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This paper addresses the pilot-aided multiuser least square (LS) channel estimation for the uplink of multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. The systems under consideration allow all users use all available subcarriers independently and thus involve multiuser interference in the frequency domain. Direct application of the known pilot-aided single-user channel estimation methods to these systems is prohibited, requiring much more new investigations. The decentralized and centralized channel estimation algorithms are developed according to different multiuser scenarios. Optimal multiuser pilots are proposed, especially for centralized estimation methods with respect to the mean square error (MSE) of LS channel estimate. In addition, channel tracking algorithms are represented in terms of individual user's channels.

In this paper, we design an optimal training scheme for multi-input multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems under spatially correlated time- and frequency- (doubly) selective fading channels. We first develop the optimal pilot symbols and placement of pilot clusters with respect to the minimum mean square error (MMSE) of the linear channel estimate. We then derive the optimal power allocation for pilot symbols in a two-water-level way: by maximizing the averaged capacity lower bound, how much power to be allocated for training is determined subject to the global water level (or the constraint of total transmit power); subsequently, pouring power to the pilot symbols with an approximately optimal water-filling scheme subject to the local water level (or the constraint of assigned power for training). In addition, for a particular OFDM size, the optimal number of pilot clusters is derived by maximizing the capacity lower bound and by minimizing the channel estimate's MMSE.

Tailored for the best effort service, the present paper deals with dynamic allocation of subcarriers, rate and power resources based on channel state information (CSI) for multiuser MIMO-OFDM systems with trellis code modulation (TCM). Users rely on adaptive modulation and power modes that they select in accordance with the finite-rate feedback (FRF) they receive from the base station (BS). The BS uses CSI to maximize a generic concave utility of average rates while adhering to power constraints imposed on every user. We utilize a stochastic approximation primal-dual algorithm for on-line scheduling and resource allocation. The resultant optimum resource allocation depends on the current channel realization and optimally calculates dual prices. The simulation results show that the system throughput of proposed algorithm is not only good but also can effectively reduce the feedback overhead and the adoption of TCM can improve the system throughput greatly comparing to uncoded system.

This paper presents a modified implementation of QR decomposition for multiple input multiple output-orthogonal frequency division multiplexing (MIMO-OFDM) detection based on the Givens rotation method. The QR decomposition hardware is constructed using the coordinate rotation digital computer (CORDIC) algorithm operating with fewer gate counts and lower power consumption than do triangular systolic array (TSA) structures. Accurate signal transmission is essential to wireless communication systems. Thus, a more effective data detection algorithm and precise channel estimation method play vital roles in MIMO systems. Implementing data detection with QR decomposition helps reduce the complexity of MIMO-OFDM detection. Implementation results reveal that the proposed recursive QR decomposition (RQRD) architecture has lower clock latency than do TSA structures, and has a smaller hardware area than do Gram–Schmidt structures.

Orthogonal frequency division multiplexing (OFDM) is a multicarrier transmission system that can achieve high data rate over wireless channels. At the same time, multiple input multiple output OFDM (MIMO-OFDM) in wireless communication systems has been exposed to offer significant improvement over wireless technology by providing transmit diversity. It has become a promising technique for high-performance 5G broadband wireless communications. However, the main problem associated with MIMO-OFDM is that its signal exhibits high peak-to-average power ratio (PAPR), which causes nonlinear distortion and consequently performance degradation. Besides, PAPR carries weaknesses such as an increase in power consumption of high power amplifier (HPA) and analog to digital converter (ADC). Thus, 5G base stations will push up power requirements because energy consumption grows with the number of transceiver elements. So, mobile operators must find the right compromise that, on the one hand, guarantees a certain level of performance to a data flow, and, on the other hand, the energy cost generated for the deployment of the network. For this, as part of the management of power consumption, we propose MIMO constant envelope OFDM (MIMO-CE-OFDM) technique. In this work, we used MIMO-CE-OFDM to mitigate the nonlinear effect of HPA and ADC. To perform practical simulations, we have used COST 2100 MIMO channel model. In this paper, a MIMO-CE-OFDM system has been presented and analyzed under COST 2100 channel model conditions. Simulation results are given to illustrate the performance of $2\times 2$ MIMO-CE-OFDM in the presence of both HPA and ADC nonlinearity. This work shows that the effect of nonlinearity is shown to be negligible on MIMO-CE-OFDM signal.

TD-LTE (Time division duplex-long term evolution) is a mainstream standard of the fourth generation mobile communication led by China. In this paper, a TD-LTE base-station signal is proposed to achieve passive detection of targets as external radiation sources. First, it is proposed that the detection distance of the passive radar can be enhanced using multi-static radar according to the characteristics of TD-LTE. The processing of passive radar signal of TD-LTE is then given. By reconstructing the reference signal according to the 3GPP standard, pure multi-base station signals can be attained without demodulation. Finally, the characteristics of the ambiguity function of OFDM (orthogonal frequency division multiplexing) signal is analyzed. The reasons and positions of side peaks are deduced, and the ambiguity is resolved via frequency domain synthesis. The availability of the method proposed is proven using actual signals.