In this paper, the sea shortwave ground wave communication was studied and discussed as a supplementary communication method of the intelligent marine communication network. In order to solve the problems existing in the shortwave ground wave communication in the offshore field, a new adaptive bit allocation OFDM technique is proposed with low calculation complexity. The computer simulation results showed that the proposed method is superior to the ordinary OFDM system. The simulation results provided the theoretical basis for the sea shortwave ground wave communication, and had the significance of engineering realization.
At present, intelligent marine communication networks were constructed by many countries. In general sense, the intelligent marine communication networks were regarded as a kind of connection between physical ocean world and virtual information sea world, and lots of researches concerned communication methods for different channels were conducted (Eklof F L T et al, 2001, Ghaffari A, 2014, Norris A, 2007, Perera L P et al, 2012, Sarabandi K et al, 2001, Unni S et al, 2015, Xu B et al, 2012, Xu G et al, 2014) in recent years. Among them, sea shortwave ground wave communication as one of the commonly used sea surface electromagnetic communication methods was seldom mentioned. In this paper, the shortwave ground wave communication channel of the sea was studied (Andreja R et al, 2014, Abdollah D et al, 2018, Eklof F L T et al, 2001, Ghaffari A, 2014, Norris A, 2007, Perera L P et al, 2012, Sarabandi K et al, 2001, Unni S et al, 2015, Xu B et al, 2012, Xu G et al, 2014,) and the suitable transmission technology-adaptive bit allocation Orthogonal Frequency Division Multiplexing (OFDM) technology-was applied under this kind of channel. The algorithm used in the OFDM technology is a method to improve the data transmission efficiency by low calculation complexity under the channel condition. There are some bit loading methods that have been used to the underwater OFDM communication systems. The algorithm showed by Abdollah D et al (2018) described an optimal method by the optimization criterion of minimization of bit error rate (BER). First step, the optimal relay was selected by calculating minimum BER. Second step the optimal power loading vectors solution was found by Karush-Kuhn-Tucker (KKT) conditions at the relay nodes. The design mentioned by Andreja R et al (2014) presented two bit loading methods on the criterion of maximizing the system throughput under a target average BER. And the methods must be applied with channel estimation. To an extent, the system performance is depends on the channel estimation performance. Most of the classic bit allocation algorithms have high degree of complexity for good transmission performance. Simple Block Loading Algorithm (SBLA) algorithm presented by Grunheid H R R et al (2000) is a low complexity bit allocation algorithm. In this paper, an improved algorithm of SBLA is studied and discussed. To the original algorithm, the new algorithm has been modified in the SNR calculation formula, which can be more accurate guidance for each group of subcarriers used by the modulation. After determining the modulation scheme of each group of sub-carriers, the sub-carriers in each group are numbered to divide the sub-carriers into active sub-carriers and non-active sub-carriers. The data can be transmitted by using active sub-carriers and active sub-carriers index mode. Under the premise of a certain output power and data rate, the new algorithm can improve the system performance of the original algorithm.
