8,082 research outputs found
Simple Semi-Grant-Free Transmission Strategies Assisted by Non-Orthogonal Multiple Access
Grant-free transmission is an important feature to be supported by future
wireless networks since it reduces the signalling overhead caused by
conventional grant-based schemes. However, for grant-free transmission, the
number of users admitted to the same channel is not caped, which can lead to a
failure of multi-user detection. This paper proposes non-orthogonal
multiple-access (NOMA) assisted semi-grant-free (SGF) transmission, which is a
compromise between grant-free and grant-based schemes. In particular, instead
of reserving channels either for grant-based users or grant-free users, we
focus on an SGF communication scenario, where users are admitted to the same
channel via a combination of grant-based and grant-free protocols. As a result,
a channel reserved by a grant-based user can be shared by grant-free users,
which improves both connectivity and spectral efficiency. Two NOMA assisted SGF
contention control mechanisms are developed to ensure that, with a small amount
of signalling overhead, the number of admitted grant-free users is carefully
controlled and the interference from the grant-free users to the grant-based
users is effectively suppressed. Analytical results are provided to demonstrate
that the two proposed SGF mechanisms employing different successive
interference cancelation decoding orders are applicable to different practical
network scenarios
Impact of NOMA on Age of Information: A Grant-Free Transmission Perspective
The aim of this paper is to characterize the impact of non-orthogonal
multiple access (NOMA) on the age of information (AoI) of grant-free
transmission. In particular, a low-complexity form of NOMA, termed
NOMA-assisted random access, is applied to grant-free transmission in order to
illustrate the two benefits of NOMA for AoI reduction, namely increasing
channel access and reducing user collisions. Closed-form analytical expressions
for the AoI achieved by NOMA assisted grant-free transmission are obtained, and
asymptotic studies are carried out to demonstrate that the use of the simplest
form of NOMA is already sufficient to reduce the AoI of orthogonal multiple
access (OMA) by more than 40%. In addition, the developed analytical
expressions are also shown to be useful for optimizing the users' transmission
attempt probabilities, which are key parameters for grant-free transmission
On Secure NOMA-Aided Semi-Grant-Free Systems
Semi-grant-free (SGF) transmission scheme enables grant-free (GF) users to
utilize resource blocks allocated for grant-based (GB) users while maintaining
the quality of service of GB users. This work investigates the secrecy
performance of non-orthogonal multiple access (NOMA)-aided SGF systems. First,
analytical expressions for the exact and asymptotic secrecy outage probability
(SOP) of NOMA-aided SGF systems with a single GF user are derived. Then, the
SGF systems with multiple GF users and a best-user scheduling scheme is
considered. By utilizing order statistics theory, closed-form expressions for
the exact and asymptotic SOP are derived. Monte Carlo simulation results
demonstrate the effects of system parameters on the SOP of the considered
system and verify the accuracy of the developed analytical results. The results
indicate that both the outage target rate for GB and the secure target rate for
GF are the main factors of the secrecy performance of SGF systems
Advanced NOMA Assisted Semi-Grant-Free Transmission Schemes for Randomly Distributed Users
Non-orthogonal multiple access (NOMA) assisted semi-grant-free (SGF)
transmission has recently received significant research attention due to its
outstanding ability of serving grant-free (GF) users with grant-based (GB)
users' spectrum, {\color{blue}which can greatly improve the spectrum efficiency
and effectively relieve the massive access problem of 5G and beyond networks.
In this paper, we investigate the performance of SGF schemes under more
practical settings.} Firstly, we study the outage performance of the best user
scheduling SGF scheme (BU-SGF) by considering the impacts of Rayleigh fading,
path loss, and random user locations. Then, a fair SGF scheme is proposed by
applying cumulative distribution function (CDF)-based scheduling (CS-SGF),
which can also make full use of multi-user diversity. Moreover, by employing
the theories of order statistics and stochastic geometry, we analyze the outage
performances of both BU-SGF and CS-SGF schemes. Results show that full
diversity orders can be achieved only when the served users' data rate is
capped, which severely limit the rate performance of SGF schemes. To further
address this issue, we propose a distributed power control strategy to relax
such data rate constraint, and derive closed-form expressions of the two
schemes' outage performances under this strategy. Finally, simulation results
validate the fairness performance of the proposed CS-SGF scheme, the
effectiveness of the power control strategy, and the accuracy of the
theoretical analyses.Comment: 41 pages, 8 figure
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