Modulation and Demodulation Technology

Modulation and Demodulation Technology

Modulation is the process of processing the encoded information of a signal source into a form suitable for channel transmission. In general, the signal source encoding information (source) contains a DC component and a lower frequency component, called baseband signal . Baseband signals are often not used as transmission signals, so the baseband signal must be transformed into a band with a very high frequency relative to the baseband frequency to be suitable for channel transmission. The band communication number is called the modulated signal, and the baseband signal is called the modulation signal. Modulation is achieved by changing the amplitude, phase, or frequency of the high-frequency carrier, which changes with the baseband signal. Demodulation is the process of extracting baseband signals from the carrier to be processed and understood by the intended recipient (the message House).

Modulation can be divided into two types: linear modulation and nonlinear modulation. Linear modulation includes amplitude modulated (AM), suppressed carrier bilateral amplitude modulation (DSB-SC), single sideband amplitude modulation (SSB), residual sideband amplitude modulation (VSB), etc. The nonlinear amplitude modulation has strong anti-jamming performance, including FM (FM), Frequency shift keying (FSK), Phase shift keying (PSK), differential phase shift keying (DPSK) and so on. Linear modulation is characterized by not altering the original spectral structure of the signal, while the nonlinear modulation alters the original spectral structure of the signal. Depending on the modulation, the modulation can be divided into continuous modulation and pulse modulation. According to modulation technology, can be divided into analog modulation technology and digital modulation technology, the main difference is: Analog modulation is the carrier signal of certain parameters of continuous modulation, at the receiving end of the carrier signal modulation parameters continuous estimation, and digital modulation is the carrier signal of some discrete state to characterize the transmitted information, Only the discrete modulation parameters of the carrier signal are detected at the receiving end.

Modern communication technology to digital development, the main discussion in this digital modulation and demodulation technology . Compared with the amplitude modulation, frequency modulation and phase modulation in the analog modulating system, there are three ways of amplitude keying, frequency shift keying and phase shift keying in the digital modulation system, in which the phase-shift keying modulation has the characteristics of strong anti-noise capability and narrow bandwidth, and is widely used in digital equipment. Digital modulation can be divided into two types of binary modulation mode and multi-input modulation mode, the main difference is: The former is the use of binary digital signal to modulate the carrier's amplitude, frequency or phase, the latter is the use of multi-input digital signal to modulate the carrier amplitude, frequency or phase. The frequency band utilization of the multi-input modulation is greatly improved, but at the same time of the interference level, the multilevel decision is more error-prone than the second-level, so the anti-jamming ability of the multi-input modulation is reduced. Simple induction, multi-input modulation and binary modulation compared to the following features: first, at the same code element transfer rate, the information transfer rate of the system is significantly higher than the binary system, for example, the four binary is binary twice times, octal binary binary is three times times, and the second is at the same information rate, Because the transfer rate of the multi-input code element is lower than the binary, the duration is longer than the binary length, that is, increasing the width of the code element will increase the code element Energy and reduce the inter-code interference caused by the channel characteristics. The third is that the multi-input modulation insufficiency is at the same time as the interference level, Therefore, the multilevel decision is more error-prone than the two-level, that is, the anti-jamming ability is reduced, moreover, the multi-input receiver is more complex than the binary system. Several modern modulation and demodulation techniques are described below.

Quadrature amplitude Modulation (QAM)

In modern communication, it has been one of the focus of attention to improve the spectrum utilization. Quadrature amplitude modulated QAM (quadrature amplitude modulation) is a kind of modulation method with high frequency spectrum utilization, which has been widely used in medium and large capacity digital microwave communication system, CATV network high-speed data transmission, satellite communication system and so on. In the mobile communication, with the appearance of micro-honeycomb and pico-honeycomb, the channel transmission characteristics have changed greatly. In the past, the orthogonal amplitude modulation which can not be applied in the traditional cellular system has aroused people's attention. Quadrature amplitude Modulation (OFDM) is a two-sided band modulation using two independent baseband digital signals to suppress the carrier of the two mutually orthogonal coaxial carriers, and uses this modulated signal to realize the dual parallel digital information transmission in the same bandwidth with orthogonal spectral properties.

QAM signal modulation schematic diagram. In order to suppress the out-of-band radiation of the modulated signal, the baseband signal of the L-level is also subjected to a pre-modulated low-pass filter.

The QAM signal can also adopt the Quadrature coherent demodulation method and its demodulator schematic diagram. Multi-level decision-making and detection of multi-electric pinky with signal.

Minimum frequency shift keying (MSK)

Digital frequency modulation and digital phase modulation, because the modulated signal envelope is constant, it is advantageous in the non-linear characteristic channel transmission. The frequency of frequency-shift keying signal is low because of its phase discontinuity and large frequency deviation. MSK (Minimum Frequency Shift keying) is a special form of binary continuous phase FSK, which can effectively improve the above mentioned shortcomings, MSK is called the minimum frequency shift keying, sometimes also called fast frequency shift keying (FFSK). The so-called "minimum" means that the modulation can be obtained by the minimum modulation index (0.5) quadrature signal, and "fast" refers to the given same frequency band, the MSK can be higher than the 2PSK data transmission rate, and in the band outside the spectral component is faster than the 2PSK attenuation. The MSK signal has the following characteristics:

1. The MSK signal is a constant envelope signal;

2. At the time of the code element conversion, the phase of the signal is continuous, and the signal phase based on the carrier phase is changed by the inside of a code element ±;

3. During a code element period, the signal should include an integer multiple of the One-fourth carrier period, the frequency offset of the signal equals the corresponding modulation exponent h=0.5.

DQPSK Pi/4 modulation

DQPSK PI/4 modulation is an orthogonal phase shift keying modulation method, which synthesizes the advantages of two modulation modes of QPSK and OQPSK. DQPSK has a smaller envelope fluctuation than QPSK, and in the case of multipath expansion and fading, DQPSK performance is better than OQPSK. DQPSK is able to use incoherent demodulation, which makes the receiver implementation greatly simplified. DQPSK has been used in digital cellular mobile communication systems in North America and Japan. The modulator block diagram is as follows:

In order to suppress the out-of-band power radiation of the modulated signal, the phase branch signal and the Quadrature branch signal IK and qk are passed through a low-pass filter with linear and square-root amplitude-frequency characteristics before orthogonal modulation.

The demodulation block diagram is:

FM frequency detector can also be used to detect, the block diagram is as follows:

OFDM Technology

The digital modulation and demodulation method discussed above is a serial system, and a system corresponding to the serial system is a parallel system. It is a high rate of information flow through the string/and transform, divided into a number of low-speed parallel data streams, and then each low-rate data using a separate carrier modulation and superimposed together constitute the sending signal, this system is also known as a multicarrier transmission system. In parallel system, orthogonal frequency Division multiplexing (OFDM) is an efficient modulation technique, which has strong ability to resist multipath propagation and frequency selective fading, and high spectral efficiency, so it has been studied in depth. OFDM (orthogonal Frequency Division Multiplexing) system has been successfully applied to the terrestrial broadcasting system of high-speed digital loop HDSL, asymmetric digital loop ADSL and HDTV in the Access network. In the field of mobile communication, OFDM is one of the technologies for the third generation and fourth generation mobile communication systems. Multi-carrier transmission system schematic diagram:

OFDM is an efficient modulation technology, the basic principle is to spread the transmitted data stream to many sub-carriers, so that the signal rate of the sub-carrier is greatly reduced, which can improve the anti-multipath and anti-fading ability. In order to improve the spectrum utilization, the sub-carrier spectrum in OFDM mode is orthogonal to each other, and the sub-carriers are separated by the correlation demodulation technology at the receiving end, and the influence of inter-code interference is eliminated.

Reference Link: http://www.eefocus.com/bai_jh/blog/12-09/284835_410b6.html

Modulation and Demodulation Technology