###### Abstract  
Medium Access Control (MAC) delay which occurs between the anchor node's transmissions is one of the error sources in underwater localization. In particular, in AUV localization, the MAC delay significantly degrades the ranging accuracy. The Cramer-Rao Low Bound (CRLB) definition theoretically proves that the MAC delay significantly degrades the localization performance. This paper proposes underwater localization combined with multiple access technology to decouple the localization performance from the MAC delay. Towards this goal, we adopt hyperbolic frequency modulation (HFM) signal that provides multiplexing based on its good property, high-temporal correlation. Owing to the multiplexing ability of the HFM signal, the anchor nodes can transmit packets without MAC delay, i.e., simultaneous transmission is possible. In addition, the simulation results show that the simultaneous transmission is not an optional communication scheme, but essential for the localization of mobile object in underwater.  
A 20181835610.3390/s1802035610.  
handled, but it just remains as future works in most studies [2]. In addition, in recently proposed AUV localization, the impact of MAC delay is not mentioned at all [3; 4; 5; 6; 7].  
This paper proposes AUV localization combined with the multiple access system. The ultimate solution to decouple the localization performance from the MAC delay is to guarantee the simultaneous transmission to the ANs. The simultaneous transmission allows the ANs to transmit the packet without waiting time, and thus the MAC delay can be removed. For the the simultaneous transmission, multiple access technologies can be used. Among multiple access technologies, we adopt frequency division multiple access (FDMA) combined with hyperbolic frequency modulation (HFM) signal. The HFM is a kind of nonlinear frequency modulation scheme, and its frequency varies during the symbol duration. The good property of HFM signal, high-temporal correlation, stems from the frequency varying. Owing to the high-temporal correlation, the multiple HFM signals which arrive at the receiver at the same time can be separated using a bank of matched filter. In other words, the different ANs are able to transmit their packet simultaneously without collision [8; 9]. Accordingly, the localization performance can be decoupled from the MAC delay.  
To investigate the impact of MAC delay on the AUV localization, we define Cramer-Rao Low Bound (CRLB) of the localization algorithm. CRLB is a metric indicating the theoretical optimal performance of the estimator [10]; consequently the CRLB definition is used to evaluate the proposed localization algorithm. In this paper, the CRLB is used to analyze the AUV localization performance, according to the length of MAC delay. The CRLB which increases as the MAC delay increase reveals that the MAC delay should be properly handled in AUV localization. After the error analysis, we introduce how to transmit the packet using the HFM signal for the localization. Although we present time of arrival (ToA)-based AUV localization in this paper, our system can be applied in all kinds of ranging techniques such as time differences of arrival (TDoA), and angle of arrival (AoA).  
The rest of this paper is organized as follows. Section 2 shows the motivation of this paper and the impact of MAC delay on the AUV localization. Section 3 theoretically analyzes the relationship between the MAC delay and localization performance using CRLB definition. Section 4 introduces AUV localization adopting FDMA combined with the HFM signal. The simulation results show that the MAC delay should be removed in the AUV localization, and the HFM signal is a good solution. Finally, we conclude our paper with future works.