In a conventional multi-carrier communication system, the entire system band is divided into a number of mutually separated sub-channels (carriers). There is a certain guard interval between carriers, and the receiver receives the required information after separating each subchannel through a filter. This can avoid different channels interfering with each other, but at the expense of frequency utilization. Moreover, when the number of subchannels is large, it is almost impossible to separate the settings of the filters of the subchannel signals.
In the middle of the last century, a multi-carrier communication scheme with band aliasing was proposed, and the carrier frequencies orthogonal to each other were selected as subcarriers, which is what we call OFDM. This "orthogonal" represents the exact mathematical relationship between carrier frequencies. According to this assumption, OFDM can fully utilize the channel bandwidth and avoid the use of high-speed equalization and anti-burst noise errors. OFDM is a special multi-carrier communication scheme in which a single user's information stream is serial/parallel converted into multiple low-rate code streams, each of which is transmitted with one sub-carrier. Instead of bandpass filters to separate subcarriers, OFDM uses Fast Fourier Transform (FFT) to select waveforms that remain orthogonal even if aliased.
OFDM is a high-speed transmission technology in a wireless environment. The frequency response curve of the wireless channel is mostly non-flat, and the main idea of ​​OFDM technology is to divide a given channel into many orthogonal subchannels in the frequency domain, use one subcarrier for modulation on each subchannel, and transmit each subcarrier in parallel. . Thus, although the total channel is non-flat and frequency selective, each subchannel is relatively flat, and narrowband transmission is performed on each subchannel, and the signal bandwidth is smaller than the corresponding bandwidth of the channel, so that the signal waveform can be greatly eliminated. Interference. Since the carriers of each subchannel are orthogonal to each other in the OFDM system, their spectrums overlap each other, which not only reduces mutual interference between subcarriers, but also improves spectrum utilization.
OFDM technology belongs to the multi-carrier modulation (MulTI-Carrier ModulaTIon, MCM) technology. In some literatures, OFDM and MCM are mixed, which is actually not tight enough. MCM and OFDM are commonly used in wireless channels. The difference is that OFDM technology specifically divides channels into orthogonal subchannels and has high channel utilization. MCM can be a more variety of channel division methods.
The introduction of OFDM technology is actually to improve the spectrum utilization of the carrier, or to improve the modulation of multiple carriers. It is characterized in that each subcarrier is orthogonal to each other, so that the spectrum after the spread spectrum modulation can overlap each other, thereby reducing Mutual interference between subcarriers. After the modulation is completed for each carrier, in order to increase the throughput of the data and increase the speed of data transmission, it uses a processing technique called HomePlug to combine all the carriers of the data signal bits to be transmitted. Combine a large number of individual signals into one independent transmission signal for transmission. In addition, one of the important reasons why OFDM has attracted much attention is that it can replace the multi-carrier modulation and demodulation with discrete Fourier transform/discrete Fourier transform (IDFT/DFT).
OFDM enhances the ability to resist frequency selective fading and to resist narrowband interference. In a single carrier system, a single fading or interference may result in the entire link being unavailable, but in a multi-carrier OFDM system, only a small fraction of the carrier is affected. In addition, the use of error correcting codes can also help them recover information on some carriers. By reasonably selecting the subcarrier positions, the spectral waveform of the OFDM can be kept flat while ensuring orthogonality between the carriers.
Although OFDM is still a frequency division multiplexing (FDM), it is completely different from the past FDM. The receiver of OFDM is actually a set of demodulators implemented by FFT. It moves the different carriers to zero frequency and then integrates in one symbol period. The other carrier signals are orthogonal to the integrated signal and therefore do not affect the information extraction. The data transmission rate of OFDM is also related to the number of subcarriers.
The modulation method used by each carrier of OFDM can be different. Each carrier can select different modulation modes according to different channel conditions, such as BPSK, QPSK, 8PSK, 16QAM, 64QAM, etc., based on the principle of optimal balance between spectrum utilization and bit error rate. We can achieve maximum spectral efficiency by choosing the best modulation that meets a certain bit error rate. The frequency selective fading of the wireless multipath channel will greatly reduce the received signal power, often reaching as much as 30 dB, and the signal to noise ratio will also drop significantly. In order to improve spectrum utilization, a modulation scheme that matches the signal-to-noise ratio should be used. Reliability is the basic evaluation indicator for the normal operation of communication systems. Therefore, many communication systems tend to choose BPSK or QPSK modulation to ensure the signal-to-noise ratio requirement under the worst conditions of the channel, but the spectrum efficiency of these two modulation methods is very low. . OFDM technology uses adaptive modulation to select different modulation schemes depending on the quality of the channel conditions. For example, when the terminal is close to the base station, the channel condition is generally better, and the modulation mode can be converted into 16QAM-64QAM (spectrum efficiency 4 to 6 bit/s/Hz) by BPSK (spectral efficiency 1 bit/s/Hz), and the spectrum of the entire system. The utilization rate will be greatly improved. Adaptive modulation can increase system capacity, but it requires that the signal must contain a certain amount of overhead bits to inform the receiver of the modulation method that should be used to transmit the signal. The terminal also periodically updates the modulation information, which also adds more overhead bits.
OFDM also employs a coordinated operation of power control and adaptive modulation. When the channel is good, the transmit power is constant, the modulation mode (such as 64QAM) can be enhanced, or the transmit power can be reduced in a low modulation mode (such as QPSK). Power control and adaptive modulation are balanced. That is to say, for a transmitting station, if it has a good channel, a higher modulation scheme such as 64QAM can be used if the transmission power remains unchanged; if the power is reduced, the modulation scheme can be correspondingly reduced, using QPSK. Ways, etc.
Adaptive modulation requires the system to have a timely and accurate understanding of the performance of the channel. If a strong modulation scheme is used on a poor channel, then a high bit error rate will occur, affecting the availability of the system. The OFDM system can use pilot signals or reference code words to test the quality of the channel. Send a codeword of known data, measure the signal-to-noise ratio of each channel, and determine the most suitable modulation method based on this signal-to-noise ratio.
What is OFDM?
The English full name of OFDM is called Orthogonal Fre-quency Division MulTIplexing, which means Orthogonal Frequency Division Multiplexing. This technology is the basis of the HPP Alliance Powerline Alliance industry specification, which uses a discontinuous multi-tone technique to combine a large number of signals at different frequencies, called carriers, into a single signal to complete the signal transmission. Since this technology has the ability to transmit signals under clutter interference, it is often used in transmission media that are susceptible to external interference or poor resistance to external interference.
In fact, OFDM is not a new technology developed today. The application of OFDM technology has been used for nearly 40 years, mainly for military wireless high-frequency communication systems. However, the structure of an OFDM system is very complex, which limits its further promotion. Until the 1970s, discrete Fourier transforms were used to implement modulation of multiple carriers, simplifying the system structure and making OFDM technology more practical. In the 1980s, people studied how to apply OFDM technology to high-speed MODEM. Since the 1990s, research on OFDM technology has penetrated into broadband data transmission over wireless FM channels. 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.
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