49 research outputs found
User-Centric Interference Nulling in Downlink Multi-Antenna Heterogeneous Networks
In heterogeneous networks (HetNets), strong interference due to spectrum
reuse affects each user's signal-to-interference ratio (SIR), and hence is one
limiting factor of network performance. In this paper, we propose a
user-centric interference nulling (IN) scheme in a downlink large-scale HetNet
to improve coverage/outage probability by improving each user's SIR. This IN
scheme utilizes at most maximum IN degree of freedom (DoF) at each macro-BS to
avoid interference to uniformly selected macro (pico) users with
signal-to-individual-interference ratio (SIIR) below a macro (pico) IN
threshold, where the maximum IN DoF and the two IN thresholds are three design
parameters. Using tools from stochastic geometry, we first obtain a tractable
expression of the coverage (equivalently outage) probability. Then, we analyze
the asymptotic coverage/outage probability in the low and high SIR threshold
regimes. The analytical results indicate that the maximum IN DoF can affect the
order gain of the outage probability in the low SIR threshold regime, but
cannot affect the order gain of the coverage probability in the high SIR
threshold regime. Moreover, we characterize the optimal maximum IN DoF which
optimizes the asymptotic coverage/outage probability. The optimization results
reveal that the IN scheme can linearly improve the outage probability in the
low SIR threshold regime, but cannot improve the coverage probability in the
high SIR threshold regime. Finally, numerical results show that the proposed
scheme can achieve good gains in coverage/outage probability over a maximum
ratio beamforming scheme and a user-centric almost blank subframes (ABS)
scheme.Comment: Transactions on Wireless Communications (under revision). arXiv admin
note: text overlap with arXiv:1504.0528
5G Ultra-dense networks with non-uniform Distributed Users
User distribution in ultra-dense networks (UDNs) plays a crucial role in
affecting the performance of UDNs due to the essential coupling between the
traffic and the service provided by the networks. Existing studies are mostly
based on the assumption that users are uniformly distributed in space. The
non-uniform user distribution has not been widely considered despite that it is
much closer to the real scenario. In this paper, Radiation and Absorbing model
(R&A model) is first adopted to analyze the impact of the non-uniformly
distributed users on the performance of 5G UDNs. Based on the R&A model and
queueing network theory, the stationary user density in each hot area is
investigated. Furthermore, the coverage probability, network throughput and
energy efficiency are derived based on the proposed theoretical model. Compared
with the uniformly distributed assumption, it is shown that non-uniform user
distribution has a significant impact on the performance of UDNs.Comment: 14 pages, 10 figure
Towards a Realistic Assessment of Multiple Antenna HCNs: Residual Additive Transceiver Hardware Impairments and Channel Aging
Given the critical dependence of broadcast channels by the accuracy of
channel state information at the transmitter (CSIT), we develop a general
downlink model with zero-forcing (ZF) precoding, applied in realistic
heterogeneous cellular systems with multiple antenna base stations (BSs).
Specifically, we take into consideration imperfect CSIT due to pilot
contamination, channel aging due to users relative movement, and unavoidable
residual additive transceiver hardware impairments (RATHIs). Assuming that the
BSs are Poisson distributed, the main contributions focus on the derivations of
the upper bound of the coverage probability and the achievable user rate for
this general model. We show that both the coverage probability and the user
rate are dependent on the imperfect CSIT and RATHIs. More concretely, we
quantify the resultant performance loss of the network due to these effects. We
depict that the uplink RATHIs have equal impact, but the downlink transmit BS
distortion has a greater impact than the receive hardware impairment of the
user. Thus, the transmit BS hardware should be of better quality than user's
receive hardware. Furthermore, we characterise both the coverage probability
and user rate in terms of the time variation of the channel. It is shown that
both of them decrease with increasing user mobility, but after a specific value
of the normalised Doppler shift, they increase again. Actually, the time
variation, following the Jakes autocorrelation function, mirrors this effect on
coverage probability and user rate. Finally, we consider space division
multiple access (SDMA), single user beamforming (SU-BF), and baseline
single-input single-output (SISO) transmission. A comparison among these
schemes reveals that the coverage by means of SU-BF outperforms SDMA in terms
of coverage.Comment: accepted in IEEE TV