Preface

The field of information and communication technologies continues to evolve and grow in both the research and the practical domains. However, energy efficiency is an aspect in communication technologies that until recently was only considered for embedded, mobile or handheld battery constraint devices. Today, and driven by cost and sustainability concerns about the energy and carbon footprint of the IT infrastructure we see energy efficiency becoming a pervasive issue that is considered in all information technology areas starting at the circuit level to device architecture and platforms to the system level of whole datacenters management.

Reducing the energy consumption of networks and communication devices has always been, and presumably will stay, a significant challenge for the designers, developers and the operators. This challenge is mainly because of the typical tradeoff between striving for always achieving a better performance to cope with the growing workload demand and the increased energy consumption associated with these performance guarantees. With energy consumption becoming an increasingly important design criterion, new techniques, designs and algorithms are needed to optimize this tradeoff between energy consumption and performance.

Looking toward the future, it is evident that the use of networks and communication technologies will continue to grow exponentially with more users adopting them every day and more innovative usages being developed continuously to the extent that these technologies are transformed into a commercial commodity. As a result, quantifying, understanding and improving their energy footprint are very timely and vital topics.

This book contains six chapters authored by a group of internationally well know experienced researchers. It is designed to cover a wide range of topics and to reflect the present state of the art in the field of energy-efficiency for networks and communication technologies.

> **Sameh Gobriel**  Circuits and Systems Research Lab, Intel Labs, Intel Corporation USA

**1** 

**Self-Cancellation of Sampling** 

Zhen Gao1 and Mary Ann Ingram2

*2Georgia Institute of Technology* 

*1P.R. China 2USA* 

**Frequency Offsets in STBC-OFDM** 

**Based Cooperative Transmissions** 

*1Tsinghua University, Tsinghua Research Institute of Information Technology,* 

Orthogonal frequency division multiplexing (OFDM) is a popular modulation technique for wireless communications (Heiskala & Terry, 2002; Nee & Prasad, 2000). Because OFDM is very effective for combating multi-path fading with low complex channel estimation and equalization in the frequency domain, the OFDM-based cooperative transmission (CT) with distributed space-time coding becomes a very promising approach for achieving spatial diversity for the group of single-antenna equipped devices (Shin et al., 2007; Li & Xia, 2007; Zhang, 2008; Li et al., 2010). Duo to the spacial diversity gain, CT is an energy efficient transmission technique, which can be used in sensor networks, cellular networks, or even

However, OFDM systems are sensitive to sampling frequency offset (SFO), which may lead to severe performance degradation (Pollet, 1994). In OFDM based CTs, because the oscillator for DAC on each relay is independent, multiple SFOs exist at the receiver, which is a very difficult problem to cope with (Kleider et al., 2009). The common used correction method for single SFO is interpolation/decimation (or named re-sampling), which is a energy consuming procedure. And what is more important is that, because the re-sampling of the received signal can only correct single SFO, it seems helpless to multiple SFOs in the case of OFDM based CTs. Although the estimation of multiple SFOs in OFDM-based CT systems has been addressed by several researchers (Kleider et al., 2009; Morelli et al., 2010), few contributions have addressed the correction of multiple SFOs in OFDM-based CT systems so far to our knowledge. One related work is the tracking problem in MIMO-OFDM systems (Oberli, 2007), but it is assumed that all transmitting branches are driven by a common

To provide an energy efficient solution to the synchronization problem of SFOs in OFDM based CTs, in Section 2 of this chapter, we firstly introduce a low-cost self-cancellation scheme that we have proposed for single SFO in conventional OFDM systems. Then we will show in the Section 3 that, the combination of the self-cancellation for single SFO and the re-

satellite networks, to improve the communication quality or coverage.

sampling clock, so there is still only one SFO at the receiver.

**1. Introduction** 

*Tsinghua National Laboratory for Information Science and Technology* 
