Sidelink positioning research predominantly focuses on the snapshot positioning problem, often within the mmWave band. Only a limited number of studies have delved into vehicle-to-anything (V2X) tracking within sub-6 GHz bands. In this paper, we investigate the V2X sidelink tracking challenges over sub-6 GHz frequencies. We propose a Kalman-filter-based tracking approach that leverages the estimated error covariance lower bounds (EECLBs) as measurement covariance, alongside a gating method to augment tracking performance. Through simulations employing ray-tracing data and super-resolution channel parameter estimation, we validate the feasibility of sidelink tracking using our proposed tracking filter with two novel EECLBs. Additionally, we demonstrate the efficacy of the gating method in identifying line-of-sight paths and enhancing tracking performance.

In the context of joint communication and sensing (JCAS), communication systems can be used for sensing, where parameters such as time delay can be determined from channel estimation. These time delays enable applications such as ranging and localization. Various studies show that environmental conditions significantly impact sensing performance. Thus practical measurements within the actual environment are essential to overcome the limitations of simulation models. To accurately evaluate the real-world performance of sensing algorithms, a software-defined radio (SDR)-based experimental platform was developed in previous work for indoor measurements with co-located user equipments (UEs). This paper introduces extensions to the platform to enable long-range measurements. Using the enhanced platform, two-way ranging (TWR) measurements are performed in a vehicle-to-vehicle (V2V) setup to evaluate the realistic accuracy using cross-correlation (CCR)-and superresolution path delay estimation (SPDE) algorithms for ranging applications. The measurement results confirm that SPDE can improve accuracy in scenarios where the signal-to-noise ratio (SNR) and signal integrity are assured.

In this paper, we investigate sub-6 GHz V2X sidelink positioning scenarios in 5G vehicular networks through a comprehensive end-to-end methodology encompassing ray-tracing-based channel modeling, novel theoretical performance bounds, high-resolution channel parameter estimation, and geometric positioning using a round-trip-time (RTT) protocol. We first derive a novel, approximate Cramér-Rao bound (CRB) on the connected road user (CRU) position, explicitly taking into account multipath interference, path merging, and the RTT protocol. Capitalizing on tensor decomposition and ESPRIT methods, we propose high-resolution channel parameter estimation algorithms specifically tailored to dense multipath V2X sidelink environments, designed to detect multipath components (MPCs) and extract line-of-sight (LoS) parameters. Finally, using realistic ray-tracing data and antenna patterns, comprehensive simulations are conducted to evaluate channel estimation and positioning performance, indicating that sub-meter accuracy can be achieved in sub-6 GHz V2X with the proposed algorithms.

5G sidelink technology recently presented its unique potential for precise positioning using time delay estimates, although environmental conditions and the reference signal used highly influence the performance. Given the complexity of simulating all potential environmental impacts in a particular scenario, carrying out measurements within the specific environment becomes necessary. This paper introduces a flexible experimental platform to support research in designing the protocol, waveform, and time delay estimation algorithms, offering configurable transmitting signals and radio frequency (RF) parameters. Furthermore, this platform can emulate potential timing errors due to hardware constrains, offering a more practical understanding of 5G sidelink ranging applications.

This paper leverages the information bottleneck method to design receivers for ISI channels working with coarsely quantized messages. The proposed equalizer is based on the forward-backward algorithm. In contrast to conventional approaches, state reliability information is exchanged with a finite alphabet message instead of a real-valued probability vector. Moreover, each node update performs only a single lookup instead of many arithmetic operations. The lookup table construction aims at maximizing the preserved relevant mutual information. We propose a symmetric KL-means IB algorithm applied in an iterative discrete density evolution procedure. Based on the resulting distributions, a new static design technique creates three reusable symmetric lookup tables to perform forward, backward and final node updates. This way, coarse quantization is an integral part of the system design in the first place rather than a separate follow-up process. Two different ISI channel setups show that the proposed equalizers can achieve comparable performance to the high-resolution alternatives. Remarkably, the state metrics require an order of magnitude fewer bits, which potentially improves area and energy efficiency. Also, a lookup table sharing approach is presented to spread the implementation cost across sub-block equalizers operating in parallel.

This paper uses the reconstruction-computation-quantization (RCQ)paradigm to decode low-density parity-check (LDPC) codes. RCQ facilitates dynamic non-uniform quantization to achieve good frame error rate (FER) performance with very low message precision. For message-passing according to a flooding schedule, the RCQ parameters are designed by discrete density evolution. Simulation results on an IEEE 802.11 LDPC code show that for 4-bit messages, a flooding Min Sum RCQ decoder outperforms table-lookup approaches such as information bottleneck (IB) or Min-IB decoding, with significantly fewer parameters to be stored. Additionally, this paper introduces layer-specific RCQ, an extension of RCQ decoding for layered architectures. Layer-specific RCQ uses layer-specific message representations to achieve the best possible FER performance. For layer-specific RCQ, this paper proposes using layered discrete density evolution featuring hierarchical dynamic quantization (HDQ) to design parameters efficiently. Finally, this paper studies field-programmable gate array (FPGA) implementations of RCQ decoders. Simulation results for a (9472, 8192) quasi-cyclic (QC) LDPC code show that a layered Min Sum RCQ decoder with 3-bit messages achieves more than a 10% reduction in LUTs and routed nets and more than a 6% decrease in register usage while maintaining comparable decoding performance, compared to a 5-bit offset Min Sum decoder.

This paper proposes a finite-precision decoding method for low-density parity-check (LDPC) codes that features the three steps of Reconstruction, Computation, and Quantization (RCQ). Unlike Mutual-Information-Maximization Quantized Belief Propagation (MIM-QBP), RCQ can approximate either belief propagation or Min-Sum decoding. MIM-QBP decoders do not work well when the fraction of degree-2 variable nodes is large. However, sometimes a large fraction of degree-2 variable nodes is used to facilitate a fast encoding structure, as seen in the IEEE 802.11 standard and the DVB-S2 standard. In contrast to MIM-QBP, the proposed RCQ decoder may be applied to any off-the-shelf LDPC code, including those with a large fraction of degree-2 variable nodes. Simulations show that a 4-bit Min-Sum RCQ decoder delivers frame error rate (FER) performance within 0.1 dB of floating point belief propagation (BP) for the IEEE 802.11 standard LDPC code in the low SNR region. The RCQ decoder actually outperforms floating point BP and Min-Sum in the high SNR region were FER less than 10-5. This paper also introduces Hierarchical Dynamic Quantization (HDQ) to design the time-varying non-uniform quantizers required by RCQ decoders. HDQ is a low-complexity design technique that is slightly sub-optimal. Simulation results comparing HDQ and optimal quantization on the symmetric binary-input memoryless additive white Gaussian noise channel show a mutual information loss of less than 10-6bits, which is negligible in practice.

The new 5G communications standard increases data rates and supports low-latency communication that places constraints on the computational complexity of channel decoders. 5G low-density parity-check (LDPC) codes have the so-called protograph-based raptor-like (PBRL) structure which offers inherent rate-compatibility and excellent performance. Practical LDPC decoder implementations use message-passing decoding with finite precision, which becomes coarse as complexity is more severely constrained. Performance degrades as the precision becomes more coarse. Recently, the information bottleneck (IB) method was used to design mutual-information-maximizing lookup tables that replace conventional finite-precision node computations. The IB approach exchanges messages represented by integers with very small bit width. This paper extends the IB principle to the flexible class of PBRL LDPC codes as standardized in 5G. The extensions include puncturing and rate-compatible IB decoder design. As an example of the new approach, a 4-bit information bottleneck decoder is evaluated for PBRL LDPC codes over a typical range of rates. Frame error rate simulations show that the proposed scheme outperforms offset min-sum decoding algorithms and operates very close to double-precision sum-product belief propagation decoding.

We consider a trainable point-to-point communication system, where both transmitter and receiver are implemented as neural networks (NNs), and demonstrate that training on the bit-wise mutual information (BMI) allows seamless integration with practical bit-metric decoding (BMD) receivers, as well as joint optimization of constellation shaping and labeling. Moreover, we present a fully differentiable neural iterative demapping and decoding (IDD) structure which achieves significant gains on additive white Gaussian noise (AWGN) channels using a standard 802.11n low-density parity-check (LDPC) code. The strength of this approach is that it can be applied to arbitrary channels without any modifications. Going one step further, we show that careful code design can lead to further performance improvements. Lastly, we show the viability of the proposed system through implementation on software-defined radios (SDRs) and training of the end-to-end system on the actual wireless channel. Experimental results reveal that the proposed method enables significant gains compared to conventional techniques.

Recently, a novel lookup table based decoding method for binary low-density parity-check codes has attracted considerable attention. In this approach, mutual-information-maximizing lookup tables replace the conventional operations of the variable nodes and the check nodes in message passing decoding. Moreover, the exchanged messages are represented by integers with very small bit width. A machine learning framework termed the information bottleneck method is used to design the corresponding lookup tables. In this paper, we extend this decoding principle from binary to non-binary codes. This is not a straightforward extension but requires a more sophisticated lookup table design to cope with the arithmetic in higher order Galois fields. Provided bit error rate simulations show that our proposed scheme outperforms the log-max decoding algorithm and operates close to sum-product decoding.

In practical implementations, message passing decoding of LDPC codes has to be implemented with finite precision, i.e., the messages are quantized with a small number of bits. This results in a significant performance degradation with respect to decoding with high-precision messages. Recently, we have proposed so-called information bottleneck decoders to design finite-precision decoders with error-correction performance close to high-precision belief-propagation decoding. Earlier works solely focus on the design of information bottleneck decoders for regular LDPC codes or specifically optimized irregular LDPC codes. In this paper, we extend the concept of information bottleneck decoders to irregular LDPC with arbitrary degree distribution. We show that this extension is not straightforward and requires an additional information-optimum step. Therefore, we devise a novel intermediate construction step which we call message alignment. Exemplary numerical simulations using an irregular LDPC code taken from the IEEE 802.11 standard show that incorporating message alignment in the construction yields a 4-bit information bottleneck decoder which performs only 0.15 dB worse than a double-precision belief propagation decoder and outperforms a min-sum decoder.

We present an information-theoretic approach to discrete signal processing called information-optimum signal processing for the example of a wireless receiver comprising LDPC decoding, channel estimation and detection. All operations are replaced by simple lookup tables and all messages which are exchanged between detection stages are unsigned integers. The lookup tables are designed offline using the information bottleneck concept of preserving relevant information. We show that the approach allows for simple signal processing with coarse quantization while achieving virtually the same performance as conventional signal processing approaches with high resolution, e.g. double precision. The contribution of the paper is a tutorial style explanation of the concept as well as an exemplary survey of applications and performance results which illustrate the potential of the concept.

Mutual information maximizing clustering techniques, like the information bottleneck method, enable message passing based on compressed but highly informative beliefs. In this paper, we apply this concept to joint maximum a-posteriori detection problems in sensor networks. We show that by leveraging the information bottleneck method both the amount of exchanged data and the complexity of the operations performed in the involved sensor nodes respectively the fusion center is significantly reduced. In the considered network, distributed sensor nodes quantize their measurements and forward only cluster indices instead of high-precision cluster representatives to a fusion center. Due to a spatial distribution of the sensor nodes, the quantizers in the sensor nodes are optimized to the actual, varying measurement conditions. Thus, the meaning of a cluster index is sensor-dependent and cannot be uniquely recaptured if the transmitting sensor is unknown. Using a technique which we call message alignment we resolve this ambiguity without transmitting additional information to the fusion center. Additionally, we present a novel iterative message alignment algorithm to solve the generalized message alignment problem. Although only 4-bit integer-valued cluster indices are transmitted, included simulations show that our proposed system encounters no considerable performance degradation compared to an optimum maximum a-posteriori detection strategy.

Recent works describe the design of discrete decoders for low-density parity-check codes by application of mutual information maximizing clustering algorithms in discrete density evolution. In the resulting discrete message passing decoders only integers are exchanged and node operations become simple lookup operations. Earlier works only describe discrete decoders for binary modulation schemes. This paper presents a new technique called message alignment which enables to design discrete decoders for higher-order modulation schemes. First, we design a channel output quantizer for quadrature amplitude modulation with the Information Bottleneck method. The quantizer attempts to preserve the relevant information on the modulation symbols. Afterwards, we illustrate that the assignment of several bits to one modulation symbol does not allow straightforward decoder design with the available discrete density evolution technique. The proposed message alignment solves this problem. The resulting discrete decoder is compared with state-of-the-art decoders using bit error rate simulations.

Starting from a discrete density evolution scheme originally introduced by Brian M. Kurkoski et al. which we improved by applying the Information Bottleneck method, we recently presented results on message passing decoders for Low Density Parity Check codes that have much lower complexity than state of the art decoders. In the decoders all node operations are replaced by discrete message mappings of unsigned integers what yields a great complexity reduction. Anyway the decoders perform very close to belief propagation decoding. New included simulation results prove that using a 4 bit integer architecture these decoders loose only 0.1 dB over Eb/No in comparison to an exact belief propagation decoder applied to the quantized output of a Gaussian channel. The most important contribution of this paper is the derivation of the message mapping decoders from the sum-product algorithm. Until now these decoders are assumed to not be linked to this algorithm. In order to reveal the hidden connection, we explain the decoding principle of the message mapping decoders in general factor graphs.

A generic design method for low complexity receivers is presented. The method pairs factor graphs and the Information Bottleneck method in one framework. Consequently, the method is called Information Bottleneck Graphs. The main idea of Information Bottleneck Graphs is optimizing the flow of relevant information through the signal processors. In contrast to most topical receivers with high precision signal processing units, Information Bottleneck Graphs yield receivers purely working on unsigned integers. All signal processing degenerates to lookup operations in tables of integers. Information Bottleneck Graphs are exemplarily applied to develop a complete coherent receiver including analog-to-digital conversion, channel estimation and decoding of Low Density Parity Check codes that only works on unsigned integers. This receiver uses recently introduced discrete decoders for Low Density Parity Check codes.