Deliverable 3

8.2Distributed Coding for OFDM-Based Transmission in Cooperative Broadcast Networks

8.2.1System Description

We consider the wireless relay network depicted in Figure . We consider an OFDM transmission waveform where the available bandwidth is divided into N_FFT subcarriers. N_BS subcarriers are allocated to the base station transmission and N_R subcarriers to the relay with N_FFT=N_BS+N_R.

_{Figure : A wireless relay network: a base station broadcasts to a mobile station with the help of a relay.}
At the base station BS, the information sequence u_BS, of length k_BS bits, is encoded by encoder C_BS of rate R_BS into codeword c_BS of length n_BS=k_BS/R_BS bits. Codeword c_BS is modulated and then mapped onto OFDM symbol x_BS with N_BS subcarriers and then transmitted over a wireless channel. Here, we use the binary phase shift keying (BPSK) modulation; however, our scheme can be extended to higher order modulations. The transmitter at the base station is depicted in Figure . Due to the broadcast nature of the wireless channel, the relay receives a noisy version of symbol x_BS, denoted by y_BS,R, from the base station BS. It then cooperates with the base station BS by transmitting its own parity sequence x_R to the mobile station MS. The mobile station decodes the information of the base station by jointly exploiting the received sequence y_BS,MS from the base station and the sequence y_R,MS from the relay.

_{Figure : a) The transmitter at the base station. b) The receiver and the transmitter at the relay node.}

Based on the two observations y_BS,MS and y_R,MS from the base station and the relay, respectively, an iterative decoding is performed at the mobile station.

The proposed relaying scheme can be regarded to as a decode-and-forward scheme. The relay receives a noisy version of symbol x_BS from the base station, it decodes it, and generates an estimate of codeword c_BS. The estimated bits are then interleaved by an interleaver Π. The bits at the output of the interleaver are encoded by a recursive inner encoder C_e of rate R_e punctured to rate R_I=R_e/ρ through a puncturer. ρ denotes the permeability ratio of the puncturer, giving the ratio of bits that survive puncturing. Parameter ρ determines the amount of redundancy transmitted by the relay and can be adjusted according to requirements in terms of performance, throughput and/or power. The resulting codeword is then modulated by BPSK modulation and mapped onto OFDM symbol x_R with N_R subcarriers and transmitted to the mobile station.

The subcarriers can be optimally allocated between the base station and the relay in order to achieve higher rates [143]. Furthermore, the coding/decoding scheme can be extended to scenarios where multiple relays aid the communication from the base station to the mobile station. If successful decoding is achieved at the relays, the relays process the received signal according to the encoding strategy described above and SFN-like transmission at the relays, i.e., the relays are allocated the same subcarriers N_R in order to efficiently exploit the radio spectrum [143].

8.2.2Numerical Results

The performance of the proposed scheme is evaluated through simulations. The 4-state, rate-1/2, recursive convolutional encoder with generator polynomials (1,5/7) in octal form is used at the base station. The 4-state, rate-1, recursive convolutional encoder with generator polynomial (5/7) is used at the relay.

In Figure we give bit error rate (BER) results of the proposed scheme for block length k_BS=1024 bits over a Rayleigh fast fading channel as a function of γ_BS,MS. An S-random interleaver and a maximum of ten decoding iterations were considered. We also assumed that d_BS,R=(1/4)d_BS,MS and d_R,MS=(3/4)d_{BS,MS .}

A total number of N_FFT=4096 subcarriers is considered. N_BS=2048 subcarriers and parameter ρ is set so that N_R=2048 subcarriers. Therefore, the effective global rate of the overall system R_eff=1/4. For comparison purposes, we also consider the non-cooperation scenario where the base station transmits to the mobile station without the help of the relay.

For fair comparison, we plot the non-cooperative curve assuming that the base station transmits with a rate 1/4 which corresponds to the effective rate of the cooperative system. We consider the use of convolutional codes (CCs) and turbo codes (TCs) at the base station. The proposed distributed turbo-like code shows a significant gain with respect to the non-cooperation scenario. It also outperforms the non-cooperation scheme where a turbo code is used at the base station.

We also consider a scenario where the transmission from the base station to the mobile station is assisted by a gap filler which simply amplifies the received signal from the base station and forwards it to the mobile station. The proposed cooperative scheme presents better performance in terms of error rate compared to the gap filler scenario. The observed gain is due to the iterative decoding process in the proposed scheme.

_{Figure : BER curves of the non-cooperative system and the turbo-like coding scheme over Rayleigh fast fading channel. kBS=1024 bits, NFFT=4096 subcarriers, Reff=1/4, 10 iterations. The dashed curve corresponds to a broadcast scenario employing gap}