Successful implementation of the Orthogonal Frequency Division Multiple
Access (OFDMA) method (Lee, 1989; Proakis, 2000; Van Nee and Prasad, 2000; and Li
and Cimini, 2001) in some systems, such as Digital Audio Broadcasting (DAB), Digital
Video Broadcasting-Terrestrial (DVB-T), Digital Video Broadcasting-Handheld (DVB-H),
Worldwide Interoperability for Microwave Access (WiMax) and Wireless Fidelity (WiFi), have in
the past raised expectations that a new cellular system, for example, Long-Term
Evolution (LTE), could be based on this method (Dardari and Tralli, 1999; and Ohmori et al., 2000). The main purpose of this article is to show the enormous difficulties with
which OFDMA is implemented in cellular systems.
Problems associated with mobile communication systems are more evident
with the increase of frequency up to 10 GHz and vehicle speed up to 300 km/h. To
date, these problems are treated by using channel estimations, pilot signal transmission
and effective Error Correction Codes (ECC). However, these methods significantly
reduce the system spectral efficiency. The high level of reflected signals found in any
modern big city are the main reasons for the use of increased symbol duration, which,
as a result, makes the system more sensitive to the Doppler Effect (DE). In the
case discussed below, DE causes orthogonality deterioration, which in turn leads to
the appearance of Inter Carrier Interference (ICI), since the phase of Pilot signals is
not only dependent on channel conditions, but also on the neighboring carrier phases. |
