Preface

In June 2001, operators and equipment vendors in the communications ecosystem founded the nonprofit WiMAX Forum, an industry-led organization aimed at harmonizing broad‐ band wireless access standards, and certifying interoperability among devices from different manufacturers.

Nowadays, about 10 years later, the WiMAX technology is a mature and affordable solution for high-speed IP-based 4G mobile broadband, fully supporting bandwidth-intensive serv‐ ices, such as high-speed Internet access and television, as well as less bandwidth-demanding but more latency-sensitive services, such as voice-over-IP calls.

The deployment of WiMAX networks, which are based on the IEEE 802.16-2004 Air Inter‐ face Standard, started in many countries in 2005, as soon as manufacturers released the first equipments. The early WiMAX networks were aimed at providing fixed broadband wireless services in a wide geographical scale and proved to be a really effective solution for the es‐ tablishment of wireless metropolitan area networks.

In February 2006, the IEEE802.16e-2005 amendment gave a further extension to the poten‐ tialities of this technology, introducing a number of features aimed at supporting also users mobility; in this way, the so-called Mobile-WIMAX profile was born. The result was a com‐ plete standard family that specified the air interface for both fixed and mobile broadband wireless access, enabling thus the convergence of fixed and mobile networks through a com‐ mon wide area radio access technology.

Since then, the interest in WiMAX has increased dramatically; nowadays, this technology is at work in more than 100 countries, providing both residential and mobile connectivity in urban areas as well as remote locations, due to the relatively low costs associated with its deployment (in comparison with 3G, HSDPA).

The parallel development of the 4G Long Term Evolution (LTE) wireless access technology, currently deployed in many countries, is not expected to displace WiMAX, which had a first-mover advantage. It is likely, in fact, that the two technologies will be complementary.

As LTE networks begin to roll out, it is extremely likely, for instance, that WiMAX will be used as the wireless backhaul for those networks, leaving to LTE the access provision. More‐ over, WiMAX seems preferable for DSL replacement, especially in developing countries and rural regions, where wireline broadband technologies are not available. For these reasons, both the industrial and the scientific communities keep on developing and investigating Wi‐ MAX, with the final aim to get the best from this technology.

In this book, a collection of selected papers is presented, which covers several aspects of the WiMAX technology, investigated by specialists working in universities and research centres all over the world.

Chapter 1 is focused on multiuser (MU) multiple input multiple output (MIMO) diversity techniques. It provides a comprehensive overview of various MIMO strategies and a critical discussion on the recent research in multiuser MIMO communications with emphasis on the design of precoders. State-of-the-art in research on MU-MIMO precoding and multi-cell processing is presented. A new precoder design and its application in WiMAX setting is also provided.

In chapter 2 the issue of the high peak-to-average power ratio (PAPR) that affects WiMAX is investigated, with particular reference to the adoption of companding as a PAPR mitigation strategy. Impairments from nonlinear distortion at the transmitter, multipath and noise are considered.

Chapter 3 deals with the adoption of mesh architectures within WiMAX, and provides an overview of the main challenges of the WiMAX mesh mode, with a focus on routing proto‐ cols and the effect of quality of service mechanisms on scenarios with mobile clients.

Chapter 4 investigates the handover mechanism, with particular reference to the cell selec‐ tion scheme. A multi-criteria selection scheme is proposed, based on the experienced carrierto-interference plus noise ratio, the required bandwidth and the expected congestion delay.

In chapter 5 the issue of coordinated authentication in a heterogeneous network environ‐ ment is investigated. A Coordinated Robust Authentication (CRA) Mechanism is presented, with the ability to use a single set of credentials with any network.

Finally, Chapter 6 reviews the Multicast and Broadcast re-keying Algorithm (MBRA) of IEEE 802.16e, that ensures the confidentiality of communications. The operational efficiency, scalability and 1-affects-n phenomenon are discussed.
