Thomas Zasowski

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.

Noncoherent ultra-wideband systems

The need for low-complexity devices with low-power consumption motivates the application of suboptimal noncoherent ultra-wideband (UWB) receivers. This article provides an overview of the state of the art of recent research activities in this field. It introduces energy detection and autocorrelation receiver front ends with a focus on architectures that perform the initial signal processing tasks in the analog domain, such that the receiver does not need to sample the UWB received signals at Nyquist rate. Common signaling and multiple access schemes are reviewed for both front ends. An elaborate section illustrates various performance tradeoffs to highlight preferred system choices. Practical issues are discussed, including, for low-data-rate schemes, the allowed power allocation per pulse according to the regulators ruling and the estimated power consumption of a receiver chip. A large part is devoted to signal processing steps needed in a digital receiver. It starts with synchronization and time-of-arrival estimation schemes, introduces studies about the narrowband interference problem, and describes solutions for high-data-rate and multiple access communications. Drastic advantages concerning complexity and robustness justify the application of noncoherent UWB systems, particularly for low-data-rate systems.

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.

Hardware Aware Optimization of an Ultra Low Power UWB Communication System

A wireless body area network with an average throughput of 500 kbps based on ultra-wideband pulse position modulation is considered. For a long battery autonomy a hardware aware system optimization with respect to the specific applications at hand is essential. A key feature to achieve power savings is low duty cycle signaling, and its effectiveness when combined with burst-wise transmission at high peak data rate. Exploiting this observation, an ultra-low power system is presented, jointly optimized with respect to application and hardware specific aspects. Based on an exhaustive survey of the state of the art literature, its power consumption is estimated significantly below 1 mW.

Partial Channel State Information and Intersymbol Interference in Low Complexity UWB PPM Detection

We consider an UWB PPM based wireless body area network with an average throughput of about 1 Mbps. For a long battery autonomy a low duty cycle operation of the nodes and thus a high peak data rate is essential. Due to the moderate path loss a peak data rate in excess of 50 Mbps would be feasible within the FCC transmit power constraints. With current low complexity PPM detectors, such as the energy detector, the peak data rate is constrained to much lower values, because they are very sensitive to intersymbol interference (ISI). In this paper we constrain our attention to low complexity detectors for UWB PPM, which utilize an observation window of one symbol duration to generate the decision variables for the subsequent symbol decoder. The key contribution is a family of detectors, which utilize partial channel state information (CSI) to improve the robustness to ISI. Specifically we treat the following cases of partial CSI: (i) no CSI, (ii) average power delay profile (APDP), (iii) instantaneous power delay profile (IPDP). To further improve the performance in presence of ISI, a simple post-detection maximum-likelihood sequence estimator (MLSE) is introduced. Finally performance results are given, that highlight the tradeoff between complexity and performance covered by the proposed detection schemes

Performance of UWB Receivers with Partial CSI Using a Simple Body Area Network Channel Model

Ultra wideband (UWB) communication is a very promising candidate for the use in wireless body area networks (BAN). The high UWB peak data rate allows for medium average data rates in combination with a very low duty cycle, which is the key for a very low power consumption. Devices in a wireless BAN require low complexity. Hence, mainly non-coherent receivers such as energy detector and transmitted-reference receiver are suited. In this paper, the symbol-wise maximum-likelihood (ML) detectors for pulse position modulation (PPM) and transmitted reference pulse amplitude modulation (TR PAM) are derived assuming partial channel state information (CSI) at the receiver. Additionally, also the ML detectors for a combination of PPM and TR PAM are presented. The performance of the derived receiver structures is evaluated using a novel BAN channel model not distinguishing line-of-sight and non line-of-sight situations. This simple channel model is based on 1100 channel measurements in the frequency range between 2 and 8 GHz, which were measured in an anechoic chamber. Using the BAN channel model, performance of the derived receiver structures is evaluated showing that the knowledge of the average power delay profile (APDP) at the receiver improves performance substantially. Requiring only slightly more complexity such receivers are a well suited alternative to non-coherent receivers for the use in a BAN.

Performance of UWB Systems using a Temporal Detect-and-Avoid Mechanism

According to the draft versions of the European and Japanese regulation authorities, UWB systems have to use detect and avoid (DAA) mechanisms to avoid interference with existing wireless services. Since most existing wireless services such as GSM, WLAN, and Bluetooth transmit their data burst-wise, we propose UWB transmission between adjacent bursts of such systems. Thus, not only interference from UWB systems is avoided but also interference to them. We refer to this kind of DAA mechanism as temporal DAA. Because the time of UWB transmission is reduced with such an approach, we investigate the performance of UWB systems using temporal DAA. Based on time domain measurements of different interference scenarios we show that with UWB still reasonable data rates are achieved and strict latency time requirements can be met

Synchronization scheme for low duty cycle UWB impulse radio receiver

Ultra-wide band (UWB) communication shows great potential for low-power communication for wireless sensor or body area network (BAN) applications. In particular, noncoherent receivers can be implemented with very low complexity. However, impulse radio and low duty cycle signaling involve stringent requirements on timing recovery. Standard synchronization algorithms might not be applicable due to constraints on memory capacity, clock accuracy and the sampling frequency of the receiver. Therefore, we present a scheme for burst detection and joint frame and symbol synchronization where both transmitter and receiver respect the low-duty cycle requirements. Furthermore, a subsampling analog-to-digital converter with a free running clock is assumed to meet low-power constraints. Burst detection is based on correlation with a known synchronization sequence. For symbol synchronization digital reconstruction of the symbol timing is applied, based on an FIR interpolation filter. Finally, it can be seen from performance results with real measured BAN channels that the presented synchronization algorithm is very well suited for the use in such applications.