Application of OFDM technology in emergency communication system
1 Overview In recent years, whether it is rescue work for natural disasters, epidemic prevention work for public health incidents, or order maintenance work for security incidents, the relevant departments of public incidents have placed increasingly high demands on emergency response incidents. Corresponding departments are faced with a new test in terms of early warning programs and software for organization and coordination, as well as the hardware of corresponding communication equipment and management command systems. Therefore, building an efficient and applicable emergency communication system to provide the public with more timely rescue services has become an important and urgent topic at present. As the core technology of the fourth generation mobile communication technology, OFDM technology, its multi-carrier transmission distance and image signal fluency are superior to single-carrier technology, suitable for real-time communication emergency communication systems that emphasize wireless voice and wireless video. 2 OFDM technology and characteristics OFDM technology is orthogonal frequency division multiplexing technology. It has been proposed for 40 years. It is very similar to the FDM technology that has been widely used. OFDM technology distributes high-speed data streams to relatively low rates through serial / parallel conversion. Several frequency sub-channels are used for transmission. The first practical application is military wireless high-frequency communication links. Compared with traditional technology, OFDM technology has the following advantages: 1) By serial / parallel conversion of the high-rate data stream, the duration of the data symbols on each subcarrier is relatively increased, thereby effectively reducing the interference between the symbols due to the time dispersion of the wireless channel, thereby reducing the reception In some cases, the complexity of the internal equalizer may not even use the equalizer, but only insert the cyclic prefix to eliminate the adverse effects of inter-symbol interference. 2) Due to the orthogonality between the sub-carriers in OFDM, the spectrums of the sub-channels are allowed to overlap with each other. Therefore, the OFDM system can make maximum use of spectrum resources. The bandwidth utilization ratio of OFDM technology and traditional FDM technology is shown in Figure 1. As can be seen from Figure 1, the traditional FDM technology requires a large frequency interval between the two channels to prevent interference, which reduces the overall spectrum utilization, and the orthogonal multiplexing of subcarriers using OFDM technology Technology greatly reduces the protection bandwidth and improves spectrum utilization. 3) The orthogonal modulation and demodulation of each sub-channel can be achieved by using inverse discrete Fourier transform and discrete Fourier transform, and when the number of subcarriers is large, it can also be achieved by using inverse fast Fourier transform and Fast Fourier Transform. 4) The physical layer of the OFDM system supports asymmetric high-rate data transmission. Different transmission rates in the uplink and downlink can be achieved by using different data sub-channels. 5) OFDM technology is easy to use in combination with multiple access methods. However, since there are multiple orthogonal subcarriers in the OFDM system, and the output signal is a superposition of multiple subchannel signals, there are also some disadvantages compared with the traditional technology: 1) Easily affected by frequency deviation. Since the spectrums of the sub-channels cover each other, this places strict requirements on the orthogonality between them. The wireless signal spectrum shift caused by the time-varying wireless channel or the frequency deviation between the local oscillator of the transmitter and the receiver will destroy the orthogonality between the subcarriers of the OFDM system, resulting in Inter-subcarrier interference. 2) There is a higher peak-to-average power ratio. The output of a multi-carrier system is the superposition of multiple subchannel signals. Therefore, if the phases of multiple signals are consistent, the instantaneous power of the resulting superimposed signal will be much higher than the average power of the signal, resulting in a larger peak-to-average power ratio. This imposes high requirements on the linearity of the amplifier in the transmitter, so it may cause signal distortion and change the frequency spectrum of the signal, resulting in the destruction of the orthogonality between the sub-channels and interference. The performance of the system deteriorates.
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Figure 1 Comparison of bandwidth utilization between FDM and OFDM. 