| 1. |
- Bezuglov, NN, et al.
(author)
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Time-dependent radiative transfer in magneto-optical traps
- 2003
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In: Physical Review A (Atomic, Molecular and Optical Physics). - 1050-2947. ; 68:6: 063415
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Journal article (peer-reviewed)abstract
- We investigate radiation-imprisonment (RI) phenomena in a system of cold atoms trapped in magneto-optical traps (MOTs) when cooling lasers are switched off for a short period. The problem is formulated within the framework of the Streater-Cooper-Sandle equation that belongs to the Markov class and accounts for photon frequency redistribution in multiple scattering processes. On the basis of a semiclassical approach, we derive explicit analytical solutions for the spectral problem of the RI equation neglecting Doppler effects. The analysis reveals peculiarities in emission profiles under initial partial saturation of the resonance transition by the probe laser pulse. Comparison between theoretical and experimental data on effective radiative lifetimes obtained in Cs and Rb MOTs confirms the validity of the discussed nonstationary model of RI in cold samples. For small detunings, the Streater-Cooper-Sandle equation agrees with the experimental data, while in the case of large detunings, the theory does not agree, probably due to non-Markovian effects.
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| 2. |
- Cassioli, D, et al.
(author)
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Effects of spreading bandwidth on the performance of UWB RAKE
- 2003
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In: IEEE International Conference on Communications, 2003. ICC '03.. - 0780378024 ; , s. 3545-3549
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Conference paper (peer-reviewed)abstract
- We consider an ultra-wide bandwidth system using reduced-complexity RAKE receivers, which are based on either selective (called SRake) or partial (called PRake) combining of a subset of the available resolved multipath components. We investigate the influence of the spreading bandwidth on the system performance using the two considered types of RAKE receivers. We show that, for a fix number of RAKE fingers and a fix transmit power, there is an optimum bandwidth. This optimal bandwidth increases with the number of RAKE fingers, and is higher for an SRake than for a Prake. We also investigate the effects of the fading statistics (Rayleigh or Nakagami) on the optimal spreading bandwidth. We find that the optimal spreading bandwidth is approximately the same for both types of fading, but that the actual performance of an SRake can be better or worse in Rayleigh fading (compared to Nakagami), depending on the spreading bandwidth and the number of fingers.
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| 3. |
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| 4. |
- Digham, F, et al.
(author)
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Joint Pilot and Data Loading Technique for MIMO Systems Operating with Covariance Feedback
- 2004
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In: Fifth IEE International Conference on 3G Mobile Communication Technologies, 2004. 3G 2004.. - 0863413889 ; , s. 24-28
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Conference paper (peer-reviewed)abstract
- We consider a multiple input multiple output (MIMO) system wherein channel correlation as well as noisy channel estimation are both taken into account. For a given block fading duration, pilot-assisted channel estimation is carried out with the aid of a minimum mean square error estimator. The pilot sequences and data are jointly optimized to maximize the ergodic capacity while fulfilling a total transmit energy constraint. Using the optimization results, we study the effect of various system parameters and different types of channel knowledge. We show that systems with channel covariance knowledge and imperfect channel estimates can even outperform those with perfect channel estimation at the receiver, but without any channel knowledge at the transmitter. Further, we show that the signal to noise ratio and the block fading duration determine the qualitative behavior of ergodic capacity as a function of antenna spacing or angular spread. Finally, we find that the optimum pilot-to-data power ratio is always greater than one. It increases as the total transmit power decreases or as the block fading length increases.
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| 5. |
- Mehta, N B, et al.
(author)
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Rate of MIMO Systems with CSI at Transmitter and Receiver from Pilot-Aided Estimation
- 2004
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In: IEEE 60th Vehicular Technology Conference, VTC 2004-Fall.. - 1090-3038. - 0780385217 ; 3, s. 1575-1579
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Conference paper (peer-reviewed)abstract
- We consider realistic multiple input multiple output antenna systems operating over spatially colored channels with instantaneous, albeit imperfect, channel state information at the receiver and only covariance knowledge available at the transmitter. We focus on pilot-aided channel estimation in which the receiver uses minimum mean square error channel estimation. For such a setup, our goal is then to design both the pilot and data sequences optimally to maximize the information rates achievable over the channel. We first demonstrate that the estimation error cannot be modeled as additive white Gaussian noise. We adopt a lower bound on channel capacity with imperfect channel knowledge and show that an optimum design leads to a matching of the eigenspaces of the pilots and of the data to the eigenspace of the channel. Furthermore, the ranks of the pilot and data covariance matrices need to be equal, and the optimal training duration need only equal the above rank. The assignment of powers to the different modes of transmission can then be obtained numerically. The paper, in essence, extends the results in the literature that assume perfect channel knowledge at the receiver, and shows that fully exploiting covariance knowledge leads to a visible improvement in capacity when compared to schemes that assume no channel knowledge, whatsoever, at the transmitter.
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| 6. |
- Tufvesson, Fredrik, et al.
(author)
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Ultra-wideband communication using hybrid matched filter correlation receivers
- 2004
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In: IEEE Vehicular Technology Conference. - 1550-2252. ; 59:3, s. 1290-1294
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Conference paper (peer-reviewed)abstract
- For ultra-wideband (UWB) communication, Rake receivers using matched filter detection show the best performance but they are complex to realize due to the inherent channel estimation problem and combining problem. Transmitted reference schemes, on the other hand, allow for a less complex receiver structure that is able to combine the energy from different multipath components without channel estimation. These schemes, however, show a performance loss due to non-linear operations on noise terms (generation of noise-noise crossterms) when forming the decision variable. This paper describes a receiver structure for UWB communication capable of capturing the energy from different paths without the need for channel estimation. The proposed hybrid matched filter correlation receivers reduce the performance loss, due to the noise-times-noise term.
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