Wireless OFDM Systems

Transcription

1 Wireless OFDM Systems Lecture by Prof. Robert W. Heath Jr. Telecommunications and Signal Processing Research Center The University of Texas at Austin

2 OFDM & Applications Standard Meaning Carrier Freq. Rate (Mbps) Applications DAB Digital Audio Broadcasting FM Audio broadcasting DVB-T Digital Video Broadcasting UHF Digital TV broadcasting IEEE a Wireless LAN 5.2GHz 6-54 Wireless networks IEEE Fixed Wireless Access 2.1GHz Internet/voice access Orthogonal Frequency Division Multiplexing (OFDM) Digital modulation scheme Wireless counterpart to discrete multitone transmission Used in a variety of applications o Broadcast o High-speed internet access 22-2

3 Wireless Digital Communication System Message Source Transmitter Encoder Modulator Pulseshape exp(j2π f c t) Carrier frequency Examples: MHz FM radio Analog cellular 900MHz Digital cellular 1.8GHz Raised-cosine pulseshaping filter 22-3

4 Wireless Digital Communication System (continued) Propagation TX h c (t) noise RX Receiver Message Sink Pulseshape Demodulator Decoder Remove carrier exp(-j2π f c t) 22-4

5 Multipath Propagation Simple Model α 0 α 1 α 2 α α 0 α 2 h c (t) = k=0 K-1 α k δ(t - τ k ) α k : path gain (complex) τ 0 = 0 normalize relative delay of first path k = τ k - τ 0 difference in time-of-flight 22-5

6 Equivalent Propagation Channel h eff (t) = g tr (t) h c (t) g rx (t) convolution transmit filters multipath channel receive filters Effective channel at receiver Propagation channel TX / RX filters h c (t) typically random & changes with time Must estimate & re-estimate channel 22-6

7 Impact of Multipath: : Delay Spread & ISI T s 4T s t/t s t/t s T s t/t s -0.2 Max delay spread = effective number of symbol periods occupied by channel Requires equalization to remove resulting ISI 22-7

8 Effective Delay Spread Delay spread depends on difference in path lengths Effective delay spread: function of the maximum difference Pico cell Micro cell Macro cell Cell size 100m 5km 20km Max Delay Spread 300ns 15us 40us Sampling period T s determines effect of delay spread T s Channel taps Application a DVB-T DAB 50ns 160ns 600ns WLAN Audio TV broadcast Radio waves travel ~ 1ns / ft 22-8

9 Multicarrier Modulation Divide broadband channel into narrowband subchannels No ISI insubchannels 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, HyperLAN II Proposed for IEEE , considered for 4G channel magnitude carrier subchannel Subchannels are 312 khz wide in a and HyperLAN II frequency 22-9

10 An OFDM Symbol N subsymbols X 0 X 1 X 2 X N-1 N-point Inverse FFT x 0 x 2 x 3 x N-1 one symbol N complex samples 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 Key difference with DMT N input symbols! Why? Bandpass transmission allows for complex waveforms Transmit: y(t) = Re{(I(t)+jQ(t)) exp(j2π f c t)} = I(t) cos(2π f c t) Q(t) sin(2 π f c t) 22-10

11 An OFDM Modem N subchannels 2N real samples Bits S/P quadrature amplitude modulation (QAM) encoder N-IFFT add cyclic prefix P/S D/A + transmit filter TRANSMITTER RECEIVER multipath channel N subchannels 2N real samples P/S QAM demod decoder invert channel = frequency domain equalizer N-FFT S/P remove cyclic prefix Receive filter + A/D 22-11

12 Forthek th carrier: Frequency Domain Equalization x k =H k s k +v k where H k = n=0 N-1 h k (nt s )exp(j2πkn/n) Frequency domain equalizer x k s k H k -1 Noise enhancement factor H k 2 H -1 k 2 bad k good k 22-12

13 DMT vs. OFDM DMT Channel changes very slowly ~ 1000s Subchannel gains known at transmitter Bitloading (sending more bits on good channels) increases throughput OFDM Channel may change quickly ~ 10ms Not enough time to convey gains to transmitter Forward error correction mitigates problems on bad channels DMT: Send more data here magnitude OFDM: Try to code so bad subchannels can be ignored frequency 22-13

14 Coded OFDM (COFDM) Error correction is necessary in OFDM systems Forward error correction (FEC) Adds redundancy to data stream Examples: convolutional codes, block codes Mitigates the effects of bad channels Reduces overall throughput according to the coding rate k/n Automatic repeat request (ARQ) Adds error detecting ability to data stream Examples: 16-bit cyclic redundancy code Used to detect errors in an OFDM symbol Bad packets are retransmitted (hopefully the channel changes) Usually used with FEC Minus: Ineffective in broadcast systems 22-14

15 Typical Coded OFDM Encoder FEC Bitwise Interleaving Reed-Solomon and/or convoluational code Data bits Parity bits Rate 1/2 Intersperse coded and uncoded bits Symbol Mapping Map bits to symbols 22-15

16 Example: IEEE a IEEE employs adaptive modulation Code rate & modulation depends on distance from base station Overall data rate varies from 6Mbps to 54Mbps Reference: IEEE Std a

17 Typical COFDM Decoder Frequency-domain equalization Symbol demapping Produce soft estimate of each bit Improves decoding Symbol Demapping Deinterleaving Decoding 22-17

18 FCC manages spectrum Specifies PSD mask Adjacent channel interference Roll-off requirements Implications to OFDM Zero tones on edge of band Time domain windowing smoothes adjacent symbols Spectrum Shaping Adjacent channel Inband Zero tones IEEE a Ofdm symbol Reference: Std a frequency 22-18

19 Ideal Channel Estimation Wireless channels change frequently ~ 10ms Require frequent channel estimation Many systems use pilot tones known symbols Givens k,fork=k 1,k 2,k 3, solvex k = l=0l h l e -j2π k l/n s k for h l Find H k = l=0l h l e -j2π kl/n (significant computation) More pilot tones Better noise resiliance Lower throughput (pilots are not informative) magnitude Pilot tones frequency 22-19

20 Channel Estimation Via Interpolation More efficient approach is interpolation Algorithm For each pilot k i find H ki =x ki /s ki Interpolate unknown values using interpolation filter H m = α m,1 H k1 + α m,2 H k2 + Comments Longer interpolation filter: more computation, timing sensitivity Typical 1dB loss in performance in practical implementation magnitude frequency 22-20

21 OFDM and Antenna Diversity Wireless channels suffer from multipath fading Antenna diversity is a means of compensating for fading Example Transmit Delay Diversity h 1 (t) OFDM Modulator h 2 (t) Delay Equivalent channel is h(t) = h 1 (t) + h 2 (t-d) More channel taps = more diversity ChooseDlargeenough 22-21

22 OFDM and MIMO Systems Multiple-input multiple-output (MIMO) systems Use multiple transmit and multiple receive antennas Creates a matrix channel OFDM Modulator OFDM Modulator H(t) Joint Demodulator Equivalent system for k th tone x k = H k s k + v k Vector inputs & outputs! (more info see WSEL homepage) 22-22

23 Why OFDM in Broadcast? Enables Single Frequency Network (SFN) Multiple transmit antennas geographically separated Enables same radio/tv channel frequency throughout a country Creates artificially large delay spread OFDM has no problems! 20km

24 Why OFDM for High-Speed Internet Access? High-speed data transmission Large bandwidths -> high rate, many computations Small sampling periods -> delay spread becomes a serious impairment Requiresmuch lower BER than voice systems OFDM pros Takes advantage of multipath through simple equalization OFDM cons Synchronization requirements are much more strict Requires more complex algorithms for time / frequency synch Peak-to-average ratio PAR is approximately 10 log N (db) Large signal peaks require higher power amplifiers Amplifier cost grows nonlinearly with required power 22-24

25 Case Study: IEEE a WLAN System parameters FFT size: 64 Number of tones used 52 (12 zero tones) Number of pilots 4 (data tones = 52-4 = 48 tones) Bandwidth: 20MHz Subcarrier spacing : f = 20MHz / 64 = khz OFDM symbol duration: T FFT =1/ f =3.2us Cyclic prefix duration: T GI =0.8us Signal duration: T signal =T FFT +T GI CP s y m b o l i T GI T FFT 22-25

26 Case Study: IEEE a WLAN Modulation: BPSK, QPSK, 16-QAM, 64-QAM Codingrate:1/2,2/3,3/4 FEC: K=7 (64 states) convolutional code Frequency band (GHz) Maximum Output Power (6dBi antenna gain) mw