Optimal Channel Coding Structures for Fast Acquisition Signals in Harsh Environment Conditions
Lorenzo Ortega Espluga, Charly Poulliat, Marie Laure Boucheret, Marion Aubault-Roudier, Hanaa AlBitar, TéSa, France
In recent works    , the interest for reducing the Time-To-First-Fix (TTFF) on the GNSS system has motivated the design of new channel coding schemes, which decrease the time to retrieve the Clock and Ephemerides Data (CED), also called Time-To-Data (TTD). In  and , the coding schemes exploit both serial concatenation and the Maximum Distance Separable (MDS) property in order to retrieve the information data as fast as possible and to improve the demodulation threshold of the data. In , the coding schemes exploit both the MDS property and the full diversity in order to both reduce the complexity of the decoder and improve the error correcting performance in hash environments, while the information data is retrieved as fast as possible. Finally in , the coding scheme exploits not only the MDS property and the full diversity, but also the rate-compatible property  is added in the coding scheme design. Thanks to the rate-compatible property, an outstanding enhancement of the error correction capabilities and the demodulation threshold is achieved.
In this paper, we provide the methodology to construct channel coding structures able to provide MDS property, full diversity property and rate-compatible property. Thanks to those combined properties, the decoder is capable to reduce the TTD and to provide enhanced error correction capabilities and lower demodulation threshold under low Carrier to Noise ratio (C/N0) environments, urban environments and pulsed jamming environments. Moreover, in order to ensure the robustness of the CED, error detecting techniques are combined with the channel coding structure, as it was already the case in    .
For this purpose, in order to construct the channel coding structures, we start by modeling the message structure (information bits and redundant bits) under the non-ergodic channel assumption   (commonly presented as block fading channel and which can be seen as an extension of the already presented  erasure channel model). Accordingly, the information and the redundant bits from the channel coding encoder are divided into different data blocks and each block is weighted by a fading coefficient. Modeling the message structure under the erasure channel assumption , helps us to finely describe how the CED can be retrieved under lack of received data (labelled as erased data) and also, as a direct consequence, to describe the method to reduce the TTD.
Moreover, this model enables us to provide the requirements to obtain the three desired channel coding properties: the MDS, the full diversity and the rate-compatible properties. The MDS property allows to retrieve k data information units from any k free error data units (no matter if it is information or redundant data units). It must be noted that, in this case, the information units correspond to the block defined by the message structure design. The second full diversity property allows creating an error correction code structure to lower the resilience degradation under rough environments. The last rate-compatible property allows to improve the error correction capabilities and the demodulation threshold by combining different data units and tuning a lower final channel coding rate (providing lower rate involves better error correction capabilities since more data units can be used to decode the information data) at the decoder.
In this paper, the construction of two channel coding structures to provide MDS property, full diversity property and rate-compatible property is presented:
• The first construction is a rate-compatible root LDPC code of rate 1/3.
• The second construction is a rate-compatible root LDPC code of rate 1/4.
Both error channel coding structures are simulated and compared with the GPS L1C subframe 2 error correcting scheme under the AWGN channel assumption, the urban channel assumption and the pulsed jamming assumption.
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