79 research outputs found
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-Digital Precoding Revisited Under Realistic RF Modeling
In this letter, we revisit hybrid analog-digital precoding systems with emphasis on the modeling of their radio-frequency (RF) losses, to realistically evaluate their benefits in 5G system implementations. We focus on fully-connected analog beamforming networks (FC-ABFNs) and on discrete Fourier transform implementations, and decompose these as a bank of commonly used RF components. We then model their losses based on their S-parameters. Our results reveal that the performance and energy efficiency of hybrid precoding systems are severely affected once these, commonly ignored, losses are considered in the overall design. In this context, we also show that hybrid precoder designs similar to Butler matrices are capable of providing better performances than FC-ABFN for systems with a large number of RF chains
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
Energy efficiency of mmWave massive MIMO precoding with low-resolution DACs
With the congestion of the sub-6 GHz spectrum, the interest in massive
multiple-input multiple-output (MIMO) systems operating on millimeter wave
spectrum grows. In order to reduce the power consumption of such massive MIMO
systems, hybrid analog/digital transceivers and application of low-resolution
digital-to-analog/analog-to-digital converters have been recently proposed. In
this work, we investigate the energy efficiency of quantized hybrid
transmitters equipped with a fully/partially-connected phase-shifting network
composed of active/passive phase-shifters and compare it to that of quantized
digital precoders. We introduce a quantized single-user MIMO system model based
on an additive quantization noise approximation considering realistic power
consumption and loss models to evaluate the spectral and energy efficiencies of
the transmit precoding methods. Simulation results show that
partially-connected hybrid precoders can be more energy-efficient compared to
digital precoders, while fully-connected hybrid precoders exhibit poor energy
efficiency in general. Also, the topology of phase-shifting components offers
an energy-spectral efficiency trade-off: active phase-shifters provide higher
data rates, while passive phase-shifters maintain better energy efficiency.Comment: Published in IEEE Journal of Selected Topics in Signal Processin
Low-Complexity Hybrid Beamforming for Massive MIMO Systems in Frequency-Selective Channels
Hybrid beamforming for frequency-selective channels is a challenging problem
as the phase shifters provide the same phase shift to all of the subcarriers.
The existing approaches solely rely on the channel's frequency response and the
hybrid beamformers maximize the average spectral efficiency over the whole
frequency band. Compared to state-of-the-art, we show that substantial sum-rate
gains can be achieved, both for rich and sparse scattering channels, by jointly
exploiting the frequency and time domain characteristics of the massive
multiple-input multiple-output (MIMO) channels. In our proposed approach, the
radio frequency (RF) beamformer coherently combines the received symbols in the
time domain and, thus, it concentrates signal's power on a specific time
sample. As a result, the RF beamformer flattens the frequency response of the
"effective" transmission channel and reduces its root mean square delay spread.
Then, a baseband combiner mitigates the residual interference in the frequency
domain. We present the closed-form expressions of the proposed beamformer and
its performance by leveraging the favorable propagation condition of massive
MIMO channels and we prove that our proposed scheme can achieve the performance
of fully-digital zero-forcing when number of employed phase shifter networks is
twice the resolvable multipath components in the time domain.Comment: Accepted to IEEE Acces
Hybrid Analog-Digital Millimeter-Wave MU-MIMO Transmission with Virtual Path Selection
In this letter, we propose a low-complexity hybrid precoding and combining design for the mmWave MUMIMO transmission, applicable to both fully-connected and subconnected structures. Analog precoding and combining schemes are firstly designed, where a joint approach, a decoupled approach, and a sub-optimal approach are proposed to harvest the array gain. Virtual path selection is performed to maximize the channel gain of the analog effective channel. Then, based on the effective channel, a low-dimensional zero-forcing (ZF) precoding is applied in the baseband to manage the interference. The simulation results show that the proposed techniques offer an enhanced performance-complexity trade-off compared to both existing hybrid schemes and fully digital schemes
Beam division multiple access for millimeter wave massive MIMO: Hybrid zero-forcing beamforming with user selection
Massive multiple-input multiple-output (MIMO) systems are considered a promising solution to minimize multiuser interference (MUI) based on simple precoding techniques with a massive antenna array at a base station (BS). This paper presents a novel approach of beam division multiple access (BDMA) which BS transmit signals to multiusers at the same time via different beams based on hybrid beamforming and user-beam schedule. With the selection of users whose steering vectors are orthogonal to each other, interference between users is significantly improved. While, the efficiency spectrum of proposed scheme reaches to the performance of fully digital solutions, the multiuser interference is considerably reduced
Energy-Efficient System Design for Future Wireless Communications
The exponential growth of wireless data traffic has caused a significant increase in the power consumption of wireless communications systems due to the higher complexity of the transceiver structures required to establish the communication links. For this reason, in this Thesis we propose and characterize technologies for improving the energy efficiency of multiple-antenna wireless communications. This Thesis firstly focuses on energy-efficient transmission schemes and commences by introducing a scheme for alleviating the power loss experienced by the Tomlinson-Harashima precoder, by aligning the interference of a number of users with the symbols to transmit. Subsequently, a strategy for improving the performance of space shift keying transmission via symbol pre-scaling is presented. This scheme re-formulates complex optimization problems via semidefinite relaxation to yield problem formulations that can be efficiently solved. In a similar line, this Thesis designs a signal detection scheme based on compressive sensing to improve the energy efficiency of spatial modulation systems in multiple access channels. The proposed technique relies on exploiting the particular structure and sparsity that spatial modulation systems inherently possess to enhance performance. This Thesis also presents research carried out with the aim of reducing the hardware complexity and associated power consumption of large scale multiple-antenna base stations. In this context, the employment of incomplete channel state information is proposed to achieve the above-mentioned objective in correlated communication channels. The candidate’s work developed in Bell Labs is also presented, where the feasibility of simplified hardware architectures for massive antenna systems is assessed with real channel measurements. Moreover, a strategy for reducing the hardware complexity of antenna selection schemes by simplifying the design of the switching procedure is also analyzed. Overall, extensive theoretical and simulation results support the improved energy efficiency and complexity of the proposed schemes, towards green wireless communications systems
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