323 research outputs found
Impact of User Pairing on 5G Non-Orthogonal Multiple Access
Non-orthogonal multiple access (NOMA) represents a paradigm shift from
conventional orthogonal multiple access (MA) concepts, and has been recognized
as one of the key enabling technologies for 5G systems. In this paper, the
impact of user pairing on the performance of two NOMA systems, NOMA with fixed
power allocation (F-NOMA) and cognitive radio inspired NOMA (CR-NOMA), is
characterized. For FNOMA, both analytical and numerical results are provided to
demonstrate that F-NOMA can offer a larger sum rate than orthogonal MA, and the
performance gain of F-NOMA over conventional MA can be further enlarged by
selecting users whose channel conditions are more distinctive. For CR-NOMA, the
quality of service (QoS) for users with the poorer channel condition can be
guaranteed since the transmit power allocated to other users is constrained
following the concept of cognitive radio networks. Because of this constraint,
CR-NOMA has different behavior compared to F-NOMA. For example, for the user
with the best channel condition, CR-NOMA prefers to pair it with the user with
the second best channel condition, whereas the user with the worst channel
condition is preferred by F-NOMA
On the Performance of NOMA with Hybrid ARQ
In this paper, we investigate the outage performance of hybrid automatic
repeat request with chase combining (HARQ-CC) assisted downlink non-orthogonal
multiple access (NOMA) systems. A closed-form expression of the individual
outage probability and the diversity gain are obtained firstly. Based on the
developed analytical outage probability, a tradeoff between the minimum number
of retransmissions and the transmit power allocation coefficient is then
provided for a given target rate. The provided simulation results demonstrate
the accuracy of the developed analytical results. Moreover, it is shown that
NOMA combined with the HARQ-CC can achieve a significant advantage when only
average channel state information is known at the transmitter. Particularly,
the performance of the user with less transmit power in NOMA systems can be
efficiently improved by utilizing HARQ-CC
OTFS-NOMA: An Efficient Approach for Exploiting Heterogenous User Mobility Profiles
This paper considers a challenging communication scenario, in which users
have heterogenous mobility profiles, e.g., some users are moving at high speeds
and some users are static. A new non-orthogonal multiple-access (NOMA)
transmission protocol that incorporates orthogonal time frequency space (OTFS)
modulation is proposed. Thereby, users with different mobility profiles are
grouped together for the implementation of NOMA. The proposed OTFS-NOMA
protocol is shown to be applicable to both uplink and downlink transmission,
where sophisticated transmit and receive strategies are developed to remove
inter-symbol interference and harvest both multi-path and multi-user diversity.
Analytical results demonstrate that both the high-mobility and low-mobility
users benefit from the application of OTFS-NOMA. In particular, the use of NOMA
allows the spreading of the high-mobility users' signals over a large amount of
time-frequency resources, which enhances the OTFS resolution and improves the
detection reliability. In addition, OTFS-NOMA ensures that low-mobility users
have access to bandwidth resources which in conventional OTFS-orthogonal
multiple access (OTFS-NOMA) would be solely occupied by the high-mobility
users. Thus, OTFS-NOMA improves the spectral efficiency and reduces latency
NOMA Assisted Wireless Caching: Strategies and Performance Analysis
Conventional wireless caching assumes that content can be pushed to local
caching infrastructure during off-peak hours in an error-free manner; however,
this assumption is not applicable if local caches need to be frequently updated
via wireless transmission. This paper investigates a new approach to wireless
caching for the case when cache content has to be updated during on-peak hours.
Two non-orthogonal multiple access (NOMA) assisted caching strategies are
developed, namely the push-then-deliver strategy and the push-and-deliver
strategy. In the push-then-deliver strategy, the NOMA principle is applied to
push more content files to the content servers during a short time interval
reserved for content pushing in on-peak hours and to provide more connectivity
for content delivery, compared to the conventional orthogonal multiple access
(OMA) strategy. The push-and-deliver strategy is motivated by the fact that
some users' requests cannot be accommodated locally and the base station has to
serve them directly. These events during the content delivery phase are
exploited as opportunities for content pushing, which further facilitates the
frequent update of the files cached at the content servers. It is also shown
that this strategy can be straightforwardly extended to device-to-device
caching, and various analytical results are developed to illustrate the
superiority of the proposed caching strategies compared to OMA based schemes
A novel power allocation scheme under outage constraints in NOMA systems
In this letter, we study a downlink non-orthogonal multiple access (NOMA) transmission system, where only the average channel state information (CSI) is available at the transmitter. Two criteria in terms of transmit power and user fairness for NOMA systems are used to formulate two optimization problems, subjected to outage probabilistic constraints and the optimal decoding order. We first investigate the optimal decoding order when the transmitter knows only the average CSI, and then, we develop the optimal power allocation schemes in closed form by employing the feature of the NOMA principle for the two problems. Furthermore, the power difference between NOMA systems and OMA systems under outage constraints is obtained
Outage performance for dynamic power allocation in hybrid non-orthogonal multiple access systems
In this letter, we propose a novel dynamic power allocation scheme for hybrid non-orthogonal multiple access (DH-NOMA). The exact expressions for the outage probabilities and their asymptotic approximations at high signal-to-noise ratio are derived in DH-NOMA systems. Both Monte Carlo simulations and analytical results are used to compare the proposed scheme with the existing works, such as NOMA based on fixed power allocation (F-NOMA) and cognitive radio inspired NOMA (CR-NOMA). The provided results demonstrate that the DH-NOMA scheme can always achieve better outage performance than the traditional orthogonal multiple access, and achieve a more balanced tradeoff between the two user individual rates than F-NOMA and CR-NOMA
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
Globally Optimal Beamforming Design for Integrated Sensing and Communication Systems
In this paper, we propose a multi-input multi-output (MIMO) beamforming
transmit optimization model for joint radar sensing and multi-user
communications, where the design of the beamformers is formulated as an
optimization problem whose objective is a weighted combination of the sum rate
and the Cram\'{e}r-Rao bound (CRB), subject to the transmit power budget
constraint. The formulated problem is challenging to obtain a global solution,
because the sum rate maximization (SRM) problem itself (even without
considering the sensing metric) is known to be NP-hard. In this paper, we
propose an efficient global branch-and-bound algorithm for solving the
formulated problem based on the McCormick envelope relaxation and the
semidefinite relaxation (SDR) technique. The proposed algorithm is guaranteed
to find the global solution for the considered problem, and thus serves as an
important benchmark for performance evaluation of the existing local or
suboptimal algorithms for solving the same problem.Comment: 5 pages, 2 figures, submitted for possible publicatio
- …