OFDM (Orthogonal Frequency Division MulTIplexing) is orthogonal frequency division multiplexing technology. In fact, OFDM is MCM MulTI-CarrierModulaTIon, a type of multi-carrier modulation. The main idea is to divide the channel into several orthogonal sub-channels, convert the high-speed data signals into parallel low-speed sub-data streams, and modulate to transmit on each sub-channel. Orthogonal signals can be separated by using related technologies at the receiving end, which can reduce the mutual interference ICI between sub-channels. The signal bandwidth on each sub-channel is smaller than the associated bandwidth of the channel, so each sub-channel can be regarded as a flat fading, which can eliminate inter-symbol interference. And because the bandwidth of each sub-channel is only a small part of the original channel bandwidth, channel equalization becomes relatively easy.
1. Basic principles
OFDM-OFDM (Orthogonal Frequency Division MulTIplexing) is orthogonal frequency division multiplexing technology. In fact, OFDM is MCM Multi-Carrier Modulation, a type of multi-carrier modulation. The main idea is to divide the channel into several orthogonal sub-channels, convert the high-speed data signals into parallel low-speed sub-data streams, and modulate to transmit on each sub-channel. Orthogonal signals can be separated by using related technologies at the receiving end, which can reduce the mutual interference ICI between sub-channels. The signal bandwidth on each sub-channel is smaller than the associated bandwidth of the channel, so each sub-channel can be regarded as a flat fading, which can eliminate inter-symbol interference. And because the bandwidth of each sub-channel is only a small part of the original channel bandwidth, channel equalization becomes relatively easy. In the process of evolution to B3G / 4G, OFDM is one of the key technologies. It can combine diversity, spatio-temporal coding, interference and inter-channel interference suppression, and smart antenna technology to maximize system performance. Including the following types: V-OFDM, W-OFDM, F-OFDM, MIMO-OFDM, multi-band-OFDM.
2. Development history
The fourth generation mobile communication system In the 1970s, Weistein and Ebert and others developed a complete multi-application using discrete Fourier transform (DFT) and fast Fourier method (FFT). The carrier transmission system is called orthogonal frequency division multiplexing (OFDM) system.
OFDM is the abbreviation of orthogonal frequency division multiplexing. Orthogonal frequency division multiplexing is a special multi-carrier transmission scheme. OFDM uses discrete Fourier transform (DFT) and its inverse transform (IDFT) method to solve the problem of generating multiple mutually orthogonal subcarriers and recovering the original signal from the subcarriers. This solves the problem of multi-carrier transmission system transmission and transmission. The application of fast Fourier transform makes the complexity of multi-carrier transmission system greatly reduced. From this OFDM technology began to be practical. However, the application of OFDM system still requires a large number of complicated digital signal processing processes. At that time, there was still a lack of powerful digital processing components, so the OFDM technology has not been developed rapidly.
OFDM baseband transceiver block diagram In recent years, the rapid development of integrated digital circuits and digital signal processing devices, as well as the increasing urgency of high-speed wireless communication requirements, OFDM technology has once again received attention.
The concept of using parallel data transmission and frequency division multiplexing (FDM) has been proposed in the 1960s. In 1970, the United States invented and applied for a patent. The idea is to use parallel data and subchannels to overlap frequency division multiplexing to eliminate the dependence on high-speed equalization. It is used to resist impulse noise and multipath distortion, and can be fully utilized. bandwidth. This technology was initially mainly used in military communication systems. But for quite a long time in the future, the pace of OFDM theory towards practice slowed down. Since OFDM subcarriers are orthogonal to each other, FFT is used to achieve this modulation, but in practical applications, the complexity of real-time Fourier transform equipment, the stability of transmitter and receiver oscillators, and the linearity requirements of RF power amplifiers, etc. The factor part becomes the restrictive condition for the realization of OFDM technology. In the 1980s, MCM made breakthrough progress. Large-scale integrated circuits made the realization of FFT technology no longer an insurmountable obstacle. Some other difficult difficulties were also solved. Since then, OFDM has embarked on The stage of communication is gradually moving towards the field of high-speed digital mobile communication [1].
3. Application
OFDM baseband signal processing schematic diagram Due to the technical achievability, in the 1990s, OFDM was widely used in various digital transmission and communication, such as mobile wireless FM channel, high bit rate digital subscriber line system (HDSL), not Symmetric digital subscriber line system (ADSL), very high bit rate digital subscriber line system HDSI], digital audio broadcasting (DAB) system, digital video broadcasting (DVB) and HDTV terrestrial broadcasting system. In 1999, IEEE802.lla passed a SGHz wireless local area network standard, in which OFDM modulation technology was adopted as a physical layer standard, making the transmission rate up to 54MbPs. In this way, it can provide 25MbPs wireless ATM interface and 10MbPs Ethernet wireless frame structure interface, and support voice, data and image services. Such a rate can fully meet various indoor and outdoor applications. The European Telecommunications Organization (ETsl) broadband radio frequency access network LAN standard HiperiLAN2 also sets OFDM as its modulation standard technology.
OFDM improves spectrum efficiency In 2001, IEEE802.16 passed the wireless metropolitan area network standard. The standard can be divided into two types of line-of-sight and non-line-of-sight depending on the frequency band used. Among them, the 2-11GHz licensed and unlicensed frequency bands are used. Due to the long wavelength in this band, it is suitable for non-line-of-sight propagation. At this time, the system will have a strong multipath effect, and there are interference problems in the unlicensed band. It adopts OFDM modulation which has obvious advantages in resisting multipath effect, frequency selective fading or narrow-band interference. The multiple access method is OFDMA. Then, the IEEE802.16 standard is developing every year. In February 2006, IEEE802.16e (mobile broadband wireless metropolitan area network access air interface standard) formed the final publication. Of course, the modulation used is still OFDM.
Pilot insertion method In November 2004, according to the requirements of many mobile communication operators, manufacturers and research institutes, 3GPP adopted the project work called LongTerm Evolution (LTE), or "3G Long Term Evolution". The project aims at formulating technical specifications for 3G evolved systems. After intense discussion and hard integration, 3GPP finally selected the basic transmission technology of LTE in December 2005, namely downlink OFDM and uplink SC (single carrier off FDMA. OFDM was selected as the downlink standard due to the maturity of the technology. A consensus was reached soon. As for the selection of upstream technology, some equipment vendors believe that the OFDM peak-to-average ratio (PAPR) will increase the terminal ’s power amplifier cost and power consumption, limit the terminal ’s use time, and some believe that it can be filtered. Peak shaving and other methods limit the peak-to-average ratio. However, after discussion, the final uplink still uses the SC-FDMA method. The 3G standard with China ’s independent intellectual property rights—TD-SCDMA has also proposed TD-CDM in the LTE evolution plan. The OFDM scheme B3G / 4G is a goal proposed by the ITU and hopes to be implemented in 2010. The goal of B3G / 4G is to support downlink data transmission rates of up to 100 Mb / S in high-speed mobile environments and up to 100 Mb / S in indoor and static environments. The downlink data transmission rate of IGb / S. OFDM technology will also play an important role [2].
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