Christian Hoene

Measurements of a wireless link in an industrial environment using an IEEE 802.11-compliant physical layer

The design and simulation of coding schemes, medium access control (MAC), and link-layer protocols for future industrial wireless local area networks can be supported by some understanding of the statistical properties of the bit error patterns delivered by a wireless link (which is an ensemble of transmitter, channel, receiver, modems). The authors present results of bit error measurements taken with an IEEE 802.11-compliant radio modem in an industrial environment. In addition to reporting the most important results, they draw some conclusions for the design of MAC and link-layer protocols. Furthermore, they show that the popular Gilbert/Elliot model and a modified version of it are a useful tool for simulating bit errors on a wireless link, despite their simplicity and failure to match certain measured statistics.

Four-way TOA and software-based trilateration of IEEE 802.11 devices

In this paper we present an algorithm that measures the round trip times of IEEE 802.11 MAC packets in order to trilaterate WLAN nodes.We show that if we use packet sequences (e.g., RTS, CTS, DATA and ACK) instead of packet pairs (e.g., DATA and ACK), the TOA measurements can be implemented more easily and a higher observation rate can be achieved. In addition, the receiver and any listening third node can measure the round trip times. We present the open source software ldquoGoodtryrdquo that implements this algorithm and overcomes the low clock resolution and other implementation constraints of off-the-shelf IEEE 802.11 cards. Experimental results show that our software-based trilateration has an accuracy of about four meters.

A perceptual quality model intended for adaptive VoIP applications

Quality models predict the perceptual quality of services as they calculate subjective ratings from measured parameters. In this article, we present a new quality model that evaluates Voice over IP (VoIP) telephone calls. In addition to packet loss rate, coding mode and delay, it takes into account the impairments due to changes in the transmission configuration (e.g. switching the coding mode or re-scheduling the playout time). Moreover, this model can be used at run time to control the transmission of such calls. It is also computationally efficient and open source. To demonstrate the potential of our model, we apply it to select the ideal coding and packet rate in bandwidth-limited environments. Furthermore, we decide, based on model predictions, whether to delay the playout of speech frames after delay spikes. Delay spikes often occur after congestion and cause packets to arrive too late. We show a considerable improvement in perceptual speech quality if our model is applied to control VoIP transmissions. Copyright

Measuring the impact of slow user motion on packet loss and delay over IEEE 802.11b wireless links

IEEE 802.11b compliant WLAN technology is increasingly used for cordless telephone services. Often, a WLAN phone is moved during a call. In the present paper we explore to what extent slow user motion influences the wireless link quality. We conducted extensive measurements with speech over commercial WLAN equipment using an experimental environment enforcing controlled motion. Our experiments show that - in contrast to the common assumption - an increase of motion speed can result in a better link quality: The packet loss rate and variance, measured after link-layer retransmissions, decreases. In addition, our measurements demonstrate that the modulation type, the maximal number of retransmissions, the experimental setting, and even the quality of power supply are the dominant influences on link quality.

Voice over IP: improving the quality over wireless LAN by adopting a booster mechanism: an experimental approach

The performance of unreliable voice transmission (Voice over IP) over wireless links is measured not by the throughput but by the perceptual speech quality. The speech quality is impaired by packet losses, which are common on wireless links, and by high transmission delays. In this paper, we describe the design and implementation of a novel Speech Property Based Booster that improves the quality of voice over wireless LANs. This booster is in compliance with existing standards and is transparent to other protocols. It uses characteristics of human speech production and features of modern audio codecs to distinguish packets regarding their importance for perceptual quality. Important packets are protected at the link layer by three mechanisms: selective packet loss recovery, redundant transmission and a hybrid solution. These mechanisms have been evaluated using an experimental set-up with commercial wireless LAN equipment. We made measurements of the objective audio quality and analyzed the effects of packet losses, both due to real wireless channels and late packet arrivals. Our experiments show that the booster increases the quality of voice best with the hybrid solution and that the performance of Voice over IP can be improved further.

Predicting the Perceptual Service Quality Using a Trace of VoIP Packets

We present an instrumental approach on how to assess the perceptual quality of voice transmissions in IP-based communication networks. Our approach is end-to-end and uses combinations of common codecs, loss concealment algorithms, playout schedulers, and ITU’s quality assessment algorithms E-Model and PESQ. It is the first method that takes the impact of playout rescheduling and non-random packet loss distributions into account. Non-random packet losses occur if a rate-distortion optimized multimedia streaming algorithm forwards packets dependent on the packets’ importance.

Precise time of flight measurements in IEEE 802.11 networks by cross-correlating the sampled signal with a continuous Barker code

Wireless LAN devices can be used for two-way time of arrival (TOA) measurements even with off-the-shelf hardware that has only a measurement resolution of 1 µs. However, to get a good locating and tracking performance the send and arrival times as well as the durations of IEEE 802.11 MAC packets must be measured precisely. In this paper, we calculate a cross-correlation between the received and sampled signal and a time continuous Barker code sequence to determine the TOA. Our adaptive sampling offset algorithm helps to efficiently obtain a subsample resolution. We implemented our algorithm and an IEEE 802.11 receiver as a Software-Defined-Radio (SDR) program. In our experiments, the algorithm needs fewer observations in comparison to previously published approaches that measure the time of flight between two WLAN devices.

Voicing-aware parametric speech quality models over VoIP networks

This paper describes novel parametric speech quality models which subsume the effect of packet loss distribution and voicing feature of missing signal waves. Speech quality estimate models for voiced and unvoiced loss location patterns are developed following multiple statistical regression analysis of measurements gathered from a built speech quality assessment framework. The overall speech quality is estimated by combining voiced and unvoiced speech quality estimate scores using an expression calibrated using a large number of speech samples. The input parameters namely, mean loss durations and ratios for voiced and unvoiced packets, of speech quality estimate models are extracted at run-time using a new voicing-aware packet loss Markov model. This chain, calibrated at run-time, finely models bursty packet loss behavior over voiced and unvoiced missing speech waves. Performance evaluation study shows that our voicing-aware speech quality estimate models clearly outperform voicing-agnostic speech quality models in terms of accuracy over a wide range of conditions.

Are QoE Requirements for Multimedia Services Different for Men and Women? Analysis of Gender Differences in Forming QoE in Virtual Acoustic Environments

In recent years, the Quality of Experience (QoE) notion has become a major research theme within the telecommunication community. QoE provides an assessment around a human perception, feeling, performance and behavior. Normally, research studies on multimedia, Quality of Experience (QoE) and gender differences are carried out separately. To develop multimedia solutions covering QoE aspects, keeping the gender differences in mind, are the need of time. In current study, we are interested in knowing how QoE is shaped in Virtual Acoustic Environment (VAE) for male and female. We present experimental test results which provide interesting findings that both male and female have difference in their performance and perception in locating concurrent talkers in small and big sized virtual conferencing rooms. The middle-sized virtual room was suitable for teleconferencing because both male and female participants’ performance and perception converge to similar trend and obtained better QoE values.

Calculating Relative Clock Drifts Using IEEE 802.11 Beacons

This paper proposes an approach to calculate the relative clock drift occurring between two communicating nodes in an WLAN system using IEEE 802.11 beacon frames. The IEEE 802.11 beacon frames contain a timestamp field that tells the time at which the packet was sent from the transmitting node. If a IEEE 802.11 packet is received, the WLAN card stores a timestamp that precisely depicts the time at which the packet was received. Using these timestamps, we developed an algorithm that calculates the relative clock drift between two nodes and conducted many experimental measurements using offthe-shelf WLAN cards showing the clock drifts under different circumstances. Knowing the relative clock drifts is important for two-way time-of-arrival (TOA) location tracking.