# Active Channel

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### Seunghyup Yoo – One of the best experts on this subject based on the ideXlab platform.

• ##### improvement of on off current ratio in hbox tio _ rm x ActiveChannel tfts using hbox n _ 2 hbox o plasma treatment
IEEE Electron Device Letters, 2009
Co-Authors: Jae-woo Park, Dongyun Lee, Hakyoung Kwon, Seunghyup Yoo

Abstract:

Without sacrificing the on-current in the transfer characteristics, we have successfully reduced the off-current part by the optimal N2O plasma treatment to improve the on-off-current ratio in n-type titanium oxide ( TiOx) ActiveChannel thin-film transistors. While the high-power (275 W) N2O plasma treatment oxidizes the whole TiOx Channel and results in the reduction of both on- and off-current, the optimized low-power (150 W) process makes the selective oxidation of the top portion in the Channel and reduces only the off-current significantly. Increase in on-off ratio by almost five orders of magnitude is achieved without change in on-current by using the presented method.

• ##### Improvement of On–Off-Current Ratio in $\hbox{TiO}_{\rm x}$ Active–Channel TFTs Using $\hbox{N}_{2} \hbox{O}$ Plasma Treatment
IEEE Electron Device Letters, 2009
Co-Authors: Jae-woo Park, Dongyun Lee, Hakyoung Kwon, Seunghyup Yoo

Abstract:

Without sacrificing the on-current in the transfer characteristics, we have successfully reduced the off-current part by the optimal N2O plasma treatment to improve the on-off-current ratio in n-type titanium oxide ( TiOx) ActiveChannel thin-film transistors. While the high-power (275 W) N2O plasma treatment oxidizes the whole TiOx Channel and results in the reduction of both on- and off-current, the optimized low-power (150 W) process makes the selective oxidation of the top portion in the Channel and reduces only the off-current significantly. Increase in on-off ratio by almost five orders of magnitude is achieved without change in on-current by using the presented method.

• ##### New n-Type $\hbox{TiO}_{2}$ Transparent ActiveChannel TFTs Fabricated With a Solution Process
IEEE Electron Device Letters, 2008
Co-Authors: Jae-woo Park, Seunghyup Yoo

Abstract:

New metal-oxide thin-film transistors (MOxTFTs) with a solution-processed TiO2 transparent Active Channel are fabricated with a novel doping process that consists of a deposition of an ultrathin Ti layer on TiO2 films and a brief rapid thermal annealing. Contrary to an as-prepared device which does not show any appreciable TFT actions, devices with the proposed process exhibit a clear n-type TFT behavior with a saturation mobility of 0.12 cm2 V-1ldr s-1 and a threshold voltage of 11 V. A solution processibility and a low-cost manufacturability of TiO2 make the presented TFTs potentially attrActive for cost-sensitive applications.

### Giuseppe Caire – One of the best experts on this subject based on the ideXlab platform.

• ##### fdd massive mimo via ul dl Channel covariance extrapolation and ActiveChannel sparsification
IEEE Transactions on Wireless Communications, 2019
Co-Authors: Mahdi Barzegar Khalilsarai, Saeid Haghighatshoar, Giuseppe Caire

Abstract:

We propose a novel method for massive multiple-input multiple-output (massive MIMO) in frequency division duplexing (FDD) systems. Due to the large frequency separation between uplink (UL) and downlink (DL) in FDD systems, Channel reciprocity does not hold. Hence, in order to provide DL Channel state information to the base station (BS), closed-loop DL Channel probing, and Channel state information (CSI) feedback is needed. In massive MIMO, this typically incurs a large training overhead. For example, in a typical configuration with $M \simeq 200$ BS antennas and fading coherence block of $T \simeq 200$ symbols, the resulting rate penalty factor due to the DL training overhead, given by $\max \{0, 1 – M/T\}$ , is close to 0. To reduce this overhead, we build upon the well-known fact that the angular scattering function of the user Channels is invariant over frequency intervals whose size is small with respect to the carrier frequency (as in current FDD cellular standards). This allows us to estimate the users’ DL Channel covariance matrix from UL pilots without additional overhead. Based on this covariance information, we propose a novel sparsifying precoder in order to maximize the rank of the effective sparsified Channel matrix subject to the condition that each effective user Channel has sparsity not larger than some desired DL pilot dimension ${\sf T_{dl}}$ , resulting in the DL training overhead factor $\max \{0, 1 – {\sf T_{dl}}/ T\}$ and CSI feedback cost of ${\sf T_{dl}}$ pilot measurements. The optimization of the sparsifying precoder is formulated as a mixed integer linear program , that can be efficiently solved. Extensive simulation results demonstrate the superiority of the proposed approach with respect to the concurrent state-of-the-art schemes based on compressed sensing or UL/DL dictionary learning.

• ##### FDD Massive MIMO: Efficient Downlink Probing and Uplink Feedback via ActiveChannel Sparsification
2018 IEEE International Conference on Communications (ICC), 2018
Co-Authors: Mahdi Barzegar Khalilsarai, Saeid Haghighatshoar, Xinping Yi, Giuseppe Caire

Abstract:

In this paper, we propose a novel method for efficient implementation of a massive Multiple-Input Multiple- Output (massive MIMO) system with Frequency Division Duplexing (FDD) operation. Our main objective is to reduce the large overhead incurred by Downlink (DL) common training and Uplink (UL) feedback needed to obtain Channel state information (CSI) at the base station. Our proposed scheme relies on the fact that the underlying angular distribution of a Channel vector, also known as the angular scattering function, is a frequency-invariant entity yielding a ULDL reciprocity and has a limited angular support. We estimate this support from UL CSI and interpolate it to obtain the corresponding angular support of the DL Channel. Finally we exploit the estimated support of the DL Channel of all the users to design an efficient Channel probing and feedback scheme that maximizes the total spectral efficiency of the system. Our method is different from the existing compressed-sensing (CS) based techniques in the literature. Using support information helps reduce the feedback overhead from O(s logM) in CS techniques to O(s) in our proposed method, with s andM being sparsity order of the Channel vectors and the number of base station antennas, respectively. Furthermore, in order to control the Channel sparsity and therefore the DL common training and UL feedback overhead, we introduce the novel concept of Active Channel sparsification. In brief, when the fixed pilot dimension is less than the required amount for reliable Channel estimation, we introduce a pre-beamforming matrix that artificially reduces the effective Channel dimension of each user to be not larger than the DL pilot dimension, while maximizing both the number of served users and the number of probed angles. We provide numerical experiments to assess the performance of our method and compare it with the state-of-the-art CS technique.

• ##### fdd massive mimo via ul dl Channel covariance extrapolation and ActiveChannel sparsification
arXiv: Information Theory, 2018
Co-Authors: Mahdi Barzegar Khalilsarai, Saeid Haghighatshoar, Giuseppe Caire

Abstract:

We propose a novel method for massive Multiple-Input Multiple-Output (massive MIMO) in Frequency Division Duplexing (FDD) systems. Due to the large frequency separation between Uplink (UL) and Downlink (DL), in FDD systems Channel reciprocity does not hold. Hence, in order to provide DL Channel state information to the Base Station (BS), closed-loop DL Channel probing and feedback is needed. In massive MIMO this incurs typically a large training overhead. For example, in a typical configuration with $M \simeq 200$ BS antennas and fading coherence block of $T \simeq 200$ symbols, the resulting rate penalty factor due to the DL training overhead, given by $\max\{0, 1 – M/T\}$, is close to 0. To reduce this overhead, we build upon the observation that the Angular Scattering Function (ASF) of the user Channels is invariant over the frequency domain. We develop a robust and stable method to estimate the users’ DL Channel covariance matrices from pilots sent by the users in the UL. The resulting DL covariance information is used to optimize a sparsifying precoder, in order to limit the effective Channel dimension of each user Channel to be not larger than some desired DL pilot dimension $T_{\rm dl}$. In this way, we can maximize the rank of the effective sparsified Channel matrix subject to a desired training overhead penalty factor $\max\{0, 1 – T_{\rm dl} / T\}$. We pose this problem as a Mixed Integer Linear Program, that can be efficiently solved. Furthermore, each user can simply feed back its $T_{\rm dl}$ pilot measurements. Thus, the proposed approach yields also a small feedback overhead and delay. We provide simulation results demonstrating the superiority of the proposed approach with respect to state-of-the-art “compressed DL pilot” schemes based on compressed sensing.

### Jae-woo Park – One of the best experts on this subject based on the ideXlab platform.

• ##### improvement of on off current ratio in hbox tio _ rm x ActiveChannel tfts using hbox n _ 2 hbox o plasma treatment
IEEE Electron Device Letters, 2009
Co-Authors: Jae-woo Park, Dongyun Lee, Hakyoung Kwon, Seunghyup Yoo

Abstract:

Without sacrificing the on-current in the transfer characteristics, we have successfully reduced the off-current part by the optimal N2O plasma treatment to improve the on-off-current ratio in n-type titanium oxide ( TiOx) ActiveChannel thin-film transistors. While the high-power (275 W) N2O plasma treatment oxidizes the whole TiOx Channel and results in the reduction of both on- and off-current, the optimized low-power (150 W) process makes the selective oxidation of the top portion in the Channel and reduces only the off-current significantly. Increase in on-off ratio by almost five orders of magnitude is achieved without change in on-current by using the presented method.

• ##### Improvement of On–Off-Current Ratio in $\hbox{TiO}_{\rm x}$ Active–Channel TFTs Using $\hbox{N}_{2} \hbox{O}$ Plasma Treatment
IEEE Electron Device Letters, 2009
Co-Authors: Jae-woo Park, Dongyun Lee, Hakyoung Kwon, Seunghyup Yoo

Abstract:

Without sacrificing the on-current in the transfer characteristics, we have successfully reduced the off-current part by the optimal N2O plasma treatment to improve the on-off-current ratio in n-type titanium oxide ( TiOx) ActiveChannel thin-film transistors. While the high-power (275 W) N2O plasma treatment oxidizes the whole TiOx Channel and results in the reduction of both on- and off-current, the optimized low-power (150 W) process makes the selective oxidation of the top portion in the Channel and reduces only the off-current significantly. Increase in on-off ratio by almost five orders of magnitude is achieved without change in on-current by using the presented method.

• ##### New n-Type $\hbox{TiO}_{2}$ Transparent ActiveChannel TFTs Fabricated With a Solution Process
IEEE Electron Device Letters, 2008
Co-Authors: Jae-woo Park, Seunghyup Yoo

Abstract:

New metal-oxide thin-film transistors (MOxTFTs) with a solution-processed TiO2 transparent Active Channel are fabricated with a novel doping process that consists of a deposition of an ultrathin Ti layer on TiO2 films and a brief rapid thermal annealing. Contrary to an as-prepared device which does not show any appreciable TFT actions, devices with the proposed process exhibit a clear n-type TFT behavior with a saturation mobility of 0.12 cm2 V-1ldr s-1 and a threshold voltage of 11 V. A solution processibility and a low-cost manufacturability of TiO2 make the presented TFTs potentially attrActive for cost-sensitive applications.