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's a key player in Wi-Fi products and is considered a strong candidate for fourth-generation mobile communication systems. Basic Principles of OFDM Technology The core idea behind OFDM is to split high-speed data streams into multiple lower-rate sub-streams using a serial-to-parallel conversion process. 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. A guard interval is often inserted between OFDM symbols, which is longer than the maximum delay spread of the channel, helping to minimize inter-symbol interference. A cyclic prefix is typically used as this guard interval, preventing inter-channel interference from multipath effects. Additionally, OFDM uses orthogonal carriers, allowing their spectra to overlap without causing interference, thus improving spectral efficiency. Basic Working Process of OFDM At the transmitter, the incoming data is converted from serial to parallel, with the number of parallel streams equal to the number of subcarriers. Each subcarrier sequence is encoded and represented as complex numbers. These are then modulated onto corresponding IFFT bins. The IFFT transforms the signal into the time domain, generating discrete OFDM samples. The relationship between subcarrier frequency and symbol rate is crucial here. Subcarrier frequencies are spaced at intervals of 1/NT, and each subcarrier’s symbol rate is 1/NT. The modulation effect results in a sinc-like shape, ensuring that the zero points of one subcarrier align with the peaks of others, enabling non-interfering overlapping of signals and efficient bandwidth usage. Characteristics of OFDM Technology One of the main advantages of OFDM is its ability to resist inter-symbol interference caused by multipath propagation. Compared to single-carrier systems with equalizers, OFDM has much lower implementation complexity. In channels with slow variations, OFDM can optimize bit allocation on each subcarrier based on the signal-to-noise ratio, significantly boosting data transmission capacity. OFDM is also more resilient to impulse noise because the demodulation process spans a long symbol period, spreading out the noise’s impact. High spectral efficiency is another benefit, as OFDM signals are composed of multiple overlapping sinc-shaped spectra. However, OFDM has some disadvantages compared to traditional single-carrier systems. It is more sensitive to carrier frequency offset and timing errors. Additionally, the high peak-to-average power ratio (PAPR) in OFDM systems requires highly linear amplifiers. Coherent demodulation also necessitates accurate channel estimation. To address these challenges, current research focuses on three key areas: frequency offset estimation, PAPR reduction, and advanced channel estimation algorithms. Application of OFDM Technology in Wi-Fi Products A prominent example of OFDM application is IEEE 802.11a, a standard that uses OFDM for WLAN. Operating in the 5 GHz band, it supports data rates ranging from 6 Mb/s to 54 Mb/s. To adapt to varying fading conditions, different coding rates are applied to different subcarriers. Common code rates include 1/2, 2/3, and 3/4. The 1/2 rate uses convolutional coding with a constraint length of 7 and generator polynomials (133, 171), while the others are obtained through puncturing. The following table lists the eight modes supported by IEEE 802.11a, along with seven modes supported by HIPERLAN/2 for comparison.

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