André Girard

Near-Optimal and Efficient Heuristic Algorithms for Resource Allocation in MISO-OFDM Systems

MISO-OFDM systems enable the allocation of resources in the frequency and spatial domain to increase the data throughput of wireless systems. In this paper, we initially present a near-optimal solution to the MISO-OFDM Resource Allocation (RA) problem. This solution performs an exhaustive search over all possible user sets and then uses a dual-based approach to optimally assign power to the selected user sets. However, this algorithm requires computing the pseudo-inverse of all subchannel matrices. In order to reduce its computational complexity we derive an efficient heuristic RA algorithm. We first propose an approximation method that only computes the determinant of the squared subchannel matrices and selects the sets with maximum determinant per subcarrier. We then further reduce the computational cost by using a greedy search method which decreases the number of candidate users for each subcarrier. We present simulation results which show that the performance gap of the approximate method is under 0.6\% with respect to the near-optimal solution, and under 1.4\% when we use the greedy search method pre-selecting 4 sets. These results show that the proposed heuristic algorithms can achieve a performance very close to the optimal at a low computational cost even with a large number of users in the cell.

Effective Bandwidth Evaluation for VoIP Applications in IEEE 802.11 Networks

The convergence of different classes of traffic with different priorities over the wireless network has become a reality. To insure that the users of key applications such as VoIP are satisfied with the service they receive, one must insure that the QoS criteria for these applications, such as delay, jitter or packet loss, are met This in turn means that some form of connection admission control must be used. In this paper, we show how the notion of effective bandwidth that had been previously used in wired systems can be used for CAC in WLAN IEEE 802.11b and 802.11g networks. Effective bandwidth simplifies connection admission since a new application can be accepted on a link whenever its effective bandwidth is lower than the bandwidth still available on the link. The paper presents empirical results obtained by extensive simulations showing that the admission region is nearly linear so that it is possible to design an effective linear CAC policy based solely on the effective bandwidth of a connection. We also propose a more practical method that relates to per packet effective bandwidth.

Calculation of packet jitter for non-poisson traffic

The packet delay variation, commonly called delay jitter, is an important quality of service parameter in IP networks especially for real-time applications. In this paper, we propose the exact and approximate models to compute the jitter for some non-Poisson FCFS queues with a single flow that are important for recent IP network. We show that the approximate models are sufficiently accurate for design purposes. We also show that these models can be computed sufficiently fast to be usable within some iterative procedure, e.g., for dimensioning a playback buffer or for flow assignment in a network.

Dual-based bounds for resource allocation in zero-forcing beamforming OFDMA-SDMA systems

We consider multi-antenna base stations using orthogonal frequency-division multiple access and space division multiple access techniques to serve single-antenna users. Some users, called real-time users, have minimum rate requirements and must be served in the current time slot while others, called non real-time users, do not have strict timing constraints and are served on a best-effort basis. The resource allocation (RA) problem is to find the assignment of users to subcarriers and the transmit beamforming vectors that maximize the total user rates subject to power and minimum rate constraints. In general, this is a nonlinear and non-convex program and the zero-forcing technique used here makes it integer as well, exact optimal solutions cannot be computed in reasonable time for realistic cases. For this reason, we present a technique to compute both upper and lower bounds and show that these are quite close for some realistic cases. First, we formulate the dual problem whose optimum provides an upper bound to all feasible solutions. We then use a simple method to get a primal-feasible point starting from the dual optimal solution, which is a lower bound on the primal optimal solution. Numerical results for several cases show that the two bounds are close so that the dual method can be used to benchmark any heuristic used to solve this problem. As an example, we provide numerical results showing the performance gap of the well-known weight adjustment method and show that there is considerable room for improvement.

Efficient Heuristic for Resource Allocation in Zero-Forcing OFDMA-SDMA Systems with Minimum Rate Constraints

Multi-antenna OFDMA-SDMA systems provide the required high spectral efficiency and flexibility to support the ever increasing data rates requirements of real-time multimedia applications in future wireless access systems. However, the resource allocation process becomes extremely complex because of the large number of degrees of freedom and the strict timing requirement of real-time traffic. In this paper, we propose heuristics to efficiently solve the zero-forcing OFDMA-SDMA resource allocation problem and provide, when feasible, guaranteed service to users with minimum rate requirements. The heuristics combine both rate-constrained power allocation and subcarrier reassignment algorithms. We compare the heuristics performance against an upper bound and other methods proposed in the literature and find that, although they have a slightly lower sum rate performance, they support a wider range of minimum rates while significantly reducing the computational complexity, making them suitable for usage in real-time systems.

A method for the integrated design of reliable GMPLS networks

The increase of bandwidth demand for new Internet applications suggests mapping directly IP over the WDM layer. Since reliability is such a critical issue in these broadband networks, we propose an integrated design method which addresses the problem of survivability as viewed from the IP/MPLS layers but taking into account the failure mechanisms in the optical layer. This approach is becoming practical because of the emergence of GMPLS as a multilayer control plane that can support the signaling required for coordinating the restoration mechanisms in multiple layers. The model relies on network calculus to evaluate a QoS metric as actually perceived by end users and computes a preplanned restoration scheme to recover from failures. We discuss the numerical implementation, the convergence and the solutions produced by the algorithm and show that the resulting network can provide the prescribed QoS guarantees for all failure states.

On the relationship between packet jitter and buffer loss probabilities

Controlling the jitter buffer at the receiver side is a key component in multimedia and real-time services. A jitter buffer is used to control the packet rate and to compensate for the delay variation, commonly called jitter, by the tradeoff between delay and loss. In this paper, we present simulation results for the relation between the over- and underflow probabilities and the jitter. We then show how we can control a jitter buffer based on an estimation of the variance of the arrival process and a fast computation model for the jitter in this buffer.