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The main objective of the present design has been the development of a HF band communication system providing digital voice transmission within around 3 kHz bandwidth, through ionospheric reflection, mainly, but not only, for aeronautical environment. Interactivity, as a basic design constraint, has shown that standard structures using single carrier modulations are not suitable at all because their reliability is mainly based on the use of powerful channel coding schemes and very long interleavers with delays of several seconds. On the other hand, OFDM (orthogonal frequency division multiplex) systems are more robust techniques facing the multipath effect as the main distorting source of this propagation mechanism. The combination of this characteristic with spread spectrum CDMA (code division multiple access)-like schemes provides an extreme robust modem without the need of powerful codes neither long interleavers to guarantee an specific data rate and quality of service. Therefore, we are able to design a high data rate system with the possibility of providing interactive digital communication.