303 research outputs found
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
Performance of NOMA systems with HARQ-CC in finite blocklength
Abstract. With the advent of new use-cases requiring high reliability and low-latency, transmission with finite blocklength becomes inevitable to reduce latency. In contrast to classical information-theoretic principles, the use of finite blocklength results in a non-negligible decoder error probability. Hybrid automatic repeat request (HARQ) procedures are used to improve the accuracy in decoding by exploiting time-diversity at the expense of increased latency. Thus, achieving high reliability and low-latency are Pareto-optimal, which calls for a trade-off between the two. Concurrently, non-orthogonal multiple access (NOMA) has gained widespread attention in research due to the ability to outperform its counterpart, orthogonal multiple access (OMA) in terms of spectral efficiency and user fairness.
This thesis investigates the performance of a two-user downlink NOMA system using HARQ with chase combining (HARQ-CC) in finite blocklength unifying the three enablers. First, an analytical framework is developed by deriving closed-form approximations for the individual average block error rate (BLER) of the near and the far user. Based upon that, the performance of NOMA is discussed in comparison to OMA, which draws the conclusion that NOMA outperforms OMA in terms of user fairness. Further, asymptotic expressions for average BLER are derived, which are used to devise an algorithm to determine such minimum blocklength and power allocation coefficients for NOMA that satisfies reliability targets for the users. NOMA has a lower blocklength in high transmit signal-to-noise ratio (SNR) conditions, leading to lower latency than OMA when reliability requirements in terms of BLER for the two users are in the order of 10^(-5)
Achievable Diversity Order of HARQ-Aided Downlink NOMA Systems
The combination between non-orthogonal multiple access (NOMA) and hybrid
automatic repeat request (HARQ) is capable of realizing ultra-reliability, high
throughput and many concurrent connections particularly for emerging
communication systems. This paper focuses on characterizing the asymptotic
scaling law of the outage probability of HARQ-aided NOMA systems with respect
to the transmit power, i.e., diversity order. The analysis of diversity order
is carried out for three basic types of HARQ-aided downlink NOMA systems,
including Type I HARQ, HARQ with chase combining (HARQ-CC) and HARQ with
incremental redundancy (HARQ-IR). The diversity orders of three HARQ-aided
downlink NOMA systems are derived in closed-form, where an integration domain
partition trick is developed to obtain the bounds of the outage probability
specially for HARQ-CC and HARQ-IR-aided NOMA systems. The analytical results
show that the diversity order is a decreasing step function of transmission
rate, and full time diversity can only be achieved under a sufficiently low
transmission rate. It is also revealed that HARQ-IR-aided NOMA systems have the
largest diversity order, followed by HARQ-CC-aided and then Type I HARQ-aided
NOMA systems. Additionally, the users' diversity orders follow a descending
order according to their respective average channel gains. Furthermore, we
expand discussions on the cases of power-efficient transmissions and imperfect
channel state information (CSI). Monte Carlo simulations finally confirm our
analysis
Physical layer security for NOMA: requirements, merits, challenges, and recommendations
Non-orthogonal multiple access (NOMA) has been
recognized as one of the most significant enabling technologies
for future wireless systems due to its eminent spectral efficiency,
its ability to provide an additional degree of freedom for ultra
reliable low latency communications (URLLC), and grant free
random access. Meanwhile, physical layer security (PLS) has got
much attention for future wireless communication systems due to
its capability to efficiently complement the cryptography-based
algorithms for enhancing overall security of the communication
system. In this article, security design requirements for downlink
power domain NOMA and solutions provided by PLS to fulfil
these requirements are discussed. The merits and challenges
which were encountered while employing PLS to NOMA are
identified. Finally, future recommendations and prospective so lutions are also presented.No sponso
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