OFDM (orthogonal frequencydivisionmulTIplexing) Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier transmission technology mainly applied to digital video broadcasting systems, MMDS (mulTIchannelmulTIpointdistribuTIonservice) multi-channel multi-point distribution services and WLAN services, and next-generation land mobile communication systems.
First, the basis of OFDMOFDM is a multi-carrier digital modulation technique that encodes data into a radio frequency signal. Unlike conventional single-carrier techniques, such as AM/FM (Amplitude Modulation/Frequency Modulation), which transmit a single signal with a single frequency at a time, OFDM simultaneously transmits multiple high-speed signals over a specially calculated orthogonal frequency. This result is as efficient as the use of bandwidth in bursts of noise and other interference.
The traditional FDM (Frequency Division Multiplexing) theory divides the bandwidth into several subchannels, with a guard band in the middle to reduce interference, and they simultaneously transmit data. For example: cable TV systems and analog radio broadcasts, the receiver must be tuned to the appropriate station.
OFDM systems require much less bandwidth than traditional FDM systems. Due to the use of interference-free orthogonal carrier technology, no guard bands are required between individual carriers. This makes the use of the available spectrum more efficient. In addition, OFDM technology can dynamically allocate data on subchannels. To achieve maximum data throughput, multi-carrier modulators can intelligently allocate more data to sub-channels with less noise.
The application of OFDM to overcome inter-symbol and adjacent-channel interference techniques dates back to the mid-1960s. However, the practical application of OFDM has long been limited by the speed and efficiency of fast Fourier transformers. Today, the maturity of high-performance PLD (Programmable Logic Device) technology has created the current application of OFDM.
Modern single-carrier modulation methods such as integrated amplitude modulation (QAM) or integral phase shift keying modulation (QPSK) combine basic amplitude modulation, frequency modulation, and phase modulation techniques to provide higher noise rejection and better system throughput. The use of increased complex modulation techniques requires high performance digital logic, but also allows system builders to achieve higher signal-to-noise ratios and spectral efficiency close to the first-in-far limit.
Second, the application of OFDMRecently, OFDM has been adopted in several European wireless communication applications, such as ETSI standard digital audio broadcasting (DAB) and terrestrial digital video broadcasting (DVB-T). In the United States, OFDM is applied to MMDS (Multipoint Multi-Channel Distributed Service). The GPRS application standard IEEE 802.11a and the ETSI (European Telecommunications Standards Council) HiperLAN/2 standard also use OFDM as the modulation method. Wired applications also employ OFDM-based systems such as discrete multi-tone systems and cable modulator applications in xDSL.
The AT&T fixed wireless broadband subscriber service based on OFDM is planned to reach 15 million by the end of 2002. AT&T and Nortel Networks are considering the feasibility of fourth-generation wireless networks, with EDGE (Enhanced Data for Global Evolution) as the uplink and OFDM as the downlink.
The advantage of using OFDM at the physical layer for these applications is the simplification of narrowband channels, high system throughput, and noise rejection.
Third, OFDM structureThe OFDM structure can be divided into a forward error correcting encoder, an interleaver, a constellation map, a serial-to-parallel converter and a receiving portion of the inverse fast Fourier transformer, a parallel-serial converter, and a cyclic prefix according to an OFDM data processing flow. Insert, shape finite excitation response filter, digital-to-analog conversion and other modules.
OFDM modulation uses channel coding to suppress multipath effects, data symbols are mapped onto a corresponding constellation (like QPSK, QAM), and the resulting I and Q values ​​are stored in the buffer and the inverse fast Fourier transform (IFFT) is applied. The IFFT is modulated on a quadrature carrier. The data is ready to be sent and serialized in addition to a multipath effect plus a cyclic prefix. The processed signal is sent to the antenna and sent out.
1. functional module
(1) Forward Error Correction (ForwardErrorCorrection)
The channel coding uses a Reed-Solomon code, a convolutional error correcting code, a Viterbi code or a TURBO code.
(2) Interleaver
The interleaver is used to reduce burst errors in the data channel, and the interleaved data is mapped to a corresponding constellation through a serial to parallel converter.
(3) constellation diagram (omitted)
Multi-carrier OFDM is considered to be superior to N independent sub-bands modulated by a single carrier. The constellation maps the symbols to the corresponding constellation points. This process produces IQ values ​​that are filtered and sent to the IFFT for transformation.
(4) Buffering
Used to store the IQ value before being sent to IFFT. The IFFT can quickly and efficiently apply the discrete Fourier transform function and mathematically generate orthogonal carriers for OFDM transmission. The core of OFDM is IFFT. IFFT modulates each subchannel to a high precision orthogonal carrier. The channelized data is injected into a parallel string buffer, and the serial data is prepared for transmission by DAC conversion.
(5) parallel to serial converter
Used to convert parallel data to serial data.
(6) Cyclic prefix
The cyclic prefix creates a guard band for a single OFDM symbol individual that is dropped in the SNR edge loss to greatly reduce ISI. A Shaped Finite Excitation Response Filter (ShaperFIRFilter) is used to shape the signal.
2. Advantages of OFDM technology
(1) A large amount of data can be transmitted even in a narrowband bandwidth. OFDM technology can separate at least 1000 digital signals at the same time, and the ability to operate safely around the interfering signals will directly threaten the further development of the CDMA technology that has already become popular in the market. It is because of this special The signal "penetration capability" makes OFDM technology popular with European communication operators and mobile phone manufacturers, such as Cisco Systems of California, Flarion College of New York, and Lucent Institute of Technology, at the Wi-LAN Institute of Technology in Canada. Also started using this technology.
(2) OFDM technology can continuously monitor sudden changes in communication characteristics on a transmission medium. Since the ability of a communication path to transmit data changes over time, OFDM can dynamically adapt to it and turn on and off the corresponding carrier. To ensure continuous and successful communication;
(3) The technique can automatically detect which particular carrier has a high signal attenuation or interference pulse under the transmission medium, and then adopt appropriate modulation measures to enable successful communication of the carrier at the specified frequency;
(4) OFDM technology is particularly suitable for use in high-rise buildings, densely populated and geographically prominent areas, and areas where signals are spread. High-speed data transmission and digital voice broadcasting all want to reduce the impact of multipath effects on the signal.
(5) The biggest advantage of OFDM technology is against frequency selective fading or narrowband interference. In a single carrier system, a single fading or interference can cause the entire communication link to fail, but in a multi-carrier system, only a small fraction of the carriers are subject to interference. Error correction codes can also be used for these subchannels for error correction.
(6) It can effectively resist interference between signal waveforms and is suitable for high-speed data transmission in multipath environments and fading channels. When frequency selective fading occurs in the channel due to multipath transmission, only the subcarriers falling in the frequency band recess and the information carried by them are affected, and the other subcarriers are not damaged, so the overall BER performance of the system is better. many.
(7) It has strong anti-fading capability through joint coding of each subcarrier. The OFDM technology itself has utilized the frequency diversity of the channel, and if the fading is not particularly severe, there is no need to add a time domain equalizer. By jointly coding the individual channels, system performance can be improved.
(8) OFDM technology is highly resistant to narrowband interference because these interferences only affect a small portion of the subchannels.
(9) An OFDM implementation method based on IFFT/FFT can be selected;
(10) The channel utilization rate is very high, which is especially important in a wireless environment with limited spectrum resources; when the number of subcarriers is large, the spectrum utilization rate of the system tends to 2Baud/Hz.
3. Two defects of OFDM technology
(1) Very sensitive to frequency offset and phase noise.
(2) The peak-to-average power ratio is relatively large, and an increase in this ratio reduces the power efficiency of the RF amplifier.
In the design and manufacture of specific equipment, various manufacturers have taken different measures to offset their impact. In recent years, with the development of DSP chip technology, Fourier transform/inverse transform, 64/128/256QAM technology adopted by high-speed Modem, raster coding technology, soft decision technology, channel adaptive technology, insertion protection period, and reduced balance calculation With the gradual introduction of mature technologies such as quantity, OFDM as a high-speed transmission technology that can effectively resist interference between signal waveforms will be more widely used in the field of broadband mobile communication.
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