TOWARD LAYERLESS COOPERATION AND RATE CONTROL IN WIRELESS MULTI-ACCESS CHANNELS

In wireless networks, a transmitted message may successfully reach multiple nodes simultaneously, which is referred to as the Wireless Multicast Advantage. As such, intermediate nodes have the ability to capture the message and then contribute to the communication toward the ultimate destination by cooperatively relaying the received message. This enables cooperative communication, which has been shown to counteract the effects of fading and attenuation in wireless networks. There has been a great deal of work addressing cooperative methods and their resulting benefits, but most of the work to date has focused on physical-layer techniques and on information-theoretic considerations. While compatible with these, the main thrust of this dissertation is to explore a new approach by implementing cooperation at the network layer. First, we illustrate the idea in a multi-hop multi-access wireless network, in which a set of source users generate packets to deliver to a common destination. An opportunistic and dynamic cooperation protocol is proposed at the network level, where users with a better channel to the destination have the capability and option to relay packets from users that are farther afield. The proposed mode of cooperation protocol is new and relies on MAC/Network-level of relaying, but also takes into account physical-layer parameters that determine successful reception at the destination and/or the relay. We explicitly characterize the stable throughput and average delay performance. Our analysis reveals that cooperation at the network layer leads to substantial performance gains for both performance metrics. Next, on top of the network-layer cooperation, we investigate enhanced cooperative techniques that exploit more sophisticated physical-layer properties. Specifically, we consider dynamic decode-and-forward, superposition coding, and multipacket reception capability, and we quantify the extent to which the enhancement techniques can further improve the stable throughput region. Then we revert back to the two-user multi-access channel with single-packet reception, which has been extensively studied in the case of no cooperation. After cooperation is permitted between the two users, we revisit the relationship between the stability region and the throughput region under both scheduled access and random access schemes. Finally, we shift our focus from the packet-level to bit-level multi-access channels. By exploiting the bit-nature of a packet, we create a bridge between traditional physical-layer-based transmission rates and classical MAC/Network-layer-based throughput rates. We first obtain the closed form of the stability region in bits/slot. Then, as a separate, but related issue, we look at the minimum delivery time policy; for any initial queue size vector, the optimal policy that empties all bits in the system within the shortest time is characterized

The Effect of Cooperation at the Network Protocol Level ∗

There has been a great deal of attention on cooperative communication which exploits the spatial diversity among antennas belonging to multiple terminals. Most of the existing work focuses on the physical layer and shows how message relaying can improve the Shannon capacity region, outage probability, diversity order, etc. But it is possible to use relays in simple, innovative ways that depend on the protocol properties at the medium access control (MAC) and network layers. In this paper we build upon prior work on such relay use by considering sets of nodes in simple topology configurations in which reaching a common destination is accomplished through direct links as well as relayed transmissions. Each non-destination node generates its own traffic for the destination but the nodes that are closer to the destination have the capability and option to relay packets from nodes farther afield. Channel quality is modeled by a reception probability which injects the physical layer property into upper layer design and analysis. We consider bursty arrival processes and we characterize the stable throughput region and delay performance at each node. We show that a proposed cooperation strategy can lead to improved performance for both work-conserving and Time Division Multiple Access (TDMA) MAC protocols. The innovative elements in this work are the balance between own and relayed traffic at each node and the fact that the performance improvement is in part due to the concentration of the queues of failed packets into fewer virtual queues. 1

Relationship between facet tropism and facet joint degeneration in the sub-axial cervical spine

Abstract Background Facet tropism is the angular asymmetry between the left and right facet joint orientation. Although debatable, facet tropism was suggested to be associated with disc degeneration, facet degeneration and degenerative spondylolisthesis in the lumbar spine. The purpose of this study was to explore the relationship between facet tropism and facet degeneration in the sub-axial cervical spine. Methods A total of 200 patients with cervical spondylosis were retrospectively analyzed. Facet degeneration was categorized into 4 grade: grade I, normal; grade II, degenerative changes including joint space narrowing, cyst formation, small osteophytes (3 mm) without fusion of the joint; grade IV, bony fusion of the facet joints. Facet orientations and facet tropisms with respect to the transverse, sagittal and coronal plane were calculated from the reconstructed cervical spine, which was based on the axial CT scan images. The paired facet joints were then categorized into three types: symmetric, moderated tropism and severe tropism. Univariate and multivariate analysis were performed to evaluate the relationship between any demographic and anatomical factor and facet degeneration. Results The mean age of enrolled patients was 46.23 years old (ranging from 30 to 64 years old). There were 114 males and 86 females. The degrees of facet degeneration varied according to cervical levels and ages. Degenerated facet joints were most common at C2-C3 level and more common in patients above 50 years old. The facet orientations were also different from level to level. By univariate analysis, genders, ages, cervical levels, facet orientations and facet tropisms were all significantly different between the normal facets and degenerated facets. However, results from multivariate logistic regression suggested only age and facet tropism with respect to the sagittal plane were related to facet degeneration. Conclusion Facet degeneration were more common at C2-C3 level. Older age and facet tropism with respect to the sagittal plane were associated with the facet degeneration

On combating the half-duplex constraint in modern cooperative networks: Protocols and techniques

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