Martin Kubisch

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.

Distributed algorithms for transmission power control in wireless sensor networks

In a wireless, multi-hop sensor network, choosing transmission power levels has an important impact on energy efficiency and network lifetime. Two algorithms for dynamically adjusting transmission power level on a per-node basis are proposed here. Network lifetime, convergence speed as well as resulting network connectivity are used as figures of merit for these two algorithms. They have been evaluated in an indoor sensor environment. The network lifetime metrics of these two local algorithms are also benchmarked against power control algorithms using global information. We show that these local algorithms outperform fixed power level assignment and that the lifetime achieved by them is usually within a factor of two of globally computed solution while being scalable.

Multi-rate Relaying for Performance Improvement in IEEE 802.11 WLANs

It is well known that the presence of nodes using a low data transmit rate has a disproportionate impact on the performance of an IEEE 802.11 WLAN. ORP is an opportunistic relay protocol that allows nodes to increase their effective transmit rate by replacing a low data rate transmission with a two-hop sequence of shorter range, higher data rate transmissions, using an intermediate node as a relay. ORP differs from existing protocols in discovering relays experimentally, by optimistically making frames available for relaying. Relays identify themselves as suitable relays by forwarding these frames. This approach has several advantages compared with previously proposed relay protocols: Most importantly, ORP does not rely on observations of received signal strength to infer the availability of relay nodes and transmit rates. We present analytic and simulation results showing that ORP improves the throughput by up to 40% in a saturated IEEE 802.11b network.

Applying Ad-Hoc Relaying to Improve Capacity, Energy Efficiency, and Immission in Infrastructure-Based WLANs

In classical infrastructure-based wireless systems such as access point-equipped wireless LANs, all mobile terminals communicate directly with the access point. In order to improve the capacity or energy efficiency of such systems, the use of mobile terminals as intermediate relays has been proposed. The rationale is that intermediate relays reduce the communication distance and hence the emitted power. Therefore, relaying could also reduce electromagnetic immission. To assess these potential benefits, we study the effectiveness of various relaying algorithms in a uniform, HiperLAN/2-based system model that has been amended by relaying functionality. These algorithms jointly select intermediate relay terminals and assign transmission power as well as modulation to mobile terminals. The energy efficiency of a point-to-point communication is indeed improved by relaying, however, this effect only marginally transfers to scenarios taking into account several terminals. Nevertheless, it is still possible to extend the lifetime of a network by taking into account available battery capacities.

A geometric derivation of the probability of finding a relay in multi-rate networks

Relaying can improve performance of a wireless network, especially when transmission modes with different distance/cost tradeoffs are available. Examples of such modes include data rates or transmission power. This paper geometrically analyzes the probability that a high-cost direct transmission can be replaced by a combination of low-cost relay transmissions. The main result of the analysis is a technology-agnostic characterization of a communication systems amenability to relaying strategies and some recommendations for how to structure such systems

A receiver based protecting protocol for wireless multi-hop networks

Nowadays most medium access protocols designed for wireless ad hoc networks are based on collision avoidance strategies like the CSMA/CA based IEEE 802.11 protocol. But these types of protocols are not designed for multi-hop scenarios -- the efficiency of the channel utilization is too low which results in, among others, large packet delays. One popular approach to increase the channel utilization is to reserve time slots along a transmission path, thus having a scheduled access. However, a major problem is interference from nearby nodes, although these nodes are not on the same route. This might lead to destruction of ongoing data receptions. In this paper we suggest a new reservation protocol, called JamTDMA. It offers protection against this effect by advertising the reservations in a larger neighborhood. We will show that this protocol allows to improve the rate of successfully received packets while assuring an upper bound for the end-to-end delay.

A MAC Protocol for Wireless Sensor Networks with Multiple Selectable, Fixed-Orientation Antennas

One of the open issues in sensor network research is the efficient operation while having energy constraint sensor nodes and dense networks. One potential approach to improve the energy usage is the application of directed data transmission / reception in order to improve the energy usage and decreases the probability of transmission conflicts. To do so, appropriate medium access control protocols are required that decide which antennas to use for sending and receiving, in a distributed fashion. We extend existing protocols for selectable antennas and study their performance. As a main result of this performance evaluation, we derive recommendations on how many antennas per node should be used and how high the performance benefits for an increasing number of antennas is. We show considerable improvements in number of retries (up to 87 %) and per-hop delay (up to 24 %), depending on load characteristics and network density.

Are Classes of Nodes with Different Power Amplifiers Good for Wireless Multi-hop Networks?

It has been shown that optimization of wireless network operations can be achieved by transmission power control of wireless nodes. Controlling the transmission power is mostly performed as a regulation of the final power amplifier, which is the major power sink. Depending on the desired transmission power level, currently popular amplifiers work at different levels of efficiency: highest in case of maximum emitted power, lower in case of reduced emitted power.