Cooperative Communication in Wireless OFDMA Multi-Hop Networks
Today, wireless communication mainly takes place in a centrally controlled manner, whereby the sender and the receiver are connected over a single wireless connection. In order to enable connections over wide areas, usually, an access point is used that is connected to a wired backbone and enables a connection to a large network. This type of communication has many advantages, but also struggles with certain limitations, especially in terms of coverage in particular scenarios or at certain places that are difficult to cover with wired infrastructure. Wireless multi-hop networks offer a low-cost opportunity to enable wide-area communication, without the need for a centrally controlled wired infrastructure. In wireless multi-hop networks, each network node serves as a potential relay for messages that cannot be transmitted via a direct connection. However, traditional routing methods for multi-hop networks, which rely on a data transmission along a fixed predefined sequence of nodes, are limited in the achievable data throughput and are not suitable for data-intense applications. Furthermore, the routing paths often do not provide a reliable basis for a stable connection over a long period of time. In this project, Corridor-based Routing is investigated in which routing paths are widened such that each stage of the path spans multiple cooperating forwarding nodes. Instead of following a fixed routing path, a corridor offers multiple forwarding nodes per hop among which the data can be divided. Due to the varying positions of the nodes, different channel states occur on the available links. Thereby, a foundation is given for diversity gains for data throughput. In combination with Orthogonal Frequency Division Multiple Access (OFDMA), which is the basis for most of the current and future communication standards, the corridor enables efficient usage of the available frequency resources. In OFDMA, the available bandwidth is divided into narrow orthogonal subcarriers which can be assigned to different links according to the current channel states. The required channel state information is provided only locally within a stage of the corridor in order to enable an efficient allocation of the resources which takes place independently from the remaining stages. Therewith, a link with a preferably high channel capacity can be found for each subcarrier and therefore, a corresponding high data throughput can be achieved.
The operation of Corridor-based Routing requires the selection of adequate nodes for each stage of the corridor. For this purpose, a method is proposed to determines the expected lifetime of relevant links based on locally exchanged control messages. This information is then taken into account for the selection of suitable nodes. Thereby, a stable corridor is generated that serves as a support structure for the later data transmission. In addition, a proactive maintenance protocol is proposed that checksand renews the corridor structure in order to prevent link breakages before they take place. It is shown that the corridor used as a support structure for data transmission enables significant improvements in terms of the potential transmission capacity, as well as the connection stability, compared to traditional routing methods.
A major challenge in Corridor-based Routing is the resource allocation within the local stages of the corridor. To guarantee an interference-free communication, each available subcarrier needs to be allocated exclusively to a single forwarding node. To achieve the highest possible data throughput, the channel quality, as well as the data buffer levels of the nodes, need to be taken into account. Since channel states are changing over time, a dynamic adaptation of the resource allocation is required. Based on a Markov-Decision-Process model and using dynamic programming, a resource allocation policy is derived that minimizes the expected number of required time slots to forward the available data. To handle large state spaces in the underlying model, a state approximation technique is proposed. Nevertheless, the allocation procedure requires large amounts of computing and storage capacities and is limited to models with a small number of variables and states. Therefore, a suboptimal heuristic resource allocation scheme is presented, in which the decision making is based on a channel-state-comparative metric. It is shown that the suboptimal heuristic approach performs close to the optimal approach in terms of the achievable data throughput.
Finally, the operation of Corridor-based Routing with fountain codes is investigated. In contrast to traditional channel coding, fountain codes allow for an automatic adaptation of the data transmission rate to the corresponding channel. With fountain codes, a transmitter can theoretically generate an infinite number of encoded symbols from a given set of information bits. Receivers can recover the original data from an arbitrary subset of these encoded symbols as far as the accumulated mutual information from the received signals is sufficient and exceeds the entropy of the original data. Fountain codes open up the possibility to efficiently exploit weak links that go beyond the boundaries of the corridor stages to improve the performance of the transmission process. Distant nodes overhear transmissions and thus, the required transmission time in subsequent stages can be reduced. To this end, suitable methods for the selection and scheduling of coded data packets and the allocation of the subcarriers are proposed which significantly increase the achievable data throughput through the exploitation of inter-stage links. In addition, a forwarding scheme is proposed that extends the cooperation among the nodes and enables data forwarding through distributed multiantenna transmissions. To this end, the restriction of exclusive usage of subcarriers is canceled. By a suitable preprocessing of the transmit signals, which is adapted to the corresponding channel states, a beamforming effect can be achieved which results in an improved signal level at the corresponding receiver. However, this transmission scheme requires the availability of the same data packets at several transmitters, which goes along with an additional effort that needs to be spent. A procedure is proposed that enables distributed multi-antenna transmissions and includes them in Corridor-based Routing in a profitable way compared to the exclusive usage of subcarriers.