On the complexity of sphere decoding in digital communications

Sphere decoding has been suggested by a number of authors as an efficient algorithm to solve various detection problems in digital communications. In some cases, the algorithm is referred to as an algorithm of polynomial complexity without clearly specifying what assumptions are made about the problem structure. Another claim is that although worst-case complexity is exponential, the expected complexity of the algorithm is polynomial. Herein, we study the expected complexity where the problem size is defined to be the number of symbols jointly detected, and our main result is that the expected complexity is exponential for fixed signal-to-noise ratio (SNR), contrary to previous claims. The sphere radius, which is a parameter of the algorithm, must be chosen to ensure a nonvanishing probability of solving the detection problem. This causes the exponential complexity since the squared radius must grow linearly with problem size. The rate of linear increase is, however, dependent on the noise variance, and thus, the rate of the exponential function is strongly dependent on the SNR. Therefore sphere decoding can be efficient for some SNR and problems of moderate size, even though the number of operations required by the algorithm strictly speaking always grows as an exponential function of the problem size.     

Micro- and nanofabrication processes for hybrid synthetic and biological system fabrication

The application of micro- and nanofabrication processes to the development of hybrid synthetic and biological systems may enable the production of new devices such as controllable transporters, gears, levers, micropumps, or microgenerators powered by biological molecular motors. However, engineering these hybrid devices requires fabrication processes that are compatible with biological materials such as kinesin motor proteins and microtubules. In this paper, the effects of micro- and nanofabrication processing chemicals and resists on the functionality of casein, kinesin, and microtubule proteins are systematically examined to address the important missing link of the biocompatibility of micro- and nanofabrication processes needed to realize hybrid system fabrication. It is found that both casein, which is used to prevent motor denaturation on surfaces, and kinesin motors are surprisingly tolerant of most of the processing chemicals examined. Microtubules, however, are much more sensitive. Exposure to the processing chemicals leads to depolymerization, which is partially attributed to the pH of the solutions examined. When the chemicals were diluted in aqueous buffers, a subset of them no longer depolymerized microtubules and in their diluted forms still worked as resist removers. This approach broadens the application of micro- and nanofabrication processes to hybrid synthetic and biological system fabrication.     

Multiservice allocation for multiaccess wireless systems

This paper discusses principles for allocating multiple bearer services onto different subsystems in multiaccess wireless systems. Based on the included subsystem's multiservice capacities, under certain constraints near-optimum subsystem service allocations that maximize combined multiservice capacity are derived through simple optimization procedures. These favorable service allocations are either extreme points where services, as far as possible, are allocated to the subsystems best at supporting them, or they are characterized by the relative efficiency of supporting services being equal in all subsystems. The consequences of this include that services should typically be mixed in subsystems with convex capacity regions and isolated in subsystems with concave capacity regions. Simple user assignment algorithms based on this are also discussed. Additionally, illustrating the main findings of the analysis, some system examples are given, including a case study with combined global system for mobile communications (GSM) and wideband code-division multiple-access (WCDMA) systems. The gain of using the proposed service allocation principles compared to a reference case of maintaining equal service mixes in all subsystems depends on the shape of the subsystem capacity regions; the more different the capacity regions, the larger the gain. In the GSM and WCDMA case study, capacity gains of up to 100% in terms of supported data users for a fixed voice traffic load are achieved.     

Frequency shift keyed narrowband interference rejection: optimal exponential weighting factor for the RLS algorithm

Cochannel narrowband interference can limit the performance of direct sequence spread spectrum (DSSS) and high frequency (HF) systems. Narrowband interference (NBI) can be single tone, chirped or frequency shift keyed (FSK) in nature and numerous techniques for its removal have been proposed. Linear adaptive prediction filters based on autoregressive modelling have been suggested owing to their ability to perform in a non-stationary environment. In the FSK narrowband interference case, adaptive filters are susceptible to excess residual errors owing to instantaneous frequency step changes and the finite convergence time required for the filter to adapt to a new interference frequency. The signal degradation owing to this type of interference becomes greater in high SNR regimes and has been found to be a function of the frequency parameters of the FSK interference signal. The paper discusses the convergence and frequency tracking properties of the recursive least squares (RLS) adaptive lattice filter using a posteriori estimation errors in the presence of FSK narrowband interference. An optimal exponential weighting factor that balances convergence time and steady state error is derived for this case of NBI. Results are compared to those of a previously proposed fast converging minimum frequency error (FCMFE) RLS lattice filter.     

A method of determining the effective axial demagnetizing factor of an irregular hexagonal magnetic insulator

The shape demagnetizing factors of a planar magnetic insulator are of some interest in the design of a number of microwave ferrite components. One means of experimentally evaluating these quantities, in the case of a disk geometry, is to measure the Kittel line and to make use of a standard identity. Another method, introduced here, is to extract the axial demagnetizing factor by having recourse to the intersections of the split branches of a gyromagnetic resonator. The letter includes measurements on the axial demagnetizing factor of an irregular hexagonal geometry for three different values of saturation magnetization.     

On the stationary distribution of speed and location of random waypoint

In "Stationary distributions for the random waypoint mobility model" (TMC, Vol. 3, No, 1), Navidi and Camp find the stationary distribution of the random waypoint model, with or without pause on a rectangular area. In this short note, we show that, under the stationary regime, speed and location are independent.     

Arc root mobility on piezoelectrically actuated contacts in miniature circuit breakers

A novel contact opening mechanism has been developed using a piezoelectric actuator to open the contacts in a low contact opening velocity circuit breaker. The arc control on the contacts is critical for successful current interruption (10/sup 3/-10/sup 4/ A) in low voltage (<250V) devices. Previous work has shown how arc root commutation from the contact region into the arc chamber is affected by arc chamber materials, contact materials and the gap behind the moving contact for contact velocities between 1ms/sup -1/ and 10ms/sup -1/. This work is extended using a commercially available piezoelectric actuator to open the contacts. Contact opening speeds are assessed and the arc root mobility is characterized under this operating regime. A flexible test apparatus and solid-state high-speed arc imaging system are used to gather data on the arc root during the opening of the contacts. New experimental results are presented on the anode and cathode root velocity and arc root motion in an arc chamber with piezoelectrically actuated contact opening. These results can be used to improve the design of high current low voltage circuit breakers suitable for piezoelectric actuation.     

Security issues in hybrid networks with a satellite component

Satellites are expected to play an increasingly important role in providing broadband Internet services over long distances in an efficient manner. Most future networks will be hybrid in nature - having terrestrial nodes interconnected by satellite links. Security is an important concern in such networks, since the satellite segment is susceptible to a host of attacks, including eavesdropping, session hijacking and data corruption. In this article we address the issue of securing communication in satellite networks. We discuss various security attacks that are possible in hybrid satellite networks, and survey the different solutions proposed to secure data communications in these networks. We look at the performance problems arising in hybrid networks due to security additions like Internet security protocol (IPSec) or secure socket layer (SSL), and suggest solutions to performance-related problems. We also point out important drawbacks in the proposed solutions, and suggest a hierarchical key-management approach for adding data security to group communication in hybrid networks.     

Mutual information-based analysis of JPEG2000 contexts.

Context-based arithmetic coding has been widely adopted in image and video compression and is a key component of the new JPEG2000 image compression standard. In this paper, the contexts used in JPEG2000 are analyzed using the mutual information, which is closely related to the compression performance. We first show that, when combining the contexts, the mutual information between the contexts and the encoded data will decrease unless the conditional probability distributions of the combined contexts are the same. Given I, the initial number of contexts, and F, the final desired number of contexts, there are S(I, F) possible context classification schemes where S(I, F) is called the Stirling number of the second kind. The optimal classification scheme is the one that gives the maximum mutual information. Instead of using an exhaustive search, the optimal classification scheme can be obtained through a modified generalized Lloyd algorithm with the relative entropy as the distortion metric. For binary arithmetic coding, the search complexity can be reduced by using dynamic programming. Our experimental results show that the JPEG2000 contexts capture the correlations among the wavelet coefficients very well. At the same time, the number of contexts used as part of the standard can be reduced without loss in the coding performance.     

Linear demosaicing inspired by the human visual system.

There is an analogy between single-chip color cameras and the human visual system in that these two systems acquire only one limited wavelength sensitivity band per spatial location. We have exploited this analogy, defining a model that characterizes a one-color per spatial position image as a coding into luminance and chrominance of the corresponding three colors per spatial position image. Luminance is defined with full spatial resolution while chrominance contains subsampled opponent colors. Moreover, luminance and chrominance follow a particular arrangement in the Fourier domain, allowing for demosaicing by spatial frequency filtering. This model shows that visual artifacts after demosaicing are due to aliasing between luminance and chrominance and could be solved using a preprocessing filter. This approach also gives new insights for the representation of single-color per spatial location images and enables formal and controllable procedures to design demosaicing algorithms that perform well compared to concurrent approaches, as demonstrated by experiments.