Bernhard Wegmann

The atoms of vision: Cartesian or polar?

The inherent structure of the encoding in early stages of the visual system is investigated from a combined information-theoretical, psychophysical, and neurophysiological perspective. We argue that the classical modeling in terms of linear spatial filters is equivalent to the assumption of a Cartesian organization of the feature space of early vision. We show that such a linear Cartesian feature space would be suboptimal for the exploitation of the statistical redundancies of natural images since these have a radially separable probability-density function. Therefore a more efficient representation can be obtained by a nonlinear encoding that yields a feature space with polar organization. This prediction of the information-theoretical approach regarding the organization of the feature space of early vision is confirmed by our psychophysical measurements of basic discrimination capabilities for elementary Gabor patches, and the necessary nonlinear operations are shown to be closely related to cortical gain control and to the phase invariance of complex cells. Finally, we point out some striking similarities between the polar representation in visual cortex and basic image-coding strategies pursued in shape-gain vector quantization schemes.

Statistical dependence between orientation filter outputs used in a human-vision-based image code

We present an image coding scheme based on the properties of the early stages of the human visual system. The image signal is decomposed via even and odd symmetric frequency and orientation selective bandpass filters in analogy to the quadrature phase simple cell pairs in the visual cortex. The resulting analytic signal is transformed into a local amplitude and local phase representation in order to achieve a better match to its signal statistics. Both intra filter dependencies of the analytic signal and inter filter dependencies between different orientation filters are exploited by a suitable vector quantization scheme. Inter orientation filter dependencies are demonstrated by means of a statistical evaluation of the multidimensional probability density function. The results can be seen as an empirical confirmation of the suitability of vector quantization in subband coding. Instead of generating a code book by use of an conventional designalgorithm we suggest a feature specific partitioning of the multidimensional signal space matched to the properties of human vision. Using this coding scheme satisfactory image quality can be obtained with about 0. 78 bit/pixel.

Dynamic channel allocation in UMTS terrestrial radio access TDD systems

The TDD operation mode of UMTS allows highly dynamic and various configurations of the physical layer time frame. A key issue in developing access methodologies to the available spectrum is an optimal management of the rare radio resources. The application of interference based dynamic resource allocation is evaluated and compared to fixed channel allocation. The dynamic allocation is based on an attempt to offer connections instantaneous quality of service. Moreover, it allows rearrangement of the allocated resources in order to increase the connection quality in case of a possible impending connection breakdown. As a decentralised algorithm, dynamic channel allocation offers among its performance gain the ease of implementation while reducing the network planning.

Feature-specific vector quantization of images

A new interband vector quantization of a human vision-based image representation is presented. The feature specific vector quantizer (FVQ) is suited for data compression beyond second-order decorrelation. The scheme is derived from statistical investigations of natural images and the processing principles of biological vision systems, the initial stage of the coding algorithm is a hierarchical, and orientation-selective, analytic bandpass decomposition, realized by even- and odd-symmetric filter pairs that are modeled after the simple cells of the visual cortex. The outputs of each even- and odd-symmetric filter pair are interpreted as real and imaginary parts of an analytic bandpass signal, which is transformed into a local amplitude and a local phase component according to the operation of cortical complex cells. Feature-specific multidimensional vector quantization is realized by combining the amplitude/phase samples of all orientation filters of one resolution layer. The resulting vectors are suited for a classification of the local image features with respect to their intrinsic dimensionality, and enable the exploitation of higher order statistical dependencies between the subbands. This final step is closely related to the operation of cortical hypercomplex or end-stopped cells. The codebook design is based on statistical as well as psychophysical and neurophysiological considerations, and avoids the common shortcomings of perceptually implausible mathematical error criteria. The resulting perceptual quality of compressed images is superior to that obtained with standard vector quantizers of comparable complexity.

Capacity improvement through random timeslot opposing (RTO) algorithm in cellular TDD systems with asymmetric channel utilisation

In this paper the performance of the RTO algorithm at different asymmetries favouring uplink (UL) and downlink (DL) is studied. The RTO algorithm is a dynamic channel allocation (DCA) algorithm that yields a significant interference reduction (min. 3 dB, max. 20 dB) compared to a fixed channel assignment (FCA) algorithm. The FCA algorithm used, emulates an equivalent FDD system which has no same-entity interference. Thus, the existence of different interference modes in TDD is exploited constructively to avoid jamming. The RTO algorithm shows an excellent performance for an asymmetry favouring the DL. This is a very useful result as future high data rate services which, for example, make use of multimedia streaming technologies require significantly higher capacity in the DL than in the UL. If the asymmetry. for example, is 14:2 (14 time slots (TSs) used for DL and 2 TSs for UL). the performance is about 15 dB better than with an asymmetry of 2:12 (2 TSs used for DL and 14 TSs for UL). It is important to note that the FCA algorithm results in greater interference for all rates of asymmetry investigated.

Interference diversity through random time slot opposing (RTO) in a cellular TDD system

In this paper a new dynamic channel assignment (DCA) technique for a time division duplex (TDD) air-interface is proposed. This DCA exploits the fact that a cellular TDD system experiences additional interference scenarios compared with a frequency division duplex (FDD) air-interface. The principle of interference diversity is utilised to reduce the total interference from neighbouring cells. This is achieved by a new DCA algorithm which is named: random TS opposing (RTO) algorithm. It is demonstrated that even for up to 90 % of all users, the new RTO algorithm yields a significant interference reduction (minimum 3 dB) compared with systems that are operated using disjoint uplink and downlink channels through the entire network (e.g. the FDD mode). The gains are in the range of 3 dB - 10 dB with the largest gains obtained in the uplink direction. The RTO algorithm enables great flexibility since cell independent asymmetry can be supported without loss of system performance.

Efficient image sequence coding by vector quantization of spatiotemporal bandpass outputs

A coding scheme for image sequences is designed in analogy to human visual information processing. We propose a feature-specific vector quantization method applied to multi-channel representation of image sequences. The vector quantization combines the corresponding local/momentary amplitude coefficients of a set of three-dimensional analytic band-pass filters being selective for spatiotemporal frequency, orientation, direction and velocity. Motion compensation and decorrelation between successive frames is achieved implicitly by application of a non-rectangular subsampling to the 3D-bandpass outputs. The nonlinear combination of the outputs of filters which are selective for constantly moving one- dimensional (i.e. spatial elongated) image structures allows a classification of the local/momentary signal features with respect to their intrinsic dimensionality. Based on statistical investigations a natural hierarchy of signal features is provided. This is then used to construct an efficient encoding procedure. Thereby, the different sensitivity of the human vision to the various signal features can be easily incorporated. For a first example, all multi- dimensional vectors are mapped to constantly moving 1D-structures.

Analysis of power control target levels in UTRA-TDD

Power control (PC) refers to the possibility to modify the transmission power with the aim of reducing co-channel interference and improve the system capacity. This paper describes the results of a dynamic simulation model used to analyze the behavior of different slow power control mechanisms in the time division/code division multiple access (TD/CDMA) system. As a performance measure the carrier-to-interference ratio (CIR) outage probability is used. Two types of slow power control algorithms have been implemented: (i) PC based on power received from the transmitter (level-based PC), and (ii) PC based on CIR at the receiver (CIR-based PC). The results have shown that in terms of system capacity the optimal target of the CIR-based PC is dependent on the traffic density while the target of the level based is independent from the system load.