Effect of tap spacing on the performance of direct-sequencespread-spectrum RAKE receiver

The effect of tap spacing on the performance of a RAKE receiver is analyzed analytically in a frequency-selective fading channel. A continuous time multipath fading channel model is used for the analysis, and the expression of the correlation between the desired signals, interference signals, and noise signals at the output of each branch of the RAKE receiver is derived for various chip waveforms. Since the noise components of each branch signal are correlated to each other, an optimum combining rule based on the maximum-likelihood criterion is derived to gain utmost performance. It is shown that the performance of the system can be improved by setting the tap spacing of the RARE receiver below the chip duration when the bandwidth of the transmitted signal is larger than the inverse of the chip duration. Also, it is shown that the normalized capacity of the system can be increased by using a chip waveform occupying wider bandwidth, which takes advantage of the increased diversity gain merits of a wide-band code-division multiple-access system at the same chip rate. It is noted that the derived combining rule gives diversity gain against the fading process as well as noise whitening processing gain against multiple-access interference at the same time     

Reproducing consistent wireless protocol performance across environments

Full scale experimentation with wireless networks in deployment environments is difficult. Therefore a common validation technique is to test a prototype network in a convenient environment prior to deployment. In this paper, we consider the problem of obtaining comparable protocol performance when the test and deployment environments differ in RF propagation environment and/or inter-node spacing. To achieve comparable protocol behavior in the two settings, we propose the concept of “link usage spectrum”. Based on the hypothesis that the link usage spectrum is a gross predictor for network performance, we show how to replicate in the test setting the link usage spectrum of the protocol that is expected in the deployment setting. We show our technique for achieving comparable protocol behavior via experiments and simulations in multiple indoor and outdoor propagation environments. The link usage spectrum is protocol specific; we illustrate for a family of protocols how the link usage spectrum is calculated analytically, from the protocol metric for choosing forwarding links in the network, and how power scaling can be used to match the link usage spectrum across networks.     

Analysis of TFCI coding performance of WCDMA systems

In WCDMA system, transport format combination indicator (TFCI) is encoded by an encoder based on Reed-Muller code and transmitted in a time-multiplexed fashion. When the number of TFCI information bits is less than the number of input bits to encoder, subset of basis sequences should be used. In this paper, we analyze the upper bound of word error rate (WER) of TFCI coding and find the optimal subset of basis sequences that minimize the bound. Since the bound of the WER is dependent on the channel correlation, the basis sequences to mitigate the effect of channel correlation are selected. From the result, a basis sequence with uniformly distributed bit of '1' is firstly chosen in optimal subset of basis sequences. The choice of basis sequences is more significant for high mobile speed.     

Chip warpage model for reliability prediction of delamination failures

A new experimental method to predict reliability for ACA type packages under temperature cycling is developed and proposed. The method introduces a new damage parameter that can be easily measured by experiment. It is proved that the linear elastic parameter, dw/dT which represents the rate of change of chip warpage with respect to temperature, efficiently reflects the common failure mechanism of ACA type packages, the interfacial delamination between the chip and the adhesive. It is demonstrated, both experimentally and numerically, that the size of delamination affects the warpage behavior of the chip. The dw/dT of the chip is monitored in real time using laser interferometers under thermal fatigue cycles up to 3000. The gradual decrease in warpage due to progressive increase in delamination is clearly emerged. As a result, a reliability curve that can predict the size of delamination and remained life is obtained. The new long-term reliability prediction method developed in this study can be applied to various advanced packages, e.g. underfilled flip–chip or TSV stacked chip packages, that embrace interfacial delamination as primary failure mode.     

Base station location in a cellular CDMA system

We consider a cellular CDMA system in which blocking is enforced when the relative interference exceeds a certain threshold level. This paper addresses a radio network design problem in such a CDMA system. Given the data of call‐traffic distributed over the service area and potential sites of base stations, the objective of the problem is to locate base stations so as to minimize the associated cost for establishing base stations while keeping the probability of blocking under control. We develop an efficient algorithm for solving the design problem. Computational experiments with real‐world data are conducted to show both the efficiency and the practicality of the proposed design method. This revised version was published online in August 2006 with corrections to the Cover Date.     

Watt-level Ka- and Q-band MMIC power amplifiers operating at lowvoltages

Ka- and Q-band watt-level monolithic power amplifiers (PAs) operating at a low drain bias of 3.6 V are presented in this paper. Design considerations for low-voltage operation have been carefully studied, with an emphasis on the effect of device models. The deficiency of conventional table-based models for low-voltage operation is identified. A new nonlinear device model, which combines the advantages of conventional analytical models and table-based models, has been developed to circumvent the numerical problems and, thus, to predict optimum load impedance accurately. The model was verified with load-pull measurements at 39 GHz. To implement a low-voltage 1-W monolithic-microwave integrated-circuit amplifier, careful circuit design has been performed using this model. A Q-band two-stage amplifier showed 1-W output power with a high power gain of 15 dB at 3.6-V drain bias. The peak power-added efficiency (PAE) was 28.5% and 1-dB compression power (P_{1 dB}) was 29.7 dBm. A Ka-band two-stage amplifier showed a P_{1 dB} of 30 dBm with 24.5-dB associated gain and 32.5% PAE. Under very low dc power conditions (P_dc<2 W, V_ds=3.4 V), the amplifiers showed 29-dBm output power and PAE close to 36%, demonstrating ultimate low-power operation capability. To the best of our knowledge, this is the first demonstration of watt-level PA's under 3.6-V operation at 26 and 40 GHz. Compared with the published data, this work also represents state-of-the-art performance in terms of power gain, efficiency, and chip size     

Characterizing Overlay Multicast Networks and Their Costs

Overlay networks among cooperating hosts have recently emerged as a viable solution to several challenging problems, including multicasting, routing, content distribution, and peer-to-peer services. Application-level overlays, however, incur a performance penalty over router-level solutions. This paper quantifies and explains this performance penalty for overlay multicast trees via: 1) Internet experimental data; 2) simulations; and 3) theoretical models. We compare a number of overlay multicast protocols with respect to overlay tree structure, and underlying network characteristics. Experimental data and simulations illustrate that the mean number of hops and mean per-hop delay between parent and child hosts in overlay trees generally decrease as the level of the host in the overlay tree increases. Overlay multicast routing strategies, overlay host distribution, and Internet topology characteristics are identified as three primary causes of the observed phenomenon. We show that this phenomenon yields overlay tree cost savings: Our results reveal that the normalized cost L(n)/U(n) is propn^0.9 for small n, where L(n) is the total number of hops in all overlay links, U(n) is the average number of hops on the source to receiver unicast paths, and n is the number of members in the overlay multicast session. This can be compared to an IP multicast cost proportional to n^0.6 to n^0.8     

Microlens for efficient coupling between LED and optical fiber

A new fabrication method of a microlens is proposed that can be easily applied to optical devices and microlens systems. The proposed microlens is formed by self-surface tension and cohesion of UV curing material. Since the microlens is hardened by short time UV exposure, the fabrication process is very simple. Integration with surface emitting-light emitting diode (SE-LED) results in enhanced coupling to optical fiber with coupling efficiency larger than the conventional case by 1.5 times. We also made a hemispheric microlensed fiber using this method. Compared with a typical arc-lensed fiber and a flat-end fiber, the coupling efficiency is improved to 18% and 40%, respectively     

A power-factor controller for single-phase PWM rectifiers

A novel power-factor controller for single-phase pulsewidth modulated rectifiers is proposed. The unity power-factor controller for a sinusoidal input current is derived using the feedback linearization concept. Two active switches and two diodes are utilized for AC-to-DC power conversion. Experimental results obtained on a 4 kW prototype are discussed     

High-Temperature Stability of Thermoelectric Skutterudite In0.25Co3FeSb12

The thermal stability of skutterudite-based thermoelectric modules is of great importance since they are used at elevated temperatures. This study examined the high-temperature stability of In-filled and Fe-doped skutterudites (In_0.25Co₃FeSb₁₂) as a function of the following aging variables: atmosphere (vacuum and air), temperature, and time. Sb-based oxides are produced preferentially on exposure to high temperatures in air. The oxide layer produced during aging at 823?K in air was much thinner than that produced during aging at 723?K in air. The formation of InSb is believed to retard the oxidation of Sb and act as an obstacle to the growth of the oxide layer. The CoSb₃-based skutterudites were stable at 823?K if they were not exposed to air, and InSb phases were not produced in the In_0.25Co₃FeSb₁₂ skutterudites.