Deliverable 3

8.3A New Decode-Rotate-and-Forward Strategy for Cooperative Relay Networks

8.3.1System Description

We propose a new relaying strategy, called decode-rotate-and-forward for wireless relay networks. We consider a scenario where a source communicates with a destination with the help of a relay. The source and the relay are allowed to transmit simultaneously.

We consider a full-duplex relay, i.e., the relay is allowed to transmit and receive simultaneously.

We consider the wireless relay network depicted in Figure . The network consists of a source S which communicates statistically independent data to a single destination D cooperating through a relay R.

Except for the first transmission, the source and the relay transmit simultaneously. At source S, the information sequence u_s, of length k bits, is encoded by encoder C_s of rate R_s into codeword c_s. Codeword c_s is then modulated into x_s and transmitted over a wireless channel. For simplicity, we first consider BPSK modulation.

_{Figure : Simultaneous transmission: the source and the relay transmit simultaneously; the relay forwards a coded version of the data source of the previous slot.}

Due to the broadcast nature of the wireless channel the relay receives a noisy version of codewords c_s, denoted by y_s,r, from the source. During the same time slot, the relay transmits a new version x_r of the source data received in the previous slot. The destination receives the signals from the source and the relay simultaneously and decodes the information of the source by jointly exploiting the received sequence y_d from the source and the relay simultaneously. The transmission scheduling is illustrated Figure .
The relay receives a noisy version of codewords c_s from the source, it decodes it, and generates an estimate of the information bits u_s. The estimated sequence û_s is then interleaved into ũ_s by an interleaver П, and re-encoded by another encoder C_r into codeword c_r. Codeword c_r is then modulated into x_r using BPSK modulation rotated by an angle φ. Here, we consider a rotation angle φ=90°. The proposed distributed coding scheme is depicted in Figure .

_{Figure : Block diagram of the proposed decode-rotate-and-forward strategy: Encodings at the source and the relay. Equivalent distributed constellation between the source and the relay.}

The modulation used in the proposed scheme can be seen as a distributed higher order modulation between the source and the relay (here, the resulting higher order modulation is a QPSK modulation) where the BPSK modulation at the source accounts for modulation on the I axis (Inphase axis) of the QPSK modulation and the BPSK modulation at the relay accounts for modulation on the Q axis (Quadrature axis). The equivalent distributed constellation is depicted in Figure .

At the destination D, a higher order demodulator is used to jointly demodulate the received sequence y_d and generate the channels log-likelihood ratios (LLRs) LLR(x_s) and LLR(x_r). Here, since BPSK modulation is used at the source and the relay, a QPSK demodulator is considered. LLR(x_s) and LLR(x_r) are then passed to decoders C^-1_s and C^-1_r of the source code and the relay code, respectively. Decoding of the source information is performed in the classic way of a distributed turbo code [139], i.e., in an iterative fashion by exploiting the additional redundancy provided by the relay.

This work has recently been filed as a patent and is currently under review at the National Institute of Intellectual Properties (INPI France).

8.3.2Numerical Results

The performance of the proposed scheme is evaluated through simulations. For simplicity, the 4-state, rate-1/2, recursive convolutional encoder with generator polynomial (1,5/7)₈ is used at the source and the relay. However, different constituent encoders at the sources and the relay can be considered. An S-random interleaver П and a maximum of ten decoding iterations are assumed.

For comparison purposes, we also consider the non-cooperation scenario where a source transmits to the destination without the help of the relay. For fair comparison, we consider that the source uses an encoder rate R=1/4 and QPSK modulation in order to be at the same spectral efficiency of our proposed scheme.
In Figure , we give BER results for the proposed decode-rotate-and-forward scheme over AWGN channels as a function of γ_sd and a block length k=128 bits. We also assume that d_rd=d_sd and we assume that the source-to-relay link is reliable enough to guarantee a low error probability at the relay which is a necessary condition for the decode-and-forward strategy [141], and therefore for the new decode-rotate-and-forward strategy. In Figure , BPSK modulation is used at the source and a rotated BPSK of angle φ=90° is used at the relay. The resulting constellation is a QPSK constellation as described in Section 8.3.1.

The proposed distributed coding/modulation scheme shows a significant gain with respect to the non-cooperation scenario. For instance, a gain of almost 3 dB is observed at BER=10^-3 compared to direct transmission. Moreover, it allows at efficiently exploiting the radio channel and achieves higher spectral efficiency compared to schemes time or frequency sharing transmission.