4,809 research outputs found
Massive MU-MIMO Downlink TDD Systems with Linear Precoding and Downlink Pilots
We consider a massive MU-MIMO downlink time-division duplex system where a
base station (BS) equipped with many antennas serves several single-antenna
users in the same time-frequency resource. We assume that the BS uses linear
precoding for the transmission. To reliably decode the signals transmitted from
the BS, each user should have an estimate of its channel. In this work, we
consider an efficient channel estimation scheme to acquire CSI at each user,
called beamforming training scheme. With the beamforming training scheme, the
BS precodes the pilot sequences and forwards to all users. Then, based on the
received pilots, each user uses minimum mean-square error channel estimation to
estimate the effective channel gains. The channel estimation overhead of this
scheme does not depend on the number of BS antennas, and is only proportional
to the number of users. We then derive a lower bound on the capacity for
maximum-ratio transmission and zero-forcing precoding techniques which enables
us to evaluate the spectral efficiency taking into account the spectral
efficiency loss associated with the transmission of the downlink pilots.
Comparing with previous work where each user uses only the statistical channel
properties to decode the transmitted signals, we see that the proposed
beamforming training scheme is preferable for moderate and low-mobility
environments.Comment: Allerton Conference on Communication, Control, and Computing,
Urbana-Champaign, Illinois, Oct. 201
Aspects of Favorable Propagation in Massive MIMO
Favorable propagation, defined as mutual orthogonality among the
vector-valued channels to the terminals, is one of the key properties of the
radio channel that is exploited in Massive MIMO. However, there has been little
work that studies this topic in detail. In this paper, we first show that
favorable propagation offers the most desirable scenario in terms of maximizing
the sum-capacity. One useful proxy for whether propagation is favorable or not
is the channel condition number. However, this proxy is not good for the case
where the norms of the channel vectors may not be equal. For this case, to
evaluate how favorable the propagation offered by the channel is, we propose a
``distance from favorable propagation'' measure, which is the gap between the
sum-capacity and the maximum capacity obtained under favorable propagation.
Secondly, we examine how favorable the channels can be for two extreme
scenarios: i.i.d. Rayleigh fading and uniform random line-of-sight (UR-LoS).
Both environments offer (nearly) favorable propagation. Furthermore, to analyze
the UR-LoS model, we propose an urns-and-balls model. This model is simple and
explains the singular value spread characteristic of the UR-LoS model well
Non-Coherent Cooperative Communications Dispensing with Channel Estimation Relying on Erasure Insertion Aided Reed-Solomon Coded SFH M-ary FSK Subjected to Partial-Band Interference and Rayleigh Fading
The rationale of our design is that although much of the literature of cooperative systems assumes perfect coherent detection, the assumption of having any channel estimates at the relays imposes an unreasonable burden on the relay station. Hence, non-coherently detected Reed-Solomon (ReS) coded Slow Frequency Hopping (SFH) assisted M -ary Frequency Shift Keying (FSK) is proposed for cooperative wireless networks, subjected to both partial-band interference and Rayleigh fading. Erasure insertion (EI) assisted ReS decoding based on the joint maximum output-ratio threshold test (MO-RTT) is investigated in order to evaluate the attainable system performance. Compared to the conventional error-correction-only decoder, the EI scheme may achieve an Eb/N0 gain of approximately 3dB at the Codeword Error Probability, Pw , of 10-4 , when employing the ReS (31, 20) code combined with 32-FSK modulation. Additionally, we evaluated the system’s performance, when either equal gain combining (EGC) or selection combining (SC) techniques are employed at the destination’s receiver. The results demonstrated that in the presence of one and two assisting relays, the EGC scheme achieves gains of 1.5 dB and 1.0 dB at the Pw of 10-6 , respectively, compared to the SC arrangement. Furthermore, we demonstrated that for the same coding rate and packet size, the ReS (31, 20) code using EI decoding is capable of outperforming convolutional coding, when 32-FSK modulation is considered, whilst LDPC coding had an edge over the above two schemes
A review of the use of information and communication technologies for dietary assessment
Presently used dietary-assessment methods often present difficulties for researchers and respondents, and misreporting errors are common. Methods using information and communication technologies (ICT) may improve quality and accuracy. The present paper presents a systematic literature review describing studies applying ICT to dietary assessment. Eligible papers published between January 1995 and February 2008 were classified into four assessment categories: computerised assessment; personal digital assistants (PDA); digital photography; smart cards. Computerised assessments comprise frequency questionnaires, 24 h recalls (24HR) and diet history assessments. Self-administered computerised assessments, which can include audio support, may reduce literacy problems, be translated and are useful for younger age groups, but less so for those unfamiliar with computers. Self-administered 24HR utilising computers yielded comparable results as standard methods, but needed supervision if used in children. Computer-assisted interviewer-administered recall results were similar to conventional recalls, and reduced inter-interviewer variability. PDA showed some advantages but did not reduce underreporting. Mobile phone meal photos did not improve PDA accuracy. Digital photography for assessing individual food intake in dining facilities was accurate for adults and children, although validity was slightly higher with direct visual observation. Smart cards in dining facilities were useful for measuring food choice but not total dietary intake. In conclusion, computerised assessments and PDA are promising, and could improve dietary assessment quality in some vulnerable groups and decrease researcher workload. Both still need comprehensive evaluation for micronutrient intake assessment. Further work is necessary for improving ICT tools in established and new methods and for their rigorous evaluatio
Cell-Free Massive MIMO versus Small Cells
A Cell-Free Massive MIMO (multiple-input multiple-output) system comprises a
very large number of distributed access points (APs)which simultaneously serve
a much smaller number of users over the same time/frequency resources based on
directly measured channel characteristics. The APs and users have only one
antenna each. The APs acquire channel state information through time-division
duplex operation and the reception of uplink pilot signals transmitted by the
users. The APs perform multiplexing/de-multiplexing through conjugate
beamforming on the downlink and matched filtering on the uplink. Closed-form
expressions for individual user uplink and downlink throughputs lead to max-min
power control algorithms. Max-min power control ensures uniformly good service
throughout the area of coverage. A pilot assignment algorithm helps to mitigate
the effects of pilot contamination, but power control is far more important in
that regard.
Cell-Free Massive MIMO has considerably improved performance with respect to
a conventional small-cell scheme, whereby each user is served by a dedicated
AP, in terms of both 95%-likely per-user throughput and immunity to shadow
fading spatial correlation. Under uncorrelated shadow fading conditions, the
cell-free scheme provides nearly 5-fold improvement in 95%-likely per-user
throughput over the small-cell scheme, and 10-fold improvement when shadow
fading is correlated.Comment: EEE Transactions on Wireless Communications, accepted for publicatio
Tunneling into Nonequilibrium Luttinger Liquid with Impurity
We evaluate tunneling rates into/from a voltage biased quantum wire
containing weak backscattering defect. Interacting electrons in such a wire
form a true nonequilibrium state of the Luttinger liquid (LL). This state is
created due to inelastic electron backscattering leading to the emission of
nonequilibrium plasmons with typical frequency . The
tunneling rates are split into two edges. The tunneling exponent at the Fermi
edge is positive and equals that of the equilibrium LL, while the exponent at
the side edge is negative if Coulomb interaction is not too strong.Comment: 4+ pages, 5 figure
Simple Combined Model for Nonlinear Excitations in DNA
We propose a new simple model for DNA denaturation bases on the pendulum
model of Englander\cite{A1} and the microscopic model of Peyrard {\it et
al.},\cite{A3} so called "combined model". The main parameters of our model
are: the coupling constant along each strand, the mean stretching
of the hydrogen bonds, the ratio of the damping constant and driven force
. We show that both the length of unpaired bases and the velocity
of kinks depend on not only the coupling constant but also the
temperature . Our results are in good agreement with previous works.Comment: 6 pages, 10 figures, submitted to Phys. Rev.
An Excess of Jupiter Analogs in Super-Earth Systems
We use radial velocity observations to search for long-period gas giant
companions in systems hosting inner super-Earth (1-4 R_Earth, 1-10 M_Earth)
planets to constrain formation and migration scenarios for this population. We
consistently re-fit published RV datasets for 65 stars and find 9 systems with
statistically significant trends indicating the presence of an outer companion.
We combine these RV data with AO images to constrain the masses and semi-major
axes of these companions. We quantify our sensitivity to the presence of
long-period companions by fitting the sample with a power law distribution and
find an occurrence rate of 39+/-7% for companions 0.5-20 M_Jup and 1-20 AU.
Half of our systems were discovered by the transit method and half were
discovered by the RV method. While differences in RV baselines and number of
data points between the two samples lead to different sensitivities to distant
companions, we find that occurrence rates of gas giant companions in each
sample are consistent at the 0.5 level. We compare the frequency of
Jupiter analogs in these systems to the equivalent rate from field star surveys
and find that Jupiter analogs are more common around stars hosting
super-Earths. We conclude that the presence of outer gas giants does not
suppress the formation of inner super-Earths, and that these two populations of
planets instead appear to be correlated. We also find that the stellar
metallicities of systems with gas giant companions are higher than those
without companions, in agreement with the well-established metallicity
correlation from RV surveys of field stars.Comment: published in A
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