Application of OFDM Technology in Wi-Fi Products

In recent years, Orthogonal Frequency Division Multiplexing (OFDM) has gained significant attention due to its ability to effectively combat multipath interference and enhance system capacity. This technology is widely used in various applications such as digital audio broadcasting (DAB), digital video broadcasting (DVB), and wireless local area networks (WLAN). It is particularly prominent in Wi-Fi products and is considered a strong candidate for fourth-generation mobile communication systems. Basic Principles of OFDM Technology The core concept behind OFDM is to split high-speed data streams into multiple lower-rate sub-streams using a serial-to-parallel conversion. These sub-streams are then transmitted over several sub-channels. By increasing the symbol period in each sub-channel, the impact of time dispersion caused by multipath delay can be reduced. Additionally, a guard interval is inserted between OFDM symbols, typically longer than the maximum delay spread of the channel, to minimize inter-symbol interference. A cyclic prefix is commonly used as this guard interval, helping to avoid inter-carrier interference caused by multipath propagation. The orthogonal nature of the carriers allows their spectra to overlap without causing interference, significantly improving spectral efficiency. Basic Working Process of OFDM On the transmitting side, the incoming data is converted from serial to parallel format, with the number of parallel streams matching the number of subcarriers. Each subcarrier sequence is encoded, modulated, and transformed into a complex representation. These signals are then mapped onto the corresponding IFFT bins, and an inverse fast Fourier transform (IFFT) is applied to generate a time-domain OFDM signal. The key to successful OFDM modulation lies in the relationship between subcarrier frequency and symbol rate. Each subcarrier is spaced at intervals of 1/NT, and the symbol rate is 1/NT per second. The resulting signal exhibits a sinc-like shape, ensuring that the null points of one subcarrier align with the peaks of others, enabling efficient spectrum usage without interference. Characteristics of OFDM Technology One of the main advantages of OFDM is its robustness against inter-symbol interference caused by multipath effects, with lower implementation complexity compared to single-carrier systems using equalizers. In slowly varying channels, OFDM can optimize bit allocation on each subcarrier based on the signal-to-noise ratio, significantly boosting system capacity. It is also more resistant to impulse noise due to the long symbol period over which demodulation occurs, spreading the impact of noise. High spectral efficiency is another benefit, as the overlapping sinc-shaped spectra allow for tight packing of subcarriers. However, OFDM has some disadvantages. It is more sensitive to carrier frequency offset and timing errors than single-carrier systems. Additionally, the high peak-to-average power ratio (PAPR) of OFDM signals requires highly linear power amplifiers. Coherent demodulation also necessitates accurate channel estimation. To address these challenges, research focuses on three key areas: frequency offset estimation, PAPR reduction, and advanced channel estimation algorithms. Application of OFDM Technology in Wi-Fi Products OFDM has been widely adopted in Wi-Fi standards, such as IEEE 802.11a, which operates in the 5 GHz band. This standard uses OFDM as its physical layer technology, offering data rates ranging from 6 Mb/s to 54 Mb/s. To adapt to different fading environments, it employs different coding rates—1/2, 2/3, and 3/4—on various subcarriers. The 1/2 code rate uses convolutional coding with a constraint length of 7 and generator polynomials (133, 171), while the other rates are obtained through puncturing. The table below shows the eight operational modes supported by IEEE 802.11a, alongside seven modes from HIPERLAN/2 for comparison.

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