In this letter, a wireless local multipoint distribution system (LMDS) at millimeter waves for the last-mile broad-band distribution to users of interactive services is investigated. The system analyzed employs a coded orthogonal frequency-division multiplexing transmission scheme with frequency-division multiplexing (FDM) and/or time-division multiplexing techniques and adaptive carrier allocation to counteract the effects of the wireless communication channel. The idea of deploying the reverse channel (exploited by the user for interactivity purposes) to provide channel information to the broadcasting transmitter is introduced. System performance is evaluated for an urban microcellular radio system in an actual propagation environment and the choices of the optimum multiplexing technique and carrier allocation algorithm are discussed in the case of ideal feedback. It is found that a pure FDM technique combined with an adaptive carrier allocation algorithm giving priority to users having the largest path loss leads to the best performance.

This paper presents a theoretical characterization of nonlinear distortion effects in orthogonal frequency division multiplexing (OFDM) transmission systems. In the theoretical framework developed, it is shown that the effects on the decision variables of the in-band distortion introduced by a bandpass memoryless nonlinearity can be described by means of a complex gain and an additive Gaussian term with zero mean and suitable variance; analytical expressions for gain and variance are given. The conditions which allow this description are emphasized and discussed. As a consequence, a completely analytical procedure to evaluate error probability is also obtained and illustrated using OFDM/discrete multitone modulation (DMT) systems with rectangular pulse shaping; for the soft-envelope limiter nonlinearity, a closed form is derived. A comparison with simulation results is carried out to verify the accuracy of this method.

In this paper a wireless local multipoint distribution system (LMDS) at 30 GHz for the last mile broadband distribution to users of interactive services is investigated. The system we propose users a coded OFDM transmission scheme with FDM and/or TDM multiplexing techniques and adaptive carrier allocation to counteract the effects of the wireless communication channel. An urban microcell in an actual propagation environment is analyzed by means of a ray-tracing tool. Coverage probability is defined and investigated and the choices of the optimum multiplexing technique and carrier allocation algorithm are discussed. It is found that a pure FDM technique with adaptive carrier allocation and priority to users having the largest path loss leads to the best performance.

This paper proposes a new technique to reduce the non-linear distortion effects in orthogonal frequency division multiplexing (OFDM) systems. It exploits virtual carriers, which are modulated by using properly defined "dummy symbols", to obtain a transmitted signal with reduced envelope variations. With this method no additional redundancy is introduced, thus preserving the spectrum efficiency, and no changes at the receiver are required. To evaluate the dummy symbols some optimization criteria are investigated; an optimum algorithm and a low complexity iterative algorithm are proposed and compared from the point of view of the system's performance. The effectiveness of this technique is tested by considering some typical high power amplifier (HPA) models and by evaluating the performance degradation. A comparisons with other techniques proposed in the literature is also carried out.