Low-density parity-check (LDPC) codes together with belief propagation (BP) decoding yield exceptional error correction capabilities in the large block length regime. Yet, there remains a gap between BP decoding and maximum likelihood decoding for short block length LDPC codes. In this context, ensemble decoding schemes yield both reduced latency and good error rates. In this paper, we propose subcode ensemble decoding (SCED), which employs an ensemble of decodings on different subcodes of the code. To ensure that all codewords are decodable, we use the concept of linear coverings and explore approaches for sampling suitable ensembles for short block length LDPC codes. Monte-Carlo simulations conducted for three LDPC codes demonstrate that SCED improves decoding performance compared to stand-alone decoding and automorphism ensemble decoding. In particular, in contrast to existing schemes, e.g., multiple bases belief propagation and automorphism ensemble decoding, SCED does not require the NP-complete search for low-weight dual codewords or knowledge of the automorphism group of the code, which is often unknown.

In this paper, we analyze the formation of small stopping sets in joint factor graphs describing a frame-asynchronous two-user transmission. Furthermore, we propose an algorithm to completely avoid small stopping sets in the joint factor graph over the entire range of symbol delays. The error floor caused by these stopping sets is completely mitigated. Our key observation is that, while the order of bits in the codeword is irrelevant in a single-user environment, it turns out to be crucial in an asynchronous, unsourced two-user system. Subsequently, our algorithm finds a reordering of variable nodes which avoids the smallest stopping set in the joint graph. We show that further improvements can be achieved when girth optimization of the single-user graphs by progressive edge growth (PEG) is used in combination with our proposed algorithm. Starting with a randomized code construction with optimized degree distribution, our simulation results show that PEG followed by the proposed algorithm can improve the average per user probability of error in a noiseless channel by almost two orders of magnitude for a broad range of frame delays.

The automorphism groups of various linear codes are extensively studied yielding insights into the respective code structure. This knowledge is used in, e.g., theoretical analysis and in improving decoding performance, motivating the analyses of endomorphisms of linear codes. In this work, we discuss the structure of the set of transformation matrices of code endomor-phisms, defined as a generalization of code automorphisms, and provide an explicit construction of a bijective mapping between the image of an endomorphism and its canonical quotient space. Furthermore, we introduce a one-to-one mapping between the set of transformation matrices of endomorphisms and a larger linear block code enabling the use of well-known algorithms for the search for suitable endomorphisms. Additionally, we propose an approach to obtain unknown code endomorphisms based on automorphisms of the code. Furthermore, we consider ensemble decoding as a possible use case for endomorphisms by introducing endomorphism ensemble decoding. Interestingly, EED can improve decoding performance when other ensemble decoding schemes are not applicable.

Quantum low-density parity-check (QLDPC) codes are among the most promising candidates for future quantum error correction schemes. However, a limited number of short to moderate-length QLDPC codes have been designed and their decoding performance is sub-optimal with a quaternary belief propagation (BP) decoder due to unavoidable short cycles in their Tanner graphs. In this paper, we propose a novel joint code and decoder design for QLDPC codes. The constructed codes have a minimum distance of about the square root of the block length. In addition, it is, to the best of our knowledge, the first QLDPC code family where BP decoding is not impaired by short cycles of length 4. This is achieved by using an ensemble BP decoder mitigating the influence of assembled short cycles. We outline two code construction methods based on classical quasi-cyclic codes and finite geometry codes. Numerical results demonstrate outstanding decoding performance over depolarizing channels.

This paper investigates the theoretical analysis of intrinsic message passing decoding for generalized product codes (GPCs) with irregular degree distributions, a generalization of product codes that allows every code bit to be protected by a minimum of two and potentially more component codes. We derive a random hypergraph-based asymptotic performance analysis for GPCs, extending previous work that considered the case where every bit is protected by exactly two component codes. The analysis offers a new tool to guide the code design of GPCs by providing insights into the influence of degree distributions on the performance of GPCs.

Low Power Wide Area Networks (LPWAN) technologies in the 2.4 GHz band offer the opportunity to employ multiple antennas at the sensor, which can be used to create antenna diversity. In this paper, we present our software-defined, interference-robust, and highly mobile channel sounder architecture and experimentally evaluate the long-range outdoor-to- outdoor MIMO channel in a typical Smart City environment. Analyzing the measurements we have obtained over a wide area, we estimate path loss, coherence bandwidth, and antenna correlations. Link budget calculations show that, despite the higher frequency compared to popular LPWAN technologies such as LoRa or SigFox, multi-kilometer links are feasible. Furthermore, the comparison of an IEEE 802.15.4 PHY designed for LPWANs with a recently proposed multi-antenna scheme indicates that considerable performance gains are possible through antenna diversity schemes, even with rather high antenna correlation.

We present the design of a low-complexity air interface for the uplink in Low Power Wide Area Networks (LPWANs) based on a recently published Differential Spatial Modulation scheme for two transmit antennas achieving full transmit diversity (FD-DSM). Extending the authors' work, we derive an optimized assignment from bits to space-time matrices comparable to Gray coding, which improves the bit error rate by up to 1 dB. Furthermore, FD-DSM's superiority over single-antenna differential PSK schemes as used in current LPWAN technologies is evaluated in extensive simulations and confirmed in uncoded and coded scenarios, where a convolutional code with soft-input Viterbi decoding is employed. It is shown that existing systems are outperformed by at least 5 dB for packet error rates below 1%. Our system utilizes the DSSS technique to achieve the enormous processing gains required to enable links over many kilometers. We propose the insertion of time gaps between chip sequences to allow for the rise and fall time of conventional, spectrally-efficient pulse shaping filters and to thus facilitate antenna switching without spectral broadening, a challenging issue in Spatial Modulation (SM) systems. The resulting rate loss is negligible but allows the realization of practical SM systems with the theoretical minimum of a single RF chain.

Low Power Wide Area Networks (LPWANs) are an emerging paradigm for low-cost and long-range connection of sensor networks. While most technologies focus on sub-GHz frequencies for their low path loss, the 2.4 GHz band is also an interesting choice due to its high bandwidth and global availability. In this paper, we report our results from a measurement campaign performed in Karlsuhe, Germany, where we investigate the interference background in different scenarios typical for Smart City applications. Using our software implementation of the IEEE 802.15.4 LECIM DSSS PHY and the recorded noise and interference, we also quantify its impact on the performance of a practical LPWAN waveform in terms of packet error rate and effectively available bandwidth at different received signal strengths. Even though it turns out that link budget calculations based on AWGN might be too optimistic, promising results are presented for a two-way single-channel network with up to two retransmissions.

This paper presents a weighted least squares algorithm for passive localization using Time Difference of Arrival in multipath scenarios. The algorithm is based on the direct or one-step approach for position estimation using distributed sensors and requires no knowledge about the number of received multipaths, the transmitted signal or the transmit time. Delayed signal paths arriving due to multipath propagation are considered and treated as interference to the localization problem. Simulation results show a better and more robust performance of the algorithm, compared to conventional two-step localization algorithms.

Industrial environments often exhibit harsh propagation conditions for wireless transmissions. This paper investigates the influence of different interference characteristics on the performance of IEEE 802.15.4 using the OQPSK and Chirp Spread Spectrum (CSS) PHY. Bit error rates are determined for AWGN, Rayleigh fading and self-interference scenarios. GNU Radio, a Software Defined Radio framework, serves as simulation environment. In addition to the implementation of the CSS transmitter and receiver, a low-complexity synchronization algorithm, which only consists of a correlator and a second order Phase Locked Loop (PLL), and its limitations are presented. Furthermore, MAC layer simulations are performed for beacon-enabled (slotted) as well as nonbeacon- enabled (unslotted) mode to make a statement about achievable throughput and latency in the presence of interference. It shows that the CSS PHY clearly surpasses the OQPSK PHY in throughput. Moreover, the simulation results are confirmed by the derivation of the theoretical maximum throughput for the investigated PHY layers.

This work analyzes the gains of cooperative relaying in interference-limited networks, in which outages can be due to interference and fading. A stochastic model based on point process theory is used to capture the spatial randomness present in contemporary wireless networks. Using a modification of the diversity order metric, the reliability gain of selection decode-and-forward is studied for several cases. The main results are as follows: the achievable spatial-contention diversity order (SC-DO) is equal to one irrespective of the type of channel which is due to the ineffectiveness of the relay in the MAC-phase (transmit diversity). In the BC-phase (receive diversity), the SC-DO depends on the amount of fading and spatial interference correlation. In the absence of fading, there is a hard transition between SC-DO of either one or two, depending on the system parameters.

Geolocation is an important topic nowadays. Among other applications, geolocation can be used by frequency regulators to monitor the time, frequency and space domain. In this case of a passive geolocation system, Time Difference of Arrival (TDOA) is often used. In this paper we analyze a geolocation system based on a combination of TDOA and Received Signal Strength Differences (RSSD). The benefit of combining the two measurement types is shown using the Cramer Rao Lower Bound (CRLB). The suitability of the Unscented Kalman Filter (UKF) as an algorithm for the hybrid geolocation system is investigated. Simulation results show that by using the UKF, the instability of RSSD geolocation, that is often caused by bad geometry, is compensated. The UKF nearly approaches the CRLB and greatly outperforms the previously proposed least squares hybrid solution. Using the UKF, the information about the signal power can be definitely used to enhance the position estimation accuracy.

We study the interference and outage statistics in a slotted Aloha ad hoc network, where the spatial distribution of nodes is non-stationary and isotropic. In such a network, outage probability and local throughput depend on both the particular location in the network and the shape of the spatial distribution. We derive in closed-form certain distributional properties of the interference that are important for analyzing wireless networks as a function of the location and the spatial shape. Our results focus on path loss exponents 2 and 4, the former case not being analyzable before due to the stationarity assumption of the spatial node distribution. We propose two metrics for measuring local throughput in non-stationary networks and discuss how our findings can be applied to both analysis and optimization.

Automatic classification of the modulation type of an unknown communication signal is a challenging task, with applications in both commercial and military contexts, such as spectrum surveillance, cognitive radio, and electronic warfare systems. Most of the automatic modulation classification (AMC) algorithms found in the literature assume that the signal of interest has been transmitted using a single antenna. In this paper, a novel AMC algorithm for multiple input multiple output (MIMO) signals is proposed, which employs fourth-order cumulants as features for classification. First, perfect channel state information (CSI) is assumed. Subsequently, a case of more practical relevance is considered, where the channel matrix is unknown and has to be estimated blindly by employing independent component analysis (ICA). The performance of the proposed classification algorithm is investigated through simulations and compared with an average likelihood ratio test (ALRT) which can be considered as optimum in the Bayesian sense, but has a very high computational complexity.

An analysis of narrow- and broadband interference robustness within an On-Off Keying/Binary Pulse Position Modulation based noncoherent multiband impulse radio ultra-wideband communication system is presented. Using the energy detector's processing gain closed-form expressions of the noise and interference related second order moment statistics at the output of an energy detection receiver are derived. This allows separate statements on the relative modulation specific processing gain with respect to various interference parameters.

This paper studies the performance of ad hoc networks with local FDMA scheduling using stochastic point processes. In such networks, the Poisson assumption is not justified due to interdependencies between points introduced by scheduling. For this reason, an upper bound on the second reduced moment measure is derived. Using this result, two lower bounds on the success probability are given, based on the second order product density and a non-homogeneous Poisson approximation. The relative performance of local FDMA is compared to random channel access. It is shown that the relative outage probability reduction of local FDMA highly depends on the SIR threshold as well as on the ratio of transmission distance to orthogonalization distance. If these two quantities are small, the improvement is high; the number of channels has only a minor effect on the relative improvement.

This paper investigates robustness of a non-coherent multi-band impulse radio ultra-wideband communication system with respect to narrow-band interference. A statistical analysis of interference at the energy detector output gives insight into its impact and system dependency. Furthermore, the potential of interference mitigation is shown and verified with respect to the processing gain.

We derive the outage capacity of a bursty version of the amplify-and-forward (BAF) protocol for small signal-to-noise ratios when incremental relaying is used. We show that the ratio between the outage capacities of BAF and the cut-set bound is independent of the relay position and that BAF is outage optimal for certain conditions on the target rate R. This is in contrast to decode-and-forward with incremental relaying, where the relay location strongly determines the performance of the cooperative protocol. We further derive the outage capacity for a network consisting of an arbitrary number of relay nodes. In this case the relays transmit in subsequent partitions of the overall transmission block and the destination accumulates signal-to-noise ratio until it is able to decode.

We present the ε-outage capacity in the low signal-to-noise ratio (SNR) regime of an incremental relaying network. The network consists of one source, one destination, and two half-duplex relays. The one-bit feedback from the destination node is considered to be imperfect and is modeled as binary symmetric channel. The basic idea is that in each block only one terminal transmits depending on the feedback information in order to achieve a better use of the degrees of freedom of the channel. We derive a closed-form expression of the ε-outage capacity and show that the quality of the feedback link has a great influence on the ε-outage capacity of the network. We further examine an extension to the "usual" incremental relaying protocol by allowing the relays to overhear the transmissions of each other in order to increase their own decoding probability. This novel incremental relaying protocol performs close to the cut-set bound (within 0.5 bit/s/Hz).

We derive the e-outage capacity of a wireless relay network where relaying only takes place if the destination has not been able to decode the source message. We refer to this scheme as incremental relaying. The relays use space-time block coding and perform decode-and-forward. We compare incremental relaying to transmit diversity in terms of e-outage capacity and signal-to-noise ratio gain. It is demonstrated that incremental relaying outperforms transmit diversity only in certain regions that determine the relay locations. Moreover, our results confirm the well-known fact that decode-and-forward is especially preferable if the relays are located close to the source.

We present the e-outage capacity of incremental relaying at low signal-to-noise ratios (SNR) in a wireless cooperative network with slow Rayleigh fading channels. The relay performs decode-and-forward and repetition coding is employed in the network, which is optimal in the low SNR regime. We derive an expression on the optimal relay location that maximizes the e-outage capacity. It is shown that this location is independent of the outage probability and SNR but only depends on the channel conditions represented by a path-loss factor. We compare our results to the e-outage capacity of the cut-set bound and demonstrate that the ratio between the e-outage capacity of incremental relaying and the cut-set bound lies within 1/¿2 and 1. Furthermore, we derive lower bounds on the e-outage capacity for the case of K relays.

On-off-keying with energy detection is a promising candidate when aiming at simple receiver concepts, due to the fact that simple energy detection reduces receiver complexity. On the other hand, data rates are low and interference mitigation by correlation is no longer possible. This paper proposes and analyzes a method for mitigating asynchronous interferers in energy detection receivers by adapting the maximum likelihood decision rule. It is shown that detection in the presence of interferers is improved and detection in absence of interferers is not declined.

A critical obstacle for ultra wideband (UWB) communications is conformity to restrictions set on the allowed interference with other wireless devices. Using trains of N amplitude modulated basic pulses gives an FIR filter like design when disregarding the power spectral density (PSD) of the basic pulse. These leads to implementation losses and should be avoided. We quantify these losses and introduce an FIR filter design using semidefinite programming which can incorporate the basic pulse PSD by introducing non-constant upper bounds in the design. This leads to optimal designs and increases available signal power at no extra implementation complexity, just by choosing more optimal filter coefficients

Investigations performed for future applications in various research areas allow users to communicate over more than one radio access technology with one single device. Thus, user terminals need methods to specify the quality of service provided by a radio access technology in order to decide about the radio access technology being used for a certain service or even to consider a possible change of an operator. In this paper a quality of service measure is proposed that allows the evaluation of services by certain service parameters, as, for example, a data transmission may be judged by the bit error rate, the overall network data rate or the delay. The measure defined in this paper bases on simple observation of frames resp. packets and can be applied for the assessment of any service property whose validity can be characterized by a simple decision whether or not a given quality of service was fulfilled.

The interference of ultra wideband (UWB) signals on conventional radio systems, called victims, is still an open issue. In this paper we show that not only the interference power but also the amplitude statistics have significant influence on the bit error rate (BER) of the victim. The amplitude probability distribution (APD) of UWB signals at the channel and inside the victim's receiver is studied analytically and verified by simulation. It is shown under which boundary conditions the UWB signal looses its pulsed behavior inside the victim's receiver. From this an interference model for the additional BER caused by UWB is proposed. The theoretical model is evaluated by simulations with a WLAN and a UMTS victim

While the importance of multi-tier architectures for enterprise information systems is widely accepted and their benefits are well published, the systematic migration from monolithic legacy systems toward multi-tier architectures is known to a much lesser extent. In this paper we present a pattern on how to re-use elements of legacy systems within multi-tier architectures, which also allows for a smooth migration path. We report on experience we made with migrating existing municipal information systems towards a multitier architecture. The experience is generalized by describing the underlying pattern such that it can be re-used for similar architectural migration tasks. The emerged Dublo pattern is based on the partial duplication of business logic among legacy system and newly deployed application server. While this somehow contradicts the separation-of-concerns principle, it offers a high degree of flexibility in the migration process and allows for a smooth transition. Experience with the combination of outdated database technology with modern server-side component and Web services technologies is discussed. In this context, we also report on technology and architecture selection processes.