内容ni官方培训labview rf lesson3b

1、Lesson 3B: Orthogonal Frequency Division Multiplexing (Optional)Orthogonal Frequency Division Multiplexing (OFDM) OverviewOFDM in 802.11a and 802.11gOFDM and Orthogonal CarriersOFDM Frequency Spectrum vs. Time DomainOFDM and EVMMultiplication in Time Domain Convolution in Frequency Domain*f1xTime Do

2、mainFrequency Domain1/TFFT bin spacing is 1/TThe FFT of Sinusoid Convolved with Rectangle*f1A. Orthogonal Frequency Division Multiplexing (OFDM) OverviewOFDM was invented in 1970 as a multicarrier modulation.OFDM grew in popularity after real-time FFT was possible (because very fast processors were

3、needed).High-rate data is achieved by sharing parallel low-rate modulators.Carrier spacing is chosen to avoid adjacent carrier interference (orthogonality):Data is coded, time and frequency interleaved for protection.Resilient to multipath fading and narrowband interferers.802.11a and 802.11g have t

4、he same structure, so OFDM can be used in both standards.Multicarrier signaling format:Each of the 52 orthogonal carriers has a low symbol rate.The sand-castle shape is the aggregate response of 52 closely spaced, modulated carriers.B. OFDM in 802.11a and 802.11gOFDM ExamplePowerFrequencyOFDM Advant

5、agesModulation type (BPSK, QPSK, 16-QAM or 64-QAM) is dependent on data rate (6, 9, 12, 18, 24, 36, 48, and 54Mbps).OFDM increases efficiency because carrier spacing is reduced (orthogonal carriers overlap).Equalization is simplified.Data transfer rate can be scaled to conditions.OFDM DisadvantagesH

6、igher peak to average power ratio (PAPR):Stresses the power amplifier more than other systemsAmplifier must be very linearGreater sensitivity to phase noiseCarriers are closely spaced; any noise (jitter) makes it more difficult for the receiver to separate carriers into their proper locations Requir

7、es very clean (low phase noise) clock and LOMore expensive transmitters and receiversSystems required to be backwards compatible (tri-band chips with 802.11a, 802.11b, 802.11g in a single package)OFDM Digital Communications System802.11a and 802.11g Specifications802.11a OFDM occupied bandwidth312.5

8、 kHz x 52 subcarriers + subcarrier 0 = 16.56 MHzSubcarriersData: 48Pilot: 4 (21, 7, +7, +21)Unused: 1 (subcarrier 0)Carrier numbering convention:-2+2+10-1-3+3. .-24-25+25+26+24.-26FFT bin spacing is 1/TThe FFT of OFDM ReceiversNulls land On Bin if the tone is On Bin.802.11a OFDM receiver FFT: FFT bi

9、n spacing = Carrier spacing = 312.5 kHz Sample rate = bin spacing x # bin = 20 MSa (64 bins, complex) Capture time = 1/bin spacing = 3.2 s1/TC. OFDM and Orthogonal CarriersEach carrier spectrum has a sin(x)/x shapeDesigned for closely spaced carriers to overlapNulls in each carriers spectrum land at

10、 the center of all other carriers for Zero Intercarrier Interference (ICI).Phase noise is critical because if zeros move around, then they may not properly align.WiMax OFDM Frequency Domain UtilizationQI64-QAMb0b1b2b3b4b51001001011001111001101000101000111000011000001001001011011011111011101010101010

11、111010011010001011001111011111111111101110101110111110011110001111001101011101111101101100101100111100011100001101000101010101110101100100100100110100010100000101000111010111110111100110100110110110010110000111000011010011110011100010100010110010010010000011000001010001110001100000100000110000010000

12、0000064-QAM Constellation Bit EncodingHow might we build this?Eight I levelsEight Q levels802.11a/g Symbol GenerationCoded bit streamMap to constellationIQ0111013+5jDo this 48 times.3+5jPut into frequency bin.Add 4 pilots and 12 0+0j1-7j1+1j-5+5j7-5j3-1j-1-1j64 point inverse FFT= 64 frequency binsCo

13、py points to guard intervalGISame pointsPulse ShapingFcFc-1/TsFc+1/TsPower Spectral Density of Transmitted SignalSquareRoot raised cosineOFDM Modulation ParametersIEEE Std 802.11i-2003 EditionD. OFDM Frequency Spectrum vs. Time DomainTop graphShows frequency spectrum All 52 carriers/pilotsBottom gra

14、phShows time domain over three, 8 microsecond slotsTime varying / pulsed natureTiming Synchronization Sequence Slot 1Lets examine the first slot (8 microseconds)Training sequenceEvery fourth carrier transmittedEasy for receiver to detectProvides coarse tuning for receiverChannel Estimation Sequence

15、Slot 2Every carrier is transmitted:Equal amplitudeEqual phaseUsed to fine-tune the equalizerlinear errors/multipathNo signal is transmitted on center channel.Data/Payload Slot 3Data in time domain resembles noise.Data in frequency domain clearly depicts OFDM signal.Demonstration 3-1: Orthogonal Freq

16、uency Division Multiplexing (OFDM)Observe the PAP (Peak to Average Power) ratio challenge when using OFDM.Demonstration 3-2: OFDM Base Band Modulation and DemodulationObserve how the transmitter builds and transmits an OFDM signal and how the receiver demodulates the received signal.Typical 64-QAM EVM Modulation plotEVM Single-Carrier ErrorsOn