Modulation and demodulation technology in mobile communication
第3章 移动通信中的调制解调技术
Chapter 3 Modulation and Demodulation Technology in Mobile Communication
3.1 Overview
Modulation is the process of encoding the source information of the message. Its purpose is to match the signal carrying the information with the channel characteristics and effectively use the channel.
Multipath fading, Doppler frequency expansion; the increasing number of users, and the crowding of wireless channel spectrum have a major impact on the choice of modulation
Main factors affecting the choice of modulation method
Frequency band utilization: In digital modulation, the bandwidth efficiency ηb is commonly used to represent its utilization efficiency of spectrum resources, which is defined as ηb = Rb / B, where Rb is the bit rate and B is the bandwidth of the wireless signal.
Power efficiency: refers to the minimum signal power (or minimum signal-to-noise ratio) required to maintain information accuracy
Constant envelope of modulated signal
Easy demodulation
Out-of-band radiation: generally required to reach -70 to -60dB
3.2 Minimum frequency shift keying MSK
FSK with continuous phase
2FSK signal
Note: Under the same modulation index h, the bandwidth of CPFSK is narrower than the bandwidth of 2FSK. This means that the former has higher frequency band efficiency than the latter. As the modulation index h increases, the bandwidth of the signal also increases. Considering the efficiency of the frequency band, the modulation index h should not be too large. But too small is not conducive to signal detection because the two signal frequencies are too close. So it should be considered from their correlation coefficients and signal bandwidth
Phase path, frequency and power spectrum of MSK signal
Phase path
MSK frequency relationship
MSK power spectrum
MSK signal generation and demodulation
The working process of MSK modulator
① Differential encoding of the input binary data signal
② After serial / parallel conversion, it is divided into two signals Ik and Qk interleaved by one symbol width
③Use the weighting functions cos (πt / 2Ts) and sin (πt / 2Ts) respectively to the two data signals Ik
Weighted with Qk
④The two weighted signals are respectively modulated to the orthogonal carrier cosωct and sinωct
⑤Add the two modulated signals obtained and pass the band-pass filter to get MSK
signal
3.3 Gaussian minimum frequency shift keying GMSK
3.3.1 Transmission characteristics of Gaussian filters
GMSK is the MSK of the baseband signal passing through the Gaussian low-pass filter
Frequency characteristic H (f) and impulse response h (t)
3.3.2 GMSK signal waveform and phase path
3.3.3 Modulation and demodulation of GMSK signal
modulation
demodulation
GMSK can be demodulated by coherent method or non-coherent method. Here we introduce a one-bit delay differential demodulation method (non-coherent demodulation). Let the received signal be
3.3.4 GMSK power spectrum
3.4 QPSK modulation
3.4.1 Two-phase modulation BPSK
The power spectrum BPSK signal is a linear modulation. When the baseband waveform is NRZ code, its power spectrum is shown in Figure 3.23
3.4.2 Four-phase modulation QPSK
QPSK signal generation
QPSK demodulation
QPSK signal power spectrum and bandwidth
The envelope characteristic and phase jump of QPSK signal
3.4.3 Offset QPSK—OQPSK
3.4.4 π / 4-QPSK
Overview of π / 4-QPSK
π / 4-QPSK modulation is a compromise between OQPSK and QPSK at the actual maximum phase change. In p / 4-QPSK, the maximum phase change is limited to ± 135 degrees, while QPSK is ± 180 degrees, and OQPSK is ± 90 degrees. Therefore, the band-limited p / 4-QPSK signal has a better constant envelope property than the band-limited QPSK, but it is more sensitive to the envelope change than OQPSK.
In the case of multipath spreading and fading, p / 4-QPSK performs better than OQPSK
π / 4-QPSK can be demodulated using coherent or non-coherent methods. Non-coherent detection will greatly simplify the design of the receiver. After using differential encoding, p / 4-QPSK can become p / 4-DQPSK.
π / 4-QPSK signal has the characteristics of good spectrum characteristics, high power efficiency, strong anti-interference ability, etc.
π / 4-DQPSK signal generation
Phase differential encoding
Phase jump of π / 4-DQPSK signal
Demodulation of pi / 4-DQPSK
3.5 Higher order modulation
3.5.1 The principle of digitally modulated signal space
If M signals with limited energy are mapped onto the N-dimensional vector space, the M mapping points in space are called constellation points, and the vector space is called signal space. In the vector space, it is easy to describe two indicators to measure the performance of bit errors, the correlation between signals and the Euclidean distance. The greater the correlation between symbols, the smaller the Euclidean distance, and the worse the error performance. In general, the higher the modulation order, the smaller the Euclidean distance. However, due to the limitation of frequency resources, the modulation method must use a relatively high order. In order to ensure the performance of the link under high spectral efficiency, measures such as strong error control technology and increased power can be used to compensate for the error performance. M-ary digital modulation can generally be divided into MASK, MPSK, MQAM and MFSK, which are linear modulation without memory. If combined with the vector space representation of the signal, it can be understood that these different modulation methods are due to the use of different sets of orthogonal functions. It is generally considered that the order is higher order modulation. The three modulation modes MASK, MQAM and MPSK have the same spectrum utilization rate when the information rate and M value are the same. Because MPSK's anti-noise performance is better than MASK, 2PSK and QPSK have been widely used. And the ASK signal modulates the amplitude of the carrier, so it is not suitable for fading channels. At that time, MQAM's anti-noise performance is better than that of MPSK, so the higher order modulation is generally in the form of QAM.
3.5.2 M-ary digital modulation and higher-order modulation
. M-ary Phase Shift Keying (MPSK)
2. Quadrature amplitude modulation (QAM)
The vector space of the MASK signal is one-dimensional, and the vector space of the MPSK signal is two-dimensional. As the modulation order increases, the Euclidean distance between symbols decreases. Then, if the plane of the two-dimensional vector space can be fully utilized, increasing the number of points of the constellation without reducing the Euclidean distance can increase the spectrum utilization rate, thus leading to a quadrature amplitude modulation method QAM that jointly controls the amplitude and phase of the carrier. The QAM method is also the most used in high-order modulation, which will be highlighted below.
3.5.3 Application of high-order modulation in 3G and 4G
There are many applications of high-order modulation in high-speed data transmission systems. In order to improve spectrum efficiency, high-order modulation is widely used in broadband wireless communication systems such as LTE, HSPA, and 802.11n. These modulation techniques are combined with channel coding to form an adaptive modulation coding (AMC) scheme, which has become a key technology for B3G and 4G mobile communications. The following describes each modulation method from the aspect of mobile communication standard application.
After 1986, because the practical linear high power amplifier has made breakthrough progress, people have once again paid attention to the simple and easy BPSK and QPSK. As the evolution of GSM, EDGE adopts 8PSK modulation.
3G standard cdma20001x, WCDMA, and TD-SCDMA all use QPSK, and TD-SCDMA also introduces 8PSK. With the development of error control technology, when it evolves to the HSPA stage, both TDD systems and FDD systems have introduced high-order 16QAM and 64QAM modulation methods. The evolution of cdma20001x to the EV-DO stage has also introduced 8PSK, 16QAM, 64QAM and other high-order modulation methods. As the mainstream standard of B3G, LTE introduces 16QAM and 64QAM modulation methods to improve the spectrum utilization data channel, and the control channels adopt BPSK and QPSK modulation methods
3.6 Orthogonal Frequency Division Multiplexing
In practical applications, the OFDM system can automatically test the transmission quality of subcarriers, and accordingly adjust the transmission power and number of transmitted bits of the subchannels in time, so that the transmission rate of each subchannel reaches the optimal state.
OFDM is widely used in high-speed data transmission on wired or wireless channels, such as ADSL on digital subscriber loops, IEEE802.11a and HIPERLAN-2 on wireless local area networks, digital broadcasting, high-definition television, and fourth-generation mobile communications ( 4G, mainly including LTE-A and IEEE802.16m) systems, etc.
OFDM has the problem that the peak power and average power ratio (PAR) of the transmitted signal is too large and the problem of destroying the orthogonality of subcarriers due to Doppler spectrum spread.
