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Multisource SWIPTbased coded cooperation:rate compatible codes and codeword splitting protocol
EURASIP Journal on Wireless Communications and Networking volume 2020, Article number: 54 (2020)
Abstract
To achieve a high reliable and energysaving green communication, we investigate a multisource simultaneous wireless information and power transfer (SWIPT)based coded cooperation where the relay can realize information decoding and energy harvesting. Firstly, a class of naturally rate compatible lowdensity paritycheck (LDPC) codes–quasicyclic repeataccumulate (QCRA) codes is introduced, and the joint paritycheck matrix corresponding to the QCRA codes employed by the multiple sources and relay is deduced. Based on the joint paritycheck matrix, we jointly design the QCRA codes to cancel all the short girth cycles. Then, by exploiting the rate compatible characteristic of QCRA codes, we propose a new SWIPT protocol—codeword splitting protocol for the proposed system, which has the characteristics of lower complexity, higher efficiency, no strictly bit synchronization limitation, and less hardware requirement. The results show that the bit error rate (BER) performance of the proposed system employing jointly designed QCRA codes clearly outperforms that of general RA codes. Theoretical analysis and numerical simulations also demonstrate the superiority of the proposed codeword splitting protocol.
Introduction
Currently, the area of information and communication technology (ICT) plays an important role in the global energy consumption and the greenhouse gas emission [1,2,3]. In nextgeneration wireless communication, an unprecedented number of devices will be served and huge amounts of application will be provided, which would result in much more serious challenges. To achieve green communications, it is extremely urgent to explore energysaving technologies and energy efficiency protocols for nextgeneration wireless communication. Recently, simultaneous wireless information and power transfer (SWIPT) technology [4,5,6] which can realize information decoding (ID) and energy harvesting (EH) simultaneously have drawn great attention. In SWIPTbased communication systems, the nodes can harvest the energy from radio frequency (RF) signals for transmission. It is energysaving and reduces the carbon footprint. Hence, SWIPT technology is a new approach to the green communications. Furthermore, the lifetime of nodes with SWIPT technology is not restricted by the grid energy or battery, and they can be placed in the hardtoreach areas to reduce the communication blind zones and enlarge the coverage.
Researchers have investigated various SWIPTbased communication systems such as cognitive radio network [7], multipleinput multipleoutput (MIMO) system [8], and cooperative communication [9]. To exploit the spatial diversity, cooperative communication forms a virtual MIMO by sharing the antennas of different nodes. Hence, SWIPTbased cooperative communication can not only overcome the power limitation but also achieve spatial diversity.
The three main protocols for cooperative communication are the amplifyandforward (AF) [10], decodeandforward (DF) [11], and coded cooperation [12]. The AF protocol has the advantage of lower implementation complexity. The DF protocol performs better than the AF protocol when the sourcerelay channel is good enough. Coded cooperation protocol combines channel coding and cooperative technology. It obtains both coding gain and spatial diversity gain and achieves the best reliable performance.
References [13,14,15] investigated SWIPTbased AF cooperative communications. In [13], an AF cooperative communication was considered, where the relay uses the energy harvested from RF signals to assist the source in transmitting information. Furthermore, the power splitting protocol and time switching protocol were proposed to implement SWIPT. For an AFMIMO cooperative communication in [14], the source and relay adopted orthogonal spacetime block codes and designed the joint optimal precoders to achieve tradeoffs between information decoding and energy harvesting. Based on directional modulation, secure SWIPTbased AF relay network was investigated in [15]. To maximize the secrecy rate, the authors built a twinlevel optimization problem and then solved it using a onedimensional search and semidefinite relaxation. References [16,17,18] studied SWIPTbased DF cooperative communications. Multiantenna relayassisted SWIPT for twohop DF cooperative transmission was considered in [16]. To maximize the achievable rate, the authors formulated a joint problem of power allocation and power splitting at the multiantenna relay. Reference [17] investigated SWIPT for a DF fullduplex relay network and studied two models for the battery, i.e., the virtual harvestusestore model and harvestuse model. Reference [18] formulated a distributed precoding problem for SWIPTbased DF MIMO relay networks. A noncooperative game was established when only local channel state information (CSI) was required, and the existence and uniqueness of the pure strategy Nash equilibrium solution were proved.
The existing works about SWIPTbased cooperative communication mostly focus on the AF protocol or DF protocol. Generally, compared with AF protocol or DF protocol, coded cooperation protocol is much more suitable when extremely reliable communication is required. Hence, SWIPTbased coded cooperation is an effective way to realize extremely reliable and green communication. However, to the best of our knowledge, the references about SWIPTbased coded cooperation are relatively scarce. Lowdensity paritycheck (LDPC) code [19, 20] is adopted as channel coding for the data channel in 5G standard [21]. Repeataccumulate (RA) code [22, 23], as a special class of LDPC codes, not only has the merits of high coding gain, low memory consumption, simple encoding/decoding, and low energy consumption but also possess naturally rate compatible characteristic, which is very suitable for the multisource SWIPTbased coded cooperation. Reference [24] investigated the energy harvestingbased RAcoded cooperative MIMO, where single source and relay are just considered. Hence, the application scenarios are strictly limited. Furthermore, in [24], the relay harvests energy by SWIPT technology via the antenna switching protocol, and it does not apply in the single antenna case. We will consider the multisource SWIPTbased RA coded cooperation and propose a new efficient and lower complexity SWIPT protocol which applies both in the single antenna and multiple antenna cases.
Methods
In this paper, to achieve a high reliable and energysaving green communication, we focus on the multisource SWIPTbased coded cooperation. To improve the reliability, quasicyclic RA (QCRA) codes are introduced to the system, and the QCRA codes employed by the sources and relay are jointly designed to further improve the coding gain. To save the energy and achieve the green target, SWIPT technology is implemented at the relay. A new efficient and lower complexity SWIPT protocol is further proposed and investigated. The main contributions are summarized as follows:
 (1)
We briefly introduce the rate compatible QCRA codes, and we deduce the joint paritycheck matrix corresponding to the QCRA codes employed by the multiple sources and relay in the multisource SWIPTbased coded cooperation.
 (2)
Based on the joint paritycheck matrix, we jointly design the QCRA codes to cancel all the short girth cycles. We decompose the joint paritycheck matrix into two parts, i.e., QC part and quasidiagonal (QD) part, and then we propose the algorithm and theorems to design the QC part to cancel all the girth4 cycles in the joint paritycheck matrix absolutely.
 (3)
By exploiting the rate compatible characteristic of QCRA codes, we propose a new SWIPT protocol—codeword splitting protocol, which is only operated one time during the whole codeword period. It has the characteristics of lower complexity, higher efficiency, no strictly bit synchronization limitation, and less hardware requirement.
The rest of this paper is organized as follows. In Section 2.1, the general fundamental principle of multisource SWIPTbased QCRA coded cooperation is presented. Section 2.2 mainly deals with joint design of QCRA codes for the proposed system. Section 2.3 describes the new SWIPT protocol—codeword splitting protocol. Simulation results and discussion are given in Section 3. Finally, Section 4 concludes the whole paper.
System description
For simplicity, the twosource SWIPTbased coded cooperation is considered in Fig. 1, which can be extended easily to multisource multirelay scenarios. Two sources (S_{1}, S_{2}) and the destination (D) are powered by external power supply such as the grid. However, the relay (R) cannot access the external power supply, and it harvests the energy from the RF signals from two sources via SWIPT technology. In time slot 1, at the source S_{1}, a codeword c_{1} = [s_{1}p_{1}] of the first QCRA encoder (QCRA1) is sent simultaneously to R and D over the broadcast channel. Similarly, in time slot 2, at the source S_{2}, a codeword c_{2} = [s_{2}p_{2}] of the second QCRA encoder (QCRA2) is sent simultaneously to R and D. s_{i} and p_{i} (i = 1, 2) are the information bits and check bits of c_{i}, respectively. In time slot 3, firstly, the relay R decodes the information and harvests energy from the two incoming signals of S_{1} and S_{2} via SWIPT technology. Traditionally, the relay combines the decoded messages by network coding [25], such as bitwise exclusive OR (XOR) operation in the Galois field GF(2). However, this operation will inevitably introduce girth4 cycles in the joint paritycheck matrix. Hence, in the proposed system, the relay R cascades the information bits s_{1} and s_{2} rather than combines them by network coding and then encodes the cascaded message by another QCRA encoder (QCRAR). Because the information bits s_{1} and s_{2} have been sent to the destination by S_{1} and S_{2}, to improve the transmission efficiency, R transmits only additional paritycheck bits to the destination by utilizing the harvested energy.
Joint design of rate compatible codes for the multisource SWIPTbased coded cooperation
In this section, firstly, we briefly introduce a kind of naturally rate compatible LDPC codes–QCRA codes. Then, we jointly design the QCRA codes employed by the multiple sources and relay in the SWIPTbased coded cooperation to further improve the coding gain and accelerate the decoding convergence.
QCRA codes—a kind of rate compatible LDPC codes
Assume the sparse paritycheck matrix of a QCRA code has the form as H = [A D]. D is a quasidiagonal matrix in which all elements are zero except the elements of the principal diagonal and the elements immediately below this diagonal. A is a quasicyclic sparse matrix which is constructed based on the base matrix and exponent matrix [26]. We refer to A and D as the quasicyclic (QC) part and quasidiagonal (QD) part, respectively. The paritycheck matrix is shown as follows:
where
\( {\boldsymbol{I}}_{B\times B}^{\Big({p}_{j,l\Big)}}\kern0.6em \) is an identity matrix I_{B × B}with p_{j, l}rightcyclicshift.
Definition 1: Let the base matrix M(A) and exponent matrix E(A) of A in H be defined as follows:
where
Joint design of QCRA codes
Assume H_{1} = [A D_{1}], H_{2} = [B D_{2}], and H_{R} = [C_{1}C_{2} D_{3}] are the paritycheck matrices corresponding to QCRA1, QCRA2, and QCRAR employed by S_{1}, S_{2}, and R, respectively_{,} and their corresponding codewords are c_{1} = [s_{1}p_{1}], c_{2} = [s_{2}p_{2}], and c_{R} = [s_{1}s_{2} p_{R}]. From the viewpoint of the SWIPTbased coded cooperation system, the overall codeword at the destination is c = [c_{1}c_{2} p_{R}]. Define the joint paritycheck matrix of c as \( \tilde{\boldsymbol{H}} \). We have \( \tilde{\boldsymbol{H}}\boldsymbol{c}=0 \). According to the following paritycheck relationship
the joint paritycheck matrix \( \tilde{\boldsymbol{H}} \) is achieved as:
The joint Tanner graph corresponding to the joint paritycheck matrix \( \tilde{\boldsymbol{H}} \) is illustrated in Fig. 2.
While we analyze the short cycles in the joint Tanner graph corresponding to \( \tilde{\boldsymbol{H}} \), exchanging any two columns of \( \tilde{\boldsymbol{H}} \) does not influence the status of the cycles. For simplicity, we firstly exchange the second and third columns of \( \tilde{\boldsymbol{H}} \) and then decompose \( \tilde{\boldsymbol{H}} \) into \( {\tilde{\boldsymbol{H}}}_{\mathrm{Q}\ \mathrm{C}} \) and \( {\tilde{\boldsymbol{H}}}_{\mathrm{Q}\ \mathrm{D}} \).
To further improve the coding gain, we design the joint paritycheck matrix \( \tilde{\boldsymbol{H}} \) to cancel all the girth4 cycles in the joint Tanner graph. Firstly, we jointly design the base matrix of QC part \( \mathbf{M}\left({\tilde{\boldsymbol{H}}}_{\mathrm{Q}\kern0.1em \mathrm{C}}\right) \) by Algorithm 1 to cancel the girth4 cycles as much as possible. Secondly, if there are still remaining girth4 cycles in \( \mathbf{M}\left({\tilde{\boldsymbol{H}}}_{\mathrm{Q}\kern0.1em \mathrm{C}}\right) \), we further design the exponent matrix of QC part \( \mathbf{E}\left({\tilde{\boldsymbol{H}}}_{\mathrm{Q}\kern0.1em \mathrm{C}}\right) \)by Theorem 1 and Theorem 2 to cancel all the girth4 cycles absolutely [27].
Theorem 1: Assume there is a remaining girth4 cycle in \( \mathbf{M}\left({\tilde{\boldsymbol{H}}}_{\mathrm{Q}\kern0.1em \mathrm{C}}\right) \), whose corresponding shift values are p_{j, l}, p_{j + k, l}, p_{j, l + t}, and p_{j + k, l + t}. To avoid girth4 cycles in \( {\tilde{\boldsymbol{H}}}_{\mathrm{Q}\ \mathrm{C}} \), a necessary and sufficient condition that they should satisfy is
The proof is referred in [26].
Theorem 2: Assume there are two upper and lower adjacent “1”s in \( \mathbf{M}\left({\tilde{\boldsymbol{H}}}_{\mathrm{Q}\kern0.1em \mathrm{C}}\right) \), whose corresponding shift values are p_{j, l}, p_{j + 1, l}. To cancel girth4 cycles between \( {\tilde{\boldsymbol{H}}}_{\mathrm{Q}\ \mathrm{C}} \) and \( {\tilde{\boldsymbol{H}}}_{\mathrm{Q}\ \mathrm{D}} \), a necessary and sufficient should be satisfied is
The proof in detail can be referred in [24].
Codeword splitting protocol for the multisource SWIPTbased coded cooperation
In the proposed system, assume two sources employ the jointly designed QCRA codes with the same code length N = K + M and code rate r = K/N = K/(K + M), and the codeword c = (s_{1}, s_{2}, ⋯, s_{K}, p_{1}, p_{2}, ⋯, p_{M})is shown in Fig. 3. We explore the rate compatible characteristic of QCRA codes. Assume the quality of S_{1}R channels is improved, it can support the code rate up to r_{1} = K/(K + m). The so called codeword splitting protocol for SWIPT technology is described as follows. The codeword c = (s_{1}, s_{2}, ⋯, s_{K}, p_{1}, p_{2}, ⋯, p_{M}) is split into two parts, i.e., the subcodeword \( \hat{\boldsymbol{c}}=\left({s}_1,{s}_2,\cdots, {s}_K,{p}_1,{p}_2,\cdots, {p}_m\right) \) with code rate r_{1}and the remaining M_{EH} = M − mcheck bits (p_{m + 1}, p_{m + 2}, ⋯, p_{M}). The subcodeword \( \hat{\boldsymbol{c}} \) is only needed for information decoding, and the remaining M − m check bits are used for energy harvesting.
As shown in Fig. 4, for the power splitting protocol, the power of the received signal corresponding to each bit of the codeword is separated into two parts, one for information decoding and the other for energy harvesting. It is difficult to be implemented in hardware, and the energy utilization efficiency is limited. For the time switching protocol, the ID and EH modes have to be switched during each bit transmission period. The implementation complexity is high, and the strictly bit synchronization is required. The antenna switching protocol does not apply in the single antenna case. It can be seen that the power splitting protocol and the time switching protocol are both carried out during each bit period. However, the proposed codeword splitting protocol is only operated one time during the whole codeword period. Hence, the codeword splitting protocol is with the characteristics of lower complexity, higher efficiency, no strictly bit synchronization limitation, and less hardware requirement.
Results and discussion
We investigate the performance of multisource SWIPTbased coded cooperation by numerical simulations in this section. S_{1}R, S_{2}R, S_{1}D, S_{2}D, and RD are all additive white Gaussian noise (AWGN) channels. The signal to noise ratios (SNRs) of S_{1}D and S_{2}D are the same. The SNRs of S_{1}R and S_{2}R determine how many bits from S_{1} and S_{2} can be used for EH at the relay, and the SNR of RD depends on the number of bits for EH at the relay. The joint iterative decoding algorithm [28] and binary phase shift keying (BPSK) modulation are assumed at the destination. For twosource SWIPTbased coded cooperation, QCRA codes at the sources and relay are given in Table 1.
BER comparison of SWIPTbased jointly designed QCRA coded cooperation and general RA coded cooperation
We compare the BER performance of SWIPTbased jointly designed QCRA coded cooperation and general RA coded cooperation. General RA codes and jointly designed QCRA codes have the same code length and code rate codes as shown in Table 1. Their paritycheck matrices both have quasicyclic structure. The relay uses subcodeword with 300 bits for ID and the rest 100 check bits for EH. It is shown in Fig. 5 when the number of decoding iterations is one, the BER curves of the jointly designed QCRA codes and the general RA codes are almost the same. It is because the extrinsic information is not exchanged sufficiently during the iterative decoding, and the influence of short cycles does not appear obviously. Figure 5 also illustrates that the BER performance of the jointly designed QCRA coded cooperation clearly outperforms that of general RA coded cooperation when the number of decoding iterations is two or ten. This is because in the jointly designed QCRA codes, all girth4 cycles are cancelled, and there are neither girth4 cycles in the single QCRA codes nor girth4 cycles between them. Hence, when the joint iterative decoding is implemented at the destination, higher coding gain is achieved, and the gain increases with the number of decoding iterations rising.
BER comparison of SWIPTbased QCRA coded cooperation with codeword splitting protocol and power splitting protocol
In this part, we compare the proposed SWIPTbased QCRA coded cooperation with codeword splitting protocol and the power splitting protocol in [17]. The relay uses subcodeword with 300 bits for ID and the rest 100 check bits for EH. The number of decoding iterations is ten. For a fair comparison, in the power splitting protocol, we assume 1/4 power of each bits of the total codeword is used to harvest energy. When the energy utilization ratio is η = 1, it is shown in Fig. 6 that the BER performance of the codeword splitting protocol is almost the same with the power splitting protocol. For the power splitting protocol, as described in Section 2.3, firstly, the relay has to split the power bit by bit for ID and EH and then collects the EH energy from all bits for transmission. Hence, compared with the codeword splitting protocol, the energy utilization ratio of the power splitting protocol actually decreases. Furthermore, the time delay at the relay is longer, and the complexity is higher. When the energy utilization ratio of the power splitting protocol decreases to 0.9 or 0.6, the BER performance of codeword splitting protocol is much superior to that of the power splitting protocol.
BER performance of SWIPTbased QCRA coded cooperation exploiting rate compatible characteristic
For the codeword splitting protocol, at the relay, the number of check bits for EH adapts to the quality of S_{i}R channels. We assume the quality of S_{1}R channel is fixed and it can support the code rate up to r_{1} = 2/3. The quality of S_{2}R channel varies and it adapts to support the code rate r_{2} = 2/3, 4/5, 1. By the rate compatible characteristic of QCRA codes, M_{EH} = M − m = 100, 150, 200 check bits can be exploited for EH, respectively. In Fig. 7, it is shown that the BER of the proposed system decreases sharply with the quality of S_{2}R channel becomes higher from supporting code rate r_{2} = 2/3 to supporting code rate r_{2} = 1. This is because the more check bits exploited for EH by the rate compatible characteristic of QCRA codes, the more energy harvested at the relay.
BER comparison of SWIPTbased coded cooperation and point to point system over AWGN channels or Rayleigh fading channels
In this part, we compare the BER performance of SWIPTbased coded cooperation and pointtopoint (noncooperation) system over AWGN channels or Rayleigh fading channels, which are block fading with perfect channel state information at the destination. The fading coefficient for each channel remains constant over each codeword. SWIPTbased coded cooperation employs the jointly designed QCRA codes as shown in Table 1, and the relay uses subcodeword with 300 bits for ID and the rest 100 check bits for EH. The pointtopoint system without SWIPTbased relay employs single QCRA code whose girth4 cycles are also cancelled. The number of decoding iterations is ten.
In Fig. 8, it is shown that the BER performance of SWIPTbased coded cooperation clearly outperforms that of the pointtopoint system over AWGN channels. For example, at the SNR = 3 dB, compared with the pointtopoint system, the BER of the coded cooperation drops from about 2 × 10^{−5} to 8 × 10^{−6}. It demonstrates the superiority of the investigated scheme over AWGN channels. We also compare them over Rayleigh fading channels. It is demonstrated that the BER performance of coded cooperation is much better than that of the pointtopoint system. For example, at the BER = 10^{3}, it achieves about 2 dB gain. What is more, the results also show that the SWIPTbased coded cooperation achieves a higher diversity gain.
Conclusion
In this paper, we have investigated the multisource SWIPTbased QCRA coded cooperation. It combines the SWIPT, advanced channel coding, and cooperation technologies. Hence, the investigated system can achieve high coding gain, spatial diversity gain, and high energy efficiency. It is a new approach to the high reliable and energysaving green characteristics of the next generation wireless communications. We deduced the joint paritycheck matrix corresponding to the QCRA codes employed by the multiple sources and relay, based on which we jointly designed the rate compatible QCRA codes to cancel all the girth4 cycles. Furthermore, by exploring the rate compatible characteristic of QCRA codes, we proposed a new SWIPT protocol—codeword splitting protocol for the proposed system. Theoretical analysis and numerical simulations demonstrated the superiority of the designed QCRA codes and the proposed codeword splitting protocol.
Availability of data and materials
The authors declare that all the data and materials in this manuscript are available from the author.
Abbreviations
 AF:

Amplifyandforward
 AWGN:

Additive white Gaussian noise
 BER:

Bit error rate
 BPSK:

Binary phase shift keying
 DF:

Decodeandforward
 EH:

Energy harvesting
 ICT:

Information and communication technology
 ID:

Information decoding
 LDPC:

Lowdensity paritycheck
 MIMO:

Multipleinput multipleoutput
 QCRA:

Quasicyclic repeataccumulate
 SWIPT:

Simultaneous wireless information and power transfer
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Acknowledgements
The authors wish to thank the editor and the anonymous reviewers for their valuable suggestions on improving this paper.
Funding
This work was supported in part by the National Natural Science Foundation of China (61501256, 61501250), the Natural Science Foundation of Jiangsu Province (BK20150857), the NUPTSF (NY219073), and the China Scholarship Council (201608320093).
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Shunwai Zhang is the main writer of this paper and proposed the main idea. Lingjun Kong and Jun Li revised and checked the whole manuscript. All authors read and approved the final manuscript.
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Shunwai Zhang received the B.E. degree in Electronic Engineering from Nanjing University of Technology, Nanjing, China, in 2008, and the Ph.D. degree in Telecommunications from Nanjing University of Aeronautics and Astronautics, Nanjing, China, in 2014. He joined Nanjing University of Posts and Telecommunications as an Assistant Professor since 2014, and then as an Associate Professor since 2017. From 2016 to 2017, he was a research scientist with the Department of Electrical Engineering, University of Texas at Dallas, USA. He has authored or coauthored over 20 research papers. His research interest includes advanced channel coding, cooperative communications, and green communications.
Lingjun Kong received the M.S. and the Ph.D. degree in Electrical Engineering from Beijing Jiaotong University, Beijing, China, in 2007 and 2011, respectively. He had been a research fellow at Nanyang Technical University from Feb. 2012 to Nov. 2013, and a visiting scholar in the Center for Memory Recording Research (CMRR) at University of California, San Diego, USA, from Dec. 2016 to Dec. 2017. He is currently an associate professor with College of Telecommunication and Information Engineering, Nanjing University of Posts and Telecommunications, Nanjing, China. His current research interests include coding theory, communication theory, and signal processing for various data storage and wireless communication systems.
Jun Li received Ph.D. degree in Electronic Engineering from Shanghai Jiao Tong University, Shanghai, P. R. China, in 2009. From January 2009 to June 2009, he worked in the Department of Research and Innovation, Alcatel Lucent Shanghai Bell as a research scientist. From June 2009 to April 2012, he was a postdoctoral fellow at the School of Electrical Engineering and Telecommunications, the University of New South Wales, Australia. From April 2012 to June 2015, he is a research fellow at the School of Electrical Engineering, the University of Sydney, Australia. From June 2015 to now, he is a professor at the School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing, China. His research interests include network information theory, channel coding theory, wireless network coding, and cooperative communications.
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Zhang, S., Kong, L. & Li, J. Multisource SWIPTbased coded cooperation:rate compatible codes and codeword splitting protocol. J Wireless Com Network 2020, 54 (2020). https://doi.org/10.1186/s1363802001664x
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Keywords
 Coded cooperation
 Codeword splitting protocol
 LDPC codes
 QCRA codes
 SWIPT