OFDM has been the accepted standard for digital TV broadcasting. European DAB and DVB-T standards use OFDM. HIPERLAN 2 standard is also using OFDM techniques and so is the 5 GHz extension of IEEE 802.11 standard. ADSL and VDSL use OFDM. More recently, IEEE 802.16 has standardized OFDM for both Fixed and Mobile WiMAX.
In communication channels, frequency selective fading occurs when channel introduces time dispersion and the delay spread is larger than the symbol duration. The delay spread is the time spread between the arrival of the first and last multipath signal received at the receiving end. As a result, it will lead to intersymbol interference (ISI), where a received symbol can be influenced by previous symbol. With high data rate, the symbol duration can be very small. Therefore, the number of symbols that are affected by ISI can be large. In general, when the delay spread is less than the symbol length, the channel is flat fading. On the other hand, when the delay spread is much larger than the symbol length, the channel is frequency selective fading.
In fact, frequency selective fading is difficult to be compensated because the fading charateristics is random and not predictable due to the ever-changing environments. Due to this reason, multicarrer modulation (MCM) techniques such as orthogonal frequency division multiplexing (OFDM) and discrete multitone (DMT) modulation have become popular for high-speed wireline and wireless communication systems due to its robustness against frequency selective channels. The concept of MCM is to partition a broadband channel into a large number of narrowband subcahnnels, thus converting a frequency-selective channel into a collection of frequency-flat channels. In other words, we generate closely-spaced orthogonal subcarriers which are used to carry data, where the data is divided into several parallel data streams or channels, one for each subcarriers. Each subcarrier is modulated with a conventional modulation scheme, such as quadrature amplitude modulation (QAM), at a low symbol rate, thus maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth.
As such, the transmission can achieve a high data rate while maintaining long symbol duration which reduces the effects of ISI. Reducing the effects of ISI yields a simple equalization. The equalization is performed in each subchannel by scaling back the received signal by an appropriate factor. Therefore, we can have a simple receiver structure. The implementations of MCM make use of an inverse fast Fourier transform (IFFT) for the modulation and a fast Fourier transform (FFT) for the demodulation to create orthogonal subchannels. After the information bits are grouped, coded and modulated, they are segmented by the serial-to-parallel converter before being fed into N-point IFFT to obtain the time domain OFDM symbols.
At the receiver, after passing through the analog-to-digital (A/D) and removing CP, the signals are converted from serial to parallel. Then N-point FFT is used to transform the data back to frequency domain. Finally, the information bits are obtained through the channel equalization and demodulation process.
In communication channels, frequency selective fading occurs when channel introduces time dispersion and the delay spread is larger than the symbol duration. The delay spread is the time spread between the arrival of the first and last multipath signal received at the receiving end. As a result, it will lead to intersymbol interference (ISI), where a received symbol can be influenced by previous symbol. With high data rate, the symbol duration can be very small. Therefore, the number of symbols that are affected by ISI can be large. In general, when the delay spread is less than the symbol length, the channel is flat fading. On the other hand, when the delay spread is much larger than the symbol length, the channel is frequency selective fading.
In fact, frequency selective fading is difficult to be compensated because the fading charateristics is random and not predictable due to the ever-changing environments. Due to this reason, multicarrer modulation (MCM) techniques such as orthogonal frequency division multiplexing (OFDM) and discrete multitone (DMT) modulation have become popular for high-speed wireline and wireless communication systems due to its robustness against frequency selective channels. The concept of MCM is to partition a broadband channel into a large number of narrowband subcahnnels, thus converting a frequency-selective channel into a collection of frequency-flat channels. In other words, we generate closely-spaced orthogonal subcarriers which are used to carry data, where the data is divided into several parallel data streams or channels, one for each subcarriers. Each subcarrier is modulated with a conventional modulation scheme, such as quadrature amplitude modulation (QAM), at a low symbol rate, thus maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth.
As such, the transmission can achieve a high data rate while maintaining long symbol duration which reduces the effects of ISI. Reducing the effects of ISI yields a simple equalization. The equalization is performed in each subchannel by scaling back the received signal by an appropriate factor. Therefore, we can have a simple receiver structure. The implementations of MCM make use of an inverse fast Fourier transform (IFFT) for the modulation and a fast Fourier transform (FFT) for the demodulation to create orthogonal subchannels. After the information bits are grouped, coded and modulated, they are segmented by the serial-to-parallel converter before being fed into N-point IFFT to obtain the time domain OFDM symbols.
At the receiver, after passing through the analog-to-digital (A/D) and removing CP, the signals are converted from serial to parallel. Then N-point FFT is used to transform the data back to frequency domain. Finally, the information bits are obtained through the channel equalization and demodulation process.
(From my thesis introduction.)
