Two-way amplify-and-forward relaying with carrier offsets in the absence of CSI: differential modulation-based schemes
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Two-way amplify-and-forward relaying with carrier offsets in the absence of CSI: differential modulation-based schemes Wu Zhuo*† , Li Guangxiang*† and Wang Tao*†
Abstract In this paper, differential modulation (DM) schemes, including single differential and double differential, are proposed for amplify-and-forward two-way relaying (TWR) networks with unknown channel state information (CSI) and carrier frequency offsets caused by wireless terminals in high-speed vehicles and trains. Most existing work in TWR assumes perfect channel knowledge at all nodes and no carrier offsets. However, accurate CSI can be difficult to obtain for fast varying channels, while increases computational complexity in channel estimation and commonly existing carrier offsets can greatly degrade the system performance. Therefore, we propose the two schemes to remove the effect of unknown frequency offsets for TWR networks, when neither the sources nor the relay has any knowledge of CSI. Simulation results show that the proposed differential modulation schemes are both effective in overcoming the impact of carrier offsets with linear computational complexity in the presence of high mobility. Keywords: Bidirectional relay communication; Amplify-and-forward; Differential; Carrier offsets
Introduction Two-way relaying (TWR) has attracted much interest recently [1-7], where two source terminals communicate with each other through an intermediate relay. Both amplify-and-forward (AF) and decode-and-forward (DF) relaying schemes under one-way relaying have been extended to TWR [3,4]. In the DF protocol, the relay first decodes the information transmitted from both sources in the multiple-access (MA) phase, performs binary network coding to the decoded signal, then broadcasts the network-coded signal back to the sources in the broadcast (BC) phase. If the relay cannot decode the information correctly, erroneous relaying will cause significant performance degradation. For the AF-based TWR, the relay amplifies the superimposed signal received from the two sources and then broadcasts it back in the BC phase. AFbased TWR is particularly useful in wireless networks, since the wireless channel acts as a natural implementation of network coding by summing the wireless signals *Correspondence: [email protected]; [email protected]; [email protected] † Equal contributors Key Laboratory of Specialty Fiber Optics and Optical Access Networks, Shanghai University, Yanchang Road, Shanghai 200072, China
over the air. Therefore, we will focus on the AF-based TWR in this paper. There has been some work investigating TWR using AF [4-6], referred to as analog network coding (ANC). However, most of the existing work assumes that perfect channel state information (CSI) is known at all transmission links. Although in some scenarios, the CSI is likely to be acquired through the use of pilot signals, it may be very difficult to obtain accurate CSI when the channel coefficients vary fast. Moreover, conventional estimation methods do not work for AF-bas
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