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O F D M M o d u l a t o r
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- Điều hướng trang này:
- D i v e r s i t y T r a n s m i t t e r R F B 1 R F B 1 R F B
- D e c o d e r 1 D e c o d e r 2 2 1 1 P - 1 1 2 P - 1 2 - D
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M o d u l a t o r O F D M M o d u l a t o r R F B 1 R F B 1 R F B 2 R F B 2 s 1 s 2 D i v e r s i t y T r a n s m i t t e r R F B 1 R F B 1 R F B 2 R F B 2 D 1 ( J ) + D 2 ( J ) + n 1 n 2 O F D M M o d u l a t o r O F D M M o d u l a t o r O F D M D e m o d u l a t o r O F D M D e m o d u l a t o r O F D M D e m o d u l a t o r O F D M D e m o d u l a t o r D e c o d e r 1 D e c o d e r 2 2 1 1 P - 1 1 2 P - 1 2 - D 1 e s t i m a t i o n D 2 e s t i m a t i o n T u r b o D i v e r s i t y O F D M R e c e i v e r y 1 y 2 + + C o d e 1 C o d e 2 1 2 u - - F a d i n g C h a n n e l û P P P P P Figure 3.13: Iterative diversity receiver for OFDM systems. 3.3.2 Turbo Diversity A soft-input soft-output (SISO) decoder which allows for propagating extrinsic in- formation is the key element of the turbo decoding principle [6]. Therefore, an optimum maximum-a-posteriori symbol-by-symbol estimator like the Bahl-Cocke- Jelinek-Raviv (BCJR) algorithm for turbo codes or the sum-product algorithm [7] for LDPC codes have be applied, calculating for each bit c k the LLR conditioned on the received sequence Λ (c k |r) = ln (P (c k = 1|r) /P (c k = 0|r)) (3.20) According to the turbo decoding principle, extrinsic information Λ e is exchanged between the decoding stages, where for Λ e any a-priori information has to be sub- tracted from the BCJR output reliability information, see Fig. 3.13. The iteration process can be visualized by EXIT charts, where the mutual infor- mation (MI) of one decoder is plotted versus its a-priori input, i.e., the MI of the other decoder [9]. Using optimum decoders, the knowledge of the MI contained in its a-priori information is sufficient to derive the MI of Λ e delivered by this decoder. Figure 3.13 motivates the similarity of TC and TD. Consequently, EXIT chart methods considering the constituent LDPC codes allow for analyzing the iteration process of TD. In this case EXIT functions are derived from the parameters of LDPC codes, namely coefficients of degree distributions of variable nodes λ(x) and of check 56 3 Link-Level Aspects nodes ρ(x), denoted here shortly as (λ, ρ) [7]. Properties of TD can be examined using a scenario with two diversity paths, where the SNR of both paths differ by ∆SNR dB. 3.3.3 Optimization for Turbo Diversity Proper LDPC code selection is the crucial issue of TD system design. Here we assume identical codes for both diversity branches. In the consecutive, different code optimization methods are considered. The idea of the design method No. 1 is depicted in Fig. 3.14. The tunnel between the EXIT function of the first decoder T D 1,n and the inverse EXIT function T −1 D 2,n should be open. Taking into account the criterion of the minimal area gives rise to the definition of the first design method [2]. tải về 7.55 Mb. Chia sẻ với bạn bè của bạn:
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