Throughput of ADSL modems

This paper considers the throughput of ADSL (Asymmetric Digital Subscriber Line) modems, used for high-speed data transmission over relatively unreliable connections, e.g. copper telephone wires. The modem technique uses an error correcting code and interleaving. The settings include a grouping factor $S$ which affects the amount of data per code word, the number $R$ of redundant bytes per code word and the interleave depth $D$. The influence of these parameters on both the effective data transmission rate and the resulting error rate in the received signal are determined for two error situations: random errors and bursts of errors. An approximate analysis for the random error case of the throughput of a TCP (Transport Control Protocol) connection using an ADSL modem shows that maximum throughput is obtained for the highest values of $S$ and $R$

Orbital alignment cross sections by stimulated emission probing : the state-to-state Ca Rydberg process Ca(4s17d^1D_2)+Xe->Ca(4s18p^1P_1)+Xe

The initial state alignment effect vs relative velocity is measured for a state-to-state Ca Rydberg collisional energy transfer process. The stimulated emission detection method is used to determine the alignment effect for the n,1-changing transition: Ca(4s17d1D2) + Xe -->Ca(4s18p1P1) + Xe+deltaE = -1.7cm-1. The rate of electronic energy transfer in this state-changing collision is observed to vary with the direction of the Rydberg electron charge cloud relative to the collision axis. Both the expected cos(4beta) and cos(2beta) dependencies are observed. The alignment data are analyzed to obtain the relative cross sections for the individual Ca(1D2) magnetic sublevels. The values of the m-sublevel cross sections sigma0 : sigma1 : sigma2 are 1.13+/-0.02:1.11+/-0.02:0.83+/-0.02. Qualitative interpretations of the relative cross sections in terms of both molecular (van der Waals) Born-Oppenheimer potentials and the impulse approximation are presented

Autoionization widths for Ne*(3s)-Ar and Ne**(3p)-Ar collisions

Following Jones and Dahler [Phys. Rev. A 37, 2916 (1988)] we study the theory of the process of ionization within the framework of the Feshbach projection-operator formalism, with the discrete fine-structure states (before ionization) and the continuum states (after ionization) as the two separate subspaces of Hilbert space. For the Ne*(3s)-Ar and Ne**(3p)-Ar systems we have calculated ab initio autoionization widths. In the approximation in which only two electrons play an active role, the coupling matrix element between the two subspaces is a linear combination of two-center two-electron integrals for the exchange and the radiative mechanisms. In general, our calculations support the semiempirical results of Morgner [J. Phys. B 18, 251 (1985)] for the coupling matrix elements as derived from experimental data. The autoionization widths GJ',JO are presented for initial states |J,O> of the Ne*(*) atom and final states |J'> of the Ar+ ion. They show a pronounced O dependence and a strong correlation of initial and final states, e.g., |0,0>¿|3/2> and |2,2>¿|3/2>. For the Ne*(3s) states the two-center two-electron calculation is in good agreement with the two-state basis of Driessen et al. [Phys. Rev. A 42, 4058 (1990)], based on a one-electron overlap approximation. In a semiclassical model the polarized-atom ionization cross sections are calculated. For the Ne**(3p)-Ar system the energy dependence of both the polarization effect and the average cross-section value is reproduced very well, in contrast to the one-electron approximation. For the Ne*(3s)-Ar system we have to conclude that the ab initio results cannot explain all experimental observed features, such as the large cross-section ratio Q(P30)/Q(P32)

Orbital alignment cross sections by stimulated emission probing : the state-to-state Ca Rydberg process Ca(4s17d^1D_2)+Xe->Ca(4s18p^1P_1)+Xe

The initial state alignment effect vs relative velocity is measured for a state-to-state Ca Rydberg collisional energy transfer process. The stimulated emission detection method is used to determine the alignment effect for the n,1-changing transition: Ca(4s17d1D2) + Xe -->Ca(4s18p1P1) + Xe+deltaE = -1.7cm-1. The rate of electronic energy transfer in this state-changing collision is observed to vary with the direction of the Rydberg electron charge cloud relative to the collision axis. Both the expected cos(4beta) and cos(2beta) dependencies are observed. The alignment data are analyzed to obtain the relative cross sections for the individual Ca(1D2) magnetic sublevels. The values of the m-sublevel cross sections sigma0 : sigma1 : sigma2 are 1.13+/-0.02:1.11+/-0.02:0.83+/-0.02. Qualitative interpretations of the relative cross sections in terms of both molecular (van der Waals) Born-Oppenheimer potentials and the impulse approximation are presented

Alignment probing of Rydberg states by stimulated emission

The possibility of probing the collisions of aligned Rydberg atoms by stimulated emission is assessed with studies of a polarized state and a new measurement of a collisional alignment effect in atomic Ca. The stimulated emission method uses a laser to dump the desired state to a lower level which subsequently fluoresces. The technique can be used to obtain populations and polarization dependent information. First, the method is tested by applying it to an aligned Ca(4s17d¿1D2) state. Alignment curves are measured when the initial state is prepared with both parallel and perpendicular relative polarizations. The experimentally observed alignment compares well with that derived from theoretical considerations of a saturated stimulated transition. Second, a two-vector collisional alignment experiment (initial state and relative velocity vector) is performed to study the energy transfer process Ca(4s7d¿1D2)+He¿Ca(4s6f¿1F3)+He+¿E=17.7 cm-1, and alignment effects are measured by both stimulated emission and conventional direct fluorescence detection. A preference for the ¿m¿=1 and 2 initial states is observed in the relative cross sections. Essentially identical data are obtained with the two detection methods when elliptically polarized light is used for the stimulated emission detection method. The stimulated emission technique can provide alignment and population information of the final states, making it an excellent new tool for both three-vector correlation experiments and state-to-state Rydberg transitions

Alignment probing of Rydberg states by stimulated emission

The possibility of probing the collisions of aligned Rydberg atoms by stimulated emission is assessed with studies of a polarized state and a new measurement of a collisional alignment effect in atomic Ca. The stimulated emission method uses a laser to dump the desired state to a lower level which subsequently fluoresces. The technique can be used to obtain populations and polarization dependent information. First, the method is tested by applying it to an aligned Ca(4s17d¿1D2) state. Alignment curves are measured when the initial state is prepared with both parallel and perpendicular relative polarizations. The experimentally observed alignment compares well with that derived from theoretical considerations of a saturated stimulated transition. Second, a two-vector collisional alignment experiment (initial state and relative velocity vector) is performed to study the energy transfer process Ca(4s7d¿1D2)+He¿Ca(4s6f¿1F3)+He+¿E=17.7 cm-1, and alignment effects are measured by both stimulated emission and conventional direct fluorescence detection. A preference for the ¿m¿=1 and 2 initial states is observed in the relative cross sections. Essentially identical data are obtained with the two detection methods when elliptically polarized light is used for the stimulated emission detection method. The stimulated emission technique can provide alignment and population information of the final states, making it an excellent new tool for both three-vector correlation experiments and state-to-state Rydberg transitions