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—This paper addresses user coexistence in cognitive radio systems by taking advantage of opportunities that arise during ARQ retransmission. It is shown that if these opportunities are properly exploited, nontrivial rates can be made available to a secondary (cognitive) pair while impinging little or no interference on the primary pair. This can be(More)
— This paper addresses the multiplexing loss that occurs in relay networks due to causality of relays and the half-duplex constraint. We devise relay selection methods to recover the multiplexing loss in decode-and-forward (DF) relay networks, while requiring very little feedback (merely bits/relay). Two network topologies are studied: First the case is(More)
— In this paper, we present a method for estimating the Doppler spread in mobile orthogonal frequency division multiplexing (OFDM) systems. The estimation is based on finding the autocorrelation function of time domain channel estimates over several OFDM symbols. In OFDM systems channel estimation is popularly performed in frequency domain. Channel(More)
This paper investigates the degrees of freedom of the interference channel in the presence of a dedicated MIMO relay. The relay is used to manage the interference at the receivers. We pose a fundamental question: What benefits are achieved by exploiting the direct links from the sources to the receivers and whether a two-hop strategy suffices considering(More)
This paper investigates the degrees of freedom of the interference channel in the presence of a dedicated multiple-input multiple-output (MIMO) relay. The relay is used to manage the interference at the receivers. It is assumed that all nodes including the relay have channel state information only for their own links and that the relay has <i>M</i> &#x2265;(More)
This paper considers the problem of user coexistence in cognitive radio networks operating under fading. The core idea of this work is to exploit the opportunities occurring during retransmissions of a primary system to allow the coexistence of cognitive radios. Several protocols are proposed depending on the amount of the channel knowledge available at the(More)
The relay channel with private messages (RCPM) is a generalized relay channel model where in addition to the traditional communication from source to destination (assisted by relay), the source has a private message for the relay, and the relay has a private message for the destination. This paper develops coding strategies for this channel based on(More)
Relay networks face a fundamental challenge in terms of spectral efficiency, because relays must repeat the source information. To address this key problem, this paper presents two relay selection protocols that salvage spectral efficiency by adroitly leveraging the multiple relays. Both protocols have the feature of letting the source transmit all or most(More)
In this paper we address cooperation in wideband channels. We first discuss a straight forward extension of existing cooperation protocols. The outage expressions for the wideband cooperation are cataloged. We also consider the case where a limited amount of channel state feedback is allowed. We explore the different ways that this side information can be(More)