1 Introduction Communication signal generator is an important link in the simulation and simulation of electromagnetic environment and the design of communication system, and it is also a basic element in the research of communication and communication countermeasure system. The communication signal generator is usually composed of a combination of hardware circuits, which has the disadvantages of large hardware scale, large capital investment, and difficulty in expansion. In "software radio", the hardware circuit provides the basic signal path, and the transceiver function of the radio system can be realized by software programming, and the system function can also be expanded by modifying some software and hardware circuits. Therefore, designing a signal generator based on the idea of ​​"software radio" is the key to "universality". In the communication signal generator, the minimum frequency shift keying (MSK) signal generated by the basic hardware circuit is the basis for generating subsequent Gaussiian Filtered Minimum Shift Keying (GMSK) signals. Therefore, the MSK modulation signal based on the quadrature modulation method is discussed, which can provide a reference for the digital signal generated by the communication signal generator. 2 MSK signal based on quadrature modulation method 2.1 Principle of quadrature modulation method Ideally, using the idea of ​​"software radio", computer operation generates a sample value of any communication signal, and the sample value is converted into any desired modulation communication signal after D / A conversion. However, when the communication signal bandwidth is large, directly generating radio frequency signals not only requires a large amount of data, but also requires high hardware performance. Because a large amount of data comes from the sampling of the carrier frequency signal, and the sampling data of the baseband signal is much smaller than the sampling data of the carrier frequency signal, some characteristics of the baseband signal are generated as sample values ​​through software operations, and then through D / A conversion and loading Frequency signals combine to produce the required modulated communication signals, avoiding a lot of data processing. The orthogonal modulation method is that for any modulated narrowband communication signal can be expressed as: In the type, amplitude r (t) and phase θ (t) characterize the baseband signal. Expand formula (1) to get: Let I (t) = r (t) cosθ (t), Q (t) = r (t) sinθ (t), as seen from equation (2), as long as I (t) and Q (t) are passed Function, the baseband signal can be transferred at a lower data transmission rate. I (t) and Q (t) are orthogonally multiplied with the carrier frequency signal to generate an arbitrary modulation signal. 2.2 Overview of MSK MSK is a special continuous-phase frequency shift keying (Frequency-Shift Keying, FSK for short), whose maximum frequency shift is 1/4 of the bit rate. That is, MSK is a continuous phase FSK with a modulation coefficient of 0.5. The frequency at which the MSK modulated wave is transmitted in any element is determined by two adjacent symbols. When two adjacent symbol data are the same, f1 is sent, and when two symbol data are different, f2 is sent, then: In the formula, k is a positive integer, and t0 is the symbol width. Therefore, the power spectral density of the MSK signal is relatively concentrated and the frequency band utilization rate is high; the frequency band is wide and the channel effectiveness is high. In the frequency hopping spread spectrum communication, the frequency hopping point is increased; Ping still rolls off rapidly, which is suitable for power-limited and nonlinear amplification applications; the bit error rate of MSK modulation is close to the best PSK modulation in digital systems, with high reliability; the demodulation and synchronization circuits are simple. Based on the above characteristics, MSK can be widely used in mobile communication systems. 2.3 Generating principle of MSK signal in general communication signal generator The general communication signal generator uses the orthogonal modulation method to generate the MSK modulated signal. From equation (2), the MSK signal uses the orthogonal modulation method: In the formula, an and bn are the odd and even sequences of the input binary data, Tb is the data bit duration, kTb≤t≤ (k + 1Tb). It can be known from equation (4) that the phase of the MSK signal has continuous modulation characteristics. The quadrature modulation method is used to generate the MSK signal. Its block diagram is shown in Figure 1. A PON consists of an Optical Line Termination (OLT) at the communication company's office and a number of Optical Network Units (ONUs) near end users. Typically, up to 32 ONUs can be connected to an OLT. The passive simply describes the fact that optical transmission has no power requirements or active electronic parts once the signal is going through the network. Passive Optical Network(PON Series) Passive Optical Network,GPON Technology,PON Network ,PON Series Shenzhen N-net High-Tech Co.,Ltd , http://www.nnetswitch.com
N-NET supply Ethernet Passive Optical Network (EPON) and Gigabit-Capable Passive Optical Network(GPON),EPON and GPON are popular versions of passive optical networks (PONs). All PON systems have essentially the same theoretical capacity at the optical level. The limits on upstream and downstream bandwidth are set by the electrical overlay, the protocol used to allocate the capacity and manage the connection.