Channel Surfing Managing Propagation in Broadband Wireless Systems

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1 Channel Suring Managing Propagation in Broadband Wireless Systems SCV Antennas and Propagation Society Meeting January 21 st, 2010 Derek K. Shaeer, Ph.D. IC Design Manager, InvenSense, Inc.

2 Outline Wireless Channels OFDM Modulation OFDMA Multiple Access in WiMAX Systems Implementation Challenges: What Limits Orthogonality Summary 2 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

3 The Mobile Broadband Wireless Channel PROPRIETARY & CONFIDENTIAL Mobile broadband wireless channels have a number o unattractive attributes Slow large-scale ades (shadow ading) Fast small-scale l ades (Rayleigh ading) Frequency-selective due to multipath Time-varying due to mobility The deining issue or mobile broadband is how to manage the channel while providing ever increasing data rates But, it s not all bad news Frequency-selective channels have some beneits: Channel diversity Spatial multiplexing (e.g. spatial rate 2 MIMO) Tx p ail p ail p ail p ail Rx In broadband systems we take advantage o these characteristics to achieve higher throughputs and link diversity p << ail, tot = p 4 ail p ail 3 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

4 Path Loss Due to Fading 2 4πd d L p 10 + λ d 0 0 ( d ) = 10log n log + Xσ X ρ Path loss between isotropic antennas Free-space loss to re. distance, d 0 d n loss rom d 0 to d Log-normal random variable (Shadow Fading) Rayleigh distributed random variable (Rayleigh Fading) Shadow ading describes the attenuation o received power due to large-scale eects Buildings, oliage, ground relections, etc Path loss typically increases as d n, where n~3-5. Log-normal random component, with std-deviation o 8dB or greater! Rayleigh ading is a small-scale eect occurring due to carrier summation among multipath components Fast ading in single-carrier systems occurs roughly every λ/2 distance Multi-carrier systems take advantage o reduced correlation across requency 20-30dB needs to be budgeted or Rayleigh ades 4 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

Shadow Fading Example Measurements taken

5 Shadow Fading Example Measurements taken in and around eight suburban homes. 815MHz Dipole RX & TX antennas Measurement results show: Distance exponent ~ Log-normal σ ~ 10dB Signal 1000t ~ 27dB below what ree-space propagation would predict Donald C. Cox, R.R. Murray and A.W. Norris, 800-MHz Attenuation Measured In and Around Suburban Houses, AT&T Bell Laboratories Technical Journal, vol. 63, no. 6, July-August 1984, pp Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

6 Rayleigh Fading Example Indoor 900MHz Fades as deep as 30dB observed Note the small distance scale over which signal levels vary Donald C. Cox, Universal Digital Portable Radio Communications, Proceedings o the IEEE, vol. 75, no. 4, April 1987, pp Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

7 Eects o Multipath Multiple paths rom Tx converge on Rx Each path has dierent delay (ISI) Each path has dierent carrier phase (Rayleigh ading) Relections occurring beyond the cell boundary can be signiicant Assume d 4 roll-o Path loss along 3x nominal length rays is about -20dB relative to the direct path. Delay-spread due to 3x path lengths: Motion o the Rx antenna All paths experience Doppler shit Worst-case Doppler spread due to velocity v away rom Tx antenna and toward a relecting object: T r r c 2 M, 3 x = 2v 2 D = λ Tx 1 2 3r ellipse Rx 3 v 7 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

8 Multipath, Delay Spread and Coherence BW PROPRIETARY & CONFIDENTIAL Coherence BW: Wireless channel has a time-varying impulse response. Power delay proile gives the long-term response w/ expected power at each tap Received signals are spread in time. Maximum delay spread, T M, determined by the range o path lengths RMS delay spread, σ T, is a common metric This causes ISI inter-symbol intererence Coherence BW is the width o the power spectrum o the impulse response B C Gives a measure o the bandwidth o signals that can be sent over the channel without equalization. 1 4σ T 8 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

Delay Spread Examples [1] B. Sklar, Rayleigh Fading Channels, in The Mobile Communications Handbook, CRC Press, 1999, pp. 18-1 to 18-39. [2] Donald C.

9 Delay Spread Examples [1] B. Sklar, Rayleigh Fading Channels, in The Mobile Communications Handbook, CRC Press, 1999, pp to [2] Donald C. Cox, Universal Digital Portable Radio Communications, Proceedings o the IEEE, vol. 75, no. 4, April 1987, pp Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

10 Time-Variance, Doppler Spectrum and Coherence Time Coherence Time: T C 1 2 D Dual o delay spread test: t Apply a CW tone to the channel Frequency-domain impulse Received signals are spread in requency. Maximum requency spread, D, determined by the Doppler shit This causes ICI inter-carrier intererence Coherence time is the spacing between zero-crossings o the time autocorrelation unction Gives a measure o the interval over which the channel appears to be time-invariant. 10 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

11 Dense-Scatterer Model Doppler Spectrum PROPRIETARY & CONFIDENTIAL Uniormly distributed angle o arrival Doppler shit w/ Velocity V P R = 2 v = sinα = λ π 2 PR 2 π 2 D π dα sinα Change o variables yields integral over P R = D D π P R 1 2 D 2 D d Doppler spectrum: S D ( ) = π D P R D 11 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

12 Scattering Functions Wireless channels scatter energy in both requency and time. Scattering Function: S( τ,ν ) Delay Power Spectrum: ( τ ) S ( τ ν ) S =, d ν Doppler Power Spectrum: S ( ν ) S( τ ν ) =, dτ 0 [1] E. Biglieri, J. Proakis and S. Shamai, Fading Channels: inormation-theoretic and Communications Aspects, IEEE Transactions on Inormation Theory, vol. 44, no. 6, October 1998, pp [2] B. Sklar, Rayleigh Fading Channels, in The Mobile Communications Handbook, CRC Press, 1999, pp to Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

13 Equivalent Model Tapped delay line model. Time-varying taps modeled as stationary (wide-sense) uncorrelated random variables with speciied Doppler power spectra. [1] E. Biglieri, J. Proakis and S. Shamai, Fading Channels: inormation-theoretic and Communications Aspects, IEEE Transactions on Inormation Theory, vol. 44, no. 6, October 1998, pp Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

14 ITU Channel Models Time Domain 1 ITU Indoor Oice Channel A ITU Pedestrian Channel A ITU Vehicular Channel A T M ~ 170ns T M ~ 190ns T M ~ 2µs 0.8 Rel. Pow wer Rel. Pow wer Rel. Pow wer Delay (us) Delay (us) Delay (us) 1 ITU Indoor Oice Channel B 1 ITU Pedestrian Channel B 1 ITU Vehicular Channel B. Power Rel Rel. Power 0.8 T M ~ 300ns T M ~ 3µs T M ~ 15µs Rel. Power Delay (us) Delay (us) Delay (us) Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

15 ITU Channel Models Frequency Domain Channel Amplitude Response H 2.5 PedA PedB 2 PROPRIETARY & CONFIDENTIAL Cha annel Amplitu ude Tone Index 15 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

16 Outline Wireless Channels OFDM Modulation OFDMA Multiple Access in WiMAX Systems Implementation Challenges: What Limits Orthogonality Summary 16 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

17 Optimal Symbol Period To transmit symbols while avoiding ISI and ICI, the symbol period should satisy the ollowing condition: That is: 1 2 D >> T S >> T Symbols should be short enough to avoid ast ades Symbols should be long enough that ISI is insigniicant M A reasonable choice: T ~ T 2 r v S M, 3x 2 D = 0 Example: Assume 1-km cell radius, 100-km/h speed, 2.5-GHz carrier requency 1 2 D ~ 2.2ms >> T S ~ 120µ s >> T ~ 6. 7µ s M 17 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

18 The Concept o a Cyclic Preix Avoid ISI PROPRIETARY & CONFIDENTIAL A single-carrier transmitted symbol is a carrier burst whose amplitude and phase carry the desired inormation. Note that a single symbol contains an integer number o sub-carrier cycles. The beginning o the received symbol is corrupted by multipath. Distortion persists until the homogeneous response o the channel dies out. Add a cyclic preix to the symbol Copy the end o the symbol to the beginning. Discard the CP at the receiver end. 18 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

19 OFDM Signals Symbol period, T S, is too long to support a high data rate with a single subcarrier. Combine multiple subcarriers to be transmitted simultaneously This way, we can scale up the data rate while maintaining a long symbol period. Use a cyclic preix to avoid any distortion due to ISI rom multipath. Subcariers can carry multiple bits / symbol using high order modulation (e.g. 16-QAM) as SNR permits. Orthogonal Frequency Division Multiplexing (OFDM) 19 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

20 The Concept o Orthogonality Avoid ICI PROPRIETARY & CONFIDENTIAL With multiple subcarriers present, there is potential or ICI. Spectral side-lobes o a given sub-carrier can interere with adjacent sub-carriers. To avoid this, we construct the composite signal such that all subcarriers are orthogonal. The condition or doing so is that the symbol should contain an integer number o cycles o each and every sub-carrier (neglecting the CP). In the presence o multipath, the cyclic extension o the symbol with the CP maintains the orthogonality o all subcarriers. This ensures that the transmitted OFDM symbol will be ree o ICI. 20 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

21 Mathematical Description o OFDM Signals PROPRIETARY & CONFIDENTIAL A Single OFDM Symbol: s k ( ) () =+ Nused 1 2 j2π ( ) = 0 t j2πk SC t TCP t Re e c ke k = ( N 1) 2, k 00 used : RF carrier requency 0 k c T 0 k CP SC N used : OFDM subcarrier requency spacing : OFDM subcarrier index : Complex number representing the data on subcarrier k : Cyclic preix (guard) time : Number o subcarriers in use 21 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

22 OFDM Modulation and Demodulation (MS) PROPRIETARY & CONFIDENTIAL Through the use o CP, the modulator becomes a simple IFFT. Serial data are encoded and assigned to requency bins. IFFT generates the assembly o sub-carriers. On the Rx side, do the reverse. Rx processing is generally much more complex: synchronization, intererence mitigation, channel estimation, antenna combining, etc.. 22 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

23 Envelope PDF / PAPR 2-D Gaussian Random Variable Envelope PDF 2-D Gaussian Random Variable PDF ure Component Rayleigh Distribution PDF Quadrat In-Phase Component Quadrature Component In-Phase Component With a large number o sub-carriers, the complex OFDM signal statistics approach that o a 2-D Gaussian random variable. Central limit theorem Amplitude o the complex signal is Rayleigh distributed. ib t d 23 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

24 Implications o High-PAPR 10 0 Rayleigh Envelope CCDF -20 EVM o OFDM Signal Sg Ater Clipping Envelope CCDF F EVM Due to Clipping (db) PAPR (db) PAPR (db) Rayleigh-distributed signals have high peak-to-average ratio. With inite headroom, compression will clip the signals, which degrades EVM. Need to maintain about 9dB PAPR or clipped EVM < -45dB. PAR Peak-to-Average Ratio : The PAR o the complete RF signal. PAPR Peak-to-Average Power Ratio : The PAR o the RF envelope. 24 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

25 Outline Wireless Channels OFDM Modulation OFDMA Multiple Access in WiMAX Systems Implementation Challenges: What Limits Orthogonality Summary 25 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

26 WiMAX Modulation Schemes and Data Rates Bandwidth RCE 5 MHz Ch Data Rates 10 MHz Ch Data Rates MCS (db) Downlink Uplink Downlink Uplink QPSK ½ CC, 6x QPSK ½ CC, 4x QPSK ½ CC, 2x QPSK ½ CC, 1x QPSK ¾ CC QAM ½ CC QAM ¾ CC QAM ½ CC QAM 2/3 CC QAM ¾ CC MCS Modulation and Coding Scheme RCE Relative Constellation Error S. Lloyd and L. Jalloul, e WiMAX Key System and Circuit Design Issues, Workshop WK151, RFIC Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

27 Multiple-Access w/ OFDMA Transmitted OFDM Signals Tx1 Rx1 Received OFDMA Signal User #1 User #2 User #3 User #4 Tx2 Tx3 Tx4 Multiple-Access Users share a common Tx requency. Users receive unique subcarrier allocations. Users individual id allocations are orthogonal. Signals combine at the Rx antenna and are received simultaneously. Beneits Flexible bandwidth allocation High capacity Frequency diversity 27 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

28 Sub-Carrier Assignment Strategies (Simpliied) Channel #1 Channel #2 BAMC Band Adaptive Modulation and Coding Contiguous allocations Suitable or quasi-stationary stationary channels Enables multi-user diversity gains MIMO (Rev. 2.0) FUSC / PUSC Full (Partial) Usage o Sub-Carriers Allocations are dispersed over requency (diversity) Suitable or ading channels Frequency diversity MIMO BAMC User #1 User #2 FUSC / PUSC User #1 User #2 28 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

29 WiMAX MIMO Modes / Diversity Techniques PROPRIETARY & CONFIDENTIAL Maximum Ratio / MMSE Combining Multi-antenna Rx diversity technique Improved SNR Opportunistic i Scheduling (Multi-User Diversity) i Works well or slow-ading channels w/ good channel eedback Reduced susceptibility to ading Beam Forming Good or slow-ading channels w/ good channel eedback However, can also be applied or closely-spaced antennas w/ aster ade rates Improved SNR MIMO Space Time Coding Same data transmitted on multiple antennas and at multiple time instances Each antenna is coded such that data components are transmitted at dierent times Improved diversity Easy decoding (Alamouti algorithm) MIMO Spatial Multiplexing Dierent data streams transmitted on each antenna Increased channel capacity (~doubled or 2x2 system) 29 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

30 Outline Wireless Channels OFDM Modulation OFDMA Multiple Access in WiMAX Systems Implementation Challenges: What Limits Orthogonality Summary 30 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

31 The Challenge o Maintaining Orthogonality Eect on Subcarrier 1 Input OFDM Spectrum 1 PROPRIETARY & CONFIDENTIAL = 0 FFT ICI Orthogonality is easily lost > 0 Frequency osets Preamble acilitates carrier requency & symbol timing estimation UL requency accuracy: < +/- 2% o SC spacing (RCT requirement) Frequency q y tracking error is typically y much less than this Doppler spreading (Rayleigh ading) Sub-carrier spacing selection is critical Symbol windowing can be used to reduce sub-carrier side-lobes Not used in WiMAX. Distributed pilot tones used or timing and requency error estimation. 31 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

32 Common Wideband Receiver Architectures PROPRIETARY & CONFIDENTIAL BPF PLL LNA 2x Freq Conv I,Q I,Q LPF I,Q ADC C = VCO 2 -or- 1 C = VCO 1 ± K Direct Conversion Frequency planning Run VCO at oset rom carrier Div-by-2 Oset-LO generation D.C. osets IIP2 BPF LNA 2x 4x I,Q I,Q I,Q LPF I/Q PLL I/Q /K I/Q C ADC 1 = VCO 1 ± K Sliding IF Frequency planning Run VCO at oset rom carrier Dual-conversion LO power consumption D.C. oset issues are alleviated somewhat 32 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

33 A Typical Wireless PLL Fractional N Σ Modulated Loop Fine requency resolution & support or multiple crystal requencies Coarse Tuning Minimize Kvco to keep spurious under control Kvco Compensation Maintain loop dynamics over VCO ine-tuning range Freq. Conversion For direct-conversion i systems, VCO is o-requency 33 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

34 Orthogonality and PLL Phase Noise Signal alling in the n th requency bin: Q n = T s 1 j 2πn t x() t e T s 0 dt 2 sinc n I Equivalent requency-domain power weighting unction: σ 2 n 2 2 n = X ( ) sinc d Q Phase noise produces two eects Common Phase Error Phase noise impressed on each sub-carrier causes symbol rotation o that sub-carrier Net noise correlated among all sub-carriers Phase Noise ICI (Reciprocal Mixing) Phase noise impressed on each sub-carrier causes ICI to neighboring sub-carriers Net noise is uncorrelated between sub-carriers I 34 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

35 Calculating RCE Due to Phase Noise Assume that the data on each subcarrier is random and uncorrelated among subcarriers. RMS noise alling on n th subcarrier is the superposition o ICI due to subcarriers to the let and right. Number o subcarriers on each side varies with n Deine an eective SSB P/N weighting unction Total RCE is approx. given by: 2 σ ε = 2 N min L ( ) W ( ) e d where min is set by the Rx carrier requency tracking loop bandwidth. In practice, RCE is slightly less, accounting or pilot boosting. k = N 2 n N 2 m = N 2 + n N 2 35 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

36 Rx Spurious Emissions LNA BPF LPF ADC 1 XO harmonics LO Leaka kage -and ndrence Reere spurs 2 3 Oset spurs I/Q C = VCO 1 1 ± M K PFD CP LF /M Type Spur Frequency DIV /K 1 2 n n XO ± m C XO 3 n ± m M ± p K C VCO VCO In a direct-conversion conversion Rx, VCO typically runs at a large requency oset rom carrier to avoid time-varying DC osets Many sources o spurs need to be managed In-band: XO harmonics and reerence spurs Out-o-band: Oset LO generation spurs (radiated and conducted) 36 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

37 Rx Spurious Limits Emissions Limits are speciied in CEPT/ERC recommendation 74-01E spur Reception > 0 S n splatter Spurs received by the Rx chain have to be compared to the sub-carrier sensitivity -129dBm / S.C. I spurs are o-requency, then spectral splatter results. A single spur can eect multiple sub- carriers! XO harmonics are particularly vexing. WiMAX Speciication Conditions Min Typ Max Units In-Band Spurious Re. LNA Input -135 dbm Out-o-Band Spurious Re. LNA Input -59 dbm 37 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

38 Outline Wireless Channels OFDM Modulation OFDMA Multiple Access in WiMAX Systems Implementation Challenges: What Limits Orthogonality Summary 38 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

39 Summary Wireless channels present a challenging environment or broadband communication where signals are scattered in time and requency Time-variance / mobility Multipath th Fading (shadow and Rayleigh) OFDM modulation is well-suited to broadband communication over wireless channels Tolerant o multipath delay-spread Tolerant o Rayleigh ading OFDMA modulation provides a scalable solution or multiple-access in broadband wireless systems Flexible bandwidth allocation / multiple access Flexible network requency planning Compatible with advanced diversity / MIMO techniques Transciever challenges or maintaining orthogonality were outlined Timbase accuracy Spurious intererence (sel- or external-) PLL phase noise 39 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

40 Reerences 1. Donald C. Cox, R.R. Murray and A.W. Norris, Universal Digital Portable Radio Communications, Proceedings o the IEEE, vol. 75, no. 4, April 1987, pp Donald C. Cox, 800-MHz Attenuation ti Measured In and Around Suburban b Houses, AT&T Bell Laboratories Technical Journal, vol. 63, no. 6, July-August 1984, pp B. Sklar, Rayleigh Fading Channels, in The Mobile Communications Handbook, CRC Press, 1999, pp to E. Biglieri, J. Proakis and S. Shamai, Fading Channels: inormation-theoretic and Communications Aspects, IEEE Transactions on Inormation Theory, vol. 44, no. 6, October 1998, pp P802.16Rev2/D1 (October 2007) (Revision o IEEE Std as amended by IEEE Std and IEEE Std e-2005) 5. S. Lloyd and L. Jalloul, e WiMAX Key System and Circuit Design Issues, Workshop WK151, RFIC WiMAX Forum Mobile System Proile Release 1.0 Approved Speciication (Revision 1.5.0: ) 7. WiMAX ForumTM Mobile Radio Conormance Tests (MRCT)Release 1.0 Approved Speciication (Revision i 2.0.0) 0) 40 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

41 Reerences 8. Wilma Forum User Equipment Coexistence Emission Mask or MHz Band or 10 MHz and 5 MHz Channels 9. CEPT/ERC/RECOMMENDATION 74-01E 10. I. Vassiliou, et al, A single-chip digitally calibrated GHz 0.18-mm CMOS transceiver or a wireless LAN, IEEE Journal o Solid-State Circuits, vol. 38, no. 12, December J.K. Cavers, New methods or adaptation o quadrature modulators and demodulators in ampliier linearization circuits, IEEE Transactions on Vehicular Technology, vol. 46, no. 3, pp , August M. Locher, A low power, high perormance BiCMOS MIMO/diversity direct conversion transceiver IC or WiBro/WiMAX (802.16e), IEEE 2007 Custom Integrated Circuits Conerence, pp J. Stott, The eects o phase noise in COFDM, EBU Technical Review, Summer P802.16Rev2/D1 (October 2007) (Revision o IEEE Std as amended by IEEE Std and IEEE Std e-2005) 15. WiMAX Forum Mobile System Proile Release 1.0 Approved Speciication (Revision 1.5.0: ) 16. CEPT/ERC/RECOMMENDATION 74-01E 41 Channel Suring Derek K. Shaeer, Ph.D. January 21, 2010

ECE5984 Orthogonal Frequency Division Multiplexing and Related Technologies Fall Mohamed Essam Khedr. Channel Estimation

ECE5984 Orthogonal Frequency Division Multiplexing and Related Technologies Fall 2007 Mohamed Essam Khedr Channel Estimation Matlab Assignment # Thursday 4 October 2007 Develop an OFDM system with the