Digital Compensation for Analog Front-Ends: A New Approach to Wireless Transceiver Design

2

New Air Interfaces

Because of the limited frequency bandwidth, on the one hand, and the potential limited power of terminal stations, on the other hand, spectral and power efficiencies of future communication systems should be as high as possible. New air interfaces need to be developed to meet the new user requirements.

The OFDM modulation has been selected for WLAN communication systems because it enables the low complexity equalization of the multi-path channel in the frequency domain (van Nee et al. 1999). In Section 2.1, we review the principles of OFDM and give a detailed mathematical description of the system. An interesting alternative air interface for WLANs, called signal-carrier frequency domain equalization (SC-FDE), is also introduced in Section 2.2. Multiple antenna technologies are finally combined to OFDM to improve the system reliability and capacity (see Section 2.3).

Cellular systems of the third generation (3G) are based on the recently emerged direct-sequence code division multiple access (DS-CDMA) technique (Ojanperä and Prasad 1998). DS-CDMA offers a potentially high system capacity and interesting networking abilities, such as soft hand-over. However, the DS-CDMA system suffers from interference (inter-symbol interference (ISI) and multi-user interference (MUI)) caused by multi-path propagation, leading to a high loss in spectral efficiency. We show in Section 2.4 that DS-CDMA can be combined with OFDM and SC-FDE to deal better with the multi-path propagation. ...