Carlos Andrés Viteri-Mera

Interference-nulling time-reversal beamforming for mm-Wave massive MIMO systems

We present a novel beamforming technique for multi-user frequency-selective indoor channels called interference-nulling time-reversal (INTR). The beamformer is implemented with a pre-filter designed to minimize the inter-user interference present in conventional time-reversal (TR). Moreover, our technique relies on a large number of antennas at the transmitter to mitigate inter-symbol interference, enabling low computational complexity receivers. We demonstrate that INTR outperforms previous TR techniques with respect to average bit error rate per user and achievable sum rate. Thus, INTR can be used for space division multiple access in no-line-of-sight mm-wave massive MIMO, providing remarkable diversity and multiplexing gains.

Equalized Time Reversal Beamforming for Frequency-Selective Indoor MISO Channels

Time-reversal (TR) is a beamforming technique for wireless frequency-selective channels, which has received increasing attention due to its high energy efficiency and low computational complexity. In this paper, we present two contributions on TR beamforming for single-user indoor wideband MISO systems. First, we provide novel analyses of a baseband TR system using two commonly used indoor propagation channel models. We derive closed-form approximations for the inter-symbol interference (ISI) with these channel models in order to characterize the influence of propagation conditions (such as the power-delay profile, delay spread, and bandwidth) on TR performance metrics. In particular, we analyze spatial focusing and time compression performance of TR beamforming, and their impact on the bit error rate (BER). As a second contribution, we introduce an equalized TR (ETR) technique that mitigates the ISI of conventional TR. The proposed ETR utilizes a zero-forcing pre-equalizer at the transmitter in a cascade configuration with the TR pre-filter. Unlike previous approaches to ISI mitigation in TR systems, we derive theoretical performance bounds for ETR and show that it greatly enhances the BER performance of conventional TR with minimal impact to its beamforming capabilities. By means of numerical simulations, we verify our closed-form approximations and show that the proposed ETR technique outperforms conventional TR with respect to the BER under any SNR.

Performance comparison of time-reversal beamforming in wireless local area networks

We present a performance comparison of conventional time-reversal and equalized time-reversal beamforming techniques for wireless local area network scenarios with different delay spreads and signal bandwidths. A comparison among various modulation schemes is also provided for each technique.

Space-Time Block Diagonalization for Frequency-Selective MIMO Broadcast Channels

The most important linear precoding method for frequency-flat MIMO broadcast channels is block diagonalization (BD) which, under certain conditions, attains the same nonlinear dirty paper coding channel capacity. However, BD is not easily translated to frequency-selective channels, since space-time information must be included in the transceiver design. In this paper, we demonstrate that BD is possible in frequency-selective MIMO broadcast channels to eliminate inter-user interference and derive the conditions on the number of transmit antennas and the transmission block length (as functions of the number of users and channel delay spread) for the existence of BD precoders. We also propose three different approaches to mitigate/eliminate inter-symbol interference in block transmissions: time-reversal-based BD (TRBD), equalized BD (EBD), and joint processing BD (JPBD). We show that any transmit-processing-only method (including TRBD and EBD) yields zero diversity and multiplexing gains (high SNR regime). We also demonstrate that JPBD, which uses linear processing at the transmitter and the receiver, approximates full multiplexing gain for a sufficiently large transmit block length, and show its diversity-multiplexing tradeoff. Extensive numerical simulations show that the achievable rate and probability of error performance of all the proposed techniques remarkably improve that of conventional time-reversal beamforming. Moreover, JPBD provides the highest achievable rate region for frequency-selective MIMO broadcast channels.

Regularized time-reversal beamforming for mm-Wave massive MIMO systems

We present a novel beamforming technique for multi-user indoor wireless communications called regularized time-reversal (RTR). A comparison with other time-reversal techniques is provided with respect to bit error rate and spatial focusing performance in mm-wave massive MIMO systems. We verify that the proposed technique can be used for space-division multiple access in such scenarios.

Space-time focusing performance of time-reversal beamforming in rich-scattering indoor channels

We analyze the space-time focusing performance of conventional time-reversal (TR) beamforming in single-user wireless scenarios. We use two indoor wideband channel models to investigate, under uncorrelated scattering, spatial focusing performance of TR increases versus bandwidth and delay spread. We also examine the time focusing performance w.r.t. variations in channel parameters and number of antennas.

Analysis of time-reversal MUSIC beamforming for indoor WLAN systems

We present a performance comparison of conventional time-reversal and time-reversal with multiple signal classification (MUSIC) beamforming techniques for multiuser wireless local area network (WLAN) scenarios with different delay spreads and signal bandwidths. A comparison among various modulation schemes is also provided for each technique.

Polarimetrie interference-alignment beamforming

We introduce the concept of Polarimetric interference-alignment (PIA) beamforming for multiuser MIMO channels. By using vector antennas, we harness the orthogonal polarization components of electromagnetic waves to provide a signal space for the interference-alignment technique proposed by Cadambe and Jafar. We show that such beamformer exists and provide preliminary comments regarding its operation and performance.

Feasibility analysis of polarimetric-interference alignment beamforming in rich-scattering indoor channels

We analyze the feasibility of polarimetric interference-alignment (PIA) beamforming in rich-scattering indoor channels. PIA harnesses the orthogonal polarization components of electromagnetic waves to provide a signal space for the interference alignment technique proposed by Cadambe and Jafar. We demonstrate that PIA is feasible under common cross-polarization discrimination values and scattering conditions by using a polarimetric multiuser MIMO channel model.