Ofdmpres2 Tutorial on OFDM and MIMO OFDM

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Text of Ofdmpres2 Tutorial on OFDM and MIMO OFDM

  • 1. An Introduction To OFDM John Wiss 1
  • 2. OFDM Origins OFDM is a modulation type whereby the information coming from a single source is intentionally split up into many carriers (subcarriers) to combat multipath effects and to add a dimension of diversity to the transmit signal Initial work in 1960s for military applications OFDM stands for Orthogonal Frequency Division Multiplexing Orthogonal in the sense that each information-bearing subcarrier may be demodulated without interference from adjacent subcarriers Frequency Division in the sense that the subcarriers are generated as frequency-disjoint coupled carriers with fixed spacing and modulation type Multiplexed in the sense they are synthesized into a single channel Error-correction coding essential because some subcarriers may be in deep fades so encoding of all information bits with interleaving necessary 2
  • 3. OFDM-BASED SYSTEMS IEEE 802.11a was the first wireless networking standard with widespread usage in unlicensed 5 GHz band providing up to 54 Mbps 16.67 MHz wide channel containing 52 subcarriers with BPSK, QPSK, 16-QAM, 64-QAM modulation on each subcarrier OFDM symbol rate of 250 ksym/sec and up to 288 bits/symbol of encoded information IEEE 802.11g is a dual mode system fusing 802.11b WiFi and 802.11a except at a lower frequency band (unlicensed 2.4 GHz) UWB- Multiband OFDM is the leading contender for ultra-wideband communications (>100 Mbps) Hopped OFDM over three bands to spread energy for short range high-speed communications Spatially diverse Vector OFDM (VOFDM) Multiple Input/Multiple Output (MIMO) diversity to combat flat fading Wideband Networking Waveform for military communications 4-G Cellular IEEE 802.16 Metropolitan Area Networking--OFDMA 3
  • 4. One QAM symbol per subcarrier 4
  • 5. Why Use OFDM? There are a few ways to cope with multipath channels: Use spread spectrum techniques to allow separation of paths Direct Sequence Spreading (CDMA/DSSS)Low bps/Hz Use Frequency Hopping (Bluetooth, GSM)MAC inefficiencies/complex Use high bandwidth single-carrier signal Large delay spread vs Tsym Use lower data rates to allow channel to be flat over signal bandwidth The problem is that the signal bandwidth needs to be as narrow as possible to mitigate multipath (the symbol period needs to be as long as possible), Unless The signal is made bandwidth-inefficient in order to perform combining of dispersed rays (e.g., RAKE combining) The optimal signal would be one which has high bandwidth efficiency and immunity to channel variations due to multipath Lower complexity equalization for given delay spread than single-carrier modulation OFDM allows tightly packed carriers to convey information orthogonally and with high bandwidth efficiency 5
  • 6. Equalization Complexity Lets compare equalization complexity of a single carrier (SC) system with the same total bandwidth as an OFDM system subject to similar channel conditions Frequency domain equalization (via FFT) for OFDM Rx Decision feedback equalization for SC Rx OFDM complexity grows slightly greater than linear with BW*Delay Spread product Single carrier complexity grows quadratically with BW*Delay Spread product Single Carrier Case GMSK or OQPSK 24 Mbps Delay Spread = 250 nsec Requires 20 FF, 20 FB taps OFDM Case QAM 52-OFDM 24 Mbps (QAM 16) Delay Spread = 250 nsec 64 Point FFT 52 Subcarriers Guard Interval = 800 nsec Rsym = 250 kHz FEEDFORWARD FILTER (FFF) Cmplex Data In D Tap 0 D Tap 1 D Tap 2 D FEEDBACK FILTER (FBF) Tap 3 D [ . ]* X X Tap FB2 + X From other Taps FFF D + X + From FBF Taps LMS + Tap FB3 1 SPS Error Eq. Output Slicer Decisions + + D(k) {Training, Demod Decisions} Tap FB1 Tap FB0 [ . ]* 4 6 Single Carrier DFE is 10 times as complex!!! D X D + Adj Proc. GMSK Eq (LMS) = 2*20*24*10 (960E6) rmult/sec OFDM (radix 4)= 96*106 = (96E6) cmult/4sec = 96E6 rmult/sec D Tap 4 Peamble Sequence 6 Error X
  • 7. Indoor Channels ETSI/BRAN Low Mobility Indoor 7
  • 8. Indoor Channels (Cont) Small Room Fading (amb. Motion) Tunnel Fading (amb. Motion) k = 0 Gaussian, k = Rayleigh k is the ratio of power of the direct path to the scattered paths 8
  • 9. Outdoor Channels Tx Height=15 m, 3.5 GHz/Outdoor Vs Antenna Height12 &15dBi antennas 3km Separation 9
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  • 11. -- Channel can be inverted (Equalized) by spectrum division 11
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  • 16. OFDM Channel Estimation Each subcarrier corrected by a single tap (complex) equalizer Typically have training symbols with to train frequency domain equalizer Sometimes have embedded pilot subcarriers that are used to estimate channel Uniformly spaced throughout symbol to allow interpolation between pilots to correct data-bearing subcarriers Data subcarriers corrected post-FFT in frequency domain Additional performance by weighting soft decisions based on channel estimates 16
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  • 18. Insert Short Train Standard (52-Carrier) Mode Spectrum For Various Output Backoffs 10 0 -10 -20 -30 10 dB OBO -40 12 dB OBO -50 -60 Linear PA -70 -80 -2.0E+7 -1.5E+7 -1.0E+7 -5.0E+6 0.0E+0 5.0E+6 Frequency (MHz) 1.0E+7 1.5E+7 -8 -6 -4 -2 0 2 4 6 8 -8 -6 -4 -2 0 2 4 6 8 -8 -6 -4 -2 0 2 4 6 8 I 1 I Conv. Encode PN Data 0 Interleave Subcarrier Mapper QAM Map Q Add Cyclic Prefix IFFT Spectral Window Digital To Analog Polyphase Interpolate Oscillator Phase Noise Effects PA Nonlinearity Effects To Channel 10 0 -10 Normalized Pout Vs Pin GaAsTEK ITT6401FM -20 DAC Compensate Insert Long Train 1.2 -30 -40 Phase PSD Sxx(f) -50 A0cos(2 f1t+ 1 (t)) -60 -70 -80 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1 A1cos(2 f2t+ 2(t)) A3cos(2 f3t+ 3(t)) 0.8 Y=1 for (X>2.0) 0.6 0.4 Y = 0.0781X^3-0.5285X^2+1.245X for (0 0 (1) Theory (2) Simulation 200 -30 0 1.0E-7 -200 -40 -400 -50 -60 -512 Additive Noise + AGC -600 7400 -312 -112 88 288 I DQM Decimate Q Filter Analog To Digital Fs = 40,80M Short Train Correl Noise Average Decimate By 2 Filter Frequenc y Respons e 10 RADIX 2 STAGE 128-Points Multipath Resistant Detection RADIX 8 FFT A STAGE 1 {64-Points} Orthogonalizer RADIX 8 FFT B STAGE 1 {64-Points} Mag 0 By 2 -10 -20 -40 7550 7600 7650 7700 7750 7800 20 21 22 23 24 Ecarr/No (dB) RADIX 8 FFT A STAGE 2 {64-Points} RADIX 8 FFT B STAGE 2 {64-Points} Channel Inverter Unscramble 64-pt Transform -50 -60 UNSCRAMBLE 128-pt Transform Phase Noise Reduction Algorithm Slicer Adaptive Filters -70 -80 -90 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Frequency (1.0 = Ny quist) 0.8 0.9 1 Decimate By 4 Filter Frequenc y Respons e 20 By 4 0 -20 250 -40 200 -60 150 -80 -100 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Frequency (1.0 = Ny quist) 0.8 0.9 1 100 50 0 -50 -100 0 500 1000 1500 25 26 27 28 Equalization Tap Update AGC -30 7500 Programmable FFT Mag Gen Test Statistic 1.0E-8 7450 488 Burst Detector/Orthogonalizer/Synchronizer Amplitude Spectrum (dB) Q 8 6 4 2 0 -2 -4 -6 -8 Pilot Carriers Amplitude Spec trum (dB) Su bca rrie r# 8 6 4 2 0 -2 -4 -6 -8 8 6 4 2 0 -2 -4 -6 -8 10 0 -10 -20 -30 -40 -50 -60 -70 -80 Real System Up to 64 QAM Per Carrier 2000 2500 Time (Samples) 3000 3500 4000 18 De-Interleave And Decode PN Data Out 3 2.0E+7
  • 19. OFDM Waveform/Receiver 802.11a/g uses training symbols that can be used to estimate/correct gain frequency offsets and to equalize the channel and Pilot carriers to correct phase offsets during the data symbols VOFDM uses dense training structure in the data symbols to allow for interpolation between pilots to equalize data-bearing carriers 19
  • 20. The Peak to Average Power (PAPR) Problem 802.11a/g consists of 52 subcarriers which are modulated using Digital rather than analog techniques forcing output PA backoffs of ~5-9 dB from P1 The peak signal envelope is the vector sum of the instantaneous amplitude and phases of the 52 subcarriers rotating at nF where n ranges from 26 to +26 (in the case of 802.11a) The PAPR is more dependent on the number of subcarriers than the modulation on each subcarrier when the #subcarriers grow QAM 64 52-Carrier OFDM W/Shaping Cr