Type or paste a DOI name into the text box. Please time division multiplexing theory pdf this error screen to 158. OFDM was introduced by Chang of Bell Labs in 1966. The following list is a summary of existing OFDM-based standards and products.
For further details, see the Usage section at the end of the article. The advantages and disadvantages listed below are further discussed in the Characteristics and principles of operation section below. High spectral efficiency as compared to other double sideband modulation schemes, spread spectrum, etc. Can easily adapt to severe channel conditions without complex time-domain equalization.
In OFDM, the sub-carrier frequencies are chosen so that the sub-carriers are orthogonal to each other, meaning that cross-talk between the sub-channels is eliminated and inter-carrier guard bands are not required. Almost the whole available frequency band can be utilized. 1,000 samples per symbol by FFT. Nyquist bandwidth requirement would be even lower. If no guard interval was applied, this would result in a base band complex valued signal with a sample rate of 1 MHz, which would require a baseband bandwidth of 0. The orthogonality allows for efficient modulator and demodulator implementation using the FFT algorithm on the receiver side, and inverse FFT on the sender side.
The time to compute the inverse-FFT or FFT transform has to take less than the time for each symbol. The computational demand approximately scales linearly with FFT size so a double size FFT needs double the amount of time and vice versa. As a comparison an Intel Pentium III CPU at 1. 6 GHz does it in 387 µs. The guard interval also eliminates the need for a pulse-shaping filter, and it reduces the sensitivity to time synchronization problems. A simple example: If one sends a million symbols per second using conventional single-carrier modulation over a wireless channel, then the duration of each symbol would be one microsecond or less. This imposes severe constraints on synchronization and necessitates the removal of multipath interference.