Defne Aktas

Distributed Downlink Beamforming With Cooperative Base Stations

In this paper, we consider multicell processing on the downlink of a cellular network to accomplish ldquomacrodiversityrdquo transmit beamforming. The particular downlink beamformer structure we consider allows a recasting of the downlink beamforming problem as a virtual linear mean square error (LMMSE) estimation problem. We exploit the structure of the channel and develop distributed beamforming algorithms using local message passing between neighboring base stations. For 1-D networks, we use the Kalman smoothing framework to obtain a forward-backward beamforming algorithm. We also propose a limited extent version of this algorithm that shows that the delay need not grow with the size of the network in practice. For 2-D cellular networks, we remodel the network as a factor graph and present a distributed beamforming algorithm based on the sum-product algorithm. Despite the presence of loops in the factor graph, the algorithm produces optimal results if convergence occurs.

Distributed space-time filtering for cooperative wireless networks

Significant performance gains can be leveraged in wireless networks by allowing the different nodes to cooperate. Cooperative transmission strategies attempt to realize the performance gains possible in multi-input multi-output (MIMO) fading channels by modeling the cooperating nodes as virtual antennas. However, in contrast to the point-to-point MIMO scenario, efficient cooperative schemes must address the distributed implementation challenge. For example, individual cooperating nodes may not be aware of their partners. We formulate the problem of maximizing the diversity advantage subject to the constraint of unknown message state information at the cooperating transmitter(s). We argue that our formulation can be used to model different relevant scenarios in wireless networks (e.g., fault tolerant applications, energy efficient sensor networks). In this setting, we propose a novel space-time filtering (STF) approach that achieves the optimal tradeoff between diversity advantage and receiver complexity. We further compare this approach with existing space-time coding approaches, highlighting the benefits of STF in the distributed implementation setting. Our arguments are supported by simulation results that demonstrate the performance gains possible with the proposed scheme in certain representative scenarios.

Distance spectrum analysis of space-time trellis-coded Modulations in quasi-static Rayleigh-fading channels

In this correspondence, we propose an algorithm for computing the distance spectrum of a space-time trellis code achieving maximal diversity gain in quasi-static fading channels. We further present a state reduction technique for trellis codes that can reduce the complexity of the distance spectrum computation. We provide numerical results supporting the empirical evidence that a truncated union bound obtained from the distance spectrum provides an accurate characterization of the relative performance ordering of different space-time trellis codes and, therefore, it offers a tool for better space-time trellis code design.

Multiuser scheduling for MIMO wireless systems

Significant throughput gains can be leveraged in multiuser wireless systems by taking advantage of the spatial diversity provided by multiple antennas at the transmitter and/or receiver and the multiuser diversity inherent in the system. For delay tolerant data applications, this gain in system throughput can be realized through a scheduling scheme where the resources are allocated to the user experiencing the most favorable channel. In this paper, we investigate scheduling algorithms for the downlink of a multiple input multiple output (MIMO) multiuser wireless system. We propose a novel scheduling scheme that exploits spatial and multiuser diversities available in the channel. We provide numerical results demonstrating that by allowing more bits in the feedback channel, the proposed scheme enjoys significant performance gains over the existing schemes with comparable complexity. Furthermore, compared to schemes requiring perfect channel information at the base station, the proposed scheme has less complexity at the expense of a loss in throughput.

On the design and maximum-likelihood decoding of space-time trellis codes

In this letter, we present a simple generalization of the maximum ratio combining principle for space-time coded systems. This result leads to a maximum-likelihood decoder implementation that does not depend on the number of receive antennas and avoids the loss in performance incurred in the decoders proposed by Tarokh and Lo (1998) and Biglieri et al. The insights offered by this decoding rule allow for a simple and elegant proof for the space-time code design criterion in systems with large number of receive antennas. We further present an upper bound on probability of error that captures the dependence of space-time code design on the number of receive antennas. Finally, we present a computationally efficient approach for constructing space-time trellis codes that exhibit satisfactory performance in systems with variable number of receive antennas.

Noncoherent space-time coding: An algebraic perspective

The design of space-time signals for noncoherent block-fading channels where the channel state information is not known a priori at the transmitter and the receiver is considered. In particular, a new algebraic formulation for the diversity advantage design criterion is developed. The new criterion encompasses, as a special case, the well-known diversity advantage for unitary space-time signals and, more importantly, applies to arbitrary signaling schemes and arbitrary channel distributions. This criterion is used to establish the optimal diversity-versus-rate tradeoff for training based schemes in block-fading channels. Our results are then specialized to the class of affine space-time signals which allows for a low complexity decoder. Within this class, space-time constellations based on the threaded algebraic space-time (TAST) architecture are considered. These constellations achieve the optimal diversity-versus-rate tradeoff over noncoherent block-fading channels and outperform previously proposed codes in the considered scenarios as demonstrated by the numerical results. Using the analytical and numerical results developed in this paper, nonunitary space-time codes are argued to offer certain advantages in block-fading channels where the appropriate use of coherent space-time codes is shown to offer a very efficient solution to the noncoherent space-time communication paradigm.

Downlink Beamforming Under Individual SINR and Per Antenna Power Constraints

In this paper we consider the problem of finding the optimum beamforming vectors for the downlink of a multiuser system, where there are individual signal to interference plus noise ratio (SINR) targets for each user. Majority of the previous work on this problem assumed a total power constraint on the base stations. However, since each transmit antenna is limited by the amount of power it can transmit due to the limited linear region of the power amplifiers, a more realistic constraint is to place a limit on the per antenna power. Yu and Lan proposed an iterative algorithm for computing the optimum beamforming vectors minimizing the power margin over all antennas under individual SINR and per antenna power constraints. However, from a system designer point of view, it may be more desirable to minimize the total transmit power rather than minimizing the power margin, especially when the system is not symmetric. Reformulating the transmitter optimization problem to minimize the total transmit power subject to individual SINR constraints on the users and per antenna power constraints on the base stations, the algorithm proposed by Yu and Lan is modified. Performance of the modified algorithm is compared with existing methods for various cellular array scenarios.

Coherent space-time codes for noncoherent channels

A new algebraic formulation for the diversity advantage design criterion for arbitrary space-time signals in noncoherent block fading channels is developed. It is shown that the new criterion encompasses, as a special case, the well-known diversity advantage criterion for unitary space-time signaling. Using the proposed criterion, the optimal diversity-vs-rate tradeoff is derived for training based noncoherent signaling schemes. Our results are then specialized to the class of affine space-time signals which allow for an efficient polynomial complexity decoder. Within this class, new space-time constellations based on the threaded algebraic space-time (TAST) framework are proposed. These codes achieve the optimal diversity-vs-rate tradeoff and outperform previously proposed codes in the considered scenarios as demonstrated by numerical results. Using these analytical and numerical results, we argue that non-unitary space-time codes offer certain advantages in block fading channels and the appropriate use of coherent space-time codes is shown to offer a very efficient solution to the noncoherent space-time communication paradigm.

Multiuser scheduling for multiple antenna systems

Significant throughput gains can be leveraged in multiuser wireless systems by taking advantage of the spatial diversity provided by multiple antennas and the multiuser diversity inherent in the system. For delay tolerant data applications, this gain in system throughput can be realized through a scheduling scheme where the resources are allocated to the user experiencing the most favorable channel. In this paper, we investigate scheduling algorithms for the downlink of a multiple antenna multiuser wireless system. We propose a novel scheduling scheme that exploits spatial and multiuser diversities available in the channel. We provide numerical results demonstrating that by allowing more bits in the feedback channel, the proposed scheme enjoys significant performance gains over the existing schemes with comparable complexity. Furthermore, compared to schemes requiring perfect channel information at the base station, the proposed scheme has less complexity at the expense of a loss in throughput.

On capacity and coding for segmented deletion channels

We consider binary deletion channels with a segmentation assumption which appears to be suited for more practical scenarios. Unlike the binary independent and identically distributed (i.i.d.) deletion channel where each bit is independently deleted with an equal probability, the segmentation assumption prohibits certain transmitted bits to be deleted, i.e., in a block of bits of a certain length, only a limited number of deletions can occur. We first propose several upper and lower capacity bounds for the segmented deletion channel. Then we focus on an interleaved concatenation of an outer low-density parity check (LDPC) code with error-correction capabilities and an inner marker code with synchronization capabilities over these channels. With the help of a specifically designed maximum-a-posteriori (MAP) detector, we demonstrate reliable transmission at higher code rates than the existing ones reported in the literature.