Font Size: a A A

Research On Key Technologies Of MC-CDMA

Posted on:2004-10-01Degree:DoctorType:Dissertation
Country:ChinaCandidate:Q B HuangFull Text:PDF
GTID:1118360095462817Subject:Computer software and theory
Abstract/Summary:PDF Full Text Request
The concept of MC-CDMA was proposed in 1993, and now lots of experts are developing it. The thesis mainly researches on single-user detection and multi-user detection techniques, and has obtained some theoretical and applied results.There are lots of key technologies about single user detection, such as synchronization between receiver and transmitter, channel estimation and diversity. The thesis mainly focuses on equalization in frequency diversity, which includes MMSE equalization and controlled equalization etc. But the MMSE equalization needs information about signal-to-noise ratio, and controlled equalization needs a threshold determined by channel fading, and these are difficult to be implemented. The thesis tries to make them easier.Multi-user detection includes optimal detection, linear detection and nonlinear detection. In nonlinear detection, there are lots of literatures about interference cancellation scheme. Interference cancellation detection is an iterative procedure, and its first stage is the conventional single user detection, so the first stage may have high bit error probability. Unfortunately, detection error in the first stage will have bad effects on the detection of subsequent stages. Taking this into account, the thesis proposed a comparing interference cancellation scheme, which compares all the presumptions so that obtain the signal that is most probable to be transmitted.The innovative works in this thesis mainly include the three aspects:1) To MMSE equalization, the thesis proposes to use the reference symbol of an OFDM frame to estimate noise power, and use it to detection of the data symbol in the OFDM frame. As an OFDM frame contains a reference symbol and 64 data symbols, to slow fading channel, the estimation is accurate enough, and it can be applied to MMSE equalization.2) To improve controlled equalization, the thesis proposes energy controlled equalization scheme. The scheme has defined an energy percentage threshold, and through it, we can get the fading threshold in controlled equalization. Because energy percentage threshold is universal to all kinds of channels, the scheme is universal too. In addition, we can adopt feedback in slow fading channel, with that the transmitter can redistribute energy to all subcarriers, so that increases signal-to-noise ratio at receiver. Two distribution schemes are proposed,including equal-number distribution and even distribution. Experiments show that effects of the two schemes don't differ much, and increase the system performance about l~2dB. Furthermore, adaptive energy controlled equalization can obtain the optimal threshold and increase the system performance more than 2dB.3) To improve serial interference cancellation multi-user detection, the thesis proposes a comparing serial interference cancellation scheme, including least error serial interference cancellation and compareand-backdate serial interference cancellation. The main idea of them is to make presumptions of the signals that users transmitted, then determine reliability of all the presumptions by detection and choose the most reliable presumption as the output. The time complexity of them is a little high, but they can attain the single-user-system performance. If the number of users is n, time complexity of least error serial interference cancellation is O(n2n-1), and the time complexity of compare-and-backdate serial interference cancellation is O(2n-1). Time complexity of the improved compare-and-backdate serial interference cancellation is between them, but the detection delay of it is short. If the processing speed of hardware is quick enough, the scheme can be implemented.
Keywords/Search Tags:MC-CDMA, Deep fading, Controlled equalization, Multi-user detection, interference cancellation
PDF Full Text Request
Related items