Lecture 4 - Spread Spectrum Technologies
Spread Spectrum Technologies (1 September, 2006) February 2005 Copyright 2005 All Rights Reserved Objectives Upon completion of this chapter you will be able to: q Define spread spectrum technologies and how they are used q Describe modulation and the different data rates q Explain and compare FHSS, DSSS and OFDM q List the factors that impact signal throughput and range February 2005 Copyright 2005 All Rights Reserved Spread Spectrum q Spread spectrum is a communication technique that spreads a narrowband communication signal over a wide range of frequencies for transmission then de-spreads it into the original data bandwidth at the receive q Spread spectrum is characterized by: wide bandwidth and low power q Jamming and interference have less effect on Spread spectrum because it is: Resembles noise Hard to detect Hard to intercept February 2005 Copyright 2005 All Rights Reserved Narrowband vs Spread Spectrum Narrowband (High Peak Power) Power Spread Spectrum (Low Peak Power) Frequency February 2005 Copyright 2005 All Rights Reserved Narrow Band vs Spread Spectrum q Narrow Band Uses only enough frequency spectrum to carry the signal High peak power Easily jammed q Spread Spectrum The bandwidth is much wider than required to send to the signal Low peak power Hard to detect Hard to intercept Difficult to jam February 2005 Copyright 2005 All Rights Reserved Spread Spectrum Use q In the 1980s FCC implemented a set of rules making Spread Spectrum available to the public Cordless Telephones Global Positioning Systems (GPS) Cell Phones Personal Communication Systems Wireless video cameras q Local Area Networks Wireless Local Area Networks (WLAN) Wireless Personal Area Network (WPAN) Wireless Metropolitan Area Network (WMAN) Wireless Wide Area Network (WWAN) February 2005 Copyright 2005 All Rights Reserved FCC Specifications q The Code of Federal Regulations (CFR) Part 15 originally only described two spread spectrum techniques to be used in the licensed free Industrial, Scientific, Medical (ISM) band, 2.4 GHz, thus 802.11 and 802.11b Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence spread Spectrum (DSSS) q Orthogonal Frequency Division Multiplexing (OFDM) was not covered by the CFR and would have required licensing 802.11a, employing OFDM, was created to work in the 5GHz Unlicensed National Information Infrastructure (UNII) q In May, 2001 CFR, Part 15 was modified to allow alternative "digital modulation techniques" This resulted in 802.11g which employs OFDM in the 2.4 GHz range February 2005 Copyright 2005 All Rights Reserved Wireless LAN Networks q Wireless LANs RF spread spectrum management techniques Frequency Hopping Spread Spectrum (FHSS) Operates in the 2.4 Ghz range Rapid frequency switching – 2.5 hops per second w/ a dwell time of 400ms A predetermined pseudorandom pattern Fast Setting frequency synthesizers Direct Sequence Spread Spectrum (DSSS) Operates in the 2.4 GHz range Digital Data signal is inserted into a higher data rate chipping code p A Chipping code is a bit sequence consisting of a redundant bit pattern p Barker, Gold, M-sequence and Kasami codes are employed Orthogonal Frequency Division Multiplexing (OFDM) Operates in both the Ghz and 2.4 GHz range with a data rate of between and 54 Mbps 802.11a divides each channel into 52 low-speed sub-channels V 48 sub-channels are for data while the other are pilot carriers The modulation scheme can be either BPSK, QPSK or QAM depending upon the speed of transmission February 2005 Copyright 2005 All Rights Reserved FCC Radio Spectrum Band Name Range Usage VLF 10 kHz - 30 kHz Cable Locating Equipment LF 30 kHz - 300 kHz Maritime Mobile Service MF 300 kHz - MHz Aircraft navigation, ham radio and Avalanche transceivers HF and MHz - 30 MHz CB radios, CAP, Radio telephone, Radio Astronomy VHF Cars, 30 MHz - 328.6 MHZ Cordless phones, Televisions, RC Aircraft, police and business radios UHF radios, wireless 328.6 MHz - 2.9 GHz police radios, fire radios, business cellular phones, GPS, paging, networks and cordless phones SHF 2.9 GHz - 30 GHz Doppler weather radar, satellite communications EHF 30 GHz and above Radio astronomy, military systems, vehicle radar systems, ham radio February 2005 Copyright 2005 All Rights Reserved ISM Frequency Bands UHF ISM 902 - 928 Mhz S-Band - Ghz S-Band ISM (802.11b) 2.4 - 2.5 Ghz C-Band - Ghz C-Band Satellite downlink 3.7 - 4.2Ghz C-Band Radar (weather) 5.25 - 5.925 Ghz C-Band ISM (802.11a) 5.725 - 5.875 Ghz C-Band satellite uplink 5.925-6.425 Ghz X-Band 8-12 Ghz X-Band Radar (police/weather) 9.5-10.55 Ghz Ku-band 12-18 Ghz Ku-band Radar (Police) 13.5-15 Ghz 15.7-17.7 Ghz ISM - Industrial, Scientific and Medical February 2005 Copyright 2005 All Rights Reserved 10 802.11a q IEEE 802.11a Standard Orthogonal Frequency Division Multiplexing (OFDM) Operates in the 5.0 GHz band It Operates in the Unlicensed National Information Infrastructure (UNII) 200 channels ( channels 1-199) spaced MHz apart Supported data rates are 6, 9, 12, 18, 24, 36, 48, and 54, MBps 6, 12, and 24 are mandatory All others are optional 75-80 Feet 64 users /Access Point February 2005 Copyright 2005 All Rights Reserved 44 802.11a Network Channel Assignments Area Frequency Band USA U-NII Lower Band 36 5.180 Ghz (5.150-5.250 Ghz) 40 5.200 Ghz 44 5.220 Ghz 48 5.240 Ghz U-NII Middle Band 52 5.260 Ghz (5.250 – 5.350 Ghz) 56 5.260 Ghz 60 5.280 Ghz 64 5.320 Ghz U-NII Upper Band 149 5.745 Gh (5.725 – 5.825) 153 5.765 Ghz 157 5.785 Ghz 161 5.805 Ghz USA USA Channel Center Frequency NOTE: U-NII : Unlicensed National Information Infrastructure 802.11a is specific to the US February 2005 Copyright 2005 All Rights Reserved 45 OFDM q A mathematical process that allows 52 channels to overlap without losing their orthogonality (individuality) 48 sub-channel are used for data Each sub-channel is used to transmit data sub-channel are used as pilot carriers The pilot sub-channels are used to monitor path shift and shifts in sub-channel frequencies (Inter Carrier Interference (ICI)) OFDM OFDM selects channels that overlap but not interfere with one another Channels are separated based upon orthogonality February 2005 Copyright 2005 All Rights Reserved 46 802.11a Channels q 802.11a use the lower and middle UNII GHz bands to create channels Each Channel is 20 MHz each Each channel is broken into 52 sub-channels with each sub-channel 300 KHz each 48 Sub-channels are used to transmit data sub-channels are used as Pilot carriers to monitor the channel 52 Sub-Channels for each channels Each channel is 20 MHz wide Lower and Middle UNII frequency band Channels February 2005 Lower UNII Band Middle UNII Band Copyright 2005 All Rights Reserved 47 OFDM Modulation February 2005 Copyright 2005 All Rights Reserved 48 Modulation Background q In order to properly understand OFDM modulation we need to a quick review of various modulation techniques James Clark Maxwell, 1864, first developed the idea that electromagnetic magnetic waves arose as a combination electric current and magnetic field – an electromagnetic wave Heinrich Hertz , in 1880s, developed the first Radio Frequency device that sent and received electromagnetic waves over the air The name Hertz (Hz) was given to the unit of frequency measurement representing one complete oscillation of an electromagnetic wave This is also called cycle per second Kilohertz = thousands of cycles per second Megahertz = millions of cycles per second Gigahertz = billions cycles per second February 2005 Copyright 2005 All Rights Reserved 49 Modulation Background Contd q The oscillating electromagnetic wave, also called a sine wave, is shown below q This wave can be used as a carrier signal to carry information q The information can be imposed upon the carrier through a process called modulation which is accomplished by modifying one of three physical wave characteristic These physical characteristics are: Amplitude – The height of the wave Frequency – the number of oscillation (cycles) per second Phase – the starting point of the wave (when compared to the starting point of the previous wave q The are two major types of modulation schemes: Analog and Digital Frequency Sine Wave Phase February 2005 Amplitude Copyright 2005 All Rights Reserved 50 Analog Modulation Amplitude Modulation varies the height of the carrier wave Change in Amplitude Frequency Modulation varies the number of oscillation (waves) per second Phase Modulation changes the starting point of the wave Change in Frequency Change in Phase February 2005 = 1800 Phase Change = No Phase Change Copyright 2005 All Rights Reserved 51 Digital Modulation Amplitude Shift Keying (ASK) changes the amplitude of the carrier wave to represent a or Frequency Shift Keying (FSK) changes the frequency of the carrier wave to represent a or Phase Shift Keying (PSK) changes the phase of the carrier wave to represent a or 1 = 1800 Phase Change = No Phase Change 180 degree phase change February 2005 Copyright 2005 All Rights Reserved 52 Phase Modulation Extended Phase Modulation changes the starting point of the wave Change in Phase = 1800 Phase Change = No Phase Change 900 BPSK Phase shift can also be represented on an x/y axis constellation such that: = 1800 Phase Change ( π radians) = No Phase Change 180o In this instance we can transmit bit for every phase shift This is called Binary Phase Shift Keying (BPSK) in 802.11a February 2005 0o = 1800 Phase Change ( π radians) = No Phase Change 2700 Copyright 2005 All Rights Reserved 53 QUADRATURE AMPLITUDE MODULATION (QAM) bits/phase 900 01 Quadrature Phase Shift Keying (QPSK) extends this technique to transmit two bits for every phase shift 00 = 350 Phase Change 00 135o QPSK 180o 0o 01 = 1350 Phase Change 225o 315o 11 11 = 2250 Phase Change 10 2700 10 = 3150 Phase Change bits/phase Quadrature Amplitude Modulation (QAM) generalizes these techniques to encode information in both phase (by employing PSK techniques such as BPSK and QPSK) with amplitude For example, in the diagram a right, each quadrature contains amplitudes (16 levels) and can therefore transmit bits per phase February 2005 35o 900 QAM 0010 0110 0111 0100 0001 0101 0011 0000 180o 0o 1100 1101 1111 1001 1110 Copyright 2005 All Rights Reserved 1010 270 1000 1011 54 QAM Extended 900 In the diagram at right, each quadrature contains amplitudes (64 levels) and can therefore transmit bits per phase 180o February 2005 0o 2700 Copyright 2005 All Rights Reserved 55 Summary of OFDM Encoding/Modulation 64 Phase shifts can encode bits /phase shift resulting is a transmission rate of either 48 or 54 Mbps depending upon the number of sub-channels (R) used for error correction Coding Rate (R) is the ratio of sub-channels carrying data to sub-channels carrying error correction code E.G., 1/2 would indicate that 24 sub-channels (1/2 X 48 = 24) are being used for error correction while the remaining 24 sub-channels are used for data transmission The Length of the each Symbol is equal to number of sub-carriers times the bits /transition e.g., 48 X = 288 February 2005 Copyright 2005 All Rights Reserved 56 Summary of OFDM Encoding/Modulation February 2005 Copyright 2005 All Rights Reserved 57 End of Lecture February 2005 Copyright 2005 All Rights Reserved 58 ... - 928 Mhz S-Band - Ghz S-Band ISM (802.11b) 2 .4 - 2.5 Ghz C-Band - Ghz C-Band Satellite downlink 3.7 - 4. 2Ghz C-Band Radar (weather) 5.25 - 5.925 Ghz C-Band ISM (802.11a) 5.725 - 5.875 Ghz C-Band... C-Band satellite uplink 5.92 5-6 .42 5 Ghz X-Band 8-1 2 Ghz X-Band Radar (police/weather) 9. 5-1 0.55 Ghz Ku-band 1 2-1 8 Ghz Ku-band Radar (Police) 13. 5-1 5 Ghz 15. 7-1 7.7 Ghz ISM - Industrial, Scientific... All Rights Reserved 44 802.11a Network Channel Assignments Area Frequency Band USA U-NII Lower Band 36 5.180 Ghz (5.15 0-5 .250 Ghz) 40 5.200 Ghz 44 5.220 Ghz 48 5. 240 Ghz U-NII Middle Band 52