19 research outputs found
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
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
Channel Measurements and Analysis for Very Large Array Systems At 2.6 GHz
Abstract in UndeterminedVery large MIMO is a technique that potentially can offer large network capacities in multi-user scenarios where the users are equipped only with single antennas. In this paper we are investigating channel properties for a realistic, though somewhat extreme, outdoor base station scenario using a large array. We present measurement results using a 128 element linear array base station and 26 different user position in line-of-sight (LOS) and 10 different user position in non line-of-sight (NLOS). We analyze the Ricean K-factor, received power levels over the array, antenna correlation and eigenvalue distributions. We show that the statistical properties of the received signal vary significantly over the large array. Near field effects and the non-stationarities over the array help decorrelating the channel for different users, thereby providing a favorable channel conditions with stable channels and low interference for the considered single antenna users
Hybrid beamforming for massive MIMO systems.
Massive multiple-input multiple-output (MIMO) technology is considered as one of the enabling technologies to scale up the data rates for the future communication systems. Traditional MIMO systems employ digital beamforming where each antenna element is equipped with one radio frequency (RF) chain. When the number of the antennas are scaled up, the cost and power consumption of massive MIMO systems also increase significantly. Recently, hybrid analog-and-digital beamformers have attracted a lot of attention as a cost effective approach to benefit from the advantages of massive MIMO. In hybrid structure, a small number of RF chains are connected to a large number of antennas through a network of phase shifters. The optimal hybrid beamforming problem is a complex nonconvex optimization due to the nonconvex constraint imposed by phase shifters. The overall objective of this thesis is to provide simple and effective hybrid beamforming solutions for narrowband point-to-point and multiuser massive MIMO scenarios. Firstly, hybrid beamforming problem for a point-to-point communication system with perfect channel state information (CSI) is investigated, and an effective codebook based hybrid beamforming with low resolution phase shifters is proposed which is suitable for sparse scattering channels. Then, by leveraging the properties of massive MIMO, an asymptotically optimal hybrid beamforming solution as well as its closed-form formula will be presented. It will be shown that the proposed method is effective in both sparse and rich scattering propagation environments. In addition, the closed-form expression and lower-bounds for the achievable rates are derived when analog and digital phase shifters are employed. Secondly, hybrid beamforming problem to maximise the total sum-rate for the downlink of multiuser MIMO is investigated, and an effective solution as well as its closed-form expression for this system is proposed. The presented solutions for the single-antenna and multiantenna scenarios are shown to be effective as they can achieve a similar sum-rate as digital beamforming can reach. In addition, it is shown that the proposed technique with low-cost low resolution phase shifters at the RF beamformer demonstrates a comparable performance to that of a hybrid beamformer with an expensive analog beamformer. Finally, two novel hybrid beamforming techniques are proposed to reduce the power consumption at the RF beamformer. Defining a threshold level, it is shown that half of the phase shifters could be turned off without a performance loss when the wireless channel matrix is modeled by Rayleigh fading. Then, we reduce the number of the phase shifters by using a combination of phase shifters and switches at the RF beamformer. The proposed methods can significantly reduce the power consumption as switches, in general, have lower power consumption compared to phase shifters. It is noted that the presented algorithms and the closed-form expressions of their performance are derived by using the asymptotic properties of the elements of the singular vectors for the rich scattering channel matrix
Hybrid beamforming with reduced number of phase shifters for massive MIMO systems
In this paper, two novel hybrid beamforming methods are proposed to reduce the cost and power consumption of hybrid beamformers with a subconnected phase shifter network structure in massive multiple-input multiple-output systems. This is achieved by replacing some of the phase shifters with switches, which, in general, are cheaper and have lower power consumption compared to phase shifters. The proposed methods and the closed-form expressions of their performance are derived according to the properties of the elements of the singular vectors of the channel matrix. In the first approach, it is shown that by combining the subconnected phase shifter network with a fully connected switch architecture, the number of the phase shifters can be reduced up to 50%, while the spectral efficiency is preserved. Then, in order to simplify the structure of the switch network, the fully connected switches are replaced by a subconnected switch network, e.g., binary switches. The analytical and simulation results indicate that just by using 25% of phase shifters, 90% spectral efficiency can be achieved. Finally, simulation results indicate that a similar behavior is observed when the wireless channel is considered to be sparse or correlated
Delay spread properties in a measured massive MIMO system at 2.6 GHz
Massive multiple-input multiple-output (MIMO) systems, where the base station (BS) is equipped with a large number of antennas and the mobile devices have a single antenna, can significantly enhance the system performance. In many wireless systems inter symbol interference (ISI) due to delay dispersion of the channel can dramatically affect the demodulation process of the received signals. Precoding in massive MIMO can reduce or eliminate ISI, while still exploiting the spatial diversity. In this paper, we investigate how much standard linear precoders help the ISI mitigation by shortening the rms delay spread. In order to evaluate the system performance, we measured a massive MIMO channel with 128 antennas at the base station and 36 users, including 26 line-of-sight and 10 non line-of-sight, each equipped with a single antenna. We also compare the results from the measurements with an independent identically distributed Gaussian channel with an exponentially decaying average power delay profile
Channel Measurement and Analysis for Polarimetric Wideband Outdoor Scenarios at 26 GHz: Directional vs Omni-Directional
This paper presents the measurement results and
analysis for outdoor wireless propagation channels at 26 GHz
over 2 GHz bandwidth for two receiver antenna polarization
modes. The angular and wideband properties of directional
and virtually omni-directional channels, such as angular spread,
root-mean-square delay spread and coherence bandwidth, are
analyzed. The results indicate that the reflections can have a significant
contribution in some realistic scenarios and increase the
angular and delay spreads, and reduce the coherence bandwidth
of the channel. The analysis in this paper also show that using
a directional transmission can result in an almost frequencyflat
fading channel over the measured 2 GHz bandwidth; which
consequently has a major impact on the choice of system design
choices such as beamforming and transmission numerology
Indoor Wideband Directional Millimeter Wave Channel Measurements and Analysis at 26 GHz, 32 GHz, and 39 GHz
This paper presents details of the wideband directional propagation
measurements of millimetre-wave (mmWave) channels
in the 26 GHz, 32 GHz, and 39 GHz frequency bands
in an indoor typical office environment. More than 14400
power delay profiles (PDPs) were measured across the 26
GHz band and over 9000 PDPs have been recorded for the
32 GHz and 39 GHz bands at each measurement point. A
mmWave wideband channel sounder has been used, where
signal analyzer and vector signal generator was employed.
Measurements have been conducted for both co- and crossantenna
polarization. The setup provided 2GHz bandwidth
and the mechanically steerable directional horn antenna with
8 degrees beamwidth provides 8 degrees of directional resolution
over the azimuth for 32 GHz and 39 GHz while 26
GHz measurement setup provides the angular resolution of
5 degrees. Measurements provide path loss, delay and spatial
spread of the channel. Large-scale fading characteristics,
RMS delay spread, RMS angular spread, angular and
delay dispersion are presented for three mmWave bands
for the line-of-sight (LoS) scenario
Millimeter-Wave Directional Path Loss Models in the 26 GHz, 32 GHz, and 39 GHz Bands for Small Cell 5G Cellular System
This paper presents empirically-based large-scale propagation path loss models for small cell fifth generation (5G) cellular system in the millimeter-wave bands, based on practical propagation channel measurements at 26 GHz, 32 GHz, and 39 GHz. To characterize path loss at these frequency bands for 5G small cell scenarios, extensive wideband and directional channel measurements have been performed on the campus of the University of Surrey. Close-in reference (CI), and 3GPP path loss models have been studied, and large-scale fading characteristics have been obtained and presented
Polarimetric Wideband Directional Channel Measurement and Analysis for Outdoor Small Cell Scenarios at 32 GHz and 39 GHz
Wideband millimeter-wave (mmWave) directional
propagation measurements were conducted in the 32 GHz and
39 GHz bands in outdoor line-of-sight (LoS) small cell scenarios.
The measurement provides spatial and temporal statistics that
will be useful for small-cell outdoor wireless networks for future
mmWave bands. Measurements were performed at two outdoor
environments and repeated for all polarization combinations.
Measurement results show little spread in the angular and delay
domains for the LoS scenario. Moreover root-mean-squared
(RMS) delay spread at different polarizations show small difference
which can be due to specific scatterers in the channel