Basic theory of adaptive data transmission

Abstract

This paper presents the basic theory of adaptive data transmission. First we describe a bit stream which has to be transmitted on line or in the air. In the transmission of the bit stream we use digital modulation methods symbol by symbol. We describe one symbol with a waveform, that contains several bits. The adaptive modulation method is used. It means that we can adapt the generated symbol waveform to the analogue channel used. The modulation is made by a software algorithm which converts each symbol to a specific amplitude-phase constellation point of the known carrier and uses a proper symbol time. Depending on the channel bandwidth B (wired or radio) we have one or several transmission carriers in use for the optimal Shannon capacity. Depending on the channel characteristics or signal to noise ratio we can select the amplitude phase constellation optimally. We describe then the detection and the demodulation of the symbol waveform and use there the discrete Fourier transform (DFT). First the received waveform is sampled and the number of samples is selected by the sampling frequency. We evaluate the proper sample number used in the DFT calculations, which give the estimates of the transmitted symbols of each carrier (amplitude, phase and frequency). Finally we summarize our results using the Shannon capacity formula B*/sub 2/log(S/N+1) and bit error rate (BER) calculations in the evaluations. We discuss the adaptive modulation method properties and functionality found in the simulations and field tests. We find out, that the high performance of the adaptive modulation method is generated by the waveforms designed in the way that they occupy fully the channel in use. The full capacity of any channel is used by selection of proper multiple carriers (bandwidth). The full QoS (S/N) of the channel is applied by the selection of the best amplitude-phase constellation and symbol time selection. These features of the modulation method makes it adaptive, which we could design only using software modem technology. The result is the full practical Shannon's capacity for any analog transmission case. The channel may be a radio, satellite, and wired telecommunication channel which has a limited bandwidth for transmission. Also any coded digital voice transmission can be used for data transmission with the adaptive modem because it produces and decodes data as voice signal designed within the wanted bandwidth. We believe that the adaptive modem is a cornerstone for modern data transmission and thus it may be used in many applications.