We propose fiber-to-the-room deployments as a new centralized infrastructure to distribute Wi-Fi radio nodes and, introduce centralized radio resource control to increase data rates and reduce latency. We define and implement a functional split between lower MAC and upper PHY in the 802.11 protocol stack, where the main fiber-to-the-room unit implements the medium access control protocol for all radio nodes. An optical in-building transport network interconnects subordinate fiber-to-the-room units, which are located in different rooms, implementing only the physical layer and the radio front-end. With a centralized MAC, all stations can be assigned mutually orthogonal time slots, with separate down-link and up-link transmissions. Our evaluation framework starts from detailed building models, using realistic channel models to conduct system-level simulation studies. With a centralized round-robin scheduling approach, our results show a decrease in worst case latencies of up to 84 %, yielding a more deterministic traffic management.

This paper reports on the first real-time implementation of reliable optical wireless communications in an field programmable gate array and laboratory trials using the high-bandwidth physical layer defined in IEEE Std 802.15.13-2023. Orthogonal frequency-division multiplexing with adaptive bitloading is implemented in real-time to adapt the link to time-varying channel conditions. Clipping is used to increase the transmission range. Optical front-ends are made of multiple, off-the-shelf high-power light-emitting diodes and large-area photodiodes which enable ranges of several meters for short-range mobile applications. High reliability is observed in a point-to-point scenario. Packet error rates of 3 • 10−7and a peak throughput of 599.2 Mbit/s are measured. Error-free real-time operation is demonstrated for a series of 108packets at distances up to 3.5 m.

High-Speed digital-to-analog converters (DACs) are essential components in optical transmitters for global communication networks. As the demand for increased analog bandwidth grows, the commonly used, cost-effective CMOS technology is approaching its limitations. A promising solution is the interleaving of multiple DAC outputs through the nonlinear operation of an analog multiplexer (AMUX) in silicon-germanium technology. In this work, we present a behavioral model for 4:1 AMUX ICs, which reduces run times for system-level simulations by several orders of magnitude compared to layout-level IC simulations. We fit the model parameters using reference data from a 4:1 AMUX layout simulation while utilizing a stochastic algorithm to autonomously search the parameter space. The results show a close match of the reference data with the model output with low overfitting tendency, but also considerable variability in parameter influence on model accuracy.

5G New Radio (NR) is the de facto global standard for 5G mobile networks. 5G NR was developed by 3GPP and designed to provide new kinds of mobile services across very large geographical areas. One of the main challenges that evolutions of 5G NR must face is the limited radio frequency resources that are available for Beyond-5G mobile connectivity. Optical wireless bands have been already considered for Wireless Local Area Network (WLAN) technologies, where they are denoted as Light Fidelity (Li-Fi). However, and in order to make optical wireless an integral part of Beyond 5G mobile standards, it is convenient to first assess the performance that is feasible when an actual 5G NR waveform is transmitted over an optical wireless frequency band. This paper presents the first experimental proof-of-concept for the transmission of a 5G NR frame over an LED-based optical wireless link. The obtained results are very motivating, as the 5G NR data rates that would be feasible over optical wireless links are very similar to the ones observed nowadays over the n78 RF band (centered around 3.5 GHz, with 100 MHz bandwidth). However, the most important difference is that the spectrum of the optical wireless signal will be reusable in very small cells of radius not larger than few meters, with the possibility of deploying many remote optical units for ultra-densification in indoor service areas without notable inter-cell interference.

An increasing number of industrial Internet-ofthings applications requires wireless transmission of control and data packets. A combination of optical wireless communication (also denoted as LiFi), for deterministic wireless access, and powerline communication, as a cost-effective backbone, is considered promising to enable easy installation of small wireless cells. This paper reports on work in progress from an initial measurement campaign to characterize the cascaded LiFi-over-powerline link in physical layer. The combined channel and noise have been studied in a practical setup for the first time. The frequency response was measured up to 100 MHz under various channel conditions to capture multi-path and attenuation effects across different cable lengths and free-space distances. Also, interference and noise were evaluated up to 300 MHz, revealing contributions from both the PLC and LiFi segments.

Light Fidelity (LiFi) networks transmit information via light waves and are an interesting alternative to Radio Frequency networks: as light can be confined easily, LiFi provides better performance and makes eavesdropping attacks much more difficult. A core application of LiFi networks is self-contained and local networks among a group of autonomous devices, e.g., in industrial or medical environments. Cryptographic protocols are used to secure these networks, however the key exchange sometimes relies solely on the confineability of light signals and sends key material in plain over the network. This is clearly not desirable from a security perspective and newer standards recommend key exchange protocols to establish shared keys. A crucial part in any authenticated key exchange protocol is how to bootstrap trust, e.g., by assuming a PKI, pre-installed keys or an out-of-band-channel. Well established solutions exist, but they are not ideal for the type of self-contained networks targeted by LiFi communication. In this work we investigate how the physical properties of a LiFi channel can be used to replace these mechanisms, resulting in a more convenient and also more efficient solution for key exchange. To this end we propose a new type of secret-less key exchange (SEKA) that does not rely on any pre-shared secrets, and instead runs in two phases: a short bootstrap phase where we make stronger assumptions on the physical security, ruling out active attacks. This can be realized by putting all devices in a closed room, taking advantage of the light's confineability feature. The bootstrap phase is followed by a more classical key-exchange phase, where the actual key material gets exchanged in the presence of active attacks - relying on the shared states from the bootstrap phase. We formally define this new type of key-exchange protocol which offers authenticated key exchange with post-compromise security without relying on pre-shared secrets. We then show that a simpler and more efficient version of the signed Diffie-Hellmann protocol, now relying on MACs instead of signatures for the mutual authentication, can be proven secure in our model. Finally, a proof-of-concept implementation of the SEKA protocol is evaluated in a testbed demonstrating the efficiency gains of our approach.

Today's automotive communication systems use baseband pulse modulation and time-division multiple access. Future in-vehicle networks will need to serve a growing number of devices in a multipoint-to-multipoint topology. Therefore, we present a new automotive bus system which allows enhanced channel adaptation and fine-granular multi-user access based on orthogonal frequency division multiple access (OFDMA). We highlight the advantages of OFDMA in automotive applications and sketch the concept. In this paper, we deliver a proof-of-concept for the OFDMA-based automotive communication bus by using an experimental real-time FPGA prototype.

We assess the impact of synchronization errors in distributed multiple-input multiple-output (D-MIMO) LiFi systems when using Ethernet as fronthaul. To this end, we perform a sensitivity analysis of a distributed multiple-input single-output (D-MISO) LiFi system model by running link level simulations of a joint transmission scheme in a worst-case scenario. Specifically, we investigate the impact of timing errors between distributed transmitters (TXs) on the frame synchronization rate and the block error rate (BLER) for the two IEEE Std 802.15.13 physical layers (PHYs), It is found that timing errors should not exceed the inverse system bandwidth in order to enable reliable communication. This allows a quantification of the synchronization accuracy requirements of distributed TXs in a D-MIMO realization of IEEE 802.15.13 networks using Ethernet as fronthaul. We also show that the required synchronization accuracy is achieved by using the IEEE 1588 Precision Time Protocol (PTP) together with synchronous Ethernet (SyncE).

This paper proposes handover-aware scheduling solutions for multi-cell small-and large-scale visible-light communication networks, enabling soft handover. For this, we coordinate the transmissions at the access points (APs), serving the users in different time slots based on their locations with respect to the AP coverage areas for an efficient utilization of the resources. For scenarios where coverage in large areas is needed, we additionally propose clustering solutions to decrease the handover rate. Compared with non-coordinated schemes, the proposed soft handover techniques offers improved performance in terms of user achievable throughput and link reliability.

In this paper, we evaluate the effectiveness of compressed-sensing-based channel estimation in LiFi orthogonal frequency-division multiplex (OFDM) systems with multiple-input multiple-output (MIMO). The sparseness of LiFi channels suggests that it is beneficial to utilize compressed sensing which has been investigated also for radio communications. In this paper, we formulate the mathematical problem of compressed-sensing for feedback generation in OFDM-based LiFi systems. We use well-known algorithms for both, sparse pilot design and sparse recovery and apply them to LiFi OFDM channels. We evaluate the effectiveness of these algorithms by measuring the achievable mean squared error (MSE) and vary the total numbers of subcarriers as well as the number of pilot subcarriers.

We presentHy-Fi, a system that aggregates light fidelity (LiFi) and radio frequency (RF)-based communication on the 802.11 (WiFi) physical layer by utilizing the MIMO capabilities in IEEE 802.11-compliant commodity WiFi chips.Hy-Fiis based on two key ideas. First, we use inexpensive commodity hardware to facilitate direct transmission of WiFi waveforms over the optical wireless channel, as this is proposed in the IEEE P802.11bb task group. Second, we use the MIMO signal processing from WiFi to aggregate LiFi and radio signals directly at the physical layer.Hy-Fiwas implemented as a prototype and evaluated in a small testbed. Experimental results reveal that our approach offers excellent robustness against signal fading, blockage and external interference in both, the optical and radio channels making it suitable for applications with very strict requirements to the packet delay and loss ratio. Moreover, the two channels, LiFi and RF, can be aggregated to double the link capacity in the best case. Finally, we demonstrate howHy-Ficould be used as wireless access technology in next-generation indoor enterprise networks providing both high capacity and seamless mobility.

In this paper, suitable estimation sequences and algorithms for the simultaneous channel estimation in an optical wireless communication system are assessed. We focus on a multi-user-MISO setup and an on-off-keying based pulsed-modulation physical layer. The performance of the tested configurations is evaluated using simulations under the constraints of imperfect time synchronization between the transmitters and band-pass distortions. Furthermore, a new family of binary channel estimation sequences, named Variable Sampling Factor (VSF) sequences, is proposed with the goal of enabling simultaneous estimation even between terminals using different sample clock rates. Despite a high variance of channel gains in the used model, the best presented algorithm is able to detect 6 out of 8 transmitters on average.

Multiple standardization activities have been started for optical wireless communications. In this paper, we summarize the ongoing work in the IEEE P802.15.13 task group. The goal is to make optical wireless communication usable for specialty applications, e.g., in industrial wireless networks. The current draft standard defines a medium access control (MAC) sublayer, supporting deterministic channel access for low latency/jitter through dynamic time division multiple access (TDMA), distributed multiple-input multiple-output (MIMO) operation for increased robustness, as well as two physical layers for high spectral efficiency and long reach, respectively. Currently, the specification is being finalized for publication. It is prepared for future updates to add support for the group of time-sensitive networking (TSN) standards. In this paper, we provide an overview over the upcoming standard.

We demonstrate on-off-keying optical wireless transmissions according to the IEEE P802.15.13 PM-PHY, in which we replace the 8b10b line-coding by guided and non-guided data scramblers and compensate the remaining high-pass distortions through a frequency domain equalizer. The cyclic prefix length is identified as a critical parameter in balancing performance and transmission overhead, especially at low symbol rates. This is due to the high-pass characteristics of AC-coupled frontends causing baseline variation, effectively creating very long impulse responses. With a cyclic prefix length of 800 ns, 8b10b can be replaced by a scrambler and overhead is reduced by 50 %, while still maintaining an error-free transmission for all symbol rates from 12.5 to 200 MBd.

Positioning technology is considered to be an essential tool for future Internet of Things (IoT) applications, especially for the smart factory. Compared to traditional wireless network positioning systems, LiFi (Light Fidelity) localization technology has a greater potential to provide the sub-meter level accuracy required for smart manufacturing applications, due to its low multi-path and flat channel characteristics. In this paper, we evaluate a LiFi positioning system, based on the ITU-T G.9991 standard for optical wireless communication (OWC), for the first time in a factory environment. Our results show a 3-D positioning error of about 5 cm and a further improvement down to about 3 cm by introducing correction factor. The correction factor is based on an initial calibration of the system, to address the observed dependency of the ranging error vs. the angle between transmitter and receiver. The investigated approach demonstrates the possibility of further development of LiFi systems for the positioning of products and navigation vehicles in future smart factories.

Mobility is a crucial aspect when using optical wireless technology (LiFi) in an Industry 4.0 manufacturing environment. This paper presents initial results with the wireless network concept developed in the SESAM-project combining LiFi and radio frequency (RF) prototypes. In a first stage, the performance of a high-speed LiFi cell providing spatial diversity is evaluated when mobility issues are taken into account. As a second step, we demonstrate mobility in the whole manufacturing hall with a software-defined network solution. This enables a low latency vertical handover between a proprietary RF solution, denoted as EchoRing, and the LiFi cell.

The ongoing shop floor digitalization confronts manufacturers with challenges in computer networks. Wi-Fi became a standard for enabling mobile applications in factories but suffers from operating in the shared industrial, scientific and medical (ISM) radio band, which makes Wi-Fi networks prone to interference from wireless networks that operate in parallel. Consequently, Wi-Fi is a less reliable wireless network technology for smart factories. LiFi and 5G are alternatives that do not rely on ISM bands. With a combination of 5G and LiFi we reach flexibility, reliability, wide coverage, and high throughput for fixed and mobile devices in factories. In this paper, we demonstrate the use of LiFi as a non-3GPP access technology for the 5G core network and describe our initial testing of the N3IWF implementation. Besides, this paper describes a multistage demonstrator set-up, which allows to conduct initial validation tests in a distributed testbed environment. This approach enables the direct migration of developed system components into a real factory environment considering the rough environment of manufacturing halls.

In this paper, we report for the first time on a new channel modelling technique for multi-link LiFi scenarios. By considering simplified numerical calculation, it models the links between multiple optical frontends and multiple mobile devices much faster than previous approaches. For the first two diffuse reflections, we replace ray tracing method by frequency domain channel modelling technique. For the other higher order diffuse reflections, we use a well-established model based on the integrating sphere. For validation of our new approach, we performed distributed 4x2 MIMO channel measurements. Comparison of simulation and measurement yields a relative mean square error below 5 percent for the signals with a free line-of-sight. Our new technique enables efficient modeling of mobile scenarios and analyzing statistical properties of the LiFi channels.

Optical wireless communications gains new interest in the recent years, due to high potential for use cases in the industrial internet of things. In order to address lower power consumption requirements at the mobile transmitter, an on-off-keying based physical layer has been developed by the IEEE task group 802.15.13, supporting a wide range of symbol rates from 12.5 to 200 MBaud. While providing robustness against high-pass filtering effects in analogue optical frontends, the currently used 8b10b line-coding causes a large overhead, notably limiting the achievable throughput. In this work, we investigate scrambler-based alternatives for the line-coding that reduce overhead at the cost of less-balanced signaling. We analyze statistical properties and thoroughly test the performance using a frontend-model derived from measurements on an optical frontend prototype. Accordingly, the 64b67b scheme is proposed as an optional line coding in the IEEE P802.15.13 project.

We present Hy-Fi, a system which combines light fidelity (LiFi) and radio based on the WiFi physical layer waveform by using the MIMO features available in IEEE 802.11-compliant commodity chip sets. Hy-Fi is based on two key ideas. First, we use inexpensive COTS hardware to facilitate direct transmission of WiFi waveforms over the optical wireless channel, as this is proposed in the IEEE P802.11bb task group. Second, we use the MIMO signal processing to aggregate LiFi and radio signals at the physical layer. The system was implemented as a prototype and evaluated in a small testbed. Experimental results show that our approach offers robustness against signal blockage (Shadowing) and external interference in both, the optical and RF channels. Moreover, the two media (LiFi and WiFi) can be aggregated to double the capacity in the best case.

Most literature on the physical layer (PHY) for optical wireless communication (OWC) considers the down-link from lighting infrastructure to mobile devices, and focuses on achieving high data rates by combining large bandwidth and high spectral efficiency. However, some applications, such as the corresponding up-link, device-to-device (D2D) communication, and Internet of Things (IoT), rely on energy-efficient transmission due to limited battery power. According to Shannon's theorem, this can be achieved through using a large bandwidth with power-efficient modulation. The IEEE P802.15.13 task group recently defined the pulsed modulation (PM) PHY with these objectives in mind. The key is to enable robust synchronization and header detection by means of binary modulation. Block-based transmission with a cyclic prefix (CP) and frequency domain equalization (FDE) are used to enable a large bandwidth. In this paper, we present a thorough performance evaluation of the PM PHY. It is demonstrated that, by using an advanced synchronization scheme and FDE, packets can reliably be transmitted at clock rates up to 200 MHz over line-of-sight (LOS) and strong multi-path OWC channels considered typical for industrial wireless scenarios.

We assess the performance of two distinct functional splits based on latency/latency variation and mapping efficiency, both individually and in unison. By considering hardware-offloading possibilities for a low-layer split (especially a pre-resource mapper split) using an option-6 software-based local thermal equilibrium split as an example, we show how data rate, Ethernet frame size and, in general, traffic generation characteristics will be very important aspects in the design of the future Ethernet mapping function. Then, an integrated Ethernet fronthaul with legacy and new/evolved split functionality operating at 100 Gb/s link rate is presented with the state-of-the art sub-100-ns latency variation for a timing-protocol flow. This is achieved through the application of a gap-filling aggregator, used for the first time in such a mobile fronthaul application.

This paper reports on a field trial in a robotic manufacturing cell. Channel measurements and transmission experiments were conducted, assisting the development of a light-emitting diode-based optical wireless communication (OWC) system tailored for the needs of industrial wireless applications. These are moderate data rates, reliability, and low latency. Since dedicated radio spectrum is hardly available, the usage of light is an interesting alternative. Due to its spatial confinement, OWC links provide enhanced security and are difficult to jam from outside a building-a particularly interesting property for industrial applications. We performed broadband distributed $8\times 6$ multiple-input multiple-output (MIMO) channel measurements in a manufacturing cell. Results confirm that the signal-to-noise ratio is sufficient if a line-of-sight (LOS) is available and bandwidth is limited to a few megahertz. However, small movements and rotations may lead to sudden fades of 10-20 dB, when an LOS is blocked. To improve reliability, antenna diversity concepts are applied. Results show that for reliable communication, MIMO diversity schemes are indispensable as it increases the probability of a free LOS significantly. Based on these insights, we suggest an OWC design tailored to industrial wireless applications.

A next generation optical wireless communications standard is currently established, denoted as IEEE 802.15.7r1, targeting data rates between 1 Mbit/s and 10 Gbit/s. Selecting an appropriate transmission scheme is one of the critical tasks. Following this issue, this paper gives a review on PAM-FDE in VLC. Based on PAM-FDE laboratory measurements, we demonstrate the need to include highpass filtering into the propagation model. In this paper, numerical analyses investigate the impact of the resulting baseline wander on the performance. We include a multi-level 5S6S coding technique in order to prepare PAM-FDE for practice. Finally, research topics are summarized.

Link adaptation is recognized as a powerful tech- nique for enhancing spectral efficiency over wireless channels. While its theoretical aspects are well understood, it is not clear yet how to practically deal with limited feedback. In this paper, we study a frequency-selective link adaptation scheme for orthogonal frequency division multiplexing (OFDM). The modulation order is adjusted per resource block (smallest time-frequency unit assigned to a user), while a joint coding rate is used for all variable-length codewords, comprising resource blocks with different modulations. In order to meet the required coding rate, adaptive puncturing is applied after a fixed-rate Turbo encoder. Link adaptation is guided by an effective signal-to-noise ratio (SNR), which is calculated over several subcarriers by a mapping function. 3GPP LTE compliant link-layer simulations were performed for optimizing the SNR interface so that the spectral efficiency is maximized under a codeword error rate constraint and tradeoffs between feedback savings and spectral efficiency losses were elaborated. The result is a thoroughly tested link-layer abstraction for 3GPP LTE including adaptive modulation and bit-interleaved coding that can be also used for system-level simulations towards future mobile radio systems.

Coordination of base stations is a promising technique for reducing inter‐cell interference in next‐generation cellular systems. For the downlink of such systems, the authors study the degradation of zero‐forcing precoded transmissions caused by imperfect channel estimation and channel variation over time. They derive exact expressions for the mean power of the desired signal received by an arbitrary user and of the interference caused by the mismatched precoder. By using these expressions, they show that for attaining a given signal‐to‐interference power ratio (SIR) these systems are limited by a maximum feedback delay of updated channel state information. Finally, they present a procedure to calculate the maximum tolerable feedback delay and to determine the separation between the non‐precoded pilot symbols for guaranteeing a desired SIR.

For reducing costs in drilling technology, seismic prediction while drilling (SPWD) is envisioned. SPWD needs a fast data link bringing up the seismic data from bottomhole to the ground. In this paper, we propose a flexible and easy-to-use acoustic channel model for long drill strings. The model enables efficient design of adaptive OFDM communication links and prediction of achievable data rates for variable string dimensions. We describe acoustic wave propagation by the S-parameters of the drill string modelled as a series of alternating short and long resonators due to segments of constant acoustic impedance. All segments have been parametrised and the final channel is a concatenation of all its segments. We verify the new model by comparison with measurements on a 55 m long drill string. By using our model, the properties of a manifold of real drill pipes with variable dimensions can be predicted. We investigate the impact of length variations typical for rough drilling applications. For efficient communications over 1.5 km, length variations of the screwed tool joints should be limited to a few centimetres while the pipe length may vary up to one meter.

Singe-carrier frequency-division multiple access (SC-FDMA) enables simplified equalization and flexible multiuser scheduling in the frequency domain. Compared to orthogonal frequency division multiple access (OFDMA), it has reduced envelope fluctuations in the same bandwidth. SC-FDMA is widely used in mobile communications, where the focus is on spectral efficiency. Here, we consider an evolution towards better energy efficiency. We propose evolved transmission schemes combining energy-efficient modulation and frequency-domain filtering. Using Gaussian minimum-shift keying (GMSK) and filtered π/2 binary phase-shift keying (BPSK), the peak-to-average power ratio (PAPR) can be reduced down to 0 and 1.9 dB, respectively, while penalties at the receiver side are negligible.

In this paper, we study the spatial structure of multiple-input multiple-output (MIMO) links in small urban macro-cells at 2.6 GHz. We resolve the double-directional structure of the radio channel. Also we compute common metrics used to characterize MIMO links, i.e. the structure of singular values and the resulting capacity. In a line-of-sight (LOS) link, we find that local scattering is not enough to create full-rank MIMO channels. Even behind a sector, we observe a low-rank channel. In a non-line-of-sight (NLOS) scenario, although we resolve several multi-paths with individual delay, direction of departure (DoD) and direction of arrival (DoA), the impact of local scattering is limited. Singular values indicate few more degrees of freedom for NLOS channels, but less than for a random matrix. We can model the spatial degrees of freedom in small cells better by assuming a random but small number of nearly specular paths feeding some local scattering widely spread in azimuth and elevation. Based on our results, we discuss modeling aspects and the value of large antenna arrays in mobile networks.

Base station cooperation is a promising technique for next generation cellular systems as it reduces intercell interference. In this paper, we investigate to what extent performance degradation due to the feedback delay can be compensated by advanced channel prediction. We present a new method based on a Doppler-delay model of the time-variant radio channel. In a first step, significant channel taps are extracted in the delay domain. In a second step, a high-resolution algorithm extracts the major Doppler frequency components for each channel tap. In this way, a shorter history of the channel is needed, as compared to the standard discrete Fourier transform (DFT) technique. Prediction is performed by imposing the estimated parameters into the channel model and extrapolating into future. Evaluation over the extended spatial channel model (SCME) shows that the channel mean square error can be reduced by roughly 10 dB for feedback delays between 5 and 10 ms, which translates into signal to interference ratio (SIR) gains between 5 and 15 dB, also depending on the number of jointly served terminals. Finally, it is shown that, even if the channel state information (CSI) feedback is quantized, almost the same performance can be achieved as if based on perfect channel knowledge.

In this paper, we study adaptive feedback compression of explicit multi-cell channel state information (CSI) for joint transmission coordinated multi-point (JT CoMP) in future mobile networks. The scheme comprises of three steps: First, the strongest cells are clustered using a network-defined threshold. Next, the strongest taps are selected using a threshold related to noise and out-of-cluster interference. Finally, adaptive quantization is applied so that the channel mean square error (MSE) remains sufficiently small. We show that the number of quantization bits grows linearly with the signal to noise ratio (SNR) in dB, as required by information theory. Results indicate that explicit CSI from all relevant cells around a terminal can be provided using fw kbits per reporting interval in 20 MHz bandwidth. The scheme is implemented and tested in real time.

In this paper, we report first acoustic communications experiments along deep drill strings enabling data rates up to 20 kbit/s. To reduce costs for geothermal energy, search for water-bearing fault zones in the rocks hundreds of meters below the ground will be assisted by seismic prediction while drilling (SPWD) enabling control of the drilling direction. The SPWD tool needs higher data rates to the ground. In contrast to common mud-pulse telemetry offering few bit/s, our approach is based on acoustic wave propagation along the steel wall of the drill string. Based on waveguide theory, we develop a tractable channel model for drill-string communication predicting the pass and stopbands created by regular screw joints between the pipes. For data transmission, we use closed-loop adaptive orthogonal frequency division multiplexing (OFDM) following the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE). First results indicate that, in this way, much higher data rates can be realized in future deep drilling applications.

In this paper we derive information theoretic results for asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) in an intensity modulated direct detection (IM/DD) optical communication system subject to a range of constraints. ACO-OFDM is a form of OFDM designed for IM/DD systems. It is an effective solution to intersymbol interference (ISI) caused by a dispersive channel and also requires less optical power than conventional optical modulation formats. Although the classical Shannon capacity formula cannot be applied directly to an IM/DD system, we show that when ACO-OFDM is used in an IM/DD system, it can be adapted to calculate the information rate of the data-carrying odd frequency subcarriers. As a result conventional water filling techniques can be used for a frequency selective channel. These results are applied to indoor wireless systems using realistic parameters for the transmitter, receiver and channel. The maximum rate at which data can be transmitted depends on the channel, the electrical bandwidth and the transmitted optical power. Even when there is no line of sight (LOS) path, when the electrical bandwidth is limited to 50 MHz and the average optical power is limited to 0.4 W, data rates of approximately 80 Mbit/s can theoretically be achieved.

The goal of this paper is to provide a rigorous information-theoretic analysis of subnetworks of interference networks. We prove two coding theorems for the compound multiple-access channel (MAC) with an arbitrary number of channel states. The channel state information at the transmitters is such that each transmitter has a finite partition of the set of states and knows which element of the partition the actual state belongs to. The receiver may have arbitrary channel state information. The first coding theorem is for the case that both transmitters have a common message and that each has an additional private message. The second coding theorem is for the case where rate-constrained, but noiseless transmitter cooperation is possible. This cooperation may be used to exchange information about channel state information as well as the messages to be transmitted. The cooperation protocol used here generalizes Willems' conferencing. We show how this models base station cooperation in modern wireless cellular networks used for interference coordination and capacity enhancement. In particular, the coding theorem for the cooperative case shows how much cooperation is necessary in order to achieve maximal capacity in the network considered.

We evaluate the interference limited spectral efficiency of a small cellular network using measurements with 3 sites at an ISD of 750 m. Each 3-sector site is equipped with cross-polarized panel antennas. We provide an efficient method to estimate the MIMO capacity from the measured data. We then compare the capacities of SISO and 2x2 MIMO with and without interference. At a fixed SNR of 10 dB, our cross-polarized interference free 2x2 setup achieves 5.7 bps/Hz which compares well to findings from Manhattan. For the best serving BS, the median 2x2 capacity including the received power and a reuse factor of 9 is 1.7 bps/Hz. This value increases to 3 bps/Hz when using frequency reuse 3 and 4.7 bps/Hz with full reuse.

Base station cooperation is considered as a promising approach to increase the quality of service (QoS) in uplink. Base stations are connected via high-capacity backhaul links, which makes it possible to perform joint detection through information sharing among several base stations. Joint detection broadens the feasible signal to interference ratio (SIR) region, in which any SIR assignment for all users can be concurrently supported. Thus it provides a higher system throughput. We characterized the Pareto-optimal boundary of the feasible SIR region of power control with base station cooperation by utilizing the property of concave interference function. We suggest a distributed power control algorithm that exploits the broader feasible SIR region and optimizes the SIR assignment. Depending on the feasibility of the network to support cooperation, we analyze 3 degrees of cooperation: no cooperation, limited cooperation, and full cooperation.

We report on measurements at 2.53 GHz with 20 MHz bandwidth in two triple-sectored urban macro-cell deployments in Berlin and Dresden, Germany. These measurements assist the design of multiuser and cooperative transmission and detection techniques promising to reduce the interference in future cellular systems. We study the geometry factor and the Top-N power statistics in the non-cooperative case and compare the results with a commonly used spatial channel model. Furthermore we observe that the median delay parameters in a coherent single frequency network deployment are roughly 1.5 times higher than in an isolated cell while the maximum excess delays are similar in both cases. Finally we study the correlation of the channel response for different antenna spacings at the base station. When the sector antennas are close together, the correlation is generally high which is also predicted by the channel model, but with spacings of about 20 m the correlation significantly decreases.

We consider the downlink of a multi-user MIMO OFDM system in a cellular environment, where users are scheduled to the transmission resources in frequency and space. Targeting a practical solution, we assume the use of fixed DFT- based pre-coding beams at the base stations and linear receivers at the multi-antenna terminals. After having received feedback on the frequency-selective interference conditions from all terminals, each base station schedules the terminals to their resources of highest quality while following a fair scheduling policy. It is shown that this rather simple practical concept enables spatial multiplexing transmission for all users in the system, in particular even for users at cell edge. This results in a capacity scaling that is similar to the one known for isolated point-to-point links. Finally, the system is shown to achieve a performance close to the ideal case if proper techniques to estimate the desired and interfering channels are applied.

In this paper we investigate the time and frequency synchronization as well as the sector and cell search for the 3GPP long term evolution (LTE) downlink. The proposed algorithms rely on the synchronization and cell-specific reference signals and are hence compliant to most recent 3GPP specifications. Time and (fractional) frequency offset coarse synchronization are performed in the time domain with the cyclic prefix based autocorrelation, while the sector and cell as well as the integer frequency offset are estimated in the frequency domain by using the primary and secondary synchronization signals. To improve the cell detection reliability, the estimated cell is afterwards confirmed through the cell-specific reference signal. All algorithms are evaluated under multipath channel conditions and an initial carrier frequency mismatch. Compromising high performance with reasonable implementation complexity, we draw a practical solution for an LTE receiver.

It has been recently shown that base station cooperation may yield great capacity improvement in downlink multiple antenna cellular networks. However, the proposed solutions assume that there is a central processing unit which coordinates the information exchange and determines the optimal resource allocation of the overall cellular network. Whilst the benefits of base station cooperation are large, computational burden of the central unit can be significant. Thus distributed solutions are desirable. This paper suggests a distributed solution for base station cooperation via block-diagonalization and dual-decomposition to maximize the weighted sum network capacity under per-antenna power constraint. The block-diagonalization pre-coding matrix is determined separately at each base station. It enables the full potential of base station cooperation by determining a trade-off between inter-cell interference mitigation, spatial multiplexing and macro diversity. The power allocation problem is formulated as a network utility maximization (NUM) problem. By looking at its Lagrangian dual problem, the decomposable structure of the optimization problem is revealed. This leads to a distributed and iterative algorithm that converges to the global optimum. The advantage of macro diversity in addition to inter-cell interference mitigation and spatial multiplexing in base station cooperation context is studied and shows superior performance in terms of a higher capacity increase with lower variance.

In this paper, we investigate the polarization characteristics of the multiple-input multiple-output (MIMO) channels. The cross polarization discrimination (XPD) is found to change over time, while its variation in frequency domain is relatively small. The probability density function (PDF) of the XPD follows a Gaussian distribution. Analysis and measurement results show that the polarization preserves better in the line of sight (LOS) scenario. In the case of non line of sight (NLOS) scenario, the polarization of the signals is destroyed due to multiple reflections, diffractions or scattering. The difference between the XPDs of the vertically polarized and horizontally transmission highly depends on the specific propagation environment between the transmitter and the receiver. Comparison of the capacity in different scenarios further shows that the polarization diversity gain is less in NLOS scenarios compared with that in LOS scenarios.

This paper proposes a relay channel model which takes into account the influence of relay location and antenna configuration. Simulation results show that when relay is in the line-of-sight (LOS) propagation environment, the channel capacity will degrade. To improve the performance of the channel, water-filling (WF) algorithm is used to allocate the power, simulation results shows that the WF-based relay channel outperforms the uniform power allocation channel. The radiation pattern of the relay antenna is also an importance factor which affects the channel capacity in this case. The widely used Rayleigh channel model overestimates the performance of the channel compared with our model when the location of the relay is not in the scenario where Ricean K-factor is zero. Measurements have been done at 2.53 GHz to investigate the above-rooftop and ground-level relay channels. Results indicate that above-rooftop deployment is very efficient to improve the capacity and coverage.

Multicellular radio systems are often limited due to the presence of cochannel interference. Proposed physical layer concepts, e.g. coordinated joint transmission and interference rejection combining, try to strengthen the signal while combating the interference. However, the performance may be limited by the available channel knowledge. We provide a concept for multi-cell channel estimation in the downlink applicable for for both physical layer concepts. This concept uses virtual pilots based on block-orthogonal sequences, e.g. Hadamard.

In this paper, we compare linear receivers for DFT-spread MIMO-OFDM Systems. It is proven mathematically and illustrated by numerical simulations that the advanced DFT RAKE receiver with linear multiuser detection is equivalent to the popular frequency-domain equalization (FDE) with IDFT despreading. This result holds also for multi-antenna systems. As a consequence, broadband wireless transmission chains with high diversity can be implemented with low complexity at the receiving end. The benefit of using the minimum mean-square error (MMSE) criterion in the FDE is also illustrated.

In this paper, we present real-time broadband multi-user multi-antenna measurements in typical indoor and outdoor scenarios. For the first time, essential frequency-selective functions of a new medium access control (MAC) layer, namely fair resource assignment to multiple users, spatial mode selection and adaptive modulation, have been implemented in real-time on standard digital signal processing hardware. The new MAC layer is steered over wireless feedback and control channels in a closed loop manner. Our implementation uses parameters close to the forthcoming long-term evolution (LTE) of the 3G air interface, thus illustrating the feasibility of the new functions in next-generation cellular radio systems. The paper describes our real-time implementation and reports several test results. Benefits of the frequency-selective multi-user MIMO MAC are validated in realistic mobile propagation environments.

A simple algorithmic scheme is proposed which calculates multiple variable-length FFTs/IFFTs at once by a single fixed-length one. The algorithm exploits the fact that the transform of a periodic signal is a line spectrum, the linearity property of FFT/IFFT and the shift theorem. The overall algorithm complexity is higher than calculating separate variable- length transforms, but the ease of implementation makes the scheme well-suited for FPGA based rapid prototyping systems. Because in SC-FDMA systems multiple variable-length transforms are needed for different users, the algorithm can be used to implement multiuser multiplexing. The scheme can be applied to DFT/IDFT as well as FFT/IFFT implementations.

In future wireless networks, cooperation between the base stations may reduce the effects of intercell interference and enhance the capacity per cell. In this paper, the potential of such cooperation is evaluated using measurements of the information-theoretic channel capacity at 5.2 GHz in 4 overlapping urban macrocells with 5 users. In each link, a 10-transmit 16-receive antenna configuration is used. Results for individual links between terminals and base stations, valid for 60% of the scenarios, indicate that the direct signal has a significant impact on the channel structure. The direct signal contributes two spatial degrees of freedom due to the polarization. Rich scattering is observed when the direct signal is blocked. Results for the multiuser multicell scenario demonstrate for the first time experimentally that the intercell interference leads to an enhanced rank of the joint multiuser multicell channel matrix. Compared to a conventional cellular system, where a fixed assignment of different radio resources is used in adjacent cells in order to avoid the interference, it is better to exploit the interference by cooperation. This enhances the spectral efficiency by a factor of five in our measurement scenario.

This paper addresses piecewise frequency domain interpolation for OFDMA channels when raw estimates are given on a sparse grid of pilot carriers in a fraction of the spectrum only. Starting from the optimal linear interpolation which needs a-priori information on the power delay profile and the SNR, we develop robust suboptimal schemes that do not require such information. One scheme is based on singular value decomposition and it allows the estimation of the SNR. In practice, however, it is difficult to obtain the required information on the power delay profile. To ensure robustness of the filter to mismatched channel parameters, a favorable assumption of the power delay profile is suggested for the filter design. It is further shown that a full-spectrum interpolation assembled from multiple piecewise interpolation results has a dramatically reduced complexity which makes it suitable for real-time implementation.

OFDM-SDMA systems are intended to allow multiple users to communicate simultaneously with a base station in the same frequency band. In these systems, each user signal may be distorted by an independent carrier frequency offset (CFO), which will result in a severe degradation of the system performance if no CFO compensation is carried out. The standard compensation algorithms known from single-user SISO systems do not apply in this context, hence novel compensation techniques have to be introduced. We analyse the influence of multi-users' CFOs on the OFDM-SDMA signal transmission and develop a scheme for their compensation with low complexity. Due to a disturbance of the inherent structure of the OFDM system, the scheme is not capable to compensate for all the effects generated by the CFOs. The remaining interferences may be treated as additional noise, thus setting tight limits for the size of the CFOs which are supported by the system.

In this paper we analyze 5.2 GHz channel measurement data in an urban macro-cell scenario in order to figure out the likelihood of a free line-of-sight and the statistics of the Ricean K-factor. We introduce a physical method to obtain this information from the Doppler delay spectrum taking GPS information into account. It is found, that for short distances the direct signal is available in 62% of all cases. However, the likelihood reduces at larger distances. In addition the average K-factor reduces with increasing distance. Measured distributions are fitted and parameters are provided for use in channel modelling

This paper investigates the value of spatial diversity for the synchronisation in MIMO-OFDM systems. A preamblebased synchronisation scheme exploiting the available spatial diversity of a MIMO system is proposed and evaluated by simulation. It is shown that synchronisation becomes significantly more reliable compared to a system where only single antennas are used within the synchronisation process. The largest share of this improvement is achieved by exploiting receive diversity based on maximum ratio combining of the Schmidl-Cox correlation results at each receiving antenna; exploitation of transmit diversity leads to moderate further improvements.

In this paper, we investigate the optimal length of the training and data phase for continuously time-varying channels that maximize the achievable information rate. It is shown that both lengths depend not only on the Doppler spread of the channel but also on the noise power and the number of transmit antennas whereas the number of receive antennas and the channel condition have only a small influence. It turns out that the optimal coherence interval depends strongly on the SNR, whereas the optimal ratio between training and data phase is nearly independent on the SNR.

The implementation of adaptive channel inversion into a real-time single carrier MIMO test-bed is reported for the first time. Since no joint detection is possible at the distributed users, all signal preprocessing is performed at the transmitter which adapts the transmission links to all the mobile terminals depending on the actual channel situation. This means that the number of reliably supportable users is controlled as well as the modulation, ensuring BER targets for all users. The implemented algorithms are described and measurement results have shown the stable sum rate throughput of several 10 Mbit/s for a MIMO base station with 4 transmit antennas which supports up to 4 mobile terminals simultaneously. Since the transmission is based on optimum feedback of the channel knowledge to the transmitter, the results given here can be understood as a kind of an upper practical performance bound for the application of linear channel pre-distortion.

The rake receiver is investigated for spatial multiplexing (SM) in wide-band code-division multiple access systems. In general, the SM increases the data rate and the diversity order as well. The appropriate maximum likelihood detector is derived in this paper and it is shown that the diversity order is related to the product of the numbers of receive antennas and taps. With realistic numbers similar diversity orders may be achieved also with reduced complexity detection schemes. Finally, a standard-conformal scrambling technique is proposed to enhance the performance and to equalize it among the full set of orthogonal variable spreading factor (OVSF) sequences used in the third generation cellular system.

In this paper we develop a unique framework for evaluating the required repetition rate of channel estimation for multiple-input multiple-output (MIMO) systems in continuous fading radio channels. An analytical formula for the interference due to the temporal variation of the channel coefficients is given. This makes it possible to evaluate the time interval in which the channel has to be estimated again in order to keep the error probability below a desired threshold. This interval depends not only on the Doppler spread of the channel but also on the antenna configuration, detection algorithm and modulation scheme.

The irreducible error, introduced by the time dispersion of the radio channel for multiple-input multiple-output (MIMO) systems with flat fading detection algorithms are investigated analytically and by link level simulations. Using an adequate synchronisation method, the error floor is determined only by the intersymbol interference, for which a formula is derived. The maximal data rate is not only determined by the value of this interference, but also by the modulation scheme, antenna configuration and channel condition.

A high-speed wireless system (/spl ges/100 Mb/s) for indoor infrared (IR) communications via the line of sight is described and feasibility is shown in an experimental demonstrator. A diffuse link is used for connectivity, and tracked directed links are used for high-speed communications. The transmitter is made of a laser diode array in combination with multiple-beam forming optics. For the receiver (Rx), a wide-angle lens, and an avalanche photodiode array are used. For the diffuse link, the signals from all pixels in the array are combined. Pixels are selectively addressed to realize directed links. Fast electronic tracking of a directed link is possible by switching the signal path onto the right pixel in the array. Diffuse link, directed link, position detection, and tracking can be realized with one and the same transceiver hardware. A favorite system design is derived from constraints due to the IR channel, eye safety, lenses, photodetectors, and the overall system complexity. The experimental system shows some key features, namely 155-Mb/s wireless transmission over a distance of nearly 2 m with electronic tracking at an imaging IR Rx. Electronic tracking of IR links, thus, allows both high data rates and high capacity for wireless access in small office and home environments.

We study linear pre-processing for multi-antenna systems when the channel is perfectly known at the transmitter. In particular, the capacity of linear and adaptive channel inversion is compared to the optimum water-filling. When equal numbers of antennas are used at the transmitter and at the receiver, the outage probability with linear inversion is rather large. Outage is remarkably reduced when more antennas are used at the transmitter. Adaptive inversion approximates the water-filling capacity at small transmitter power and so it is robust against fading correlation. The average transmitter power with linear inversion depends on the numbers of antennas at the transmitter and at the receiver. For Rayleigh fading, we compare the bit error performance to V-BLAST.

Equalization in the time domain is a well-known technique for combating intersymbol interference (ISI) in frequency selective channels. We derive the general structure of the linear zero forcing (ZF) equalizing filters for multiple input multiple output (MIMO) communication systems with more outputs than inputs. These filters are finite impulse response (FIR) filters in general and their structure is determined by the number of inputs and outputs. The relation between the spatial diversity of the system and the necessary filter length is worked out and it is shown, that an optimal time lag in the filter will improve the performance considerably.

The paper discusses channel inversion which is a spatial equalization technique when channel state information is available at the transmitter. Channel inversion is a straightforward concept without iterations and it might be useful when the data transmission is critical with time e.g. high data rate applications. We discuss performance degradation caused by channel estimation errors, clipping due to the limited range of the transmitted power and the effect of cochannel interference. These results give an insight into the technical constraints of this transmission technique and show how these critical issues can be limited or reduced.

In this work, we study a single-user MIMO transmission link in flat and frequency selective Rayleigh fading. The impact of the time dispersion in the channel on the performance of the signal separation algorithms is analyzed. We discuss the mismatch case in which the receiver assumes a flat-fading channel but is confronted actually with a frequency-selective MIMO channel. We give an analytical formula for the uncoded bit error rate in the mismatch case for zero-forcing detection. It is shown how to generalize the well known signal detection algorithms for flat-fading MIMO channels to the frequency-selective case. In particular, the performance and complexity of these algorithms is compared. A suboptimal algorithm with reduced complexity is proposed and compared to the optimal algorithms. All theoretical results are confirmed by numerical simulations.

We study the relation between antenna spacing and capacity of MIMO channels for indoor environments using a ray tracing program. It has been confirmed also by measurements that in rich scattering environments an antenna spacing below 0.5/spl lambda/ is sufficient to reach nearly the full capacity predicted for multiple-antenna arrays in ideal and uncorrelated Rayleigh fading channels.

A simple analytical model of the wireless infrared communication channel in indoor environments is presented. The infrared signal is modeled as the combination of a diffuse component and a line-of-sight (LOS) or direct component. For the diffuse component alone, the properties of the channel are found using Ulbricht's integrating sphere. When a LOS component is also present, the transfer function depends upon the Rician factor K given by the ratio of the electrical power in the LOS and diffuse signals after the detector. For small K, the transfer function shows notches down to low frequencies, but due to the nature of light never for zero frequency. We confirm that a K-factor /spl ges/13 dB is required also in infrared wireless links in order to support distortionless data transmission beyond 100 Mbit/s. Increasing the directivity at the receiver and/or at the transmitter improves the effective value of K. Here, we show that a moderate directivity will be sufficient for high-speed infrared communication in typical indoor scenarios.

A multiple-input multiple-output (MIMO) indoor radio system is studied to identify the origin of a typical performance degradation. When the data and overlay pilot-sequences for the channel estimation are transmitted at the same time, an error floor at high signal-to-noise ratio is normally observed. The floor is caused by channel estimation errors due to the interfering data signal. The crosstalk between the data paths can be calculated, approximately, and it is shown that the shape of the bit error curves can be steered both with the amplitude ratio /spl eta/ between pilot and data signals and with the length L of the sequences. Near-optimum performance can be reached in this way. With L = 16383, for instance, an amplitude ratio of /spl eta/ /spl ges/ 0.15 (0.3) is sufficient for BPSK (16-QAM) modulation in a MIMO system with 8 transmit and 12 receive antennas.

Multiple input multiple output (MIMO) systems, being under intensive research nowadays, are discussed under the aspect of bit error rates (BER) that can be achieved in indoor scenarios. This paper focuses on the influence of antenna diversity, a line of sight (LOS) signal and channel estimation errors onto the BER using BPSK and 16-QAM modulation. The developed simulation environment allows us to determine the necessary accuracy for channel estimation to achieve the desired BER performance in a MIMO channel under certain constraints like number of antennas, modulation scheme etc.

An simple analytic model for the light propagation in indoor environments is presented. Ray tracing simulations confirm that the model is applicable to the wireless infrared communication channel in rooms.