Alfredo Todini

A Resource Allocator for the Uplink of Multi-Cell OFDMA Systems

We propose a simple distributed radio resource allocation algorithm for an OFDMA cellular system, which aims at minimizing the overall transmitted power subject to a rate constraint for each user. In order to reduce the problem complexity we use a single modulation; simulations show that the resulting performance degradation is negligible when the number of users is high enough. Moreover, we propose a simple distributed heuristic that, by reducing the rate constraints, steers the multicell system towards an stable resource allocation. Results show that the proposed system exhibits a great robustness to the destructive effects of multiple access interference.

On the impact of physical layer awareness on scheduling and resource allocation in broadband multicellular IEEE 802.16 systems [Radio Resource Management and Protocol Engineering for IEEE 802.16]

Multicellular networks based on the IEEE 802.16 standard appear to be very promising candidates to provide end users with broadband wireless access. However, they also pose interesting challenges in terms of radio resource management, where several design choices are not specified in the standard, intentionally left open to implementors. For this reason, we focus in this article on scheduling and resource allocation, and investigate how they could operate in a cross-layer fashion. In particular, we describe the principles of joint scheduling and resource allocation for IEEE 802.16 operating in AMC mode, and discuss the critical role played by physical layer considerations, especially intercell interference estimation and channel state awareness, in the obtained performance. This leads to identifying key open issues and possible general solutions

A Layered Architecture for Fair Resource Allocation in Multicellular Multicarrier Systems

We consider a multicell multicarrier system with frequency reuse distance that is equal to one. Allowing all cells to transmit on the whole bandwidth unveils large potential gains in terms of spectral efficiency, in comparison with conventional cellular systems. Such a scenario, however, is often deemed unfeasible because of the strong multiple access interference (MAI) that negatively affects system performance. This paper presents a layered architecture that integrates a packet scheduler with an adaptive resource allocator that was explicitly designed to take care of the MAI. Each cell performs its resource management in a distributed way with no central controller. Iterative resource allocation assigns radio channels to the users to minimize interference. Packet scheduling guarantees that all users get a fair share of resources, regardless of their position in the cell. This scheduler-allocator architecture integrates both goals and is able to self-adapt to any traffic and user configuration. An adaptive distributed load control strategy can reduce the cell load so that the iterative procedure always converges to a stable allocation, regardless of the interference. Numerical results show that the proposed architecture guarantees both high spectral efficiency and throughput fairness among flows.

Why a multichannel protocol can boost IEEE 802.11 performance

We analyse a CSMA MAC protocol for ad hoc wireless networks, that uses one control channel and a number of data channels. The data channel employed in each transmission is dynamically selected with an exchange of frames on the control channel. We present simulation results obtained with both the multichannel protocol and the IEEE 802.11 MAC protocol under different scenarios. We show that the multichannel protocol performs better than the single channel MAC under certain conditions, since the presence of hidden nodes has a smaller impact on its performance; we argue that this is mainly due to the separation operated between the control frames and the data frames.

On the effects of ARQ mechanisms on TCP performance in wireless environments

In this paper we investigate the interaction between TCP and wireless ARQ mechanisms. The aim is to understand what is the best reliability degree of the wireless link in order to guarantee TCP performance. For this purpose, we first develop a Markov model for a selective repeat ARQ protocol, widely used in the current wireless environments. Secondly, we design a cross-layer algorithm that, by exploiting the proposed model, can adapt the number of link layer transmission attempts to the end-to-end packet loss rate perceived by TCP. The interaction between TCP and link layer is evaluated in a specific case study (TCP over 3G radio access) by means of simulations carried out by using a very detailed UMTS-TDD simulator based on us. The deployment of the link layer Markov model and of the proposed algorithm allows us to derive some interesting conclusions about the design of retransmission protocols in TCP/IP network environments.

Inadequacy of the Queue-Based Max-Weight Optimal Scheduler on Wireless Links with TCP Sources

The interaction between wireless optimized scheduling algorithms and TCP congestion control mechanisms can have adverse effects on the performance of the system. We focus on the Queue Based Max-Weight (QBMW) scheduler, a scheduling strategy which is known to be throughput-optimal under unregulated traffic sources. We use fluid modeling to describe the time evolution of the congestion window size and of the wireless buffer, and show by numerical results that under TCP traffic sources the QBMW scheduling policy leads to a very unfair outcome, in which some users may be completely shut off. We also evaluate and discuss the performance achieved by other scheduling policies: the Proportional Fair (PF) scheduler, and the Queue Age (QA) scheduler, which takes account of the age of the packets stored in the wireless buffers.

Distributed radio resource allocation for the downlink of multi-cell OFDMA radio systems

This paper analyzes the problem of allocating power and subchannels in the downlink of a multi-cell, full-reuse OFDMA cellular system. We propose a distributed iterative algorithm, in which the allocation is performed independently in each cell, by maximizing the cell rate subject to a power constraint. The algorithm restricts the set of users that can be allocated on each subchannel, in order to satisfy a sufficient condition for the convergence to a stable equilibrium. Simulation results show that the proposed allocation and power control algorithm converges quickly and achieves a good spectral efficiency.

On the unfairness among TCP flows in IEEE 802.11 multi-hop ad-hoc networks

The problem of unfairness among competing flows in TCP multihop wireless networks is well known, and has often been traced to the misbehaviour of dynamic routing protocols in case of link failure. We show, through simulations, that the competition between TCP flows in a multihop wireless network leads to an unfair outcome, independently of the routing protocol involved. We focus on the impact of hidden node collisions on the performance of multi-hop TCP flows, and trace the source of the problem to collisions between RTS frames belonging to the starved flow and long data frames sent by the active flow. Such collisions lead to a rapid increase of the MAC contention window at the TCP sender of the starved flow; this, in conjunction with TCPs congestion control algorithm, allows the running flow to hold the channel for a long time, until the situation is reversed. We also show that a simple multi-channel MAC can ease the competition among flows.

Power-delay trade-off in a many-to-many wireless access

Given a set of communication links sharing a channel, delay for delivery of a given amount of backlogged data can be traded off against average power consumption, for a fixed transmission link requirement. The main result of this work is the identification of general bounds for the average power needed to deliver a given backlog within a delay constraint and their application to compare orthogonal (TDMA) and concurrent (CDMA) multiple access. Numerical examples for a collection of links belonging to an ad-hoc network are discussed. It turns out that non-orthogonal CDMA can be the only viable choice for strict (yet feasible) delay constraints, the price to be paid being a larger energy consumption. Also, CDMA offers better power-delay trade-off for low interference as well as high path loss channel scenarios.

Analytic modeling of ad hoc 802.11 networks with hidden nodes applied to a split channel solution for performance improvement

Several simulation studies have shown that the performance of IEEE 802.11 DCF in an ad hoc scenario strongly depends on the coverage and interference radii. We state and solve an analytical model for an 802.11 DCF ad hoc network, with an interference radius larger than the coverage radius. The model is developed for the study of a split channel solution, where RTS/CTS signaling is conveyed via a separate, orthogonal channel with respect to data and ACK frames. By exploiting the model we can optimize the bandwidth split of the control and data channels. Further, we compare single channel, split channel and multichannel solutions, thus highlighting that the simple split channel achieves most of the performance advantage potentially offered by a multi-channel 802.11 DCF.