1 / 36

Principles of Spread Spectrum

Principles of Spread Spectrum. Lecture 4. Objectives. List and describe the wireless modulation schemes used in IEEE WLANs Tell the difference between frequency hopping spread spectrum and direct sequence spread spectrum

eliora
Télécharger la présentation

Principles of Spread Spectrum

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Principles of Spread Spectrum Lecture 4

  2. Objectives • List and describe the wireless modulation schemes used in IEEE WLANs • Tell the difference between frequency hopping spread spectrum and direct sequence spread spectrum • Explain how orthogonal frequency division multiplexing is used to increase network throughput

  3. Introduction Figure 4-2: OSI data flow

  4. Introduction (continued) Table 4-1: OSI layers and functions

  5. OSI model

  6. WLan technologies

  7. Infrared

  8. Narrowband Transmission • Narrowband transmission used primarily by radio stations • Radio signals by nature transmit on only one radio frequency or a narrow portion of frequencies • Require more power for the signal to be transmitted • Signal must exceed noise level • Total amount of outside interference • Vulnerable to interference from another radio signal at or near same frequency • IEEE 802.11 standards do not use narrowband transmissions

  9. Narrowband Transmission (continued) Figure 4-3: Narrowband transmission

  10. Spread spectrum

  11. Spread Spectrum Transmission Figure 4-4: Spread spectrum transmission

  12. Spread Spectrum

  13. Spread Spectrum Transmission (continued) • Advantages over narrowband: • Resistance to narrowband interference • Resistance to spread spectrum interference • Lower power requirements • Less interference on other systems • More information transmitted • Increased security • Resistance to multipath distortion

  14. Two approaches for SS

  15. Frequency Hopping Spread Spectrum (FHSS) • Uses range of frequencies • Change during transmission • Hopping code: Sequence of changing frequencies • If interference encountered on particular frequency then that part of signal will be retransmitted on next frequency of hopping code • FCC has established restrictions on FHSS to reduce interference • Due to speed limitations FHSS not widely implemented in today’s WLAN systems • Bluetooth does use FHSS

  16. Frequency Hopping Spread Spectrum (continued) Figure 4-6: FHSS error correction

  17. Frequency hopping

  18. Direct Sequence

  19. Direct Sequence Spread Spectrum (DSSS) • Uses expanded redundant code to transmit data bits • Chipping code: Bit pattern substituted for original transmission bits • Advantages of using DSSS with a chipping code: • Error correction • Less interference on other systems • Shared frequency bandwidth • Co-location: Each device assigned unique chipping code • Security

  20. Direct Sequence Spread Spectrum (continued) Figure 4-7: Direct sequence spread spectrum (DSSS) transmission

  21. DSS frequency change plan

  22. Radio Modulation

  23. Orthogonal Frequency Division Multiplexing (OFDM) • With multipath distortion, receiving device must wait until all reflections received before transmitting • Puts ceiling limit on overall speed of WLAN • OFDM: Send multiple signals at same time • Split high-speed digital signal into several slower signals running in parallel • OFDM increases throughput by sending data more slowly • Avoids problems caused by multipath distortion • Used in 802.11a networks

  24. Orthogonal Frequency Division Multiplexing (continued) Figure 4-8: Multiple channels

  25. Orthogonal Frequency Division Multiplexing (continued) Figure 4-9: Orthogonal frequency division multiplexing (OFDM) vs. single-channel transmissions

  26. Comparison of Wireless Modulation Schemes • FHSS transmissions less prone to interference from outside signals than DSSS • WLAN systems that use FHSS have potential for higher number of co-location units than DSSS • DSSS has potential for greater transmission speeds over FHSS • Throughput much greater for DSSS than FHSS • Amount of data a channel can send and receive

  27. Comparison of Wireless Modulation Schemes (continued) • DSSS preferred over FHSS for 802.11b WLANs • OFDM is currently most popular modulation scheme • High throughput • Supports speeds over 100 Mbps for 802.11a WLANs • Supports speeds over 54 Mbps for 802.11g WLANs

  28. Key modulation techniques

  29. Modulation techniques used by 802.11a • Modulation techniques used to encode 802.11a data vary depending upon speed • Speeds higher than 54 Mbps may be achieved using 2X modes Table 4-7: 802.11a characteristics

  30. Physical Layer Standards (continued) Figure 4-19: Phase shift keying (PSK)

  31. Physical Layer Standards (continued) Figure 4-20: Quadrature phase shift keying (QPSK)

  32. Physical Layer Standards (continued) Figure 4-21: 16-level quadrature amplitude modulation (16-QAM)

  33. Physical Layer Standards (continued) Figure 4-22: 64-level quadrature amplitude modulation (64-QAM)

  34. Summary • Three modulation schemes are used in IEEE 802.11 wireless LANs: frequency hopping spread spectrum (FHSS), direct sequence spread spectrum (DSSS), and orthogonal frequency division multiplexing (OFDM) • Spread spectrum is a technique that takes a narrow, weaker signal and spreads it over a broader portion of the radio frequency band • Spread spectrum transmission uses two different methods to spread the signal over a wider area: FHSS and DSSS

  35. Summary (continued) • OFDM splits a single high-speed digital signal into several slower signals running in parallel

  36. Lab • 3-3 • 4-1 and 4-3 from text book

More Related