**2. A survey of various retransmission schemes**

The traditional retransmission scheme designed to combat the loss of transmission data for single-radio single-channel environments was first introduced by Lin *et al.* in [8]. Thanks to the advances of multi-band radio technologies, for many broadband wireless systems and short range communication networks, there may be many communication channels available to use. Consequently, it is natural to arrange the link layer packets to be transmitted over multiple channels to boost bandwidth. Issues regarding how to design and analyze the multi-channel transmission schemes have recently become an important research direction. A vast amount of research groups have thus started to pay attention to related topics. Individual aspects of these problems have been separately considered in many related papers [9-18]. Most literatures in this area can be conceptually categorized into 2 research directions; one is the single-radio multi-channel transmission discussed in [9-11],

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

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

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

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

The traditional retransmission scheme designed to combat the loss of transmission data for single-radio single-channel environments was first introduced by Lin *et al.* in [8]. Thanks to the advances of multi-band radio technologies, for many broadband wireless systems and short range communication networks, there may be many communication channels available to use. Consequently, it is natural to arrange the link layer packets to be transmitted over multiple channels to boost bandwidth. Issues regarding how to design and analyze the multi-channel transmission schemes have recently become an important research direction. A vast amount of research groups have thus started to pay attention to related topics. Individual aspects of these problems have been separately considered in many related papers [9-18]. Most literatures in this area can be conceptually categorized into 2 research directions; one is the single-radio multi-channel transmission discussed in [9-11],

be employed in a general CD environment for delay-sensitive flows as a case study.

delta function was proposed by Jang in [7].

references is provided in the end.

the retransmission scheme under the CD environment.

**2. A survey of various retransmission schemes** 

and the other direction is the multi-radio multi-channel transmission discussed in [12-18]. Meanwhile, for the single-radio single-channel cases, the performance results of the Automatic-Repeat-reQuest (ARQ) retransmission schemes based on Markov analysis were available in [19-22], and optimized in both the power and the packet drop probability aspects respectively were recently studied in [23]. These studies [9-23] together provide a basis for the analysis and comparison of the multi-channel transmission. Furthermore, various kinds of ARQ and Hybrid ARQ schemes designed to be employed for CD environments were explored in [24-27]. The design approaches of these related research works are elaborated as follows.

In [9], a protocol was proposed to enable hosts to utilize multiple channels by switching channels dynamically, and their simulation showed that the effective throughput was improved, especially when the network was highly congested. Centralized and distributed algorithms to perform efficient channel assignments in component-based approach were proposed and implemented in [10]. A joint multi-channel and multi-path control protocol was proposed in [11], where it combined multi-channel link layer with multi-path routing, and simulations showed that the scheme can improve the throughput significantly.

The uses of switchable interfaces and multi-channel routing were proposed in [12], and simulation results showed that the throughput in a wireless ad hoc network can be improved. In [13], a hybrid channel assignment scheme was modeled into an integer linear programming formulation, and an approximation method for simplification was also studied. In the ad hoc network related areas, [14] maximized the network throughput subject to fairness requirements using the proposed wireless network coding schemes for a variety of multi-radio multi-channel environments with different routing strategies.

Several works focusing on the ARQ schemes in multi-radio multi-channel transmissions have been discussed in [15-18]. Formulas of the link layer throughput for the Stop-and-Wait (SW), the Go-Back-N (GBN) or the Selective-Repeat (SR) schemes were derived under different assumptions of channel characteristics. The multi-channel SR ARQ scheme in [15] assumed equal transmission rate and allowed the transmitter dynamically to assign the retransmission link packet to the channel using the link packet error probability as the selection criterion, and retransmissions can continue till its success. In [16], both throughput and delay performances of the multi-channel SW, GBN, and SR ARQ schemes were investigated and validated via simulations, while all channels were assumed statistically independent and identical. Throughput analysis of the multi-channel SW, GBN, and SR ARQ schemes were generalized in [17], where the generalization took the form of packet-tochannel assignment rules, and radio channels can be with different transmission rates and different link packet error probabilities. Two fast ARQ/HARQ packet retransmission schemes have been proposed to transport delay-sensitive flows in a multi-radio multichannel environment in [18], where they can incorporate various retransmission policies, which are adjusted by the channel signal-to noise ratio (SNR) and the APDU size.

Closed form equations of the service data unit delay under the SR ARQ scheme were successfully derived and validated via simulation in [19]. An exact Markov model proposed to evaluate the delay statistics of the link packet for the SR ARQ scheme was available in [20]. The queueing models using dynamic link adaptation for the GBN and the SR ARQ schemes were formulated in [21], where the exact queue length and the delay statistics were obtained. For HARQ schemes, a Markov model was presented to analyze the SR truncated type II HARQ scheme employing Reed Solomon linear erasure block codes in [22], where the link packet throughput, error probability, and delay performance were analyzed. In [23], a suboptimal root-finding solution was developed to solve the exhaustive search for the optimization problem formulated based on the incremental-redundancy HARQ scheme.

The delay performance of several truncated ARQ and HARQ schemes in a CD environment under the assumption of Poisson arriving packets were evaluated in detail by Boujemâa in [24]. An analytical model to quantify end-to-end performances for a CD ARQ scheme in a cluster-based multi-hop wireless network was proposed by Le *et al.* in [25]. Markov models developed to evaluate the CD system were also investigated by Mahinthan *et al.* in [26] and by Issariyakul *et al.* in [27], respectively.

While papers [24-27] have widely explored various ARQ/HARQ schemes in the CD environments, the issues regarding time constraints for delay-sensitive flows were not addressed and elaborated on. Therefore, their throughput formulas did not reflect the effective throughput that must satisfy the typical delay constraints of streaming-type or realtime multimedia flows in such an environment.

Due to the aforementioned reasons, we herein propose a novel fast packet retransmission scheme, where a new approach of retransmission strategy is designed and appropriately combines the encoding/decoding mechanism presented in [18], in such a CD environment for delay-sensitive flows as a case study. In the proposed scheme, there are 2 retransmission policies that can be employed adaptively according to both the channel quality and the Application layer Protocol Data Unit (APDU) size. The retransmission is designed to be allowed only one time. Here, APDU flows in the sender are further assumed to always have a link packet ready for transmission. As a result, it is not much meaningful to analyze the packet delay involving the queueing analysis. In this paper, we only focus on the complete throughput analysis to gain the main insight of optimizing the number of channels for retransmission between the 2 proposed retransmission policies under such the CD environment. All of the derived formulas are then verified via simulations. The effective throughput of our proposed scheme is shown better than that of other CD retransmission schemes (such as [26]) and non-CD retransmission schemes.
