2,847 research outputs found
Pilot Decontamination Through Pilot Sequence Hopping in Massive MIMO Systems
This work concerns wireless cellular networks applying massive multiple-input
multiple-output (MIMO) technology. In such a system, the base station in a
given cell is equipped with a very large number (hundreds or even thousands) of
antennas and serves multiple users. Estimation of the channel from the base
station to each user is performed at the base station using an uplink pilot
sequence. Such a channel estimation procedure suffers from pilot contamination.
Orthogonal pilot sequences are used in a given cell but, due to the shortage of
orthogonal sequences, the same pilot sequences must be reused in neighboring
cells, causing pilot contamination. The solution presented in this paper
suppresses pilot contamination, without the need for coordination among cells.
Pilot sequence hopping is performed at each transmission slot, which provides a
randomization of the pilot contamination. Using a modified Kalman filter, it is
shown that such randomized contamination can be significantly suppressed.
Comparisons with conventional estimation methods show that the mean squared
error can be lowered as much as an order of magnitude at low mobility
Massive MIMO for Crowd Scenarios: A Solution Based on Random Access
This paper presents a new approach to intra-cell pilot contamination in
crowded massive MIMO scenarios. The approach relies on two essential properties
of a massive MIMO system, namely near-orthogonality between user channels and
near-stability of channel powers. Signal processing techniques that take
advantage of these properties allow us to view a set of contaminated pilot
signals as a graph code on which iterative belief propagation can be performed.
This makes it possible to decontaminate pilot signals and increase the
throughput of the system. The proposed solution exhibits high performance with
large improvements over the conventional method. The improvements come at the
price of an increased error rate, although this effect is shown to decrease
significantly for increasing number of antennas at the base station
Towards Very Large Aperture Massive MIMO: a measurement based study
Massive MIMO is a new technique for wireless communications that claims to
offer very high system throughput and energy efficiency in multi-user
scenarios. The cost is to add a very large number of antennas at the base
station. Theoretical research has probed these benefits, but very few
measurements have showed the potential of Massive MIMO in practice. We
investigate the properties of measured Massive MIMO channels in a large indoor
venue. We describe a measurement campaign using 3 arrays having different shape
and aperture, with 64 antennas and 8 users with 2 antennas each. We focus on
the impact of the array aperture which is the main limiting factor in the
degrees of freedom available in the multiple antenna channel. We find that
performance is improved as the aperture increases, with an impact mostly
visible in crowded scenarios where the users are closely spaced. We also test
MIMO capability within a same user device with user proximity effect. We see a
good channel resolvability with confirmation of the strong effect of the user
hand grip. At last, we highlight that propagation conditions where
line-of-sight is dominant can be favorable
Random Access for Massive MIMO Systems with Intra-Cell Pilot Contamination
Massive MIMO systems, where the base stations are equipped with hundreds of
antenna elements, are an attractive way to attain unprecedented spectral
efficiency in future wireless networks. In the "classical" massive MIMO
setting, the terminals are assumed fully loaded and a main impairment to the
performance comes from the inter-cell pilot contamination, i.e., interference
from terminals in neighboring cells using the same pilots as in the home cell.
However, when the terminals are active intermittently, it is viable to avoid
inter-cell contamination by pre-allocation of pilots, while same-cell terminals
use random access to select the allocated pilot sequences. This leads to the
problem of intra-cell pilot contamination. We propose a framework for random
access in massive MIMO networks and derive new uplink sum rate expressions that
take intra-cell pilot collisions, intermittent terminal activity, and
interference into account. We use these expressions to optimize the terminal
activation probability and pilot length
Design and Performance Analysis of Non-Coherent Detection Systems with Massive Receiver Arrays
Harvesting the gain of a large number of antennas in a mmWave band has mainly
been relying on the costly operation of channel state information (CSI)
acquisition and cumbersome phase shifters. Recent works have started to
investigate the possibility to use receivers based on energy detection (ED),
where a single data stream is decoded based on the channel and noise energy.
The asymptotic features of the massive receiver array lead to a system where
the impact of the noise becomes predictable due to a noise hardening effect.
This in effect extends the communication range compared to the receiver with a
small number of antennas, as the latter is limited by the unpredictability of
the additive noise. When the channel has a large number of spatial degrees of
freedom, the system becomes robust to imperfect channel knowledge due to
channel hardening. We propose two detection methods based on the instantaneous
and average channel energy, respectively. Meanwhile, we design the detection
thresholds based on the asymptotic properties of the received energy.
Differently from existing works, we analyze the scaling law behavior of the
symbol-error-rate (SER). When the instantaneous channel energy is known, the
performance of ED approaches that of the coherent detection in high SNR
scenarios. When the receiver relies on the average channel energy, our
performance analysis is based on the exact SER, rather than an approximation.
It is shown that the logarithm of SER decreases linearly as a function of the
number of antennas. Additionally, a saturation appears at high SNR for PAM
constellations of order larger than two, due to the uncertainty on the channel
energy. Simulation results show that ED, with a much lower complexity, achieves
promising performance both in Rayleigh fading channels and in sparse channels
Massive MIMO for Ultra-reliable Communications with Constellations for Dual Coherent-noncoherent Detection
The stringent requirements of ultra-reliable low-latency communications
(URLLC) require rethinking of the physical layer transmission techniques.
Massive antenna arrays are seen as an enabler of the emerging
generation systems, due to increases in spectral efficiency and degrees of
freedom for transmissions, which can greatly improve reliability under
demanding latency requirements. Massive array coherent processing relies on
accurate channel state information (CSI) in order to achieve high reliability.
In this paper, we investigate the impact of imperfect CSI in a single-input
multiple-output (SIMO) system on the coherent receiver. An amplitude-phase
keying (APK) symbol constellation is proposed, where each two symmetric symbols
reside on distinct power levels. The symbols are demodulated using a dual-stage
non-coherent and coherent detection strategy, in order to improve symbol
reliability. By means of analysis and simulation, we find an adequate scaling
of the constellation and show that for high signal-to-noise ratio (SNR) and
inaccurate CSI regime, the proposed scheme enhances receiver performance.Comment: Accepted at WSA 2018, special session on "Massive MIMO for mobile
broadband communications and new 5G services
Random Access Protocol for Massive MIMO: Strongest-User Collision Resolution (SUCR)
Wireless networks with many antennas at the base stations and multiplexing of
many users, known as Massive MIMO systems, are key to handle the rapid growth
of data traffic. As the number of users increases, the random access in
contemporary networks will be flooded by user collisions. In this paper, we
propose a reengineered random access protocol, coined strongest-user collision
resolution (SUCR). It exploits the channel hardening feature of Massive MIMO
channels to enable each user to detect collisions, determine how strong the
contenders' channels are, and only keep transmitting if it has the strongest
channel gain. The proposed SUCR protocol can quickly and distributively resolve
the vast majority of all pilot collisions.Comment: Published at the IEEE International Conference on Communications
(ICC), 2016, 6 pages, 6 figures. (c) 2016 IEEE. Personal use of this material
is permitte
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