Interference in wireless networks is one of the key capacity-limiting factors. Recently developed interference-embracing techniques show promising performance on turning collisions into useful transmissions. However, the interference-embracing techniques are hard to apply in practical applications due to their strict requirements. In this paper, we consider utilizing the interference-embracing techniques in a common scenario of two interfering sender-receiver pairs. By employing opportunistic listening and analog network coding (ANC), we show that compared to traditional ARQ retransmission, a higher retransmission throughput can be achieved by allowing two interfering senders to cooperatively retransmit selected lost packets at the same time. This simultaneous retransmission is facilitated by a simple handshaking procedure without introducing additional overhead. Simulation results demonstrate the superior performance of the proposed cooperative retransmission.
The idea of turning a collision of two simultaneous wireless transmissions into a useful transmission was first introduced in PNC @cite_5 . In particular, the authors proposed a frame-based decode-and-forward strategy in packet forwarding. In their scenario of a relay network, two nodes transmit simultaneously to a common receiver. Assuming perfect transmission synchronization at the physical layer, based on the additive nature of simultaneously arriving (EM), the receiver detects a single collided signal which is the sum of the two transmitted signals. Using a suitable mapping scheme, they show that for certain modulation schemes, there exists a mapping scheme such that the relationship between the two transmitted binary bits and the decoded binary bit follows the (XOR) principle. ANC @cite_2 was further proposed to relax the restrictions of symbol-level synchronization, carrier-frequency synchronization and carrier-phase synchronization required in PNC, which makes ANC more practical. Specifically, ANC is able to decode an unknown packet @math from a collided packet @math We use the notation @math to denote a collision of two packet transmissions. based on the known packet @math by leveraging the co-channel FM signal separation technique @cite_1 and network layer information to cancel the interference.
@cite_5 @cite_1 @cite_2
1975099099 2166383582 2152496949
Abstract This paper presents the first implementation of a two-way relay network based on the principle of physical-layer network coding (PNC). To date, only a simplified version of PNC, called analog network coding (ANC), has been successfully implemented. The advantage of ANC is that it is simple to implement; the disadvantage, on the other hand, is that the relay amplifies the noise along with the signal before forwarding the signal. PNC systems in which the relay performs XOR or other denoising PNC mappings of the received signal have the potential for significantly better performance. However, the implementation of such PNC systems poses many challenges. For example, the relay in a PNC system must be able to deal with symbol and carrier-phase asynchronies of the simultaneous signals received from multiple nodes, and the relay must perform channel estimation before detecting the signals. We investigate a PNC implementation in the frequency domain, referred to as FPNC, to tackle these challenges. FPNC is based on OFDM. In FPNC, XOR mapping is performed on the OFDM samples in each subcarrier rather than on the samples in the time domain. We implement FPNC on the universal soft radio peripheral (USRP) platform. Our implementation requires only moderate modifications of the packet preamble design of 802.11a g OFDM PHY. With the help of the cyclic prefix (CP) in OFDM, symbol asynchrony and the multi-path fading effects can be dealt with simultaneously in a similar fashion. Our experimental results show that symbol-synchronous and symbol-asynchronous FPNC have essentially the same BER performance, for both channel-coded and non-channel-coded FPNC systems. Most medium access control (MAC) mechanisms discard collided packets and consider interference harmful. Recent work on Analog Network Coding (ANC) suggests a different approach, in which multiple interfering transmissions are strategically scheduled. Receiving nodes collect the results of collisions and then use a decoding process, such as ZigZag decoding, to extract the packets involved in the collisions. In this paper, we present an algebraic representation of collisions and describe a general approach to recovering collisions using ANC. To study the effects of using ANC on the performance of MAC layers, we develop an ANC-based MAC algorithm, CMAC, and analyze its performance in terms of probabilistic latency guarantees for local packet delivery. Specifically, we prove that CMAC implements an abstract MAC layer service, as defined in [14, 13]. This study shows that ANC can significantly improve the performance of the abstract MAC layer service compared to conventional probabilistic transmission approaches. We illustrate how this improvement in the MAC layer can translate into faster higher-level algorithms, by analyzing the time complexity of a multi-message network-wide broadcast algorithm that uses CMAC. When data are transmitted in a wireless network, they reach the target receiver as well as other receivers in the neighborhood. Rather than a blessing, this attribute is treated as an interference-inducing nuisance in most wireless networks today (e.g., IEEE 802.11). Physical-layer network coding (PNC), however, has been proposed to take advantage of this attribute. Unlike "conventional" network coding which performs coding arithmetic on digital bit streams after they are decoded, PNC makes use of the additive nature of simultaneously arriving electromagnetic (EM) waves and applies the network coding arithmetic at the physical layer. As a result, the destructive effect of interference is eliminated and the capacity of networks is boosted significantly. A key requirement of PNC is synchronization among nodes, which has not been addressed previously. This is the focus of this paper. Specifically, we investigate the impact of imperfect synchronization (i.e., finite synchronization errors) on PNC. We show that with BPSK modulation, PNC still yields significantly higher capacity than straightforward network coding when there are synchronization errors. And interestingly, this remains to be so even in the extreme case where synchronization is not performed at all.