Christian Siegl

Reed–Solomon and Simplex Codes for Peak-to-Average Power Ratio Reduction in OFDM

New schemes for peak-to-average power ratio reduction in orthogonal frequency-division multiplexing (OFDM) systems are proposed. Reed-Solomon (RS) and simplex codes are employed to create a number of candidates, from which the best are selected. Thereby, in contrast to existing approaches, the codes are arranged over a number of OFDM frames rather than over the carriers, hence a combination of the principles of multiple signal representation with selection (as done in selected mapping) and the use of channel coding is present. In particular, in multiple-antenna transmission, the proposed schemes do not cause any additional delay, but due to the utilization of the dimension space, additional gains can be achieved. Moreover, the schemes are very flexible; due to the selection step, any criterion of optimality can be taken into account. Besides multiple-antenna transmission, packet transmission is briefly considered, which, moreover, covers the appealing similarities with incremental redundancy check schemes in automatic repeat request (ARQ) applications and with decoding of codes transmitted over the erasure channel. The performance of the schemes is (using some approximations) derived analytically and is covered by numerical results that are in very good agreement with the theory. Significant gains can be achieved with these very flexible and versatile methods.

Partial transmit sequences for peak-to-average power ratio reduction in multiantenna OFDM

The major drawback of orthogonal frequency-division multiplexing (OFDM) is its high peak-to-average power ratio (PAR), which gets even more substantial if a transmitter with multiple antennas is considered. To overcome this problem, in this paper, the partial transmit sequences (PTS) method—well known for PAR reduction in single antenna systems—is studied for multiantenna OFDM. A directed approach, recently introduced for the competing selected mapping (SLM) method, proves to be very powerful and able to utilize the potential of multiantenna systems. To apply directed PTS, various variants for providing a sufficiently large number of alternative signal superpositions (the candidate transmit signals) are discussed. Moreover, affording the same complexity, it is shown that directed PTS offers better performance than SLM. Via numerical simulations, it is pointed out that due to its moderate complexity but very good performance, directed or iterated PTS using combined weighting and temporal shifting is a very attractive candidate for PAR reduction in future multiantenna OFDM schemes.

Real-time pixel luminance optimization for dynamic multi-projection mapping

Using projection mapping enables us to bring virtual worlds into shared physical spaces. In this paper, we present a novel, adaptable and real-time projection mapping system, which supports multiple projectors and high quality rendering of dynamic content on surfaces of complex geometrical shape. Our system allows for smooth blending across multiple projectors using a new optimization framework that simulates the diffuse direct light transport of the physical world to continuously adapt the color output of each projector pixel. We present a real-time solution to this optimization problem using off-the-shelf graphics hardware, depth cameras and projectors. Our approach enables us to move projectors, depth camera or objects while maintaining the correct illumination, in realtime, without the need for markers on the object. It also allows for projectors to be removed or dynamically added, and provides compelling results with only commodity hardware.

Selected basis for PAR reduction in multi-user downlink scenarios using lattice-reduction-aided precoding

The application of OFDM within a multi-user downlink scenario is considered. Thereby, two problems occur. First, due to OFDM, the transmit signal exhibits a large peak-to-average power ratio (PAR). Second, the multi-user interferences have to be equalized (or precoded) at the transmitter side. In this article, we address combined precoding and PAR reduction. As precoding schemes sorted Tomlinson-Harashima precoding (sTHP) and its lattice-reduction-aided variant (LRA-THP) are considered. In order to reduce the PAR, we review the scheme selected sorting (SLS), which is a combined approach of PAR reduction and precoding with sTHP. Based on this idea, the novel PAR reduction scheme selected basis (SLB) is introduced which combines PAR reduction with the precoding approach LRA-THP. It can be shown that SLB achieves very good PAR reduction performance and hardly influences the error performance. Both schemes, SLB and SLS, are compared with simplified selected mapping (sSLM), the only PAR reduction scheme from the SLM family, which can be applied in multi-user downlink scenarios. The comparison is done on the basis that the respective schemes exhibit the same computational complexity. In terms of PAR reduction performance, it turns out that sSLM outperforms SLS, whereas the performance of sSLM and SLB is similar. Noteworthy, the great benefit of SLB or SLS is that no side information has to be communicated to the receiver as it is necessary with sSLM. Moreover, using SLB, full diversity error rate performance is possible with only low-PAR transmit signals.

Performance of peak-to-average power ratio reduction in single- and multi-antenna OFDM via directed selected mapping

Selected mapping (SLM) is a popular scheme for peak power reduction in orthogonal frequency-division multiplexing (OFDM) systems. In this letter, the performance of various versions of SLM, among them ordinary and directed SLM, in single- and multi-antenna point-to-point OFDM systems is assessed. Analytic expressions for the distribution of the PAR are derived. Numerical results cover that significant gains over conventional SLM can be achieved by directed SLM.

Peak-to-Average Power Ratio Reduction in Multi-User OFDM

In this paper variants of selected mapping are investigated for different MIMO OFDM scenarios (point-to- point, multiple-access, and broadcast channel). Particular focus is on the broadcast scenario, where transmitter side precoding is mandatory which usually increases peak-to-average power ratio (PAR). The combination of (non-linear) precoding with a selected mapping technique leads to a very high computational complexity because it operates on all carriers. This paper introduces a new PAR reducing method which affects only a subset of carriers and therefore saves complexity. Because PAR reduction is achieved by optimizing the sorting in the precoding scheme, this technique is called selected sorting.

Reproducibility of Kidney Perfusion Measurements With Arterial Spin Labeling at 1.5 Tesla MRI Combined With Semiautomatic Segmentation for Differential Cortical and Medullary Assessment.

AbstractMagnetic resonance imaging with arterial spin labeling (ASL) is a noninvasive approach to measure organ perfusion. The purpose of this study was to evaluate the reproducibility of ASL kidney perfusion measurements with semiautomatic segmentation, which allows separate quantification of cortical and medullary perfusion.The right kidneys of 14 healthy volunteers were examined 6 times on 2 occasions (3 times at each occasion). There was a 10-minute pause between each examination and a 14-day interval between the 2 occasions. Cortical, medullary, and whole kidney parenchymal perfusion was determined with customized semiautomatic segmentation software. Coefficient of variances (CVs) and intraclass correlations (ICCs) were calculated.Mean whole, cortical, and medullary kidney perfusion was 307.26 ± 25.65, 337.10 ± 34.83, and 279.61 ± 26.73 mL/min/100 g, respectively. On session 1, mean perfusion for the whole kidney, cortex, and medulla was 307.08 ± 26.91, 336.79 ± 36.54, and 279.60 ± 27.81 mL/min/100 g, respectively, and on session 2, 307.45 ± 24.65, 337.41 ± 33.48, and 279.61 ± 25.94 mL/min/100 g, respectively (P > 0.05; R2 = 0.60/0.59/0.54). For whole, cortical, and medullary kidney perfusion, the total ICC/CV were 0.97/3.43 ± 0.86%, 0.97/4.19 ± 1.33%, and 0.96/4.12 ± 1.36%, respectively. Measurements did not differ significantly and showed a very good correlation (P > 0.05; R2 = 0.75/0.76/0.65).ASL kidney measurements combined with operator-independent semiautomatic segmentation revealed high correlation and low variance of cortical, medullary, and whole kidney perfusion.

Real-time Collision Detection for Dynamic Hardware Tessellated Objects

We present a novel method for real-time collision detection of patch based, displacement mapped objects using hardware tessellation. Our method supports fully animated, dynamically tessellated objects and runs entirely on the GPU. In order to determine a collision between two objects, we first find the intersecting volume of the corresponding object oriented bounding boxes. Next, patches of both objects are tested for inclusion within this volume. All possibly colliding patches are then voxelized into a uniform grid of single bit voxels. Finally, the resulting voxelization is used to detect collisions. Testing two moderately complex models containing thousands of patches can be done in less than a millisecond making our approach ideally suited for real-time games.

OpenCL: a viable solution for high-performance medical image reconstruction?

Reconstruction of 3-D volumetric data from C-arm CT projections is a computationally demanding task. For interventional image reconstruction, hardware optimization is mandatory. Manufacturers of medical equipment use a variety of high-performance computing (HPC) platforms, like FPGAs, graphics cards, or multi-core CPUs. A problem of this diversity is that many different frameworks and (vendor-specific) programming languages are used. Furthermore, it is costly to switch the platform, since the code has to be re-written, verified, and optimized. OpenCL, a relatively new industry standard for HPC, promises to enable portable code. Its key idea is to abstract hardware in a way that allows an efficient mapping onto real CPUs, GPUs, and other hardware. The code is compiled for the actual target by the device driver. In this work we investigated the suitability of OpenCL as a tool to write portable code that runs efficiently across different hardware. The problems chosen are back- and forward-projection, the most time-consuming parts of (iterative) reconstruction. We present results on three platforms, a multi-core CPU system and two GPUs, and compare them against manually optimized native implementations. We found that OpenCL allows to share a common framework in one language across platforms. However, considering differences in the underlying architecture, a hardware-oblivious implementation cannot be expected to deliver maximal performance. By optimizing the OpenCL code for the specific hardware we reached over 90% of native performance for both problems, back- and forward-projection, on all platforms.

Asymptotic Performance Analysis and Successive Selected Mapping for PAR Reduction in OFDM

One major drawback of orthogonal frequency-division multiplexing (OFDM) systems is the high peak-to-average power ratio (PAR) of the transmit signal. In order to overcome this problem, selected mapping (SLM) is a well known method for PAR reduction. In this paper, asymptotic performance analysis of SLM is carried out and a bound on the maximum tolerable PAR values is derived. Starting with the analysis of Gaussian signalling at Nyquist rate, it is also shown that these theoretic results can be applied for QAM signalling at any oversampling rate. Based on the idea to distinguish between tolerable and nontolerable PAR values a novel variant of SLM is derived, namely successive SLM (SSLM). With this approach it is possible to significantly decrease the average number of assessed signal candidates and the average number of required side-information bits per OFDM frame as long as the threshold of tolerable PAR values is larger than the critical PAR ( for Nyquist sampled signals). As a rule of thumb, less than 2.71 (Eulers number) candidates and less than 2.57 bits of side-information are required on average.