On the Multiple-Access Channel With Common Rate-Limited Feedback

Coding scheme for discrete memoryless multicast networks with rate-limited feedback from the receivers and relays to the transmitter is proposed. The coding scheme is based on block-Markov coding, joint backward decoding and hybrid relaying strategy. In each block, the relays use partial decode-forward strategy to decode part of the source message. Meanwhile, the receivers and relays use compress-forward strategy to compress their channel outputs and send the compression indices to the transmitter through the feedback links. In the next block, after obtaining the compression indices, the transmitter sends them together with the source message. Each receiver uses backward decoding to jointly decode the source message and all compression indices. It is shown that our coding scheme generalizes Gabbai and Bross's results for the single relay channel with partial feedback, where they proposed coding schemes based on restricted decoding and deterministic partitioning. For the single relay channel with relay-transmitter feedback, our coding scheme can strictly improve on noisy network coding, distributed decode-forward coding and all known lower bounds on the achievable rate in the absence of feedback. Furthermore, motivated by the feedback coding scheme, we propose a new coding scheme for discrete memoryless multicast networks without feedback, which also improves noisy network coding and distributed decode-forward coding.

Information-theoretic broadcast channels (BCs) and multiple-access channels (MACs) enable a single node to transmit data simultaneously to multiple nodes, and multiple nodes to transmit data simultaneously to a single node, respectively. In this paper, we address the problem of link scheduling in multihop wireless networks containing nodes with BC and MAC capabilities. We first propose an interference model that extends protocol interference models, originally designed for point-to-point channels, to include the possibility of BCs and MACs. Due to the high complexity of optimal link schedulers, we introduce the Multiuser Greedy Maximum Weight algorithm for link scheduling in multihop wireless networks containing BCs and MACs. Given a network graph, we develop new local pooling conditions and show that the performance of our algorithm can be fully characterized using the associated parameter, the multiuser local pooling factor. We provide examples of some network graphs, on which we apply local pooling conditions and derive the multiuser local pooling factor. We prove optimality of our algorithm in tree networks and show that the exploitation of BCs and MACs improve the throughput performance considerably in multihop wireless networks.

This paper studies the problem of secure communication over a degraded wiretap channel p(y, z|x) = p(y|x)p(z|y) with secure feedback link of rate R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> , where X is the channel input, and Y and Z are channel outputs observed by the legitimate receiver and the wiretapper respectively. The secrecy capacity is characterized as C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> {R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> ) = maxmin{I(X; Y), I(X; Y|Z) + R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> }. p(x)equation. A capacity-achieving coding scheme is presented, in which the receiver securely feeds back fresh randomness with rate R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> , independent of the received channel output. The transmitter then uses the shared randomness as a secret key on top of Wynerpsilas coding scheme for wiretap channel without feedback. Hence, when the receiver has a means of interacting with the transmitter, he should allocate all resources to convey a new key rather than sending back the channel output. For the converse, a recursive argument is used to obtain the single-letter characterization.

In this paper, a generalized multiple access channel (MAC) model for secret key sharing between three terminals is considered. In this model, there are two transmitters and a receiver where all three terminals receive noisy channel outputs. In addition, there is a one-way public channel from the transmitters to the receiver. Each of the transmitters intends to share a secret key with the receiver by using the MAC and the public channel, where the transmitters are eavesdroppers with respect to each other. Two strategies for secret key sharing are considered, namely, the pregenerated key strategy and the two-stage key strategy. For both of them, inner and outer bounds of the secret key capacity region are derived. Furthermore, the effect of the public channel is discussed and the two strategies are compared. In both strategies, it is assumed that the channel outputs at the transmitters are only used for eavesdropping and not as inputs to the encoders. The effect of this assumption in the presence of the public channel is analyzed for some Gaussian MACs.

Purpose Open Government Data (OGD) portals aim to enhance accountability and transparency by serving as central access points for all government data at local, regional and national levels. To enhance user engagement with OGD portals, it is essential to optimize feedback mechanisms within these portals. This work aims to analyse the current status of feedback mechanisms as a foundation for their future improvement to strengthen user engagement and to advance in accountability and transparency. Design/methodology/approach This research conceptualizes feedback mechanisms in OGD portals, followed by case studies and extrapolation through the analysis to a wider range of OGD portals by means of an automated detection of input channels, output channels and the connecting flows. Findings The study reveals significant variability in the structure and formality of input and output feedback channels. Feedback mechanisms range from highly informal channels, such as social media, to formal structured channels, such as dedicated feedback forms. Our data analysis identifies three distinct clusters of countries based on the characteristics of feedback mechanisms in their OGD portals. Originality/value While existing research highlights the importance of feedback in OGD portals, it mainly focuses on the flow from users to data publishers through input channels. The novelty of this work lies in examining not only input channels but also the return flow from data publishers to users through output channels.

A secret key agreement framework involving three users is considered in which each of the users 1 and 2 intends to share a secret key with user 3 and users 1 and 2 are eavesdroppers with respect to each other. There is a generalized discrete memoryless multiple access channel (GDMMAC) from users 1 and 2 to user 3 where the three users receive outputs from the channel. Furthermore, there is a feedback channel from user 3 to users 1 and 2 through which user 3 sends information extracted from the received output from the GDMMAC to increase the key rates. We consider both noiseless and noisy feedback. In the case of noiseless feedback, a public channel of unlimited capacity from user 3 to users 1 and 2 is used only once. In the case of noisy feedback, a noisy broadcast channel (BC) from user 3 to users 1 and 2 can be repeatedly used, like GDMMAC. In both setups, inner bounds of the secret key capacity region are derived. The secret key capacity region is derived in some special cases where the channel inputs and outputs form Markov chains in certain orders. For illustration, the corresponding results are also derived and discussed for Gaussian channels. The cases with noiseless feedback, noisy feedback, and no feedback at all are compared with each other.

We consider the impact of destination cooperation on improving the achievable rate region in a multiple access channel with joint source-destination cooperation (MAC-SDC). Such cooperation may be appealing in systems with a more powerful destination than the sources, such as the uplink of cellular networks. We propose a coding scheme in which each source employs superposition block Markov encoding and partial decode-forward relaying, while the destination employs quantize-forward relaying and backward decoding. The sources partially exchange their messages using not only the direct links between them as in source cooperation, but also the feedback links from the destination, hence they are able to exchange more information at the same power by utilizing the destination as a relay. We analyze the condition on channel and power parameters such that involving destination cooperation is strictly better than just having source cooperation. Results for Gaussian channels show improvement especially when the inter-source link qualities are close to source-destination link qualities. These results provide guideline for practical systems such as cellular networks in determining when it is beneficial for the base station to cooperate with mobile users in the uplink communication.

We consider two-way wire-tap channels, where two users are communicating with each other in the presence of an eavesdropper, who has access to the communications through a multiple-access channel. We find achievable rates for two different scenarios, the Gaussian two-way wire-tap channel, (GTW-WT), and the binary additive two-way wire-tap channel, (BATW- WT). It is shown that the two-way channels inherently provide a unique advantage for wire-tapped scenarios, as the users know their own transmitted signals and in effect help encrypt the other user's messages, similar to a one-time pad. We compare the achievable rates to that of the Gaussian multiple-access wiretap channel (GMAC-WT) to illustrate this advantage.

In this paper, we study the two-user Gaussian fading Multiple Access Channel (MAC) with cooperative encoders. Two different scenarios are studied: the Gaussian fading MAC with a common message, and the Gaussian fading MAC with conferencing encoders. The throughput capacity region of these channels with partial Channel State Information (CSI) at the transmitters (CSIT) and perfect CSI at the receiver (CSIR) is established. For the Gaussian fading systems with only CSIR (transmitters have no access to CSIT), some numerical examples and simulation results are provided for Rayleigh fading models.

The growing interest in action-dependent channels motivates us to extend the study of action-dependent settings, which until now focused on point-to-point models, to multiple-access channels (MACs). In this paper, we consider a two-user, state-dependent MAC, in which one of the encoders, called the informed (cognitive) encoder, is allowed to take an action that affects the formation of the channel states. Two independent messages are to be sent through the channel: (1) a common message known to both encoders and (2) a private message known only to the informed encoder. In addition, the informed encoder has access to the sequence of channel states in a noncausal manner. Our framework generalizes the previously evaluated settings of state-dependent point-to-point channels with actions and MACs with common messages. We derive a single letter characterization of the capacity region for this setting. Using this general result, we obtain and compute the capacity region for the Gaussian action-dependent MAC. The special methods used in solving the Gaussian case are then applied to obtain the capacity of the Gaussian action-dependent point-to-point channel, a problem left open and solved now. Finally, we establish some dualities between action-dependent channel coding and source coding problems. In particular, we obtain a duality equivalence between the considered MAC setting and the rate distortion model known as successive refinement with actions. This is done by developing a set of simple duality principles that enables us to successfully evaluate the outcome of one problem given the outcome of the other.

Progressive kernel pruning with saliency mapping of input-output channels

In this work, we study two problems: three-user Multiple-Access Channel (MAC) with correlated sources, and MAC with Feedback (MAC-FB) with independent messages. For the first problem, we identify a structure in the joint probability distribution of discrete memoryless sources, and define a new common information called “conferencing common information”. We develop a multi-user joint-source channel coding methodology based on structured mappings to encode this common information efficiently and to transmit it over a MAC. We derive a new set of sufficient conditions for this coding strategy using single-letter information quantities for arbitrary sources and channel distributions. Next, we make a fundamental connection between this problem and the problem of communication of independent messages over three-user MAC-FB. In the latter problem, although the messages are independent to begin with, they become progressively correlated given the channel output feedback. Subsequent communication can be modeled as transmission of correlated sources over MAC. Exploiting this connection, we develop a new coding scheme for the problem. We characterize its performance using single-letter information quantities, and derive an inner bound to the capacity region. For both problems, we provide a set of examples where these rate regions are shown to be optimal. Moreover, we analytically prove that this performance is not achievable using random unstructured random mappings/codes.

In this paper, we first look at the achievable rate regions of the Gaussian Multiple Access Channel (G-MAC) and Gaussian Broadcast Channel (G-BC) when discrete constellations are transmitted. Focusing on the two-user case and assuming uncoded Pulse Amplitude Modulation (PAM), we show that the rate regions of G-MAC and G-BC can be improved by power conservation at the transmitter, a factor ignored in the earlier derived rate regions. We then further investigate duality between the G-MAC and the G-BC and show that a rate pair achieved in the G-MAC can be translated to a rate pair in the dual G-BC, such that the equal sum power constraint be satisfied. Due to the similarity of the rate expressions to Shannon's capacity formula, for an appropriate choice of signal to noise ratio (SNR) gap, we show that when finite constellations are used for transmission, rate regions of these two channels also have a dual relationship (the known for Gaussian alphabets). The rate region of the G-BC can therefore be characterized from the rate region of the dual G-MAC and vice versa.

We present the second part of our work on communicating correlated sources over multiple access (MAC) and interference channels (IC). Specifically, we undertake a Shannon-theoretic study of the above scenarios and focus on characterizing sufficient conditions for lossless recoverability of the sources at the decoder(s). We enhance the fixed block-length (B-L) coding technique by incorporating the technique of inducing source correlation onto channel inputs, originally discovered by Cover, El Gamal and Salehi. In contrast to the first part, performance analysis of a joint source-channel decoder poses new challenges. We enhance the earlier developed suite of coding and analytical tools to overcome these challenges and derive (simplified) single-letter characterizations for a new set of sufficient conditions for both scenarios. For both the MAC and IC problems, the derived sufficient conditions are (i) subsumed in the current known tightest, and (ii) strictly weaker for identified examples. Lastly, we propose simple `plug-in' approaches that can further weaken the derived sufficient conditions. Our findings enable us to subsume Dueck's findings (1981) and go even further for the example considered therein.

Purpose– The purpose of this paper is to present the result of an experiment examining the effects of changing input and output channel variety in an optical feedback system and their potential to give rise to novelty, as some non-trivial systems do.Design/methodology/approach– The experimental design is based on a digital computer connected to a digital photo camera (input channel), and to a monitor screen (output channel). The camera is pointed at the monitor to form a circular feedback system. Monitor and camera resolutions constitute the variable input and output variety of the system, allowing the visual investigation of effects of variety reduction and variety amplification in circular feedback systems.Findings– Results suggest that variety amplification in input and/or output channels promotes the emergence of discernible novelty in the apparent absence of a creative/generative capability or agenda within the recursive system.Research limitations/implications– An analogy between an optical feedback system and human novelty generation is shown, while generalisability of observations made beyond the described experiment remains to be established.Originality/value– A new approach to the computational modelling of aspects of human creativity is presented. Photographic investigation of variety reduction and variety amplification is new, and hoped to be of value to those wishing to examine these theoretical concepts in concrete terms.