9,821 research outputs found
User-Base Station Association in HetSNets: Complexity and Efficient Algorithms
This work considers the problem of user association to small-cell base
stations (SBSs) in a heterogeneous and small-cell network (HetSNet). Two
optimization problems are investigated, which are maximizing the set of
associated users to the SBSs (the unweighted problem) and maximizing the set of
weighted associated users to the SBSs (the weighted problem), under
signal-to-interference-plus-noise ratio (SINR) constraints. Both problems are
formulated as linear integer programs. The weighted problem is known to be
NP-hard and, in this paper, the unweighted problem is proved to be NP-hard as
well. Therefore, this paper develops two heuristic polynomial-time algorithms
to solve both problems. The computational complexity of the proposed algorithms
is evaluated and is shown to be far more efficient than the complexity of the
optimal brute-force (BF) algorithm. Moreover, the paper benchmarks the
performance of the proposed algorithms against the BF algorithm, the
branch-and-bound (B\&B) algorithm and standard algorithms, through numerical
simulations. The results demonstrate the close-to-optimal performance of the
proposed algorithms. They also show that the weighted problem can be solved to
provide solutions that are fair between users or to balance the load among
SBSs
Harmonized Cellular and Distributed Massive MIMO: Load Balancing and Scheduling
Multi-tier networks with large-array base stations (BSs) that are able to
operate in the "massive MIMO" regime are envisioned to play a key role in
meeting the exploding wireless traffic demands. Operated over small cells with
reciprocity-based training, massive MIMO promises large spectral efficiencies
per unit area with low overheads. Also, near-optimal user-BS association and
resource allocation are possible in cellular massive MIMO HetNets using simple
admission control mechanisms and rudimentary BS schedulers, since scheduled
user rates can be predicted a priori with massive MIMO.
Reciprocity-based training naturally enables coordinated multi-point
transmission (CoMP), as each uplink pilot inherently trains antenna arrays at
all nearby BSs. In this paper we consider a distributed-MIMO form of CoMP,
which improves cell-edge performance without requiring channel state
information exchanges among cooperating BSs. We present methods for harmonized
operation of distributed and cellular massive MIMO in the downlink that
optimize resource allocation at a coarser time scale across the network. We
also present scheduling policies at the resource block level which target
approaching the optimal allocations. Simulations reveal that the proposed
methods can significantly outperform the network-optimized cellular-only
massive MIMO operation (i.e., operation without CoMP), especially at the cell
edge
Energy Efficient User Association and Power Allocation in Millimeter Wave Based Ultra Dense Networks with Energy Harvesting Base Stations
Millimeter wave (mmWave) communication technologies have recently emerged as
an attractive solution to meet the exponentially increasing demand on mobile
data traffic. Moreover, ultra dense networks (UDNs) combined with mmWave
technology are expected to increase both energy efficiency and spectral
efficiency. In this paper, user association and power allocation in mmWave
based UDNs is considered with attention to load balance constraints, energy
harvesting by base stations, user quality of service requirements, energy
efficiency, and cross-tier interference limits. The joint user association and
power optimization problem is modeled as a mixed-integer programming problem,
which is then transformed into a convex optimization problem by relaxing the
user association indicator and solved by Lagrangian dual decomposition. An
iterative gradient user association and power allocation algorithm is proposed
and shown to converge rapidly to an optimal point. The complexity of the
proposed algorithm is analyzed and the effectiveness of the proposed scheme
compared with existing methods is verified by simulations.Comment: to appear, IEEE Journal on Selected Areas in Communications, 201
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