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1. Repeated Games, Optimal Channel Capture, and Open Problems for Slotted Multiple Access

   https://doi.org/10.1109/Allerton49937.2022.9929420  

   Neely, Michael J. (October 2022, 58th Annual Allerton Conference on Communication, Control, and Computing)

   This paper revisits a classical problem of slotted multiple access with success, idle, and collision events on each slot. First, results of a 2-user multiple access game are reported. The game was conducted at the University of Southern California over multiple semesters and involved competitions between studentdesigned algorithms. An algorithm called 4-State was a consistent winner. This algorithm is analyzed and shown to have an optimal expected score when competing against an independent version of itself. The structure of 4-State motivates exploration of the open question of how to minimize the expected time to capture the channel for a n-user situation. It is assumed that the system delivers perfect feedback on the number of users who transmitted at the end of each slot. An efficient algorithm is developed and conjectured to have an optimal expected capture time for all positive integers n. Optimality is proven in the special cases n ∈ {1, 2, 3, 4, 6} using a novel analytical technique that introduces virtual users with enhanced capabilities. 

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2. Wideband Temporal Spectrum Sensing Using Cepstral Features

   https://doi.org/10.1109/WoWMoM.2019.8793002  

   Cheng, Hanke; Mark, Brian L.; Ephraim, Yariv (June 2019, 2019 IEEE 20th International Symposium on "A World of Wireless, Mobile and Multimedia Networks" (WoWMoM))

   Spectrum sensing enables secondary users in a cognitive radio network to opportunistically access portions of the spectrum left idle by primary users. Tracking spectrum holes jointly in time and frequency over a wide spectrum band is a challenging task. In one approach to wideband temporal sensing, the spectrum band is partitioned into narrowband subchannels of fixed bandwidth, which are then characterized via hidden Markov modeling using average power or energy measurements as observation data. Adjacent, correlated subchannels are recursively aggregated into channels of variable bandwidths, corresponding to the primary user signals. Thus, wideband temporal sensing is transformed into a multiband sensing scenario by identifying the primary user channels in the spectrum band. However, future changes in the configuration of the primary user channels in the multiband setup cannot generally be detected using an energy detector front end for spectrum sensing. We propose the use of a cepstral feature vector to detect changes in the spectrum envelope of a primary user channel. Our numerical results show that the cepstrum-based spectrum envelope detector performs well under moderate to high signal-to-noise ratio conditions. 

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3. When is the zero-error capacity positive in the relay, multiple-access, broadcast and interference channels?

   https://doi.org/10.1109/ALLERTON.2016.7852296  

   Devroye, Natasha (September 2016, Communication, Control, and Computing (Allerton), 2016 54th Annual Allerton Conference on)

   Shannon determined that the zero-error capacity of a point-to-point channel whose channel p(y|x) has confusability graph GX|Y is positive if and only if there exist two inputs that are “non-adjacent”, or “non-confusable”. Equivalently, it is non-zero if and only if the independence number of GX|Y is strictly greater than 1. A multi-letter expression for the zero-error capacity of the channel with confusability graph GX|Y is known, and is given by the normalized limit as the blocklength n → 1 of the maximum independent set of the n-fold strong product of GX|Y. This is not generally computable with known methods. In this paper, we look at the zero-error capacity of four multi-user channels: the relay, the multiple-access (MAC), the broadcast (BC), and the interference (IC) channels. As a first step towards finding a multi-letter expression for the capacity of such channels, we find necessary and sufficient conditions under which the zero-error capacity is strictly positive. 

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4. Reconfigurable Antenna Multiple Access for 5G mmWave Systems

   https://doi.org/10.1109/ICCW.2018.8403609  

   Almasi, Mojtaba Ahmadi; Mehrpouyan, Hani; Matolak, David; Pan, Cunhua; Elkashlan, Maged (May 2018, 2018 IEEE International Conference on Communications Workshops (ICC Workshops))

   This paper aims to realize a new multiple access technique based on recently proposed millimeter- wave reconfigurable antenna architectures. To this end, first we show that integration of the existing reconfigurable antenna systems with the well-known non-orthogonal multiple access (NOMA) technique causes a significant degradation in sum rate due to the inevitable power division in reconfigurable antennas. To circumvent this fundamental limit, a new multiple access technique is proposed. The technique which is called reconfigurable antenna multiple access (RAMA) transmits only each user's intended signal at the same time/frequency/code, which makes RAMA an inter-user interference-free technique. Two different cases are considered, i.e., RAMA with partial and full channel state information (CSI). In the first case, CSI is not required and only the direction of arrival for a specific user is used. Our analytical results indicate that with partial CSI and for symmetric channels, RAMA outperforms NOMA in terms of sum rate. Further, the analytical result indicates that RAMA for asymmetric channels achieves better sum rate than NOMA when less power is assigned to users that experience better channel quality. In the second case, RAMA with full CSI allocates optimal power to each user which leads to higher achievable rates compared to NOMA for both symmetric and asymmetric channels. The numerical computations demonstrate the analytical findings. 

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5. On the Capacity of Intensity-Modulation Direct-Detection Gaussian Optical Wireless Communication Channels: A Tutorial

   https://doi.org/10.1109/COMST.2021.3120087  

   Chaaban, A. (January 2022, IEEE Communications surveys and tutorials)

   Optical wireless communication (OWC) using intensity-modulation and direct-detection (IM/DD) has a channel model which possesses unique features, due to the constraints imposed on the channel input. The aim of this tutorial is to overview results on the capacity of IM/DD channels with input-independent Gaussian noise as a model of OWC channels. It provides the reader with an entry point to the topic, and highlights some major contributions in this area. It begins with a discussion on channel models and how this IM/DD Gaussian channel model comes about, in addition to an explanation of input constraints. Then, it discusses the capacity of the single-input single-output channel, its computation, and capacity bounds and asymptotic capacity results. Then, it extends the discussion to the multiple-input multiple-output setup, and reviews capacity bounds for this channel model. Finally, it discusses multi-user channels modelled as a broadcast channel (downlink) or a multiple-access channel (uplink), with their associated capacity bounds. 

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