In this thesis, Interleaved Frequency-Division Multiple-Access (IFDMA) - an orthogonal wideband multiple-access system - is introduced which is especially suitable for mobile radio applications. Compared to other wideband multiple-access systems, IFDMA shows some interesting advantages. The two most important advantages are the realization of frequency diversity and the avoidance of multiple-access interference. The IFDMA transmission signal of a single user utilizes a group of subcarriers for transmission which are equally distributed over the whole transmission bandwidth. Thus, frequency diversity is achieved with IFDMA in the case of a transmission over a frequency-selective mobile radio channel. Since each user applies another group of subcarriers for transmission, an orthogonal frequency-division multiple-access scheme is realized. Moreover, the orthogonality is maintained even for IFDMA transmission in a time-dispersive mobile radio channel. Additional advantages of IFDMA are continuous transmission, constant envelope transmission, applicability to both downlink and uplink, and avoidance of ``worst-case'' interference from neighboring cells in the case of a cellular mobile radio scenario. A detailed mathematical description as well as a profound performance investigation of IFDMA is one of the main contributions of this thesis. IFDMA for mobile radio applications is impaired by intersymbol interference. However, the performance loss due to intersymbol interference can be easily reduced by applying standard equalization techniques at the receiver, like Maximum Likelihood (ML) equalization or the suboptimum Decision Feedback Equalization (DFE). These both standard techniques are considered for equalization within IFDMA. Simulations show that IFDMA with ML equalization outperforms Multi-Carrier Code-Division Multiple-Access (MC-CDMA) with comparable parameter choice and receiver complexity. Besides these standard techniques, a new equalization scheme is proposed and investigated in detail which performs channel estimation and equalization jointly. Taking into account the currently received transmission symbol this joint channel estimation and equalization continuously adapts the channel estimation using the fact that each received transmission symbol carries information about the transmission channel. Joint channel estimation and equalization is capable of improving the performance of IFDMA and/or reducing the required redundancy for channel estimation and/or reducing the receiver complexity compared to the standard techniques where channel estimation and equalization is done separately. This new equalization scheme is not only applicable to IFDMA, but also to any other transmission system with intersymbol interference and ``a priori'' unknown channel parameters. Joint channel estimation and equalization forms the other main part of this thesis and is described in detail mathematically as well as investigated exemplarily for IFDMA.