土曜日, 2月 04, 2006

OFDM




In FDM, the available bandwidth is divided into multiple data carriers. The data to be transmitted is then divided between these subcarriers. Because each carrier is treated independently of the others, a frequency guard band (an area of frequency between each of the carriers) must be placed around it. This guard band lowers the bandwidth efficiency because frequency is not used to carry any useful information.

In some FDM systems, up to 50 percent of the available bandwidth is wasted. In most FDM systems, individual users are segmented to a particular subcarrier; therefore, their burst rate cannot exceed the capacity of that subcarrier. If some subcarriers are idle, their bandwidth cannot be shared with other subcarriers.

The OFDM spread-spectrum technique spreads the transmitted data over a large number of RF carriers that are spaced apart at particular frequencies. This orthogonal (mutually independent or well-separated) relationship between carriers prevents the receivers demodulators from "hearing" frequencies other than their own.

OFDM carries with it many advantages. As an RF signal is transmitted, it can be reflected by objects in the environment, providing multiple signals to the receiver in varying time frames and signal strengths. This is known as multipath fading. Because OFDM is a multiple-carrier system, it has built-in frequency diversity (achieved by using multiple frequencies in parallel), which provides greater immunity to multipath environments. It is very unlikely that two frequencies will reflect (or fade) simultaneously in the same environment, so one of the signals will inevitably be successfully received.

OFDM also provides higher data rates and high spectral efficiency more information in the same amount of frequency spectrum and helps in fighting a signal's delay spread of a signal, which can limit the data rate.

Combined, these features result in OFDM systems providing better tolerance to noise, interference, and multipath situations, which in turn provides improved range and overall performance (when compared to other modulation schemes for the same frequencies).


0 Comments:

コメントを投稿

Links to this post:

リンクを作成

<< Home