| 1. |
- Aharonian, Felix, et al.
(författare)
-
Atmospheric gas dynamics in the Perseus cluster observed with Hitomi
- 2018
-
Ingår i: Publications of the Astronomical Society of Japan. - : Oxford University Press (OUP). - 0004-6264 .- 2053-051X. ; 70:2
-
Tidskriftsartikel (refereegranskat)abstract
- Extending the earlier measurements reported in Hitomi collaboration (2016, Nature, 535, 117), we examine the atmospheric gas motions within the central 100 kpc of the Perseus cluster using observations obtained with the Hitomi satellite. After correcting for the point spread function of the telescope and using optically thin emission lines, we find that the line-of-sight velocity dispersion of the hot gas is remarkably low and mostly uniform. The velocity dispersion reaches a maxima of approximately 200 km s(-1) toward the central active galactic nucleus (AGN) and toward the AGN inflated northwestern ghost bubble. Elsewhere within the observed region, the velocity dispersion appears constant around 100 km s(-1). We also detect a velocity gradient with a 100 km s(-1) amplitude across the cluster core, consistent with large-scale sloshing of the core gas. If the observed gas motions are isotropic, the kinetic pressure support is less than 10% of the thermal pressure support in the cluster core. The well-resolved, optically thin emission lines have Gaussian shapes, indicating that the turbulent driving scale is likely below 100 kpc, which is consistent with the size of the AGN jet inflated bubbles. We also report the first measurement of the ion temperature in the intracluster medium, which we find to be consistent with the electron temperature. In addition, we present a new measurement of the redshift of the brightest cluster galaxy NGC 1275.
|
|
| 2. |
- Okabe, Nobuhiro, et al.
(författare)
-
Active gas features in three HSC-SSP CAMIRA clusters revealed by high angular resolution analysis of MUSTANG-2 SZE and XXL X-ray observations
- 2021
-
Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 501:2, s. 1701-1732
-
Tidskriftsartikel (refereegranskat)abstract
- We present results from simultaneous modelling of high angular resolution GBT/MUSTANG-2 90 GHz Sunyaev-Zel'dovich effect (SZE) measurements and XMM-XXL X-ray images of three rich galaxy clusters selected from the HSC-SSP Survey. The combination of high angular resolution SZE and X-ray imaging enables a spatially resolved multicomponent analysis, which is crucial to understand complex distributions of cluster gas properties. The targeted clusters have similar optical richnesses and redshifts, but exhibit different dynamical states in their member galaxy distributions: a single-peaked cluster, a double-peaked cluster, and a cluster belonging to a supercluster. A large-scale residual pattern in both regular Compton-parameter y and X-ray surface brightness distributions is found in the single-peaked cluster, indicating a sloshing mode. The double-peaked cluster shows an X-ray remnant cool core between two SZE peaks associated with galaxy concentrations. The temperatures of the two peaks reach similar to 20-30 keV in contrast to the cool core component of similar to 2 keV, indicating a violent merger. The main SZE signal for the supercluster is elongated along a direction perpendicular to the major axis of the X-ray core, suggesting a minor merger before core passage. The S-X and y distributions are thus perturbed at some level, regardless of the optical properties. We find that the integrated Compton y parameter and the temperature for the major merger are boosted from those expected by the weak-lensing mass and those for the other two clusters show no significant deviations, which is consistent with predictions of numerical simulations.
|
|
| 3. |
- Takeuchi, Keigo, et al.
(författare)
-
A New Signaling Scheme for Large DS-CDMA Channels without CSI
- 2009
-
Ingår i: 2009 7TH INTERNATIONAL SYMPOSIUM ON MODELING AND OPTIMIZATION IN MOBILE, AD HOC, AND WIRELESS. - 9781424449200 ; , s. 478-484
-
Konferensbidrag (refereegranskat)abstract
- We propose a novel signaling scheme for wireless communication systems without channel state information (CSI). In that scheme, a bias of the occurrence probabilities of constellation points is utilized as pilot information known to the receiver, whereas pilot signals known to the receiver are sent in conventional pilot-based approaches. We evaluate the performance of the new scheme and conventional pilot-based schemes for a large direct-sequence code-division multiple-access (DS-CDMA) system, by using the replica method. It is shown that the new scheme outperforms the conventional pilot-based scheme when the amount of pilot information is large.
|
|
| 4. |
- Takeuchi, Keigo, et al.
(författare)
-
Asymptotic Performance Bounds of Joint Channel Estimation and Multiuser Detection in Frequency-Selective Fading DS-CDMA Channels
- 2008
-
Ingår i: 2008 INTERNATIONAL SYMPOSIUM ON INFORMATION THEORY AND ITS APPLICATIONS, VOLS 1-3. - 9781424420698 ; , s. 358-363
-
Konferensbidrag (refereegranskat)abstract
- We derive asymptotic upper bounds of the performance of a one-shot MMSE multiuser detector (MUD) and an optimal MUD in frequency-selective fading DS-CDMA channels with imperfect CSI at the receiver, using the replica method. We find that the performance bound of the one-shot MMSE MUD is indistinguishable from the performance of the one-shot LMMSE MUD analyzed by Evans and Tse when signal-to-noise ratio is high and the system load is small, and that the performance gap between the optimal MUD and the one-shot LMMSE MUD is also small on the same conditions.
|
|
| 5. |
- Takeuchi, Keigo, et al.
(författare)
-
Large-System Analysis of Joint Channel and Data Estimation for MIMO DS-CDMA Systems
- 2012
-
Ingår i: IEEE Transactions on Information Theory. - 0018-9448 .- 1557-9654. ; 58:3, s. 1385-1412
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents a large-system analysis of the performance of joint channel estimation, multiuser detection, and per-user decoding (CE-MUDD) for randomly spread multiple-input multiple-output (MIMO) direct-sequence code-division multiple-access (DS-CDMA) systems. A suboptimal receiver based on successive decoding in conjunction with linear minimum mean-squared error (LMMSE) channel estimation is investigated. The replica method, developed in statistical mechanics, is used to evaluate the performance in the large-system limit, where the number of users and the spreading factor tend to infinity while their ratio and the number of transmit and receive antennas are kept constant. Joint CE-MUDD based on LMMSE channel estimation is compared to several methods of one-shot channel estimation, i.e., methods that do not utilize decoded data symbols to refine the initial channel estimates, in terms of spectral efficiency. Joint CE-MUDD is found to significantly reduce the rate loss caused by transmission of pilot signals when compared to the receivers based on one-shot channel estimation. This holds particularly for multiple-antenna systems. As a result, joint CE-MUDD can provide significant performance gains compared to the receivers based on one-shot channel estimation.
|
|
| 6. |
- Takeuchi, Keigo, et al.
(författare)
-
On an Achievable Rate of Large Rayleigh Block-Fading MIMO Channels With No CSI
- 2013
-
Ingår i: IEEE Transactions on Information Theory. - 0018-9448 .- 1557-9654. ; 59:10, s. 6517-6541
-
Tidskriftsartikel (refereegranskat)abstract
- Training-based transmission over Rayleigh block-fading multiple-input multiple-output (MIMO) channels is investigated. As a training method a combination of a pilot-assisted scheme and a biased signaling scheme is considered. The achievable rates of successive decoding (SD) receivers based on the linear minimum mean-squared error (LMMSE) channel estimation are analyzed in the large-system limit, by using the replica method under the assumption of replica symmetry. It is shown that negligible pilot information is best in terms of the achievable rates of the SD receivers in the large-system limit. The obtained analytical formulas of the achievable rates can improve the existing lower bound on the capacity of the MIMO channel with no channel state information (CSI), derived by Hassibi and Hochwald, for all SNRs. The comparison between the obtained bound and a high-SNR approximation of the channel capacity, derived by Zheng and Tse, implies that the high-SNR approximation is unreliable unless quite high SNR is considered. Energy efficiency in the low-SNR regime is also investigated in terms of the power per information bit required for reliable communication. The required minimum power is shown to be achieved at a positive rate for the SD receiver with no CSI, whereas it is achieved in the zero-rate limit for the case of perfect CSI available at the receiver. Moreover, numerical simulations imply that the presented large-system analysis can provide a good approximation for not so large systems. The results in this paper imply that SD schemes can provide a significant performance gain in the low-to-moderate SNR regimes, compared to conventional receivers based on one-shot channel estimation.
|
|
| 7. |
- Takeuchi, Keigo, et al.
(författare)
-
Performance analysis of optimal channel estimation and multiuser detection in a randomly-spread CDMA channel
- 2010
-
Ingår i: Journal of Systems Science and Complexity. - : Springer Nature. - 1009-6124 .- 1559-7067. ; 23:1, s. 22-34
-
Tidskriftsartikel (refereegranskat)abstract
- This paper analyzes performance of optimal channel estimation and multiuser detection (MUD) in a block-fading code-division multiple-access (CDMA) channel on the assumptions of random spreading and large-system limit, by using the replica method developed in statistical mechanics. The authors find that the asymptotic spectral efficiency of the linear minimum mean-squared error (LMMSE) MUD which was proposed and analyzed by Evans and Tse in 2000 is indistinguishable from that of the optimal MUD for small system loads. Our results imply that performance of MUD scarcely improves even if one spends more computational cost than that of the LMMSE MUD, i.e., at most the cube of the number of users, on the above-described conditions
|
|
| 8. |
- Takeuchi, Keigo, et al.
(författare)
-
Practical Signaling with Vanishing Pilot-Energy for Large Noncoherent Block-Fading MIMO Channels
- 2009
-
Ingår i: 2009 IEEE INTERNATIONAL SYMPOSIUM ON INFORMATION THEORY, VOLS 1- 4. - 9781424443130 ; , s. 759-763
-
Konferensbidrag (refereegranskat)abstract
- We propose a randomly-biased quadrature phase shift keying (QPSK) signaling scheme for a noncoherent Rayleigh block-fading multiple-input multiple-output (MIMO) channel. In order to optimize a prior of bias, we evaluate a lower bound of the spectral efficiency of the noncoherent MIMO channel with randomly-biased QPSK signaling in the large-system limit, by using the replica method. Our main result is that randomly-biased QPSK signaling with vanishing bias is optimal in the large-system limit for any signal-to-noise ratio.
|
|
| 9. |
- Takeuchi, Keigo, et al.
(författare)
-
Replica Analysis of General Multiuser Detection in MIMO DS-CDMA Channels with Imperfect CSI
- 2008
-
Ingår i: 2008 IEEE INTERNATIONAL SYMPOSIUM ON INFORMATION THEORY PROCEEDINGS, VOLS 1-6. - 9781424422579 ; , s. 514-518
-
Konferensbidrag (refereegranskat)abstract
- We consider impacts of channel estimation errors on performance of general multiuser detectors in MIMO DS-CDMA channels. We evaluate their performance in terms of asymptotic spectral efficiency, which is obtained via decoupling structure, by using the replica method. Numerical results imply that the performance of LMMSE detection is very close to that of MMSE detection for small system loads. Furthermore, we find that the spectral efficiency of MMSE detection grows discontinuously with the length of pilot sequences for large system loads, and that the critical length is close to the optimal length. While it is indistinguishable from that of LMMSE detection for short pilot sequences, the gap between the two is significantly large if the length of pilot sequences is longer than the critical length.
|
|
| 10. |
- Vehkaperä, Mikko, et al.
(författare)
-
Analysis of Large MIMO DS-CDMA Systems With Imperfect CSI And Spatial Correlation
- 2010
-
Ingår i: 2010 IEEE INTERNATIONAL SYMPOSIUM ON INFORMATION THEORY. - 9781424478910 ; , s. 2098-2102
-
Konferensbidrag (refereegranskat)abstract
- The large system analysis of randomly spread MIMO DS-CDMA systems is provided. Correlated Rayleigh fading MIMO channels are assumed for all users. Linear multiuser detection with separate decoding and pilot-aided channel estimation are used. The results imply that with channel estimation, the performance can improve significantly as the correlation between the transmit antennas increases. No channel information at the transmitter is required, but the channel estimator needs knowlegde of the long term transmit correlation in advance. The numerical results demonstrate that in a 4 x 4 MIMO DS-CDMA system with two users per chip, high antenna correlation at the transmitter can double the ergodic spectral efficiency compared to the case of uncorrelated transmit antennas.
|
|
| 11. |
- Vehkaperä, Mikko, et al.
(författare)
-
Asymptotic analysis of iterative channel estimation and multiuser detection with soft feedback in multipath channels
- 2008
-
Ingår i: 16th European Signal Processing Conference (EUSIPCO 2008).
-
Konferensbidrag (refereegranskat)abstract
- Large system analysis of a randomly spread direct-sequencecode-division multiple-access system in a frequency-selective channelis considered. The receiver uses iterative channel estimationand multiuser detection (MUD) with soft feedback from single-userdecoders to refine both the decisions of the linear minimum meansquare error (LMMSE) MUD, and the initial pilot-based estimatesof the LMMSE channel estimator. Replica method, a tool fromstatistical physics, and density evolution with Gaussian approximationare used to obtain the asymptotic performance of the LMMSEMUD with soft parallel interference cancellation. The results indicatethat as the channel coherence time grows, the performance ofcorresponding single-user system having perfect channel state informationcan be approached even in an overloaded multiuser systemusing channel estimation and vanishing pilot overhead.
|
|
| 12. |
- Vehkaperä, Mikko, et al.
(författare)
-
How Much Training is Needed for Iterative Multiuser Detection and Decoding?
- 2009
-
Ingår i: GLOBECOM 2009 - 2009 IEEE GLOBAL TELECOMMUNICATIONS CONFERENCE, VOLS 1-8. ; , s. 2896-2901
-
Konferensbidrag (refereegranskat)abstract
- This paper studies large randomly spread direct-sequence code-division multiple-access system operating over a block fading multipath channel. Channel knowledge is obtained by a linear estimator whose initial decisions are iteratively refined by using a soft feedback from the single-user decoders. In addition to the traditional training symbol based signaling scheme, we study a novel method that utilizes a random bias in the symbol probabilities of the transmitted signal to construct the initial channel estimates. The numerical results suggest that in the large system limit, appropriate selection of the channel code and signaling method allows for successful communication with vanishing training overhead in overloaded systems if iterative channel and data estimation is performed at the receiver.
|
|
| 13. |
|
|
| 14. |
- Vehkaperä, Mikko, et al.
(författare)
-
Iterative channel estimation, detection, and decoding in large CDMA systems
- 2010
-
Ingår i: 6th International Symposium on Turbo Codes and Iterative Information Processing, ISTC 2010. - 9781424467457 ; , s. 16-23
-
Konferensbidrag (refereegranskat)abstract
- The large system analysis of randomly spread direct-sequence code-division multiple-access systems operating over frequency-selective fading channels is considered. Iterative multiuser detection and decoding (MUDD) based on generalized posterior mean estimation and single-user sum-product decoding is assumed to be used at the receiver. The channel state information (CSI) at the MUDD is mismatched and obtained by a linear channel estimator whose initial decisions are iteratively refined with the help of information feedback provided by the MUDD. Furthermore, a new training method by means of probability-biased signaling is proposed. The results indicate that in the large system limit and under certain threshold loads, a near single-user performance with perfect CSI can be achieved using a vanishing training overhead. It is also found that for the considered setups, the iterative linear minimum mean square error based channel estimator is near optimal for relatively slowly time-varying multipath fading channels.
|
|
