17-07-2014, 02:44 PM
VHDL IMPLEMENTATION OF IFFT/ FFT BLOCKS FOR OFDM
VHDL IMPLEMENTATION.pptx (Size: 758.3 KB / Downloads: 13)
Basic principle
Split the transmission bandwidth into many sub channels which are transmitted in parallel
(Ideally) Each sub channel is narrow enough so that it experiences a flat fading although the overall radio propagation environment is frequency-selective.
The time dispersion effects are less significant as the symbol duration increases
Multiple access
In data communication it is common to allow several transmitters to send information over a single communication channel, allowing several users to share a band of frequencies.
This is the concept of Multiple access.
CDMA(CODE DIVISION MULTIPLE ACCESS)
CDMA uses spread spectrum technology and a special coding scheme where the transmitter is assigned a code for multiple users to transmit over the same physical channel.
Advantage: (mobile) transmission power is very small, better call quality.
Disadvantage: As the number of users increases, the overall quality of service decreases
TDMA
Several users share the same frequency channel by dividing the signal into different time slots.
Users transmit in rapid succession, one after the other each using its own time slots.
Multiple station share same transmission medium
Non continuous transmission
Interference with other devices at frequency directly connected to the time slot length.
Orthogonal Multicarrier Frequency
Orthogonal frequency-division multiplexing (OFDM) is a method of digital modulation in which a signal is split into several narrowband channels at different frequencies.
OFDM is similar to conventional frequency-division multiplexing (FDM). The difference lies in the way in which the signals are modulated and demodulated. Priority is given to minimizing the interference, or crosstalk , among the channels and symbols comprising the data stream. Less importance is placed on perfecting individual channels.