# Philipp WalkTechnische Universität Berlin | TUB · School IV Electrical Engineering and Computer Science

Philipp Walk

Dr. rer. nat.

## About

37

Publications

3,014

Reads

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324

Citations

Introduction

Additional affiliations

June 2019 - January 2021

**Futurewei**

Position

- Researcher

March 2018 - present

September 2015 - present

## Publications

Publications (37)

We give a stability result for sparse convolutions on $\ell^2(G)\times
\ell^1(G)$ for torsion-free discrete Abelian groups $G$ such as $\mathbb{Z}$.
It turns out, that the torsion-free property prevents full cancellation in the
convolution of sparse sequences and hence allows to establish stability, that
is, injectivity with an universal lower norm...

The success of the compressed sensing paradigm has shown that a substantial reduction in sampling and storage complexity can be achieved in linear and non-adaptive estimation problems. It is therefore an advisable strategy for noncoherent information retrieval in, for example, sporadic blind and semi-blind communication and sampling problems. But,...

For several communication models, the dispersive part of a communication
channel is described by a bilinear operation $T$ between the possible sets of
input signals and channel parameters. The received channel output has then to
be identified from the image $T(X,Y)$ of the input signal difference sets $X$
and the channel state sets $Y$. The main go...

We investigate practical aspects of a recently introduced blind (noncoherent) communication scheme, called modulation on conjugate-reciprocal zeros (MOCZ). MOCZ is suitable for a reliable transmission of sporadic and short-packets at ultra-low latency and high spectral efficiency via unknown multipath channels, which are assumed to be static over t...

To provide a reliable wireless uplink for users in a given ground area, one can deploy Unmanned Aerial Vehicles (UAVs) as base stations (BSs). In another application, one can use UAVs to collect data from sensors on the ground. For a powerefficient and scalable deployment of such flying BSs, directional antennas can be utilized to efficiently cover...

To provide a reliable wireless uplink for users in a given ground area, one can deploy Unmanned Aerial Vehicles (UAVs) as base stations (BSs). In another application, one can use UAVs to collect data from sensors on the ground. For a power-efficient and scalable deployment of such flying BSs, directional antennas can be utilized to efficiently cove...

We introduce a novel blind (noncoherent) communication scheme, called modulation on conjugate-reciprocal zeros (MOCZ), pronounced as ”Moxie”, to reliably transmit sporadic short-packets over unknown wireless multipath channels. In MOCZ, the information is modulated onto the zeros of the transmitted discrete-time baseband signal’s z–transform, which...

We will investigate practical aspects for a recently introduced blind (noncoherent) communication scheme, called modulation on conjugate-reciprocal zeros (MOCZ), which enables reliable transmission of sporadic and short-packets at ultra-low latency in unknown wireless multipath channels, which are static over the receive duration of one packet. Her...

We will investigate practical aspects for a recently introduced blind (noncoherent) communication scheme, called modulation on conjugate-reciprocal zeros (MOCZ), which enables reliable transmission of sporadic and short-packets at ultra-low latency in unknown wireless multipath channels, which are static over the receive duration of one packet. Her...

In many quantization problems, the distortion function is given by the Euclidean metric to measure the distance of a source sample to any given reproduction point of the quantizer. We will in this work regard distortion functions, which are additively and multiplicatively weighted for each reproduction point resulting in a heterogeneous quantizatio...

We introduce a novel blind (noncoherent) communication scheme, called modulation on conjugate-reciprocal zeros (MOCZ), to reliably transmit short binary packets over unknown finite impulse response systems as used, for example, to model underspread wireless multipath channels. In MOCZ, the information is modulated onto the zeros of the transmitted...

We introduce a novel blind (noncoherent) communication scheme, called modulation on conjugate-reciprocal zeros (MOCZ), to reliably transmit short binary packets over unknown finite impulse response systems as used, for example, to model underspread wireless multipath channels. In MOCZ, the information is modulated onto the zeros of the transmitted...

Recently the one-dimensional time-discrete blind deconvolution problem was shown to be solvable uniquely, up to a global phase, by a semi-definite program for almost any signal, provided its autocorrelation is known. We will show in this work that under a sufficient zero separation of the corresponding signal in the $z-$domain, a stable reconstruct...

In this work we consider one-dimensional blind deconvolution with prior knowledge of signal autocorrelations in the classical framework of polynomial factorization. In particular this univariate case highly suffers from several non-trivial ambiguities and therefore blind deconvolution is known to be ill-posed in general. However, if additional auto...

Providing short-message communication and simultaneous channel estimation for sporadic and fast fading scenarios is a challenge for future wireless networks. In this work we propose a novel blind communication and deconvolution scheme by using Huffman sequences, which allows to solve three important tasks at once: (i) determination of the transmit...

Providing short-message communication and simultaneous channel estimation for sporadic and fast fading scenarios is a challenge for future wireless networks. In this work we propose a novel blind communication and deconvolution scheme by using Huffman sequences, which allows to solve three important tasks in one step: (i) determination of the trans...

In this work we characterize all ambiguities of the linear (aperiodic) one-dimensional convolution on two fixed finite-dimensional complex vector spaces. It will be shown that the convolution ambiguities can be mapped one-to-one to factorization ambiguities in the $z-$domain, which are generated by swapping the zeros of the input signals. We use th...

In this work we characterize all ambiguities of the convolution on two fixed finite-dimensional complex vector spaces. It will be shown that the convolution ambiguities correspond to factorization ambiguities in the z-domain, which are generated by swapping their zeros. We use this polynomial description to show a deterministic version of a recentl...

Pilot-aided channel estimation is nowadays a standard component in each
wireless receiver enabling coherent transmission of complex-valued
constellations, only affected by noise and interference. Whenever these
disturbances are sufficiently small and long data frames are used, high data
rates can be achieved and the resource overhead due to the pil...

Pilot-aided channel estimation is nowadays a standard component in each wireless receiver enabling coherent transmission of complex-valued constellations, only affected by noise and interference. Whenever these disturbances are sufficiently small and long data frames are used, high data rates can be achieved and the resource overhead due to the pil...

We will establish in this note a stability result for sparse convolutions on
torsion-free additive (discrete) abelian groups. Sparse convolutions on
torsion-free groups are free of cancellations and hence admit stability, i.e.
injectivity with a universal lower bound $\alpha=\alpha(s,f)$, only depending
on the cardinality $s$ and $f$ of the support...

In this note we show that stable recovery of complex-valued signals
$x\in\mathbb{C}^n$ up to global sign can be achieved from the magnitudes of
$4n-1$ Fourier measurements when a certain "symmetrization and zero-padding" is
performed before measurement ($4n-3$ is possible in certain cases). For real
signals, symmetrization itself is linear and ther...

In this paper we show that convolutions of sufficiently sparse signals always admit a non-zero lower bound in energy if oversampling of its Fourier transform is employed. This bound is independent of the signals and the ambient dimension and is determined only be the sparsity of both input signals. This result has several implications for blind sys...

In this paper we consider the design of spectrally efficient time-limited pulses for ultra-wideband (UWB) systems using an overlapping pulse position modulation scheme. For this we investigate an orthogonalization method, which was developed in 1950 by Löwdin [1, 2]. Our objective is to obtain a set of N orthogonal (Löwdin) pulses, which remain tim...

In this contribution we present a novel orthogonalization method for ultra-wideband (UWB) impulse radio transmission. Contrary to other work we utilize Löwdin's orthogonalization method which delivers a shift-orthogonal basis optimally close (in energy) to the initial pulse generating the shift-invariant space. We show that the shift-orthogonal bas...

In this contribution we present a novel method for constructing orthogonal pulses for UWB impulse radio transmission under the FCC spectral mask constraint. In contrast to previous work we combine a convex formulation of the spectral design with Lowdin's orthogonalization method [1], which delivers a shift--orthogonal basis optimally close (in ener...

The development of a UWB impulse radio front end for commercial indoor applications is a challenging task, since its components
must have constant characteristics over an ultra-wide bandwidth to achieve high performance. In reality, non-ideal behavior
cannot completely be cancelled out. For the estimation of the performance of such a non-ideal syst...

Non-ideal hardware degrades the performance in UWB systems. Especially the non-isotropic transmit antenna is critical: To meet the regulation, the maximal antenna gain must be substracted from the regulation which leads to reduced transmit power, and hence decreased signal-to-noise-ratio. This contribution briefly describes the modeling of a comple...

The performance of an impulse radio system depends on the pulse shape, modulation, coding, the front-end components, the channel and the receiver structure. In this contribution, a time hopping pulse position modulation (TH-PPM) system is investigated that operates in an indoor scenario and consists of non-ideal frontend components and a correlatio...

The performance of an impulse radio system depends on the pulse shape, modulation, coding, the frontend components, the channel and the receiver structure. In this contribution, a TH-PPM system is investigated that operates in an indoor scenario and consists of non-ideal frontend components and a correlation receiver. The question arises how the pu...

ZnSe prepared by metal organic chemical vapor deposition is used as a buffer layer in Cu(In,Ga)(S,Se)(2) solar cells without any utilization of wet chemistry. Cell efficiencies are as good as cells with the conventional CdS buffer. Stability of unencapsulated cells under damp heat conditions is somewhat lower for the alternative buffer. The first s...

CuGaSe2 CGS thin films were prepared on tin doped Indium oxide ITO coated soda lime glass substrates by thermal co evaporation to fabricate transparent solar cells. The films consisted of columnar grains with a diameter of about 1 m. The deterioration of the transparency of the ITO was observed after deposition of the CGS film. A distinct second di...

## Projects

Projects (3)

The future generation of wireless networks faces a diversity of new challenges. Trends - like the emergence of an internet of things and the tactile internet - have radically changed our thinking about a scalable wireless infrastructure. Some of the main challenges are: (i) a massive number (billions) of devices ranging from powerful smartphones to small sensor nodes must communicate over a flexible network infrastructure; (ii) diverse types of traffic should be simultaneously supported (these include high speed cellular links, managed device-to-device connections and short messages of sporadic nature; (iii) replacing the conventional cellular infrastructure by key enablers like cognitive radio, cooperating antennas/remote radio heads and cloud-based radio access. The most fundamental question “How will devices communicate in the future under such diverse conditions?”, although intensively discussed, remains unsolved. Such trends come with numerous difficult inverse problems, ranging from large-scale data science problems (to be solved using powerful centralized computers) to recovery problems of moderate to small size (to be solved on low-performance devices). A key problem here is how to acquire, communicate, and process channel information (status and control information, activity patterns etc.) in the network. Conventional calibration, synchronization, and channel estimation procedures require substantial amount of resources, such that the overhead dominates over the intended information exchange. A way out of this dilemma are noncoherent and blind strategies showing possibly random nature. Classical engineering approaches have been known for a long time, but new approaches which explicitly account for the short-message and sporadic type of data are required.

Recent theoretical developments in the field of inverse problems have also put the analysis of a whole range of practical non-linear problems within reach. Examples include blind decoding of wireless signals under channel uncertainties, recovery of images from fuzzy snapshots without precise knowledge of the blurring kernel, or more general model uncertainties in conventional compressed sensing. The unifying feature of these tasks is that the signal is accessible only through an uncalibrated system, whose description is partially unknown at the time of measurement. Mathematically, one set of parameters (the channel, the kernel, the sensing matrix) is coupled in a multiplicative way to the signal -- giving rise to an inherent bilinear structure. While in conventional CS such model uncertainties inevitably degrade the quality of the recovery, the novel approach is to combine bilinearity and compressibility in order to simultaneously estimate both the signal and the model parameters.