Abstract-This paper considers the problem of pilot contamination (PC) in large-scale multi-cell multiple-input multipleoutput (MIMO) aided orthogonal frequency division multiplexing systems. We propose an efficient scheme relying on an optimal pilot design conceived for time-domain channel estimation, which can either completely eliminate PC or significantly reduce it, depending on the channel's coherence time. This is achieved by designing an optimal pilot set to allowing us to beneficially group the users in all the cells and to assign a time-shifted pilot transmission to the different groups. Unlike the existing PC elimination schemes which require an excessively long channel coherence time, our proposed scheme is capable of completely eliminating PC under a much shorter coherence time. Moreover, the existing PC elimination schemes can no longer be used if the channel coherent time is insufficiently large. By contrast, even for extremely short channel coherent time, our scheme can still be implemented to significantly reduce PC. This is particularly beneficial for high velocity scenarios. Our simulation results demonstrate the efficiency of the proposed scheme.
Top gas recycling is considered as one of the highest potential technologies to improve reduction efficiency and correspondingly to reduce carbon consumption. As a typical nitrogen free ironmaking process, pre-reduction shaft furnace of COREX ® process (COREX ® shaft furnace) for short is suitable to adopt the technology aiming to cut down CO2 emission. Under the premise of constant total injection volume, three kinds of reducing gas injection methods are numerically studied by employing a two-dimensional mathematical model. The method that 20% of total reducing gas in volume fraction is blasted through normal inlet (NI) while the rest through down pipe inlet (PI) rather than deadman inlet (DI) could apparently improve gas flow in the inactive zone located near the bottom direct reduced iron (DRI) outlet, thus increasing DRI reduction degree to 61% under present calculation conditions. Meanwhile, either decreasing the vertical height of PI or increasing its diameter makes further improvement on furnace efficiency. After adopting top gas recycling to the shaft furnace by NI+PI method with optimal parameters, CO utilization ratio reaches above 46% when DRI reduction degree correspondingly increases by 12%, what's more, CO2 emission from the whole process is reduced by about 540 Nm 3 /tHM. The results prove that top gas recycling technology promotes reduction efficiency inside shaft furnace and greatly reduces the greenhouse gas emission, which will contribute to suppressing global warming.
The limited fronthaul capacity imposes a challenge on the uplink of centralized radio access network (C-RAN). We propose to boost the fronthaul capacity of massive multipleinput multiple-output (MIMO) aided C-RAN by globally optimizing the power sharing between channel estimation and data transmission both for the user devices (UDs) and the remote radio units (RRUs). Intuitively, allocating more power to the channel estimation will result in more accurate channel estimates, which increases the achievable throughput. However, increasing the power allocated to the pilot training will reduce the power assigned to data transmission, which reduces the achievable throughput. In order to optimize the powers allocated to the pilot training and to the data transmission of both the UDs and the RRUs, we assign an individual power sharing factor to each of them and derive an asymptotic closed-form expression of the signal-to-interference-plus-noise for the massive MIMO aided C-RAN consisting of both the UD-to-RRU links and the RRU-to-baseband unit (BBU) links. We then exploit the C-RAN architecture's central computing and control capability for jointly optimizing the UDs' power sharing factors and the RRUs' power sharing factors aiming for maximizing the fronthaul capacity. Our simulation results show that the fronthaul capacity is significantly boosted by the proposed global optimization of the power allocation between channel estimation and data transmission both for the UDs and for their host RRUs. As a specific example of 32 receive antennas (RAs) deployed by RRU and 128 RAs deployed by BBU, the sum-rate of 10 UDs achieved with the optimal power sharing factors improves 33% compared with the one attained without optimizing power sharing factors.
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