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Dr Sohail Payami


My publications

Publications

Payami Sohail, Sellathurai Mathini, Nikitopoulos Konstantinos (2019) Low-Complexity Hybrid Beamforming for Massive MIMO Systems in Frequency-Selective Channels,IEEE Access 7 pp. 36195-36206 IEEE
Hybrid beamforming for frequency-selective channels is a challenging problem, as the phase shifters provide the same phase shift to all 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 the 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 the number of employed phases shifter networks is twice the resolvable multipath components in the 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 the 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 the number of employed phases shifter networks is twice the
resolvable multipath components in the time domain.
Payami Sohail, Khalily Mohsen, Taheri Sohail, Nikitopoulos Konstantinos, Tafazolli Rahim (2020) Channel Measurement and Analysis for Polarimetric
Wideband Outdoor Scenarios at 26 GHz:
Directional vs Omni-Directional
,
EUCAP 2020
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.
Payami Sohail, Khalily Mohsen, Loh Tian Hong, Nikitopoulos Konstantinos (2020) Hybrid Beamforming with Switches and Phase Shifters over Frequency-Selective Channels,IEEE Wireless Communications Letters pp. 1-1 Institute of Electrical and Electronics Engineers (IEEE)
The recent studies on hybrid beamformers with a combination of switches and phase shifters indicate that such methods can reduce the cost and power consumption of massive multiple-input multiple-output (MIMO) systems. However, most of the works have focused on the scenarios with frequency-flat channel models. This letter proposes an effective approach for such systems in frequency-selective channels and presents the closed-form expressions of the beamformer and the corresponding sum-rates. Compared to the traditional subconnected structures, our approach with a significantly smaller number of phase shifters results in a promising performance.
Payami Sohail, Khalily Mohsen, Araghi Ali, Hong Loh Tian, Cheadle David, Nikitopoulos Konstantinos, Tafazolli Rahim (2020) Developing the First mmWave Fully-Connected Hybrid Beamformer
with a Large Antenna Array
,
IEEE Access Institute of Electrical and Electronics Engineers
Millimeter wave (mmWave) systems with effective beamforming capability play a key role
in fulfilling the high data-rate demands of current and future wireless technologies. Hybrid analog-todigital
beamformers have been identified as a cost-effective and energy-efficient solution towards deploying
such systems. Most of the existing hybrid beamforming architectures rely on a subconnected phase shifter
network with a large number of antennas. Such approaches, however, cannot fully exploit the advantages of
large arrays. On the other hand, the current fully-connected beamformers accommodate only a small number
of antennas, which substantially limits their beamforming capabilities. In this paper, we present a mmWave
hybrid beamformer testbed with a fully-connected network of phase shifters and adjustable attenuators and
a large number of antenna elements. To our knowledge, this is the first platform that connects two RF inputs
from the baseband to a 16ý 8 antenna array, and it operates at 26 GHz with a 2 GHz bandwidth. It provides
a wide scanning range of 60ý, and the flexibility to control both the phase and the amplitude of the signals
between each of the RF chains and antennas. This beamforming platform can be used in both short and
long-range communications with linear equivalent isotropically radiated power (EIRP) variation between
10 dBm and 60 dBm. In this paper, we present the design, calibration procedures and evaluations of such a
complex system as well as discussions on the critical factors to consider for their practical implementation.