Topics in Communications: Spreading in Bandwidth over Fading Channels and Tree-Based Redundancy in Mesh Networks
Thursday, February 19, 1998


The talk is divided into two parts. In the first part, we consider the effect of spreading in bandwidth on the maximum achievable information transmission rate (capacity) for fading channels. We give an overview of spreading and of fading channels, as well as of results in the area of signaling for wideband channels. We then consider two particular forms of spreading in bandwidth. The first form uses coding and signaling based on bandlimited white Gaussian noise, as would be optimal for capacity if there was additive white Gaussian noise but no fading. The second form of spreading is direct-sequence code spreading (such as is used in direct-sequence CDMA). For both forms of spreading, we establish results combining entropy and optimal estimation to determine, for typical wireless fading channels, in which regions spreading is detrimental. In the second part of the talk, we present a tree-based redundancy scheme for mesh networks. Trees are an increasingly important form of routing owing to the rise in multicast (point-to-multipoint) and incast (multipoint-to-point) communications. We give an overview of restoration of service in networks and of restoration for tree-based routes in particular. We present a way of building two redundant trees over any edge/node redundant network (i.e. any network which remains connected after the elimination of any single edge/node). The redundant trees are such that the failure of any edge/node leaves every node connected to at least one of the trees, while allowing the trees to share edges. We discuss the advantages of this scheme over existing redundancy and recovery schemes.
UC Berkeley Networking
Ashwin Pananjady and Orhan Ocal
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