Recommendation itu-r f. 1101 Characteristics of digital fixed wireless systems below about 17 ghz
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ĐỒ ÁN MÔN TRUYỀN THÔNG DI ĐỘNG K2018, Ho ng Th Hu , Chuong 2 SV, Bai-4, VNU, document tailieudaihoc
3 Multi-level coded modulationIn MLCM, each level, in set partitioning, is regarded as an independent transmission path with different minimum square distance, and a different code with different strength is applied to each level. An example of set partitioning of 16-QAM is shown in Fig. 5, where the total set of 16 states (A) is divided into sub-sets B0 and B1. In these sub-sets, the minimum square distance among each sub-set is twice that of set (A). Sub-sets B0 and B1 are divided into sub-sets C0, C2 and C1, C3 respectively. In the sub-sets Ci (i 0 to 3), the minimum square distance is 4 d 2. The same partition is done until the number of states becomes one in each sub-set. Hence, 16 states are divided into sets of sub-sets with increased minimum square distance. However, in this stage, error performance of level 1 is determined by the minimum square distance of (A) states set. Then, in order to increase the “free Euclidian distance”, dE, coding is performed to the lower level. Hence the total error performance is improved. Codes used in MLCM are not restricted to only convolutional codes. However convolutional codes are generally used for the lower levels, while other codes like block code can be used for other levels. The coding rate for MLCM can be selected rather freely because the coding rates of each level are chosen separately. For example, in the case of 16‑QAM, if the coding rate is 1/2 for level 1, 3/4 for level 2, 23/24 for level 3 and no coding for level 4, the total coding rate R becomes:
The output of each encoder is converted from parallel to serial and applied to a mapping circuit. Therefore the results of one coding correspond to a number of symbols. Consequently, the coding speed is at least half of the modulation speed. The coding gain of MLCM depends on its coding rate and coding methods. Decoding is done by the method called “multi-stage decoding”. At first, the lowest level is decoded and according to the result, the next level is decoded. Further upper levels are decoded in the same way. Simplified block diagrams of TCM and MLCM are shown in Fig. 6.
Figure 7 shows a calculated comparison between BER performance for 128-QAM systems with different redundancies. tải về 179 Kb. Chia sẻ với bạn bè của bạn:
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