**1. Introduction**

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> As implied by the word "Cooperative Diversity (CD)," mobile users in a multi-user environment can share their antennas in a manner that creates a virtual Multiple-Input Multiple-Output (MIMO) system, which can be conceptually viewed as a multichannel transmission environment in the network layer, to achieve individual or common purposes of those users. By employing CD for transmissions, the quality and reliability of users' data in wireless networks can thus be improved, mainly owning to the reason that the effect of wireless channel fading can be reduced. In this chapter, we aim to introduce existing representative retransmission schemes under various environments and further present a novel packet retransmission scheme for Quality-of-Service (QoS)-constrained applications in a general CD environment.

> Transmit diversity of MIMO systems is an important technique which can bring significant gain to wireless systems with multiple transmit antennas. This technique is clearly advantageous to be employed on a cellular base station; however, it may not be practical for other scenarios. To be more specific, due to size, cost, or hardware limitations, small handsets/cellular phones may not be able to support certain types of multiple transmit antennas. For example, the size of an antenna must be several times the wavelength of the carrier frequency. Therefore, the use of multiple antennas is not an attractive way to achieve the transmit/receiving diversity in small handsets/cellular phones. To overcome such a naturally fundamental problem, CD is in nature an effective strategy to allow a singleantenna mobile device to achieve the benefit of MIMO systems with the help of cooperative mobile devices.

> CD, which is a form of spatial diversity, is through cooperating users' (usually called partners) relaying signals to the destination. This technique is achieved without the use of additional antennas of any user. That is to say, the antennas of the sender and partners together form a multiple-transmit antenna situation. Basically, the relay mechanism can be decode-and-forward or amplify-and-forward. Moreover, CD is an emerging and powerful technique that can mitigate fading and improve robustness to interference in wireless environments. Thus, CD becomes a promising candidate for emulating MIMO systems.

Recently, many research groups have turned their attention to the CD-related topics. Individual aspects of these problems have been considered, for example, in various papers [1-4]. In [1], Mahinthan *et al.* proposed a Quadrature Signaling (QS) mechanism in the CD system for transmissions. CD transmissions considering issues related to power allocation algorithms were explored by Mahinthan *et al.* in [2-3]. In [4], Chen *et al.* exploited that the use of space-time block coding in the multi-user CD to improve the performance of the transmission in wireless local area networks. Other abounding literature survey and investigation regarding the issue related to CD including principles and applications can be referred to Ray Liu *et al.* in [5] and Fitzek *et al.* in [6], respectively. Recently, a simple method to evaluate the performance of complex networks under CD using sampling property of a delta function was proposed by Jang in [7].

However, because of fundamental physical characteristics of wireless channels, data packets often cannot be delivered to the destination successfully. As a result, the design focusing on the efficient retransmission scheme under such a CD environment still plays a highly crucial role. Due to the evolution of the communication technology, most packet retransmission schemes under CD in literatures were based on the rich results from those retransmission schemes on point-to-point transmissions. Thus, we will first provide an overview of retransmission schemes on point-to-point transmissions, and then, investigate the issue on the retransmission scheme under the CD environment.

While there have been many papers exploring various retransmission schemes in the CD environment, there were no elaborations on the issue considering the time constraint for delay-sensitive services. Consequently, in such a CD environment their throughput formulas did not reflect the effective throughput (goodput) that must satisfy the typical delay constraints of streaming-type or real-time multimedia flows. Motivated by the above point, we therefore pay our attention to design a novel fast packet retransmission scheme to be employed in a general CD environment for delay-sensitive flows as a case study.

The rest of this chapter is organized as follows. A survey of various retransmission schemes is included in Section 2. Next, Section 3 proposes a novel fast packet retransmission scheme in a general CD environment for delay-sensitive applications as a case study. Section 4 makes a summary of this chapter and suggests the future work of interest. Finally, the list of references is provided in the end.
