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Breaking Spectrum Gridlock with Cognitive Radios: An Information Theoretic Perspective
, 2008
"... Cognitive radios hold tremendous promise for increasing spectral efficiency in wireless systems. This paper surveys the fundamental capacity limits and associated transmission techniques for different wireless network design paradigms based on this promising technology. These paradigms are unified b ..."
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Cited by 265 (4 self)
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Cognitive radios hold tremendous promise for increasing spectral efficiency in wireless systems. This paper surveys the fundamental capacity limits and associated transmission techniques for different wireless network design paradigms based on this promising technology. These paradigms are unified by the definition of a cognitive radio as an intelligent wireless communication device that exploits side information about its environment to improve spectrum utilization. This side information typically comprises knowledge about the activity, channels, codebooks and/or messages of other nodes with which the cognitive node shares the spectrum. Based on the nature of the available side information as well as a priori rules about spectrum usage, cognitive radio systems seek to underlay, overlay or interweave the cognitive radios ’ signals with the transmissions of noncognitive nodes. We provide a comprehensive summary of the known capacity characterizations in terms of upper and lower bounds for each of these three approaches. The increase in system degrees of freedom obtained through cognitive radios is also illuminated. This information theoretic survey provides guidelines for the spectral efficiency gains possible through cognitive radios, as well as practical design ideas to mitigate the coexistence challenges in today’s crowded spectrum.
Communication over mimo x channels: Interference alignment, decomposition, and performance analysis
 IEEE TRANSACTIONS ON INFORMATION THEORY
, 2008
"... In a multipleantenna system with two transmitters and two receivers, a scenario of data communication, known as the X channel, is studied in which each receiver receives data from both transmitters. In this scenario, it is assumed that each transmitter is unaware of the other transmitter’s data (n ..."
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Cited by 213 (12 self)
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In a multipleantenna system with two transmitters and two receivers, a scenario of data communication, known as the X channel, is studied in which each receiver receives data from both transmitters. In this scenario, it is assumed that each transmitter is unaware of the other transmitter’s data (noncooperative scenario). This system can be considered as a combination of two broadcast channels (from the transmitters ’ points of view) and two multipleaccess channels (from the receivers ’ points of view). Taking advantage of both perspectives, two signaling schemes for such a scenario are developed. In these schemes, some linear filters are employed at the transmitters and at the receivers which decompose the system into either two noninterfering multipleantenna broadcast subchannels or two noninterfering multipleantenna multipleaccess subchannels. The main objective in the design of the filters is to exploit the structure of the channel matrices to achieve the
Real Interference Alignment: Exploiting the Potential of Single Antenna Systems
, 2009
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Interference alignment on the deterministic channel and application to gaussian networks
"... Abstract—An interference alignment example is constructed for the deterministic channel model of the user interference channel. The deterministic channel example is then translated into the Gaussian setting, creating the first known example of a fully connected Gaussian user interference network w ..."
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Cited by 75 (23 self)
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Abstract—An interference alignment example is constructed for the deterministic channel model of the user interference channel. The deterministic channel example is then translated into the Gaussian setting, creating the first known example of a fully connected Gaussian user interference network with single antenna nodes, real, nonzero and constant channel coefficients, and no propagation delays where the degrees of freedom outerbound is achieved. An analogy is drawn between the propagation delay based interference alignment examples and the deterministic channel model which also allows similar constructions for the user channel as well. I.
Aiming perfectly in the dark  blind interference alignment through staggered antenna switching
 IEEE Transactions on Signal Processing
, 2011
"... We propose a blind interference alignment scheme for the vector broadcast channel where the transmitter is equipped with M antennas and there are K receivers, each equipped with a reconfigurable antenna capable of switching among M preset modes. Without any knowledge of the channel coefficient value ..."
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Cited by 48 (7 self)
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We propose a blind interference alignment scheme for the vector broadcast channel where the transmitter is equipped with M antennas and there are K receivers, each equipped with a reconfigurable antenna capable of switching among M preset modes. Without any knowledge of the channel coefficient values at the transmitters and with only mild assumptions on the channel coherence structure we show that MKM+K−1 degrees of freedom are achievable. The key to the blind interference alignment scheme is the ability of the receivers to switch between reconfigurable antenna modes to create short term channel fluctuation patterns that are exploited by the transmitter. The achievable scheme does not require cooperation between transmit antennas and is therefore applicable to the M ×K X network as well. Only finite symbol extensions are used, and no channel knowledge at the receivers is required to null the interference. ar X iv
Real Interference Alignment with Real Numbers
, 2009
"... A novel coding scheme applicable in networks with single antenna nodes is proposed. This scheme converts a single antenna system to an equivalent Multiple Input Multiple Output (MIMO) system with fractional dimensions. Interference can be aligned along these dimensions and higher Multiplexing gains ..."
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Cited by 46 (2 self)
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A novel coding scheme applicable in networks with single antenna nodes is proposed. This scheme converts a single antenna system to an equivalent Multiple Input Multiple Output (MIMO) system with fractional dimensions. Interference can be aligned along these dimensions and higher Multiplexing gains can be achieved. Tools from the field of Diophantine approximation in number theory are used to show that the proposed coding scheme in fact mimics the traditional schemes used in MIMO systems where each data stream is sent along a direction and alignment happens when several streams arrive at the same direction. Two types of constellation are proposed for the encoding part, namely the single layer constellation and the multilayer constellation. Using the single layer constellation, the coding scheme is applied to the twouser X channel and the threeuser Gaussian Interference Channel (GIC). In case of the twouser X channel, it is proved that the total DegreesofFreedom (DOF), i.e. 4, of 3 the channel is achievable almost surely. This is the first example in which it is shown that a time invariant single antenna system does not fall short of achieving its total DOF. For the threeuser GIC, it is shown that the DOF of 4 is achievable almost surely. 3 Using the multilayer constellation, the coding scheme is applied to the symmetric threeuser GIC. Achievable DOFs are derived for all channel gains. As a function of the channel gain, it is observed that the DOF is everywhere discontinuous. In particular, it is proved that for the irrational channel gains the achievable DOF meets the upper bound 3. For the rational gains, 2 the achievable DOF has a gap to the available upper bounds. By allowing carry over from multiple layers, however, it is shown that higher DOFs can be achieved.
Cellular Interference Alignment with Imperfect Channel Knowledge
"... Abstract—Interference alignment is evaluated as a technique to mitigate intercell interference in the downlink of a cellular network using OFDMA. The sum mutual information achieved by interference alignment together with a zeroforcing receiver is considered, and upper and lower bounds are derived ..."
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Cited by 39 (5 self)
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Abstract—Interference alignment is evaluated as a technique to mitigate intercell interference in the downlink of a cellular network using OFDMA. The sum mutual information achieved by interference alignment together with a zeroforcing receiver is considered, and upper and lower bounds are derived for the case of imperfect channel knowledge. The sum mutual information achieved by interference alignment when the base stations share their information about the channels is shown to compare favorably to the achievable sumrate of methods where the base stations do not cooperate, even under moderately accurate knowledge of the channel state. I.
Subspace alignment chains and the degrees of freedom of the threeuser MIMO interference channel
 IEEE Trans. Inf. Theory
, 2014
"... We show that the 3user MT ×MR MIMO interference channel where each transmitter is equipped with MT antennas and each receiver is equipped with MR antennas has d(M,N) 4 ..."
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Cited by 36 (12 self)
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We show that the 3user MT ×MR MIMO interference channel where each transmitter is equipped with MT antennas and each receiver is equipped with MR antennas has d(M,N) 4
On the Degrees of Freedom of Finite State Compound Wireless Networks  Settling a Conjecture by Weingarten et. al.
, 2009
"... We explore the degrees of freedom (DoF) of three classes of finite state compound wireless networks in this paper. First, we study the multipleinput singleoutput (MISO) finite state compound broadcast channel (BC) with arbitrary number of users and antennas at the transmitter. In prior work, Weing ..."
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Cited by 32 (19 self)
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We explore the degrees of freedom (DoF) of three classes of finite state compound wireless networks in this paper. First, we study the multipleinput singleoutput (MISO) finite state compound broadcast channel (BC) with arbitrary number of users and antennas at the transmitter. In prior work, Weingarten et. al. have found inner and outer bounds on the DoF with 2 users. The bounds have a different character. While the inner bound collapses to unity as the number of states increases, the outer bound does not diminish with the increasing number of states beyond a threshold value. It has been conjectured that the outer bound is loose and the inner bound represents the actual DoF. In the complex setting (all signals, noise, and channel coefficients are complex variables) we solve a few cases to find that the outer bound – and not the inner bound – of Weingarten et. al. is tight. For the real setting (all signals, noise and channel coefficients are real variables) we completely characterize the DoF, once again proving that the outer bound of Weingarten et. al. is tight. We also extend the results to arbitrary number of users. Second, we characterize the DoF of finite state scalar (single antenna nodes) compound X networks with arbitrary number of users in the real setting. Third, we characterize the DoF of finite state scalar compound interference networks with arbitrary number of users in both the real and complex setting. The key finding is that scalar interference networks and (real) X networks do not lose any DoF due to channel uncertainty at the transmitter in the finite state compound setting. The finite state compound MISO BC does lose DoF relative to the perfect CSIT scenario. However, what is lost is only the DoF benefit of joint processing at transmit antennas, without which the MISO BC reduces to an X network.