2,712 research outputs found
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
Downlink Precoding for Massive MIMO Systems Exploiting Virtual Channel Model Sparsity
In this paper, the problem of designing a forward link linear precoder for
Massive Multiple-Input Multiple-Output (MIMO) systems in conjunction with
Quadrature Amplitude Modulation (QAM) is addressed. First, we employ a novel
and efficient methodology that allows for a sparse representation of multiple
users and groups in a fashion similar to Joint Spatial Division and
Multiplexing. Then, the method is generalized to include Orthogonal Frequency
Division Multiplexing (OFDM) for frequency selective channels, resulting in
Combined Frequency and Spatial Division and Multiplexing, a configuration that
offers high flexibility in Massive MIMO systems. A challenge in such system
design is to consider finite alphabet inputs, especially with larger
constellation sizes such as . The proposed methodology is next
applied jointly with the complexity-reducing Per-Group Processing (PGP)
technique, on a per user group basis, in conjunction with QAM modulation and in
simulations, for constellation size up to . We show by numerical results
that the precoders developed offer significantly better performance than the
configuration with no precoder or the plain beamformer and with
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