Explore the vast range of titles available.

In this paper, we propose an energy-efficient resource allocation scheme for the downlink of decode- and-forward relay-assisted orthogonal frequency division multiple access systems. The resource allocation is designed based on imperfect channel state information at the transmitter. We jointly optimize the power allocation, data rate allocation, and subcarrier allocation to maximize the system energy efficiency （EE）. We formulate the EE problem as a probabilistic mixed non-convex optimization problem, in which the individual power constraints and the outage probability requirements are considered. An iterative algorithm based on bisection method is given to transform the optimization problem into a standard concave form. Also, we consider a proportional fairness design for the EE maximization problem. Simulation results illustrate the effectiveness of the proposed algorithm.

Adaptively adjusting system parameters including bandwidth, transmit power and mode to maximize the \"Bits per-Joule\" energy efficiency (BPJ-EE) in the downlink MIMO systems with imperfect channel state information at the transmitter (CSIT) is considered in this article. By mode, we refer to choice of transmission schemes i.e., singular value decomposition (SVD) or block diagonalization (BD), active transmit/receive antenna number and active user number. We derive optimal bandwidth and transmit power for each dedicated mode at first, in which accurate capacity estimation strategies are proposed to cope with the imperfect CSIT caused capacity prediction problem. Then, an ergodic capacity-based mode switching strategy is proposed to further improve the BPJ-EE, which provides insights into the preferred mode under given scenarios. Mode switching compromises different power parts, exploits the trade-off between the multiplexing gain and the imperfect CSIT caused inter-user interference and improves the BPJ-EE significantly.

One of the challenges to implementing downlink multiuser multiple-input multiple-output in practical systems is to overcome the detrimental impact of the imperfect channel state information at transmitter (CSIT). Researchers have derived a low bound to describe the imperfect CSIT's impact on the achievable rate. However, this low bound is not tight, especially when the quality of CSIT is poor. A tighter low bound using the exponential integral is proposed. A strict proof is given and the simulation results verify the tightness.

For this paper, we analyze the achievable sum rate of zero-forcing (ZF) pre-coding and Maximum Ratio Transmission (MRT) pre-coding with Matrix Normalization in massive MIMO system with Imperfect CSIT. We compare the performances of these two pre-codings and find that ZF pre-coding outperforming MRT pre-coding in the high SNR region while MRT pre-coding outperforming ZF pre-coding in the low SNR region. Then we derive the threshold of the pre-coding selection and provide the procedure of pre-coding schemes selection.