Volker Pohl

A physical model of the wireless infrared communication channel

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 Ulbrichts 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.

Performance of MIMO systems with channel inversion

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.

Capacity of MIMO systems with closely spaced antennas

Wide-band radio channel measurements at 5.2 GHz with four transmit and four receive antennas at variable element spacing are reported, aiming to evaluate the potential of compact antenna arrays at mobile terminals. We show that, for an element spacing d<0.5/spl middot//spl lambda/ (down to 0.2/spl middot//spl lambda/), the link capacity is not smaller than that for much larger d. This is explained by the observation that mutual coupling changes the radiation patterns of closely spaced antenna elements, individually. Compact multi-antenna terminals may thus become practical.

Continuous flat-fading MIMO channels: achievable rate and optimal length of the training and data phases

In this paper, we propose a simple framework for evaluating the required repetition rate of channel estimation for multiple-input multiple-output (MIMO) systems in continuous slowly fading radio channels. An analytical formula for the interference due to the temporal variation of the channel coefficients is given and verified by link level simulations based on synthetic and measured impulse responses. The proposed interference model makes it possible to optimize the lengths of the training and data phases under different performance criteria. As an example, we investigate the bit-error rate performance of MIMO systems with zero-forcing detection and determine the time interval, after which the channel has to be estimated again in order to keep the error probability below a desired threshold. In the second part, the lengths of the training and data phases is optimized based on the information-theoretical capacity. It is shown that these lengths depend not only on the Doppler spread of the channel, but also on the antenna configuration and noise power.

Antenna spacing in MIMO indoor channels

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.

Electronic tracking for wireless infrared communications

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.

How often channel estimation is needed in MIMO systems

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.

Performance of a MIMO system with overlay pilots

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.

High capacity antennas for MIMO radio systems

The capacity of radio systems with multiple-transmit and multiple-receive antennas may be improved by antenna arrangements which reduce the effective Rician factor. Channel measurements indicate that the capacity in indoor environments may then be close to the independent Rayleigh fading channel even for large numbers of antennas. High capacities may be achieved with such antennas also in outdoor environments. Experiments in the same scenarios indicate significant improvements compared to patch arrays with an element spacing of /spl lambda//2.

Bit error rates for a MIMO system in Rayleigh and Rician channels

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.