5 research outputs found
Downlink Performance of Superimposed Pilots in Massive MIMO systems
In this paper, we investigate the downlink throughput performance of a
massive multiple-input multiple-output (MIMO) system that employs superimposed
pilots for channel estimation. The component of downlink (DL) interference that
results from transmitting data alongside pilots in the uplink (UL) is shown to
decrease at a rate proportional to the square root of the number of antennas at
the BS. The normalized mean-squared error (NMSE) of the channel estimate is
compared with the Bayesian Cram\'{e}r-Rao lower bound that is derived for the
system, and the former is also shown to diminish with increasing number of
antennas at the base station (BS). Furthermore, we show that staggered pilots
are a particular case of superimposed pilots and offer the downlink throughput
of superimposed pilots while retaining the UL spectral and energy efficiency of
regular pilots. We also extend the framework for designing a hybrid system,
consisting of users that transmit either regular or superimposed pilots, to
minimize both the UL and DL interference. The improved NMSE and DL rates of the
channel estimator based on superimposed pilots are demonstrated by means of
simulations.Comment: 28 single-column pages, 6 figures, 1 table, Submitted to IEEE Trans.
Wireless Commun. in Aug 2017. Revised Submission in Feb. 201
Spectral and Energy Efficiency of Superimposed Pilots in Uplink Massive MIMO
Next generation wireless networks aim at providing substantial improvements
in spectral efficiency (SE) and energy efficiency (EE). Massive MIMO has been
proved to be a viable technology to achieve these goals by spatially
multiplexing several users using many base station (BS) antennas. A potential
limitation of Massive MIMO in multicell systems is pilot contamination, which
arises in the channel estimation process from the interference caused by
reusing pilots in neighboring cells. A standard method to reduce pilot
contamination, known as regular pilot (RP), is to adjust the length of pilot
sequences while transmitting data and pilot symbols disjointly. An alternative
method, called superimposed pilot (SP), sends a superposition of pilot and data
symbols. This allows to use longer pilots which, in turn, reduces pilot
contamination. We consider the uplink of a multicell Massive MIMO network using
maximum ratio combining detection and compare RP and SP in terms of SE and EE.
To this end, we derive rigorous closed-form achievable rates with SP under a
practical random BS deployment. We prove that the reduction of pilot
contamination with SP is outweighed by the additional coherent and non-coherent
interference. Numerical results show that when both methods are optimized, RP
achieves comparable SE and EE to SP in practical scenarios.Comment: 32 pages, 12 figures, 3 tables. Submitted in March 2017 to IEEE
Transactions on Wireless Communication
Massive MIMO systems for 5G: a systematic mapping study on antenna design challenges and channel estimation open issues
The next generation of mobile networks (5G) is expected to achieve high data rates, reduce latency, as well as improve the spectral and energy efficiency of wireless communication systems. Several technologies are being explored to be used in 5G systems. One of the main promising technologies that is seen to be the enabler of 5G is massive multiple-input multiple-output (mMIMO) systems. Numerous studies have indicated the utility of mMIMO in upcoming wireless networks. However, there are several challenges that needs to be unravelled. In this paper, the latest progress of research on challenges in mMIMO systems is tracked, in the context of mutual coupling, antenna selection, pilot contamination and feedback overhead. The results of a systematic mapping study performed on 63 selected primary studies, published between the year 2017 till the second quarter of 2020, are presented. The main objective of this secondary study is to identify the challenges regarding antenna design and channel estimation, give an overview on the state-of-the-art solutions proposed in the literature, and finally, discuss emerging open research issues that need to be considered before the implementation of mMIMO systems in 5G networks
Space station systems: A bibliography with indexes (supplement 10)
This bibliography lists 1,422 reports, articles, and other documents introduced into the NASA scientific and technical information system between July 1, 1989 and December 31, 1989. Its purpose is to provide helpful information to researchers, designers and managers engaged in Space Station technology development and mission design. Coverage includes documents that define major systems and subsystems related to structures and dynamic control, electronics and power supplies, propulsion, and payload integration. In addition, orbital construction methods, servicing and support requirements, procedures and operations, and missions for the current and future Space Station are included