orthogonal frequency division multiplexing(OFDM)

orthogonal frequency division multiplexing(OFDM)
its orthogonal frequency multiplexing topic basicallly in digital signal processing , network signal and system , it also helpful in engineering course either electrical or electronics and communication engineering.

1. OFDM ORTHOGONAL FREQUENCY DIVIDION MULTIPLEXING Submitted to- Mr. Hemant Kumar Meena Assistant Professor Electrical Engineering Department Submitted by- Toshim Kumar (2013UEE1593) Manish Gupta (2013UEE1762) Bankesh Mehta (2013UEE1614) Khinya Ram (2013UEE1597)

2. OFDM was invented more than 40 years. OFDM has been adopted for several technologies. Asymmetric Digital Subscriber Line(ADSL) services. Digital Audio broadcast(DAB). Digital Terrestrial television broadcast. History of OFDM

3. Principle of OFDM Data to be transmitted is spreaded over a large number of carriers. E ach carrier modulated at a low rate . C arriers are orthogonal to each other. D ivides the total available bandwidth in the spectrum into sub-bands for multiple carriers to transmit in parallel . C ombines a large number of low data rate carriers to construct a composite high data rate communication system.

4. ORTHOGONALITY Two conditions must be considered for the orthogonality between the subcarriers. Each subcarrier has exactly an integer number of cycles in the FFT interval. The number of cycles between adjacent subcarriers differs by exactly one.

5. Orthogonality Time domain Frequency domain Example of four subcarriers within one OFDM symbol Spectra of individual subcarriers

6. FFT Based ofdm system

7. OFDM Implementation x= [1,0,1,1,0,0…] x 1 =[1,0] x 2 =[1,1] x 3 =[0,0] …….. d 1 =[-1] d 2 =[-i] d 3 =[1] ……..

8. IMPLEMENTATION Discrete Fourier transform (DFT) and inverse DFT (IDFT) processes are useful for implementing these orthogonal signals. Note that DFT and IDFT can be implemented efficiently by using fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT), respectively. In the OFDM transmission system, N-point IFFT is taken for the transmitted symbols so as to generate , the samples for the sum of N orthogonal subcarrier signals. Let y[n] denote the received sample that corresponds to x[n] with the additive noise w[n] (i.e., y[n] =x[n]+w[n]). Taking the N-point FFT of the received samples, , the noisy version of transmitted symbols can be obtained in the receiver.

9. Cyclic Prefix Insertion Because wireless communications systems are susceptible to multi-path channel reflections, a cyclic prefix is added to reduce ISI. A cyclic prefix is a repetition of the first section of a symbol that is appended to the end of the symbol. In addition, it is important because it enables multi-path representations of the original signal to fade so that they do not interfere with the subsequent symbol.

10. Guard Interval and Cyclic Extension Two different sources of interference can be identified in the OFDM system. Intersymbol interference (ISI) is defined as the crosstalk between signals within the same sub-channel of consecutive FFT frames, which are separated in time by the signaling interval T. Inter-carrier interference (ICI) is the crosstalk between adjacent subchannels or frequency bands of the same FFT frame.

11. Guard Interval and Cyclic Extension For the purpose to eliminate the effect of ISI, the guard interval could consist of no signals at all . Guard interval (or cyclic extension) is used in OFDM systems to combat against multipath fading. T g :guard interval T delay-spread : multi path delay spread T g > T delay-spread In that case, however, the problem of inter-carrier interference (ICI) would arise. The reason is that there is no integer number of cycles difference between subcarriers within the FFT interval.

12. Guard Interval and Cyclic Extension Delay spread Environment Delay Spread Home < 50 ns Office ~ 100 ns Manufactures 200 ~ 300 ns Suburban < 10 us

13. Guard Interval and Cyclic Extension

14. Guard Interval and Cyclic Extension To eliminate ICI, the OFDM symbol is cyclically extended in the guard interval. This ensures that delayed replicas of the OFDM symbol always have an integer number of cycles within the FFT interval, as long as the delay is smaller than the guard interval.

15. OFDM signal generation 12/9/2015 17

16. OFDM signal generation in digital domain Define complex base-band signal u(t) as follows Perform N times sampling in period T 18 u(k) = IFFT (d n ) = IFFT(a n + jb n )

17. OFDM modulator 12/9/2015 19 M A P S / P I-DFT P / S Real ＢＰＦ AIR Bit stream Imag

18. OFDM demodulator 12/9/2015 20 T u n e r S / P DFT P / S A / D LPF Channel π/2 LPF D E M A P Bit Stream

19. OFDM demodulation 12/9/2015 21 d n = FFT(u(k))