Broadband Powerline Communications Networks Design phần 6

Sau đó, nếu từ nhận được không liên quan đến mã C, hội chứng của nó là bằng số không được thể hiện bởi Eq. (4,36), và do đó, không có lỗi được phát hiện. Trong trường hợp khác, nơi các hội chứng khác không, bộ giải mã phải có hành động để sửa chữa sai sót. Tuy nhiên, | 130 Broadband Powerline Communications Networks OFDM symbols OFDM symbols Figure OFDM TDMA network based on the OFDM building an OFDM TDMA transmission system Lind99 WongCh99 . In this case the network resources are divided into time slots each of them carrying an integer number of OFDM symbols Fig. . The length of the time slots can be fixed or variable but the number of OFDM symbols within a time slot has to be an integer. Some of the OFDM subcarriers can fail because of the disturbances . because of the long-term narrowband noise Sec. or they can operate with variable data rates if bit loading is applied. In both cases the entire network capacity changes dynamically according to the actual disturbance conditions. An OFDM symbol includes a particular number of bits bytes and carries a specific amount of user data payload. Thus if the network capacity is decreased the payload of an OFDM symbol is reduced as well. There are the following two solutions to keep the payload of an OFDM symbol constant There are a number of so-called spare subcarriers that can be used in the case of failures or capacity decrease. However if the disturbance conditions are more convenient at the moment the spare subcarriers remains unused which is not efficient. The duration of OFDM symbols is dynamically changed according to the current network capacity and availability of the subcarriers. Thus the duration of the OFDM symbols is varied so that an OFDM symbol always carries a fixed amount of payload bytes. However after each capacity change the system has to be again synchronized to adapt to the lengths of the time slots and to fit an integer number of OFDM symbols. To avoid the change of both symbol and time slot duration the size of user data transmitted within a time slot can be variable to fit within an OFDM symbol according to the actual network conditions and its currently available transmission capacity. Data Segmentation The division of the transmission .

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