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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
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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 • Explain how orthogonal frequency division multiplexing is used to increase network throughput
Introduction Figure 4-2: OSI data flow
Introduction (continued) Table 4-1: OSI layers and functions
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
Narrowband Transmission (continued) Figure 4-3: Narrowband transmission
Spread Spectrum Transmission Figure 4-4: Spread spectrum transmission
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
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
Frequency Hopping Spread Spectrum (continued) Figure 4-6: FHSS error correction
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
Direct Sequence Spread Spectrum (continued) Figure 4-7: Direct sequence spread spectrum (DSSS) transmission
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
Orthogonal Frequency Division Multiplexing (continued) Figure 4-8: Multiple channels
Orthogonal Frequency Division Multiplexing (continued) Figure 4-9: Orthogonal frequency division multiplexing (OFDM) vs. single-channel transmissions
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
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
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
Physical Layer Standards (continued) Figure 4-19: Phase shift keying (PSK)
Physical Layer Standards (continued) Figure 4-20: Quadrature phase shift keying (QPSK)
Physical Layer Standards (continued) Figure 4-21: 16-level quadrature amplitude modulation (16-QAM)
Physical Layer Standards (continued) Figure 4-22: 64-level quadrature amplitude modulation (64-QAM)
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
Summary (continued) • OFDM splits a single high-speed digital signal into several slower signals running in parallel
Lab • 3-3 • 4-1 and 4-3 from text book