The presence of signal outage, due to shadowing and blockage, is expected to be the main bottleneck in millimeter wave (mmWave) networks. Moreover, with the anticipated vision that mmWave networks would have a dense deployment of base stations, interference from strong line-of-sight base stations increases too, thus further increasing the probability of outage. To address the issue of reducing outage, this paper explores the possibility of base station cooperation in the downlink of a mmWave heterogenous network. The main focus of this work is showing that, in a stochastic geometry framework, cooperation from randomly located base stations decreases outage probability. With the presumed vision that less severe fading will be experienced due to highly directional transmissions, one might expect that cooperation would increase the coverage probability; our numerical examples suggest that is in fact the case. Coverage probabilities are derived accounting for: different fading distributions, antenna directionality and blockage. Numerical results suggest that coverage with base station cooperation in dense mmWave systems and with no small scale fading considerably exceeds coverage with no cooperation. In contrast, an insignificant increase is reported when mmWave networks are less dense with a high probability of signal blockage and with Rayleigh fading.
This paper considers the K-user cognitive interference channel with one primary and K − 1 secondary/cognitive transmitters with a cumulative message sharing structure, i.e., cognitive transmitter i ∈ [2 : K] knows non-causally all messages of the users with index less than i. We propose a computable outer bound valid for any memoryless channel. We first evaluate the sum-rate outer bound for the high-SNR linear deterministic approximation of the Gaussian noise channel. This is shown to be capacity for the 3-user channel with arbitrary channel gains and the sum-capacity for the symmetric K-user channel.Interestingly, for the K user channel having only the K-th cognitive transmitter know all other messages is sufficient to achieve capacity, i.e., cognition at transmitters 2 to K − 1 is not needed. Next, the sumcapacity of the symmetric Gaussian noise channel is characterized to within a constant additive and multiplicative gap. The proposed achievable scheme for the additive gap is based on Dirty Paper Coding and can be thought of as a MIMO-broadcast scheme where only one encoding order is possible due to the message sharing structure. As opposed to other multiuser interference channel models, a single scheme suffices for both the weak and strong interference regimes. With this scheme, the generalized degrees of freedom (gDoF) is shown to be a function of K, in contrast to the non cognitive case and the broadcast channel case. Interestingly, it is show that as the number of users grows to infinity the gDoF of the K-user cognitive interference channel with cumulative message sharing tends to the gDoF of a broadcast channel with a K-antenna transmitter and K single-antenna receivers. The analytical additive additive and multiplicative gaps are a function of the number of users. Numerical evaluations of inner and outer bounds show that the actual gap is less than the analytical one.
To be considered for an IEEE Jack Keil Wolf ISIT Student Paper Award. This paper characterizes the ergodic capacity of the fading multiple-input single-output (MISO) channel with per-antenna power constraints (PerPC) with perfect Channel State Information (CSI) at all terminals. This turns out to be the sum-capacity achieving strategy for the ergodic fading Gaussian overlay cognitive interference channel (EGCIFC) in the strong interference regime. The EGCIFC is a two-user timevarying interference channel in which a primary / licensed transmitter and a secondary / cognitive transmitter share the same spectrum and where the cognitive transmitter has noncausal knowledge of the primary user's message. The MISO and the EGCIFC results are verified numerically for the case of independent Rayleigh fading gains. Different achievable strategies, corresponding to different amount of CSI, are compared to show the performance of the derived PerPC optimal power allocation.
This paper provides a survey of the state-of-the-art information theoretic analysis for overlay multi-user (more than two pairs) cognitive networks and reports new capacity results. In an overlay scenario, cognitive / secondary users share the same frequency band with licensed / primary users to efficiently exploit the spectrum. They do so without degrading the performance of the incumbent users, and may possibly even aid in transmitting their messages as cognitive users are assumed to possess the message(s) of primary user(s) and possibly other cognitive user(s). The survey begins with a short overview of the two-user overlay cognitive interference channel. The evolution from two-user to three-user overlay cognitive interference channels is described next, followed by generalizations to multi-user (arbitrary number of users) cognitive networks. The rest of the paper considers K-user cognitive interference channels with different message knowledge structures at the transmitters.Novel capacity inner and outer bounds are proposed. Channel conditions under which the bounds meet, thus characterizing the information theoretic capacity of the channel, for both Linear Deterministic and Gaussian channel models, are derived.The results show that for certain channel conditions distributed cognition, or having a cumulative message knowledge structure at the nodes, may not be worth the overhead as (approximately) the same capacity can be achieved by having only one global cognitive user whose role is to manage all the interference in the network. The paper concludes with future research directions.In this paper, we survey the fundamental limits of communication for a multi-user overlay cognitive networks with an arbitrary number of secondary / cognitive user(s) having non-causal message knowledge of primary user(s). The users transmit in the same frequency band and thus in general interfere with one another. The performance metric considered is the information theoretic notion of channel capacity. In other words, we are interested in the maximum rate of communication for which arbitrarily small probability of error can be achieved by every user, which may be seen as a benchmark when building practical systems. We will focus on results for general memoryless and practically relevant additive white Gaussian noise (AWGN) [2] channel models, as well as high signal to noise ratio (SNR) approximations of Gaussian is currently an Associate Editor for the IEEE Transactions on Cognitive Communications and Networking. Her research focuses on multi-user information theory and applications to cognitive and software-defined radio, radar, relay and two-way communication networks.
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