博士 / 國立交通大學 / 電子工程系所 / 95 / In CDMA systems, the narrowband message signals of different users are discriminated by multiplying the spreading signals with large bandwidth. The multiple access interference (MAI) is introduced when spreading signals are non-orthogonal. In the cellular environment, if the power of each user within a cell is not controlled appropriately, error performance of the user with small received power can be dramatically decreased by the user with large received power, i.e., the near-far problem occurs. A technique known as multiuser detection (MUD) can be employed to mitigate the MAI. The successive interference cancellation (SIC) is considered a promising technique among the MUDs due to its simplicity and superior error performance in fading environment.
To make SIC a practical technique in uplink WCDMA systems, we first analyze the ordering method which has large influence on the performance of SIC. Then we present techniques to alleviate the drawbacks of SIC. Furthermore, techniques to increase the system capacity are proposed.
It has been shown that the ordering method has a great effect on the performance of SIC. Three ordering methods are discussed and compared in the aspect of the implementation issues (such as reordering frequency, processing delay, latency, and computational complexity), and error performance related parameters (such as pilot-to-traffic amplitude ratio, cancellation-ordering method, grouping interval, received power distribution ratio and channel estimation as well as timing estimation errors). In addition to considering the single-rate system, a generalized pilot-channel aided SIC scheme is presented to apply to multirate communications.
SIC has several drawbacks: sensitive to channel estimation error due to error propagation from stage to stage, longer processing delay than parallel interference cancellation (PIC), and complicated power control. In the uplink of WCDMA systems, the pilot-channel signals can be employed to reduce channel estimation errors. However, the traffic-channel signals are always interfered by other users’ pilot and traffic signals even without any fading. This interference can be alleviated by employing pilot-channel signal removal (PCSR) technique. To shorten processing delay, a pipeline scheme is proposed. It is shown in the thesis that even with the equal power control profile, the SIC with properly chosen ordering method still outperforms multistage partial PIC (PPIC).
In addition to considering pure SIC in uplink WCDMA systems, an adaptable scheme with the ability of adapting its structure according to the environment and channel condition is presented. The processing delay and computational complexity can be adjusted based on system loading and required performance. The proposed scheme combines SIC and PPIC for data detection and performs refined channel estimation. The processing delay is shorter than pure SIC with reasonable hardware, and better error performance on both channel parameter estimation and user data detection are achieved.
To extend the SIC technique to turbo-coded systems, an iterative IC with ordered SIC at front-end is proposed. To avoid unnecessary computation, the ordering information obtained from SIC front-end is utilized in a low-complexity stopping criterion with high efficiency. In addition to bit-wise interference cancellation, the SIC technique is also applied to code-block-wise interference cancellation. And, only the bits in incorrect blocks should be preceded to the next outer iteration. As a result, huge amount of computational complexity can be saved, and better performance is achieved.
| Identifer | oai:union.ndltd.org:TW/095NCTU5428052 |
| Date | January 2007 |
| Creators | 唐之璇 |
| Contributors | Che-Ho Wei, 魏哲和 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | en_US |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 144 |
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