What is the basic principle of OFDM?

OFDM (Orthogonal Frequency Division Multiplexing) is an orthogonal frequency division multiplexing technology. In fact, OFDM is a MCM multicarrier modulation. Its main idea is to divide the channel into several orthogonal sub-channels, convert high-speed data signals into parallel low-speed sub-data streams, and modulate them for transmission on each sub-channel. At the receiving end, orthogonal signals can be separated by using correlation technology, and the mutual interference ICI between sub-channels can be reduced. The signal bandwidth on each subchannel is smaller than the correlation bandwidth of the channel, so the signal bandwidth on each subchannel can be regarded as flat fading, so that the inter-symbol interference can be eliminated. Moreover, because the bandwidth of each subchannel is only a small part of the original channel bandwidth, channel equalization becomes relatively easy. In the evolution to B3G/4G, OFDM is one of the key technologies, which can combine diversity, space-time coding, interference and inter-channel interference suppression and smart antenna technology to maximize system performance. Include the following types: V-OFDM, W-OFDM, F-OFDM, MIMO-OFDM.

The development history of 1. 1

In 1970s, Weinstein and Ebert developed a complete multi-carrier transmission system by using discrete Fourier transform (DFT) and fast Fourier method (FFT), which is called orthogonal frequency division multiplexing (OFDM) system.

OFDM is the abbreviation of orthogonal frequency division multiplexing. Orthogonal frequency division multiplexing (OFDM) is a special multi-carrier transmission scheme. In OFDM, Discrete Fourier Transform (DFT) and its inverse transform (IDFT) are used to solve the problems of generating multiple orthogonal subcarriers and recovering the original signal from the subcarriers. This solves the problem of transmission and transmission in multi-carrier transmission system. The application of fast Fourier transform greatly reduces the complexity of multi-carrier transmission system. From then on, OFDM technology began to be practical. However, the application of OFDM system still needs a lot of complex digital signal processing processes, and there were few devices with powerful digital processing functions at that time, so OFDM technology has not developed rapidly.

In recent years, with the rapid development of integrated digital circuits and digital signal processing devices, and the increasingly urgent demand for high-speed wireless communication, OFDM technology has been paid attention to again. In 1960s, the concept of using parallel data transmission and frequency division multiplexing (FDM) was put forward. 1970, the United States applied for and invented a patent. Its idea is to use frequency division multiplexing of parallel data and subchannels to eliminate dependence on high-speed equalization, resist impulse noise and multipath distortion, and make full use of bandwidth. This technology was originally mainly used in military communication systems. However, for a long time, the pace of OFDM theory moving towards practice has slowed down. Because the subcarriers of OFDM are orthogonal to each other, FFT is used to realize this modulation. However, in practical application, the complexity of real-time Fourier transform equipment, the stability of transmitter and receiver oscillators, and the linearity requirement of RF power amplifier have become the restrictive factors of OFDM technology. In 1980s, MCM made a breakthrough. Large-scale integrated circuits make the implementation of FFT technology no longer an insurmountable obstacle, and some other difficulties that are difficult to achieve have been partially solved. Since then, OFDM has stepped onto the communication stage and gradually entered the field of high-speed digital mobile communication.

Application of 1.2

Due to the realization of technology, OFDM was widely used in various digital transmission and communication in 1990s, such as mobile wireless FM channel, high bit rate digital subscriber line system (HDSL), asymmetric digital subscriber line system (ADSL), very high bit rate digital subscriber line system (HDSI), digital audio broadcasting (DAB), digital video broadcasting (DVB) and HDTV terrestrial transmission system. 1999, IEEE802.lla passed a wireless LAN standard, in which OFDM modulation technology was adopted as the physical layer standard, so that the transmission rate could reach 54MbPs. In this way, 25MbPs wireless ATM interface and 10MbPs Ethernet wireless frame structure interface can be provided to support voice, data and image services. This rate can fully meet indoor and outdoor applications. The LAN standard of ETSl broadband RF access network also defines OFDM as its modulation standard technology.

200 1, IEEE802. 16 passed the standard of wireless metropolitan area network, which can be divided into line-of-sight and non-line-of-sight according to different frequency bands. Licensed and unlicensed frequency bands are used, because the wavelength in this frequency band is long and suitable for NLOS propagation. At this time, the system will have strong multipath effect, but there is still interference problem in the unauthorized frequency band, so the system adopts OFDM modulation which has obvious advantages in resisting multipath effect, frequency selective fading or narrowband interference, and the multiple access mode is OFDMA. Then, the standard of IEEE802. 16 is developing every year. In February, 2006, IEEE802. 16e (Mobile Broadband Wireless MAN Access Air Interface Standard) came into being as the final publication. Of course, the modulation method used is OFDM.

On June 5438+065438+ 10, 2004, according to the requirements of many mobile communication operators, manufacturers and research institutions, 3GPP adopted a project called Long Term Evolution (LTE), namely "3G Long Term Evolution". The goal of this project is to formulate the technical specifications of 3G evolution system. After intense discussion and difficult integration, 3GPP finally chose the basic transmission technology of LTE from June 5438 to February 2005, namely downlink OFDM and uplink SC. Due to the maturity of technology, OFDM was selected as the downlink standard, and soon reached an understanding of * * *. In the choice of uplink technology, due to the peak-to-average ratio (PAPR) of OFDM, some equipment manufacturers think that it will increase the power amplifier cost and power consumption of the terminal and limit the service time of the terminal, while others think that PAPR can be limited by filtering, peak clipping and other methods. The goal of B3G/4G is to support the downlink data transmission rate as high as 100Mb/S in high-speed mobile environment and 1Gb/S in indoor and static environment. In 20 10, the world's first TD-LTE-A large-scale experimental network will be opened to the media at the Shanghai World Expo. 4G is a combination of technologies based on OFDM and MIMO, but the overall structure is different. There are two sets of standards based on OFDM and MIMO, one is IEEE802- 16M, and the other is LTE-Advanced. OFDM technology is one of the key core technologies.

Advantages and disadvantages of 1.4

Advantages: OFDM has many technical advantages as follows. Used in 3G and 4G, it has many advantages in communication:

(1) OFDM technology can also send a large amount of data in a narrow bandwidth, and can separate at least 1000 digital signals at the same time. Its ability to operate safely around the interfered signals will directly threaten the further development and growth of CDMA technology which has been popular in the market at present. It is precisely because of this special signal "penetrating ability" that OFDM technology is deeply loved and welcomed by European communication operators and mobile phone manufacturers, such as Cisco Systems, new york Institute of Technology and Lucent Institute of Technology in California, and Willand Institute of Technology in Canada.

(2) OFDM technology can continuously monitor the sudden change of communication characteristics on the transmission medium. Because the ability of communication path to transmit data will change with time, OFDM can dynamically adapt, turn on and off the corresponding carriers to ensure continuous and successful communication. This technology can automatically detect which specific carrier signal in the transmission medium has large attenuation or interference pulse, and then take appropriate modulation measures to make the carrier with specified frequency successfully communicate.

(3) OFDM technology is especially suitable for high-rise buildings, densely populated and geographically prominent places, and areas where signals propagate. Both high-speed data transmission and digital voice broadcasting hope to reduce the influence of multipath effect on signals.

The biggest advantage of OFDM technology is its resistance to frequency selective fading or narrowband interference. In a single carrier system, a single fading or interference will lead to the failure of the whole communication link, but in a multi-carrier system, only a few carriers will be interfered. Error correcting codes can also be used to correct these subchannels.

(5) OFDM technology can effectively resist the interference between signal waveforms and is suitable for high-speed data transmission in multipath environment and fading channels. When the channel is frequency-selective fading due to multipath transmission, only the subcarriers and the information they carry are affected, and other subcarriers are not damaged, so the overall bit error rate performance of the system is much better.

(6) OFDM technology has strong anti-fading ability through the joint coding of each subcarrier. OFDM technology itself makes use of the frequency diversity of the channel. If the fading is not particularly serious, there is no need to add a time domain equalizer. By jointly coding each channel, the system performance can be improved.

(7) OFDM technology can make the channel utilization rate high, which is especially important in the wireless environment with limited spectrum resources; When the number of subcarriers is large, the spectrum utilization rate of the system tends to 2 baud/Hz.

Disadvantages: Although OFDM has the above advantages, its signal modulation mechanism also makes OFDM signals have some disadvantages in the transmission process:

(1) is very sensitive to phase noise and carrier frequency offset.

This is a very fatal shortcoming of OFDM technology. The whole OFDM system requires very strict orthogonality between subcarriers. Any small carrier frequency offset will destroy the orthogonality between subcarriers and cause ICI. Similarly, phase noise will also lead to the rotation and diffusion of symbol constellation points, thus forming ICI. However, single carrier systems do not have this problem. Phase noise and carrier frequency offset will only reduce the received signal-to-noise ratio and will not cause mutual interference.

(2) The peak-to-average ratio is too large

OFDM signals are composed of multiple subcarrier signals, which are independently modulated by different modulation symbols. Compared with the traditional constant envelope modulation method, OFDM modulation has a higher peak factor. Because OFDM signal is the sum of many small signals, the phase of these small signals is determined by the data sequence to be transmitted. For some data, these small signals may be in phase, but superimposed in amplitude to produce a large instantaneous peak amplitude. However, if the PAPR is too large, it will increase the complexity of A/D and D/A and reduce the efficiency of RF power amplifier. At the same time, at the transmitter, the maximum output power of the amplifier limits the peak value of the signal, which will cause interference within the OFDM frequency band and between adjacent frequency bands.

(3) the required linear range is wide

Because the peak to average power ratio (PAPR) of OFDM system is larger, it is more sensitive to nonlinear amplification, so OFDM modulation system requires higher linear range of amplifier than single carrier system.