OFDM definition
The English name of OFDM is Orthogonal Fre-quency Division MulTIplexing, and the Chinese meaning is orthogonal frequency division multiplexing technology. This technology is the basis of the HPA (HomePlug Powerline Alliance) industry specification. It uses a discontinuous multi-tone technology to combine a large number of signals in different frequencies called carriers into a single signal to complete signal transmission. Because this technology has the ability to transmit signals under clutter interference, it is often used in transmission media that is susceptible to external interference or has poor resistance to external interference.
In fact, OFDM is not a new technology developed today. The application of OFDM technology has a history of nearly 40 years and is mainly used in military wireless high-frequency communication systems. However, the structure of an OFDM system is very complicated, which limits its further promotion. Until the 1970s, people used discrete Fourier transform to realize the modulation of multiple carriers, simplified the system structure, and made OFDM technology more practical. In the 1980s, people studied how to apply OFDM technology to high-speed modems. Since the 1990s, the research of OFDM technology has gone deep into the broadband data transmission on the wireless FM channel. At present, OFDM technology has been widely used in broadcast audio and video fields and civil communication systems. The main applications include: asymmetric digital subscriber loop (ADSL), ETSI standard digital audio broadcasting (DAB), digital video broadcasting ( DVB), high-definition television (HDTV), wireless local area network (WLAN), etc.
Basic principles of OFDM
In fact, OFDM is MCM MulTI-CarrierModulaTIon, a kind 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 ISI 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.
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. The carriers in OFDM are orthogonal to each other. Each carrier has an integer number of carrier cycles within a symbol time. The zero point of the spectrum of each carrier overlaps with the zero point of the adjacent carrier. This reduces interference between carriers. Due to the partial overlap between the carriers, it improves the frequency band utilization rate compared to traditional FDMA.
During the OFDM propagation process, the high-speed information data stream is allocated to several sub-channels at a relatively low rate through serial-to-parallel conversion, and the symbol period in each sub-channel is relatively increased, which can reduce the delay caused by the multipath delay of the wireless channel. The resulting time dispersion causes intersymbol interference to the system. In addition, due to the introduction of guard interval, in the case where the guard interval is greater than the maximum multipath delay spread, the inter-symbol interference caused by multipath can be eliminated to the greatest extent. If the cyclic prefix is ​​used as the guard interval, inter-channel interference caused by multipath can also be avoided.
In the past frequency division multiplexing (FDM) system, the entire bandwidth is divided into N sub-bands, and the sub-bands do not overlap. In order to avoid mutual interference between sub-bands, protection bandwidth is usually added between the bands, but this will reduce the spectrum utilization rate . In order to overcome this shortcoming, OFDM uses N overlapping sub-bands, and the sub-bands are orthogonal, so the signal can be received without separating the spectrum at the receiving end. One of the main advantages of OFDM systems is that orthogonal subcarriers can be modulated and demodulated using fast Fourier transform (FFT / IFFT). For the N-point IFFT operation, N2 complex multiplications need to be implemented. Using the common 2-based IFFT algorithm, the complex multiplication is only (N / 2) log2N, which can significantly reduce the computational complexity.
The addition of a guard interval at the transmitting end of the OFDM system is mainly to eliminate ISI caused by multipath (the orthogonality between sub-carriers is destroyed and interference between different sub-carriers is generated). The method is to fill in the cyclic prefix within the guard interval of the OFDM symbol to ensure that the number of waveform cycles contained in the delay copy of the OFDM symbol in the FFT cycle is also an integer. In this way, a signal with a delay less than the guard interval will not generate ISI during demodulation.
OFDM application
Due to the technical feasibility, 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), asymmetric 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 5GHz wireless local area network standard, in which OFDM modulation technology was adopted as the 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.
In 2001, IEEE802.16 passed the wireless metropolitan area network standard, which can be divided into 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.
In November 2004, according to the requirements of many mobile communication operators, manufacturers and research institutions, 3GPP adopted the project work called LongTermEvoluTIon (LTE), which is called "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. IGb / S downlink data transmission rate, and OFDM technology will also play an important role.
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