# A Fine Fast Acquisition Scheme for a Communication and Navigation Fusion System
## 1 Introduction  
The global navigation satellite systems (GNSS) provides accurate outdoor positioning and timing information for both military and civilian applications as the most widely used navigation system. However, there is no prevailing technology for GNSS-deprived areas such as urban canyons and indoor environments, where satellite signals are severely attenuated or blocked [1; 2; 3]. The ubiquity of cellular communication networks provides widely signal coverage, which can be used for positioning purposes to be a complementary solution in GNSS-deprived areas [4]. The fundamental positioning techniques utilized in cellular networks are classified as proximity, angle of arrival (AOA), signal strength, and signal propagation time [3]. These techniques have several typical applications in cellular networks from 1G to 5G such as the enhanced 911 (E911) location requirements and positioning reference signal (PRS) developed in 4G system [5; 6]. However, the poor positioning accuracy of these applications cannot meet the development of location-based service (LBS) requirements [7; 8]. CNFS is developed to realize high-accuracy localization in constrained conditions based on signal propagation time technique. CNFS multiplexes the communication and navigation signal in the same frequency band and allocates different power levels for the two signals. This superposition technique has been applied in China's mobile multimedia broadcasting (CMMB) system for fine outdoor and indoor localization [9; 10; 11; 12]. Direct-sequence spread spectrum code division multiple access (DSSS-CDMA) system employing binary phase-shift keying (BPSK) modulation is used in thepositioning system in CNFS. Signal propagation time, typically represented by time difference of arrival (TDOA) or time of arrival (TOA), is estimated by exploiting the correlation properties of the pseudorandom noise (PRN) sequences. As with GNSS positioning receiver, the first operation performed by a CNFS positioning receiver is the signal acquisition, which can provide a rough estimation of the signal transmitted by the fusion signal transmitter. Signal transmitters of cellular network have dense distribution but a very small signal coverage of every transmitter, which leads to high demands on the speed of signal processing. Meanwhile, the distribution property and complex environments cause huge power differences of receiver received signals from different transmitters, and then cause the fluctuation of detection performance in acquisition stage. Fluctuation in detection performance means an increase in probability of false alarm (PFA). The positioning receiver usually takes a long time, which is defined as false alarm penalty time (FAPT), to re-enter the acquisition status if false alarm occurs [13; 14; 15]. This additional FAPT significantly increases the mean acquisition time (MAT). On the other hand, signal acquisition is the most time-consuming procedure in the signal processing of positioning receiver and has a significant impact on the time to first fix (TTFF) [15]. Therefore, it is important to realize a fast and stable acquisition in CNFS. Typical acquisition system for DSSS-CDMA applications described in literature [16; 17; 18; 19] are based on the evaluation and processing of the CAF. Some undesired components such as noise and interference degrade the readability of the CAF so further processing is needed. The whole procedure of acquisition can be split into two main specific aspects: the CAF evaluation and the signal detection. In order to realize fast and stable acquisition, a number of techniques have been utilized based on either or both the two aspects. Serial search, parallel search and the hybrid of both are the basic search techniques to test every hypothesis in the 2D search space. Serial search confirms the hypothesis with lowest computational load but longest time consumption [20; 21].