141 research outputs found
An Optimization Perspective of the Superiority of NOMA Compared to Conventional OMA
While existing works about non-orthogonal multiple access (NOMA) have
indicated that NOMA can yield a significant performance gain over orthogonal
multiple access (OMA) with fixed resource allocation, it is not clear whether
such a performance gain will diminish when optimal resource
(Time/Frequency/Power) allocation is carried out. In this paper, the
performance comparison between NOMA and conventional OMA systems is
investigated, from an optimization point of view. Firstly, by using the idea of
power splitting, a closed-form expression for the optimum sum rate of NOMA
systems is derived. Then, with rigorous mathematical proofs, we reveal the fact
that NOMA can always outperform conventional OMA systems, even if both are
equipped with the optimal resource allocation policies. Finally, computer
simulations are conducted to validate the accuracy of the analytical results.Comment: 28 pages, 8 figures, submitted to IEEE Transactions on Signal
Processin
On the Performance Gain of NOMA over OMA in Uplink Communication Systems
In this paper, we investigate and reveal the ergodic sum-rate gain (ESG) of
non-orthogonal multiple access (NOMA) over orthogonal multiple access (OMA) in
uplink cellular communication systems. A base station equipped with a
single-antenna, with multiple antennas, and with massive antenna arrays is
considered both in single-cell and multi-cell deployments. In particular, in
single-antenna systems, we identify two types of gains brought about by NOMA:
1) a large-scale near-far gain arising from the distance discrepancy between
the base station and users; 2) a small-scale fading gain originating from the
multipath channel fading. Furthermore, we reveal that the large-scale near-far
gain increases with the normalized cell size, while the small-scale fading gain
is a constant, given by = 0.57721 nat/s/Hz, in Rayleigh fading
channels. When extending single-antenna NOMA to -antenna NOMA, we prove that
both the large-scale near-far gain and small-scale fading gain achieved by
single-antenna NOMA can be increased by a factor of for a large number of
users. Moreover, given a massive antenna array at the base station and
considering a fixed ratio between the number of antennas, , and the number
of users, , the ESG of NOMA over OMA increases linearly with both and
. We then further extend the analysis to a multi-cell scenario. Compared to
the single-cell case, the ESG in multi-cell systems degrades as NOMA faces more
severe inter-cell interference due to the non-orthogonal transmissions.
Besides, we unveil that a large cell size is always beneficial to the ergodic
sum-rate performance of NOMA in both single-cell and multi-cell systems.
Numerical results verify the accuracy of the analytical results derived and
confirm the insights revealed about the ESG of NOMA over OMA in different
scenarios.Comment: 51 pages, 7 figures, invited paper, submitted to IEEE Transactions on
Communication
Improving performance of far users in cognitive radio: Exploiting NOMA and wireless power transfer
In this paper, we examine non-orthogonal multiple access (NOMA) and relay selection strategy to benefit extra advantage from traditional cognitive radio (CR) relaying systems. The most important requirement to prolong lifetime of such network is employing energy harvesting in the relay to address network with limited power constraint. In particular, we study such energy harvesting CR-NOMA using amplify-and-forward (AF) scheme to improve performance far NOMA users. To further address such problem, two schemes are investigated in term of number of selected relays. To further examine system performance, the outage performance needs to be studied for such wireless powered CR-NOMA network over Rayleigh channels. The accurate expressions for the outage probability are derived to perform outage comparison of primary network and secondary network. The analytical results show clearly that position of these nodes, transmit signal to noise ratio (SNR) and power allocation coefficients result in varying outage performance. As main observation, performance gap between primary and secondary destination is decided by both power allocation factors and selection mode of single relay or multiple relays. Numerical studies were conducted to verify our derivations.Web of Science1211art. no. 220
Uplink Non-Orthogonal Multiple Access with Finite-Alphabet Inputs
This paper focuses on the non-orthogonal multiple access (NOMA) design for a
classical two-user multiple access channel (MAC) with finite-alphabet inputs.
We consider practical quadrature amplitude modulation (QAM) constellations at
both transmitters, the sizes of which are assumed to be not necessarily
identical. We propose to maximize the minimum Euclidean distance of the
received sum-constellation with a maximum likelihood (ML) detector by adjusting
the scaling factors (i.e., instantaneous transmitted powers and phases) of both
users. The formulated problem is a mixed continuous-discrete optimization
problem, which is nontrivial to resolve in general. By carefully observing the
structure of the objective function, we discover that Farey sequence can be
applied to tackle the formulated problem. However, the existing Farey sequence
is not applicable when the constellation sizes of the two users are not the
same. Motivated by this, we define a new type of Farey sequence, termed punched
Farey sequence. Based on this, we manage to achieve a closed-form optimal
solution to the original problem by first dividing the entire feasible region
into a finite number of Farey intervals and then taking the maximum over all
the possible intervals. The resulting sum-constellation is proved to be a
regular QAM constellation of a larger size. Moreover, the superiority of NOMA
over time-division multiple access (TDMA) in terms of minimum Euclidean
distance is rigorously proved. Furthermore, the optimal rate allocation among
the two users is obtained in closed-form to further maximize the obtained
minimum Euclidean distance of the received signal subject to a total rate
constraint. Finally, simulation results are provided to verify our theoretical
analysis and demonstrate the merits of the proposed NOMA over existing
orthogonal and non-orthogonal designs.Comment: Submitted for possible journal publicatio
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