Cooperative relaying is a novel technique for wireless communications promising gains in throughput and energy efficiency. The basic idea sounds simple: A device transmits a data signal to a destination. A third device overhears this transmission and relays the signal to the destination as well. Finally, the destination combines the two received signals to improve decoding.
“Using communication techniques where devices cooperate to transmit signals over the air exploits a new, distributed form of spatial diversity that mitigates the negative effects of signal fading and interference,” states Christian Bettstetter. A team of researchers around Bettstetter has been working on this topic for several years. They have made various scientific advances.
The task of relay selection is an important building block to realize cooperative relaying in practice. Devices located between sender and potential receiver must agree in a distributed manner as to which of them will act as relay and thus promote wireless communication. To perform this task, Helmut Adam developed a relay selection protocol that is backwards compatible to the standard IEEE 802.11 WLAN protocol. In collaboration with Orange Labs, France, he specified this protocol and filed its main ideas in two patents. An inherent feature of his approach is a method that estimates the number of potential relays around a sender.
His colleague, Nikolaj Marchenko, took a more theoretical approach. He used Markov chains to model and evaluate relaying protocols taking into account the achieved throughput as well as the required energy and signaling traffic for communications. “My goal is to find the best-suited relaying technique for a particular environment, and I am also exploiting cooperative relaying for multicast communications,” he says.
Their research, however, exceeds mathematical models and equations. Results have been implemented and tested in real-world setups. A programmable hardware platform is employed to perform measurements in vehicle surroundings and in industrial halls. “These measurements reveal as to which scenarios our relaying solution yields an improved performance compared to existing techniques,” researcher Günther Brandner points out. Developing such a prototype is sometimes very demanding. All three lower layers of the protocol stack have to be programmed on an FPGA-based board. The researchers hope that their solutions will be implemented into standards and products.
H. Adam, E. Yanmaz, and C. Bettstetter. Medium access with adaptive relay selection in cooperative wireless networks. IEEE Transactions on Mobile Computing, 2014.
N. Marchenko, T. Andre, G. Brandner, W. Masood, and C. Bettstetter. An experimental study of selective cooperative relaying in industrial wireless sensor networks. IEEE Transactions on Industrial Informatics, 2014.
N. Marchenko and C. Bettstetter. Cooperative ARQ with relay selection: An analytical framework using semi-Markov processes. IEEE Transactions on Vehicular Technology, 2014.
G. Brandner, U. Schilcher, T. Andre, and C. Bettstetter. Packet delivery performance of simple cooperative relaying in real-world car-to-car communications. IEEE Wireless Communications Letters, 2012.