7 research outputs found
Non-Orthogonal Contention-Based Access for URLLC Devices with Frequency Diversity
We study coded multichannel random access schemes for ultra-reliable
low-latency uplink transmissions. We concentrate on non-orthogonal access in
the frequency domain, where users transmit over multiple orthogonal subchannels
and inter-user collisions limit the available diversity. Two different models
for contention-based random access over Rayleigh fading resources are
investigated. First, a collision model is considered, in which the packet is
replicated onto available resources, of which are received
without collision, and treated as diversity branches by a maximum-ratio
combining (MRC) receiver. The resulting diversity degree depends on the
arrival process and coding strategy. In the second model, the slots subject to
collisions are also used for MRC, such that the number of diversity branches
is constant, but the resulting combined signal is affected by multiple
access interference. In both models, the performance of random and
deterministic repetition coding is compared. The results show that the
deterministic coding approach can lead to a significantly superior performance
when the arrival rate of the intermittent URLLC transmissions is low.Comment: 2019 IEEE 20th International Workshop on Signal Processing Advances
in Wireless Communications (SPAWC) - Special Session on Signal Processing for
NOMA Communication System
How URLLC can Benefit from NOMA-based Retransmissions
Among the new types of connectivity unleashed by the emerging 5G wireless
systems, Ultra-Reliable Low Latency Communication (URLLC) is perhaps the most
innovative, yet challenging one. Ultra-reliability requires high levels of
diversity, however, the reactive approach based on packet retransmission in
HARQ protocols should be applied carefully to conform to the stringent latency
constraints. The main premise of this paper is that the NOMA principle can be
used to achieve highly efficient retransmissions by allowing concurrent use of
wireless resources in the uplink. We introduce a comprehensive solution that
accommodates multiple intermittently active users, each with its own HARQ
process. The performance is investigated under two different assumptions about
the Channel State Information (CSI) availability: statistical and
instantaneous. The results show that NOMA can indeed lead to highly efficient
system operation compared to the case in which all HARQ processes are run
orthogonally
Random Access Protocol with Channel Oracle Enabled by a Reconfigurable Intelligent Surface
The widespread adoption of Reconfigurable Intelligent Surfaces (RISs) in
future practical wireless systems is critically dependent on the design and
implementation of efficient access protocols, an issue that has received less
attention in the research literature. In this paper, we propose a grant-free
random access (RA) protocol for a RIS-assisted wireless communication setting,
where a massive number of users' equipment (UEs) try to access an access point
(AP). The proposed protocol relies on a channel oracle, which enables the UEs
to infer the best RIS configurations that provide opportunistic access to UEs.
The inference is based on a model created during a training phase with a
greatly reduced set of RIS configurations. Specifically, we consider a system
whose operation is divided into three blocks: i) a downlink training block,
which trains the model used by the oracle, ii) an uplink access block, where
the oracle infers the best access slots, and iii) a downlink acknowledgment
block, which provides feedback to the UEs that were successfully decoded by the
AP during access. Numerical results show that the proper integration of the RIS
into the protocol design is able to increase the expected end-to-end throughput
by approximately 40% regarding the regular repetition slotted ALOHA protocol.Comment: 30 pages, 7 figures, journal pape
A Framework for Control Channels Applied to Reconfigurable Intelligent Surfaces
The research on Reconfigurable Intelligent Surfaces (RISs) has dominantly
been focused on physical-layer aspects and analyses of the achievable
adaptation of the propagation environment. Compared to that, the questions
related to link/MAC protocol and system-level integration of RISs have received
much less attention. This paper addresses the problem of designing and
analyzing control/signaling procedures, which are necessary for the integration
of RISs as a new type of network element within the overall wireless
infrastructure. We build a general model for designing control channels along
two dimensions: i) allocated bandwidth (in-band and out-of band) and ii) rate
selection (multiplexing or diversity). Specifically, the second dimension
results in two transmission schemes, one based on channel estimation and the
subsequent adapted RIS configuration, while the other is based on sweeping
through predefined RIS phase profiles. The paper analyzes the performance of
the control channel in multiple communication setups, obtained as combinations
of the aforementioned dimensions. While necessarily simplified, our analysis
reveals the basic trade-offs in designing control channels and the associated
communication algorithms. Perhaps the main value of this work is to serve as a
framework for subsequent design and analysis of various system-level aspects
related to the RIS technology.Comment: Submitted to IEEE TWC, the copyright may be transferred without
further notic
Common Message Acknowledgments: Massive ARQ Protocols for Wireless Access
Massive random access plays a central role in supporting the Internet of
Things (IoT), where a subset of a large population of users simultaneously
transmit small packets to a central base station. While there has been much
research on the design of protocols for massive access in the uplink, the
problem of providing message acknowledgments back to the users has been
somewhat neglected. Reliable communication needs to rely on two-way
communication for acknowledgement and retransmission. Nevertheless, because of
the many possible subsets of active users, providing acknowledgments requires a
significant amount of bits. Motivated by this, we define the problem of massive
ARQ (Automatic Retransmission reQuest) protocol and introduce efficient methods
for joint encoding of multiple acknowledgements in the downlink. The key idea
towards reducing the number of bits used for massive acknowledgements is to
allow for a small fraction of false positive acknowledgments. We analyze the
implications of this approach and the impact of acknowledgment errors in
scenarios with massive random access. Finally, we show that these savings can
lead to a significant increase in the reliability when retransmissions are
allowed since it allows the acknowledgment message to be transmitted more
reliably using a much lower rate.Comment: Accepted by IEEE Transactions on Communication
EMF Exposure Mitigation in RIS-Assisted Multi-Beam Communications
This paper proposes a method for reducing {third-party} exposure to
electromagnetic fields (EMF) by exploiting the capability of a reconfigurable
intelligent surfaces' (RIS) to manipulate the electromagnetic environment. We
consider users capable of multi-beam communication, such that a user can use a
set of different propagation paths enabled by the RIS. The optimization
objective is to find propagation alternatives that allow to maintain the target
quality of service while minimizing the level of EMF at surrounding
non-intended users (NUEs). We provide an evolutionary heuristic solution based
on Genetic Algorithm (GA) for power equalization and multi-beam selection of a
codebook at the Base Station. Our results show valuable insights into how
RIS-assisted multi-beam communications can mitigate EMF exposure with minimal
degradation of the spectral efficiency