Mohammad Madihian

A flexible downlink scheduling scheme in cellular packet data systems

Fast downlink scheduling algorithms play a central role in determining the overall performance of high-speed cellular data systems, characterized by high throughput and fair resource allocation among multiple users. We propose a flexible channel-dependent downlink scheduling scheme, named the (weighted) alpha-rule, based on the system utility maximization that arises from the Internet economy of long-term bandwidth sharing among elastic-service users. We show that the utility as a function of per-user mean throughput naturally derives the alpha-rule scheme and a whole set of channel-dependent instantaneous scheduling schemes following different fairness criteria. We evaluate the alpha-rule in a multiuser CDMA high data rate (HDR) system with space-time block coding (STBC) or Bell Labs layered space-time (BLAST) multiple-input multiple-output (MIMO) channel. Our evaluation shows that it works efficiently by enabling flexible tradeoff between aggregate throughput, per-user throughput, and per-user resource allocation through a single control parameter. In other words the Alpha-rule effectively fills the performance gap between existing scheduling schemes, such as max-C/I and proportional fairness (PF), and provides an important control knob at the media-access-control (MAC) layer to balance between multiuser diversity gain and location-specific per-user performance.

Multi-hop wireless backhaul networks: a cross-layer design paradigm

Multihop wireless backhual networks are emerging as a cost-effective solution to provide ubiquitous and broadband access to meet the rapidly increasing demands of multimedia applications. In this paper, we consider the joint optimal design of routing, medium access control (MAC) scheduling and physical layer resource allocation for such networks, where beamforming antenna arrays are equipped at the physical layer. The notion of transmission set (TS) is introduced to separate the physical layer operations from those at the upper layers; and a column generation approach is employed to efficiently identify the TSs. We then apply the dual decomposition method to decouple the routing and scheduling subproblems, which are performed at different layers and are coordinated by a pricing mechanism to achieve the optimal overall system objective. To efficiently support multimedia traffic, an admission control criterion is considered for the system objective. The performance of the proposed scheme is verified by simulation results, and the impact of the physical layer capabilities on the network performance is evaluated. We also discuss the implementation issues of the cross-layer scheme based on the IEEE 802.16 mesh mode.

LDPC-coded cooperative relay systems: performance analysis and code design

We treat the problem of designing low-density parity-check (LDPC) codes to approach the capacity of relay channels. We consider an efficient analysis framework that decouples the factor graph (FG) of a B-block transmission into successive partial FGs, each of which denotes a two-block transmission. We develop design methods to find the optimum code ensemble for the partial FG. In particular, we formulate the relay operations and the destination operations as equivalent virtual MISO and MIMO systems, and employ a binary symmetric channel (BSC) model for the relay node output. For AWGN channels, we further develop a Gaussian approximation for the detector output at the destination node. Jointly treating the relay and the destination, we analyze the performance of the LDPC-coded relay system using the extrinsic mutual information transfer(EXIT) chart technique. Furthermore, differential evolution is employed to search for the optimum code ensemble. Our results show that the optimized codes always outperform the regular LDPC codes with a significant gain; in the AWGN case, when Protocol-II is employed and the relay is close to the source, the optimized code performs within 0.1dB to the capacity bound.

Design of Rate-Compatible Irregular Repeat Accumulate Codes

We consider the design of efficient rate-compatible (RC) irregular repeat accumulate (IRA) codes over a wide code rate range. The goal is to provide a family of RC codes to achieve high throughput in hybrid automatic repeat request (ARQ) scheme for high-speed data packet wireless systems. As a subclass of low-density parity-check codes, IRA codes have an extremely simple encoder and a low-complexity decoder while providing capacity approaching performance. We focus on a hybrid design method which employs both puncturing and extending. We propose a simple puncturing method based on minimizing the maximal recoverable step of the punctured nodes. We also propose a new extending scheme for IRA codes by introducing the degree-1 parity bits for the lower rate codes and obtaining the optimal proportions of extended nodes through density evolution analysis. The throughput performance of the designed RC-IRA codes in hybrid ARQ is evaluated for both AWGN and block fading channels. Simulation results demonstrate that our designed RC codes offer good error correction performance over a wide rate range and provide high throughput, especially in the high and low signal-to-noise ratio regions.

Optimal resource allocation in multi-hop OFDMA wireless networks with cooperative relay

We consider an optimal resource allocation strategy for cooperative relaying-enabled OFDMA multi-hop wireless networks. A cross-layer optimization problem is formulated that maximizes the balanced end-to-end throughput under the routing and the PHY/MAC constraints. A dual method is employed to solve the problem efficiently and optimally. A cooperative relaying technique is incorporated into the framework by introducing virtual links and nodes. Half-duplex operation of the radios is assumed, and mutual interference between the links is explicitly modeled to allow maximal spatial reuse of the spectral resources. Numerical results are provided to demonstrate how the bottleneck phenomenon typical in multi-hop networks can be alleviated by the proposed technique to yield significant improvement in the throughput.

Cross-Layer Design of Wireless Multihop Backhaul Networks With Multiantenna Beamforming

A cross-layer design approach is considered for joint routing and resource allocation for the physical (PHY) and the medium access control (MAC) layers in multihop wireless backhaul networks. The access points (APs) are assumed to be equipped with multiple antennas capable of both transmit and receive beamforming. A nonlinear optimization problem is formulated, which maximizes the fair throughput of the APs in the network under the routing and the PHY/MAC constraints. Dual decomposition is employed to decouple the original problem into smaller subproblems in different layers, which are coordinated by the dual prices. The network layer subproblem can be solved in a distributed manner and the PHY layer subproblem in a semidistributed manner. To solve the PHY layer subproblem, an iterative minimum mean square error (IMMSE) algorithm is used with the target link signal-to-interference-and-noise-ratio (SINR) set dynamically based on the price generated from the upper layers. A scheduling heuristic is also developed, which improves the choice of the transmission sets over time. Simulation results illustrate the efficacy of the proposed cross-layer design.

Distributed Joint Routing and Medium Access Control for Lifetime Maximization of Wireless Sensor Networks

A distributed joint routing and medium access control (MAC) algorithm is proposed for lifetime maximization of wireless sensor networks. By adopting the flow contention graph model and the resulting MAC constraints, the problem can be formulated into a linear programming problem, which can be solved distributively using dual decomposition. However, the message passing overhead of such a solution is high, since the information exchange must occur among the interfering links as well as the communicating links. In this work, the MAC layer constraints are relaxed in the form of a penalty function, which facilitates distributed optimization using only the collision statistic that each node can accumulate essentially at no extra cost. The resulting algorithm solves a convex optimization problem by a distributed primal-dual approach, where the network layer problem is solved in the dual domain, and the MAC layer problem is solved in the primal domain.

Downlink scheduling schemes in cellular packet data systems of multiple-input multiple-output antennas

High-speed cellular data systems demand fast downlink scheduling algorithms and multiple-input multiple-output (MIMO) techniques. The associated multiuser diversity and antenna diversity play a central role in achieving high system throughput and fair resource allocation among multiple users. For such systems we evaluate the cross-layer interactions between channel-dependent scheduling schemes and MIMO techniques, such as space-time block coding (STBC) or Bell Laboratories Layered Space-Time (BLAST), and propose a new scheduling algorithm named the alpha-rule. The evaluation shows that the STBC/MIMO provides a reliable channel but at a certain cost of spectral efficiency. Comparatively BLAST/MIMO provides larger capacity and enables higher scheduling throughput. Thus BLAST/MIMO may be a more suitable technique for high-rate packet data transmission at the physical layer. At the medium access control (MAC)-layer, the alpha-rule is shown to be more flexible or efficient to exploit the diversity gains than the exiting max-C/I or proportionally fair (PF) scheduling schemes. It enables online tradeoff between aggregate throughput, per-user throughput, and per-user resource allocation.

Performance comparison of MF and MMSE combined iterative soft interference canceller and V-BLAST technique in MIMO/OFDM systems

This paper proposes a complexity reduced iterative soft interference canceller (ISIC) to realize broadband mobile transmission in MIMO/OFDM systems and compares its performance with the V-BLAST technique. In the proposed scheme, we apply a minimum mean square error (MMSE) filter only in the first iteration and utilize a matched filter (MF) in the second and later iterations to reduce computational complexity by suppressing residual interference in the first iteration. Computer simulation results confirm that the proposed ISIC can achieve almost equal performance to the MMSE-based ISIC with multiplications reduced by 85 % and outperforms the V-BLAST receiver by about 2 dB in four transmit and receive antennas systems under exponentially distributed twelve path conditions.

Design of Linear Dispersion Codes for Practical MIMO-OFDM Systems

In this work, we consider the problem of designing space-time-frequency linear dispersion (LD) codes in wideband multiple-input multiple-output (MIMO) antenna systems employing orthogonal-frequency-division-multiplexing (OFDM). Three criteria are presented and discussed in detail, which involve: 1) minimizing the average uncoded block error rate, 2) maximizing the ergodic mutual information, and 3) a two-step procedure considering the optimization of the ergodic mutual information as well as the average uncoded block error rate, respectively. For any set of subcarriers, any number of OFDM symbol intervals, any number of transmit/receive antennas and any statistical fading channel model, the corresponding optimized LD code matrices are numerically computed via a stochastic gradient descent algorithm assuming either maximum likelihood or linear zero-forcing decoding. As an application of the proposed criteria, code design examples are presented for practical next-generation communication systems which operate over a realistic 3GPP spatial channel model. The robustness of the resulting codes over several operating conditions is comprehensively demonstrated via numerical simulations