Low Complex OFDM Synchronization in Power Line Communication for Flight Control System in Aircraft
Abstract
The flight control system (FCS) over power line communication (PLC) is subject to strong real time constraints and harsh aircraft operating conditions. Then, low complex and accurate synchronization procedure have to be adapted. In this paper, we propose a synchronization procedure performed in two phases and we focus on the operating phase with a synchronization using the received data to estimate the sampling frequency offset. This estimation is performed by using two estimators based on the maximum likelihood principle. The first estimator performs the estimation on one orthogonal frequency division multiplexing (OFDM) transmission symbol. The second estimator performs the estimation on 20 successive OFDM symbols and uses the estimation of the previous estimator. The performances of the estimator are compared to the Cramér-Rao lower bound (CRLB). The proposed synchronization procedure reached the CRLB and satisfies the FCS constraints over PLC
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Introduction
A lot of previous works deal with the question of synchronization in orthogonal frequency-division multiplexing (OFDM) [1–3] and digital multi-tone (DMT) [4], applied to power line communication (PLC) systems [5,6]. Many solutions are based on pilot sub-carriers which allows to have the parameter estimation at each OFDM symbol [7–9]. In this case, some subcarriers are kept for the synchronization. Another possibility is to use OFDM pilot symbols. These OFDM pilot symbols are sent regularly to estimate the sampling time and frequency offset. For example, the Schmidl-Cox algorithm [10] and its improvement [11–13] allows such kind of estimation and sampling frequency offset correction with one or two OFDM symbols. The high synchronization accuracy, compatible with OFDM transmission, can then be reached using common oscillator in electronic systems [14].
The power line communication (PLC) has been proposed for the flight control system (FCS) in aircrafts to reduce the wiring complexity [15]. The OFDM transmission has been proposed [16, 17] to ensure the security requirement, which is undamental in FCS. The FCS transmission must satisfy unfavorable conditions and should be deterministic, non-sporadic, continuous and with low latency adapted to the FCS loop. Conventional communication techniques cannot be used in this context. For example, the reliability enhancement with retransmission mechanisms is not possible. In the case of shared channel between multiple transmitters or receivers, the multiple access strategy should be deterministic. Concerning the synchronization procedure, the pilot symbols are not compatible with the continuity requirement and increase adversely the latency. Furthermore, the pilot sub-carriers yield to a bit-rate reduction and are not suitable in secure high bit-rate. The constraints of FCS have been taken into account to design an OFDMPLC system [17], but the synchronization should also be adapted to this specific FCS. Moreover, and under operating conditions, temperature variations and vibrations induce a drift between the local transmitter and receiver oscillators which can cause a sampling frequency offset.
Conclusion
In this paper a sampling frequency synchronization system has been proposed for a PLC system in the FCS aeronautic environment. To solve the FFT drift of the OFDM communication system, the sampling frequency synchronization system is performed in two phases. The first one is the initialization phase during the switching of the aircraft, which allows precise channel estimation and synchronization based on conventional OFDM receiver algorithms. The second one, is the correction of the FFT drift using the phase of the received data every 20 OFDM symbols. To do that, two estimators using the maximum likelihood have been proposed. The first one estimates the sampling frequency offset of one OFDM symbol. The second one estimates the sampling frequency offset of 20 OFDM symbols using the estimation of the first estimator. This estimation allows to have a stable and simple synchronization system up until 20 ppm of drift between transmitter and receiver oscillators. To increase the robustness of the synchronization strategy, a super-frame structure is proposed, which allows a re-estimation of the channel and a resynchronization with an accuracy of one ppm at the beginning of each superframe. The interest to consider a super-frame is to have a long stable period and propose a simple synchronization system during the operating phase.