938 research outputs found
Random Access in Massive MIMO by Exploiting Timing Offsets and Excess Antennas
Massive MIMO systems, where base stations are equipped with hundreds of
antennas, are an attractive way to handle the rapid growth of data traffic. As
the number of user equipments (UEs) increases, the initial access and handover
in contemporary networks will be flooded by user collisions. In this paper, a
random access protocol is proposed that resolves collisions and performs timing
estimation by simply utilizing the large number of antennas envisioned in
Massive MIMO networks. UEs entering the network perform spreading in both time
and frequency domains, and their timing offsets are estimated at the base
station in closed-form using a subspace decomposition approach. This
information is used to compute channel estimates that are subsequently employed
by the base station to communicate with the detected UEs. The favorable
propagation conditions of Massive MIMO suppress interference among UEs whereas
the inherent timing misalignments improve the detection capabilities of the
protocol. Numerical results are used to validate the performance of the
proposed procedure in cellular networks under uncorrelated and correlated
fading channels. With UEs that may simultaneously become active
with probability 1\% and a total of frequency-time codes (in a given
random access block), it turns out that, with antennas, the proposed
procedure successfully detects a given UE with probability 75\% while providing
reliable timing estimates.Comment: 30 pages, 6 figures, 1 table, submitted to Transactions on
Communication
Massive MIMO is a Reality -- What is Next? Five Promising Research Directions for Antenna Arrays
Massive MIMO (multiple-input multiple-output) is no longer a "wild" or
"promising" concept for future cellular networks - in 2018 it became a reality.
Base stations (BSs) with 64 fully digital transceiver chains were commercially
deployed in several countries, the key ingredients of Massive MIMO have made it
into the 5G standard, the signal processing methods required to achieve
unprecedented spectral efficiency have been developed, and the limitation due
to pilot contamination has been resolved. Even the development of fully digital
Massive MIMO arrays for mmWave frequencies - once viewed prohibitively
complicated and costly - is well underway. In a few years, Massive MIMO with
fully digital transceivers will be a mainstream feature at both sub-6 GHz and
mmWave frequencies. In this paper, we explain how the first chapter of the
Massive MIMO research saga has come to an end, while the story has just begun.
The coming wide-scale deployment of BSs with massive antenna arrays opens the
door to a brand new world where spatial processing capabilities are
omnipresent. In addition to mobile broadband services, the antennas can be used
for other communication applications, such as low-power machine-type or
ultra-reliable communications, as well as non-communication applications such
as radar, sensing and positioning. We outline five new Massive MIMO related
research directions: Extremely large aperture arrays, Holographic Massive MIMO,
Six-dimensional positioning, Large-scale MIMO radar, and Intelligent Massive
MIMO.Comment: 20 pages, 9 figures, submitted to Digital Signal Processin
Joint Channel-and-Data Estimation for Large-MIMO Systems with Low-Precision ADCs
The use of low precision (e.g., 1-3 bits) analog-to-digital convenors (ADCs)
in very large multiple-input multiple-output (MIMO) systems is a technique to
reduce cost and power consumption. In this context, nevertheless, it has been
shown that the training duration is required to be {\em very large} just to
obtain an acceptable channel state information (CSI) at the receiver. A
possible solution to the quantized MIMO systems is joint channel-and-data (JCD)
estimation. This paper first develops an analytical framework for studying the
quantized MIMO system using JCD estimation. In particular, we use the
Bayes-optimal inference for the JCD estimation and realize this estimator
utilizing a recent technique based on approximate message passing. Large-system
analysis based on the replica method is then adopted to derive the asymptotic
performances of the JCD estimator. Results from simulations confirm our
theoretical findings and reveal that the JCD estimator can provide a
significant gain over conventional pilot-only schemes in the quantized MIMO
system.Comment: 7 pages, 4 figure
Generalized Spatial Modulation in Large-Scale Multiuser MIMO Systems
Generalized spatial modulation (GSM) uses transmit antenna elements but
fewer transmit radio frequency (RF) chains, . Spatial modulation (SM)
and spatial multiplexing are special cases of GSM with and
, respectively. In GSM, in addition to conveying information bits
through conventional modulation symbols (for example, QAM), the
indices of the active transmit antennas also convey information bits.
In this paper, we investigate {\em GSM for large-scale multiuser MIMO
communications on the uplink}. Our contributions in this paper include: ()
an average bit error probability (ABEP) analysis for maximum-likelihood
detection in multiuser GSM-MIMO on the uplink, where we derive an upper bound
on the ABEP, and () low-complexity algorithms for GSM-MIMO signal detection
and channel estimation at the base station receiver based on message passing.
The analytical upper bounds on the ABEP are found to be tight at moderate to
high signal-to-noise ratios (SNR). The proposed receiver algorithms are found
to scale very well in complexity while achieving near-optimal performance in
large dimensions. Simulation results show that, for the same spectral
efficiency, multiuser GSM-MIMO can outperform multiuser SM-MIMO as well as
conventional multiuser MIMO, by about 2 to 9 dB at a bit error rate of
. Such SNR gains in GSM-MIMO compared to SM-MIMO and conventional MIMO
can be attributed to the fact that, because of a larger number of spatial index
bits, GSM-MIMO can use a lower-order QAM alphabet which is more power
efficient.Comment: IEEE Trans. on Wireless Communications, accepte
Two-Stage LASSO ADMM Signal Detection Algorithm For Large Scale MIMO
This paper explores the benefit of using some of the machine learning
techniques and Big data optimization tools in approximating maximum likelihood
(ML) detection of Large Scale MIMO systems. First, large scale MIMO detection
problem is formulated as a LASSO (Least Absolute Shrinkage and Selection
Operator) optimization problem. Then, Alternating Direction Method of
Multipliers (ADMM) is considered in solving this problem. The choice of ADMM is
motivated by its ability of solving convex optimization problems by breaking
them into smaller sub-problems, each of which are then easier to handle.
Further improvement is obtained using two stages of LASSO with interference
cancellation from the first stage. The proposed algorithm is investigated at
various modulation techniques with different number of antennas. It is also
compared with widely used algorithms in this field. Simulation results
demonstrate the efficacy of the proposed algorithm for both uncoded and coded
cases.Comment: 5 pages, 4 figure
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