Variable packet splitting transmission in multirelay cooperative communications with DF and DAF for SCFDMA
 Yuta Ida^{1, 2}Email author,
 ChangJun Ahn^{2},
 Takeshi Kamio^{1},
 Hisato Fujisaka^{1} and
 Kazuhisa Haeiwa^{1}
https://doi.org/10.1186/16871499201364
© Ida et al.; licensee Springer. 2013
Received: 31 July 2012
Accepted: 18 February 2013
Published: 11 March 2013
Abstract
In recent years, cooperative communications are widely studied. Cooperative communications can obtain the space diversity as multipleinput multipleoutput (MIMO) systems. In cooperative communications, the relay method is important as decodeandforward (DF) and decodeamplifyforward (DAF). The multirelay cooperative communications can improve the system performance. In the multirelay cooperative communications, the optimum packet splitting method is effective. Moreover, the multirelay cooperative communications can more improve the system performance by using the power allocation (PA). However, the PA method requires large feedback information (FBI). To solve this problem, in this article, we propose the optimum packet splitting method based on the time domain channel state information (CSI) in the multirelay cooperative communication with DF and DAF.
Keywords
Introduction
As the next generation standard of the mobile communications, a longterm evolution (LTE)Advanced is standardized [1]. LTEAdvanced supports broadband data services with the maximum transmission rate of about 0.1–1 Gbps. In a LTEAdvanced, single carrier frequency division multiple access (SCFDMA) has been adopt in the uplink communication, and orthogonal frequency division multiple access (OFDMA) has been adopt in the downlink communication [2]. SCFDMA can achieve a low peak average power ratio (PAPR) compared with OFDMA [3–5]. Since a low PAPR of SCFDMA can reduce the cost and the power consumption of communication, SCFDMA is suitable transmission method for the uplink communication.
In recent years, cooperative communications are widely studied [6–9]. Cooperative communications use the relay node except the source and destination nodes. By using the relay node, cooperative communications can obtain the space diversity as multipleinput multipleoutput (MIMO) systems. For the problem of PAPR, the signals of each antenna have to process in the same device for MIMO systems. As a result, MIMO systems increase PAPR. On the other hand, the signals process in the independent device as the relay node for cooperative communications. Therefore, cooperative communications mitigate the problem of PAPR compared with MIMO systems [10, 11]. In cooperative communications, the relay method is important as amplifyandforward (AF), decodeandforward (DF), and decodeamplifyforward (DAF) [12–21]. AF is the simplest method. This is because it is only adjust the amplitude of the signal in the relay node [12–14]. However, since AF is poor in a frequency selective fading environment, the system performance is significantly degraded. To solve this problem, DF and DAF have been proposed [15–21]. DF and DAF detect the received signal in the relay node. As a result, DF and DAF are strong in a frequency selective fading environment. For the deference of DF and DAF, DF detects the received signal by using the hard decision [15, 16]. On the other hand, DAF detects the received signal by using the soft decision [17–21]. In this article, we use DF and DAF, and compare their performances.
In cooperative communications, the multirelay cooperative communications have been proposed [22]. This system can improve the system performance to obtain more strong diversity by using several relay nodes. However, if the channel condition of relays is bad, the system performance is degraded. To solve this problem, the optimum packet splitting has been proposed in MIMO systems [23]. The optimum packet splitting method splits the packet by considering the channel condition and sends from each transmit antenna. As a result, the optimum packet splitting method can improve the system performance. However, the optimum packet splitting is not considered in the multirelay cooperative communications. Therefore, in this article, we propose the optimum packet splitting in the multirelay cooperative communication systems. Cooperative communications can improve the system performance by using the power allocation (PA) [24–28]. This method is also effective in the multirelay cooperative communications [26]. Therefore, [27] has adopt the waterfilling power allocation (WF/PA) method. The WF/PA method mitigates the deep faded channel by using the optimum power [24–27]. However, the PA method requires large feedback information (FBI) since it has to feedback the channel state information (CSI) of each sampling point in a frequency domain. To solve these problems, in this article, we propose the optimum packet splitting method based on the time domain CSI in the multirelay cooperative communication with DF and DAF.
Common notations that E·, (·)^{∗} denote the ensemble average operation, a complex conjugate, respectively.
System model
In this article, we assume the multirelay cooperative communications as shown in Figure 1. Here, we define the notation as follows.

SD is the link between the source and destination nodes.

SR is the link between the source and relay nodes.

RD is the link between the relay and destination nodes.

SRD is the link between the source, relay, and destination nodes.
Channel model
Source node
where T_{ g }is the GI length.
Relay node
where ω_{n,s r}(k) is the MMSE weight. After the detection, the detected signal ${\xfb}_{n}\left(k\right)$ is also modulated and transmits to the destination node by using decodeandforward (DF) or decodeamplifyforward (DAF).
Destination node
MMSE equalization
where ${\sigma}_{n}^{2}$ is the noise power [29].
Proposed variable splitting method
In this section, we explain the waterfilling power allocation (WF/PA) method and the proposed variable splitting method.
Waterfilling power allocation (WF/PA)
From Equation (15), the WF/PA method can improve the system performance. However, the WF/PA method requires large feedback information (FBI). To mitigate this problem, we propose the variable splitting method.
Proposed variable splitting method
The proposed variable splitting method is performed based on the time domain CSI. For the time domain CSI, the amplitudes are constant. From this characteristic, since FBI of the proposed method becomes only one, the proposed method can reduce FBI compared with the WF/PA method.
By using Equations (21) and (22), the proposed method can achieve the optimum packet splitting.
Computer simulation results
Simulation parameters
Data modulation  QPSK 

Data detection  Coherent 
Symbol duration  2 μ s 
Number of data symbols  80 
Number of sampling points  48 
Number of relay nodes  0, 2 
Guard interval  6 sample times 
Fading  2 paths Rayleigh fading 
Doppler frequency  5 Hz 
FEC  Convolutional code 
(R=1/2, $\mathcal{K}=7$) 
Conclusion
In this article, we have proposed the variable packet splitting method in the multirelay cooperative communications with DF and DAF. The proposed method obtains the space diversity by using the multirelay cooperative communications. Moreover, since the proposed method uses the error correcting code and interleaving, obtains also the coding gain. However, if the channel condition of relays is bad, the system performance is degraded. Therefore, the proposed method splits the packet to obtain the space and coding gains. Moreover, the proposed method uses the selection parameters α and γ to obtain the maximum gains. As a result, the proposed method obtains the maximum coding gain due to the space diversity. From the simulation results, the proposed method has decided the optimum value of selection parameters α and γ, and achieved the best BER performance. Moreover, the proposed method of DAF has shown the good BER performance compared with DF.
Declarations
Authors’ Affiliations
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