Receiver

Abstract: We present a model for time-varying communication single-access and multiple-access channels without feedback. We consider the difference between mutual information when the receiver knows the channel perfectly and mutual information when the receiver only has an estimate of the channel. We relate the variance of the channel measurement error at the receiver to upper and lower bounds for this difference in mutual information. We illustrate the use of our bounds on a channel modeled by a Gauss-Markov process, measured by a pilot tone. We relate the rate of time variation of the channel to the loss in mutual information due to imperfect knowledge of the measured channel.

Abstract: We consider the situation in which a transmitter attempts to communicate reliably over a discrete memoryless channel, while simultaneously ensuring covertness (low probability of detection) with respect to a warden, who observes the signals through another discrete memoryless channel. We develop a coding scheme based on the principle of channel resolvability, which generalizes and extends prior work in several directions. First, it shows that irrespective of the quality of the channels, it is possible to communicate on the order of $\sqrt {n}$ reliable and covert bits over $n$ channel uses if the transmitter and the receiver share on the order of $\sqrt {n}$ key bits. This improves upon earlier results requiring on the order of $\sqrt {n}\log n$ key bits. Second, it proves that if the receiver’s channel is better than the warden’s channel in a sense that we make precise, it is possible to communicate on the order of $\sqrt {n}$ reliable and covert bits over $n$ channel uses without a secret key. This generalizes earlier results established for binary symmetric channels. We also identify the fundamental limits of covert and secret communications in terms of the optimal asymptotic scaling of the message size and key size, and we extend the analysis to Gaussian channels. The main technical problem that we address is how to develop concentration inequalities for low-weight sequences. The crux of our approach is to define suitably modified typical sets that are amenable to concentration inequalities.

Abstract: A multicast network system utilizes a high speed link, such as a satellite link, to multicast multimedia information from the Internet to a plurality of receivers, such as personal computers Information from selected web sites is organized into 'channels' and provided to a multicast network for multicast transmission to the receivers Updated channel information is also periodically provided The receivers store the received channel such that a user can access the web page content in the channel at hard disk speed Preferably, a conditional access system ensures that only authorized receivers receive the channels The present invention also preferably includes a lower speed two-way connection to the Internet (such as dial up modem) which is used to report usage information and/or subscription information back to the web sites The present invention also provides 'seamless' or automatic access to this connection to allow the user to retrieve any information that has not been received and stored The receiver also manages use of memory space and other applications that may be active on the receiver to ensure that the receipt and processing of the multicast information does not interfere with receiver operation