OFDM Advantages Orthogonal Frequency Division OFDM is a spectrally efficient Multiplexing (OFDM) modulation technique It is conveniently implemented using IFFT and FFT operations Mary Ann Ingram It handles frequency selective channels ECE 4823 well when combined with error correction coding OFDM Disadvantages OFDM Concept More complex than single-carrier Suppose the symbol length is Tu modulation Sinusoidal signals differing in frequency Requires a more linear power amplifier by 1/Tu will be orthogonal over the period Tu ⎛ 1 ⎞ ts +Tu ⎡ − j2π ⎜ f + ⎟t ⎤ ⎜ o T ⎟ []e j2πfot ⎢e ⎝ u ⎠ ⎥dt = 0 ∫ ⎢ ⎥ ts ⎣ ⎦ Conceptual Transmitter Block Diagram Subcarriers b0 (t) “1” Each branch X X b0 (t) corresponds to a b0 (t) 0 Tu j2πfot subcarrier j2πfot b (t) e e 1 “1” OFDM Subcarriers are b (t) X b (t) X 1 Σ “Useful Part” separated by 1/Tu 1 Σ Tu sec long Hz 0 Tu j2π ()fo +1/Tu t j2π ()fo +1/Tu t e Each subcarrier e bN −1(t) “0” M modulates a M 0 Tu bN −1(t) X different symbol bN −1(t) X can be QAM j2π ()f +[]N −1 /T t bk j2π ()f +[N −1]T t e o u e o u 1 One OFDM Symbol Subcarrier Spectra Each modulated subcarrier has a spectrum in the shape of a sinc squared ⎛ k ⎞ N −1 ⎧ − j2π ⎜ fo + ⎟t function ⎪ b (t)e ⎝ Tu ⎠ 0 < t < T s(t) = ⎨∑ k u ⎪k =0 ⎩ 0 otherwise Cyclic Prefix Generation Cyclic Prefix Appended To avoid losing the power from echoes, The length of the cyclic prefix, also known as a copy of the end is appended to the the Guard Interval, D, is supposed to be beginning of the “useful” part longer than the excess delay of the longest significant echo Tu put a copy of this part in the ∆ Tu front DSRC/WAVE Timing-Related Parameters Effects of Multipath Delays A delayed echo of each subcarrier adds either constructively or destructively to its un-delayed version, creating a flat-faded version of that h(t,τ ) subcarrier All echoes completely fill the Tu window τ ∆ Tu 2 Cyclic Prefix Removal Fading After the symbol has been received and The subcarrier spacing is typically much stored in a buffer, the cyclic prefix is less than the coherence bandwidth removed, leaving only the modulated Channel is and faded subcarriers nearly flat for each |H(f)| subcarrier Each subcarrier sees only flat fading f Tu stems indicate subcarrier frequencies Received OFDM Symbol, After Correlator Demodulator Block Cyclic Prefix Has Been Removed Diagram (Baseband) 1 TU For a static, frequency selective X • dt H ()f b (t) + n (t) T ∫ o 0 0 channel, U 0 e j2πfot 1 TU ⎛ 1 ⎞ X • dt H⎜ f + ⎟b (t) + n (t) ⎛ k ⎞ ∫ ⎜ o ⎟ 1 1 ⎧N −1 ⎜ ⎟ TU 0 T ⎛ k ⎞ − j2π ⎜ fo + ⎟t r(t) ⎝ U ⎠ ⎪ ⎝ Tu ⎠ bk (t)H⎜ fo + ⎟e + n(t) 0 < t < Tu r(t) = ⎨∑ ⎜ ⎟ j2π ( fo +1/Tu )t k=0 ⎝ Tu ⎠ e ⎪ ⎩ 0 otherwise M 1 TU ⎛ N −1⎞ X • dt H⎜ f + ⎟b (t) + n (t) T ∫ ⎜ o ⎟ N −1 N −1 U 0 ⎝ TU ⎠ j2π ( f +[N −1]T )t e o u FFT Channel Estimation These correlations are conveniently In order to detect the bits, the effects of performed by the Fast Fourier channel gains must be compensated Transform (FFT) The subcarrier channel gains are normally The modulation is performed by the estimated using a preamble and pilot tones inverse FFT (IFFT) Simplest approach is to just divide the kth There are very fast and efficient demodulated output by an estimate of implementations of the FFT and IFFT, ⎛ k ⎞ which is a big reason for the popularity ⎜ ⎟ H⎜ fo + ⎟ of OFDM ⎝ TU ⎠ 3 Forward Error-Correction Preamble Structure Coding (FEC) The Preamble comprises 12 training The information is typically FEC symbols, 10 short ones and 2 longer encoded and interleaved prior to ones modulation The bits carried by faded subcarriers might be detected in error without the FEC With FEC and interleaving, erroneous bits may be correctable, thereby providing frequency diversity Modes of DSRC/WAVE DSRC PPDU Structure Modulation and Coding Rate depend on channel quality Data Rate Modulation Coding Rate Coded Bits per Coded Bits Data Bits per Mbits/s (R) Subcarrier per OFDM OFDM (NBPSC) Symbol Symbols (NCBPS) (NDBPS) 3 BPSK 1/2 1 48 24 4.5 BPSK 3/4 1 48 36 6 QPSK 1/2 2 96 48 9 QPSK 3/4 2 96 72 12 16-QAM 1/2 4 192 96 18 16-QAM 3/4 4 192 144 24 64-QAM 2/3 6 288 192 27 64-QAM 3/4 6 288 216 Summary OFDM is a multi-carrier modulation technique Each subcarrier carries BPSK, QPSK or QAM The subcarriers are so close, that each subcarrier sees a flat-faded channel The Guard Interval ensures that successive OFDM symbols do not interfere with each other FEC and interleaving provide frequency diversity OFDM modulation and demodulation is conveniently performed by fast DSP operations (FFT and IFFT) 4.