Design Study for a Quasisynchronous CDMA Sensor Data Collection System: An LEO Satellite Uplink Access Technique Based on GPS

Yijun Chen, Sheng Ding, Zhouchen Xie, Zhuangping Qi, Xuwen Liang
2015 International Journal of Distributed Sensor Networks  
With the development of the LEO satellite communication technology, highly dependable wireless communication and sensor data collection using LEO satellites have been getting much attention for emergency, marine research, and forest fire disaster in the remote region. The satellite system is expected to have the following features: rapid production, low cost, and fast construction of the satellite network. In this paper, a QS-CDMA uplink access technique in the LEO satellite is presented and
more » ... cussed, which is focused on the local clocks using GPS 1PPS timing signals and the Doppler compensation for terminal uplink. The spreading code with length of 1023, which is used for the uplink preamble, selects the shift-m-sequence that can greatly reduce the MAI and increase the number of simultaneous access users. A novel analysis method for the accuracy of clock synchronization and a novel method for the estimation of Doppler shift and propagation delay are presented. These methods are used to guide the specific hardware implementation of the QS-CDMA LEO satellite sensor data collection system. Through simulations and experiments, it results in that this system structure can drastically reduce the complexity in implementing the acquisition in the satellite and increase the adaptability of the satellite system in different environments. 2 International Journal of Distributed Sensor Networks demonstrated. However, the network control station is hard to construct for marine research in the middle of ocean. Meanwhile, with the increasing of user requirements for data rate and the number of simultaneous access users, the Gold sequences cannot satisfy these requirements, because the higher date rate is, the more stringently the requirements for timing jitter are. The more simultaneous access users are, the worse the performance of BER is, a situation caused by the cross-correlation of the Gold sequence. The satellite wide-band CDMA consists of synchronous downlink and quasisynchronous uplink [2]. The system also requires the Base Station to synchronize system clock. A slotted quasisynchronous CDMA access system has been proposed in [3] , and the prime application of the system is short message services. The slotted QS-CDMA system requires the control earth station to provide accurate slot synchronization reference. In [1-3], previous research had not provided analysis method for the accuracy of clock synchronization and just made a request for system synchronization. Previous research had not provided estimation and analysis method for Doppler shift and propagation delay either. The modern trend in digital communications is to synchronize with the local time, whose synchronous errors arise from different allowed uncertainties in communication systems. Global Positioning System (GPS) receivers are used by conveying the reference time to the locked-clock loop via the one-pulse-per-second (1PPS) output. It was noticed that the average time error produced by the receivers varied over a range of about 150 nanoseconds (ns) in 2002 [4] . The current production as published showed that jumps are at the 10 ns level [5] . An optimal synchronization of local clocks by GPS one-pulse-per-second (1PPS) timing signals is specified in [6] , which use predictive FIR filter. The application of Kalman filter for clock synchronization is proposed in [7] . Nowadays the clock synchronization technique based on GPS has reached a high degree of accuracy. Generally, in severe Doppler environment, such as the LEO satellite communication, the pseudorandom (PN) code acquisition for the direct-sequence spread spectrum (DSSS) communication is hard to accomplish. The existence of carrier and code Doppler results in a prolonged acquisition process, and it also increased hardware complexity due to the need for a two-dimensional (chip delay and frequency) search structure of code and carrier synchronization. The Doppler characterization for LEO satellites is analyzed in [8] . The direct-sequence spread spectrum code acquisition in the presence of Doppler shift was investigated in [9] . In [10], Doppler compensation loop structure was proposed. In [9, 10], some algorithms have been used to compensate for the Doppler shift by satellites. Due to low-power consumption, which is characteristic of LEO satellites, the acquisition and synchronization structure should meet the requirements of decreasing hardware complexity. At the same time, the satellites need to serve as many users as possible, but the complexity of hardware implementation will limit the number of users. In this paper, we propose a novel quasisynchronous CDMA (QS-CDMA) transmission scheme based on GPS to increase system capacity, decrease hardware complexity of
doi:10.1155/2015/421745 fatcat:2pzxfiq3n5agpcjdu4bfw5zyie