Vijay G. Subramanian

Downlink scheduling and resource allocation for OFDM systems

We consider scheduling and resource allocation for the downlink of a OFDM-based wireless network. During each time-slot the scheduling and resource allocation problem involves selecting a subset of users for transmission, determining the assignment of available subcarriers to selected users, and for each subcarrier determining the transmission power and the coding and modulation scheme used. We address this in the context of a utility-based scheduling and resource allocation scheme presented in earlier papers. Scheduling and resource allocation is determined by solving an optimization problem, which is convex for a reasonable model of the feasible rates. By exploiting the structure of this problem, we give optimal and sub-optimal algorithms for its solution. We provide simulation results comparing different algorithms and parameter settings.

Downlink Scheduling and Resource Allocation for OFDM Systems

We consider scheduling and resource allocation for the downlink of a OFDM-based wireless network. During each time-slot the scheduling and resource allocation problem involves selecting a subset of users for transmission, determining the assignment of available subcarriers to selected users, and for each subcarrier determining the transmission power and the coding and modulation scheme used. We address this in the context of a utility-based scheduling and resource allocation scheme presented in earlier papers. Scheduling and resource allocation is determined by solving an optimization problem, which is convex for a reasonable model of the feasible rates. By exploiting the structure of this problem, we give optimal and sub-optimal algorithms for its solution. We provide simulation results comparing different algorithms and parameter settings.

Broad-band fading channels: signal burstiness and capacity

Medard and Gallager (2002) showed that very large bandwidths on certain fading channels cannot be effectively used by direct sequence or related spread-spectrum systems. This paper complements the work of Medard and Gallager. First, it is shown that a key information-theoretic inequality of Medard and Gallager can be directly derived using the theory of capacity per unit cost, for a certain fourth-order cost function, called fourthegy. This provides insight into the tightness of the bound. Secondly, the bound is explored for a wide-sense-stationary uncorrelated scattering (WSSUS) fading channel, which entails mathematically defining such a channel. In this context, the fourthegy can be expressed using the ambiguity function of the input signal. Finally, numerical data and conclusions are presented for direct-sequence type input signals.

Joint Scheduling and Resource Allocation in CDMA Systems

In this paper, the scheduling and resource allocation problem for the downlink in a code-division multiple access (CDMA)-based wireless network is considered. The problem is to select a subset of the users for transmission and for each of the users selected, to choose the modulation and coding scheme, transmission power, and number of codes used. We refer to this combination as the physical layer operating point (PLOP). Each PLOP consumes different amounts of code and power resources. The resource allocation task is to pick the ¿optimal¿ PLOP taking into account both system-wide and individual user resource constraints that can arise in a practical system. This problem is tackled as part of a utility maximization problem framed in earlier papers that includes both scheduling and resource allocation. In this setting, the problem reduces to maximizing the weighted throughput over the state-dependent downlink capacity region while taking into account the system-wide and individual user constraints. This problem is studied for the downlink of a Gaussian broadcast channel with orthogonal CDMA transmissions. This results in a tractable convex optimization problem. A dual formulation is used to obtain several key structural properties. By exploiting this structure, algorithms are developed to find the optimal solution with geometric convergence.

Class and channel condition based weighted proportional fair scheduler

In this paper we outline a scheme to perform packet-level scheduling and resource allocation at the wireless node that takes into account the notions of both efficiency and fairness and presents a means to explore the trade-off between these two notions. As a part of this scheme we see the scheduling problem as deciding not just the packet transmission schedule but also the power allocation, the modulation and coding scheme allocation and the spreading code determination since the latter three directly influence the radio resources consumed. Using a utility maximization formulation based on the data-rates that the mobiles can transmit at, we decide on the weights for a weighted proportionally fair allocation based scheduling algorithm. We conclude with a simulation based performance analysis for infinitely-backlogged sources on a UMTS system.

Optimal Scheduling for OFDMA Systems

We consider a scheduling and resource allocation problem for the downlink of an OFDMA-based wireless network, where the channel estimation error is modeled by a self-noise term in the decoding process. During each time-slot this involves selecting a subset of users for transmission, determining the assignment of available subcarriers to selected users, and for each subcarrier determining the transmission power and the coding and modulation scheme used. We address this in the context of a utility-based scheduling scheme presented in earlier papers. This results in an optimization problem, which is convex for a reasonable model of the feasible rates. By exploiting the structure of this problem, we develop optimal and sub-optimal algorithms for its solution. We provide simulation results comparing different algorithms and parameter settings.

Max-min fairness in 802.11 mesh networks

In this paper, we establish that the rate region of a large class of IEEE 802.11 mesh networks is log-convex, immediately allowing standard utility fairness methods to be generalized to this class of networks. This creates a solid theoretical underpinning for fairness analysis and resource allocation in this practically important class of networks. For the special case of max-min fairness, we use this new insight to obtain an almost complete characterization of the fair rate allocation and a remarkably simple, practically implementable method for achieving max-min fairness in 802.11 mesh networks.

Capacity and reliability function for small peak signal constraints

The capacity and the reliability function as the peak constraint tends to zero are considered for a discrete-time memoryless channel with peak constrained inputs. Prelov and van der Meulen (1993) showed that under mild conditions the ratio of the capacity to the squared peak constraint converges to one-half the maximum eigenvalue of the Fisher information matrix and if the Fisher information matrix is nonzero, the asymptotically optimal input distribution is symmetric antipodal signaling. Under similar conditions, it is shown in the first part of the paper that the reliability function has the same asymptotic shape as the reliability function for the power-constrained infinite bandwidth white Gaussian noise channel. The second part of the paper deals with Rayleigh-fading channels. For such channels, the Fisher information matrix is zero, indicating the difficulty of transmission over such channels with small peak constrained signals. Asymptotics for the Rayleigh channel are derived and applied to obtain the asymptotics of the capacity of the Marzetta and Hochwald (1999) fading channel model for small peak constraints, and to obtain a result of the type of Medard and Gallager for wide-band fading channels.

Delay performance of CSMA in networks with bounded degree conflict graphs

We analyze packet delay in CSMA-based random access schemes in networks under the protocol interference model. Using a stochastic coupling argument we identify a subset of the throughput-region where queue lengths can be bounded uniformly for all network sizes. This conclusion provides a throughput-region of interest for delay sensitive applications and suggests that delay bounds based on mixing time analyses may be loose.

Log-convexity of rate region in 802.11e WLANs

In this paper we establish the log-convexity of the rate region in 802.11 WLANs. This generalises previous results for Aloha networks and has immediate implications for optimisation based approaches to the analysis and design of 802.11 wireless networks.