Multiuser diversity is a form of diversity inherent in a wireless network, provided by independent time-varying channels across the different users. The diversity benefit is exploited by tracking the channel fluctuations of the users and scheduling transmissions to users when their instantaneous channel quality is near the peak. The diversity gain increases with the dynamic range of the fluctuations and is thus limited in environments with little scattering and/or slow fading. In such environments, we propose the use of multiple transmit antennas to induce large and fast channel fluctuations so that multiuser diversity can still be exploited. The scheme can be interpreted as opportunistic beamforming and we show that true beamforming gains can be achieved when there are sufficient users, even though very limited channel feedback is needed. Furthermore, in a cellular system, the scheme plays an additional role of opportunistic nulling of the interference created on users of adjacent cells. We discuss the design implications of implementing this scheme in a complete wireless system.

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This paper proposes a synchronous device discovery solution for ad-hoc networks based on the observations that time synchronization, along with an FDM based channel resource allocation, can lead to gains in terms of energy consumption, discovery range, and the number of devices discovered. These attributes are important for the success of proximity-aware networking, where devices autonomously find peer-groups over human mobility scales. In this paper, we develop the PHY and MAC protocols to enable autonomous device discovery. Using both simulations and stochastic-geometry based analysis, we validate our design, and argue that there can be significant gains over a conventional Wi-Fi based solution.

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Written by the pioneers of Flash-OFDM, arguably the first commercially developed OFDMA-based mobile broadband system in the world, this book teaches OFDMA from first principles, enabling readers to understand mobile broadband as a whole. The book examines the key requirements for data-centric mobile; how OFDMA fits well with data networks; why mobile broadband needs to be IP-based; and how to bridge communications theory to real-world air interface design and make a good system choice between performance and complexity. It also explores the future of wireless technologies beyond conventional cellular architecture. One of the key challenges faced by newcomers to this field is how to apply the wireless communications theory and principles to the real world and how to understand sophisticated commercial systems such as LTE. The authors use their firsthand experience to help graduate students, researchers and professionals working on 4G to bridge the gap between theory and practice.