Quadrature Amplitude Modulation(QAM)

Quadrature Amplitude Modulation(QAM) DVB system as the digital modulator sle equipment, receiving from encoders, multiplex, DVB, gateway, and video server, and other equipment TS flow, undertake RS code, convolution code and QAM digital Modulation and the output can be directly in rf signal transmission cable network, also can choose according to need intermediate frequency output. With its flexible configuration and superior performance indicators, widely used in digital cable TV transmission fields and digital MMDS system.

introduction

Orthogonal amplitude keying is a kind of two kinds of amplitude modulated signal (2ask and 2psk) rendezvous methods of single channel, so will double expansion effective bandwidth. Quadrature am is used to pulse amplitude, especially in the wireless network applications.

Quadrature am signals have two of the same frequency carrier phase difference, but a quarter of 90 degrees (from integral term), cycle. A signal that I signal, another signal that Q signals. A signal from mathematical Angle can be expressed as the sine, another is expressed as cosine. Two were prepared in the carrier launch has been mixed. Arrive destination, carrier was separated, data were extracted and then and primitive modulation information phase blend.

With two road QAM is independent of baseband signal two interrelated with orthogonal to undertake inhibition frequency modulated carrier wave amplitude, use this bilateral belt already adjustable signal spectrum in the same bandwidth, realize the orthogonality within two road parallel digital information transmission. The modulation mode usually have binary QAM (4QAM), four disables QAM (l6QAM), octal QAM 64QAM),... ( , the corresponding spatial signals vector map called constellation chart, endpoint there were 4, 16, 64,... A vector endpoints. Level several m and the relationship between signal state m for 4QAM, when two is equal to signal amplitude road, which produces, demodulation, performance and phase with the same. 4PSK vector are

principle

Orthogonal amplitude modulation in QAM (by), the data signals are orthogonal two carrier wave amplitude variation said. Analog signal phase modulation and digital signal PSK (phase shift keying) can be considered amplitude unchanged, only the phase change of orthogonal amplitude modulation special. Therefore, analog signal phase modulation and digital signal PSK (phase shift keying) can also be considered QAM exceptions, because they essentially phase modulation. Here mainly discuss the QAM digital signals, although the analog signal QAM also has a lot of applications, such as NTSC and PAL systems using orthogonal system of TV of sub-carrier different color component. QAM

QAM is a kind of vector modulation, the input bit first mapping (generally USES gray yards) to a complex plane (constellation), forming a plural modulation symbols, then will I, Q component of symbols (corresponding after plane of reality department and the imaginary part, also is the horizontal and vertical direction) adopt corresponding modulation amplitude modulation, in the mutual orthogonal (the time of two carrier (orthogonal) coswt and sinwt). Such and amplitude modulation (AM), its spectrum efficiency compared will increase 1 times. Joint QAM modulation amplitude, phase is the technology, it also used the carrier of the amplitude and phase to transfer information bit, so in minimum distance under the same conditions can achieve higher utilization ratio, the band has reached 1024 - the highest QAM QAM (1,024 sample points). Sample point number, the more its transmission efficiency, such as the higher with 16 sample point of 16 - QAM signal, each sample point represents a vector state, 16 - QAM has 16 states, every four binary number one of 16 states stipulates the normal, 16 - specified in QAM 16 carrier and phase combination, 16 - each symbols and cycle QAM transmit 4 bits.

The principle of QAM modulator is sending data in bits/symbols encoder (i.e. string - and converter) in into two road, each for the original two way signal 1/2, then respectively and a pair of orthogonal modulation components after factor, summation output. The receiver complete opposite process, orthogonal demodulation out two opposite streaming, equalizer compensation caused by channel, ruling device recognition distortion of plural signals and mapping back into the original binary signals. As shown in figure 4-2 shows is QAM modulation 16 - schematic. As the modulated signal input binary data flow through string - and transform into four road after concurrent data flow. These four route data, combining the two opposite respectively, converted into two multilevel converters road 4 level data. For example, 00 convert - 3,01 convert - 1,10 convert 1,11 converted into three. The two ways 4 level data g1 (t) and the g2 (t) on carrier cos2 respectively sin2 PI PI FCT has grown and on modulation, and add FCT has grown, can get 16 - QAM signal.

Adopt QAM modulation technology, channel bandwidth at least equal to code for timing recovery rate, $, need another bandwidth, need to increase 15% or so. Compared with other modulation technology QAM coding, with can make full use of bandwidth, anti-noise ability strong, etc. But QAM modulation technology used for ADSL's main problem is how to adapt to the different between the performance of the phone lines bigger difference. To obtain a more ideal job characteristics, QAM receiver need a and the sending end with the same frequency spectrum and corresponding characteristics of the input signal is used for decoding, QAM receiver using adaptive equalizer to compensate for transmission process signal, so the method of distortion of ADSL system's complexity QAM from its adaptive equalizer.

When the demand for data transmission rate above 8 - can offer PSK once, use QAM modulation method. Because QAM constellation points than PSK constellation points more dispersed, constellation points distance between so bigger, so can provide better transmission performance. But QAM constellation points of amplitude is not exactly the same, so it can simultaneously correct the modem need testing phase and amplitude, unlike PSK demodulation need only testing phase, this increases the complexity of QAM modem.