1. February 2005Copyright 2005 All Rights Reserved1 Spread Spectrum Technologies (1 September, 2006)

2. February 2005Copyright 2005 All Rights Reserved2 lDefine spread spectrum technologies and how they are used lDescribe modulation and the different data rates lExplain and compare FHSS, DSSS and OFDM lList the factors that impact signal throughput and range Objectives Upon completion of this chapter you will be able to:

3. February 2005Copyright 2005 All Rights Reserved3 Spread Spectrum l 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. l Spread spectrum is characterized by: 4 wide bandwidth and 4 low power l Jamming and interference have less effect on Spread spectrum because it is: 4 Resembles noise 4 Hard to detect 4Hard to intercept

4. February 2005Copyright 2005 All Rights Reserved4 Narrowband vs Spread Spectrum Frequency Power Spread Spectrum (Low Peak Power) Narrowband (High Peak Power)

5. February 2005Copyright 2005 All Rights Reserved5 Narrow Band vs Spread Spectrum l Narrow Band 4 Uses only enough frequency spectrum to carry the signal 4 High peak power 4 Easily jammed l Spread Spectrum 4 The bandwidth is much wider than required to send to the signal. 4 Low peak power 4Hard to detect 4Hard to intercept 4 Difficult to jam

6. February 2005Copyright 2005 All Rights Reserved6 Spread Spectrum Use l In the 1980s FCC implemented a set of rules making Spread Spectrum available to the public. 4 Cordless Telephones 4 Global Positioning Systems (GPS) 4 Cell Phones 4 Personal Communication Systems 4 Wireless video cameras l Local Area Networks 4 Wireless Local Area Networks (WLAN) 4 Wireless Personal Area Network (WPAN) 4 Wireless Metropolitan Area Network (WMAN) 4 Wireless Wide Area Network (WWAN)

7. February 2005Copyright 2005 All Rights Reserved7 FCC Specifications l 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. 4 Frequency Hopping Spread Spectrum (FHSS) and 4 Direct Sequence spread Spectrum (DSSS) l Orthogonal Frequency Division Multiplexing (OFDM) was not covered by the CFR and would have required licensing. 4 802.11a, employing OFDM, was created to work in the 5GHz Unlicensed National Information Infrastructure (UNII) l In May, 2001 CFR, Part 15 was modified to allow alternative "digital modulation techniques". 4 This resulted in 802.11g which employs OFDM in the 2.4 GHz range

8. February 2005Copyright 2005 All Rights Reserved8 Wireless LAN Networks l Wireless LANs RF spread spectrum management techniques 4 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. 4 Direct Sequence Spread Spectrum (DSSS) * Operates in the 2.4 GHz range * Digital Data signal is inserted into a higher data rate chipping code. k A Chipping code is a bit sequence consisting of a redundant bit pattern. k Barker, Gold, M-sequence and Kasami codes are employed 4 Orthogonal Frequency Division Multiplexing (OFDM) * Operates in both the 5 Ghz and 2.4 GHz range with a data rate of between 6 and 54 Mbps. * 802.11a divides each channel into 52 low-speed sub-channels ] 48 sub-channels are for data while the other 4 are pilot carriers. * The modulation scheme can be either BPSK, QPSK or QAM depending upon the speed of transmission.

9. February 2005Copyright 2005 All Rights Reserved9 FCC Radio Spectrum VLF10 kHz - 30 kHzCable Locating Equipment LF30 kHz - 300 kHzMaritime Mobile Service. MF300 kHz - 3 MHzAircraft navigation, ham radio and Avalanche transceivers. HF3 MHz - 30 MHzCB radios, CAP, Radio telephone, and Radio Astronomy. VHF30 MHz - 328.6 MHZCordless phones, Televisions, RC Cars, Aircraft, police and business radios. UHF328.6 MHz - 2.9 GHzpolice radios, fire radios, business radios, cellular phones, GPS, paging, wireless networks and cordless phones. SHF2.9 GHz - 30 GHzDoppler weather radar, satellite communications. EHF30 GHz and aboveRadio astronomy, military systems, vehicle radar systems, ham radio. Band Name Range Usage

10. February 2005Copyright 2005 All Rights Reserved10 ISM Frequency Bands UHF ISM902 - 928 Mhz S-Band2 - 4 Ghz S-Band ISM (802.11b)2.4 - 2.5 Ghz C-Band4 - 8 Ghz C-Band Satellite downlink3.7 - 4.2Ghz C-Band Radar (weather)5.25 - 5.925 Ghz C-Band ISM (802.11a)5.725 - 5.875 Ghz C-Band satellite uplink5.925-6.425 Ghz X-Band8-12 Ghz X-Band Radar (police/weather)9.5-10.55 Ghz Ku-band12-18 Ghz Ku-band Radar (Police)13.5-15 Ghz 15.7-17.7 Ghz ISM - Industrial, Scientific and Medical

11. February 2005Copyright 2005 All Rights Reserved11 FHSS

12. February 2005Copyright 2005 All Rights Reserved12 Frequency Hopping Spread Spectrum l Carrier changes frequency (HOPS) according to a pseudorandom Sequence. 4 Pseudorandom sequence is a list of frequencies. The carrier hops through this lists of frequencies. 4 The carrier then repeats this pattern. 4 During Dwell Time the carrier remains at a certain frequency. 4 During Hop Time the carrier hops to the next frequency. 4 The data is spread over 83 MHz in the 2.4 GHz ISM band. 4 This signal is resistant but not immune to narrow band interference.

13. February 2005Copyright 2005 All Rights Reserved13 Frequency Hopping Spread Spectrum An Example of a Co-located Frequency Hopping System

14. February 2005Copyright 2005 All Rights Reserved14 FHSS Contd l The original 802.11 FHSS standard supports 1 and 2 Mbps data rate. 4 FHSS uses the 2.402 – 2.480 GHz frequency range in the ISM band. 4 It splits the band into 79 non-overlapping channels with each channel 1 MHz wide. 4 FHSS hops between channels at a minimum rate of 2.5 times per second. Each hop must cover at least 6 MHz 4The hopping channels for the US and Europe are shown below.

15. February 2005Copyright 2005 All Rights Reserved15 FHSS Contd l Dwell Time 4 The Dwell time per frequency is around 100 ms ( The FCC specifies a dwell time of 400 ms per carrier frequency in any 30 second time period). 4 Longer dwell time = greater throughput. 4 Shorter dwell time = less throughput l Hop Time 4 Is measured in microseconds (us) and is generally around 200-300 us.

16. February 2005Copyright 2005 All Rights Reserved16 FHSS Contd l Gaussian Frequency Shift Keying 4 The FHSS Physical sublayer modulates the data stream using Gaussian Frequency Shift Keying (GFSK). 4 Each symbol, a zero and a one, is represented by a different frequency (2 level GFSK) 4 two symbols can be represented by four frequencies (4 level GFSK). 4 A Gaussian filter smoothes the abrupt jumps between frequencies. f c + f d2 f c + f d1 f c - f d1 f c – f d2 10110100 fcfc

17. February 2005Copyright 2005 All Rights Reserved17 FHSS Disadvantages l Not as fast as a wired Lan or the newer WLAN Standards l Lower throughput due to interference. 4FHSS is subject to interference from other frequencies in the ISM band because it hops across the entire frequency spectrum. l Adjacent FHSS access points can synchronize their hopping sequence to increase the number of co- located systems, however, it is prohibitively expensive.

18. February 2005Copyright 2005 All Rights Reserved18 DSSS

19. February 2005Copyright 2005 All Rights Reserved19 Direct Sequence Spread Spectrum l Spread spectrum increases the bandwidth of the signal compared to narrow band by spreading the signal. l There are two major types of spread spectrum techniques: FHSS and DSSS. 4 FHSS spreads the signal by hopping from one frequency to another across a bandwidth of 83 Mhz. 4 DSSS spreads the signal by adding redundant bits to the signal prior to transmission which spreads the signal across 22 Mhz. * The process of adding redundant information to the signal is called Processing Gain. * The redundant information bits are called Pseudorandom Numbers (PN).

20. February 2005Copyright 2005 All Rights Reserved20 Direct Sequence Spread Spectrum l DSSS works by combining information bits (data signal) with higher data rate bit sequence (pseudorandom number (PN)). l The PN is also called a Chipping Code (eg., the Barker chipping code) l The bits resulting from combining the information bits with the chipping code are called chips - the result- which is then transmitted. * The higher processing gain (more chips) increases the signal's resistance to interference by spreading it across a greater number of frequencies. * IEEE has set their minimum processing gain to 11. The number of chips in the chipping code equates to the signal spreading ratio. * Doubling the chipping speed doubles the signal spread and the required bandwidth.

21. February 2005Copyright 2005 All Rights Reserved21 Signal Spreading 4The Spreader employs an encoding scheme (Barker or Complementary Code Keying (CCK). 4 The spread signal is then modulated by a carrier employing either Differential Binary Phase Shift Keying (DBPSK), or Differential Quadrature Phase Shift Keying (DQPSK). 4 The Correlator reverses this process in order to recover the original data.

22. February 2005Copyright 2005 All Rights Reserved22 l Fourteen channels are identified, however, the FCC specifies only 11 channels for non-licensed (ISM band) use in the US. l Each channels is a contiguous band of frequencies 22 Mhz wide with each channel separated by 5 MHz. 4 Channel 1 = 2.401 – 2.423 (2.412 plus/minus 11 Mhz). 4 Channel 2 = 2.406 – 2.429 (2.417 plus/minus 11 Mhz). l Only Channels 1, 6 and 11 do not overlap DSSS Channels

23. February 2005Copyright 2005 All Rights Reserved23 Spectrum Mask l A spectrum Mask represents the maximum power output for the channel at various frequencies. l From the center channel frequency, 11 MHz and 22 MHZ the signal must be attenuated 30 dB. l From the center channel frequency, outside 22 MHZ, the signal is attenuated 50 dB. ± ± ±

24. February 2005Copyright 2005 All Rights Reserved24 DSSS Frequency Assignments Channel 1 2.412 GHz Channel 6 2.437 GHz Channel 11 2.462 GHz 25 MHz l The Center DSSS frequencies of each channel are only 5 Mhz apart but each channel is 22 Mhz wide therefore adjacent channels will overlap. l DSSS systems with overlapping channels in the same physical space would cause interference between systems. 4 Co-located DSSS systems should have frequencies which are at least 5 channels apart, e.g., Channels 1 and 6, Channels 2 and 7, etc. 4 Channels 1, 6 and 11 are the only theoretically non-overlapping channels.

25. February 2005Copyright 2005 All Rights Reserved25 DSSS Non-overlapping Channels 4 Each channel is 22 MHz wide. In order for two bands not to overlap (interfere), there must be five channels between them. 4 A maximum of three channels may be co-located (as shown) without overlap (interference). 4 The transmitter spreads the signal sequence across the 22 Mhz wide channel so only a few chips will be impacted by interference.

26. February 2005Copyright 2005 All Rights Reserved26 DSSS Encoding and Modulation

27. February 2005Copyright 2005 All Rights Reserved27 DSSS Encoding and Modulation l DSSS (802.11b) employs two types of encoding schemes and two types of modulation schemes depending upon the speed of transmission. l Encoding Schemes 4Barker Chipping Code: Spreads 1 data bit across 11 redundant bits at both 1 Mbps and 2 Mbps 4Complementary Code Keying (CCK): * Maps 4 data bits into a unique redundant 8 bits for 5.5 Mbps * Maps 8 data bits into a unique redundant 8 bits for 11 Mbps. l Modulation Schemes 4 Differential Binary Phase Shift Keying (DBPSK): Two phase shifts with each phase shift representing one transmitted bit. 4 Differential Quadrature Phase Shift Keying (DQPSK): Four phase shifts with each phase shift representing two bits.

28. February 2005Copyright 2005 All Rights Reserved28 DSSS Encoding

29. February 2005Copyright 2005 All Rights Reserved29 Barker Chipping Code l 802.11 adopted an 11 bit Barker chipping code. l Transmission. 4 The Barker sequence, 10110111000, was chosen to spread each 1 and 0 signal. * The Barker sequence has six 1s and five 0s. 4 Each data bit, 1 and 0, is modulo-2 (XOR) added to the eleven bit Barker sequence. * If a one is encoded all the bits change. * If a zero is encoded all bits stay the same. l Reception. 4 A zero bit corresponds to an eleven bit sequence of six 1s. 4 A one bit corresponds to an eleven bit sequence of six 0s.

30. February 2005Copyright 2005 All Rights Reserved30 Barker Sequence One Bit 1 0 1 0 1 1 0 1 1 1 0 0 0 1 0 1 1 0 1 1 1 0 0 0 Chipping Code (Barker Sequence) Original Data Spread Data 0 1 0 0 1 0 0 0 1 1 1 1 0 1 1 0 1 1 1 0 0 0 Six 0s = 1 Six 1s = 0 One Bit 10110111000

31. February 2005Copyright 2005 All Rights Reserved31 Direct Sequence Spread Spectrum Contd

32. February 2005Copyright 2005 All Rights Reserved32 Complementary Code Keying (CCK) l Barker encoding along with DBPSK and DQPSK modulation schemes allow 802.11b to transmit data at 1 and 2 Mbps l Complementary Code Keying (CCK) allows 802.11b to transmit data at 5.5 and 11 Mbps. l CCK employs an 8 bit chipping code. 4 The 8 chipping bit pattern is generated based upon the data to be transmitted. * At 5.5 Mbps, 4 bits of incoming data is mapped into a unique 8 bit chipping pattern. * At 11 Mbps, 8 bits of data is mapped into a unique 8 bit chipping pattern.

33. February 2005Copyright 2005 All Rights Reserved33 Complementary Code Keying (CCK) Contd l To transmit 5.5 Mbps 4 data bits is mapped into 8 CCK chipping bits.. l The unique 8 chipping bits is determined by the bit pattern of the 4 data bits to be transmitted. The data bit pattern is: 4 b0, b1, b2, b3 4 b2 and b3 determine the unique pattern of the 8 bit CCK chipping code. Note: j represents the imaginary number, sqrt(-1), and appears on the imaginary or quadrature axis of the complex plane.

34. February 2005Copyright 2005 All Rights Reserved34 Complementary Code Keying (CCK) Contd lTo transmit 5.5 Mbps 4 data bits is mapped into 8 CCK chipping bits.. lThe unique 8 chipping bits is determined by the bit pattern of the 4 data bits to be transmitted. The data bit pattern is: 4 b0, b1, b2, b3 4 b0 and b1 determine the DQPSK phase rotation that is to be applied to the chip sequence. 4 Each phase change is relative to the last chip transmitted.

35. February 2005Copyright 2005 All Rights Reserved35 Complementary Code Keying (CCK) Contd l To transmit 11 Mbps 8 data bits is mapped into 8 CCK chipping bits. l The unique 8 chipping bits is determined by the bit pattern of the 8 data bits to be transmitted. The data bit pattern is: 4 b0, b1, b2, b3, b4, b5, b6,b7 4 b2, b3, b4,b5, b6 and b7 selects one unique pattern of the 8 bit CCK chipping code out of 64 possible sequences. 4 b0 and b1 are used to select the phase rotation sequence.

36. February 2005Copyright 2005 All Rights Reserved36 DSSS Modulation

37. February 2005Copyright 2005 All Rights Reserved37 Differential Binary Phase Shift Keying (DBPSK) 0 Phase Shift 4A Zero phase shift from the previous symbol is interpreted as a 0. 4A 180 degree phase shift from the previous symbol is interpreted as a 1. 180 degree Phase Shift Previous carrier symbol

38. February 2005Copyright 2005 All Rights Reserved38 Differential Quadrature Phase Shift Keying (DQPSK) 4A Zero phase shift from the previous symbol is interpreted as a 00. Previous carrier symbol 0 Phase Shift 4A 90 degree phase shift from the previous symbol is interpreted as a 01. 4A 180 degree phase shift from the previous symbol is interpreted as a 11. 4A 270 degree phase shift from the previous symbol is interpreted as a 10. 90 Phase Shift 180 Phase Shift 270 Phase Shift

39. February 2005Copyright 2005 All Rights Reserved39 DSSS Summary 1Barker Coding 11 chips encoding 1 bitDBPSK 2Barker Coding 11 chips encoding 1 bit DQPSK 5.5CCK Coding 8 chips encode 8 bitsDQPSK 11CCK Coding 8 chips encode 4 bitsDQPSK Data Rate Encoding Modulation

40. February 2005Copyright 2005 All Rights Reserved40 FHSS vs DSSS l DSSS is more susceptible to narrow band noise. 4 DSSS channel is 22 Mhz wide whereas 4 FHSS is 79 Mhz wide. l The FCC regulated that DSSS use a maximum of 1 watt of transmitter power in Pt-to-Multipoint system. l DSSS costs less then FHSS l FHSS can have more systems co-located than DSSS. 4 DSSS systems have the advantage in throughput l The Wi-Fi alliance tests for DSSS compatibility 4 No such testing alliance exists for FHSS.

41. February 2005Copyright 2005 All Rights Reserved41 FHSS vs DSSS contd l DSSS generally has a throughput of 5-6 Mbps while FHSS is generally between 1-2 Mbps. l Both FHSS and DHSS are equally insecure. l DSSS has gained much wider acceptance due to its low cost, high speed and interoperability. 4This market acceptance is expected to accelerate. 4 FHSS advancement includes HomeRF and 802.15 (WPAN) (Bluetooth), however, it is expected to not advance into the enterprise.

42. February 2005Copyright 2005 All Rights Reserved42 Co-location Comparison 54 Mbps OFDM Date Rate in Mbps

43. February 2005Copyright 2005 All Rights Reserved43 OFDM

44. February 2005Copyright 2005 All Rights Reserved44 802.11a l IEEE 802.11a Standard. 4 Orthogonal Frequency Division Multiplexing (OFDM). 4 Operates in the 5.0 GHz band. 4 It Operates in the Unlicensed National Information Infrastructure (UNII). 4 200 channels ( channels 1-199) spaced 5 MHz apart. 4 Supported data rates are 6, 9, 12, 18, 24, 36, 48, and 54, MBps. 4 6, 12, and 24 are mandatory. All others are optional. 4 75-80 Feet 4 64 users /Access Point

45. February 2005Copyright 2005 All Rights Reserved45 802.11a Network Channel Assignments Area Frequency Band Channel Center Frequency USAU-NII Lower Band365.180 Ghz (5.150-5.250 Ghz)405.200 Ghz 44 5.220 Ghz 48 5.240 Ghz USAU-NII Middle Band525.260 Ghz (5.250 – 5.350 Ghz)565.260 Ghz 60 5.280 Ghz 64 5.320 Ghz USAU-NII Upper Band1495.745 Gh (5.725 – 5.825) 1535.765 Ghz 1575.785 Ghz 1615.805 Ghz NOTE: 1. U-NII : Unlicensed National Information Infrastructure. 2. 802.11a is specific to the US.

46. February 2005Copyright 2005 All Rights Reserved46 OFDM l A mathematical process that allows 52 channels to overlap without losing their orthogonality (individuality). 4 48 sub-channel are used for data 2 Each sub-channel is used to transmit data 4 4 sub-channel are used as pilot carriers. 2 The pilot sub-channels are used to monitor path shift and shifts in sub-channel frequencies (Inter Carrier Interference (ICI)). OFDM 4 OFDM selects channels that overlap but do not interfere with one another. 4 Channels are separated based upon orthogonality.

47. February 2005Copyright 2005 All Rights Reserved47 802.11a Channels Lower UNII BandMiddle UNII Band l 802.11a use the lower and middle UNII 5 GHz bands to create 8 channels. 4 Each Channel is 20 MHz each. 4 Each channel is broken into 52 sub-channels with each sub-channel 300 KHz each. * 48 Sub-channels are used to transmit data * 4 sub-channels are used as Pilot carriers to monitor the channel 8 Channels 52 Sub-Channels for each 8 channels Each channel is 20 MHz wide Lower and Middle UNII frequency band

48. February 2005Copyright 2005 All Rights Reserved48 OFDM Modulation

49. February 2005Copyright 2005 All Rights Reserved49 Modulation Background l In order to properly understand OFDM modulation we need to do a quick review of various modulation techniques. 4 James Clark Maxwell, 1864, first developed the idea that electromagnetic magnetic waves arose as a combination electric current and magnetic field – an electromagnetic wave. 4 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

50. February 2005Copyright 2005 All Rights Reserved50 Modulation Background Contd l The oscillating electromagnetic wave, also called a sine wave, is shown below. l This wave can be used as a carrier signal to carry information. l 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: 4 Amplitude – The height of the wave 4 Frequency – the number of oscillation (cycles) per second. 4 Phase – the starting point of the wave (when compared to the starting point of the previous wave. l The are two major types of modulation schemes: Analog and Digital Amplitude Frequency Phase Sine Wave

51. February 2005Copyright 2005 All Rights Reserved51 Analog Modulation 4Amplitude Modulation varies the height of the carrier wave. 4Frequency Modulation varies the number of oscillation (waves) per second 4Phase Modulation changes the starting point of the wave. Change in Phase Change in Frequency Change in Amplitude 1 = 180 0 Phase Change 0 = No Phase Change

52. February 2005Copyright 2005 All Rights Reserved52 Digital Modulation 1 = 180 0 Phase Change 0 = No Phase Change 4Amplitude Shift Keying (ASK) changes the amplitude of the carrier wave to represent a 0 or 1. 4Frequency Shift Keying (FSK) changes the frequency of the carrier wave to represent a 0 or 1. 4Phase Shift Keying (PSK) changes the phase of the carrier wave to represent a 0 or 1. 180 degree phase change

53. February 2005Copyright 2005 All Rights Reserved53 Phase Modulation Extended 4Phase Modulation changes the starting point of the wave. Change in Phase 1 = 180 0 Phase Change 0 = No Phase Change 90 0 270 0 180 o 0o0o 10 4Phase shift can also be represented on an x/y axis constellation such that: 4In this instance we can transmit 1 bit for every phase shift. 4This is called Binary Phase Shift Keying (BPSK) in 802.11a π radians) 1 = 180 0 Phase Change ( 0 = No Phase Change π radians) 1 = 180 0 Phase Change ( 0 = No Phase Change BPSK

54. February 2005Copyright 2005 All Rights Reserved54 QUADRATURE AMPLITUDE MODULATION (QAM) 90 0 270 0 00 135 o 01 11 10 35 o 315 o 225 o 180 o 0o0o 2 bits/phase 4 Quadrature Phase Shift Keying (QPSK) extends this technique to transmit two bits for every phase shift. 0000 0001 0011 0010 0110 0111 0101 0100 1100 1101 1111 1110 1010 1011 10011000 90 0 270 0 180 o 0o0o 4 bits/phase 4Quadrature Amplitude Modulation (QAM) generalizes these techniques to encode information in both phase (by employing PSK techniques such as BPSK and QPSK) with amplitude. 4 For example, in the diagram a right, each quadrature contains 4 amplitudes (16 levels) and can therefore transmit 4 bits per phase. 00 = 35 0 Phase Change 01 = 135 0 Phase Change 11 = 225 0 Phase Change 10 = 315 0 Phase Change QPSK QAM

55. February 2005Copyright 2005 All Rights Reserved55 QAM Extended 4In the diagram at right, each quadrature contains 8 amplitudes (64 levels) and can therefore transmit 6 bits per phase. 90 0 270 0 180 o 0o0o

56. February 2005Copyright 2005 All Rights Reserved56 Summary of OFDM Encoding/Modulation 4 64 Phase shifts can encode 6 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. 4 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. 4 The Length of the each Symbol is equal to number of sub-carriers times the bits /transition. e.g., 48 X 6 = 288.

57. February 2005Copyright 2005 All Rights Reserved57 Summary of OFDM Encoding/Modulation

58. February 2005Copyright 2005 All Rights Reserved58 End of Lecture