Information theoretic secrecy for multiple access and broadcast channels

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In current cellular systems, operators already offer not only traditional services such as voice communication,but also further multicast services or confidential services that are subject to certain secrecy constraints.Nowadays, the integration of multiple services is realized by policies that allocate different services on differentlogical channels and transmit them independently at the physical layer. In general this is quite inefficient andthus there is a trend to merge multiple coexisting services efficiently from an information theoretic point of viewso that they work on the same wireless resources. Further secrecy is realized on higher levels by applyingcryptographic techniques. Unfortunately, such techniques rely on the assumption of insufficient computationalcapabilities of non-legitimate receivers for the decoding of the confidential information. In general, however, those asumptions are difficult to verify. Thus, it is of interest to investigate alternative security measures such as physical layer secrecy techniques, which realize security at the physical layerand do not rely on such assumptions. Therefore, such techniques provide so-called unconditional security.This project analyzes information theoretic or physical layer security in multiple access and broadcastchannels. These are the basic models for the uplink and downlink in a cellular system and, thus, important formobile communication. In this project we want to analyze how to achieve information theoretic security and tocharacterize the fundamental limits. In this project we further want to analyze the impactof channel knowledge at the transmitter and receiver on the secrecy. Especially in systems with an externaleavesdropper, the assumption of perfect channel state information of the channel to the non-legitimatereceiver is quite unrealistic. Therefore, we need robust strategies that achieve security even under channeluncertainty. Another research topic is the topology of the network. In certain networks there is side informationavailable at the transmitter or receiver. We want to analyze the impact of side information on the security.