Realization of OFDM system simulation based on 16QAM modulation
【Abstract】Orthogonal Frequency Division Multiplexing (OFDM) is a modulation technology that uses multiple carriers. It has many advantages such as high spectral efficiency, effective resistance to multipath, and simple channel equalization. It is the core technology of the fourth generation of mobile communications , To study it is of great significance. Based on the basic principles of OFDM, this paper uses MATLAB platform to simulate the signal processing at each stage of the OFDM system based on hexadecimal quadrature amplitude modulation (16QAM) modulation. In the experiment, the inter-symbol crosstalk (ISI) was eliminated by adding cyclic prefix (CP), the peak-to-average power ratio (PAPR) of the system was reduced by the partial transmission sequence (PTS) method, and the training sequence (TS) was inserted. Three commonly used synchronization methods are compared. In addition, the method of channel estimation and frequency domain equalization using zero-forcing equalization under ideal synchronization conditions is studied, and the method is given under different channel conditions (Gaussian noise channel). And Rayleigh attenuation channel), the comparison curve of the influence of equalization on the bit error rate of the OFDM system, and the bit error performance of different subcarriers. Finally, the comparison curve of the influence of different 16QAM modulation and coding schemes on the BER of the OFDM system under the Gaussian noise channel is given, and a more ideal conclusion is drawn.
[Keywords] OFDM; MATLAB; synchronization method; PAPR reduction; channel estimation and zero-forcing equalization; bit error rate curve; Gray coding
1. Introduction
As the symbol rate required to be transmitted continues to increase, the transmission bandwidth becomes wider and wider. The information transmission rate requirement of today's multimedia communication has reached several Mb/s, and the transmission channel of mobile communication may be a wireless channel with severe multipath fading in large cities. In order to solve this problem, the parallel modulation system has been paid more attention again. In this situation, OFDM technology emerged as the times require and developed rapidly with digital signal processing (DSP) technology. OFDM is a modulation technology that uses multi-carrier. Its technical characteristics are easy to achieve channel equalization, reduce the complexity of the receiving end, and have strong resistance to channel frequency selective fading. It is an effective method to resist channel multipath . The main idea is to divide the channel into many orthogonal sub-channels in the frequency domain, use one sub-carrier for modulation on each sub-channel, and transmit different sub-carriers in parallel, which greatly improves the spectrum utilization. Because of its high spectrum efficiency, effective resistance to multipath fading channels, high transmission data rate, simple channel equalization, and simple implementation, it has become the core technology of the fourth generation of mobile communications, and it is of great significance to study it.
Based on the basic principles of OFDM, this article uses MATLAB platform to generate random binary signals at the sending end, and then transmit through constellation mapping, serial-parallel transformation, Hermitian mapping, IFFT, PAPR reduction, CP insertion, parallel-serial transformation, and TS addition. , After Gaussian channel or Rayleigh fading channel, after receiving at the receiving end, it undergoes synchronization, serial-to-parallel transformation, channel estimation and zero-forcing equalization, CP, FFT, Hermi symmetry, PAPR reduction post-processing, parallel-serial transformation, and solution The constellation mapping recovers the original binary data, and compares it with the transmitter to find the bit error rate. Figure 1 shows the block diagram of the OFDM system implemented in the experiment. Because of the multipath effect in the wireless transmission channel, ISI will be generated. In order to eliminate ISI, the method of adding CP is used in the experiment. In order to determine the correct start and end time of each OFDM symbol, remove the CP, and achieve OFDM frame synchronization, this article uses three commonly used symbol timing synchronization methods by inserting TS, and compares the performance of the three synchronization methods. In addition, in order to overcome the influence of frequency-selective fading and time-selective fading, and make full use of channel information, in the experiment, the method of channel estimation and frequency domain equalization using zero-forcing equalization under ideal synchronization conditions is studied, and Under different channel conditions (Gaussian noise channel and Rayleigh fading channel), the comparison curve of the influence of equalization on the bit error rate of the OFDM system and the bit error performance of different subcarriers are given. Of course, OFDM also has disadvantages. First, it is very sensitive to the frequency offset and phase noise generated by the channel. In addition, the signal peak power to average power ratio (PAPR) is large, which will reduce the efficiency of the RF power amplifier. The former can be effectively solved by adding CP. There are many ways to inhibit PAPR, and the PTS method is used in this article to effectively inhibit PAPR. At the end of the experiment, by using different 16QAM modulation and coding schemes, such as square or ring constellation diagrams, gray codes, etc., in the case of Gaussian noise channel and ideal synchronization, the influence of different schemes on the system's bit error rate curve is compared.
Figure 1 Block diagram of the OFDM system used in the experiment
The rest of this article is arranged as follows. The second part will introduce the relevant principles and implementation process of some important processing modules of the OFDM system. The third part will show the data and image results obtained from our experimental simulations and analyze them. Finally, the fourth part will draw conclusions.
Some simulation results:
Two constellation diagrams:
synchronization effect:
Gaussian white noise channel error rate comparison:
Rayleigh fading channel error rate comparison:
comprehensive comparison:
PAPR reduction effect:
different 16QAM modulation schemes (constellation diagram) error rate comparison:
more details (Code, experiment summary, etc.) For details, please see: https://download.csdn.net/download/hyl1181/12646924