A Physical-layer Network Coding Relay scheme for IEEE 802.11

1. A Physical-layer Network Coding Relay scheme for IEEE 802.11 Date: 2013-01-11 Authors:

2. Abstract • Based on joint network coding and channel coding, a physical network coding (PNC) transmission scheme for a two-way relay system is proposed. By using the broadcast nature of the wireless medium and the linear property of LDPC codes, only two time slots are required for data transmission. In the first time slot, the two users send their own data to the relay at the same time. In the second time slot, the relay performs joint network decoding and channel decoding, and then forwards the encoded bits to the two users. Simulations results show that the proposed scheme outperforms the conventional decode-and-forward (DF) Relay in term of system throughput, which is increased up to 100%.

3. Background • Device-to-device (D2D) communication is a more reasonable scheme for the traffic between a communicating STAs pair, which are closely located with limited interference to the WLAN network. D2D links have several advantages, such as low transport delay and high data rate. • One drawback of direct D2D communication is the limited range, the relay is able to extend the communication range and improve the performance of wireless systems. • Physical network coding (PNC) is a joint network and channel coding technique that can be used at the relay station to help cooperating users to improve the throughput.

4. Two way relay channel （TWRC） • A Physical-layer Network Coding (PNC) scheme is proposed to further boost the throughput and capacity of the system.

5. Diagram of 3-Phase PNC

6. Process flow • Process flow of 3-phase conventional networking coding scheme : • The information bits of STAs S1 and S2 are encoded by LDPC encoder to produce codewords c1 and c2, respectively. And then, modulate to produce complex-valued symbols x1 and x2. • Symbols x1 and x2 are sent to relay node in two different time slots, respectively. • Relay demodulates and decodes the received symbols y1 and y2 to get the estimates c1 and c2, respectively. If the estimates do not satisfy the parity check matrix of LDPC, adopt ARQ protocol and chase combining. • If satisfied, Relay performs the network coding and modulates to broadcast for the two STAs in one time slot. • Each STA demodulates and decodes the received signal.

7. Diagram of 2-Phase PNC

8. System Model of PNC • NOTES • Multiple access (MA) phase: Two STAs send signals simultaneously in one timeslot to the relay. • Broadcast phase: the relay decodes the superimposed signals and maps them to a network-coded (XOR) packet to broadcast to the two STAs.

9. Process flow • Process flow of 2-phase PNC scheme : • The information bits of STAs S1 and S2 are encoded by the same LDPC encoder to produce codewords c1 and c2, respectively. And then, modulate to produce complex-valued symbols x1 and x2. • Symbols x1 and x2 are sent to relay node at the same time in one time slot. • Relay demodulates the superimposed received signal to produce the LLRs of as LDPC decoder input, and then decodes by LDPC to get the z estimates . If the z estimates do not satisfy the parity check matrix of LDPC, adopt ARQ and chase combining. . • If satisfied, Relay modulates z and broadcasts to two STAs in one timeslot. • Each STA demodulates and decodes the received signal.

10. Detection and decoding of relay for 2-phase PNC scheme • In the MA phase, the STAs S1 and S2 send symbols x1 and x2 to relay node simultaneously. The AWGN channel and BPSK modulation is considered. The received superimposed signal of relay is , where n is the AWGN random complex variable with mean zero and variance . • Define , • Assume L denotes the LLR of ,

11. Simulation Parameters

12. Throughput performance of two STAs • We use term throughput (tp) to indicate the average number of correct received data packets in one time slot for two STAs. • The tp performance of the proposed scheme is significantly improved. • For SNR<-4 dB, the DF scheme is the best; for -4 dB<SNR<-1 dB, 3-phase PNC is the best, for SNR>-1dB, 2-phase PNC is the best, TP is increased up to 100%.where SNR = Es/ σ2.

13. Conclusions • Network coding can significantly boost the throughput and capacity of the system. • Compared to the conventional DF relay scheme, the throughput of the 2-phase PNC is increased up to 100%, and the throughput of the 3-phase PNC is increased up to 33%. • The proposed scheme can be applied to two-way high-throughput scenarios, such as short-range video communication, short-range two-person interactive 3D game and data synchronization for local devices.

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