tailieunhanh - Theory and applications of ofdm and cdma wideband wireless communications phần 6

Trong ví dụ này, m = 2 bit không và kết quả là, nhà nước cuối cùng trong các lưới mắt cáo là nhà nước không. Nó được phổ biến thực hành để bắt đầu và dừng lại với trạng thái không bởi vì nó giúp các bộ giải mã. Điều này có thể dễ dàng đạt được bằng cách thêm số không m - bit đuôi cái gọi là dòng bit hữu ích. Sơ đồ nhà nước cũng có thể mô tả các bộ mã hóa của các quốc gia và các đầu vào và các . | OFDM 203 Figure Diversity branches of the equivalent independent fading channel and normalized bandwidth X BTm 1 2 4 8. significantly to the transmission. It finds its reflection in the performance curves. Figure shows the pairwise bit error probability for K 32 and X BTm 1 2 4 8 16. The high diversity degree of the repetition code K 32 can show a high diversity gain if the equivalent channel has enough independent diversity branches of significant power. This is the case for X 16 but not for X 1 or X 2. For low X a lower repetition rate K would have been sufficient. Figure shows the bit error probability for K 10 and the same values of X. For low X the curves of Figure and are nearly identical. For higher X the curves of Figure run into a saturation that is given by the performance curve of the independent Rayleigh fading. For X 8 this limit is practically achieved. There is still a gap of nearly 2 dB in the AWGN limit at the bit error rate of 10-4. For BPSK and any linear code the probability for an error event corresponding to a Hamming distance d is given by Pd d n 2 1 -----V FT dớ i Es 1 sin2 ớ N 0 i 1 which can be upper bounded by 1 Pd T 2 d 1 i 1 1 li n. 204 OFDM Figure Bit error probabilities for 32-fold repetition diversity with X 1 2 4 8 16. Figure Bit error probabilities for 10-fold repetition diversity with X 1 2 4 8 16. OFDM 205 For the region of reasonable ES N0 those factors with k 1 do not contribute significantly to the product. Thus it is not possible to obtain tight union bounds like o Pd cdPd d dfree because Pd does not decrease as ES N0 -d if d is greater than the diversity degree of the channel that is the number of significant eigenvalues ki. The cd values grow with d and thus the union bound will typically diverge. However the diversity branch spectrum may serve as a good indicator of whether the time-frequency interleaving for a coded OFDM system is sufficient. Consider for example a