Joint Relay Selection and Analog Network Coding using Differential Modulation in Two-Way Relay Channels Free PDF Download Link White Paper

In this paper, we consider a general bi-directional relay network with two sources and N relays when neither the source nodes nor the relays know the channel state information (CSI). A joint relay selection and analog network coding using differential modulation (RS-ANC-DM) is proposed. In the proposed scheme, the two sources employ differential modulations and transmit the differential modulated symbols to all relays at the same time.

The signals received at the relay are a superposition of two transmitted symbols, which we call the analog network coded symbols. Then a single relay which has minimum sum SER is selected out of N relays to forward the ANC signals to both sources. To facilitate the selection process, in this paper we also propose a simple sub-optimal Min-Max criterion for relay selection, where a single relay which minimizes the maximum SER of two source nodes is selected.

Simulation results show that the proposed Min-Max selection has almost the same performance as the optimal selection, but is much simpler. The performance of the proposed RS-ANC-DM scheme is analyzed, and a simple asymptotic SER expression is derived. The analytical results are verified through simulations.

In a bi-directional relay network, two source nodes exchange their messages through the aid of one or multiple relays. The transmission in bi-directional relay network can take place over four, three or two time slots. In the four time slots transmission strategy, the relay helps to forward source S1’s message to source S2 in the first two time slots and source S2’s message to source S1 in the next two time slots. Four time slots transmission has been shown to be very inefficient.

When the relay receives two sources’ messages, it combines them before forwarding to the destination, which will save one time slot transmission. This three time slots transmission scheme is usually referred to as the digital network coding [1]– [3]. In this method, two source nodes transmit to the relay, separately. The relay decodes the received signals, performs binary network coding, and then broadcasts it back to both source nodes.

To further improve the spectral efficiency, the message exchange between two source nodes can actually take place in two time slots. In the first time slot, both source nodes transmit at the same time so that the relay receives a superimposed signal. The relay then amplifies the received signal and broadcasts it to both source nodes in the second time slots. This scheme is referred to as the analog network coding (ANC) [4]–[6]. Various transmission schemes and wireless network coding schemes in bi directional relay networks have been analyzed and compared in [7]–[12].

Most of existing works in bi-directional relay communications consider the coherent detection at the destination and assume that perfect channel state information (CSI) are available at the sources and relays [1]–[12]. In some scenarios, e.g. the fast fading environment, the acquisition of accurate CSI may become difficult. In this case, the non-coherent or differential modulation would be a practical solution. In a differential bi-directional relay network, each source receives a superposition of differentially encoded signals from the other source, and it has no knowledge of CSI of both channels.

 All these problems present a great challenge for designing differential modulation schemes in two-way relay channels. To solve this problem, in [13], a non-coherent receiver for two-way relaying was proposed for ANC based bi-directional relay networks. However, the schemes result in more than 3 dB performance loss compared to the coherent detection. To further improve the system performance, a differential ANC scheme was proposed in [14] and a simple linear detector was developed to recover the transmitted signals at two source nodes.

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