## 1 Introduction  
Due to their multipath interactions, UWA channels are considered one of the most challenging wireless communication environments. The slow sound speed, the sound absorption and the variability of water can produce serious interference in UWA signals. Especially in shallow water channel [1], specific multipath interference is strong enough to cause severe ISI resulting in symbol errors in UWA communication.  
TRM can be used for time compression and spatial focusing, making channel equalization and multipath interference reduction come true, in the absence of any prior knowledge [2], so it is widely studied in the field of UWA communications. [PERSON] first suggested active time reversal mirror (ATRM) [3], made definitions of its application in UWA field and did some basic theoretical analysis. However, ATRM requires two-way transmission by using a source-receive array (SRA) and a VSA, which makes it relatively complex. Each sensor of the SRA is required to both receive and transmit signals, and the signals need to be transmitted twice in the acoustic channel, resulting in a considerable lapse of time and low efficiency. In addition, two-way transmission increases the extra noise and causes the signal amplitude to fade twice. In view of the complexity of ATRM, PTRM is put forward in [4]. Compared with ATRM, PTRM requires only a VSA and one-way transmission. In [5; 6; 7; 8],some theoretical derivations of TRM technology of different array shapes are made. The research group led by [PERSON] verified the principles of TRM through a sea trial [2] in April 1996. Their work promoted the application of TRM in UWA communication. In [9; 10; 11; 12], ATRM technology is applied to UWA communication. References [13; 14; 15; 16] studied UWA communication using PTRM, and pointed out that PTRM can realize adaptive channel equalization, suppress the ISI and thereby improve the performance of UWA communication systems. In practice, there is always some residual ISI after time reversal processing. In [17; 18; 19], PTRM was followed by a single-channel decision-feedback equalizer (DFE) to eliminate the residual ISI, and it was confirmed that PTRM combined with a DFE (PTRM-DFE) can improve the performance significantly over PTRM. In [20; 21; 22], TRM was applied to multiuser or multiple-input-multiple-output (MIMO) communications due to the spatial focusing capability. Recently, with the advances in algorithm development, there is ample scope for research to be done in TRM-based communications [23].  
In summary, ATRM is both expensive and difficult to implement, and is not practical, while PTRM can obtain the same theoretical performance in a simple way. Nevertheless, PTRM needs to estimate the channel impulse response of each sensor of the VSA, which is somewhat unrealistic in complex long-range shallow water environments, because the SNR of the received signals of each sensor is too weak to estimate the channels accurately. Therefore, this paper proposes a CCVTRM technology for UWA communication and makes some theoretical derivations of this technology. The proposed method utilizes the spatial gain of the VSA to estimate the single composite channel, thus implementing the CCVTRM and achieving the temporal focusing. A long-range communication experiment in shallow water conducted in the South China Sea validates the practicability of this technology.