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

Interference and noise cancellation for joint communication radar (JCR) system based on contextual information

This paper examines the separation of wireless communication and radar signals, thereby guaranteeing cohabitation and acting as a panacea to spectrum sensing. First, considering that the channel impulse response was known by the receivers (communication and radar), we showed that the optimizing beamforming weights mitigate the interference caused by signals and improve the physical layer security (PLS) of the system. Furthermore, when the channel responses were unknown, we designed an interference filter as a low-complex noise and interference cancellation autoencoder. By mitigating the interference on the legitimate users, the PLS was guaranteed. Results showed that even for a low signal-to-noise ratio, the autoencoder produces low root-mean-square error (RMSE) values.Comment: 8 pages, 8 figure

The historical evolution of accounting in China (novissima sinica): effects of culture (2nd part).

Este artículo intenta explicar los criterios que sustentan la idea de que la contabilidad China ha sido conformada durante el pasado siglo con arreglo a factores culturales, económicos y políticos. Asimismo se propone estudiar los fenómenos históricos experimentados por la contabilidad china en el transcurso de los años, y evaluar comparativa y críticamente los efectos culturales de los mismos con las transformaciones políticas y económicas que han tenido influencia en el desarrollo de la contabilidad China. This paper attempts to argue the criteria which claim that Chinese accounting has been shaped by together with cultural, economical and political factors in the last century. This paper also aims to compare historical phenomena which occurred in Chinese accounting over the years, and then to assess comparatively or critically the effects of culture with politic and the economic transformations on the development of Chinese accounting.Contabilidad comparativa, contabilidad china, factores culturales, historia, sociología. Chinese accounting, comparative accounting, cultural factors, evolution, history, sociology.

Online Learning Models for Content Popularity Prediction In Wireless Edge Caching

Caching popular contents in advance is an important technique to achieve the low latency requirement and to reduce the backhaul costs in future wireless communications. Considering a network with base stations distributed as a Poisson point process (PPP), optimal content placement caching probabilities are derived for known popularity profile, which is unknown in practice. In this paper, online prediction (OP) and online learning (OL) methods are presented based on popularity prediction model (PPM) and Grassmannian prediction model (GPM), to predict the content profile for future time slots for time-varying popularities. In OP, the problem of finding the coefficients is modeled as a constrained non-negative least squares (NNLS) problem which is solved with a modified NNLS algorithm. In addition, these two models are compared with log-request prediction model (RPM), information prediction model (IPM) and average success probability (ASP) based model. Next, in OL methods for the time-varying case, the cumulative mean squared error (MSE) is minimized and the MSE regret is analyzed for each of the models. Moreover, for quasi-time varying case where the popularity changes block-wise, KWIK (know what it knows) learning method is modified for these models to improve the prediction MSE and ASP performance. Simulation results show that for OP, PPM and GPM provides the best ASP among these models, concluding that minimum mean squared error based models do not necessarily result in optimal ASP. OL based models yield approximately similar ASP and MSE, while for quasi-time varying case, KWIK methods provide better performance, which has been verified with MovieLens dataset.Comment: 9 figure, 29 page

A joint coded two-step multiuser detection scheme for MIMO OFDM system

Multiple-input, multiple-output (MIMO) communication is an effective scheme to improve wireless communication performance of multiuser applications. However, reliable communication in multiuser systems is affected by the presence of both multi-access interference (MAI) and inter-symbol interference (ISI) in multi-path channels. In this paper, we therefore investigate a transceiver design for a wideband multiuser-MiMO communication system, where the co-channel users are equipped with multiple transmit and multiple receive antennas. In particular, we propose a two-step interference cancellation scheme with an error correction coding technique for the receiver of a multiuser uplink system. The scheme employs orthogonal frequency division multiplexing (OFDM) modulation and space-time block codes (STBC). The receiver performs as a soft output multiuser detector based on minimum mean-squared error (MMSE) interference suppression at the first stage, and then, MAI cancellation is implemented with a bank of single-user channel decoders. The paper also includes computer simulations which help to improve the understanding of specific issues involved in the design of multiuser STBC-OFDM systems, and confirm the utility of the proposed approac