With the success of wireless technologies in consumer electronics, standard wireless technologies are envisioned for the deployment in industrial environments as well. Industrial applications involving mobile subsystems or just the desire to save cabling make wireless technologies attractive. Nevertheless, these applications often have stringent requirements on reliability and timing. In wired environments, timing and reliability are well catered for by fieldbus systems (which are a mature technology designed to enable communication between digital controllers and the sensors and actuators interfacing to a physical process). When wireless links are included, reliability and timing requirements are significantly more difficult to meet, due to the adverse properties of the radio channels. In this paper, we thus discuss some key issues coming up in wireless fieldbus and wireless industrial communication systems: 1) fundamental problems like achieving timely and reliable transmission despite channel errors; 2) the usage of existing wireless technologies for this specific field of applications; and 3) the creation of hybrid systems in which wireless stations are incorporated into existing wired systems.

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Wireless Technology in Industrial Networks

We introduce the concept of the double-directional mobile radio channel. It is called this because it includes angular information at both link ends, e.g., at the base station and at the mobile station. We show that this angular information can be obtained with synchronized antenna arrays at both link ends. In wideband high-resolution measurements, we use a switched linear array at the receiver and a virtual-cross array at the transmitter. We evaluate the raw measurement data with a technique that alternately used estimation and beamforming, and that relied on ESPRIT (estimation of signal parameters via rotational invariance techniques) to obtain superresolution in both angular domains and in the delay domain. In sample microcellular scenarios (open and closed courtyard, line-of-sight and obstructed line-of-sight), up to 50 individual propagation paths are determined. The major multipath components are matched precisely to the physical environment by geometrical considerations. Up to three reflection/scattering points per propagation path are identified and localized, lending insight into the multipath spreading properties in a microcell. The extracted multipath parameters allow unambiguous scatterer identification and channel characterization, independently of a specific antenna, its configuration (single/array), and its pattern. The measurement results demonstrate a considerable amount of power being carried via multiply reflected components, thus suggesting revisiting the popular single-bounce propagation models. It turns out that the wideband double-directional evaluation is a most complete method for separating multipath components. Due to its excellent spatial resolution, the double-directional concept provides accurate estimates of the channel's multipath-richness, which is the important parameter for the capacity of multiple-input multiple-output (MIMO) channels.

The double-directional radio channel

This paper provides an overview of the state-of-the-art radio propagation and channel models for wireless multiple-input multiple-output (MIMO) systems. We distinguish between physical models and analytical models and discuss popular examples from both model types. Physical models focus on the double-directional propagation mechanisms between the location of transmitter and receiver without taking the antenna configuration into account. Analytical models capture physical wave propagation and antenna configuration simultaneously by describing the impulse response (equivalently, the transfer function) between the antenna arrays at both link ends. We also review some MIMO models that are included in current standardization activities for the purpose of reproducible and comparable MIMO system evaluations. Finally, we describe a couple of key features of channels and radio propagation which are not sufficiently included in current MIMO models.

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Survey of channel and radio propagation models for wireless MIMO systems

To make transportation safer, more efficient, and less harmful to the environment, traffic telematics services are currently being intensely investigated and developed. Such services require dependable wireless vehicle-to-infrastructure and vehicle-to-vehicle communications providing robust connectivity at moderate data rates. The development of such dependable vehicular communication systems and standards requires accurate models of the propagation channel in all relevant environments and scenarios. Key characteristics of vehicular channels are shadowing by other vehicles, high Doppler shifts, and inherent nonstationarity. All have major impact on the data packet transmission reliability and latency. This paper provides an overview of the existing vehicular channel measurements in a variety of important environments, and the observed channel characteristics (such as delay spreads and Doppler spreads) therein. We briefly discuss the available vehicular channel models and their respective merits and deficiencies. Finally, we discuss the implications for wireless system design with a strong focus on IEEE 802.11p. On the road towards a dependable vehicular network, room for improvements in coverage, reliability, scalability, and delay are highlighted, calling for evolutionary improvements in the IEEE 802.11p standard. Multiple antennas at the onboard units and roadside units are recommended to exploit spatial diversity for increased diversity and reliability. Evolutionary improvements in the physical (PHY) and medium access control (MAC) layers are required to yield dependable systems. Extensive references are provided.

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Vehicular Channel Characterization and Its Implications for Wireless System Design and Performance

Vehicle-to-vehicle (VTV) wireless communications have many envisioned applications in traffic safety and congestion avoidance, but the development of suitable communications systems and standards requires accurate models for the VTV propagation channel. In this paper, we present a new wideband multiple-input-multiple-output (MIMO) model for VTV channels based on extensive MIMO channel measurements performed at 5.2 GHz in highway and rural environments in Lund, Sweden. The measured channel characteristics, in particular the nonstationarity of the channel statistics, motivate the use of a geometry-based stochastic channel model (GSCM) instead of the classical tapped-delay line model. We introduce generalizations of the generic GSCM approach and techniques for parameterizing it from measurements and find it suitable to distinguish between diffuse and discrete scattering contributions. The time-variant contribution from discrete scatterers is tracked over time and delay using a high resolution algorithm, and our observations motivate their power being modeled as a combination of a (deterministic) distance decay and a slowly varying stochastic process. The paper gives a full parameterization of the channel model and supplies an implementation recipe for simulations. The model is verified by comparison of MIMO antenna correlations derived from the channel model to those obtained directly from the measurements.

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A geometry-based stochastic MIMO model for vehicle-to-vehicle communications

For the identification of the time-variant, directional structure of the mobile radio channel impulse response (CIR), a broadband vector channel sounder is described. The measurement procedure relies on periodic multifrequency excitation signals, correlation processing, and joint delay-azimuth super-resolution based on the two-dimensional (2-D) unitary ESPRIT algorithm. Problems of imperfect receiver and antenna performance as well as antenna array calibration methods are discussed. Correlation analysis of the directional impulse response records is performed in the time-frequency-spatial domain and the corresponding Doppler-delay-angular domain. Results from measurements in the 5.2 GHz frequency range in an industrial environment are presented.

Identification of time-variant directional mobile radio channels

We describe a novel technique to measure the double-directional mobile radio channel. The channel is called double-directional since it includes directional spreading at both link ends, namely the base station and the mobile station. We use a double-array multiplexing technique consisting of a switched linear array at the receiver and a virtual array at the transmitter site. The measured microcellular scenarios are an open and a closed courtyard with different transmitter positions. The data are evaluated with an alternating estimation and beamforming technique that relies on (Unitary) ESPRIT estimation of positional parameters and subsequent beamforming for complex weight extraction. Usage of a two-axis crossed array at the transmitter site avoids forward-backward ambiguity. The presented exemplary results show a good agreement of the extracted directional information with the physical environment and give an indication of its scattering properties.

Array measurement of the double-directional mobile radio channel

The object of this paper is to provide data from wideband directional channel measurements carried out at 5.2 GHz in two different industrial scenarios. The channel is characterized by the temporal, azimuthal and Doppler characteristics under LOS and non-LOS situations. The results give insight into the propagation behavior for this type of environment and reveal that the installation of wireless LANs in industrial scenarios imposes important requirements on the system design.

Characterization of the directional mobile radio channel in industrial scenarios, based on wideband propagation measurements

Antenna arrays are expected to be widely used for future mobile radio applications. Therefore implementation issues are a matter of concern. Since real antenna arrays suffer from different impairments, sophisticated calibration procedures are required in order to enhance the performance. For that aim a promising calibration approach is described Its performance has been verified by simulations and measurements. Especially, the application for the direction of arrival estimation in wideband radio vector channel sounders is addressed.

Reduction of DoA Estimation Errors Caused by Antenna Array Imperfections

For the identification of the time-variant, directional structure of the mobile radio channel impulse response, a broadband vector channel sounder is described. The measurement procedure relies on periodic multifrequency excitation signals, correlation processing and joint delay-azimuth superresolution based on the ESPRIT algorithm. Problems of imperfect receiver and antenna performance and antenna array calibration methods are discussed. Results of multidimensional correlation analyses of various channel scenarios in the time-frequency-spatial domain and the corresponding Doppler-delay-angular domain are represented.

A. Schneider

Papers

Identification of time-variant directional mobile radio channels

Array measurement of the double-directional mobile radio channel

Characterization of the directional mobile radio channel in industrial scenarios, based on wideband propagation measurements

Reduction of DoA Estimation Errors Caused by Antenna Array Imperfections

Identification of time-variant directional mobile radio channels