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Public mobile radio spectrum has become a scarce resource while wide spectral ranges are only rarely used. Here, a new strategy called spectrum pooling is considered. It aims at enabling public access to these spectral ranges without sacrificing the transmission quality of the actual license owners. Unfortunately, using OFDM modulation in a spectrum pooling system has some drawbacks. There is an interaction between the licensed system and the OFDM based rental system due to the non-orthogonality of their respective transmit signals. In this paper, this interaction is described mathematically providing a quantitative evaluation of the mutual interference that leads to an SNR loss in both systems. However, this interference can be mitigated by windowing the OFDM signal in the time domain or by the adaptive deactivation of adjacent subcarriers providing flexible guard bands between licensed and rental system. It is obvious that both approaches sacrifice bandwidth of the rental system. A quantitative comparison of both approaches is given as a tradeoff between interference reduction and throughput in the rental system.

This paper proposes a new approach to apply frequency domain equalization (FDE) in the downlink of broadband CDMA cellular systems. Equalization has been recognized as a better receiving method than the RAKE receiver for CDMA downlink systems, especially in cells with a high number of users. By performing the equalization in the frequency domain, the complexity of the equalization algorithm can be significantly reduced with the help of the FFT operation. Three methods of implementing FDE in the CDMA downlink will be described: the cyclic prefix method, the zero padding method, and the overlap-cut method. In addition to its simplicity and good performance, the FDE also offers the possibility to build a multimode receiver for singlecarrier and multicarrier signals. Simulation results are presented in a TD-CDMA system with parameters taken from the UTRA-TDD standard.

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