reliability and latency levels offered by URLLC improve those
of earlier generations of cellular standards. Examples include, isochronous real-time communication for factory
and process automation in Industry 4.0 scenarios, vehicular-to-anything (V2X) communication in Automotive
sector and haptic communication for tactile Internet.
This work has been submitted to the IEEE for possible publication. Copyright may be transferred without notice, after which this version
may no longer be accessible.
2
The key design challenge for URLLC is to ensure low latency and high reliability simultaneously. In
the absence of a tight latency constraint, any desired level of reliability can be achieved by coding over
larger blocklengths and introducing sufficient redundancy, including re-transmissions. The scheduling and the
transmission delay are the two primary latency inducing components of a communication protocol at the lower
layers that can be influenced by system design. The former is the time it takes from the point a packet arrives
at the lower layer of a transmitter until it can access the channel, whereas the latter is the time it takes to
successfully deliver the message. Other sources of latency include processing delays at the transmitter/receiver,
propagation delay and queuing at higher layers.
The minimum scheduling unit in Long Term Evolution (LTE) is, in general, limited to the transmission time
interval (TTI) of one millisecond (ms). Whereas, 5G NR has introduced the concept of ‘mini-slots’ consisting
of 1 − 13 orthogonal frequency division multiplexed (OFDM) symbols, along with support for a scalable
numerology allowing the sub-carrier spacing (SCS) to be expanded up to 240 kHz. Collectively, this allows
transmissions over shorter intervals. For example, a URLLC mini-slot of 2 OFDM symbols at 60 kHz SCS
corresponds to a transmission time of only 35.7 micro seconds [1].
Access to the wireless channel is generally controlled by a grant based (GB) scheduling mechanism where
users attempting to access the channel have to first obtain an access grant through a four-way handshake
procedure. This ensures that the user has exclusive rights to the channel, thus avoiding any potential collisions,
at the expense of large latency and signalling overhead [3]. Grant-free (GF) random access, where the grant
acquisition by the user prior to transmission is skipped, is proposed as a solution to reduce the access latency [4].
With GF transmissions, a user with available traffic transmits the data (along with required control informa-
tion) in the first transmission itself. GF transmissions can be preallocated over dedicated resources, or shared
among multiple users through contention. The former is better suited for periodic traffic with a fixed pattern,
whereas the latter is more resource utilization efficient and flexible, especially in case of sporadic traffic.
GF transmissions over shared resources are subject to potential collision with other neighbouring users
transmitting simultaneously, thus jeopardizing the transmission reliability. Techniques to improve the supported
load with GF random access while ensuring high reliability and low latency is are therefore currently being
discussed in academic research and in standardization bodies [5]. State of the art solutions include GF trans-
missions with K-repetition, where a pre-defined number of replicas are transmitted, and proactive repetition,
where the transmission is proactively resent until an acknowledgment is received (also known as repetitions
with early termination) [4].
This article discusses GF random access in the uplink as an enabler for URLLC in 5G NR. The main
contribution is two-fold, namely: i) giving an overview of GF random access in 5G NR and discussing its
shortcomings, and ii) presenting two advanced GF schemes that go beyond 5G NR. In particular, the first
proposal presented in Section IV-A introduces a novel transmission scheme where dedicated resources are
allocated for the initial transmission, whereas blind retransmissions occur over shared radio resources. The
combination of non-orthogonal multiple access (NOMA) and GF access is considered next in Section IV-B.
NOMA relaxes the paradigm of orthogonal transmissions by allowing different users to concurrently share the
same physical resources in time, frequency, and space. More specifically, NOMA techniques are exploited at the
transmitter end to improve the reliability and resource efficiency, while advanced receivers are used to resolve
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3
collisions at the receiver end.
II. OVERVIEW OF URLLC DISCUSSIONS IN 3GPP
We present a brief summary of the ongoing discussion pertinent to URLLC in 3GPP Release-15, 16 and 17
in this section. The objective is to provide the reader with an overview of the important research directions in
URLLC in the context of 5G NR standardization.
The first phase of 5G NR standard covering the most basic set of use cases envisioned for 5G was completed
within Release-15 and finalized in September 2018. The second phase of 5G NR standard, bringing the full 3GPP
5G system to its completion, is targeted by Release-16 and will be finalized at the end of 2019. Currently, there
are around 25 Release-16 study items covering a variety of topics, with a number of them involving URLLC
services. Finally, Release-17 will look into emerging topics to be studied for 5G evolution systems in 2020 and
onwards.
URLLC related discussions in 3GPP NR Release-15 were primarily grouped into four different study items.
The first dealt with the support of separate channel quality indicator (CQI) and modulation and coding scheme
(MCS) tables for URLLC and the option of configuring two block error rate (BLER) targets for CQI reporting.
The second agenda item studied the potential benefits of introducing a new Downlink Control Information
(DCI) format with a smaller payload. Using a smaller DCI size permits lowering the DCI code rate, which
in turn allows robust transmission on the user plane. The necessity of Physical Downlink Control Channel
repetition, which can be useful in achieving high reliability in certain scenarios such as GF transmission, was
investigated next. The final item was a study on handling uplink multiplexing of transmission with different
reliability requirements, which considered both intra-UE and inter-UE multiplexing.
Discussions on 5G-NR URLLC in Release-16 are grouped into three different study items. The first deals with
Layer-1 enhancements, including potential control channel and processing timeline improvements. The second
studies the potential benefits of uplink inter-UE transmission prioritization and multiplexing. GF transmission,
in particular, is enabled in Release-15 by so-called “Configured Grant” operations [1]. A study item is focused
on enhancing such operations, including methods for explicit hybrid automatic repeat request acknowledgement
(HARQ-ACK), to ensure K-repetitions, and mini-slot repetitions.
URLLC studies in Release-17 will mainly focus on use cases and end-to-end performance of different
applications, such as i) audio-visual service production requiring tight synchronization and lowlatency, ii)
communication services for critical medical applications including robotic aided surgery, and iii) support for
unmanned aerial systems connectivity, identification, and tracking. The interested reader is referred to [6] for
further details.
III. THE BASICS OF GRANT-FREE RANDOM ACCESS
The conventional GB scheduling procedure in LTE networks involves exchanging multiple messages between
nodes to facilitate exclusive channel access. Due to the tight latency requirement and the associated signaling
overhead, such GB schemes are not suitable for URLLC applications. GF schemes using semi-static configu-
rations are an option to remove the signaling overhead caused by the request-followed by-grant procedure and
to reduce the latency.
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The needed control information, such as time and frequency resource allocation, MCS, power control settings
and HARQ related parameters, are configured by Radio Resource Control (RRC) signaling prior to the GF
transmissions. In the uplink, the configured devices are connected and synchronized, thus being always ready
for a URLLC transmission. The configured resources can also be shared by a number of users to increase the
resource efficiency in case of sporadic traffic. However, transmissions are then susceptible to potential collisions
from simultaneous transmissions of neighbouring nodes.
This section presents a system-level simulations based performance comparison of uplink GB and several
GF transmission procedures considering a large urban macro network.
A. The Considered Grant-Free Transmission Schemes
Four GF schemes are considered, namely reactive, K-repetition, proactive and GF scheme with power boost,
as illustrated in Figure 1. In the reactive scheme, when the UE has finalized its initial uplink data transmissions,
its signal is processed at the BS, which will then transmit a HARQ feedback (ACK/NACK). The UE re-transmits
the same payload upon reception of a NACK. The time duration of the cycle from the beginning of a transmission
until the processing of its feedback is called the HARQ round trip time (RTT). It is assumed that the BS spends
one mini-slot for processing and one mini-slot for transmitting the feedback.
The UE is configured to autonomously transmit the same packet K times before waiting for a feedback from
the BS in the K-repetition scheme. Each repetition can be identical, or consist of different redundancy versions
of the encoded data. This method eliminates the RTT latency at the expense of potential resource wastage if
the needed number of repetitions is overestimated.
Similar to the K-repetition scheme, the UE aims at repeating the initial transmission for a number of times
in the proactive scheme; with a feedback received after each transmission. This allows the UE to stop the chain
of repetitions earlier in case of a positive feedback.
The GF scheme with power boost is similar to the reactive scheme with the addition that the transmit power
of each retransmission is higher than that of the previous transmission. This is motivated by the fact that HARQ
retransmissions within the latency deadline should be prioritized to improve the success probability [7]. Here,
we discuss the applicability of improved open loop power control for GF transmissions. The considered power
control is given by: P [dBm] = min{P
max
, P
0
+ 10 log
10
(M) + αP L + g(k)}, where P
max
is the maximum
transmit power, P
0
is the target receive power per resource block, M is the number of resource blocks, α is
the fractional path loss compensation factor, P L is the path loss and the function g(k) gives a power boosting
step for the k
th
transmission.
B. Performance Evaluation Methodology
The simulation assumptions and parameters used for this study are in line with the guidelines for NR
performance evaluations presented in [2]. A total of 21 macro cells are simulated with an inter-site distance
of 500 meters, and uniformly distributed outdoor UEs. Linear minimum-mean square error with interference
rejection combining (MMSE-IRC) and multi-user detection receiver is assumed in this case. A 10 MHz band
within the 4 GHz carrier is considered. Open loop power control is used by the UE to compensate the coupling
loss. The MCS is pre-configured as very conservative (QPSK with coding rate
1
/
8
), which permits the UE
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5
P2
P1
Scheduling request
Scheduling Grant
Transmission
Transmission
Feedback (NACK)
Retransmission
Transmission (K)
Transmission (1)
…
Transmission (k)
Transmission (1)
…
Feedback (ACK)
Grant-based transmission
Reactive GF transmission
K-repeat GF transmission
Proactive GF transmission
Transmission
Feedback (NACK)
Retransmission
GF transmission with power boost
TTI
Tx
NACK
re-Tx
Feed-
back
Fig. 1. The analysed state-of-the-art Grant-Free Uplink HARQ Schemes for URLLC.
to transmit a 32 byte payload in a two OFDM symbols mini-slot using the full band. The conventional GB
scheme is considered as the baseline for comparison. The same MCS is used for the retransmissions, with
Chase combining (CC) of the retransmitted packets at the receiver end.
C. Performance Results
Results are presented in terms of the one-way latency for multiple payload transmissions. In Figure 2,
the empirical Complementary Cumulative Distribution Functions (CCDF) of the latency for the different GF
transmission schemes are shown along with the baseline GB scheme at low load (10 UEs/cell, and Poisson
arrival rate of 10 packets per second for each UE). The horizontal axis represents the latency (in ms), whereas
the vertical axis displays the outage probability.
The GF schemes clearly provide lower latencies for the same reliability compared to the GB reference. The
reactive scheme provides the best reliability for the first transmission among the GF schemes. The staircase
behaviour is caused by the HARQ RTT between the retransmissions. The K-Repetition scheme with two
repetitions presents similar shapes for the first and second consecutive transmissions, and is capable of providing
one ms latency at a reliability greater than 1 − 10
−5
in the low load case.
The Proactive scheme is able to achieve a very low outage performance. However, due to the feedback RTT,
it is not able to terminate early before at least four consecutive transmissions, resulting in excessive resource
usage.
On the other hand, HARQ with power boost can improve the outage performance of the reactive scheme.
However, the gain is limited in macro scenarios, since UEs tend to operate using maximum transmit power,
thus not being able to apply the power boost in many cases.
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