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68
User Cooperation Diversity  Part I: System Description
 IEEE TRANS. COMMUN
, 1998
"... Mobile users' data rate and quality of service are limited by the fact that, within the duration of any given call, they experience severe variations in signal attenuation, thereby necessitating the use of some type of diversity. In this twopart paper, we propose a new form of spatial diver ..."
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Cited by 669 (22 self)
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Mobile users' data rate and quality of service are limited by the fact that, within the duration of any given call, they experience severe variations in signal attenuation, thereby necessitating the use of some type of diversity. In this twopart paper, we propose a new form of spatial diversity, in which diversity gains are achieved via the cooperation of mobile users. Part I describes the user cooperation strategy while Part II focuses on implementation issues and performance analysis. Results show that, even though the interuser channel is noisy, cooperation leads not only to an increase in capacity for both users but also to a more robust system, where users' achievable rates are less susceptible to channel variations.
Exploiting Decentralized Channel State Information for Random Access
, 2002
"... We study the use of channel state information for random access in fading channels. Traditionally, random access protocols have been designed by assuming simple models for the physical layer where all users are symmetric and there is no notion of channel state. We introduce a reception model that ta ..."
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Cited by 82 (18 self)
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We study the use of channel state information for random access in fading channels. Traditionally, random access protocols have been designed by assuming simple models for the physical layer where all users are symmetric and there is no notion of channel state. We introduce a reception model that takes into account the channel states of various users. Under the assumption that each user has access to his channel state information (CSI), we propose a variant of Slotted ALOHA protocol for medium access control, where the transmission probability is allowed to be a function of the CSL The function is called the transmission control scheme. Assuming the finite user infinite buffer model we derive expressions for the maximum stable throughput of the system. We introduce the notion of asymptotic stable throughput (AST) that is the maximum stable throughput as the number of users goes to infinity. We consider two types of transmission control namely population independent transmission control (PITC) where the transmission control is not a function of the size of the network and population dependent transmission control where the transmission control is a function of the size of the network. We obtain expressions for the AST achievable with PITC. For population dependent transmission control, we introduce a particular transmission control that can potentially lead to significant gains in AST. For both PITC and PDTC, we show that the effect of transmission control is equivalent to changing the probability distribution of the channel state. The theory is then applied to CDMA networks with Linear Minimum Mean Square Error (LMMSE) receivers and Matched Filters (MF) to illustrate the effectiveness of utilizing channel state. It is shown that through the use of channel state, with an...
Optimal Energy Allocation for DelayConstrained Data Transmission over a TimeVarying Channel
, 2003
"... We seek to maximize the data throughput of an energy and time constrained transmitter sending data over a fading channel. The transmitter has a fixed amount of energy and a limited amount of time to send data. Given that the channel fade state determines the throughput obtained per unit of energy ex ..."
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Cited by 39 (5 self)
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We seek to maximize the data throughput of an energy and time constrained transmitter sending data over a fading channel. The transmitter has a fixed amount of energy and a limited amount of time to send data. Given that the channel fade state determines the throughput obtained per unit of energy expended, the goal is to obtain a policy for scheduling transmissions that maximizes the expected data throughput. We develop a dynamic programming formulation that leads to an optimal closedform transmission schedule. We then extend our approach to the problem of minimizing the energy required to send a fixed amount of data over a fading channel given deadline constraints.
Optimal Transmission Scheduling over a Fading Channel with Energy and Deadline Constraints
 IEEE Trans. Wireless Communications
, 2004
"... We seek to maximize the data throughput of an energy and time constrained transmitter sending data over a fading channel. The transmitter has a fixed amount of energy and a limited amount of time to send data. ..."
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Cited by 28 (0 self)
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We seek to maximize the data throughput of an energy and time constrained transmitter sending data over a fading channel. The transmitter has a fixed amount of energy and a limited amount of time to send data.
MIMO Multiple Access Channel with an Arbitrarily Varying Eavesdropper: Secrecy degrees of freedom
 IEEE TRANSACTIONS ON INFORMATION THEORY, FEBRUARY
, 2013
"... A twotransmitter Gaussian multiple access wiretap channel with multiple antennas at each of the nodes is investigated. The channel matrices of the legitimate users are fixed and revealed to all the terminals, whereas the channel matrices of the eavesdropper are arbitrarily varying and only known t ..."
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Cited by 13 (5 self)
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A twotransmitter Gaussian multiple access wiretap channel with multiple antennas at each of the nodes is investigated. The channel matrices of the legitimate users are fixed and revealed to all the terminals, whereas the channel matrices of the eavesdropper are arbitrarily varying and only known to the eavesdropper. The secrecy degrees of freedom (s.d.o.f.) region under a strong secrecy constraint is characterized. A transmission scheme that orthogonalizes the transmit signals of the two users at the intended receiver, and uses a singleuser wiretap code for each user, is shown to achieve the s.d.o.f. region. The converse involves establishing an upper bound on a weightedsumrate expression. This is accomplished by using induction, where at each step one combines the secrecy and multipleaccess constraints associated with an adversary eavesdropping a carefully selected group of subchannels.
Delay considerations for opportunistic scheduling in broadcast fading channels
 IEEE Trans. Wireless Commun
, 2007
"... We consider a singleantenna broadcast block fading channel with users where the transmission is packetbased. We define the (packet) delay as the minimum number of channel uses that guarantees all users successfully receive packets. This is a more stringent notion of delay than average delay and is ..."
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Cited by 13 (1 self)
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We consider a singleantenna broadcast block fading channel with users where the transmission is packetbased. We define the (packet) delay as the minimum number of channel uses that guarantees all users successfully receive packets. This is a more stringent notion of delay than average delay and is the worst case (access) delay among the users. A delay optimal scheduling scheme, such as roundrobin, achieves the delay of. For the opportunistic scheduling (which is throughput optimal) where the transmitter sends the packet to the user with the best channel conditions at each channel use, we derive the mean and variance of the delay for any and. For large and in a homogeneous network, it is proved that the expected delay in receiving one packet by all the receivers scales as, as opposed to for the roundrobin scheduling. We also show that when grows faster than, for some, then the delay scales as. This roughly determines the timescale required for the system to behave fairly in a homogeneous network. We then propose a scheme to significantly reduce the delay at the expense of a small throughput hit. We further look into the advantage of multiple transmit antennas on the delay. For a system with antennas in the transmitter where at each channel use packets are sent to different users, we obtain the expected delay in receiving one packet by all the users. Index terms: broadcast channel, fading, opportunistic scheduling, packet delay, longest queue. 1
Multiple access outerbounds and the inseparability of parallel interference channels
 IEEE Trans. Inf. Theory
"... Abstract — It is known that the capacity of parallel (multicarrier) Gaussian pointtopoint, multiple access and broadcast channels can be achieved by separate encoding for each subchannel (carrier) subject to a power allocation across carriers. In this paper we show that such a separation does no ..."
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Cited by 12 (4 self)
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Abstract — It is known that the capacity of parallel (multicarrier) Gaussian pointtopoint, multiple access and broadcast channels can be achieved by separate encoding for each subchannel (carrier) subject to a power allocation across carriers. In this paper we show that such a separation does not apply to parallel Gaussian interference channels in general. A counterexample is provided in the form of a 3 user interference channel where separate encoding can only achieve a sum capacity of log(SNR)+o(log(SNR)) per carrier while the actual capacity, achieved only by jointencoding across carriers, is 3/2 log(SNR)) + o(log(SNR)) per carrier. As a byproduct of our analysis, we propose a class of multipleaccessouterbounds on the capacity of the 3 user interference channel. I.
The ODE method and spectral theory of Markov operators
 IN STOCHASTIC THEORY AND CONTROL (LAWRENCE, KS, 2001), SER. LECTURE NOTES IN CONTROL AND INFORMATION SCIENCES
, 2002
"... We give a development of the ODE method for the analysis of recursive algorithms described by a stochastic recursion. With variability modeled via an underlying Markov process, and under general assumptions, the following results are obtained: (i) Stability of an associated ODE implies that the stoc ..."
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Cited by 11 (3 self)
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We give a development of the ODE method for the analysis of recursive algorithms described by a stochastic recursion. With variability modeled via an underlying Markov process, and under general assumptions, the following results are obtained: (i) Stability of an associated ODE implies that the stochastic recursion is stable in a strong sense when a gain parameter is small. (ii) The range of gainvalues is quantified through a spectral analysis of an associated linear operator, providing a nonlocal theory, even for nonlinear systems. (iii) A secondorder analysis shows precisely how variability leads to sensitivity of the algorithm with respect to the gain parameter. All results are obtained within the natural operatortheoretic framework of geometrically ergodic Markov processes.