Research On Adaptive Modulation System Based On MQAM

The main ideas of adaptive modulation technique in which the modulation mode is adjusted dynamicly by real-time channel estimation, when channel condition is getting better, the higher-order modulation mode is adopted, and vise versa is presented in order to realize adequate and effective use of spectrum resources.In this dissertation, low-pass equivalent scheme of bandpass signal and systems is used to do research on adaptive modulation system based on MQAM. The content is as follows:(1) The low-pass simulation model about transmitting and receiving system based on MQAM is constructed. The transmitting system mainly includes frame forming, RS encoder, differential encoder, constellation mapping, Baseband forming filter. The receiving system mainly includes matched filtering, timing recovery, adaptive equalization, carrier synchronization, constellation inverse mapping, differential decoder, RS decoder.(2) The timing recovery device is improved. Interpolation filter based on Newton interpolation formula is designed. The dissertation compares performance of the Newton interpolation filter with Lagrange interpolation filter. The conclusion is Newton interpolation filter has the same interpolation precision with Lagrange interpolation filter, but Newton interpolation filter has adjustable interpolation precision, Lagrange interpolation filter does not have this property.(3) The CMA blind equalization algorithm is improved. The improved algorithm puts phase information into cost function, solved the scatterplot chart deflection problem which is insurmountable for classical CMA blind equalization algorithm in phase deflection channel. The improved algorithm puts signal judgment value into error calculation device, it reduces the mean square error of the output signal effectively, and improves the convergence speed to a certain extent.(4) The low-pass simulation model of adaptive modulation system based on MQAM is constructed. In traditional adaptive modulation scheme, multiple connection and disassembled multiple connection circuits are indispensable for transmiting modulation mode information. The new system changes modulation mode transmission mechanism, it gets rid of the dependence on multiple connections and disassembled multiple connection circuit, and simplifies the design. The dissertation analyses the system design scheme and operation process comprehensively from frame structure, handshake protocol, modulation mode recognition, and so on. (5) The research on adaptive modulation system is based on ideal timing recovery, adaptive equalization, and carrier synchronization in the past. The dissertation analyses the influence to modulation mode conversion threshold detailedly on the basis of the low-pass simulation model of adaptive modulation system based on MQAM, which is caused by timing phase errors, fading, and carrier phase errors.