tailieunhanh - Theory and applications of ofdm and cdma wideband wireless communications phần 7

Các bit đầu tiên bên trong một khung tiêu đề, việc phân bổ bit (BAL) bảng, và các yếu tố quy mô chọn thông tin (SCFSI). Một lỗi trong nhóm này sẽ làm cho toàn bộ khung vô dụng. Vì vậy, nó là cần thiết để sử dụng một mạnh (tỷ lệ thấp) mã ở đây. | 246 OFDM Header code rate 8 24 CRC PAD code rate 8 19 Scale factors code rate 8 18 Subband samples code rate 8 14 Bit index Figure Example for an error protection profile for the audio data rate 192 kbit s. by far the biggest one. The first bits inside a frame are the header the bit allocation BAL table and the scale factor select information SCFSI . An error in this group would make the whole frame useless. Thus it is necessary to use a strong low-rate code here. The next group consists mainly of scale factors. Errors will cause annoying sounds so-called birdies but these can be concealed up to a certain point on the audio level if they are detected by a proper mechanism. The third group is the least sensitive one. It consists of subband samples. Subband sample errors cause a kind of gurgling sound. Often this will not even be noticed in a noisy car environment. A last group consists of programme-associated data PAD and the cyclic redundancy check CRC for error detection in the scale factors of the following frame . This group requires approximately the same protection as the second one. The distribution of the redundancy over the audio frame defines such an error protection profile. For DAB audio transmission 64 different protection profiles have been specified ETS 300 401 that correspond to different audio data rates from 32 kbit s and 384 kbit s and allow 5 different protection levels from PL1 the strongest to PL5 the weakest corresponding to five average code rates. Each of them requires approximately the same SNR for distortion-free audio reception. Table gives the detailed definition of the protection profile corresponding to Figure . The last column shows the number of encoded bits. Note that for each frame the trellis will be closed by tail bits. These are Table Example for an error protection profile profile PL3 for the audio data rate 192 kbit s Audio data bits Code rate Encoded bits Group 1 352 Rc 8 24 1056 Group 2 768 Rc 8 18 1758 .