Concepts of Orthogonal Frequency Division Multiplexing (OFDM) and 802.11 WLAN

Reference

Orthogonal Frequency Division Multiplexing (OFDM) because this technology is a basic building block for many of the current modulation schemes including; 802.11 WLAN, 802.16 WiMAX, and 3GPP LTE.

Introduction to OFDM - Orthogonal Frequency Division Multiplexing

OFDM) is a digital multi-carrier modulation scheme that extends the concept of single subcarrier modulation by using multiple subcarriers within the same single channel.

Rather than transmit a high-rate stream of data with a single subcarrier, OFDM makes use of a large number of closely spaced orthogonal subcarriers that are transmitted in parallel. Each subcarrier is modulated with a conventional digital modulation scheme (such as QPSK, 16QAM, etc.) at low symbol rate. However, the combination of many subcarriers enables data rates similar to conventional single-carrier modulation schemes within equivalent bandwidths.

The following figure illustrates the main concepts of an OFDM signal and the inter-relationship between the frequency and time domains.

• In the frequency domain
Multiple adjacent tones (or subcarriers) are each independently modulated with complex data. Multipath delay effect is decreased with sub-carriers. An OFDM signal is a sum of subcarriers that are individually modulated by using PSK/QDM. The symbol can be written by the sub-carriers: where : Ns is the number of subcarriers T is the symbol duration fc is the carrier frequency s(t) is the inverse Fast Fourier Transform of Ns QAM(PSK) input symbols. These Ns IFFT samples of QAM input symbols have to be multiplied by a cosine and sine of the desired frequency to produce the final OFDM signal. Guard intervals are inserted between each of the QAM symbols to prevent inter-symbol interference at the receiver caused by multi-path delay spread in the radio channel. Multiple symbols can be concatenated to create the final OFDM burst signal. The output OFDM is converted to serial and a cyclic extension is added to make the system robust to multipath propagation. Windowing is applied after to get a narrower output spectrum.

QAM encodes data on a single carrier wave, but that carrier wave is composed of 2 components: In-phase and Quadrature components. QAM performs amplitude modulation on both components, called IQ modulation. Using an IQ modulator, the signal is converted to analog, which is up-converted to the 5 GHz band, amplified, and transmitted through the antenna.

• Then in the time domain
At the receiver an FFT is performed on the OFDM symbols to recover the original data bits.

Block diagram of an OFDM transceiver