**3.1 System description**

## **3.1.1 Cooperative diversity system**

A general CD system model composing of a sender, a partner, and a receiver is considered, as shown in Fig. 1, where two cooperative users (i.e., sender and partner) transmit their information to the same destination (i.e., receiver). It is assumed that each user' device in this system only has one radio transceiver. Additionally, Orthogonal Frequency Division Multiplexing (OFDM) is employed as the underlying transmission technique.

In the present system, channels among sender, partner and receiver are modeled as nonidentical but independent Nakagami-*m* slow-fading channels corrupted by additive white Gaussian noise. The fading channels and the noise are assumed to be independent of each other.

Fig. 1. System model for two cooperative users (sender and partner) transmission.

Generally speaking, for the transmission of flows of the sender, application layer flows are composed of APDUs. We assume that an APDU consists of *s* link packets. Each link packet will be encoded (described in detail in Section 3.1.3) in sequence for transmission. Here, APDU flows in the sender are assumed to always have a link packet ready for transmission.

For the convenience of the following analysis, channels that between sender and destination, between sender and partner, and between partner and destination are denoted as channel-*j*, *j*=1,2,3, respectively.

Last, but not least, we herein choose to employ only 1 partner for study since the significant improvement of the overall system performance with CD is usually owing to the contribution of the best partner [3].
