This question appears in Mumbai University > Wireless Networks subject

Marks: 4 M

Year: May 2013

Concept of OFDM:

i. ‘Orthogonal Frequency Division Multiplexing (OFDM)’ is a modulation scheme in which a channel is divided into a number of sub-channels.

ii. In OFDM modulation, the basic principle of operation is to divide a high speed binary signal to be transmitted into a number of lower data rate subcarrier.

iii. There are 48 data subcarriers and 4 carrier pilot subcarriers defined in IEEE 802.11a. Each lower data bit stream is used to modulate a separate subcarrier from one of the channels in the 5GHz band.

iv. Inter symbol interference is generally not a concern for a lower speed carrier; however, the sub-channels may be subjected to frequency selective fading. Therefore, bit interleaving and convolution encoding is used to improve BER performance.

v. The scheme uses integer multiples of first subcarrier, which are orthogonal to each other. The modulating scheme of signal in each sub-channel is independent from other and can be different (e.g. BPSK in one channel while QPSK in other).

vi. OFDM PHY in IEEE 802.11a for WLAN has capability to transmit ay data rates up to 54Mbps. Thus, transmission of multimedia content can be considered.