In this page we (Manfred research group) would like to show you, that in strong Doppler Effect conditions (moving with speed up to 300 km/h, and high carrier’s frequency - more than 2GHz), OFDMA method in a present structure has difficult future. This conclusion is based on the following:
· The main problem of the Doppler Effect is orthogonality deterioration. Errors are created due to time and frequency changing and due to Inter Carrier Interference (ICI).
· The situation is worsening because of time and frequency synchronization problems in receiver.
· The Automatic Frequency Control (AFC) system in a receiver is not capable instantly correct decoded FFT parameters either. Therefore, an attempt is done to detect signals on transmitted frequencies, but not on frequencies received after Doppler Effect influence.
· Researchers usually assume there is no variation in Doppler Effect for one symbol time duration. Today, however, symbol duration achieves 1ms (for example in DVB-T and in DAB systems). Taking central frequency of OFDMA signal to be 1 GHz, symbol duration T = 0.1 ms and velocity V = 120 km/h. we will get the phase shifts due to delay equal 3.90. The next symbol will have phase shift 7.80 end so on – Short Delay Accumulation (SDA). Maybe this propagation error (SDA) is the most serious problem due to helplessness of the not fast enough Automatic Time Control (ATC) of a receiver.
· In the OFDMA Uplink case, the AFC and ATC are not useful at all because of different Doppler shift values from different sources.
· Due to ICI, phase nois and jitter influence a pilot signals cannot be a sufficient reference. In extremely cases the use of pilot signals does not contribute in detection correction, on the contrary, it makes it worse.
The detailed problem analyze, simulation methods and problems solving you can find in our FBS presentation and articles 5 and 11 in page References Downloads.
