# A Signaling-Free Underwater Code Division Multiple Access Scheme
## 1 Introduction  
There has been growing demand for underwater acoustic sensor networks for various military and commercial applications including oceanographic data collection, environmental monitoring, resource investigation, disaster prevention, undersea exploration, assisted navigation, distributed tactical surveillance, and mine reconnaissance [1]. To successfully implement these applications, it is important to understand the characteristics of the underwater communication channel and thus design a methodology for communication between the sensor nodes. Previous studies have found that using an acoustic signal is the most suitable approach to transport information in an underwater communications channel [2].  
However, acoustic signals have poor characteristics in underwater communication channels, such as a low propagating speed of 1500 m/s, which is five orders of magnitude lower than the terrestrial radio propagation speed, limited available bandwidth, multipath fading, temporary losses of connectivity (shadow zone), and Doppler spread [1; 3; 4]. Therefore, underwater communication suffers from a high bit error rate and long propagation delays. Enhancements or modifications for an underwater environment in terms of modulation schemes and medium access control (MAC) protocols are required to overcome the harsh underwater channel conditions.  
Code division multiple access (CDMA) is a promising underwater MAC scheme and has been intensely studied because of its advantages such as robustness against frequency-selective fading and high frequency-reuse efficiency [5; 6]. Thus, the CDMA studies have been mainly focused on spread code assignment, power control, and energy efficiency [7; 8; 9; 10; 11]. In [12; 13], code assignment and power control schemes for a transmitter-based CDMA system were proposed to achieve high network throughput, low energy consumption, and limited channel access delay in deep underwater communications. In [14], a path-oriented code assignment (POCA)-CDMA-MAC protocol was proposed to resolve the funneling effect in multi-hop underwater acoustic sensor networks where the sink discriminates data arriving from multiple routing-paths simultaneously by assigning each path a distinct spreading sequence.  
However, most CDMA systems employ a signaling procedure between the sink and sensor nodes to exchange control messages involved in power control, channel access, etc. Owing to the very limited bandwidth of acoustic channels and long propagation delays, the overhead of the signaling procedure causes severe deterioration in performance in terms of throughput and latency. Although there have been several works for freeing signaling procedure in MAC protocols, they are restricted to ALOHA- and CSMA (Carrier Sense Multiple Access)-family protocols [15; 16; 17]. In [18], the ALOHA with carrier sensing (ALOHA-CS) is proposed using a new back-off window to adapt the protocol to variable propagation delay. Similarly, to improve medium access opportunity, CSMA-ALOHA [19] adopts a random sensing duration shorter than the time required for the signal to propagate over the sensing range. However, to the best of our knowledge, the effort to remove the signaling overhead in CDMA has not been made for underwater environments.  
In this paper, we propose an underwater CDMA without any signaling procedure related to channel access and power control. Individual sensor nodes evaluate channel availability by estimating the number of nodes currently communicating with the sink, based on individual power measurement. If the estimated number of nodes is less than the system capacity, then the node tries to occupy the channel in a p-persistent manner. The estimation is mathematically driven and evaluated by computer simulation. Numerical results show that the hit probability of this estimation is almost one, and system performance is significantly improved in terms of throughput.