ENGN8637 Physical Layer Development of DVB-T2 Lecture 3 DVB-T2 Overview: Channel Modelling Part I

Transcription

1 ENGN8637 Physical Layer Development of DVB-T2 DVB-T2 Overview: Channel Modelling Part I Lecturer: Gerard Borg Gerard.bor@anu.edu.au Research School of Engineering

2 Why study channel first? We already know transmitter from en_302755v010401p Receiver fights against imperfect channels 2

3 DVB-T2 Signal Types Channel coding Date bits: A vector of binary values BICM Codeword: A vector of binary values OFDM FD OFDM symbol: A vector of complex values TD OFDM symbol: A vector of complex values 3

4 DVB-T2 Channel Types Binary symmetric channel (BSC) AWGN channel (AWGN) Uncorrelated Single-path Rayleigh channel (USRC) Multi-path Rayleigh fading channel (MRC) Multi-path Ricean fading channel (MRIC) Mobile Typical-urban 6-path channel (TU-6) FRIIS Transmission (of course) 4~7 are instances of real-world channels 1~3 mimic these channels impacts on different types of signals 4

5 Where do the channels take effect? Channel coding BICM Codeword Date bits BSC OFDM FD OFDM symbol AWGN, USRC TD OFDM symbol MRC, MRIC, MTUC 5

6 Binary Symmetric Channel Each bit is either received correctly or incorrectly (flipped) random independent transition L = 100; % codeword length p = 0.2; % transition probability code = randint(1, L, [0, 1]); % binary codeword channel = randint(1, L, [1, 100])<=100*p; % 1 for error rc = xor(code, channel); % received codeword Is independent important? Free cloud based MATLAB: with limited functionality 6

7 AWGN Channel - Not actually a channel AWGN are incurred by the imperfect receiver. IS THIS ALL THAT THERE IS TO IT? The physical channel is assumed to be perfect. WHAT DOES THIS MEAN? For BPSK: For all 2D constellations: 7

8 Uncorrelated Single-path Rayleigh Fading Channel Long but simple Uncorrelated means: For BPSK: For all 2D constellations: (We will explain the importance of this channel at the start of BICM session.) 8

9 Multipath Rayleigh Fading Channel Introduction 9

10 Multipath Rayleigh Fading Channel Profile are random Gaussian variables. So do. follows a Rayleigh distribution 10

11 Multipath Rayleigh Fading Channel Modelling TD OFDM symbol Channel Impulse Response (CIR) delay Copy-3 Copy-2 Copy-1 time 11

12 Multipath Rayleigh Fading Channel DVB-T2, Passband Contains 20 taps Time spread is 5.4 A snapshot of real-world CIR i.e., tap values are fixed It won t work for our baseband OFDM: Paths are not separated by multiples of T (See page 214 of the implementation guideline for detailed values) 12

13 Multipath Rayleigh Fading Channel DVB-T2, Baseband Solution: Merge close taps Then shift to multiples of T Result: 12 taps, time span = 50T Power normalization: 13

14 Multipath Rayleigh Fading Channel Frequency Domain N = 1024; % OFDM symbol size, FFT size h = [3, 2, 0, 0, 1]; % an instance of MRC CIR H = fft(h, N); % MRC CTF Channel Transform Function (CTF) 14

15 Multipath Ricean Fading Channel What s new? Usually a line-of-sight (LOS) path with very small delay and very large coefficient 15

16 Multipath Ricean Fading Channel DVB-T2 Add a path whose power is 10x of the sum of the Rayleigh paths: Then normalize the channel power again to obtain: 16

17 MRC and MRIC are static channels In the next lecture we will discuss mobile channels We will also discuss performance measurements TO BE CONTINUED 17