## 1 Introduction  
Accurate signal acquisition is a fundamental step for the success of signal demodulation in a variety of underwater systems. A useful method for signal acquisition is to append a linearly frequency modulated (LFM) waveform to the beginning of a packet, and then monitor the output of a receiving filter matched to the LFM signal. A sharp peak at the matched filter output marks the beginning of the received packet and allows for successful data demodulation. However, in applications where either noise is strong or signal is weak, the matched filter output does not exhibit a large peak, which can cause high demodulation error.  
A vector sensor is a multichannel sensor that simultaneously measures the vector and scalar field components. Given their multichannel nature, vector sensors are used in a variety of applications. Examples include sonar, source localization, angle of arrival estimation, beamforming, and communication [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17]. The advantages of using a vector sensor for the estimation of the direction of arrival are investigated in [1; 2; 3]. Analytical results of [4], obtained based on maximizing the directivity index, indicate that the acoustic vector component channels of vector sensors should be utilized for optimal detection. Furthermore, ref. [5] shows that a single vector sensor directivity gain can be four times higher than the directivity of a pressure sensor. Additionally, vector-scalar receivers can be used together with an interferometric method for source detection [6; 7; 8] and estimating the velocity of a noise source [9]. In addition, the processing gain of vector sensors in noise is addressed in [10], while signal detection using a vertical linear array is investigated in [11].  
The papers that utilize a vector sensor as an underwater communication receiver, e.g., [12; 13; 14; 16; 17], focus on subject matters such as demodulation, bit error rate, capacity, channel estimation, equalization, etc., rather than signal acquisition performance. Morespecifically, underwater multichannel equalization using the multiple channels of a vector sensor is proposed in [12]; space-time underwater communication channel parameters are estimated using a vector sensor in [13; 14], using a quadrilinear decomposition method, and using MUSIC and ESPRIT algorithms for a multicarrier code-division multiple access systems, respectively; underwater channel capacity bounds are calculated in [16] for a vector sensor receiver; and multi-hop cooperative underwater communication and its bit error rate and capacity are studied in [17] for a frequency-selective channel using an angle-of-arrival model for a vector sensor. However, none of these studies examine the performance of a communication receiver vector sensor at the signal acquisition phase.  
The objectives of this paper are: (a) present a ring vector sensor multichannel receiver and study its signal acquisition performance both experimentally and analytically, and (b) compare the multichannel performance with the performance offered by each individual channel of the ring vector sensor. Our solutions to achieve these objectives consist of collecting and analyzing experimental data, combined with mathematical analysis. As demonstrated later, our results allow the examination of the practical feasibility and usefulness of the proposed ring vector sensor receiver for underwater signal acquisition and packet detection.  
The rest of this paper is organized as follows. Section 2 provides the definitions for the vector and scalar signals that the ring vector sensor measures. System formulations and performance analysis for multichannel signal acquisition are presented in Section 3. These provide tools and formulas for analyzing the signal acquisition performance of the ring vector sensor receiver using experimental data in Section 4. Concluding remarks are given in Section 5.