Sign Processing for OFDM Communication Systems

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2. Plot. OFDM

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Slide 1

Flag Processing for OFDM Communication Systems Eric Jacobsen Minister of Algorithms, Intel Labs Communication Technology Laboratory/Radio Communications Laboratory July 29, 2004 With a ton of material from Rich Nicholls, CTL/RCL and Kurt Sundstrom, of obscure whereabouts

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Outline OFDM – What and Why Subcarrier Orthogonality and Spectral Effects Time Domain Comparison Equalization Signal Flow PAPR administration Cool Tricks

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Digital Modulation Schemes Single Carrier PSK, QAM, PAM, MSK, and so on. Demodulate with coordinated channel, PLLs Common Standards: DVB-S, Intelsat, GSM, Ethernet, DOCSIS Multi-Carrier OFDM, DMT Demodulate with FFT, DSP Common Standards: DVB-T, 802.11a, DAB, DSL-DMT

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What is OFDM? Orthogonal Frequency Division Multiplexing Split a high image rate information stream into N bring down rate streams Transmit the N low rate information streams utilizing N subcarriers Frequency Division Multiplexing (FDM) & Multi-Carrier Modulation (MCM) N subcarriers must be commonly orthogonal Subcarrier dispersing = f Partition accessible transfer speed into N orthogonal subchannels  Stream - N/2 Complex Baseband OFDM Signal s(t) . . . High Rate Complex Symbol Stream 1 Serial to Parallel Hold (T hold = 1/f sec) f . . . 0 - N(f)/2 (N-1)(f)/2 . . . Stream N/2-1 OFDM Conceptual Block Diagram

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Why OFDM? Decreases image rate by more than N, the quantity of subcarriers Fading per subcarrier is level, so single coefficient evening out Reduces equalizer unpredictability – O(N) rather than O(N 2 ) Fully Captures Multipath Energy For Large Channel Coherence Time, OFDM/DMT can Approach "Water Pouring" Channel Capacity Narrowband obstruction will degenerate modest number of subcarriers Effect alleviated by coding/interleaving crosswise over subcarriers Increases Diversity Opportunity Frequency Diversity Increases Adaptation Opportunities, Flexibility Adaptive Bit Loading OFDMA PAPR to a great extent free of regulation request Helpful for frameworks with versatile tweak

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Downsides of OFDM Complexity FFT for balance, demodulation Must be contrasted with multifaceted nature of equalizer Synchronization Overhead Cyclic Extension Increases the length of the image for no expansion in limit Pilot Tones Simplify balance and following for no expansion in limit PAPR Depending on the arrangement, the PAPR can be ~3dB-6dB more regrettable than a solitary transporter framework Phase clamor affectability The subcarriers are N-times smaller than an equivalent single-bearer framework Doppler Spread affectability Synchronization and EQ following can be dangerous in high doppler situations

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Subcarrier Orthogonality disentangles recuperation of the N information streams Orthogonal subcarriers = No between transporter impedance (ICI) Time Domain Orthogonality: Every subcarrier has a whole number of cycles inside T OFDM Satisfies exact numerical meaning of orthogonality for complex exponential (and sinusoidal) works over the interim [0, T OFDM ] Frequency Domain Orthogonality: ICI = 0 at f = nf 0 f Some FDM frameworks accomplish orthogonality through zero phantom cover  BW wasteful! OFDM frameworks have covered spectra with each subcarrier range having a Nyquist "zero ISI beat shape" (truly zero ICI for this situation).  BW proficient!

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OFDM Signal (Time & Frequency)

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Practical Signal Spectra Single bearer signs require separating for ghastly control. This flag has contract rolloff locales which requires long channels. OFDM spectra have actually soak sides, particularly with huge N. The PAPR is regularly higher, which may bring about more otherworldly regrowth. The blue follow is an unfiltered OFDM motion with 216 subcarriers. The red follow incorporates the impacts of a non-straight Power Amplifier.

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Time-Domain Comparisons By extraordinarily expanding the image time frame the blurring per subcarrier turns out to be level, so it can be evened out with a solitary coefficient for each subcarrier. The expansion of the cyclic prefix disposes of Inter-Symbol Interference (ISI) due to multipath.

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Frequency Domain Equalization Design System Such That T Delay Spread < T Guard and B Coherence > B Subcarrier Subcarriers are impeccably orthogonal (no ISI or ICI) Each Subcarrier encounters an AWGN channel Equalizer Complexity : Serial Data Rate = 1/T, OFDM Symbol Rate = 1/(NT) FEQ performs N complex duplicates in time NT (or 1 complex mult per time T) Time space EQ must perform MT complex duplicates in time T where M is the quantity of equalizer coefficients Channel Frequency Response (at time t) Subcarrier n Frequency

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802.11a PHY Block Diagram

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802.11a Processing 802.11a is a TDD conflict based, bursty convention Full procurement, synchronization, and EQ preparing can be performed for each blasted or "edge" The "short preparing images" give timing, AGC, assorted qualities determination, and starting transporter counterbalance The "long preparing images" give fine synchronization and channel estimation Two FFT periods permit 3dB increment in channel estimation SNR by consolidating (averaging) the assessments Tracking is encouraged by the four pilot tones

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802.11a Time/Frequency Signal Structure DATA FRAME Short Training Symbols Long Training Symbols Data Symbols 8.125 MHz … FREQUENCY 53 Subcarriers (48 information, 4 pilot, 0 @ DC) 0 … - 8.125 MHz Indicates Pilot Tone Location 800 ns 4 s TIME

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DVB-T Time/Frequency Signal Structure Since DVB-T is a consistent transmit flag, channel estimation is encouraged effectively by pivoting pilots over the subcarrier files. Interjection gives channel estimation to each subcarrier. This figure is from reference [4]

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Peak to Average Power Ratio Single Carrier Systems PAPR influenced by regulation plan, arrange, and separating Constant-envelope plans have characteristically low PAPR For instance: MSK, OQPSK PAPR increments with balance arrange e.g., 64-QAM PAPR is higher than QPSK As Raised Cosine abundance data transfer capacity diminishes, PAPR builds Squeezing the possessed range increments PAPR Multi-Carrier Systems PAPR influenced by subcarrier amount and sifting PAPR is just feebly associated with adjustment arrange PAPR increments with the quantity of subcarriers Rate of increment moderates after ~64 subcarriers The Central Limit Theorem is still your companion Whitening is extremely viable at lessening PAPR Symbol molding diminishes PAPR

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64-QAM 20% RRC 64-QAM OFDM-48 802.11a 64-QAM OFDM-240 P(PAPR < Abscissa) PAPR (dB) PAPR with 240 subcarriers N = 240 requires close to 1dB extra backoff contrasted with 802.11a, and around 3.5dB more than a solitary bearer framework. The outcomes demonstrated utilize just information brightening for PAPR decrease. Extra changes might be conceivable with different methods.

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PAPR Mitigation in OFDM Scrambling (brightening) diminishes the likelihood of subcarrier arrangement Subcarriers with normal stage increment PAPR Symbol weighting decreases the impacts of stage discontinuities at the image limits Raised Cosine Pulse weighting Works well, requires buffering Signal separating Easier to execute

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Time-Domain Weighting The stage discontinuities between images increment the span of the unearthly sidelobes. Weighting the image moves smooths them out and diminishes the sidelobe amplitudes. Ordinarily Raised-Cosine weighting Is connected. Decreased Regions This figure is useful substance from the IEEE 802.11a particular. The two-fft period case applies just to preludes for synchronization and channel estimation.

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Effect of Symbol Weighting With no RC weighting With 1% RC weighting Applying a modest piece of image weighting in the time space significantly affects PAPR. For this situation just 1% of the image time is utilized for decreasing. The blue follow is preceding the PA, the red follow after. Use of the 1% RC window meets the green transmit cover.

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Cool and Interesting Tricks OFDMA Different clients on various subcarriers Adaptive Bit Loading Seeking water filling limit Adaptation to Channel Fading Adaptation to Interference

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OFDMA Subcarrier Division The 802.16 standard depicts numerous way to execute OFDMA. In one mode every client's flag possesses bordering subcarriers which can be autonomously regulated. Another mode permutes every client's subcarriers over the band in a spreading plan so that all client's subcarriers are joined with other client's subcarriers. The principal technique takes into account versatile balance and the second strategy builds recurrence differing qualities.

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Subcarrier Division with TDM Each shading is for a particular terminal .

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Channel Frequency Response Multipath  Frequency Selective Fading v = 100 km/hr f = 2 GHz  t = 0.5 m sec Shannon's Law applies in every "level" subinterval

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High SNR At Receiver Low SNR At Receiver Sub Carriers OFDM "Image" Adaptive Bit Loading Frequency (MHz) - 5 - 4 - 3 - 2 - 1 0 1 2 3 4 5 0 6 bps/Hz - 5 4 bps/Hz - 10 Response (dB) 2 bps/Hz - 15 Deep Fade (Bad) - 20 0 bps/Hz - 25 - 30 Channel Bandwidth 64 QAM 16 QAM QPSK

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Per-Subcarrier Adaptive Modulation

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References [1] IEEE Std 802.11a-1999 [2] Robert Heath, UT at A,,22,OFDM and MIMO Systems [3] Hutter, et al, [4] Zabalegui, et al,

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Backup No! – Go forward!

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Cyclic augmentation expels ISI and ICI ! Cyclic Prefix (Guard Interval) Delay Spread Causes Inter-Symbol-Interference (ISI) and Inter-Carrier-Interference (ICI) Non-straight stage infers distinctive subcarriers encounter diverse deferral (for all intents and purposes every single genuine channel are non-direct stage) Adding a protect interim between