1. Overview of OFDM technology and its development history
Known as "fourth generation mobile communication technology", its core technology is OFDM. Orthogonal Frequency Division MulTIplexing (OFDM) is a high-speed transmission technology in a wireless environment. Mainly in the frequency domain, the given channel is divided into a plurality of orthogonal subchannels, one subcarrier is used for modulation on each subchannel, and each subcarrier is transmitted in parallel. OFDM is particularly suitable for transmitting high speed data in wireless mobile channels where multipath propagation and Doppler shift are present. It can effectively resist multipath effects, eliminate ISI, combat frequency selective fading, and have high channel utilization. OFDM can be regarded as a modulation technology and a multi-tasking technology, which is a multi-carrier (MulTIcar-rier) transmission method.
OFDM is developed from Multi-Carrier Modulation (MCM). The US military created the world's first MCM system in the 1950s and 1960s, and in 1970 it derived OFDM systems using large-scale subcarrier and frequency overlap techniques. But for a long time to come, OFDM has slowed down to practice. Since the subcarriers of OFDM are orthogonal to each other, this modulation is implemented by FFT, but in practical applications, the complexity of the real-time Fourier transform device, the stability of the transmitter and receiver oscillators, and the linearity requirements of the RF power amplifier Other factors constrain the implementation of OFDM technology. After a lot of research, in the 1980s, MCM achieved breakthrough progress, large-scale integrated circuits promoted the realization of FFT technology, and OFDM gradually entered the field of high-speed Modem and digital mobile communication. In the 1990s, OFDM began to be widely used in Europe and Australia for broadband data communication over broadcast channels, digital audio broadcasting (DAB), high definition digital television (HDTV) and wireless local area network (WLAN). With the development of DSP chip technology, the application of mature technologies such as grid coding technology, soft decision technology and channel adaptive technology, the implementation and improvement of OFMD technology is just around the corner.
2. The basic principle of OFDM technology
The main idea of ​​OFDM technology is to divide the assigned channel into a number of orthogonal subchannels, with narrowband modulation and transmission on each subchannel, the signal bandwidth being less than the associated bandwidth of the channel. The information stream of a single user of OFDM is serial/parallel converted into multiple low rate code streams (100 Hz - 50 kHz), and each code stream is transmitted with one carrier. OFDM uses a frequency hopping method to select a waveform that maintains orthogonality even if the spectrum is aliased. Therefore, OFDM has both a modulation technique and a multiplexing technique. OFDM enhances the ability to resist frequency selective fading and to resist narrowband interference. In a single carrier system, a single fading or interference can cause the entire link to be unavailable, but in a multi-carrier system, only a small fraction of the carrier is affected. The application of the error correction code can recover the information on some error-prone carriers.
OFDM allows frequencies between carriers to be aliased to each other, and FFT modulation based on carrier frequency orthogonality is adopted. Since there are no spectral components of other carriers at the center frequency of each carrier, orthogonality of each carrier can be achieved. It is not implemented by many bandpass filters, but directly at the baseband, which is one of the advantages of OFDM compared to other systems. The receiver of OFDM is actually a set of demodulator, which moves different carriers to zero frequency and integrates in one symbol period. Other carriers are orthogonal to the integrated signal and will not affect the integration result. The high data rate of OFDM is related to the number of subcarriers, and increasing the number of subcarriers can increase the data transmission rate. The modulation method of each frequency band of OFDM can be different, which increases the flexibility of the system, and OFDM is suitable for a multi-user high flexibility and high utilization communication system.
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