Basic Principles of OFDM technology

In traditional multi-carrier communication systems, the entire system band is divided into several sub-channels (carriers) that are separated from each other ). There is a certain protection interval between carriers, and the receiver uses a filter to separate each sub-channel and then receives the required information. This can avoid mutual interference between different channels, but at the expense of frequency utilization. In addition, when the number of subchannels is large, it is almost impossible to set a filter that separates the signal of each subchannel.

In the middle of the last century, we proposed a multi-carrier communication scheme with overlapping bandwidths. We chose orthogonal carrier frequencies as subcarriers, which we call OFDM. This "orthogonal" represents the exact mathematical relationship between carrier frequencies. According to this assumption, OFDM can not only make full use of channel bandwidth, but also avoid using high-speed balancing and anti-Burst Noise errors. OFDM is a special multi-carrier communication solution. The information flow of a single user is serialized/transformed into multiple low-rate bitstreams, and each bitstream is sent using one subcarrier. OFDM does not use a band-pass filter to separate subcarriers, but uses the Fast Fourier Transform (FFT) to select waveforms that can maintain orthogonal even if they are overlapped.

OFDM is a high-speed transmission technology in wireless environments. The frequency response curves of wireless channels are mostly non-flat, and the main idea of OFDM technology is to divide a given channel into many orthogonal subchannels in the frequency domain, one sub-carrier is used for modulation on each sub-channel, and each sub-carrier is transmitted in parallel. In this way, although the total channel is non-flat and frequency-selective, each sub-channel is relatively flat and performs narrowband transmission on each sub-channel, the signal bandwidth is smaller than the corresponding bandwidth of the channel, so the interference between the signal waveforms can be greatly eliminated. In the OFDM system, the carriers of subchannels are orthogonal to each other, and their spectrum overlaps with each other. This not only reduces the mutual interference between subcarriers, but also improves the spectrum utilization.

OFDM is a multi-carriermodulation (MCM) technique. In some documents, the mixing of OFDM and MCM is not strict enough. MCM and OFDM are often used in wireless channels. The difference between them is that the OFDM technology is used to divide the channels into subchannels with high channel utilization. MCM can be used to divide more channels.

The introduction of OFDM technology is to improve the spectrum utilization of the carrier, or to improve the modulation of multi-carrier, It is characteristic that each sub-carrier is orthogonal to each other, so that the spectrum after the spread spectrum modulation can overlap with each other, thus reducing the mutual interference between sub-carriers. After completing modulation for each carrier, to increase data throughput and speed up data transmission, it adopts a processing technology called homeplug, to merge and process all the carriers whose data signal locations will be sent, and merge a large number of individual signals into an independent transmission signal for sending. In addition, OFDM has received much attention. One of the important reasons is that it can use discrete Fourier inverse transformation/Discrete Fourier Transformation (idft/DFT) to replace multi-carrier modulation and decoding.

OFDM enhances the ability to resist frequency selective fading and narrow band interference. In a single carrier system, a single fading or interference may cause the entire link to become unavailable. However, in a multi-carrier OFDM system, only a small portion of the carriers are affected. In addition, the use of Error Correction Codes can also help restore information on some carriers. By selecting the sub-carrier location reasonably, the spectrum waveform of the OFDM can be flat and the orthogonal between carriers can be ensured.

Although OFDM is still a type of Frequency Division Multiplexing (OFDM), it is completely different from the previous OFDM. In fact, the OFDM receiver is a set of receivers implemented by FFT. It moves different carriers to zero frequency, and then points in a code element cycle. Because other carrier signals are orthogonal to the signals of the points, the information extraction will not be affected. The data transmission rate of OFDM is also related to the number of subcarriers.

The modulation methods used for each OFDM carrier can be different. Each carrier can select different modulation modes based on different channel conditions, such as BPSK, PSK, 8PSK, 16QAM, 64QAM, and so on. The principle is based on the optimal balance between spectrum utilization and bit error rate. We can achieve the maximum spectrum efficiency by selecting the best modulation method that satisfies a certain bit error rate. The frequency selective fading of wireless multi-path channels will greatly reduce the power of the received signal, which often reaches 30db, and the signal-to-noise ratio will also decrease significantly. To improve the spectrum utilization, the modulation method that matches the signal-to-noise ratio should be used. Reliability is the basic evaluation indicator for the normal operation of the communication system. Therefore, many communication systems tend to adopt the BPSK or PSK modulation to ensure the signal-to-noise ratio requirements under the worst channel conditions, however, the spectrum efficiency of the two modulation methods is very low. OFDM uses adaptive modulation to select different modulation modes based on the channel conditions. For example, when the terminal is close to the base station, the channel conditions are generally better, and the modulation method can be converted from BPSK (1 bit/s/Hz) to 16qam-64qam (4-bit spectrum efficiency ~ 6 bit/s/Hz), the spectrum utilization of the entire system will be greatly improved. Adaptive Modulation can expand the system capacity, but it requires that the signal must contain a certain overhead bit to inform the receiver of the modulation method used to transmit the signal. The terminal also regularly updates the modulation information, which also increases the overhead bit.

OFDM also uses power control and adaptive modulation to coordinate the operation. When the channel is good, the transmit power remains unchanged. The modulation method (such as 64QAM) can be enhanced, or the transmit power can be reduced when the channel is low (such as PSK. Power Control and adaptive modulation must be balanced. That is to say, if a transmitting station has a good channel, a high modulation scheme such as 64QAM can be used while the sending power remains unchanged; if the power is reduced, the modulation scheme can also be reduced accordingly, using the PSK method.

Adaptive Modulation requires the system to have a timely and accurate understanding of the channel performance. If a strong modulation method is used on a poor channel, a high error rate will occur, affects system availability. The OFDM system can use the pilot signal or reference code to test the channel quality. Send a code word with known data to measure the signal-to-noise ratio of each channel. The most suitable modulation method is determined based on the signal-to-noise ratio.

What is OFDM?

Orthogonal fre-quency division multiplexing, which is used in orthogonal frequency division multiplexing. This technology is the basis of the industrial specifications of the HPA Alliance (homeplug Powerline Alliance). It uses a non-consecutive multi-Tone Technology, combines a large number of signals in different frequencies called carriers into a single signal to complete signal transmission. Because this technology can transmit signals under clutter interference, it is often used in transmission media that is vulnerable to external interference or poor resistance to external interference.

In fact, OFDM is not a new technology developed today. It has been used for nearly 40 years and is mainly used in military wireless high-frequency communication systems. However, the structure of an OFDM system is very complex, which limits its further promotion. Until the 1970s S, we used Discrete Fourier Transform to implement modulation of multiple carriers, simplifying the system structure and making the OFDM technology more practical. In 1980s, we studied how to apply the OFDM technology to high-speed modem. Since 1990s, the study of OFDM technology has gone deep into the broadband data transmission over the radio frequency modulation channel. At present, OFDM technology has been widely used in broadcast audio, video and civil communication systems. Its main applications include Asymmetric Digital user loop (ADSL) ETSI standard digital audio broadcast (DAB), digital video broadcast (DVB), High Definition TV (HDTV), wireless LAN (WLAN), etc.