Cheol Mun

Interference mitigation using uplink power control for two-tier femtocell networks

This paper proposes two interference mitigation strategies that adjust the maximum transmit power of femtocell users to suppress the cross-tier interference at a macrocell base station (BS). The open-loop and the closed-loop control suppress the cross-tier interference less than a fixed threshold and an adaptive threshold based on the noise and interference (NI) level at the macrocell BS, respectively. Simulation results show that both schemes effectively compensate the uplink throughput degradation of the macrocell BS due to the cross-tier interference and that the closed-loop control provides better femtocell throughput than the open-loop control at a minimal cost of macrocell throughput.

Self-Optimized Coverage Coordination in Femtocell Networks

This paper proposes a self-optimized coverage coordination scheme for two-tier femtocell networks, in which a femtocell base station adjusts the transmit power based on the statistics of the signal and the interference power that is measured at a femtocell downlink. Furthermore, an analytic expression is derived for the coverage leakage probability that a femtocell coverage area leaks into an outdoor macrocell. The coverage analysis is verified by simulation, which shows that the proposed scheme provides sufficient indoor femtocell coverage and that the femtocell coverage does not leak into an outdoor macrocell.

Reverse-Link Interrogation Range of a UHF MIMO-RFID System in Nakagami-

In this paper, the reverse-link interrogation range (RIR) of ultrahigh-frequency-band passive radio-frequency identification (RFID) is analyzed for single-input and single-output (SISO) and multiple-input and multiple-output (MIMO) systems with maximal-ratio combining in the pinhole channel, where each channel is modeled as an arbitrarily correlated Nakagami-m distribution. Under the assumptions of perfect channel estimation and no interference, the closed-form expression of average RIR is derived, involving various parameters, such as the number of antennas, correlation, reader structure, and Nakagami- m shaping factor. The results show that the employment of multiple antennas at a reader causes the received SNR to change favorably and contributes to the improvement of the average RIR. Particularly, for the bistatic structure and Rayleigh fading (m = 0 dB), a 3 × 3 MIMO-RFID system can achieve 60% gain in the average RIR compared to the SISO-RFID system. In order to consider more realistic environments, finally, we investigated the influence of interference and imperfect channel estimation on the average RIR of the MIMO-RFID system in the uncorrelated Rayleigh fading channel.

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This paper proposes two interference mitigation strategies that adjust the maximum transmit power of femtocell users to suppress the cross-tier interference at a macrocell base station (BS). The open-loop and the closed-loop control suppress the cross-tier interference less than a fixed threshold and an adaptive threshold based on the noise and interference (NI) level at the macrocell BS, respectively. Simulation results show that both schemes effectively compensate the uplink throughput degradation of the macrocell BS and that the closed-loop control provides better femtocell throughput than the open-loop control at a minimal cost of macrocell throughput.

Fading Channels

This letter provides an analysis of the interaction between cooperative diversity and multiuser diversity in downlink channels. Through an approximation of the signal-to-noise ratio (SNR) distribution of each cooperative link using gamma distribution, we can derive an analytic expression for the average throughput of a single-cell wireless system with multiple cooperative diversity links combined with a fair-access scheduler. The proposed analytic approach is verified through comparisons with simulated results and shows that cooperative diversity has a detrimental impact on multiuser diversity.

A self-organized uplink power control for cross-tier interference management in femtocell networks

This paper analyzes the effects of polarization-mismatch and space-correlation to a multiple-input and single-output (MISO) channel which is observable in the near future cellular communications environments such as large-scale antenna arrays and small cells. The analysis is based on a polarization-mismatched and space-correlated MISO channel which is modeled from the conventional dual-polarized channel. In the MISO channel, polarization-mismatch is described by the polarization-mismatch angle which is uniformly distributed from 0 to the maximum polarization-mismatch angle and space-correlation is described by the exponential correlation model. Assuming high SNR, approximate expressions of the ergodic capacity are derived as a function of the transmit power, number of transmit antennas, maximum polarization-mismatch angle, and space-correlation coefficient in four representative environments: narrowly or widely spread polarization-mismatch angles and slightly or highly correlated channels. Further, the capacity loss introduced by polarization-mismatch and space-correlation is derived with respect to the maximum polarization-mismatch angle and space-correlation coefficient. It is shown that the capacity loss introduced by polarization-mismatch is upper bounded by 2 bit/s/Hz. Whereas, the capacity loss introduced by space-correlation increase with the number of transmit antennas and is upper bounded by 0.832 bit/s/Hz. Required resources to compensate for the capacity loss is derived as well.

Capacity of Multiuser Diversity with Cooperative Relaying in Wireless Networks

In this paper, for spatial multiplexing with limited feedback, a quantized principal component selection (QPCS) precoding scheme is proposed that achieves comparable capacity to the closed-loop multiple-input multiple-output (MIMO) and furthermore adapts to various fading channel conditions without any additional feedback bits and transmit channel state information (CSI). We propose a systematic design method for a codebook consisting of a finite number of unitary matrices based on a maximizing minimum distance criterion in the one- dimensional angular domain and show that the method outperforms the Grassmannian subspace packing method in various fading channel conditions. The proposed QPCS precoding scheme allows for adjustment of the precoding matrix based on limited feedback information on the principal vectors approximating a MIMO channel in the angular domain according to various channel conditions. Furthermore, for practical implementation of the QPCS precoding scheme, we propose a structured precoder optimization procedure and show that the proposed procedure induces a negligible capacity loss compared with the exhaustive precoder optimization, even with considerably reduced complexity.

Capacity Loss Due to Polarization-Mismatch and Space-Correlation on MISO Channel

In this paper, for spatial multiplexing with limited feedback, we propose quantized principal component selection (QPCS) precoding, wherein the active bases are selected at the receiver from a finite number of basis sets known at the both receiving and transmitting ends, conveyed to the transmitter using limited feedback, and assembled into a precoding matrix at the transmitter. The selected bases are conveyed to the transmitter using feedback information on both the index of the selected basis set, which defines the most appropriate set of coordinates for describing a multiple-input multiple-output (MIMO) channel, and the principal bases with the largest gain in the selected basis set. A systematic design method for a finite number of basis sets using the antenna array processing is presented. We show that the proposed QPCS precoding scheme provides comparable capacity with the closed-loop MIMO with full channel state information (CSI), even with limited feedback. Furthermore, for practical implementation of the QPCS precoding scheme, we propose a low complex structured precoder optimization procedure and show that the proposed procedure induces a negligible capacity loss compared with the exhaustive precoder optimization, even with considerably reduced complexity.

Quantized Principal Component Selection Precoding for Spatial Multiplexing with Limited Feedback

This work presents a quasi-three dimensional MIMO channel model to predict the polarization characteristics of an indoor radio channel. The model considers not only the effects of the polarization of the transmitting and receiving antenna elements, but also the types of indoor environment scatterers. The proposed model is based on an ellipse model and a patch scattering model. Furthermore, the channel model expands the ellipse model to a quasi-three dimensional model. The proposed model is suitable for indoor MIMO channels and can expect ergodic capacities of a multi-polarized MIMO channel, which considers polarization and 3-D geometry

Quantized Principal Component Selection Precoding for Limited Feedback Spatial Multiplexing

In this paper, the performance of a multiuser diversity system paired with a multi-element transmit antenna system is analyzed under the assumption of independent Rayleigh fading. A measure of system level performance is an achievable spectral efficiency as a function of the number of users and antennas. Spectral efficiency is obtained from the instantaneous signal-to-noise ratio (SNR) distribution combined by both transmit diversity at each link and multiuser diversity at system level when the base station transmitter adapts to channel variations using a constant power variable rate strategy. Numerical results show that closed-loop antenna techniques provide an additional gain with multiuser diversity systems due to array gain, even though space diversity gain reduces multiuser diversity gain. On the other hand, the space-time block coding (STBC) that provides full order space diversity gain only has a destructive impact on multiuser diversity.