932 research outputs found
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 Beamforming for Large Antenna Arrays with Phase Shifter Selection
This paper proposes an asymptotically optimal hybrid beamforming solution for
large antenna arrays by exploiting the properties of the singular vectors of
the channel matrix. It is shown that the elements of the channel matrix with
Rayleigh fading follow a normal distribution when large antenna arrays are
employed. The proposed beamforming algorithm is effective in both sparse and
rich propagation environments, and is applicable for both point-to-point and
multiuser scenarios. In addition, a closed-form expression and a lower-bound
for the achievable rates are derived when analog and digital phase shifters are
employed. It is shown that the performance of the hybrid beamformers using
phase shifters with more than 2-bits resolution is comparable with analog phase
shifting. A novel phase shifter selection scheme that reduces the power
consumption at the phase shifter network is proposed when the wireless channel
is modeled by Rayleigh fading. Using this selection scheme, the spectral
efficiency can be increased as the power consumption in the phase shifter
network reduces. Compared to the scenario that all of the phase shifters are in
operation, the simulation results indicate that the spectral efficiency
increases when up to 50% of phase shifters are turned off.Comment: Accepted to Transactions on Wireless Communications, 201
On Low-Resolution ADCs in Practical 5G Millimeter-Wave Massive MIMO Systems
Nowadays, millimeter-wave (mmWave) massive multiple-input multiple-output
(MIMO) systems is a favorable candidate for the fifth generation (5G) cellular
systems. However, a key challenge is the high power consumption imposed by its
numerous radio frequency (RF) chains, which may be mitigated by opting for
low-resolution analog-to-digital converters (ADCs), whilst tolerating a
moderate performance loss. In this article, we discuss several important issues
based on the most recent research on mmWave massive MIMO systems relying on
low-resolution ADCs. We discuss the key transceiver design challenges including
channel estimation, signal detector, channel information feedback and transmit
precoding. Furthermore, we introduce a mixed-ADC architecture as an alternative
technique of improving the overall system performance. Finally, the associated
challenges and potential implementations of the practical 5G mmWave massive
MIMO system {with ADC quantizers} are discussed.Comment: to appear in IEEE Communications Magazin
Secure Communication for Spatially Sparse Millimeter-Wave Massive MIMO Channels via Hybrid Precoding
In this paper, we investigate secure communication over sparse millimeter-wave (mm-Wave) massive multiple-input multiple-output (MIMO) channels by exploiting the spatial sparsity of legitimate user's channel. We propose a secure communication scheme in which information data is precoded onto dominant angle components of the sparse channel through a limited number of radio-frequency (RF) chains, while artificial noise (AN) is broadcast over the remaining nondominant angles interfering only with the eavesdropper with a high probability. It is shown that the channel sparsity plays a fundamental role analogous to secret keys in achieving secure communication. Hence, by defining two statistical measures of the channel sparsity, we analytically characterize its impact on secrecy rate. In particular, a substantial improvement on secrecy rate can be obtained by the proposed scheme due to the uncertainty, i.e., 'entropy', introduced by the channel sparsity which is unknown to the eavesdropper. It is revealed that sparsity in the power domain can always contribute to the secrecy rate. In contrast, in the angle domain, there exists an optimal level of sparsity that maximizes the secrecy rate. The effectiveness of the proposed scheme and derived results are verified by numerical simulations
MIMO Wireless Communications Under Statistical Queueing Constraints
The performance of multiple-input multiple-output (MIMO) wireless systems is investigated in the presence of statistical queueing constraints. Queuing constraints are imposed as limitations on buffer violation probabilities. The performance under such constraints is captured through the effective capacity formulation. A detailed analysis of the effective capacity is carried out in the low-power, wideband, and high signal-to-noise ratio (SNR) regimes. In the low-power analysis, expressions for the first and second derivatives of the effective capacity with respect to SNR at SNR = 0 are obtained under various assumptions on the degree of channel state information at the transmitter. Transmission strategies that are optimal in the sense of achieving the first and second derivatives are identified. It is shown that while the first derivative does not get affected by the presence of queueing constraints, the second derivative gets smaller as the constraints become more stringent. Through the energy efficiency analysis, this is shown to imply that the minimum bit energy requirements do not change with more strict limitations but the wideband slope diminishes. Similar results are obtained in the wideband regime if rich multipath fading is being experienced. On the other hand, sparse multipath fading with bounded number of degrees of freedom is shown to increase the minimum bit energy requirements in the presence of queueing constraints. Following the low-SNR study, the impact of buffer limitations on the high-SNR performance is quantified by analyzing the high-SNR slope and the power offset in Rayleigh fading channels. Finally, numerical results are provided to illustrate the theoretical findings, and to demonstrate the interactions between the queueing constraints and spatial dimensions over a wide range of SNR values
Time Reversal Communication in Rayleigh-Fading Broadcast Channels with Pinholes
The paper presents an analysis of the time reversal in independent-multipath
Rayleigh-fading channels with inputs (transmitters) and outputs
(receivers).
The main issues addressed are the condition of statistical stability, the
rate of information transfer and the effect of pinholes. The stability
condition is proved to be
for broadband channels and
for narrowband channels where is the symbol rate,
is the bandwidth and is the {\em effective} number (maybe
less than 1) of transmitters. It is shown that when the number of screens,
, is relatively low compared to the logarithm of numbers of pinholes
is given by the {\em harmonic} (or {\em inverse}) {\em sum} of
the number of transmitters and the numbers of pinholes at all screens.
The novel idea of the effective number of time reversal array (TRA) elements
is introduced to derive the stability condition and estimate the channel
capacity in the presence of multi-screen pinholes. The information rate, under
the constraints of the noise power per unit frequency and the average
total power , attains the supremum in the regime . In particular, when the
optimal information rate can be achieved with statistically stable, sharply
focused signals.Comment: Corrected typos and minor change of conten
Efficient Uniform Channel Quantization of Sparse CIR for Downlink OFDM Systems
Channel state information at the transmitter side is an important issue for wireless communications systems, namely when precoding techniques are employed. Recent works explored random vector quantization (RVQ) as a solution for limited feedback for multi-user systems equipped with multiple antennas. Despite of being a good option for narrowband channels, this method requires large complexity and is not efficient for sparse channels. To overcome these drawbacks we consider a strategy based on uniform quantization, denoted partial uniform quantization (P-UQ), where just part of channel frequency response is quantized. This allows an efficient feedback of channel frequency response from the receivers to the transmitter, by using a reduced number of quantization bits. The comparison between the proposed P-UQ-based method and RVQ performed in this paper leads to the conclusion that the most advantageous method for sparse channels is the P-UQ
- …