194 research outputs found
Intelligent Reflecting Surface Assisted Massive MIMO Communications
In a practical massive MIMO (multiple-input multiple-output) system, the
number of antennas at a base station (BS) is constrained by the space and cost
factors, which limits the throughput gain promised by theoretical analysis.
This paper thus studies the feasibility of adopting the intelligent reflecting
surface (IRS) to further improve the beamforming gain of the uplink
communications in a massive MIMO system. Under such a novel system, the central
question lies in whether the IRS is able to enhance the network throughput as
expected, if the channel estimation overhead is taken into account. In this
paper, we first show that the favorable propagation property for the
conventional massive MIMO system without IRS, i.e., the channels of arbitrary
two users are orthogonal, no longer holds for the IRS-assisted massive MIMO
system, due to its special channel property that each IRS element reflects the
signals from all the users to the BS via the same channel. As a result, the
maximal-ratio combining (MRC) receive beamforming strategy leads to strong
inter-user interference and thus even lower user rates than those of the
massive MIMO system without IRS. To tackle this challenge, we propose a novel
strategy for zero-forcing (ZF) beamforming design at the BS and reflection
coefficients design at the IRS to efficiently null the inter-user interference.
Under our proposed strategy, it is rigorously shown that even if the channel
estimation overhead is considered, the IRS-assisted massive MIMO system can
always achieve higher throughput compared to its counterpart without IRS,
despite the fact that the favorable propagation property no longer holds.Comment: Invited paper, accepted by IEEE SPAWC 202
Covariance-Based Activity Detection in Cooperative Multi-Cell Massive MIMO: Scaling Law and Efficient Algorithms
This paper focuses on the covariance-based activity detection problem in a
multi-cell massive multiple-input multiple-output (MIMO) system. In this
system, active devices transmit their signature sequences to multiple base
stations (BSs), and the BSs cooperatively detect the active devices based on
the received signals. While the scaling law for the covariance-based activity
detection in the single-cell scenario has been extensively analyzed in the
literature, this paper aims to analyze the scaling law for the covariance-based
activity detection in the multi-cell massive MIMO system. Specifically, this
paper demonstrates a quadratic scaling law in the multi-cell system, under the
assumption that the exponent in the classical path-loss model is greater than
2. This finding shows that, in the multi-cell MIMO system, the maximum number
of active devices that can be detected correctly in each cell increases
quadratically with the length of the signature sequence and decreases
logarithmically with the number of cells (as the number of antennas tends to
infinity). Moreover, in addition to analyzing the scaling law for the signature
sequences randomly and uniformly distributed on a sphere, the paper also
establishes the scaling law for signature sequences generated from a finite
alphabet, which are easier to generate and store. Moreover, this paper proposes
two efficient accelerated coordinate descent (CD) algorithms with a convergence
guarantee for solving the device activity detection problem. The first
algorithm reduces the complexity of CD by using an inexact coordinate update
strategy. The second algorithm avoids unnecessary computations of CD by using
an active set selection strategy. Simulation results show that the proposed
algorithms exhibit excellent performance in terms of computational efficiency
and detection error probability.Comment: 54 pages, 11 figures, submitted for possible publicatio
Device Activity Detection in mMTC with Low-Resolution ADC: A New Protocol
This paper investigates the effect of low-resolution analog-to-digital
converters (ADCs) on device activity detection in massive machine-type
communications (mMTC). The low-resolution ADCs induce two challenges on the
device activity detection compared with the traditional setup with assumption
of infinite ADC resolution. First, the codebook design for signal quantization
by the low-resolution ADCs is particularly important since a good codebook
design can lead to small quantization error on the received signal, which in
turn has significant influence on the activity detector performance. To this
end, prior information about the received signal power is needed, which depends
on the number of active devices . This is sharply different from the
activity detection problem in traditional setups, in which the knowledge of
is not required by the BS as a prerequisite. Second, the covariance-based
approach achieves good activity detection performance in traditional setups
while it is not clear if it can still achieve good performance in this paper.
To solve the above challenges, we propose a communication protocol that
consists of an estimator for and a detector for active device identities:
1) For the estimator, the technical difficulty is that the design of the ADC
quantizer and the estimation of are closely intertwined and doing one needs
the information/execution from the other. We propose a progressive estimator
which iteratively performs the estimation of and the design of the ADC
quantizer; 2) For the activity detector, we propose a custom-designed
stochastic gradient descent algorithm to estimate the active device identities.
Numerical results demonstrate the effectiveness of the communication protocol.Comment: Submitted to IEEE for possible publicatio
PNC Enabled IIoT: A General Framework for Channel-Coded Asymmetric Physical-Layer Network Coding
This paper investigates the application of physical-layer network coding
(PNC) to Industrial Internet-of-Things (IIoT) where a controller and a robot
are out of each other's transmission range, and they exchange messages with the
assistance of a relay. We particularly focus on a scenario where the controller
has more transmitted information, and the channel of the controller is stronger
than that of the robot. To reduce the communication latency, we propose an
asymmetric transmission scheme where the controller and robot transmit
different amount of information in the uplink of PNC simultaneously. To achieve
this, the controller chooses a higher order modulation. In addition, the both
users apply channel codes to guarantee the reliability. A problem is a
superimposed symbol at the relay contains different amount of source
information from the two end users. It is thus hard for the relay to deduce
meaningful network-coded messages by applying the current PNC decoding
techniques which require the end users to transmit the same amount of
information. To solve this problem, we propose a lattice-based scheme where the
two users encode-and-modulate their information in lattices with different
lattice construction levels. Our design is versatile on that the two end users
can freely choose their modulation orders based on their channel power, and the
design is applicable for arbitrary channel codes.Comment: Submitted to IEEE for possible publicatio
Anomalous bond softening mediated by strain-induced Friedel-like oscillations in a BC2N superlattice
The crystal structure of BC2N and the origin of its superhardness remain under constant debate, hindering its development. Herein, by evaluating the x-ray diffraction pattern, the thermodynamic stability at normal and high pressures of a series of BC2N candidates, the (111) BC2N2x2 superlattice (labeled R2u-BC2N) is identified as the realistic crystal structure of the experimentally synthesized BC2N. We further reveal that the strain-induced Friedel-like oscillations dominates the preferable slip systems of R2u-BC2N by drastically weakening the heterogenous bonds across the slip plane and thus leads to its ultralow dislocation slip resistance, which originates from the metallization triggered by the reduction in energy separation between bonding and antibonding interactions of the softened bonds. Our results rule out R2u-BC2N as the intrinsic superhard material surpassing c-BN, whereas the experimentally determined extreme hardness can be attributed to the nanocrystalline grains glued by interfacial amorphous carbon which provides a strong barrier for plastic deformation. These findings provide a view of the longstanding issue of the possible structure of experimentally observed BC2N, and establish a mechanism underlying the strain-driven electronic instability of superlattice structures, providing guidance towards rational design of superhard materials.Web of Science1066art. no. L06010
The economic status of older people’s households in urban and rural settings in Peru, Mexico and China: a 10/66 INDEP study cross-sectional survey
Few data are available from middle income countries regarding economic circumstances of households in which older people live. Many such settings have experienced rapid demographic, social and economic change, alongside increasing pension coverage. Population-based household surveys in rural and urban catchment areas in Peru, Mexico and China. Participating households were selected from all households with older residents. Descriptive analyses were weighted back for sampling fractions and non-response. Household income and consumption were estimated from a household key informant interview. 877 Household interviews (3177 residents). Response rate 68 %. Household income and consumption correlated plausibly with other economic wellbeing indicators. Household Incomes varied considerably within and between sites. While multigenerational households were the norm, older resident’s incomes accounted for a high proportion of household income, and older people were particularly likely to pool income. Differences in the coverage and value of pensions were a major source of variation in household income among sites. There was a small, consistent inverse association between household pension income and labour force participation of younger adult co-residents. The effect of pension income on older adults’ labour force participation was less clear-cut. Historical linkage of social protection to formal employment may have contributed to profound late-life socioeconomic inequalities. Strategies to formalise the informal economy, alongside increases in the coverage and value of non-contributory pensions and transfers would help to address this problem
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