33 research outputs found
Blind Estimation of Effective Downlink Channel Gains in Massive MIMO
We consider the massive MIMO downlink with time-division duplex (TDD)
operation and conjugate beamforming transmission. To reliably decode the
desired signals, the users need to know the effective channel gain. In this
paper, we propose a blind channel estimation method which can be applied at the
users and which does not require any downlink pilots. We show that our proposed
scheme can substantially outperform the case where each user has only
statistical channel knowledge, and that the difference in performance is
particularly large in certain types of channel, most notably keyhole channels.
Compared to schemes that rely on downlink pilots, our proposed scheme yields
more accurate channel estimates for a wide range of signal-to-noise ratios and
avoid spending time-frequency resources on pilots.Comment: IEEE International Conference on Acoustics, Speech and Signal
Processing (ICASSP) 201
Hybrid Analog-Digital Precoding Revisited under Realistic RF Modeling
In this paper we revisit hybrid analog-digital precoding systems with
emphasis on their modelling and radio-frequency (RF) losses, to realistically
evaluate their benefits in 5G system implementations. For this, we decompose
the analog beamforming networks (ABFN) as a bank of commonly used RF components
and formulate realistic model constraints based on their S-parameters.
Specifically, we concentrate on fully-connected ABFN (FC-ABFN) and Butler
networks for implementing the discrete Fourier transform (DFT) in the RF
domain. The results presented in this paper reveal that the performance and
energy efficiency of hybrid precoding systems are severely affected, once
practical factors are considered in the overall design. In this context, we
also show that Butler RF networks are capable of providing better performances
than FC-ABFN for systems with a large number of RF chains.Comment: 12 pages, 5 figure
Hybrid Analog and Digital Precoding: From Practical RF System Models to Information Theoretic Bounds
Hybrid analog-digital precoding is a key millimeter wave access technology,
where an antenna array with reduced number of radio frequency (RF) chains is
used with an RF precoding matrix to increase antenna gain at a reasonable cost.
However, digital and RF precoder algorithms must be accompa- nied by a detailed
system model of the RF precoder. In this work, we provide fundamental RF system
models for these precoders, and show their impact on achievable rates. We show
that hybrid precoding systems suffer from significant degradation, once the
limitations of RF precoding network are accounted. We subsequently quantify
this performance degradation, and use it as a reference for comparing the
performance of different precoding methods. These results indicate that hybrid
precoders must be redesigned (and their rates recomputed) to account for
practical factors.Comment: Accepted in Globecom 16 W
Beamspace Aware Adaptive Channel Estimation for Single-Carrier Time-varying Massive MIMO Channels
In this paper, the problem of sequential beam construction and adaptive
channel estimation based on reduced rank (RR) Kalman filtering for
frequency-selective massive multiple-input multiple-output (MIMO) systems
employing single-carrier (SC) in time division duplex (TDD) mode are
considered. In two-stage beamforming, a new algorithm for statistical
pre-beamformer design is proposed for spatially correlated time-varying
wideband MIMO channels under the assumption that the channel is a stationary
Gauss-Markov random process. The proposed algorithm yields a nearly optimal
pre-beamformer whose beam pattern is designed sequentially with low complexity
by taking the user-grouping into account, and exploiting the properties of
Kalman filtering and associated prediction error covariance matrices. The
resulting design, based on the second order statistical properties of the
channel, generates beamspace on which the RR Kalman estimator can be realized
as accurately as possible. It is observed that the adaptive channel estimation
technique together with the proposed sequential beamspace construction shows
remarkable robustness to the pilot interference. This comes with significant
reduction in both pilot overhead and dimension of the pre-beamformer lowering
both hardware complexity and power consumption.Comment: 7 pages, 3 figures, accepted by IEEE ICC 2017 Wireless Communications
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