OFDM was initially used for wired and stationary wireless communications. This lecture will examine how we can communicate information in digital form by modulation of a carrier waveform.
OFDMA supports differentiated quality of service by assigning different number of subcarriers to different users in a similar fashion as in CDMAand thus complex packet scheduling or Media Access Control schemes can be avoided. The reason why interleaving is used on OFDM is to attempt to spread the errors out in the bit-stream that is presented to the error correction decoder, because when such decoders are presented with a high concentration of errors the decoder is unable to correct all the bit errors, and a burst of uncorrected errors occurs.
However, with an increasing number of applications operating in highly mobile environments, the effect of dispersive fading caused by a combination of multi-path propagation and doppler shift is more significant.
Based on this feedback information, adaptive modulationchannel coding and power allocation may be applied across all subcarriers, or individually to each subcarrier. We can see that — when it is non-zero — each symbol consists of an integer number 4, in this example cycles of the chosen carrier frequency.
University of St. In Figures A single frequency network is a form of transmitter macrodiversity. Linear transmitter power amplifier[ edit ] An OFDM signal exhibits a high peak-to-average power ratio PAPR because the independent phases of the subcarriers mean that they will often combine constructively.
An example is shown in Figure However, time interleaving is of little benefit in slowly fading channels, such as for stationary reception, and frequency interleaving offers little to no benefit for narrowband channels that suffer from flat-fading where the whole channel bandwidth fades at the same time.
This is very beneficial in many countries, as it permits the operation of national single-frequency networks SFNwhere many transmitters send the same signal simultaneously over the same channel frequency.
FFT techniques can be used to reduce the number of multiplications for an FIR filter -based time-domain equalizer to a number comparable with OFDM, at the cost of delay between reception and decoding which also becomes comparable with OFDM.