On the Necessity of High Front-Ends in Broadband Performance RF Wireless Access Employing Multicarrier Modulations (OFDM)

AbstmctMulticarrier systems of the OFDM-type perform a frequency domain decomposition of a channel characterized by frequency selective distortion in a multitude of subchannels that are affected by frequency flat distortion. The distortion in each independent subchannel can then be compensated by simple gain and phase adjustements, somehow overcoming the need of one of the most complex components of a digital modem: the equalizer. On the other hand, coding and transmission power assignments can be applied across the subchannels in a way that resembles the Shannon water pouring argument. As the bandwidth of the subchannels is made sufficiently small one can asymptotically reach the channel capacity. Since this relatively old idea has attracted a significant interest in recent years, we prove that a careful implementation of the analog RF front-end is extremely critical to the asymptotic behavior of multicarrier transmission in approaching capacity, In fact multicarrier signals exhibit extreme vulnerability to distortions induced by RF electronics. We concentrate on the characterization of those degradations that can not be compensated at baseband by means of Digital Signal Processing and show that the throughput achievable by realistic OFDM modems is much lower than what typically advertised, particularly in transceivers with low performance analog RF front-end.