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Issues in Wireless Physical Layer

Issues in Wireless Physical Layer. A. Chockalingam Assistant Professor Indian Institute of Science, Bangalore-12 achockal@ece.iisc.ernet.in http://ece.iisc.ernet.in/~achockal. Outline. RF Spectrum Issues Wireless Channel Characteristics Combating Fading Diversity Techniques

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Issues in Wireless Physical Layer

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  1. Issues in Wireless Physical Layer A. Chockalingam Assistant Professor Indian Institute of Science, Bangalore-12 achockal@ece.iisc.ernet.in http://ece.iisc.ernet.in/~achockal

  2. Outline • RF Spectrum Issues • Wireless Channel Characteristics • Combating Fading • Diversity Techniques • Transmit Diversity • Multiple Access • Power Control • Co-channel Interference • Ultra Wideband Techniques Dept of ECE, IISc, Bangalore

  3. Radio Frequency Spectrum • Communication through electromagnetic wave propagation • Frequency Spectrum • Certain ranges of frequency • Only certain frequency spectra are usable • Limitations of atmospheric propagation effects • Technology/Device limitations • Regulatory issues • Safety hazards • Demand for spectrum far exceeds supply • Efficient use of RF spectrum is important Dept of ECE, IISc, Bangalore

  4. RF Spectrum - Some Current systems • 900 MHz Cellular Band • GSM: 890 - 915 MHz Uplink; 935 - 960 MHz Downlink • IS-54: 824 - 849 MHz Uplink; 869 - 894 MHz Downlink • PDC: 810 - 820 MHz and 1429 - 1453 MHz Uplink 940 - 960 MHz and 1477 - 1501 MHz Uplink • IS-95: 824 - 844 MHz Uplink; 869 - 889 MHz Downlink • 1800 MHz PCS Band • 1850 - 1910 MHz Uplink; 1930 - 1960 MHz Downlink • DECT: 1880 - 1900 MHz • C, Ku, L and S-Bands for SATCOM • C-band: 5.9 - 6.2 GHz Uplink; 3.7- 4.2 GHz: Downlink • Ku-band: 14 GHz Uplink; 12 GHz Downlink • L-band: 1.61 - 1.6265 GHz; S-band: 2.4835 - 2.5 GHz Dept of ECE, IISc, Bangalore

  5. Unlicensed Radio Spectrum Carrier wavelength: 33 cm 12 cm 5 cm 26 MHz 83.5 MHz 200 MHz 5.35 MHz 2.4 GHz 902 MHz 5.15 GHz 2.4835 GHz 928 MHz • Wireless LANs • Cordless phones • 802.11b • Bluetooth • Microwave Oven • 802.11a Dept of ECE, IISc, Bangalore

  6. RF Spectrum • Some forward looking developments • 300 MHz BW in the 5 GHz band made available to stimulate Wireless LAN technologies and use • Ultra wideband (UBW) technology • 60 GHz band for high-speed, short-range communications Dept of ECE, IISc, Bangalore

  7. Physical Layer Tasks • Wireless systems need to overcome one or more of the following distortions: • AWGN (receiver thermal noise) • Receiver carrier frequency and phase offset • Receiver timing offset • Delay spread • Fading (without or with LOS component) • Co-channel and adjacent interference (CCI, ACI) • Nonlinear distortion, intermodulation, impulse noise Dept of ECE, IISc, Bangalore

  8. Motivation for PHY Layer Advances • Increase channel capacity (spectral efficiency) - higher average bit rate • Increase Erlang Capacity - more users per square area • Increase reliability • Reduce Tx power • Increase range • Increase coverage Dept of ECE, IISc, Bangalore

  9. PHY Layer Advances Erlang Capacity Spectral Efficiency Transmit Diversity Spatial Multiplexing OFDM Sectorisation Link Adaptation Space-Time Coding Variable Bit-Rate Voice Activity Detection Transmit Diversity Frequency Hopping Receive Diversity Smart Beam-forming Turbo Coding Interference Suppression DS-CDMA Fixed Beamforming Power Control Range (Power Efficiency) Multi-user Detection Dynamic Channel Selection Dept of ECE, IISc, Bangalore

  10. Wireless Channel Characteristics • Free-space Transmission ( ) Rx Tx Dept of ECE, IISc, Bangalore

  11. Mobile Radio Channel • Characterized by • Free space (distance) loss • Long-term fading (shadowing) • Short-term fading (multipath fading) Dept of ECE, IISc, Bangalore

  12. Mobile Radio Channel Short Term Fading 0.1 - 1 m (10 - 100 msecs) Received Power Distance Loss Long Term Fading 10 - 100 m (1 - 10 secs) Distance, d Dept of ECE, IISc, Bangalore

  13. Distance Loss • In line-of-sight AWGN channels (AWGN: Additive White Gaussian Noise) • distance loss ,: distance between Tx and Rx • loss exponent is 2(i.e., 20 dB/decade loss) • In urban mobile radio channels • loss exponent varies between 2.5 to 5.5 • 40 dB/decade loss (typ)Rx Signal power (Based on field measurements) • Slowly varying compared to carrier wavelength • Fwd & Rev links impacted in the same way 40 dB 40 dB/decade 40 dB 1 km 10 m 100 m Dept of ECE, IISc, Bangalore

  14. Shadowing • Signals are blocked by obstacles (e.g., bridges buildings, trees, etc) • Shadow loss variation - typ log-normally distributed (Std Dev of distribution: 4 to 12 dB) • Slowly varying compared to carrier wavelength • Fwd & Rev links impacted in the same waybri Dept of ECE, IISc, Bangalore

  15. Multipath Propagation Base Station Tx. signal Rx. signal Channel Path 1 Path 2 Impulse Response Path n Mobile Frequency Response Dept of ECE, IISc, Bangalore

  16. Multipath (Short term) Fading • Time-varying impulse response • Fluctuations in received signal amplitude (typically Rayleigh distributed) • Time spread • Doppler Spread • Fade variations are fast • Rev link fading independent of Fwd link fading Signal Strength Rev link fade Fwd link fade time Dept of ECE, IISc, Bangalore

  17. Key Multipath Parameters • Delay / Frequency Characterization • Delay spread, • Coherence BW, • Time variations • Coherence time, • Doppler BW, Dept of ECE, IISc, Bangalore

  18. Delay Spread / Coherence BW • Autocorrelation function of • If we let , gives the average power output of the channel as a function of Autocorrelation FT Max. Delay Spread FT Pair Coherence Bandwidth Dept of ECE, IISc, Bangalore

  19. Delay / Frequency Characterization • Delay Spread • range of differential delay between different paths • jitter in Rx time of the signal, long echoes • results in Inter-Symbol Interference (ISI). • Need equalization to combat ISI(in unspread systems) • Provides “time Diversity” in spread systems (RAKE Combining in CDMA) • Coherence BW • BW over which fade remains constant or have strong amplitude correlation Dept of ECE, IISc, Bangalore

  20. Delay / Frequency Characterization • Frequency non-selective fading • Coherence BW > Signal BW • Frequency selective fading • Coherence BW < Signal BW: Dept of ECE, IISc, Bangalore

  21. Time Variations • Coherence Time • Time over which fade remains constant or have strong amplitude correlation • Coherence time > symbol time : Slow fading • Coherence time < symbol time : Fast fading • Doppler BW • frequency shift on the carrier frequency due to relative motion between Tx and Rx • depends on user velocity and carrier wavelength • Note: Dept of ECE, IISc, Bangalore

  22. Doppler Bandwidth : mobile velocity : carrier wavelength : carrier frequency ForMHz, m Km/h, Hz • Larger Doppler Bandwidth necessitates • Larger power control control update • rates in CDMA • Faster converging algorithms when • adaptive receivers are employed Dept of ECE, IISc, Bangalore

  23. Effect of Fading Fading AWGN Non-fading AWGN Channel: falls exponentially with increasing SNR Fading Channel: falls linearly with increasing SNR Dept of ECE, IISc, Bangalore

  24. Combating Fading Effects • Diversity techniques • Provide the receiver with multiple fade replicas of the same information bearing signal • Assume independent diversity branches • If denote the probability that the instantaneous SNR is below a given threshold on a particular diversity branch • Then, the probability that the the instantaneous SNR is below the same threshold on diversity branches is Dept of ECE, IISc, Bangalore

  25. SISO to MIMO • Single Input Single Output (SISO) • LOS point-to-point links • Single Input Multiple Output (SIMO) • Receiver diversity • Multiple Input Single Output (MISO) • Transmit diversity • Space time transmission • Multiple Input Multiple Output (MIMO) • Multiple transmitting and multiple receiving antennas Dept of ECE, IISc, Bangalore

  26. Receive Diversity Techniques • Several methods by which receive diversity can be achieved include • Space diversity • Time diversity (coding/interleaving can be viewed as a efficient way of time diversity) • Frequency diversity (multiple channels separated by more than the coherence BW) • Multipath diversity (obtained by resolving multipath components at different delays) • Angle/Direction diversity (directional antennas) • Macro diversity Dept of ECE, IISc, Bangalore

  27. Receive Diversity Combining • Method by which signals from different diversity branches are combined • Predetection Combining • Postdetection combining • With ideal coherent detection there is no difference between pre- and postdetection combining • With differentially coherent detection, there is a slight difference in performance Dept of ECE, IISc, Bangalore

  28. Receive Diversity Combining • Maximal Ratio Combining (MRC) For BPSK: • Equal Gain Combining (EGC) • Selection Combining (SC) where • Generalized Selection Combining (GSC) • Switch and Stay Combining (SSC) Dept of ECE, IISc, Bangalore

  29. Diversity Performance Fading (L=1) L=2 AWGN L=3 L=4 Average SNR • Diversity gain is maximum when the diversity branches are • uncorrelated. • Correlation between diversity branches reduces diversity gain • Diversity gain is greater for Raleigh fading than for Ricean Dept of ECE, IISc, Bangalore

  30. Transmit Diversity • Issue: Receive diversity at the mobile is difficult because of space limitations • Using multiple transmit antennas at the base station with a single receive at the mobile can give same diversity benefits • Tx. Diversity schemes • with feedback from the mobile • without feedback from the mobile Dept of ECE, IISc, Bangalore

  31. Transmit Diversity Tx Rx Dept of ECE, IISc, Bangalore

  32. Spatial Multiplexing • Use N Tx antennas and M Rx antennas (N < M) • by sending N symbols at a time Rx Tx Channel Matrix Dept of ECE, IISc, Bangalore

  33. Co-channel Interference • Frequencies reused in different cells to increase capacity • Reuse Distance: • Minimum distance between cells using same frequencies • Cell Radius: • Reuse Ratio: Dept of ECE, IISc, Bangalore

  34. Co-channel Interference • S/I : Signal-to-Interference Ratio • For same size cells, co-channel interference (CCI) becomes a function of and • Increasing reduces CCI : path loss exponent (=4 typ) : No. of co-channel cells • S/I required = 18 dB (typ) => cluster size N > 6.49 • For 7-cell reuse (N= 7), S/I = 18.7 dB Dept of ECE, IISc, Bangalore

  35. Co-Channel Interference • In FDMA/TDMA CCI determines the reuse distance • In CDMA, CCI affects the number of users supported by a BS • CCI can be reduced by • Sectorization • Power Control • Discontinuous Transmission • Frequency Hopping • Multiuser detection Dept of ECE, IISc, Bangalore

  36. Multiple Access • FDMA • AMPS • TDMA • GSM, EDGE, DECT, PHS • CDMA • IS-95, WCDMA, cdma2000 • OFDM (can be viewed as a spectrally efficient FDMA) • 802.11a, 802.11g, HiperLAN, 802.16 Dept of ECE, IISc, Bangalore

  37. OFDM Tones Carriers Power Frequency Time-slots Time Dept of ECE, IISc, Bangalore

  38. DS-CDMA vs OFDM Tx. signal Rx. signal Channel CDMA attempts to exploit “time-diversity” through RAKE receiver Impulse Response OFDM attempts to exploit “frequency-diversity” by frequency slicing Frequency Response Dept of ECE, IISc, Bangalore

  39. RAKE Receiver H*(f) Carrier L-Parallel Demodulators 90 H*(f) Dept of ECE, IISc, Bangalore

  40. RAKE Finger nTc H*(f) Carrier Initial timing from searcher Pilot Sequence Despreader Pilot Seq Tracking Loop (Early-Late Gate) 90 nTc H*(f) Dept of ECE, IISc, Bangalore

  41. Power Control • To combat the effect of fading, shadowing and distance losses • Transmit only the minimum required power to achieve a target link performance (e..g, FER) • Minimizes interference • Increases battery life • FL Power Control • To send enough power to reach users at cell edge • RL Power Control • To overcome “near-far” problem in DS-CDMA Dept of ECE, IISc, Bangalore

  42. Power Control • Types of Power Control • Open Loop Power Control • Closed loop Power Control • Open Loop Power Control (on FL) • Channel state on the FL is estimated by mobile • RL Transmit power made proportional to FL channel Loss • Works well if FL and RL are highly correlated • which is generally true for slowly varying distance and shadow losses • but not true with fast multipath Rayleigh fading • So open loop power control can effectively compensate for distance and shadow losses, and not for multipath fading Dept of ECE, IISc, Bangalore

  43. Power Control • Closed Loop Power Control (on RL) • Base station measures the received power • Compares it with the desired received power (target Eb/No) • Sends up or down command to mobile asking it to increase or decrease the transmit power • Must be performed fast enough a rate (approx. 10 times the max. Doppler BW) to track multipath fading • Propagation and processing delays are critical to loop performance Dept of ECE, IISc, Bangalore

  44. Ultra wideband (UBW) Techniques • Impulse Radio Tx (Marconi’s century old radio tx) has now emerged under the banner `ultrawideband • Reason: • mature digital techniques • practicality low power impulse radio communications • UWB • Tx and Rx of ultra-short (sub-nanosecs) electromagnetic energy impulses (or monocycles with few zero crossings) • FCC’s definition of UWB: • BW’s greater than 1.5 GHz or • or BW’s greater than 25% of the center frequency measured at 10 dB down points Dept of ECE, IISc, Bangalore

  45. UWB • Modern UWB radio is characterized by • very low effective radiated power (sub-mW range) • extremely low power spectral densities and wide bandwidths (> 1GHz) • EIRP < -41.25 dBm/MHz, with restrictions in bands below 960 MHz, between 1.99 and 10.6 GHz Dept of ECE, IISc, Bangalore

  46. UWB • Ways of generating signals having UWB characteristics • TM-UWB • Time modulated impulse stream • DS-UWB • continuous streams of PN-coded impulses (resemble CDMA signaling) • employ a chip rate commensurate with the emission center frequency • TRD-UWB • employs impulse pairs that are differentially polarity encoded by the data Dept of ECE, IISc, Bangalore

  47. UWB Capabilities • High spatial capacity • High channel capacity and scalability • Robust multipath performance • Very low transmit power • Location awareness and tracking Dept of ECE, IISc, Bangalore

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