86 research outputs found
Towards a Realistic Assessment of Multiple Antenna HCNs: Residual Additive Transceiver Hardware Impairments and Channel Aging
Given the critical dependence of broadcast channels by the accuracy of
channel state information at the transmitter (CSIT), we develop a general
downlink model with zero-forcing (ZF) precoding, applied in realistic
heterogeneous cellular systems with multiple antenna base stations (BSs).
Specifically, we take into consideration imperfect CSIT due to pilot
contamination, channel aging due to users relative movement, and unavoidable
residual additive transceiver hardware impairments (RATHIs). Assuming that the
BSs are Poisson distributed, the main contributions focus on the derivations of
the upper bound of the coverage probability and the achievable user rate for
this general model. We show that both the coverage probability and the user
rate are dependent on the imperfect CSIT and RATHIs. More concretely, we
quantify the resultant performance loss of the network due to these effects. We
depict that the uplink RATHIs have equal impact, but the downlink transmit BS
distortion has a greater impact than the receive hardware impairment of the
user. Thus, the transmit BS hardware should be of better quality than user's
receive hardware. Furthermore, we characterise both the coverage probability
and user rate in terms of the time variation of the channel. It is shown that
both of them decrease with increasing user mobility, but after a specific value
of the normalised Doppler shift, they increase again. Actually, the time
variation, following the Jakes autocorrelation function, mirrors this effect on
coverage probability and user rate. Finally, we consider space division
multiple access (SDMA), single user beamforming (SU-BF), and baseline
single-input single-output (SISO) transmission. A comparison among these
schemes reveals that the coverage by means of SU-BF outperforms SDMA in terms
of coverage.Comment: accepted in IEEE TV
Impact of User Mobility on Optimal Linear Receivers in Cellular Networks
We consider the uplink of non-cooperative multi-cellular systems deploying
multiple antenna elements at the base stations (BS), covering both the cases of
conventional and very large number of antennas. Given the inevitable pilot
contamination and an arbitrary path-loss for each link, we address the impact
of time variation of the channel due to the relative movement between users and
BS antennas, which limits system's performance even if the number antennas is
increased, as shown. In particular, we propose an optimal linear receiver (OLR)
maximizing the received signal-to-interference-plus-noise (SINR). Closed-form
lower and upper bounds are derived as well as the deterministic equivalent of
the OLR is obtained. Numerical results reveal the outperformance of the
proposed OLR against known linear receivers, mostly in environments with high
interference and certain user mobility, as well as that massive MIMO is
preferable even in time-varying channel conditions.Comment: 3 figures, 6 pages, accepted in ICC 201
Degrees of Freedom of Uplink-Downlink Multiantenna Cellular Networks
An uplink-downlink two-cell cellular network is studied in which the first
base station (BS) with antennas receives independent messages from its
serving users, while the second BS with antennas transmits
independent messages to its serving users. That is, the first and second
cells operate as uplink and downlink, respectively. Each user is assumed to
have a single antenna. Under this uplink-downlink setting, the sum degrees of
freedom (DoF) is completely characterized as the minimum of
,
, , and , where denotes
. The result demonstrates that, for a broad class of network
configurations, operating one of the two cells as uplink and the other cell as
downlink can strictly improve the sum DoF compared to the conventional uplink
or downlink operation, in which both cells operate as either uplink or
downlink. The DoF gain from such uplink-downlink operation is further shown to
be achievable for heterogeneous cellular networks having hotspots and with
delayed channel state information.Comment: 22 pages, 11 figures, in revision for IEEE Transactions on
Information Theor
Deterministic equivalent performance analysis of time-varying massive MIMO systems
© 2015 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.Delayed channel state information at the transmitter (CSIT) due to time variation of the channel, coming from the users' relative movement with regard to the BS antennas, is an inevitable degrading performance factor in practical systems. Despite its importance, little attention has been paid to the literature of multi-cellular multiple-input massive multiple-output (MIMO) system by investigating only the maximal ratio combining (MRC) receiver and the maximum ratio transmission (MRT) precoder. Hence, the contribution of this work is designated by the performance analysis/comparison of/with more sophisticated linear techniques, i.e., a minimum-mean-square-error (MMSE) detector for the uplink and a regularized zero-forcing (RZF) precoder for the downlink are assessed. In particular, we derive the deterministic equivalents of the signal-to-interference-plus-noise ratios (SINRs), which capture the effect of delayed CSIT, and make the use of lengthy Monte Carlo simulations unnecessary. Furthermore, prediction of the current CSIT after applying a Wiener filter allows to evaluate the mitigation capabilities of MMSE and RZF. Numerical results depict that the proposed achievable SINRs (MMSE/RZF) are more efficient than simpler solutions (MRC/MRT) in delayed CSIT conditions, and yield a higher prediction at no special computational cost due to their deterministic nature. Nevertheless, it is shown that massive MIMO are preferable even in time-varying channel conditions.Peer reviewe
Rate-Splitting Robustness in Multi-Pair Massive MIMO Relay Systems
Relay systems improve both coverage and system capacity. Toward this direction, a full-duplex (FD) technology, being able to boost the spectral efficiency by transmitting and receiving simultaneously on the same frequency and time resources, is envisaged to play a key role in future networks. However, its benefits come at the expense of self-interference (SI) from their own transmit signal. At the same time, massive multiple-input massive multiple-output systems, bringing unconventionally many antennas, emerge as a promising technology with huge degrees-of-freedom. To this end, this paper considers a multi-pair decode-and-forward FD relay channel, where the relay station is deployed with a large number of antennas. Moreover, the rate-splitting (RS) transmission has recently been shown to provide significant performance benefits in various multi-user scenarios with imperfect channel state information at the transmitter (CSIT). Engaging the RS approach, we employ the deterministic equivalent analysis to derive the corresponding sum-rates in the presence of interferences. Initially, numerical results demonstrate the robustness of RS in half-duplex (HD) systems, since the achievable sum-rate increases without bound, i.e., it does not saturate at high signal-to-noise ratio. Next, we tackle the detrimental effect of SI in FD. In particular, and most importantly, not only FD outperforms HD, but also RS enables increasing the range of SI over which FD outperforms HD. Furthermore, increasing the number of relay station antennas, RS appears to be more efficacious due to imperfect CSIT, since SI decreases. Interestingly, increasing the number of users, the efficiency of RS worsens and its implementation becomes less favorable under these conditions. Finally, we verify that the proposed DEs, being accurate for a large number of relay station antennas, are tight approximations even for realistic system dimensions.Peer reviewedFinal Accepted Versio
Toward a Realistic Assessment of Multiple Antenna HCNs: Residual Additive Transceiver Hardware Impairments and Channel Aging
© 2017 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.Given the critical dependence of broadcast channels by the accuracy of channel state information at the transmitter (CSIT), we develop a general downlink model with zero-forcing precoding, applied in realistic heterogeneous cellular systems with multiple-Antenna base stations (BSS). Specifically, we take into consideration imperfect CSIT due to pilot contamination, channel aging due to users relative movement, and unavoidable residual additive transceiver hardware impairments (RATHIs). Assuming that the BSS are Poisson distributed, the main contributions focus on the derivations of the upper bound of the coverage probability and the achievable user rate for this general model. We show that both the coverage probability and the user rate are dependent on the imperfect CSIT and RATHIs. More concretely, we quantify the resultant performance loss of the network due to these effects. We depict that the uplink RATHIs have equal impact, but the downlink transmit BS distortion has a greater impact than the receive hardware impairment of the user. Thus, the transmit BS hardware should be of better quality than user's receive hardware. Furthermore, we characterise both the coverage probability and user rate in terms of the time variation of the channel. It is shown that both of them decrease with increasing user mobility, but after a specific value of the normalized Doppler shift, they increase again. Actually, the time variation, following the Jakes autocorrelation function, mirrors this effect on coverage probability and user rate. Finally, we consider space-division multiple access (SDMA), single-user beamforming (SU-BF), and baseline single-input single-output transmission. A comparison among these schemes reveals that the coverage by means of SU-BF outperforms SDMA in terms of coverage.Peer reviewedFinal Accepted Versio
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