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CS 598 HL: Advanced Topics on Wireless Networks

CS 598 HL: Advanced Topics on Wireless Networks. Aug 25, 2006. Today. Wireless communications basics Modulation, channel capacity Bandwidth acquisition and management, access technologies Challenges to wireless communications Channel coding. Communication Basics.

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CS 598 HL: Advanced Topics on Wireless Networks

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  1. CS 598 HL: Advanced Topics on Wireless Networks Aug 25, 2006 Overview

  2. Today • Wireless communications basics • Modulation, channel capacity • Bandwidth acquisition and management, access technologies • Challenges to wireless communications • Channel coding Overview

  3. Communication Basics • PHY layer in ISO protocol stack • Physical media characteristics • Modulation • Channel coding symbols Channel Coding information Source Coding bits: 01010100 Modulation Transmitter analog Sender physical media analog Receiver De-modulation Receiver bits: 01010100 decoding Overview

  4. Modulation • We want to transmit bits (0 and 1) • Physical media transmit analog signals, NOT bits • Telephone line, cable • Ethernet, serial and parallel cable • Fiber • Free space • Ride digital information (0 and 1 bits) on analog signals • Modulation and demodulation Overview

  5. Three Types of Modulation • Amplitude modulation • Frequency modulation • Phase modulation Overview

  6. Amplitude Modulation 0 0 1 1 0 1 0 Overview

  7. Frequency Modulation • Frequency changes slightly Overview

  8. AM-FM Comparisons • Amplitude modulation (AM) • More susceptible to noise that affects the instantaneous strength/amplitude of signal • Used in AM radio broadcast • Digital version: ASK (amplitude shift keying) • Frequency modulation (FM) • Used in FM radio broadcast, AMPS Cellular (analog) • For AMPS analog speech, frequency changes continuously up to ±12 KHz • Digital version: FSK (frequency shift keying) Overview

  9. QPSK Bit value Phase Shift 00 None 01 1/4 10 1/2 11 3/4 QPSK 00 00 10 10 00 00 00 Phase Shift Keying Binary PSK Bit Value Phase Shift 0 None 1 1/2 3/4 phase shift BPSK 0 0 1 1 0 0 0 Overview

  10. Comparisons • Phase modulation (PM) • Can be used for analog signal with continuously varying phase • Digital version: PSK. • PSK – many variations • BPSK • Use alternative waveforms to encode 0 and 1, 1 bit per symbol • GSM uses a form of BPSK called Gaussian Minimum Shift Keying (GMSK) • 802.11 at 1Mbps uses BPSK • QPSK • Multi-level modulation, phases represent 00, 01, 10, or 11 • 2 bits per symbol • Used in IS-95 CDMA • 802.11b at 2, 5.5, and 11Mbps uses QPSK • DQPSK: Differential QPSK • Phase changes represent 00, 01, 10, or 11 • Used in IS-54 and IS-136, North American digital TDMA Overview

  11. Bit Rate • Symbol rate: • Signal parameter change speed • Number of bits per symbol: • Modulation, multi-level modulation • Bit Rate: • symbol rate * number of bits per symbol • Measured in bps (bits per second) • What limit bit rate? • Constraint on symbol rate • Constraint on number of bits per symbol Overview

  12. bandwidth modulated time frequency flow fhigh f Constraints on Symbol Rate • Modulated signal occupies certain frequency band • Bandwidth is approximately symbol rate • High symbol rate => high bandwidth signal carrier single frequency f frequency Overview

  13. Constraints on Symbol Rate • Physical media (channel) has certain bandwidth B • Frequency components beyond physical media’s bandwidth will be removed –-- inter-symbol interferences at receiver • Constraint on the speed we change the carrier signal parameters (amplitude, frequency, and/or phase) received un-modulated signal sender’s symbol rate Overview

  14. Constraint on Number of Bits per Symbol • Signal distortion due to noise and interference • Cause de-modulation error • Signal power to noise/interference power: S/N Overview

  15. Bit Rate • C = B*log2(1+S/N) b/s • B is limited by physical media bandwidth • S/N is limited by transmission power, lost, interferences, and noises. • Towards high bit-rate C: • Use high-bandwidth wire: B • Acquire more wireless bandwidth: B • Increase received signal strength: S • Decrease interferences and noise: N Overview

  16. Wired: Telephone and DSL • Use the same physical media • Work at different frequency bands, with different bandwidth upstream data voice downstream data 0~4 KHz 240K ~ 1.5M Hz 25~160K Hz B = 247 * 4K Hz B = 135 KHz 640 Kbps upstream, 8 Mbps downstream 64~640 Kbps upstream, 1.5 Mbps downstream in practice Overview

  17. Wired: Cable • Physical medium: coaxial cable 0 MHz 6 MHz / channel 1000 MHz 850 MHz + Downstream B = 6 MHz 5~65 MHz Upstream B = 2 MHz • Use QAM modulation in one channel • 30~40 Mbps, shared by up to 1000 users Overview

  18. Wireless Frequency Band • A single common physical medium: free space • Authoritative management is necessary to avoid interferences (decrease N) • FCC: Federal Communications Commission infra-red fiber optics Overview

  19. Radio Frequency Spectrum • Cellular and PCS • ISM unlicensed: Bluetooth, 802.11 Overview

  20. Cellular and PCS Spectrum Cellular PCS • 1974, FCC allocated 40MHz for cellular. 10MHz was added later • 1994, FCC allocated • 30 MHz of unlicensed PCS spectrum • 20 MHz for data (1910-1920, 2390-2400MHz), e.g., WLANs • 10 MHz for voice (1920-1930 MHz), e.g., W-PBXs • 120 MHz of licensed PCS spectrum • 6 licenses been auctioned (A-F) • A company may accumulate up to 40 MHz within an area (MTA, BTA) Overview

  21. Acquire PCS License • How FCC awarded PCS licenses? • Auctions! • FCC has completed 3 broadband-PCS license auctions: • A and B frequency blocks (99 MTA licneses) were auctioned for more than $7B, 1995 March • C block auction (492 BTA 30 MHz license), raised $10.22B in May, 1996 • D, E, and F blocks (10MHz licenses in each of 492 BTAs) raised over $2.5B in January, 1997 Overview

  22. After Bandwidth Acquisition • Make the best use of the expensive wireless bandwidth - Wireless resource management • Space-domain: cellular concept • Multiple access technologies • Frequency • Time • Code Overview

  23. Cellular Concept – Before Cellular • All 420 channels are used over entire metropolitan area • Only a max of 420 calls can be supported simultaneously • Power came down from mountain • Large area, low capacity • NY city in 1976 • 543 customers • 3700 on waiting list Overview

  24. Cellular Concept • One base station in a cell • Assign channels into cells • Guarantee min reuse distance D: the distance between a base station and the nearest base station using the same channels • Reuse factor: • N >= 1/3 (D/R)2 • Based on required S/N • Resource management for FDMA and TDMA Overview

  25. Access Technologies • Key multiple access techniques • FDMA: frequency division multiple access • Each user is assigned an individual frequency channel (carrier) • TDMA: time division multiple access • Multiple users are each assigned different time slots on a common frequency channel • CDMA: code division multiple access • Multiple users spread their energy over a common wide frequency band • Spread spectrum technology Overview

  26. FDMA • AMPS contains 832 carriers, bw 30 KHz each • Each user is assigned 2 carriers, one uplink one downlink frequency carrier time Overview

  27. TDMA • Different users are assigned different time slots in a frame frequency time frame time slot Overview

  28. TDMA • A physical channel is a time slot within one carrier • Used in IS-54 and IS-136 (North America TDMA) • Use the same 30 KHz carrier as in AMPS – backward compatible • Increase the number of channels by a factor of 6 – 6 time slots in a frame frequency carrier time frame time slot Overview

  29. Code Division Multiple Access • Multiple users (calls) share the same frequency band (v.s. FDMA) at the same time (v.s. TDMA) • Each user (call) is assigned a unique code sequence. Code sequences of different users are orthogonal. Overview

  30. CDMA w/ Direct Sequence Spread Spectrum • Occupies bw ~ N times the bw of the original signal • 1/T : chip rate (c/s) signal PN code sequence transmitted code-modulated signal Overview

  31. CDMA DSSS Demodulation received signal receiver’s PN code sequence Correlator: accumulate or integrate: Summing N chips during one “bit time” to increase confidence. 24T 48T Overview

  32. Inter-user Interferences Sender 1 • Sender 1’s PN code sequence (N=4): • PN1 = [1, -1, 1, -1] • Sender 2’s PN code sequence (N=4): • PN2 = [1, 1, -1, -1] • PN1 and PN2 are orthogonal: • PN1 · PN2 = +1 -1 -1 +1 = 0 • PN1 · PN1 = PN2 · PN2 = N = 4 • Sender 1 transmits a 3-bit string: S1 = 101, to receiver 1 • Transmitted signal: (PN1)(-PN1)(PN1) = 1,-1,1,-1,-1,1,-1,1,1,-1,1,-1 • Sender 2 transmits a 3-bit string: S2 = 011, to receiver 2 • Transmitted signal: (-PN2)(PN2)(PN2) = -1,-1,1,1,1,1,-1,-1,1,1,-1,-1 • Combined transmitted signal: • (PN1-PN2)(-PN1+PN2)(PN1+PN2) = 0,-2,2,0,0,2,-2,0,2,0,0,-2 • Receiver receives (PN1-PN2)(-PN1+PN2)(PN1+PN2) • Apply PN1 • PN1 · (PN1-PN2)(-PN1+PN2)(PN1+PN2) = (PN1 · PN1)(-PN1 · PN1)(PN1 · PN1) = 4,-4,4 101 = S1 • Apply PN2 • PN2 · (PN1-PN2)(-PN1+PN2)(PN1+PN2) = -4,4,4 011 = S2 Sender 2 Overview

  33. Walsh Hadamard Matrix • Generate orthogonal PN code sequences • W1 = [0]; W2 = [0, 0; 0, 1] • WL = [WL/2, WL/2; WL/2, W’L/2] • W4 = [ 0 0 0 0; 0 1 0 1; 0 0 1 1; 0 1 1 0 ] • All rows are mutually orthogonal • W64 used in CDMA forward link Overview

  34. CDMA Two Stage Modulation occupy BW N times that of the symbol rate PN code sequence • Multiple users communicate Overview

  35. CDMA Analogy Overview

  36. CDMA • 12 ~ 24 calls per sector per 1.25 MHz in IS-95 North America CDMA systems (cdmaOne) • Good speech quality in the presence of all co-channel interferences • Efficient frequency reuse: key to CDMA’s capacity • Inefficient if there were only one cell • Re-use EVERY frequency in EVERY sector and cell of a system provides great capacity gains! Overview

  37. W-CDMA • A U.S. CDMA PCS standard • Allow use of the maximum bandwidth spreading feasible with service provider’s spectrum. Can operate with bandwidth of • 5 MHz, or • 10 MHz, or • 15 MHz • Different from ITU’s 3G “W-CDMA” Overview

  38. Access Technology Summary • Share the allocated bandwidth B in • Frequency domain • Spatial domain • Time domain • Spatial domain sharing • Divide service area into cells • Frequency domain sharing: • Divide allocated bandwidth into frequency bands • Time domain sharing • Divide each frequency band into frames of time slots Overview

  39. “Channel” Definition • FDMA: • Certain frequency band [flow, fhigh] in one cell • Defined in frequency and spatial domain • TDMA: • Certain time slot in one frequency band in one cell • Defined in time, frequency, and spatial domains • CDMA: • Certain PN code in one (wide) frequency band • Real systems use combinations of the above Overview

  40. 2G Cellular and PCS Overview

  41. Challenges to Wireless Communications • Attenuation due to distance • Slow fading due to random environmental effects • Fast fading due to mobility and multi-path propagation • Inter-symbol interferences due to multi-path propagation Overview

  42. Attenuation • d: distance between transmitter and receiver • Free space model:  = 2 (inverse proportional to the area of propagation sphere) • Metropolitan area:  = 4 Overview

  43. Slow Fading • Environmental obstacles (shadow fading) • further decrease Preceive • Preceive rises and falls with significant changes • Over 10’s of meters • Even with constant distance from the transmitter • 3 ~ 8 dB degradation (Preceive/Ptransmit = 0.5 ~ 0.16) slow fading Overview

  44. Fast Fading • One signal travels /2 (3/2, 5/2, 7/2 etc.) farther than another signal will tend to cancel (subtract) • One signal travels  (2, 3, etc.) farther than another signal will tend to strengthen (add) Overview

  45. Fast Fading • “Fades” (subtractions) can attenuate up to 30 dB (1/1000th power) • “Adds” can be up to 10dB (10x power) • Fades come and go about every 6 inches! • Walking: 3 ft/sec • Driving: 88 ft/sec = 60 mph Overview

  46. Fast Fading • Modeled with Rayleigh statistics (Rayleigh fading): • When all signals have the same strength fast fading slow fading /2 Overview

  47. Inter-symbol Interferences • For TV: delayed version of last frame creates “Ghosts” or “echoes” Overview

  48. Challenges of Wireless Communications • Space-domain dynamics: • Attenuation: • Cellular network layout • Repeater, equalizer • Smart antenna (directional) • Slow fading • Power control • Fast fading • Time-domain dynamics: • Inter-symbol interferences Overview

  49. Dealing Multi-path Distortion • Diversity • CDMA RAKE receiver • Multiple antennas • OFDM: orthogonal frequency division multiplexing Overview

  50. CDMA RAKE Receivers • Multiple correlators synchronized to different propagation paths • Multi-path diversity combining Overview

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