Orthogonal Frequency Division Multiplexing (OFDM)

Transcription

1 Orthogonal Frequency Division Multiplexing (OFDM) Presenter: Engr. Dr. Noor M. Khan Professor Department of Electrical Engineering, Muhammad Ali Jinnah University, Islamabad Campus, Islamabad, PAKISTAN Ph: +92 (51) , Ext. 116 Fax: +92 (51) BS MS2 MS1 1

2 Single-carrier baseband communication system model 2

3 Multichannel Transmission System Model 3

4 Multicarrier System Model 4

5 Orthogonal Frequency Division Multiplexing (OFDM) 5

6 Orthogonal Frequency Division Multiplexing (OFDM) 6

7 Frequency Orthogonality 7

8 OFDM Signaling N-point IDFT of PSK or QAM data symbols 8

9 OFDM Signaling N-point IDFT of PSK or QAM data symbols The received baseband OFDM symbol The transmitted symbol X[k] can be reconstructed by the orthogonality among the subcarriers 9

10 OFDM Signaling The transmitted symbol X[k] can be reconstructed by the orthogonality among the subcarriers 10

11 OFDM Signaling The transmitted symbol X[k] can be reconstructed by the orthogonality among the subcarriers 11

12 OFDM Signaling The transmitted symbol X[k] can be reconstructed by the orthogonality among the subcarriers 12

13 OFDM Signaling 13

14 Orthogonal Frequency Division Multiplexing (OFDM) A method of encoding digital data on multiple carrier frequencies A method of digital modulation in which a signal is split into several narrowband channels at different frequencies. A digital transmission technique that uses a large number of carriers spaced apart at slightly different frequencies 14

15 OFDM: Operation Total bandwidth available within a communications system is divided into smaller non-overlapping frequency sub-bands Usually a separate data signal is associated to each frequency sub-band Passband filter at receiver extracts requested frequency sub-band / data signal 15

16 OFDMA Each terminal occupies a subset of subcarriers Subset is called an OFDMA traffic channel Each traffic channel is assigned exclusively to one user at any time 16

17 OFDM System It is a special kind of FDM The spacing between carriers are such that they are orthogonal to one another Therefore, there is no need of guard band between carriers. 17

18 OFDM: Operation 18

19 OFDM: Operation 19

20 OFDM: System Model 20

21 OFDM: System Model 21

22 OFDM: System Model 22

23 OFDM: An Example Let s we have following information bits 1, 1, -1, -1, 1, 1, 1, -1, 1, -1, -1, -1, -1, 1, -1, -1, Just convert the serial bits to parallel bits C1 C2 C3 C

24 OFDM: An Example Modulate each column with corresponding sub-carrier using BPSK Modulated signal for C1 Modulated signal for C2 Modulated signal for C3 Modulated signal for C4 24

25 OFDM: An Example Final OFDM Signal = Sum of all signal V ( t) N 1 = n= 0 I n ( t)sin(2 π nt) Generated OFDM signal, V(t) 25

26 OFDMA s Encouraging Features Can easily adapt to severe channel conditions without complex time-domain equalization. Robust against narrow-band co-channel interference. Robust against ISI and fading caused by multipath propagation. High spectral efficiency as compared to conventional modulation schemes, spread spectrum, etc. Efficient implementation using Fast Fourier Transform (FFT). Low sensitivity to time synchronization errors. Tuned sub-channel receiver filters are not required (unlike conventional FDM). 26

27 OFDMA s Discouraging Features Sensitive to Doppler shift. Sensitive to frequency synchronization problems. High peak-to-average-power ratio (PAPR), requiring linear transmitter circuitry, which suffers from poor power efficiency. Loss of efficiency caused by cyclic prefix/guard interval. 27