Abstract-The current satellite communication systems generally use TDMA signal multiplexing method to improve the frequency efficiency. The system described in this paper has build a signal monitoring platform of TDMA satellite communication system by using general-purpose instruments, the platform has a flexible structure, strong real-time and high levels intelligent. Instead of the traditional monitoring method which uses the communication quality of the entire channel to replace the quality of each time slot inside the channel, it can monitor the signal quality of all the satellite communication time slots inside the monitoring channels. Practice has proved that the platform improves the efficiency of spectrum testing staff; it has realized a high degree of standardization and generalization, also it has reliable and stable operation.
Key word: MF-TDMA spectrum monitor
INTRODUCTION
With the rapid development of economic, the satellite communication has get more and more widely application. The satellite companies which relying on the communication satellites provide satellite communication services to the general public. The satellite communication has been widely used in civil aviation, maritime transport, fire fighting, public security, as well as radio and television and other industries because of its characteristic of big coverage range, good real time nature and small effect of human factor.
The multi-carrier time division multiple access (MF-TDMA) satellite communication system is a communication system which is widely used presently[1]. The satellite data users can rent a certain bandwidth of spectrum according to the business needs. The satellite companies, based on national standards, generally provide MF-TDMA frequency band multiplexing service for the user to improve the spectral efficiency. The multi-carriers refer to divide the frequency band into different channel from the frequency, each channel use independent carrier; TDMA is to divide a segment of communication time into different time slots, the different time slot transfer different business data.
The communication can not be used because the satellite communication process is easily to be disturbed by external conditions [2, 3]. To ensure the normal communication, the satellite companies must guarantee that the basic indicators of communication signals within the bandwidth users rent, such as the center frequency carrier frequency, signal power, signal-to-noise ratio etc., are in the available range, so these key indicators must be real time monitored. Currently, the unified frequency band monitor method is adopted mostly to monitor the satellite signal, that is, only monitor the spectrum within a certain bandwidth, but not in-depth monitor each time slot, it is general think that the signal-to-noise ratio of frequency band is as same as that of each time slot. Thus, the actual situation is not the case, the different time slot in the same frequency band is used by different ground stations, so the signal-to-noise ratio of each time slot has differences, and these differences can sufficiently decided whether the communication link can be established smoothly. It is necessary to establish a kind of method so that each time slot of satellite communications can be monitored, the method can provide strong protection for the satellite user's data communications.
SIGNAL CHARACTERISTICS ANALYSIS
In order to improve bandwidth efficiency, usually the first it is carrying out the multi-carrier frequency multiplexing, creating many carriers among a wide spectrum and dividing it into many channels. Further each channel carries out TDMA, that is, dividing a certain time cycle into multiple slots, each slot is available for separate information transmission separately. TDMA satellite communication system provides time synchronization pulse to all the network users in order to standardize the start time of the slot. For example, a certain user rents 40MHz bandwidth, 20 channels are divided at first, so each channel bandwidth is 2MHz, each channel carries out TDMA next, the multiplexing time cycle is 20ms, among the 20ms, users can define the start time and the duration length of each time slot according to the communication needs, the spectrum characteristic is shown in Figure 1.
Spectrum characteristic
In the diagram, the vertical axis is frequency, it completes frequency multiplexing, the horizontal axis is time, and it describes a 20ms cycle time planning condition.
THE MAIN STRUCTURE OF SATELLITE SPECTRUM MONITORING SYSTEM
The virtual instrument control technique is adopted in the satellite spectrum monitoring [4]; the monitoring method is centering on the host computer and general instrument-assisted. As is shown in figure 2, the main structure of spectrum monitoring system mainly consists of three parts: signal connection, monitoring instrument and main control software. Signal connection part is used to transmit a received signal by an antenna to the monitoring instrument, the monitoring instrument can monitor various signal indicators，and the main control software can cooperate and control various monitoring instrument, get various indicators of MF-CDMA satellite signals, and record the information between system and the users.
Main structure of spectrum monitoring system
IV.THE WORKFLOW OF SATELLITE SPECTRUM MONITORING SYSTEM
Step 1
The user divide the Spectrum to be monitored into N-channels (Each channel bandwidth α MHz) according to current communications planning in the main control software on the host computer, and define the theoretical values of the base attributes of all the communication slots in each channel, in which the base attributes include the starting time, the slot width, the center frequency of the signal, the signal power, the signal-to-noise ratio etc., and also need to specify the TDMA repeated cycles β, the unit is ms.
Step 2
After finishing the monitoring system, the user power on the receiving antenna, and connect the MF-TDMA signals with the monitoring instrument.
Step 3
The sync signal generator demodulates the signals, and extracts synchronizing information, then outputs the signals in pulse mode.
Step 4
The main control software on the host computer remote controls the down-conversion, to let the center frequency of down-conversion aligning to it of the channel 0, and make the signal α MHz low pass filtering after down-conversion, then separate the signal of channel 0 from other channel signals, and output the signal of channel 0 to the oscilloscope.
Step 5
The real-time spectrograph is remote controlled by the main control software on the host computer, to let the center frequency aligning to it of the channel 0, and set the filtering bandwidth to α MHz, so the signals of channel 0 is collected only.
Step 6
The oscilloscope and the real-time spectrograph carry out the signal collect at the same time and finish the data records under the trigger of the synchronization pulses that generated by the sync signal generator.
Step 7
The host computer remotely reads the sampling pictures from the oscilloscope to carry out the separation of the signal slots, specific operations are as follows:
Read the βms sample point of current channel from the oscilloscope to fetch power envelope of the current channel.
Carry out the further wave filtering of the βms power envelope diagram on current channel to reduce the magnitude of the signal envelope fluctuation, in order to facilitate the follow-up slot search and improve the accuracy.
Adopt the single-threshold random lingering filtering method to search slots. A random lingering filter is designed in the processing software to reduce misjudge of the slots judgments. The filter requires that when a sample point is less than the threshold, it must ensure that a continuous R0 points lower than the threshold, then that is really happened from there are signals to no signal; similarly, there must be continuous R1 points to confirm the transformation from lower than the threshold to higher than the threshold. The time between having signal to no signal is called a slot, R0 and R1's value can be dynamically configured in the main control software.
Step 8
The host computer inputs the start time and sustained time of the slot into the real-time spectrograph, the real-time spectrograph gets the basic attribute measured values of each time slot, and transmitters the values to the host computer.
Step 9
The main control software on the host computer compares the measured values with the theoretical value, the comparison of contents includes start time, slot width, the signal center frequency, signal power, and signal-noise ratio. The system will warn the users and prompt the users to have an exception if any feature's measured values exceed the range of its theoretical values.
Step 10
According to current monitoring state, the host computer adopts the scheme that regularly stores data at normal time slots and fully stores data at alarm time slots, to complete record of the spectrum, all of the information is stored in the database server.
Step 11
the main control software on the host computer remotely controls the down-conversion, to let the center frequency of down-conversion aligning to the center frequency of down-conversion of next channel under test, and remotely controls the real-time spectrograph at the same time, to let he center frequency of down-conversion aligning to the center frequency of down-conversion of next channel under test.
The structure and testing process of this platform are completely applicable to the TDMA satellite communication system. In the monitoring system, in addition to digital down-converter need to customize [5], all other equipments are standard general-purpose equipments. And the platform has realized a high degree of standardization and generalization.
CONCLUSION
This platform has been completed development and put into use, through a period of operation, we can find the following advantages:
System structure is flexible and applicable to each frequency TDMA satellite signal's spectrum monitoring.
The system is applicable to the communication quality monitoring of all the time slots inside the frequency band.
Alarm function is complete, and can promptly notify the users at any unusual time slot.
Strong real-time, the system can complete all the time slots monitoring of full spectrum in a short time.
