171 research outputs found
Filter Bank Multicarrier for Massive MIMO
This paper introduces filter bank multicarrier (FBMC) as a potential
candidate in the application of massive MIMO communication. It also points out
the advantages of FBMC over OFDM (orthogonal frequency division multiplexing)
in the application of massive MIMO. The absence of cyclic prefix in FBMC
increases the bandwidth efficiency. In addition, FBMC allows carrier
aggregation straightforwardly. Self-equalization, a property of FBMC in massive
MIMO that is introduced in this paper, has the impact of reducing (i)
complexity; (ii) sensitivity to carrier frequency offset (CFO); (iii)
peak-to-average power ratio (PAPR); (iv) system latency; and (v) increasing
bandwidth efficiency. The numerical results that corroborate these claims are
presented.Comment: 7 pages, 6 figure
Pilot Decontamination in CMT-based Massive MIMO Networks
Pilot contamination problem in massive MIMO networks operating in
time-division duplex (TDD) mode can limit their expected capacity to a great
extent. This paper addresses this problem in cosine modulated multitone (CMT)
based massive MIMO networks; taking advantage of their so-called blind
equalization property. We extend and apply the blind equalization technique
from single antenna case to multi-cellular massive MIMO systems and show that
it can remove the channel estimation errors (due to pilot contamination effect)
without any need for cooperation between different cells or transmission of
additional training information. Our numerical results advocate the efficacy of
the proposed blind technique in improving the channel estimation accuracy and
removal of the residual channel estimation errors caused by the users of the
other cells.Comment: Accepted in ISWCS 201
Time Reversal with Post-Equalization for OFDM without CP in Massive MIMO
This paper studies the possibility of eliminating the redundant cyclic prefix
(CP) of orthogonal frequency division multiplexing (OFDM) in massive
multiple-input multiple-output systems. The absence of CP increases the
bandwidth efficiency in expense of intersymbol interference (ISI) and
intercarrier interference (ICI). It is known that in massive MIMO, different
types of interference fade away as the number of base station (BS) antennas
tends to infinity. In this paper, we investigate if the channel distortions in
the absence of CP are averaged out in the large antenna regime. To this end, we
analytically study the performance of the conventional maximum ratio combining
(MRC) and realize that there always remains some residual interference leading
to saturation of signal to interference (SIR). This saturation of SIR is
quantified through mathematical equations. Moreover, to resolve the saturation
problem, we propose a technique based on time-reversal MRC with zero forcing
multiuser detection (TR-ZF). Thus, the SIR of our proposed TR-ZF does not
saturate and is a linear function of the number of BS antennas. We also show
that TR-ZF only needs one OFDM demodulator per user irrespective of the number
of BS antennas; reducing the BS signal processing complexity significantly.
Finally, we corroborate our claims as well as analytical results through
simulations.Comment: 7 pages, 3 figure
Frequency Spreading Equalization in Multicarrier Massive MIMO
Application of filter bank multicarrier (FBMC) as an effective method for
signaling over massive MIMO channels has been recently proposed. This paper
further expands the application of FBMC to massive MIMO by applying frequency
spreading equalization (FSE) to these channels. FSE allows us to achieve a more
accurate equalization. Hence, higher number of bits per symbol can be
transmitted and the bandwidth of each subcarrier can be widened. Widening the
bandwidth of each subcarrier leads to (i) higher bandwidth efficiency; (ii)
lower complexity; (iii) lower sensitivity to carrier frequency offset (CFO);
(iv) reduced peak-to-average power ratio (PAPR); and (iv) reduced latency. All
these appealing advantages have a direct impact on the digital as well as
analog circuitry that is needed for the system implementation. In this paper,
we develop the mathematical formulation of the minimum mean square error (MMSE)
FSE for massive MIMO systems. This analysis guides us to decide on the number
of subcarriers that will be sufficient for practical channel models.Comment: Accepted in IEEE ICC 2015 - Workshop on 5G & Beyond - Enabling
Technologies and Application
Interference Localization for Uplink OFDMA Systems in Presence of CFOs
Multiple carrier frequency offsets (CFOs) present in the uplink of orthogonal
frequency division multiple access (OFDMA) systems adversely affect subcarrier
orthogonality and impose a serious performance loss. In this paper, we propose
the application of time domain receiver windowing to concentrate the leakage
caused by CFOs to a few adjacent subcarriers with almost no additional
computational complexity. This allows us to approximate the interference matrix
with a quasi-banded matrix by neglecting small elements outside a certain band
which enables robust and computationally efficient signal detection. The
proposed CFO compensation technique is applicable to all types of subcarrier
assignment techniques. Simulation results show that the quasi-banded
approximation of the interference matrix is accurate enough to provide almost
the same bit error rate performance as that of the optimal solution. The
excellent performance of our proposed method is also proven through running an
experiment using our FPGA-based system setup.Comment: Accepted in IEEE WCNC 201
CFO Estimation for OFDM-based Massive MIMO Systems in Asymptotic Regime
Massive multiple input multiple output (MIMO)
plays a pivotal role in the fifth generation (5G) wireless networks.
However, the carrier frequency offset (CFO) estimation is a challenging issue in the uplink of multi-user massive MIMO systems.
In fact, frequency synchronization can impose a considerable
amount of computational complexity to the base station (BS)
due to a large number of BS antennas. In this paper, thanks
to the properties of massive MIMO in the asymptotic regime,
we develop a simple synchronization technique and derive a
closed form equation for CFO estimation. We show that the phase
information of the covariance matrix of the received signals is
solely dependent on the users’ CFOs. Hence, if a real-valued pilot
is chosen, the CFO values can be straightforwardly calculated
from this matrix. Hence, there is no need to deal with a complex
optimization problem like the other existing CFO estimation
techniques in the literature. Our simulation results testify the
efficacy of our proposed CFO estimation technique. As we have
shown, the performance of our method does not deteriorate as
the number of users increases
Maximum-likelihood synchronization and channel estimation with multiuser detection in GFDMA
Accurate estimation and correction of channel distortions and carrier frequency
offset (CFO) are of a great importance in any multicarrier communication system. Hence, in this paper, we propose data-aided CFO and channel estimation
techniques for both multiuser uplink and downlink of the generalized frequency
division multiple access (GFDMA). Our proposed solutions jointly estimate the
CFO and channel responses based on the maximum-likelihood criterion. To simplify the implementation of the proposed estimation algorithms, we suggest a
preamble composed of two similar Zadoff-Chu training sequences in a generalized frequency division multiplexing block. It is worth mentioning that our
proposed technique can estimate both integer and fractional CFO values without any limitation on the acquisition range of CFO. In the uplink phase, each
user aligns its carrier frequency with the base station using the estimated CFO
in the downlink. However, the CFO estimates may get outdated for the uplink
transmission. Thus, residual CFOs may still remain in the received signal at the
base station. While being trivial in the downlink, CFO correction is a challenging task in the uplink. Thus, we also propose a joint CFO correction and channel
equalization technique for the uplink of GFDMA systems. Finally, we evaluate
our proposed estimation and correction algorithms in terms of estimation mean
square error and bit error rate performance through simulations
First trimester exposure to anxiolytic and hypnotic drugs and the risks of major congenital anomalies: a United Kingdom population-based cohort study.
BACKGROUND: Despite their widespread use the effects of taking benzodiazepines and non-benzodiazepine hypnotics during pregnancy on the risk of major congenital anomaly (MCA) are uncertain. The objectives were to estimate absolute and relative risks of MCAs in children exposed to specific anxiolytic and hypnotic drugs taken in the first trimester of pregnancy, compared with children of mothers with depression and/or anxiety but not treated with medication and children of mothers without diagnosed mental illness during pregnancy. METHODS: We identified singleton children born to women aged 15-45 years between 1990 and 2010 from a large United Kingdom primary care database. We calculated absolute risks of MCAs for children with first trimester exposures of different anxiolytic and hypnotic drugs and used logistic regression with a generalised estimating equation to compare risks adjusted for year of childbirth, maternal age, smoking, body mass index, and socioeconomic status. RESULTS: Overall MCA prevalence was 2.7% in 1,159 children of mothers prescribed diazepam, 2.9% in 379 children with temazepam, 2.5% in 406 children with zopiclone, and 2.7% in 19,193 children whose mothers had diagnosed depression and/or anxiety but no first trimester drug exposures. When compared with 2.7% in 351,785 children with no diagnosed depression/anxiety nor medication use, the adjusted odds ratios were 1.02 (99% confidence interval 0.63-1.64) for diazepam, 1.07 (0.49-2.37) for temazepam, 0.96 (0.42-2.20) for zopiclone and 1.27 (0.43-3.75) for other anxiolytic/hypnotic drugs and 1.01 (0.90-1.14) for un-medicated depression/anxiety. Risks of system-specific MCAs were generally similar in children exposed and not exposed to such medications. CONCLUSIONS: We found no evidence for an increase in MCAs in children exposed to benzodiazepines and non-benzodiazepine hypnotics in the first trimester of pregnancy. These findings suggest that prescription of these drugs during early pregnancy may be safe in terms of MCA risk, but findings from other studies are required before safety can be confirmed
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