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51
On compound channels with sideinformation at the transmitter
 IEEE TRANS. INF. THEORY
, 2006
"... Costa has proved that for noncausally known Gaussian interference at a power constrained transmitter communicating over an additive white Gaussian noise channel there is no capacity loss when compared to a scenario where interference is not present. For the case of a transmitter communicating over a ..."
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Cited by 14 (2 self)
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Costa has proved that for noncausally known Gaussian interference at a power constrained transmitter communicating over an additive white Gaussian noise channel there is no capacity loss when compared to a scenario where interference is not present. For the case of a transmitter communicating over a quasistatic (i.e., nonergodic) fading channel, his method does not apply. In this correspondence, we derive upper and lower bounds on the capacity of compound channels with side information at the transmitter, first for finite alphabet channels and then, based on this result, for channels on standard alphabets (this includes real alphabets). For the special case of a degenerate compound channel with only one possible realization, our bounds are equivalent to the wellknown capacity with sideinformation formula of Gel’fand and Pinsker. For the quasistatic fading channel, when fading is Ricean, we suggest a scheme based on our lower bound for which the performance is found to be relatively good even for moderatefactor. As, the uncertainty on the channel vanishes and our scheme obtains the performance of dirty paper coding, namely that the interference is perfectly mitigated. As, the proposed scheme treats the interferer as additional noise. These results may be of importance for the emerging field of cognitive radios where one user may be aware of another user’s intended message to a common receiver, but is unaware of the channel path gains.
Low Complexity Multiuser Scheduling for Maximizing Throughput
 in the MIMO Broadcast Channel,” in Proc. Allerton Conf. Commun., Contr., and Computing
, 2004
"... We consider the general multiuser downlink scheduling problem with n receivers and m transmit antennas, where data from different users can be multiplexed. It is shown that there is a throughputoptimal policy which selects a subset of users to multiplex in each scheduling interval, according to the ..."
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Cited by 14 (2 self)
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We consider the general multiuser downlink scheduling problem with n receivers and m transmit antennas, where data from different users can be multiplexed. It is shown that there is a throughputoptimal policy which selects a subset of users to multiplex in each scheduling interval, according to their queue states and current rates achievable by dirtypaper coding. However, the computation of these rates and the selection of the best ordered subset has high complexity in n. It is observed that the simpler multiplexing method of zeroforcing can achieve close to optimal throughput especially when user rate demands are asymmetric. The strong dependence of the objective function with zeroforcing precoding on the queue sizes motivates an algorithm based on ordering users according to queues and making a selection out of a reduced set of users (typically a few multiples of m), which greatly reduces the computational complexity. It is shown that searching beyond this small set is unlikely to improve the objective function. 1
On ergodic sum capacity of fading cognitive multipleaccess and broadcast channels
 IEEE Trans. Inf. Theory. Available [Online
"... ..."
Degrees of freedom in wireless multiuser spatial multiplex systems with multiple antennas
 IEEE Transactions on Communications
, 2006
"... Abstract—This letter investigates the structure of the optimal spatial multiplex scheme in a multiuser multiantenna wireless fading environment. Based on a sumcapacity criterion, this letter shows that the optimal transmission strategy in an uplink or downlink channel with n antennas at the basest ..."
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Cited by 10 (0 self)
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Abstract—This letter investigates the structure of the optimal spatial multiplex scheme in a multiuser multiantenna wireless fading environment. Based on a sumcapacity criterion, this letter shows that the optimal transmission strategy in an uplink or downlink channel with n antennas at the basestation involves more than n users at the same time. In particular, when remote users are equipped with m antennas each, the maximum number of data streams is shown to be upper bounded by n 2, with each user transmitting or receiving up to m 2 data streams. This gives a dimensioncounting interpretation for multiuser diversity. Multiple antennas at the basestation increases the total number of dimensions, thus allowing more users to transmit and receive at the same time. By contrast, multiple antennas at the remote terminal allow a single user to occupy multiple dimensions, which increases its transmission rate, but also has the potential effect of precluding simultaneous transmission by other users. Index Terms—Broadcast channel, multipleaccess channel, multipleinput multipleoutput (MIMO) system, multiuser diversity, spatial diversity, spatial multiplex. I.
Practical Costa precoding for the multiple antenna broadcast channel
 IEEE Glob. Telecom. Conf
, 2004
"... Abstract — For a multiple antenna broadcast channel, the sumrate capacity achieving transmit strategy requires the centralized transmitter to simultaneously communicate with multiple receivers. The objective of this paper is to design an implementable sumrate capacity achieving transmit strategy th ..."
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Cited by 8 (2 self)
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Abstract — For a multiple antenna broadcast channel, the sumrate capacity achieving transmit strategy requires the centralized transmitter to simultaneously communicate with multiple receivers. The objective of this paper is to design an implementable sumrate capacity achieving transmit strategy that uses a combination of beamforming and coding for known interference. For a twouser Gaussian broadcast channel, results indicate that in the context of typical QAM constellations, with Mt = 2 and Mr = 1, there is significant gain in sumrate capacity over an approach that uses only beamforming. 1 I.
Input optimization for multiantenna broadcast channels and perantenna power constraints
 in Proc. IEEE GLOBECOM
, 2004
"... Abstract — This paper considers a Gaussian multiantenna broadcast channel with individual power constraints on each antenna, rather than the usual sum power constraint over all antennas. Perantenna power constraints are more realistic because in practical implementations each antenna has its own p ..."
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Cited by 7 (0 self)
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Abstract — This paper considers a Gaussian multiantenna broadcast channel with individual power constraints on each antenna, rather than the usual sum power constraint over all antennas. Perantenna power constraints are more realistic because in practical implementations each antenna has its own power amplifier. The main contribution of this paper is a new derivation of the duality result for this class of broadcast channels that allows the input optimization problem to be solved efficiently. Specifically, we show that uplinkdownlink duality is equivalent to Lagrangian duality in minimax optimization, and the dual multipleaccess problem has a much lower computational complexity than the original problem. This duality applies to the entire capacity region. Further, we derive a novel application of Newton’s method for the dual minimax problem that finds an optimal search direction for both the minimization and the maximization problems at the same time. This new computational method is much more efficient than the previous iterative waterfillingbased algorithms and it is applicable to the entire capacity region. Finally, we show that the previous QRbased precoding method can be easily modified to accommodate the perantenna constraint. I.
Resource allocation under uncertainty using the maximum entropy principle
 IEEE TRANSACTIONS ON INFORMATION THEORY
, 2005
"... In this paper, we formulate and solve a problem of resource allocation over a given time horizon with uncertain demands and uncertain capacities of the available resources. In particular, we consider a number of data sources with uncertain bit rates, sharing a set of parallel channels with timevary ..."
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Cited by 6 (3 self)
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In this paper, we formulate and solve a problem of resource allocation over a given time horizon with uncertain demands and uncertain capacities of the available resources. In particular, we consider a number of data sources with uncertain bit rates, sharing a set of parallel channels with timevarying and possibly uncertain transmission capacities. We present a method for allocating the channels so as to maximize the expected system throughput. The framework encompasses qualityofservice (QoS) requirements, e.g., minimumrate constraints, as well as priorities represented by a userspecific cost per transmitted bit. We assume only limited statistical knowledge of the source rates and channel capacities. Optimal solutions are found by using the maximum entropy principle and elementary probability theory. The suggested framework explains how to make use of multiuser diversity in various settings, a field of recently growing interest in communication theory. It admits scheduling over multiple base stations and includes transmission buffers to obtain a method for optimal resource allocation in rather general multiuser communication systems.
Balanced Capacity of Wireline Multiuser Channels
, 2005
"... This paper analyzes the multiuser capacity of Gaussian frequencyselective wireline multiaccess channels. Both the uplink (multipleaccess channel) and downlink (broadcast channel) capacity regions are considered. The concept of balanced capacity is introduced to characterize the multiuser channel p ..."
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Cited by 6 (0 self)
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This paper analyzes the multiuser capacity of Gaussian frequencyselective wireline multiaccess channels. Both the uplink (multipleaccess channel) and downlink (broadcast channel) capacity regions are considered. The concept of balanced capacity is introduced to characterize the multiuser channel performance. Algorithms for the computation of the balanced multiuser capacity (and the associated power allocations) are proposed for an arbitrary number of users. The optimal power allocation in auser memoryless Gaussian channel is analyzed in detail, and an extension to intersymbol interference channels is given with various kinds of power constraints. Results are provided for a wireline access network with 20 users.
A Distributed Scheme for Achieving EnergyDelay Tradeoffs With Multiple Service Classes over a Dynamically Varying Network
"... We consider a dynamical probabilistic traffic model for the number of users transmitting at any time. This model captures both user mobility and traffic burstiness. Moreover, we assume no centralized controller, such as a scheduler, is available. When multiple users transmit simultaneously, multiple ..."
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Cited by 6 (0 self)
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We consider a dynamical probabilistic traffic model for the number of users transmitting at any time. This model captures both user mobility and traffic burstiness. Moreover, we assume no centralized controller, such as a scheduler, is available. When multiple users transmit simultaneously, multiple access interference affects throughput considerably. Most queue control schemes assume individual users know the states of their own queues (local queue information) along with the states of other users queues (shared queue information) and address issues of scheduling; but this sharing of information may be onerous in a practical system. While shared queue information has recently been shown [1] not to affect the capacity of such systems, it has a considerable impact on delay. We introduce a scheme, where for each user, a bit of shared queue information specifies whether its queue length is above or below a threshold. Our scheme relies on two different service classes implemented through a superposition coding scheme (first proposed in [2], further studied and expanded in [1]). The first class experiences no delay due to multiple access interference while second class requires retransmissions when such an event occurs. We show how our scheme affords an energydelay tradeoff. Moreover, when configured properly, our scheme can be can attain boundary points of the region corresponding to minimum energy with no shared queue information for 0 delay along with minimum energy subject to system stability. We derive bounds on the performance of the multiple access system using our proposed scheme by introducing Lyapunov function bounds in a manner similar to [3].
Fundamental Limits in MIMO Broadcast Channels
 IEEE J. Sel. Areas Commun
, 2007
"... Abstract — This paper studies the fundamental limits of MIMO broadcast channels from a high level, determining the sumrate capacity of the system as a function of system paramaters, such as the number of transmit antennas, the number of users, the number of receive antennas, and the total transmit ..."
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Abstract — This paper studies the fundamental limits of MIMO broadcast channels from a high level, determining the sumrate capacity of the system as a function of system paramaters, such as the number of transmit antennas, the number of users, the number of receive antennas, and the total transmit power. The crucial role of channel state information at the transmitter is emphasized, as well as the emergence of opportunistic transmission schemes. The effects of channel estimation errors, training, and spatial correlation are studied, as well as issues related to fairness, delay and differentiated rate scheduling. Index Terms — MIMO broadcast channels, sumrate capacity, asymptotics, channel state information. I.