272 research outputs found
A Joint Waveform and Precoding Design for Non-orthogonal Multicarrier Signals
In the spectrally efficient frequency division multiplexing
(SEFDM) non-orthogonal multicarrier signal, higher
spectral efficiency can be achieved at the expense of self-created
inter carrier interference (ICI). The effective interference, which
is contributed by all sub-carriers, has to be minimized and
this results in a receiver of significant complexity. In order to
mitigate the interference and simplify the receiver design, in this
work, a precoding technique, based on eigenvalue decomposition
of the sub-carrier correlation matrix, is utilised. Briefly, the
technique is based on modifying the data sent on individual
sub-carriers according to the signal quality of each, which is
based on the sub-carrier to interference ratio (ScIR) of such
sub-carrier as estimated from eigenvalue decomposition. A full
system model is presented in this paper and simulations show
that the precoding of SEFDM results in either better bit error
rate (BER) performance compared to that of an orthogonal
frequency division multiplexing (OFDM) system of the same
spectral efficiency or in higher effective bit rate relative to
an OFDM system with the same BER performance. Modelling
is done in simple Gaussian noise channels and in a static
frequency selective channel and for different modulation formats.
Results show that for the same bandwidth a 128QAM precoded
SEFDM system outperforms a 16QAM OFDM one by offering
75% bit rate increase. Furthermore, Turbo coding assisted BER
performance comparisons are investigated in this work. Using
64QAM modulated symbols, the precoded SEFDM outperforms
the typical OFDM by several dBs
Low-Complexity OFDM Spectral Precoding
This paper proposes a new large-scale mask-compliant spectral precoder
(LS-MSP) for orthogonal frequency division multiplexing systems. In this paper,
we first consider a previously proposed mask-compliant spectral precoding
scheme that utilizes a generic convex optimization solver which suffers from
high computational complexity, notably in large-scale systems. To mitigate the
complexity of computing the LS-MSP, we propose a divide-and-conquer approach
that breaks the original problem into smaller rank 1 quadratic-constraint
problems and each small problem yields closed-form solution. Based on these
solutions, we develop three specialized first-order low-complexity algorithms,
based on 1) projection on convex sets and 2) the alternating direction method
of multipliers. We also develop an algorithm that capitalizes on the
closed-form solutions for the rank 1 quadratic constraints, which is referred
to as 3) semi-analytical spectral precoding. Numerical results show that the
proposed LS-MSP techniques outperform previously proposed techniques in terms
of the computational burden while complying with the spectrum mask. The results
also indicate that 3) typically needs 3 iterations to achieve similar results
as 1) and 2) at the expense of a slightly increased computational complexity.Comment: Accepted in IEEE International Workshop on Signal Processing Advances
in Wireless Communications (SPAWC), 201
Performance Analysis of a Low-Interference N-Continuous OFDM Scheme
This paper investigates two issues of power spectrum density (PSD) and bit
error rate (BER) of an N-continuous orthogonal frequency division multiplexing
(NC-OFDM) aided low-interference time-domain scheme, when the smooth signal is
designed by the linear combination of basis signals truncated by a window.
Based on the relationship between the continuity and sidelobe decaying, the PSD
performance is first analyzed and compared, in terms of the highest derivative
order (HDO) N and the length of the smooth signal L. Since the high-order
derivative of the truncation window has the finite continuity, the N-continuous
signal has two finite continuities, which may have different continuous
derivative orders. In this case, we develop a close PSD expression by
introducing another smooth signal, which is also linearly combined by other
basis signals, to explain the sidelobe decaying related to N and L. Then, in
the context of BER, considering the multipath Rayleigh fading channel, based on
the effect of the delayed tail of the smooth signal to the received signal, we
provide a procedure for calculating the BER expressed in the form of an
asymptotic summation.Comment: 7 pages, 6 figure
Linear Precoding with Low-Resolution DACs for Massive MU-MIMO-OFDM Downlink
We consider the downlink of a massive multiuser (MU) multiple-input
multiple-output (MIMO) system in which the base station (BS) is equipped with
low-resolution digital-to-analog converters (DACs). In contrast to most
existing results, we assume that the system operates over a frequency-selective
wideband channel and uses orthogonal frequency division multiplexing (OFDM) to
simplify equalization at the user equipments (UEs). Furthermore, we consider
the practically relevant case of oversampling DACs. We theoretically analyze
the uncoded bit error rate (BER) performance with linear precoders (e.g., zero
forcing) and quadrature phase-shift keying using Bussgang's theorem. We also
develop a lower bound on the information-theoretic sum-rate throughput
achievable with Gaussian inputs, which can be evaluated in closed form for the
case of 1-bit DACs. For the case of multi-bit DACs, we derive approximate, yet
accurate, expressions for the distortion caused by low-precision DACs, which
can be used to establish lower bounds on the corresponding sum-rate throughput.
Our results demonstrate that, for a massive MU-MIMO-OFDM system with a
128-antenna BS serving 16 UEs, only 3--4 DAC bits are required to achieve an
uncoded BER of 10^-4 with a negligible performance loss compared to the
infinite-resolution case at the cost of additional out-of-band emissions.
Furthermore, our results highlight the importance of taking into account the
inherent spatial and temporal correlations caused by low-precision DACs
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