Mathias Kurth

Cooperative Opportunistic Routing Using Transmit Diversity in Wireless Mesh Networks

We propose a new high performance forwarding scheme, denoted transmit diversity based cooperative opportunistic routing (TDiCOR), that efficiently exploits multiuser and transmit diversity to improve the overall throughput in wireless multi-hop networks. Neighboring nodes cooperate through simultaneous transmissions in a MISO fashion in order to overcome the destructive effects of fading. TDiCOR uses distributed transmit diversity to increase the robustness of acknowledgements as well as data transmissions while preserving the opportunistic nature by using multiple candidates for packet relaying. We present measurements from a wireless testbed which suggest that cooperation using transmit diversity is feasible even with todays off-the-shelf hardware. At the instance of TDiCOR, we demonstrate how to realize the idea of cooperative opportunistic routing as an operational protocol and present very promising simulation results. TDiCOR outperforms traditional routing (i.e. DSR) in typical outdoor scenarios in terms of throughput by 30% and by 50% in indoor scenarios with high shadow fading, without consuming additional bandwidth or additional hardware resources.

An Opportunistic Cross-Layer Protocol for Multi-Channel Wireless Networks

In this paper, we propose a new multi-hop forwarding scheme for wireless multi-hop networks, denoted Multi-Channel Extremely Opportunistic Routing (MCExOR), that efficiently exploits radio channel characteristics and makes use of multiple RF channels. MCExOR reduces the overall number of transmissions in wireless multi-hop networks by opportunistically skipping nodes in a packets forwarding path. The use of multiple non overlapping RF channels contributes to the reduction of overall interference. In contrast to other approaches MCExOR only needs one RF transceiver per device. We present algorithms for route discovery and packet forwarding. In contrast to other multi-channel protocols the packet forwarding is decoupled from route discovery. Therefore, both protocols have low complexity and can be addressed separately. With the help of simulations we show that MCExOR significantly outperforms traditional protocols like AODV through the simultaneous use of multiple RF channels and its opportunistic behaviour. With the increasing number of RF channels the overall throughput increases superproportionally. MCExOR with 2 RF channels surpasses AODV by an average of 140%. Unlike other multi channel approaches even a single packet flow can benefit from the existence of multiple channels.

Self-Organization in Community Mesh Networks The Berlin RoofNet

A community network must be usable for inexperienced end users; thus self-organization is essential. On the one hand, we propose an approach for self-organization in ad-hoc wireless multi-hop mesh networks, where the client is fully freed from such mundane tasks as IP configuration, etc. On the other hand, the community mesh network itself is fully self-organized thus no operator or provider is required. We present the architecture of the Berlin RoofNet (BRN) and a distributed realization of services like DHCP, ARP and Internet gateway discovery and selection. In addition, results of a detailed simulation and experimental evaluation comparing our distributed hash table based approach to traditional methods are presented. We show that our approach is more reliable, efficient and responsive

Considerations on forwarder selection for opportunistic protocols in wireless networks

Opportunistic Routing gained lots of attention as a way to improve the performance of wireless multi-hop relay mesh networks. The key characteristic is its ability to take advantage of the numerous, yet unreliable wireless links in the network. The most important part of every opportunistic routing protocol is the forwarder selection algorithm. Most of the currently known protocols assume that signal paths from a sender to all candidates are independent to each other and therefore resulting in independent packet error rates. However, this assumption does not hold for spatially close candidates which are not so uncommon in indoor networks. In this paper, we present empirical measurements from our 802.11 indoor test-bed which reveal that signal paths to spatially close nodes are correlated. We believe that the loss in the radio propagation due to shadow fading is similar to spatially close nodes. For our setup we find out that a spatial correlation exists when the nodes are closer than 2 m to each other. We present a candidate set selection algorithm which is able to calculate the packet error rate of a candidate set even when the individual packet error rates are correlated, e.g. due to spatial correlation. Therefore only a simple modification to the existing ordinary link probing is required. Finally, we present modifications we made to our packet-level simulator to respect spatial correlation as well as simulation results.

Multi-Channel Opportunistic Routing in Multi-Hop Wireless Networks

We propose and investigate Multi-Channel Extremely Opportunistic Routing (MCExOR) which is a protocol that extends Extremely Opportunistic Routing by utilizing multiple RF channels in multi-hop wireless networks. Large numbers of transmissions per end-to-end delivery combined with interference are the main reasons for the low capacity of wireless multi-hop networks. MCExOR reduces the overall number of transmissions in wireless multi-hop networks by opportunistically skipping nodes in a packet’s forwarding path. The use of multiple non overlapping RF channels contributes to the reduction of overall interference. In contrast to other approaches MCExOR only needs one RF transceiver per device. We present algorithms for route discovery and packet forwarding. A significant benefit of MCExOR is that the selection of RF channels is independent of the routing function. Finally, with the help of simulations we show that MCExOR outperforms traditional protocols like ad-hoc on-demand distance vector routing through the simultaneous use of multiple RF channels. In combination with realistic radio propagation models an increase in the throughput is observed due to the opportunistic feature of MCExOR. With the increasing number of RF channels the overall throughput increases superproportionally. Unlike other multi channel approaches even a single packet flow can benefit from the existence of multiple channels.

Opportunistic Protocols in Multi-rate Environments

In recent years the research of opportunistic protocols for wireless mesh networks gained lots of attention. A great number of protocols like extreme opportunistic routing (ExOR) and multiuser diversity forwarding (MDF) was proposed. Most of the performance evaluations were conducted in a constant bit-rate environment. This paper presents simulation results of the performance of existing opportunistic protocols as well as a new opportunistic protocol called hybrid opportunistic routing (HOR) in a constant- and multi-rate environment. In a constant rate environment with a slow fading channel ExOR outperforms MDF and HOR by around 20%. This is mainly due to its small signaling overhead. ExOR is also the best choice in a fast fading channel. However, here HOR is able to outperform MDF. In a multi-rate environment our proposed ETT-RCA rate control algorithms outperforms the existing adaptive auto rate fallback (AARF) significantly. AARF is only suitable for short, high quality links. The biggest problem with ExOR is that is cannot be used together with ETT-RCA. In a slow fading channel MDF with ETT-RCA is the best choice. It outperforms ExOR with AARF by multiple times (up to 360%). In a fast fading channel HOR with ETT-RCA is the best choice for medium distances (e.g. 80% and 330% higher throughput than MDF with ETT-RCA and ExOR AARF). Only for very large distances ExOR with AARF is able to offer the highest throughput. Here in the multi-rate environment the degrees of freedom (candidate and bit-rate selection) are too large for ExOR and AARF.

On Uplink Superposition Coding and Multi-user Diversity for Wireless Mesh Networks

In recent years wireless mesh networks (WMN) gained lots of attention in research and industry. Especially, the use of WMNs for community networks proved their ability to provide a high performance Internet access while demanding little deployment planning. However, one problem became apparent. Network congestion, especially around gateway nodes (GN), can dramatically decrease the overall performance of the entire network. We present a novel approach to efficiently decrease network congestion while maintaining fairness. To achieve that goal, we combine two modern techniques, namely uplink superposition-coding and multi-user diversity to create a MAC protocol that can dramatically improve the network throughput around GN, hence extending the capabilities of the entire WMN. We will present two distributed protocols, which are suitable for ad-hoc networks without any centralized infrastructure. Extensive analysis and simulations will be presented, that show a network throughput increase of up to 88% over the use of 802.11 with the OAR rate selection algorithm. In a fading environment an additional gain of 5-13% from multi-user diversity was observed.

Network Coding for Bit Error Recovery in IEEE 802.11 Mesh Networks

Opportunistic routing (OR) relies on links of intermediate quality, i.e. packet losses are common. However, the reasons for packet losses are manifold, e.g. a received packet may contain corrupted bits. According to traditional approaches, the receiver discards the whole frame in such a case. In this paper, we present measurements from an indoor IEEE 802.11 wireless mesh network (WMN), which indicate that corrupted frames still contain a significant amount of correct data, which can be utilized. In particular, corrupted frames are common for intermediate quality links. Bit errors tend to occur in proximity, i.e. they are bursty. Furthermore, bit errors are uncorrelated across different receivers in most cases. Based on our observations, we propose a HARQ scheme for OR called Hybrid ARQ with Limited Fragmentation (HALF). It operates on a hop-by-hop manner and requires only local knowledge. Due to the bursty nature of bit errors, we are dividing frames into fragments with additional error detection. Using random linear network codes, the sender transmits incremental redundancy until one of its receivers is able to decode all fragments and therefore sends an acknowledgement packet. However, the partial information at all other receivers is not lost. Instead, to increase the throughput further, it is also used in subsequent forwarding rounds along the multi-hop route. We implemented a prototype of our protocol to evaluate its performance. With the help of detailed simulations, we analyzed the reasons why HALF significantly outperforms traditional approaches like DSR.

Opportunistic Routing with Adaptive CSMA/CA in Wireless Mesh Networks

In this paper, we present and evaluate a cross-layer protocol that integrates opportunistic routing into a CSMA wireless mesh network. We develop a MAC model and protocol with enhanced spatial reuse that enables spatial efficient anycast transmissions with more realistic assumptions about the carrier sensing capabilities. Using an optimization framework, we derive a cross-layer algorithm that solves the CSMA scheduling, opportunistic routing and flow control problem. With opportunistic routing, a tradeoff arises between spatial and multi-user diversity. The more candidates the sender uses, the higher the packet success probability, however, the more spatial resources are consumed. Within our evaluation, we illustrate how the proposed system handles the tradeoff efficiently.

Carrier sensing and receiver performance in indoor IEEE 802.11b mesh networks

In this paper, we address the following question: given a typical indoor IEEE 802.11 mesh network, how are carrier sensing, receiving and interference range related and how stable are they in time? To answer this question, we conducted broadcast measurements in the Berlin RoofNet testbed under saturated conditions using multiple simultaneous transmitters with either carrier sensing turned on and off, respectively. In contrast to several prior studies, our results indicate that wireless mesh networks are much more deterministic, and they show a high stability even under self-induced interference. Interestingly, for IEEE 802.11b at 1Mbps, the interference range of a transmitter is only slightly larger than its receiving range. On the other hand, the carrier sensing range is slightly smaller than the receiving range. In particular, a packet success rate around 10% is a reasonable good indicator for carrier sensing. In addition, we identified uncontrolled external interference and environmental mobility as the key disturbing factors causing variations in packet reception.