8 research outputs found
State-of-the-art of distributed channel assignment
Channel assignment for Wireless Mesh Networks (WMNs) attempts to increase the
network performance by decreasing the interference of simultaneous
transmissions. The reduction of interference is achieved by exploiting the
availability of fully or partially non-overlapping channels. Although it is
still a young research area, many different approaches have already been
developed. These approaches can be distinguished into centralized and
distributed. Centralized algorithms rely on a central entity, usually called
Channel Assignment Server (CAS), which calculates the channel assignment and
sends the result to the mesh routers. In distributed approaches, each mesh
router calculates its channel assignment decision based on local information.
Distributed approaches can react faster to topology changes due to node
failures or mobility and usually introduce less protocol overhead since
communication with the CAS is not necessary. As a result, distributed
approaches are more suitable once the network is operational and running.
Distributed approaches can further be classified into static and dynamic, in
regard to the modus of channel switching. In dynamic approaches, channels can
be switched on a per-packet basis, whereas in static approaches radios stay on
a specific channel for a longer period of time. Static assignments have been
more in focus, since the channel switching time for current Institute of
Electrical and Electronics Engineers (IEEE) 802.11 hardware is in the order of
milliseconds which is two orders higher than the packet transmission time.
Recently, surveys of channel assignment algorithms have been presented which
cover certain aspects of the research field. The survey in [1] introduces the
problem and presents a couple of distributed algorithms and [2] gives a broad
introduction to centralized and distributed approaches. The survey herein is
focused on distributed approaches for peer- to-peer network architectures.
This report describes the problem formulation for channel assignment in WMNs
and the fundamental concepts and challenges of this research area. We present
different distributed channel assignment algorithms and characterize them
according to a set of classification keys. Since channel assignment algorithms
may change the connectivity and therefore the network topology, they may have
a high impact on routing. Therefore, we present routing metrics that consider
channel diversity and adapt better to the multi- radio multi-channel scenario
than traditional routing metrics designed for single channel networks. The
presented algorithms are discussed and compared focusing on practical
evaluations in testbed and network environments. The implementation for real
networks is a hard and labor-intensive task because the researcher has to deal
with the complexity of the hardware, operating system, and wireless network
interface drivers. As a result, frameworks emerged in order to simplify the
implementation process. We describe these frameworks and the mechanisms used
to help researchers implementing their algorithms and show their limitations
and restrictions
Decentralized Ultra-Reliable Low-Latency Communications through Concurrent Cooperative Transmission
Emerging cyber-physical systems demand for communication technologies that enable seamless interactions between humans and physical objects in a shared environment. This thesis proposes decentralized URLLC (dURLLC) as a new communication paradigm that allows the nodes in a wireless multi-hop network (WMN) to disseminate data quickly, reliably and without using a centralized infrastructure. To enable the dURLLC paradigm, this thesis explores the practical feasibility of concurrent cooperative transmission (CCT) with orthogonal frequency-division multiplexing (OFDM). CCT allows for an efficient utilization of the medium by leveraging interference instead of trying to avoid collisions. CCT-based network flooding disseminates data in a WMN through a reception-triggered low-level medium access control (MAC). OFDM provides high data rates by using a large bandwidth, resulting in a short transmission duration for a given amount of data.
This thesis explores CCT-based network flooding with the OFDM-based IEEE 802.11 Non-HT and HT physical layers (PHYs) to enable interactions with commercial devices. An analysis of CCT with the IEEE 802.11 Non-HT PHY investigates the combined effects of the phase offset (PO), the carrier frequency offset (CFO) and the time offset (TO) between concurrent transmitters, as well as the elapsed time. The analytical results of the decodability of a CCT are validated in simulations and in testbed experiments with Wireless Open Access Research Platform (WARP) v3 software-defined radios (SDRs). CCT with coherent interference (CI) is the primary approach of this thesis.
Two prototypes for CCT with CI are presented that feature mechanisms for precise synchronization in time and frequency. One prototype is based on the WARP v3 and its IEEE 802.11 reference design, whereas the other prototype is created through firmware modifications of the Asus RT-AC86U wireless router. Both prototypes are employed in testbed experiments in which two groups of nodes generate successive CCTs in a ping-pong fashion to emulate flooding processes with a very large number of hops. The nodes stay synchronized in experiments with 10 000 successive CCTs for various modulation and coding scheme (MCS) indices and MAC service data unit (MSDU) sizes. The URLLC requirement of delivering a 32-byte MSDU with a reliability of 99.999 % and with a latency of 1 ms is assessed in experiments with 1 000 000 CCTs, while the reliability is approximated by means of the frame reception rate (FRR). An FRR of at least 99.999 % is achieved at PHY data rates of up to 48 Mbit/s under line-of-sight (LOS) conditions and at PHY data rates of up to 12 Mbit/s under non-line-of-sight (NLOS) conditions on a 20 MHz wide channel, while the latency per hop is 48.2 µs and 80.2 µs, respectively. With four multiple input multiple output (MIMO) spatial streams on a 40 MHz wide channel, a LOS receiver achieves an FRR of 99.5 % at a PHY data rate of 324 Mbit/s. For CCT with incoherent interference, this thesis proposes equalization with time-variant zero-forcing (TVZF) and presents a TVZF receiver for the IEEE 802.11 Non-HT PHY, achieving an FRR of up to 92 % for CCTs from three unsyntonized commercial devices. As CCT-based network flooding allows for an implicit time synchronization of all nodes, a reception-triggered low-level MAC and a reservation-based high-level MAC may in combination support various applications and scenarios under the dURLLC paradigm
Mobile Ad-Hoc Networks
Being infrastructure-less and without central administration control, wireless ad-hoc networking is playing a more and more important role in extending the coverage of traditional wireless infrastructure (cellular networks, wireless LAN, etc). This book includes state-of-the-art techniques and solutions for wireless ad-hoc networks. It focuses on the following topics in ad-hoc networks: quality-of-service and video communication, routing protocol and cross-layer design. A few interesting problems about security and delay-tolerant networks are also discussed. This book is targeted to provide network engineers and researchers with design guidelines for large scale wireless ad hoc networks