135 research outputs found
Relaying systems with reciprocity mismatch : impact analysis and calibration
Cooperative beamforming can provide significant performance improvement for relaying systems with the help of the channel state information (CSI). In time-division duplexing (TDD) mode, the estimated CSI will deteriorate due to the reciprocity mismatch. In this work, we examine the impact and the calibration of the reciprocity mismatch in relaying systems. To evaluate the impact of the reciprocity mismatch for all devices, the closed-form expression of the achievable rate is first derived. Then, we analyze the performance loss caused by the reciprocity mismatch at sources, relays, and destinations respectively to show that the mismatch at relays dominates the impact. To compensate the performance loss, a two-stage calibration scheme is proposed for relays. Specifically, relays perform the intra-calibration based on circuits independently. Further, the inter-calibration based on the discrete Fourier transform (DFT) codebook is operated to improve the calibration performance by cooperation transmission, which has never been considered in previous work. Finally, we derive the achievable rate after relays perform the proposed reciprocity calibration scheme and investigate the impact of estimation errors on the system performance. Simulation results are presented to verify the analytical results and to show the performance of the proposed calibration approach
Reciprocity Calibration for Massive MIMO: Proposal, Modeling and Validation
This paper presents a mutual coupling based calibration method for
time-division-duplex massive MIMO systems, which enables downlink precoding
based on uplink channel estimates. The entire calibration procedure is carried
out solely at the base station (BS) side by sounding all BS antenna pairs. An
Expectation-Maximization (EM) algorithm is derived, which processes the
measured channels in order to estimate calibration coefficients. The EM
algorithm outperforms current state-of-the-art narrow-band calibration schemes
in a mean squared error (MSE) and sum-rate capacity sense. Like its
predecessors, the EM algorithm is general in the sense that it is not only
suitable to calibrate a co-located massive MIMO BS, but also very suitable for
calibrating multiple BSs in distributed MIMO systems.
The proposed method is validated with experimental evidence obtained from a
massive MIMO testbed. In addition, we address the estimated narrow-band
calibration coefficients as a stochastic process across frequency, and study
the subspace of this process based on measurement data. With the insights of
this study, we propose an estimator which exploits the structure of the process
in order to reduce the calibration error across frequency. A model for the
calibration error is also proposed based on the asymptotic properties of the
estimator, and is validated with measurement results.Comment: Submitted to IEEE Transactions on Wireless Communications,
21/Feb/201
Temporal Analysis of Measured LOS Massive MIMO Channels with Mobility
The first measured results for massive multiple-input, multiple-output (MIMO)
performance in a line-of-sight (LOS) scenario with moderate mobility are
presented, with 8 users served by a 100 antenna base Station (BS) at 3.7 GHz.
When such a large number of channels dynamically change, the inherent
propagation and processing delay has a critical relationship with the rate of
change, as the use of outdated channel information can result in severe
detection and precoding inaccuracies. For the downlink (DL) in particular, a
time division duplex (TDD) configuration synonymous with massive MIMO
deployments could mean only the uplink (UL) is usable in extreme cases.
Therefore, it is of great interest to investigate the impact of mobility on
massive MIMO performance and consider ways to combat the potential limitations.
In a mobile scenario with moving cars and pedestrians, the correlation of the
MIMO channel vector over time is inspected for vehicles moving up to 29 km/h.
For a 100 antenna system, it is found that the channel state information (CSI)
update rate requirement may increase by 7 times when compared to an 8 antenna
system, whilst the power control update rate could be decreased by at least 5
times relative to a single antenna system.Comment: Accepted for presentation at the 85th IEEE Vehicular Technology
Conference in Sydney. 5 Pages. arXiv admin note: substantial text overlap
with arXiv:1701.0881
Mobile Cell-Free Massive MIMO: Challenges, Solutions, and Future Directions
Cell-free (CF) massive multiple-input multiple-output (MIMO) systems, which
exploit many geographically distributed access points to coherently serve user
equipments via spatial multiplexing on the same time-frequency resource, has
become a vital component of the next-generation mobile communication networks.
Theoretically, CF massive MIMO systems have many advantages, such as large
capacity, great coverage, and high reliability, but several obstacles must be
overcome. In this article, we study the paradigm of CF massive MIMO-aided
mobile communications, including the main application scenarios and associated
deployment architectures. Furthermore, we thoroughly investigate the challenges
of CF massive MIMO-aided mobile communications. We then exploit a novel
predictor antenna, hierarchical cancellation, rate-splitting and dynamic
clustering system for CF massive MIMO. Finally, several important research
directions regarding CF massive MIMO for mobile communications are presented to
facilitate further investigation.Comment: 9 pages, 4 figures, 2 tables, accepted by IEEE Wireless
Communications Magazin
Correctly Modeling TX and RX Chain in (Distributed) Massive MIMO -- New Fundamental Insights on Coherency
This letter shows that the TX and RX models commonly used in literature for
downlink (distributed) massive MIMO are inaccurate, leading also to inaccurate
conclusions. In particular, the Local Oscillator (LO) effect should be modeled
as in the transmitter chain and in the receiver chain,
i.e., different signs. A common misconception in literature is to use the same
sign for both chains. By correctly modeling TX and RX chain, one realizes that
the LO phases are included in the reciprocity calibration and whenever the LO
phases drift apart, a new reciprocity calibration becomes necessary (the same
applies to time drifts). Thus, free-running LOs and the commonly made
assumption of perfect reciprocity calibration (to enable blind DL channel
estimation) are both not that useful, as they would require too much
calibration overhead. Instead, the LOs at the base stations should be locked
and relative reciprocity calibration in combination with downlink demodulation
reference symbols should be employed
6G Radio Testbeds: Requirements, Trends, and Approaches
The proof of the pudding is in the eating - that is why 6G testbeds are
essential in the progress towards the next generation of wireless networks.
Theoretical research towards 6G wireless networks is proposing advanced
technologies to serve new applications and drastically improve the energy
performance of the network. Testbeds are indispensable to validate these new
technologies under more realistic conditions. This paper clarifies the
requirements for 6G radio testbeds, reveals trends, and introduces approaches
towards their development
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