Frank Althaus

UWB for noninvasive wireless body area networks: channel measurements and results

The paper presents UWB channel measurements from 3 to 6 GHz for a body area network (BAN) in an anechoic chamber and an office room. Both, transmit and receive antenna were placed directly on the body. Channel parameters as delay spread and path loss are extracted from the measurements and the influence of the body is highlighted. We show that in some situations there are significant echoes from the body (e.g. from the arms) and we observed deterministic echoes from the floor that could help to simplify a RAKE receiver structure. Finally, we consider the overall energy consumption of the BAN and give decision regions for singlehop and multihop links in relation to signal processing energy.

UWB signal propagation at the human head

Among different wireless solutions, ultra-wideband (UWB) communication is one promising transmission technology for wireless body area networks (WBANs). To optimize receiver structures and antennas for UWB WBANs with respect to energy efficiency and complexity, the distinct features of the body area network channel have to be considered. Thus, it is necessary to know the propagation mechanisms in the proximity of the human body. In this paper, we limit ourselves to transmission at the head, since the most important human communication organs, such as the mouth, eyes, and ears, are located there. We especially focus on the link between both ears and consider direct transmission, surface waves, reflections, and diffraction as possible propagation mechanisms. We show theoretically and by measurements, which were performed in the frequency range between 1.5-8 GHz, that direct transmission through the head is negligible due to the strong attenuation. We conclude by process of elimination that diffraction is the main propagation mechanism around the human body and verify these conclusions using a finite-difference time-domain simulation. Based on a second measurement campaign, we derive an approximation of the average power delay profile for the ear-to-ear link and calculate values for mean excess delay and delay spread. Finally, we briefly discuss the impact of the distinct ear-to-ear channel characteristic on the design of a WBAN communication system.

Propagation effects in UWB body area networks

Due to the ongoing miniaturization of electronic devices and due to a multitude of applications, wireless body area networks (WBANs) have gained much interest recently. Ultra wideband (UWB) communication is one promising transmission technology for WBANs due to reduced hardware complexity. To optimize receiver structures and antennas for UWB WBANs it is necessary to know the propagation mechanisms at the human body. In this paper, we focus on transmission at the head and consider direct transmission, surface waves, reflections, and diffraction as possible propagation mechanisms in the frequency range between 1.5-8 GHz. We show theoretically and by measurement results that the direct path is attenuated such that direct transmission through the head is negligible. Based on measurements we conclude by process of elimination that diffraction is the main propagation mechanism around the human body and that surface waves and reflections are negligible. Finally, we discuss the impact of the propagation mechanisms on the UWB WBAN communication system.

An energy efficient transmitted-reference scheme for ultra wideband communications

Transmitted-reference (TR) receivers represent a low complexity alternative to RAKE receivers, which are widely used in ultra wideband (UWB) communications. Known TR schemes are not very energy efficient, since two pulses represent one bit value only. Therefore, we present a TR pulse interval amplitude modulation (PIAM) scheme which reduces the required energy per bit and a low complexity receiver structure for TR PIAM. We investigate the performance of TR 4PIAM analytically and by means of simulation and show that higher modulation alphabets can be adapted easily. Moreover, we show that there exists an optimal correlation length for transmission over multipath channels.

UWB geo-regioning in rich multipath environment

We introduce geo-regioning as a method to achieve rough localization in asynchronous UWB networks. The ap- proach is to localize the transmitter position by means of the multipath components in the received channel impulse response (signature). To show the principle feasibility of this approach a first regioning algorithm is introduced and tested with measured data. Therefore, a measurement campaign in a rich multipath environment has been performed. A high number of signatures originating from different regions in a room have been collected. The regioning algorithm presented here is based on the a priori knowledge of the average power delay profiles of the different regions. The performance results show that almost all regions can be localized at reasonable SNR and error probability. We conclude that the geo-regioning approach is a promising alternative or supplement to classical time of arrival based approaches in UWB networks.

Ultra-wideband geo-regioning: a novel clustering and localization technique

Ultra-wideband (UWB) technology enables a high temporal resolution of the propagation channel. Consequently, a channel impulse response between transmitter and receiver can be interpreted as signature for their relative positions. If the position of the receiver is known, the channel impulse response indicates the position of the transmitter and vice versa. This work introduces UWB geo-regioning as a clustering and localization method based on channel impulse response fingerprinting, develops a theoretical framework for performance analysis, and evaluates this approach by means of performance results based on measured channel impulse responses. Complexity issues are discussed and performance dependencies on signal-to-noise ratio, a priori knowledge, observation window, and system bandwidth are investigated.

Modified pulse repetition coding boosting energy detector performance in low data rate systems

We consider ultra-wideband impulse radio (UWB-IR) low data rate (LDR) applications where a more complex cluster head (CH) communicates with many basic sensors nodes (SN). At receiver side, noncoherent energy detectors (ED) operating at low sampling clock, i.e., below 300 kHz, are focused. Drawback is that EDs suffer from significant performance losses with respect to coherent receivers. Pulse repetition coding (PRC) is a known solution to increase receiver performance at the expense of more transmit power. But in LDR systems known PRC is very inefficient due to the low receiver sampling clock. Boosting transmit power is not possible due to Federal Communications Commissions (FCC) power constraints. Hence, we present a modified PRC scheme solving this problem. Modified repetition coded binary pulse position modulation (MPRC-BPPM) fully exploits FCC power constraints and for EDs of fixed integration duration is optimal with respect to bit error rate (BER). Furthermore, MPRC-BPPM combined with ED outperforms SRAKE receivers at the expense of more transmit power and makes EDs performance robust against strong channel delay spread variations.

Pulse position precoding exploiting UWB power constraints

Due to dense multipath channels, coherent receivers in ultra-wideband impulse radio (UWB-IR) technology are high in hardware complexity and power consumption. This makes coherent receivers n downlink transmission of low data rate (LDR) applications, where a more complex node communicates with many basic sensors, unreasonable. Precoding schemes are known solutions to transfer receiver complexity to transmitter side, while maintaining receiver performance. We sketch performance and scalability potential of precoding schemes in LDR systems in presence of drastic Federal Communications Commissions (FCC) power constraints. This is done by symbol error rate (SER) analysis of an easily realizable pulse position pre-coding (PPP) scheme, supported by simulation results over measured UWB channels. Under realistic conditions, PPP gains up to 9 dB are demonstrated, and it is shown that already a few PPP pulses are sufficient to significantly reduce receiver complexity, at the expense of more transmit power.

On the Performance of UWB Geo-Regioning

Geo-regioning is an approach to localize ultra-wideband (UWB) transmitters by means of their channel impulse responses (CIRs) exploiting the capability of high temporal resolution of multipath components due to the large system bandwidth. Recently, the principle feasibility of this method was shown and a first geo-regioning algorithm was presented in (F. Althaus et al., 2005). In this paper, this algorithm is analyzed and the achievable performance in terms of pairwise error probability (PEP) is computed. In order to calculate the PEP analytically, a simple and accurate approximation of the probability density function (PDF) of a weighted sum of squared real valued Gaussian random variables (RVs) is presented. In the course of this analysis a distance measure is found, which reveals the important characteristics of individual geo-regions determining the PEP

Path-diversity for phase detection in low-cost sensor networks

Wireless sensor networks are composed by a high number of very simple nodes. A paradigm for the nodes transceiver technology is to be energy aware and cost conscious. In this paper we consider a nonlinear transceiver structure which is a key technology for low-cost and power efficient network nodes. We use a phase detection receiver with a branch dependent metric which benefits from the knowledge of the channel fading coefficients. Therefore we introduce a method to estimate the complex coefficients based on samples of the phase detector output. Since most routing concepts for sensor networks envisage multipath routing with simple forwarding strategies (e.g. flooding), it is not far from that to make use of path diversity techniques in order to improve the network reliability. We show that remarkable diversity gains can be achieved also with the nonlinear receiver structure. By means of simulations we examine path diversity techniques for 2PSK, 4PSK and 8PSK modulation where we make use of common transmit diversity schemes. For 2PSK we propose a slight modification of the Alamouti scheme (Alamouti (1998)) to improve the performance.