Modulation and Equalisation Considerations for High Performance Radio LANs

The European Telecommunications Standards Institute (ETSI) has recently defined a European standard for a high performance radio LAN (known as HIPERLAN). To operate as wired LAN replacements, these systems will operate at 5.2 GHz and support instantaneous bit rates of just under 24 Mb/s. To counteract the time dispersive nature of the indoor radio channel, the use of adaptive equalisation is suggested. In this paper a number of possible modulation and equalisation techniques are presented and, in particular, the Bit Error Rate (BER) performance of quasi-coherent GMSK combined with Decision Feedback Equalisation is explored through computer simulation. The trade off between symbol spaced and fractionally spaced equalisation is considered together with the importance of feedfoward and feedback synchronisation to the channel's power delay profile. The paper also includes a comparison of the RLS and LMS based training algorithms and compares the modem developed under the ESPRIT III LAURA project with that specified in HIPERLAN.The application of dual antenna diversity is investigated and its impact on the number of received error free data packets obtained as a function of signal leval and rms delay spread. The use of such diversity is shown to greatly improve the BER performance of a HIPERLAN modem. The problem of frequency offset is considered and modifications are proposed to the HIPERLAN frame structure to improve the receiver's tolerance to such errors. Important practical issues such as frame and symbol synchronisation, frequency offset correction and hardware implementation are discussed from both the LAURA and HIPERLAN viewpoint.     

A phenomenological model for giant magnetoresistance hysteresis inmagnetic discontinuous multilayer films

A simple phenomenological model for giant magnetoresistance (GMR) is employed for current and field parallel (CIP) to the magnetic multilayer planes in ordinary and discontinuous multilayer films. To our knowledge, it is the first model to include hysteresis in the field (H) dependence of the GMR. The computed GMR versus H curves qualitatively reproduce the GMR hysteresis seen experimentally. In particular, two GMR peaks are found to be symmetrically placed about H=0, and the GMR hysteresis curve itself is found to have an inverted butterfly shape. Also seen in the computed results is the general increase in GMR magnitude found for annealed discontinuous multilayer films. Various parameter variations are examined in the computed results. While the model reproduces GMR hysteresis quite well and the general increase in GMR for discontinuous multilayer films, it does not, in its present form, account for the oscillations seen in the GMR when the nonmagnetic layer thicknesses are varied, which is expected as a strictly quantum mechanical result     

Reduction of sodium-accelerated oxidation of silicon nitride ceramics by aluminium implantation

Norton NBD 200 silicon nitride ceramics were implanted with sodium to a dose of 7.0×10¹⁵cm^-2 at 72 keV (1 at% peak sodium content at 100 nm). The sodium-implanted samples were further implanted with aluminium to 7.3×10¹⁵cm^-2 at 87 keV (1 at% peak aluminium content at 100 nm). The implanted and unimplanted samples were oxidized in 1 atm dry oxygen at 1100 and 1300°C for 2–6 h. Profilometry and scanning electron microscopy measurements indicated that sodium implantation led to up to a two-fold increase in the oxidation rate of silicon nitride. The sodium effect was effectively neutralized when aluminium was co-implanted. The opposite effects of sodium and aluminium on the oxidation resistance of silicon nitride can be attributed to their different roles in modifying the structure and properties of the oxide formed. This revised version was published online in November 2006 with corrections to the Cover Date.     

The lattice dynamics of the Fe(tpa) (NCS)2 complex : light-induced excited spin state trapping effect

A light-induced excited spin state trapping (LIESST) experiment for a thermal gradual spin crossover complex, Fetris (2-pyridylmethyl) amine(NCS)2 or Fe(tpa) (NCS)2, was attempted for the first time. The high spin (HS) state after light inducement stayed metastable over a period of days without relaxation at 10 K. Intersystem relaxation from a high to a low spin (LS) complex occurred at 50 K after bleaching at 10 K. Investigation of the Mossbauer spectra of the LIESST and relaxation experiment indicated that the Debye–Waller factor was a correlation parameter of the HS fraction and that the co-operative effect played a role in the relaxation process for such a solid compound. This revised version was published online in November 2006 with corrections to the Cover Date.     

Influence of quantum capture and tunneling mechanisms on the dark current of bound-to-continuum quantum-well infrared photodetectors

We present a theoretical model for the dark current of bound-to-continuum quantum-well infrared photodetectors (QWIPs), by considering the field-induced mixing effect, tunneling rate and phonon scattering rate between bound and continuum states. Using this model, we can see clearly how these mechanisms significantly influence the Fermi levels of bound and continuum electrons, and thus, the dark current. Nonlinear temperature dependence of the dark current at low temperature is predicted and discussed in detail. The simulated dark currents exhibit good agreement with the experimental results, without use of parameter fitting techniques.     

Influence of the quality of the heterointerface on the cyclotron resonance spectra of InAs/(AlGa)Sb heterostructures

Cyclotron resonance is measured in solitary type-II InAs-AlGaSb quantum wells grown by molecular-beam epitaxy under various growth conditions. Quantum oscillations observed in the cyclotron resonance spectra in InAs-GaSb samples are attributed to scattering by a short-range potential due to roughness of the heterointerface. A new method based on measurement of the cyclotron resonance spectra is proposed for assessing the quality of the heterointerface. Fiz. Tekh. Poluprovodn. 31, 1246–1248 (October 1997)     

Error-resilience improvement for block-transform video coders

An algorithm is presented to enhance the resilience of block-based video-coding algorithms against channel errors. The error-resilience algorithm described suggests the use of fixed length coding to alleviate the effect of Huffman coding on the synchronisation of the decode under erroneous conditions. The proposed mechanism modifies the order of transmission of the fixed-length video parameters to increase the chance of their arrival. Synchronisation words are sent at fixed-length intervals within the bit stream to reduce the possibility of the decoder falling on similar bit patterns within the bit stream. To limit the effect of error accumulation, the motion prediction process is halted and the differential coding of motion vectors is not applied. FEC techniques are applied on some error-sensitive segments of the reordered video bitstream. The effectiveness of the proposed error-resilience algorithm is evidenced by both subjective and objective results     

Four-channel polarization-insensitive optically transparent wavelength converter

Multichannel wavelength converters may be important components in the cross-connects in future wavelength-division multiplexed (WDM) transport networks. We demonstrate a multichannel, polarization-insensitive, optically transparent wavelength converter, based on four-wave mixing in two semiconductor optical amplifiers in a polarization-diversity arrangement. Bit-error-rate (BER) measurements with four input 2.5-Gb/s WDM channels, spaced by 2 nm, show penalties for wavelength conversion less than 2.6 dB at 10/sup -9/ BER. Changes in the state of polarization of the input signals cause the output power to change by less than 1.2 dB, and the corresponding power penalties change by less than 0.9 dB.     

Wavelength selection for low-saturation pulse oximetry

Conventional pulse oximeters are accurate at high oxygen saturation under a variety of physiological conditions but show worsening accuracy at lower saturation (below 70%). Numerical modeling suggests that sensors fabricated with 735 and 890 nm emitters should read more accurately at low saturation under a variety of conditions than sensors made with conventionally used 660 and 900 nm band emitters. Recent animal testing confirms this expectation. It is postulated that the most repeatable and stable accuracy of the pulse oximeter occurs when the fractional change in photon path lengths due to perturbations in the tissue (relative to the conditions present during system calibration) is equivalent at the two wavelengths. Additionally, the penetration depth (and/or breadth) of the probing light needs to be well matched at the two wavelengths in order to minimize the effects of tissue heterogeneity. At high saturation these conditions are optimally met with 660 and 900 nm band emitters, while at low saturation 735 and 890 nm provide better performance