354 research outputs found
Power Control in Massive MIMO with Dynamic User Population
This paper considers the problem of power control in Massive MIMO systems
taking into account the pilot contamination issue and the arrivals and
departures of users in the network. Contrary to most of existing work in MIMO
systems that focuses on the physical layer with fixed number of users, we
consider in this work that the users arrive dynamically and leave the network
once they are served. We provide a power control strategy, having a polynomial
complexity, and prove that this policy stabilizes the network whenever
possible. We then provide a distributed implementation of the power control
policy requiring low information exchange between the BSs and show that it
achieves the same stability region as the centralized policy.Comment: conference paper, submitte
RIS-assisted Cell-Free MIMO with Dynamic Arrivals and Departures of Users: A Novel Network Stability Approach
Reconfigurable Intelligent Surfaces (RIS) have recently emerged as a hot
research topic, being widely advocated as a candidate technology for next
generation wireless communications. These surfaces passively alter the behavior
of propagation environments enhancing the performance of wireless communication
systems. In this paper, we study the use of RIS in cell-free multiple-input
multiple-output (MIMO) setting where distributed service antennas, called
Access Points (APs), simultaneously serve the users in the network. While most
existing works focus on the physical layer improvements RIS carry, less
attention has been paid to the impact of dynamic arrivals and departures of the
users. In such a case, ensuring the stability of the network is the main goal.
For that, we propose an optimization framework of the phase shifts, for which
we derived a low-complexity solution. We then provide a theoretical analysis of
the network stability and show that our framework stabilizes the network
whenever it is possible. We also prove that a low complexity solution of our
framework stabilizes a guaranteed fraction (higher than 78.5%) of the stability
region. We provide also numerical results that corroborate the theoretical
claims
Fairness Scheduling in Dense User-Centric Cell-Free Massive MIMO Networks
We consider a user-centric scalable cell-free massive MIMO network with a
total of distributed remote radio unit antennas serving user
equipments (UEs). Many works in the current literature assume ,
enabling high UE data rates but also leading to a system not operating at its
maximum performance in terms of sum throughput. We provide a new perspective on
cell-free massive MIMO networks, investigating rate allocation and the UE
density regime in which the network makes use of its full capability. The UE
density approximately equal to is the range in which the
system reaches the largest sum throughput. In addition, there is a significant
fraction of UEs with relatively low throughput, when serving
UEs simultaneously. We propose to reduce the number of active UEs per time
slot, such that the system does not operate at ``full load'', and impose
throughput fairness among all users via a scheduler designed to maximize a
suitably defined concave componentwise non-decreasing network utility function.
Our numerical simulations show that we can tune the system such that a desired
distribution of the UE throughput, depending on the utility function, is
achieved
Millimeter Wave Cellular Networks: A MAC Layer Perspective
The millimeter wave (mmWave) frequency band is seen as a key enabler of
multi-gigabit wireless access in future cellular networks. In order to overcome
the propagation challenges, mmWave systems use a large number of antenna
elements both at the base station and at the user equipment, which lead to high
directivity gains, fully-directional communications, and possible noise-limited
operations. The fundamental differences between mmWave networks and traditional
ones challenge the classical design constraints, objectives, and available
degrees of freedom. This paper addresses the implications that highly
directional communication has on the design of an efficient medium access
control (MAC) layer. The paper discusses key MAC layer issues, such as
synchronization, random access, handover, channelization, interference
management, scheduling, and association. The paper provides an integrated view
on MAC layer issues for cellular networks, identifies new challenges and
tradeoffs, and provides novel insights and solution approaches.Comment: 21 pages, 9 figures, 2 tables, to appear in IEEE Transactions on
Communication
Frame Structure Design and Analysis for Millimeter Wave Cellular Systems
The millimeter-wave (mmWave) frequencies have attracted considerable
attention for fifth generation (5G) cellular communication as they offer orders
of magnitude greater bandwidth than current cellular systems. However, the
medium access control (MAC) layer may need to be significantly redesigned to
support the highly directional transmissions, ultra-low latencies and high peak
rates expected in mmWave communication. To address these challenges, we present
a novel mmWave MAC layer frame structure with a number of enhancements
including flexible, highly granular transmission times, dynamic control signal
locations, extended messaging and ability to efficiently multiplex directional
control signals. Analytic formulae are derived for the utilization and control
overhead as a function of control periodicity, number of users, traffic
statistics, signal-to-noise ratio and antenna gains. Importantly, the analysis
can incorporate various front-end MIMO capability assumptions -- a critical
feature of mmWave. Under realistic system and traffic assumptions, the analysis
reveals that the proposed flexible frame structure design offers significant
benefits over designs with fixed frame structures similar to current 4G
long-term evolution (LTE). It is also shown that fully digital beamforming
architectures offer significantly lower overhead compared to analog and hybrid
beamforming under equivalent power budgets.Comment: Submitted to IEEE Transactions for Wireless Communication
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