Orthogonal frequency division multiplexing (OFDM)

Transcription

1 Orthogonal frequency division multiplexing (OFDM)

2 OFDM was introduced in 1950 but was only completed in 1960 s Originally grew from Multi-Carrier Modulation used in High Frequency military radio. Patent was granted in 1970 s Earlier OFDM wasn t popular Large arrays of sinusoidal generators and coherence demodulator Too expensive and complex. Later when DFT and IDFT became a known solution to the arrays of generators and demodulators. It was still not popular as there is no efficient method to perform the IFFT and FFT operation. Advances in VLSI technology allows implementation of fast and cheap FFT and IFFT operation drive OFDM popularity.

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4 OFDM Orthogonal Frequency Division Multiplexing Frequency Division Multiplexing -Divide the information over several carriers Instead of using one big truck When The truck is lost All is lost! Use several small trucks When one truck is lost Only a portion of the shipment is lost!

5 Concept of an OFDM signal Ch.1 Ch.2 Ch.3 Ch.4 Ch.5 Ch.6 Ch.7 Ch.8 Ch.9 Ch.10 Conventional multicarrier techniques frequency Ch.2 Ch.4 Ch.6 Ch.8 Ch.10 Ch.1 Ch.3 Ch.5 Ch.7 Ch.9 Saving of bandwidth 50% bandwidth saving Orthogonal multicarrier techniques frequency

6 OFDM changes Frequency Selective Fading to Flat Fading Channel N number of subcarrier......

7 Solution to Frequency Selective Fading When the data rate is lower = Delay spread = Symbol period = signal BW = coherence BW Frequency Selective => Flat Fading In flat fading, the amplitude varies but there is no ISI

8 Multicarrier Modulation Divide broadband channel into narrowband subchannels No ISI in subchannels if constant gain in every subchannel and if ideal sampling Orthogonal Frequency Division Multiplexing Based on the fast Fourier transform Standardized for DAB, DVB-T, IEEE a, a, HyperLAN II Considered for fourth-generation mobile communication systems channel magnitude subcarrier subchannel frequency Subchannels are 312 khz wide in a and HyperLAN II

9 OFDM Frequency Spectrum Use many carriers that are equally spaced: f k 1 k T Carrier 1 has a maximum where all other carriers are 0 f 0 s k = 0, 1,, N-1 T s = Symbol Time frequency f 1 T s f s N Hz T 4096 or 8192 s 232 s

10 OFDM Many carriers with small spacing => Long symbol time But many carriers carry a lot of information! Long symbol time is an advantage! Delay Spread (Multipath) Direct Path Delayed Path Symbol n-1 Symbol n Symbol n+1 Symbol n-1 Symbol n Symbol n+1 ISI ISI ISI = Inter Symbol Interference

11 OFDM Avoid ISI and preserve Orthogonality => Guard Interval Total Symbol length Useful Symbol length Guard Symbol n Guard Direct Path Guard Symbol n-1 Guard Symbol n Guard Symbol n+1 Delayed Path Symbol Time Guard 232μs Symbol n-1 Guard Symbol n Guard Symbol n+1 Integration Period Total Symbol Time Guard Guard 58 s 4 Total Symbol Time s Symbol n is added constructively or destructively according to phase

12 Avoid ICI and preserve Orthogonality => cyclic prefix copy copy CP s y m b o l i CP s y m b o l ( i+1) v samples N samples CP: Cyclic Prefix

13 Discrete versus Fast Fourier Transform Discrete (DFT): X k N 1 x n 0 n e 2 j. n. k N For each frequency sample k (0 to N-1) loop n (over 0 to N-1) => N 2 complex multiplications Fast (FFT, Cooley-Tukey algorithm): An efficient algorithm to calculate a DFT N.log(N) complex multiplications Example : N Discrete Fast FFT : 4096 * * ,3% with respect to DFT multiplications

14 OFDM Block Diagram

15 Main advantages High spectral efficiency. And high data rate. Efficient in multipath environments. Simple digital realization by using the FFT operation. Low complex receivers due to avoidance of ISI. Different modulation schemes can be used on individual sub-carriers.

16 x Drawbacks Large Peak to Average Ratio (PAR). Added sinusoid cause large PAR and issue of amplifier nonlinearity arises. Accurate frequency and time synchronization is required. More sensitive to Doppler spreads than single-carrier schemes. Sensitive to frequency offset and phase noise caused by imperfections in the transmitter and the receiver oscillators. Guard interval causes loss in spectral efficiency