797 research outputs found
Improving the Performance of OTDOA based Positioning in NB-IoT Systems
In this paper, we consider positioning with
observed-time-difference-of-arrival (OTDOA) for a device deployed in
long-term-evolution (LTE) based narrow-band Internet-of-things (NB-IoT)
systems. We propose an iterative expectation-maximization based successive
interference cancellation (EM-SIC) algorithm to jointly consider estimations of
residual frequency-offset (FO), fading-channel taps and time-of-arrival (ToA)
of the first arrival-path for each of the detected cells. In order to design a
low complexity ToA detector and also due to the limits of low-cost analog
circuits, we assume an NB-IoT device working at a low-sampling rate such as
1.92 MHz or lower. The proposed EM-SIC algorithm comprises two stages to detect
ToA, based on which OTDOA can be calculated. In a first stage, after running
the EM-SIC block a predefined number of iterations, a coarse ToA is estimated
for each of the detected cells. Then in a second stage, to improve the ToA
resolution, a low-pass filter is utilized to interpolate the correlations of
time-domain PRS signal evaluated at a low sampling-rate to a high sampling-rate
such as 30.72 MHz. To keep low-complexity, only the correlations inside a small
search window centered at the coarse ToA estimates are upsampled. Then, the
refined ToAs are estimated based on upsampled correlations. If at least three
cells are detected, with OTDOA and the locations of detected cell sites, the
position of the NB-IoT device can be estimated. We show through numerical
simulations that, the proposed EM-SIC based ToA detector is robust against
impairments introduced by inter-cell interference, fading-channel and residual
FO. Thus significant signal-to-noise (SNR) gains are obtained over traditional
ToA detectors that do not consider these impairments when positioning a device.Comment: Accepted in GlobeCom 2017, 7 pages, 11 figure
INTERFERENCE MANAGEMENT IN LTE SYSTEM AND BEYOUND
The key challenges to high throughput in cellular wireless communication system are interference, mobility and bandwidth limitation. Mobility has never been a problem until recently, bandwidth has been constantly improved upon through the evolutions in cellular wireless communication system but interference has been a constant limitation to any improvement that may have resulted from such evolution. The fundamental challenge to a system designer or a researcher is how to achieve high data rate in motion (high speed) in a cellular system that is intrinsically interference-limited.
Multi-antenna is the solution to data on the move and the capacity of multi-antenna system has been demonstrated to increase proportionally with increase in the number of antennas at both transmitter and receiver for point-to-point communications and multi-user environment. However, the capacity gain in both uplink and downlink is limited in a multi-user environment like cellular system by interference, the number of antennas at the base station, complexity and space constraint particularly for a mobile terminal.
This challenge in the downlink provided the motivation to investigate successive interference cancellation (SIC) as an interference management tool LTE system and beyond. The Simulation revealed that ordered successive interference (OSIC) out performs non-ordered successive interference cancellation (NSIC) and the additional complexity is justified based on the associated gain in BER performance of OSIC. The major drawback of OSIC is that it is not efficient in network environment employing power control or power allocation. Additional interference management techniques will be required to fully manage the interference.fi=Opinnäytetyö kokotekstinä PDF-muodossa.|en=Thesis fulltext in PDF format.|sv=Lärdomsprov tillgängligt som fulltext i PDF-format
Two-Layered Superposition of Broadcast/Multicast and Unicast Signals in Multiuser OFDMA Systems
We study optimal delivery strategies of one common and independent
messages from a source to multiple users in wireless environments. In
particular, two-layered superposition of broadcast/multicast and unicast
signals is considered in a downlink multiuser OFDMA system. In the literature
and industry, the two-layer superposition is often considered as a pragmatic
approach to make a compromise between the simple but suboptimal orthogonal
multiplexing (OM) and the optimal but complex fully-layered non-orthogonal
multiplexing. In this work, we show that only two-layers are necessary to
achieve the maximum sum-rate when the common message has higher priority than
the individual unicast messages, and OM cannot be sum-rate optimal in
general. We develop an algorithm that finds the optimal power allocation over
the two-layers and across the OFDMA radio resources in static channels and a
class of fading channels. Two main use-cases are considered: i) Multicast and
unicast multiplexing when users with uplink capabilities request both
common and independent messages, and ii) broadcast and unicast multiplexing
when the common message targets receive-only devices and users with uplink
capabilities additionally request independent messages. Finally, we develop a
transceiver design for broadcast/multicast and unicast superposition
transmission based on LTE-A-Pro physical layer and show with numerical
evaluations in mobile environments with multipath propagation that the capacity
improvements can be translated into significant practical performance gains
compared to the orthogonal schemes in the 3GPP specifications. We also analyze
the impact of real channel estimation and show that significant gains in terms
of spectral efficiency or coverage area are still available even with
estimation errors and imperfect interference cancellation for the two-layered
superposition system
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