CSCD 433/533 Wireless Networks

Transcription

1 CSCD 433/533 Wireless Networks Lecture 8 Physical Layer, and b,g,a,n Differences Winter

2 Topics Spread Spectrum in General Differences between b,g,a and n Frequency ranges Speed DSSS Spread Spectrum, b OFDM and n 2

3 Introduction Today, discuss physical layer of standard Many techniques have helped to increase throughput as versions evolved over time We will start with slowest and end with fastest Note, we will not cover ac Will point to some good references 3

4 FCC Regulation In 1995, Federal Communications Commission allocated several bands of wireless spectrum for use without license Industry, Science and Medical (ISM) FCC stipulated that use of spread spectrum technology would be required July 1999 the b standard was ratified Used Spread Spectrum Technology 4

5 Spread Spectrum Defined Spread spectrum is a method 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 receiver Spread spectrum is characterized by Wide bandwidth and Low power Jamming and interference have less effect on Spread spectrum because it Resembles noise Hard to detect Hard to intercept Will go into more details with a specific example DSSS.

6 Spread Spectrum Defined Spread Spectrum Transmission You are required to use spread spectrum transmission in unlicensed bands Spread spectrum transmission reduces propagation problems Especially multipath interference Spread spectrum transmission is NOT used for security in WLANs Although military does use spread spectrum transmission to make signals hard to detect 5-6

7 ISM: Industry, Science, Medical Frequency Band unlicensed frequency spectrum: 900Mhz, 2.4Ghz, 5.1Ghz, 5.7Ghz 7

8 IEEE Frequency Band Wavelength and b/g a ac 8 Early Wireless LANS

9 Physical Channels The b standard defines 14 frequency channels in 2.4GHz range Only eleven are allowed for unlicensed use by the FCC in the US, Japan allows all 14 Each channel uses "Direct Sequence Spread Spectrum" (DSSS) to spread data over channel that extends 11MHz on each side of center frequency each is 22 MHz wide Channels overlap, but there are three out of 11 channels that don't 9

10 802.11b/g Channels Channel Width = 22 MHz Channels 12 14, not sanctioned by FCC Each channel spaced 5 MHz apart Only non-overlapping channels are 1, 6 and 11 10

11 Frequency Assignments The Center frequencies of each channel are only 5 Mhz apart but each channel is 22 Mhz wide therefore adjacent channels will overlap. DSSS systems with overlapping channels in same physical space cause interference between systems. 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. Channels 1, 6 and 11 are the only theoretically non-overlapping channels. 25 MHz Channel GHz 25 MHz Channel GHz Channel GHz

12 Comparisons of Physical Layer 3 Older Versions of a b g Newest ones n and ac 12

13 Radio Communications How do you transmit Radio Signals reliably? Classic approach. Confine information carrying signal to a narrow frequency band and pump as much power as possible into signal Noise occurs as distortion in frequency band To overcome noise Ensure power of signal > noise Recall, SNR = Signal to Noise Ratio But, what if you deliberately include your own noise into signal? 13

14 Radio Communications The Unlicensed Bands Networks operate in bands which are license free, Industrial, Scientific and Medical (ISM) Requires manufacturer to file information with FCC Competing devices operate in 2.4 GHz range products Bluetooth Cordless phones X10 Protocol for home automation 14

15 Radio Communications 2.4 GHz is Unlicensed but Must obey FCC limitations on power, band use and purity of signal No regulations specify coding or modulation Thus, contention and competition between devices Solutions Stop using device, amplify its power or move it 15

16 Radio Communications Given multiple devices compete in ISM bands, how do you reliably transmit data? Spread Spectrum is one answer Radio signals are sent with as much power as allowed over a narrow band of frequency. then Spread Spectrum Used to transform radio for data Uses math functions to diffuse signal over larger range of frequencies Makes transmissions look like noise to narrowband receiver 16

17 Radio Communications Spread Spectrum continued On receiver side, signal is transformed back to narrow-band and noise is removed Spread spectrum is a requirement for unlicensed devices Minimize interference between unlicensed devices, FCC imposes limitations on power of transmissions 17

18 Radio Communications Trivia Question Who patented spread spectrum transmission and when was it patented? 18

19 Hedy Lamarr Austrian actress Hedy Lamarr became a pioneer in the field of wireless communications following her emigration to the United States With co-inventor George Anthiel, developed a "Secret Communications System" to help combat Nazis in World War II By manipulating radio frequencies at irregular intervals between transmission and reception, the invention formed an unbreakable code to prevent classified messages from being intercepted by enemy personnel Patented in

20 Spread Spectrum uses three different Spread Spectrum technologies 1. FH Frequency Hopping (FHSS) Jumps from one frequency to another in random pattern Transmits a short burst at each subchannel 2 Mbps FH or FHSS original spread spectrum technology developed in 1997 with standard However, quickly bypassed by more sophisticated spread spectrum technologies We won t cover it, since it has mostly been replaced 20

21 Spread Spectrum Continued 2. DS or DSSS Direct Sequence Spread Spectrum Took over from FHSS and allowed for faster throughput Used in b Spreads out signal over a wider path Uses frequency coding functions 3. OFDM Orthogonal Frequency Division Multiplexing Divides channel into several subchannels and encode a portion of signal across each subchannel in parallel a and g uses this technology Allows for even faster throughput than DSSS 21

22 Spread Spectrum and Examples 22

23 Spread Spectrum Code Techniques Spread-spectrum is a signal propagation technique Employs several methods Decrease potential interference to other receivers Generally makes use of noise-like signal structure to spread normally narrowband information signal over a relatively wideband (radio) band of frequencies Receiver correlates (matches) received signals to retrieve original information signal 23

24 Spread Spectrum Code Techniques Three characteristics of Spread Spectrum techniques 1. Signal occupies bandwidth much greater than that which is necessary to send the information 2. Bandwidth is spread by means of code independent of data 3. Receiver uses same code to recover the data 24

25 Spread Spectrum (SS) Code Techniques Transmitted signal takes up more bandwidth than information signal that is being modulated Name 'spread spectrum' means signals occur over full bandwidth (spectrum) of a device's transmitting frequency Military has used Spread Spectrum for many years They worry about signal interception and jamming SS signals hard to detect on narrow band equipment because the signal's energy is spread over a bandwidth of maybe 100 times information bandwidth 25

26 Spread Spectrum Techniques In a spread-spectrum system, signals spread across wide bandwidth, making them difficult to intercept and demodulate 26

27 Spread Spectrum Code Techniques Spread Spectrum signals use fast codes These special "Spreading" codes are called "Pseudo Random" or "Pseudo Noise" codes Called "Pseudo" because they are not truly random distributed noise Will look at an example of this later 27

28 Spread Spectrum Code Techniques Same code must be known in advance at both ends of the transmission channel Spreading de-spreading 28

29 General Model of Spread Spectrum System

30 Spread Spectrum Code Techniques Can see results of interference attempts, interferer signals are not recovered 30

31 DSSS and HR/DSSS 31

32 Direct Sequence Spread Spectrum - DSSS DSSS is a spread spectrum technique Modulation alters carrier wave in order to transmit a data signal (text, voice, audio, video, etc.) Phase-modulates a sine wave pseudorandomly Continuous string of pseudonoise (PN) code symbols called "chips Each of which has a much shorter duration than an information bit Each information bit is modulated by a sequence of much faster chips 32

33 Direct Sequence Spread Spectrum - DSSS Why this works... To a narrowband receiver, transmitted signal looks like noise Original signal can be recovered through correlation that reverses the process, The ratio (in db) between the spread bandwidth to the unspread bandwidth is known as Processing Gain Example 1 khz signal is spread to 100 khz Process gain would be 100,000/1,000 = 100 In decibels, 10 log10(100) = 20 db Typical SS processing gains run from 10dB to 60dB 33

34 DSSS How DSSS works Apply something called a chipping sequence to the data stream Chip is a binary digit But, spread-spectrum developers make distinction to separate encoding of data from the data itself Talk about data is bits Talk about encoding is chips or chipping sequence 34

35 DSSS Chipping sequence Also called Pseudorandom Noise Codes (PNC) Must run at a higher rate than underlying data Data bit is 0 or 1 For each bit, chip sequence is added Chip is an 11 bit code combined with a data bit to produce an 11 bit code This gets transmitted to receiver 35

36 DSSS Chipping Sequence Data Spreading Encoded Data Modulo 2-add Spreading Code Correlation Modulo2 1 Subtract Spreading Code 36

37 Figure 6.33 DSSS example

38 Direct Sequence Spread Spectrum Example

39 Code Division Multiple Access (CDMA) A multiplexing technique used with spread spectrum Given a data signal rate D Break each bit into k chips according to a fixed chipping code specific to each user Resulting new channel has chip data rate kd chips per second Can have multiple channels superimposed

40 CDMA Example

41 DSSS Chipping stream Two costs to increased chipping ratio 1. Direct cost of more expensive RF components that operate at higher frequencies 2. Amount of bandwidth required, need more 41

42 DSSS Encoding DSSS originally adopted an 11-bit Barker word Each bit encoded using entire Barker word or chipping sequence Key attribute of Barker words Have good autocorrelation properties High signal recovery possible when signal distorted by noise Correlation function operates over wide range of environments and is tolerant of propagation delay 42

43 DSSS Encoding DSSS Why 11 bits? Regulatory authorities require a 10 db processing gain in DSSS systems Using an 11 bit spreading code for each bit let meet regulatory requirements Recall The ratio (in db) between the spread baseband and the original signal is processing gain 43

44 OFDM Orthogonal Frequency Division Multiplexing 44

45 Intro to OFDM a and g based on OFDM Orthogonal Frequency Division Multiplexing Revolutionized Wi-Fi and other cellular products by allowing faster throughput and more robustness OFDM makes highly efficient use of available spectrum 45

46 OFDM Based on FDM Recall Frequency division multiplexing (FDM) is technology that transmits multiple signals simultaneously over single transmission path, such as cable or wireless system Each signal travels within its own unique frequency range (carrier) What do you recall about the efficiency of this technique? 46

47 FDM Comment FDM transmissions are least efficient since each channel can only be used by one user at a time Each User has their own channel 47

48 OFDM based on FDM OFDM, data divided among large number of closely spaced carriers "frequency division multiplex" part of name Entire bandwidth is filled from single source of data Instead of transmitting data serially for each channel, data is transferred in parallel Divided among multiple subcarriers Only small amount of data is carried on each carrier 48

49 OFDM An OFDM signal consists of Several closely spaced modulated carriers Does not use guardbands Note: When modulation of any form - voice, data, etc. is applied to a carrier Sidebands spread out on either side A receiver must be able to receive whole signal to be able to demodulate data So, when signals are transmitted close to one another typically spaced with guard frequency band between 49 them

50 Traditional View FDM with Guards Guard bands waste the spectrum Receiver filter passband: one signal selected Guards Traditional view of signals carrying modulation 50

51 OFDM Making Subcarriers Mathematically Orthogonal Breakthrough for OFDM Enables OFDM receivers to separate subcarriers via Fast Fourier Transform (FFT) Eliminates guard bands OFDM subcarriers can overlap to make full use of spectrum At Peak of each subcarrier spectrum, power in all other subcarriers is zero Subcarriers are operating at frequencies chosen to be orthogonal to each other 51

52 OFDM Shows parallel nature of subcarriers 52

53 Fast Fourier Transform Developed in the 1960's as a way to speed up the math of Fourier Transform Take analog signal, digitize it Take resulting samples and put them through FFT Essentially a digital version of a spectrum analysis of the signal FFT sorts signal components out of individual sine-wave elements of specific frequencies and amplitudes Makes FFT a good way to separate out the carriers of an OFDM signal Good reference on OFDM and FFT 53

54 Benefits of OFDM Radio signals are imperfect General challenges of RF signals include Signal-to-noise ratio, competition from other devices Multipath Effects from Physical Interference Same signal arrives at a receiver via different paths leads to several problems Fading owing to multipath effects Self-interference or Intersymbol interference or ISI 54

55 Multipath Fading Indoor and Outdoor radio channels characterized by multipath reception Sent signal contains direct line-of-sight radio wave, but also a large number of reflected radio waves Outdoors line-of-sight often blocked by obstacles, and collection of differently delayed waves received by antenna Reflected waves interfere with direct wave, causes degradation performance - Waves arrive at slightly different times, so they are out of phase with original wave 55 Randomly boosts or cancels out parts of signal!!

56 Multipath Fading 56

57 Benefits of OFDM Using multiple subcarriers is why OFDM systems more robust to Fading Fading typically decreases received signal strength at particular frequencies With many subcarriers at different frequencies affects only a few of the subcarriers at any given time Error-correcting codes provide redundant information that enables OFDM receivers to restore information lost in erroneous subcarriers a has 4 error-correcting subchannels 57

58 Self or Intersymbol Interference Intersymbol Interference (ISI) Form of signal distortion where one symbol interferes with subsequent symbols Unwanted phenomenon, previous symbols have similar effect as noise Spreading of pulse beyond its allotted time interval causes it to interfere with neighboring pulses ISI is usually caused by multipath propagation 58

59 Intersymbol Interference Symbols arrive at different times due to multipath transmission 59 Receiver must resolve timing differences by waiting for all the echos to arrive

60 Benefits of OFDM Main way to prevent Intersymbol Interference errors Transmit a short block of data (a symbol) Wait until all the multipath echoes fade before sending another symbol Waiting time often referred to as guard interval It is a time between sending subsequent symbols 60

61 Benefits of OFDM Longer guard intervals - more robust system to multipath effects But during guard interval, system gets no use from available spectrum Longer the wait, the lower the effective channel capacity Some guard interval is necessary for any wireless system Goal is to minimize that interval and maximize symbol transmission time 61

62 Benefits of OFDM OFDM meets this challenge by Dividing transmissions among multiple subcarriers Symbol transmission time is multiplied by number of subcarriers For example a, there are 52 channels, so the system has 52 times transmission capacity compared to single channel Each channel can operate at a slower rate, sends 62 fewer symbols per channel

63 OFDM vs. Single Channel 63

64 802.11a 64

65 Intro to a a was approved in September 1999, two years after standard approved Operates in 5 GHz Unlicensed National Information Infrastructure (UNII) band Spectrum is divided into three domains, Each has restrictions imposed on maximum allowed output power 65

66 ISM vs. U-NII 66

67 802.11a OFDM a Specifies 8 20 MHz channels in lower two bands Each divided into 52 sub-carriers (four of which carry pilot data) of 300-kHz bandwidth each 4 20 MHz channels are specified in upper band Receiver processes 52 individual bit streams, reconstructing original high-rate data stream Four complex modulation methods are employed, depending on data rate that can be supported by conditions between transmitter and receiver Include BPSK, QPSK, 16-QAM, and 64-QAM 67

68 802.11a Channels 68

69 802.11a Advantage Since 2.4 GHz band is heavily used, using 5 GHz band gives a advantage of less interference Disadvantage However, high carrier frequency also brings disadvantages It restricts use of a to almost line of sight, necessitating use of more access points It also means that a cannot penetrate as far as b since it is absorbed more readily, other things (such as power) being equal 69

70 802.11b vs a Path Loss Free Space Path Loss in db for 2.4 and 5 GHz Spectrums Distance (miles) 2.4 GHz 5 GHz Loss = 32.4 X 20Log(MHz) X 20Log(distance) 70

71 Range and Data Rate

72 802.11g June 2003, a third modulation standard ratified g Works in 2.4 GHz band (like b) Maximum data rate of 54 Mbit/s g hardware works with b hardware Older networks, b node significantly reduces the speed of an g network 72

73 802.11g Uses Multiple Modulation schemes OFDM for data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s, and Reverts to CCK Complimentary Code Keying like b for 5.5 and 11 Mbit/s DBPSK/DQPSK+DSSS for 1 and 2 Mbit/s Even though g operates in same frequency band as b Achieve higher data rates because it uses OFDM and better modulation 73

74 802.11g Rates, Transmission, Modulation Data Rate Mbps Trans Type Modulation 54 OFDM 64 QAM 48 OFDM 64 QAM 11 DSSS QPSK1 6 OFDM BPSK 5.5 DSSS CCK 74

75 802.11n 75

76 802.11n n is long anticipated update to WiFi standards a/b/g 4x increase in throughput Improvement in range n ratified by IEEE

77 802.11n Features n utilizes larger number of antennas Number of antennas relates to number of simultaneous streams Two receivers and two transmitters (2x2) or four receivers and four transmitters (4x4) The standards requirement is a 2x2 with a maximum two streams, but allows 4x4 77

78 802.11n Features n standard operates in 2.4-GHz, the 5GHz radio band, or both more flexibility Backward compatibility with preexisting a/b/g deployment Majority of devices and access points deployed are dual-band Operates in both 2.4-GHz and 5-GHz frequencies 78

79 802.11n Features Wireless solutions based on n standard use several techniques to improve throughput, reliability, and predictability of wireless Three primary innovations are Multiple Input Multiple Output (MIMO) technology Channel bonding (40MHz Channels) Packet aggregation Allows n solutions to achieve fivefold performance increase over a/b/g networks Details follow... 79

80 MIMO n builds on previous standards by adding multiple-input multiple-output (MIMO) MIMO uses multiple transmitter and receiver antennas to improve system performance MIMO uses additional signal paths from each antenna to transmit more information, recombine signals on the receiving end 80

81 MIMO n access points and clients transmit two or more spatial streams Use multiple receive antennas and advanced signal processing to recover multiple transmitted data streams MIMO-enabled access points use spatial multiplexing to transmit different bits of a message over separate antennas Provides greater data throughput 81

82 MIMO Technology Multiple independent streams are transmitted simultaneously to increase the data rate 82

83 MIMO Performance gain is result of MIMO smart antenna technology Allows wireless access points to receive signals more reliably over greater distances than with standard antennas Example Distance from access point for a/g client communicating with a conventional access point drops from 54 Mbps to 48 Mbps or 36 Mbps Same client communicating with MIMO access point still able to operate at 54 Mbps 83

84 Channel Bonding Most straightforward way to increase capacity of network is to increase operating bandwidth However, conventional wireless technologies limited to transmit over one of several 20-MHz channels n networks employ technique called channel bonding to combine two adjacent 20-MHz channels into a single 40-MHz channel Technique more than doubles channel bandwidth 84

85 Channel Bonding Channel bonding most effective in 5-GHz frequency given greater number of available channels 2.4-GHz frequency has only 3 non-overlapping 20-MHz channels Thus, bonding two 20-MHz channels uses two thirds of total frequency capacity So, IEEE has rules on when a device can operate in 40MHz channels in 2.4GHz space to ensure optimal performance 5 GHz has larger number of channels available for bonding 85

86 Packet Aggregation In conventional wireless transmission methods Amount of channel access overhead required to transmit each packet is fixed, regardless of the size of the packet itself As data rates increase, time required to transmit each packet shrinks Overhead cost remains same 86

87 Packet Aggregation n technologies increase efficiency by aggregating multiple data packets into single transmission frame n networks can send multiple data packets with fixed overhead cost of just a single frame Packet aggregation is more beneficial for certain types of applications such as file transfers Real-time applications (e.g. voice) don t benefit from packet aggregation because its packets would need to be interspersed at regular intervals Combining packets into larger payload would introduce unnecessary latency 87

88 Comparison 88

89 Summary From 1999 until years changes in wireless LAN technology From 5.5 Mbps to Mbps and beyond How Parallelism of data streams Increased number of antennas Resolving interference through math and multiplexing Cramming more data within limited frequencies Better modulation techniques Future 89

90 End 90

91 CSCD 433/533 Wireless Networks Lecture 8 Physical Layer, and b,g,a,n Differences Winter

92 Topics Spread Spectrum in General Differences between b,g,a and n Frequency ranges Speed DSSS Spread Spectrum, b OFDM and n 2

93 Introduction Today, discuss physical layer of standard Many techniques have helped to increase throughput as versions evolved over time We will start with slowest and end with fastest Note, we will not cover ac Will point to some good references 3

94 FCC Regulation In 1995, Federal Communications Commission allocated several bands of wireless spectrum for use without license Industry, Science and Medical (ISM) FCC stipulated that use of spread spectrum technology would be required July 1999 the b standard was ratified Used Spread Spectrum Technology 4

95 Spread Spectrum Defined Spread spectrum is a method 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 receiver Spread spectrum is characterized by Wide bandwidth and Low power Jamming and interference have less effect on Spread spectrum because it Resembles noise Hard to detect Hard to intercept Will go into more details with a specific example DSSS. 5

96 Spread Spectrum Defined Spread Spectrum Transmission You are required to use spread spectrum transmission in unlicensed bands Spread spectrum transmission reduces propagation problems Especially multipath interference Spread spectrum transmission is NOT used for security in WLANs Although military does use spread spectrum transmission to make signals hard to detect 02/03/

97 ISM: Industry, Science, Medical Frequency Band unlicensed frequency spectrum: 900Mhz, 2.4Ghz, 5.1Ghz, 5.7Ghz 7 7

98 IEEE Frequency Band Wavelength and b/g a ac 8 Early Wireless LANS 8

99 Physical Channels The b standard defines 14 frequency channels in 2.4GHz range Only eleven are allowed for unlicensed use by the FCC in the US, Japan allows all 14 Each channel uses "Direct Sequence Spread Spectrum" (DSSS) to spread data over channel that extends 11MHz on each side of center frequency each is 22 MHz wide Channels overlap, but there are three out of 11 channels that don't 9

100 802.11b/g Channels Channel Width = 22 MHz Channels 12 14, not sanctioned by FCC Each channel spaced 5 MHz apart 10 Only non-overlapping channels are 1, 6 and 11 10

101 Frequency Assignments The Center frequencies of each channel are only 5 Mhz apart but each channel is 22 Mhz wide therefore adjacent channels will overlap. DSSS systems with overlapping channels in same physical space cause interference between systems. 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. Channels 1, 6 and 11 are the only theoretically non-overlapping channels. 25 MHz Channel GHz 25 MHz Channel GHz Channel GHz 11

102 Comparisons of Physical Layer 3 Older Versions of a b g Newest ones n and ac 12

103 Radio Communications How do you transmit Radio Signals reliably? Classic approach. Confine information carrying signal to a narrow frequency band and pump as much power as possible into signal Noise occurs as distortion in frequency band To overcome noise Ensure power of signal > noise Recall, SNR = Signal to Noise Ratio But, what if you deliberately include your own noise into signal? 13

104 Radio Communications The Unlicensed Bands Networks operate in bands which are license free, Industrial, Scientific and Medical (ISM) Requires manufacturer to file information with FCC Competing devices operate in 2.4 GHz range products Bluetooth Cordless phones X10 Protocol for home automation 14

105 Radio Communications 2.4 GHz is Unlicensed but Must obey FCC limitations on power, band use and purity of signal No regulations specify coding or modulation Thus, contention and competition between devices Solutions Stop using device, amplify its power or move it 15

106 Radio Communications Given multiple devices compete in ISM bands, how do you reliably transmit data? Spread Spectrum is one answer Radio signals are sent with as much power as allowed over a narrow band of frequency. then Spread Spectrum Used to transform radio for data Uses math functions to diffuse signal over larger range of frequencies Makes transmissions look like noise to narrowband receiver 16

107 Radio Communications Spread Spectrum continued On receiver side, signal is transformed back to narrow-band and noise is removed Spread spectrum is a requirement for unlicensed devices Minimize interference between unlicensed devices, FCC imposes limitations on power of transmissions 17

108 Radio Communications Trivia Question Who patented spread spectrum transmission and when was it patented? 18

109 Hedy Lamarr Austrian actress Hedy Lamarr became a pioneer in the field of wireless communications following her emigration to the United States With co-inventor George Anthiel, developed a "Secret Communications System" to help combat Nazis in World War II By manipulating radio frequencies at irregular intervals between transmission and reception, the invention formed an unbreakable code to prevent classified messages from being intercepted by enemy personnel Patented in

110 Spread Spectrum uses three different Spread Spectrum technologies 1. FH Frequency Hopping (FHSS) Jumps from one frequency to another in random pattern Transmits a short burst at each subchannel 2 Mbps FH or FHSS original spread spectrum technology developed in 1997 with standard However, quickly bypassed by more sophisticated spread spectrum technologies We won t cover it, since it has mostly been replaced 20

111 Spread Spectrum Continued 2. DS or DSSS Direct Sequence Spread Spectrum Took over from FHSS and allowed for faster throughput Used in b Spreads out signal over a wider path Uses frequency coding functions 3. OFDM Orthogonal Frequency Division Multiplexing Divides channel into several subchannels and encode a portion of signal across each subchannel in parallel a and g uses this technology Allows for even faster throughput than DSSS 21

112 Spread Spectrum and Examples 22

113 Spread Spectrum Code Techniques Spread-spectrum is a signal propagation technique Employs several methods Decrease potential interference to other receivers Generally makes use of noise-like signal structure to spread normally narrowband information signal over a relatively wideband (radio) band of frequencies Receiver correlates (matches) received signals to retrieve original information signal 23

114 Spread Spectrum Code Techniques Three characteristics of Spread Spectrum techniques 1. Signal occupies bandwidth much greater than that which is necessary to send the information 2. Bandwidth is spread by means of code independent of data 3. Receiver uses same code to recover the data 24

115 Spread Spectrum (SS) Code Techniques Transmitted signal takes up more bandwidth than information signal that is being modulated Name 'spread spectrum' means signals occur over full bandwidth (spectrum) of a device's transmitting frequency Military has used Spread Spectrum for many years They worry about signal interception and jamming SS signals hard to detect on narrow band equipment because the signal's energy is spread over a bandwidth of maybe 100 times information bandwidth 25

116 Spread Spectrum Techniques In a spread-spectrum system, signals spread across wide bandwidth, making them difficult to intercept and demodulate 26

117 Spread Spectrum Code Techniques Spread Spectrum signals use fast codes These special "Spreading" codes are called "Pseudo Random" or "Pseudo Noise" codes Called "Pseudo" because they are not truly random distributed noise Will look at an example of this later 27

118 Spread Spectrum Code Techniques Same code must be known in advance at both ends of the transmission channel Spreading de-spreading 28

119 General Model of Spread Spectrum System 29

120 Spread Spectrum Code Techniques Can see results of interference attempts, interferer signals are not recovered 30

121 DSSS and HR/DSSS 31

122 Direct Sequence Spread Spectrum - DSSS DSSS is a spread spectrum technique Modulation alters carrier wave in order to transmit a data signal (text, voice, audio, video, etc.) Phase-modulates a sine wave pseudorandomly Continuous string of pseudonoise (PN) code symbols called "chips Each of which has a much shorter duration than an information bit Each information bit is modulated by a sequence of much faster chips 32

123 Direct Sequence Spread Spectrum - DSSS Why this works... To a narrowband receiver, transmitted signal looks like noise Original signal can be recovered through correlation that reverses the process, The ratio (in db) between the spread bandwidth to the unspread bandwidth is known as Processing Gain Example 1 khz signal is spread to 100 khz Process gain would be 100,000/1,000 = 100 In decibels, 10 log10(100) = 20 db Typical SS processing gains run from 10dB to 60dB 33

124 DSSS How DSSS works Apply something called a chipping sequence to the data stream Chip is a binary digit But, spread-spectrum developers make distinction to separate encoding of data from the data itself Talk about data is bits Talk about encoding is chips or chipping sequence 34

125 DSSS Chipping sequence Also called Pseudorandom Noise Codes (PNC) Must run at a higher rate than underlying data Data bit is 0 or 1 For each bit, chip sequence is added Chip is an 11 bit code combined with a data bit to produce an 11 bit code This gets transmitted to receiver 35

126 DSSS Chipping Sequence Data Spreading Encoded Data Modulo 2-add Spreading Code Correlation Modulo2 1 Subtract Spreading Code 36

127 Figure 6.33 DSSS example 37 37

128 Direct Sequence Spread Spectrum Example 38 One technique with direct sequence spread spectrum is to combine the digital information stream with the spreading code bit stream using an exclusive-or (XOR). Stallings DCC8e Figure 9.6 shows an example. Note that an information bit of one inverts the spreading code bits in the combination, while an information bit of zero causes the spreading code bits to be transmitted without inversion. The combination bit stream has the data rate of the original spreading code sequence, so it has a wider bandwidth than the information stream. In this example, the spreading code bit stream is clocked at four times the information rate. 38

129 Code Division Multiple Access (CDMA) A multiplexing technique used with spread spectrum Given a data signal rate D Break each bit into k chips according to a fixed chipping code specific to each user Resulting new channel has chip data rate kd chips per second Can have multiple channels superimposed 39 CDMA is a multiplexing technique used with spread spectrum. The scheme works in the following manner. We start with a data signal with rate D, which we call the bit data rate. We break each bit into k chips according to a fixed pattern that is specific to each user, called the user s code, or chipping code. The new channel has a chip data rate, or chipping rate, of kd chips per second. With CDMA, the receiver can sort out transmission from the desired sender, even when there may be other users broadcasting in the same cell. 39

130 CDMA Example 40 As an illustration we consider a simple example with k = 6. It is simplest to characterize a chipping code as a sequence of 1s and 1s. Figure 9.10 shows the codes for three users, A, B, and C, each of which is communicating with the same base station receiver, R. Thus, the code for user A is ca = <1, 1, 1, 1, 1, 1>. Similarly, user B has code cb = <1, 1, 1, 1, 1, 1>, and user C has cc = <1, 1, 1, 1, 1, 1>. We now consider the case of user A communicating with the base station. The base station is assumed to know A s code. For simplicity, we assume that communication is already synchronized so that the base station knows when to look for codes. If A wants to send a 1 bit, A transmits its code as a chip pattern <1, 1, 1, 1, 1, 1>. If a 0 bit is to be sent, A transmits the complement (1s and 1s reversed) of its code, < 1, 1, 1, 1, 1, 1>. At the base station the receiver decodes the chip patterns. If the decoder is linear and if A and B transmit signals sa and sb, respectively, at the same time, then SA (sa + sb) = SA (sa) + SA (sb) = SA (sa) since the decoder ignores B when it is using A s code. The codes of A and B that have the property that SA (cb) = SB (ca) = 0 are called orthogonal. Using the decoder, Su, the receiver can sort out transmission from u even when there may be other users broadcasting in the same cell. In practice, the CDMA receiver can filter out the contribution from unwanted users or they appear as low-level noise. However, if there are many users competing for the channel with the user the receiver is trying to listen to, or if the signal power of one or more competing signals is too high, perhaps because it is very near the receiver (the near/far problem), the system breaks down. 40

131 DSSS Chipping stream Two costs to increased chipping ratio 1. Direct cost of more expensive RF components that operate at higher frequencies 2. Amount of bandwidth required, need more 41

132 DSSS Encoding DSSS originally adopted an 11-bit Barker word Each bit encoded using entire Barker word or chipping sequence Key attribute of Barker words Have good autocorrelation properties High signal recovery possible when signal distorted by noise Correlation function operates over wide range of environments and is tolerant of propagation delay 42

133 DSSS Encoding DSSS Why 11 bits? Regulatory authorities require a 10 db processing gain in DSSS systems Using an 11 bit spreading code for each bit let meet regulatory requirements Recall The ratio (in db) between the spread baseband and the original signal is processing gain 43

134 OFDM Orthogonal Frequency Division Multiplexing 44

135 Intro to OFDM a and g based on OFDM Orthogonal Frequency Division Multiplexing Revolutionized Wi-Fi and other cellular products by allowing faster throughput and more robustness OFDM makes highly efficient use of available spectrum 45

136 OFDM Based on FDM Recall Frequency division multiplexing (FDM) is technology that transmits multiple signals simultaneously over single transmission path, such as cable or wireless system Each signal travels within its own unique frequency range (carrier) What do you recall about the efficiency of this technique? 46

137 FDM Comment FDM transmissions are least efficient since each channel can only be used by one user at a time Each User has their own channel 47

138 OFDM based on FDM OFDM, data divided among large number of closely spaced carriers "frequency division multiplex" part of name Entire bandwidth is filled from single source of data Instead of transmitting data serially for each channel, data is transferred in parallel Divided among multiple subcarriers Only small amount of data is carried on each carrier 48

139 OFDM An OFDM signal consists of Several closely spaced modulated carriers Does not use guardbands Note: When modulation of any form - voice, data, etc. is applied to a carrier Sidebands spread out on either side A receiver must be able to receive whole signal to be able to demodulate data So, when signals are transmitted close to one another typically spaced with guard frequency band between 49 them

140 Traditional View FDM with Guards Guard bands waste the spectrum Receiver filter passband: one signal selected Guards Traditional view of signals carrying modulation 50

141 OFDM Making Subcarriers Mathematically Orthogonal Breakthrough for OFDM Enables OFDM receivers to separate subcarriers via Fast Fourier Transform (FFT) Eliminates guard bands OFDM subcarriers can overlap to make full use of spectrum At Peak of each subcarrier spectrum, power in all other subcarriers is zero Subcarriers are operating at frequencies chosen to be orthogonal to each other 51

142 OFDM Shows parallel nature of subcarriers 52

143 Fast Fourier Transform Developed in the 1960's as a way to speed up the math of Fourier Transform Take analog signal, digitize it Take resulting samples and put them through FFT Essentially a digital version of a spectrum analysis of the signal FFT sorts signal components out of individual sine-wave elements of specific frequencies and amplitudes Makes FFT a good way to separate out the carriers of an OFDM signal Good reference on OFDM and FFT 53

144 Benefits of OFDM Radio signals are imperfect General challenges of RF signals include Signal-to-noise ratio, competition from other devices Multipath Effects from Physical Interference Same signal arrives at a receiver via different paths leads to several problems Fading owing to multipath effects Self-interference or Intersymbol interference or ISI 54

145 Multipath Fading Indoor and Outdoor radio channels characterized by multipath reception Sent signal contains direct line-of-sight radio wave, but also a large number of reflected radio waves Outdoors line-of-sight often blocked by obstacles, and collection of differently delayed waves received by antenna Reflected waves interfere with direct wave, causes degradation performance - Waves arrive at slightly different times, so they are out of phase with original wave 55 Randomly boosts or cancels out parts of signal!!

146 Multipath Fading 56

147 Benefits of OFDM Using multiple subcarriers is why OFDM systems more robust to Fading Fading typically decreases received signal strength at particular frequencies With many subcarriers at different frequencies affects only a few of the subcarriers at any given time Error-correcting codes provide redundant information that enables OFDM receivers to restore information lost in erroneous subcarriers a has 4 error-correcting subchannels 57

148 Self or Intersymbol Interference Intersymbol Interference (ISI) Form of signal distortion where one symbol interferes with subsequent symbols Unwanted phenomenon, previous symbols have similar effect as noise Spreading of pulse beyond its allotted time interval causes it to interfere with neighboring pulses ISI is usually caused by multipath propagation 58

149 Intersymbol Interference Symbols arrive at different times due to multipath transmission 59 Receiver must resolve timing differences by waiting for all the echos to arrive

150 Benefits of OFDM Main way to prevent Intersymbol Interference errors Transmit a short block of data (a symbol) Wait until all the multipath echoes fade before sending another symbol Waiting time often referred to as guard interval It is a time between sending subsequent symbols 60

151 Benefits of OFDM Longer guard intervals - more robust system to multipath effects But during guard interval, system gets no use from available spectrum Longer the wait, the lower the effective channel capacity Some guard interval is necessary for any wireless system Goal is to minimize that interval and maximize symbol transmission time 61

152 Benefits of OFDM OFDM meets this challenge by Dividing transmissions among multiple subcarriers Symbol transmission time is multiplied by number of subcarriers For example a, there are 52 channels, so the system has 52 times transmission capacity compared to single channel Each channel can operate at a slower rate, sends 62 fewer symbols per channel

153 OFDM vs. Single Channel 63

154 802.11a 64

155 Intro to a a was approved in September 1999, two years after standard approved Operates in 5 GHz Unlicensed National Information Infrastructure (UNII) band Spectrum is divided into three domains, Each has restrictions imposed on maximum allowed output power 65

156 ISM vs. U-NII 66

157 802.11a OFDM a Specifies 8 20 MHz channels in lower two bands Each divided into 52 sub-carriers (four of which carry pilot data) of 300-kHz bandwidth each 4 20 MHz channels are specified in upper band Receiver processes 52 individual bit streams, reconstructing original high-rate data stream Four complex modulation methods are employed, depending on data rate that can be supported by conditions between transmitter and receiver Include BPSK, QPSK, 16-QAM, and 64-QAM 67

158 802.11a Channels 68

159 802.11a Advantage Since 2.4 GHz band is heavily used, using 5 GHz band gives a advantage of less interference Disadvantage However, high carrier frequency also brings disadvantages It restricts use of a to almost line of sight, necessitating use of more access points It also means that a cannot penetrate as far as b since it is absorbed more readily, other things (such as power) being equal 69

160 802.11b vs a Path Loss Free Space Path Loss in db for 2.4 and 5 GHz Spectrums Distance (miles) 2.4 GHz 5 GHz Loss = 32.4 X 20Log(MHz) X 20Log(distance) 70

161 Range and Data Rate 71

162 802.11g June 2003, a third modulation standard ratified g Works in 2.4 GHz band (like b) Maximum data rate of 54 Mbit/s g hardware works with b hardware Older networks, b node significantly reduces the speed of an g network 72

163 802.11g Uses Multiple Modulation schemes OFDM for data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s, and Reverts to CCK Complimentary Code Keying like b for 5.5 and 11 Mbit/s DBPSK/DQPSK+DSSS for 1 and 2 Mbit/s Even though g operates in same frequency band as b Achieve higher data rates because it uses OFDM and better modulation 73

164 802.11g Rates, Transmission, Modulation Data Rate Mbps Trans Type Modulation 54 OFDM 64 QAM 48 OFDM 64 QAM 11 DSSS QPSK1 6 OFDM BPSK 5.5 DSSS CCK 74

165 802.11n 75

166 802.11n n is long anticipated update to WiFi standards a/b/g 4x increase in throughput Improvement in range n ratified by IEEE

167 802.11n Features n utilizes larger number of antennas Number of antennas relates to number of simultaneous streams Two receivers and two transmitters (2x2) or four receivers and four transmitters (4x4) The standards requirement is a 2x2 with a maximum two streams, but allows 4x4 77

168 802.11n Features n standard operates in 2.4-GHz, the 5GHz radio band, or both more flexibility Backward compatibility with preexisting a/b/g deployment Majority of devices and access points deployed are dual-band Operates in both 2.4-GHz and 5-GHz frequencies 78

169 802.11n Features Wireless solutions based on n standard use several techniques to improve throughput, reliability, and predictability of wireless Three primary innovations are Multiple Input Multiple Output (MIMO) technology Channel bonding (40MHz Channels) Packet aggregation Allows n solutions to achieve fivefold performance increase over a/b/g networks Details follow... 79

170 MIMO n builds on previous standards by adding multiple-input multiple-output (MIMO) MIMO uses multiple transmitter and receiver antennas to improve system performance MIMO uses additional signal paths from each antenna to transmit more information, recombine signals on the receiving end 80

171 MIMO n access points and clients transmit two or more spatial streams Use multiple receive antennas and advanced signal processing to recover multiple transmitted data streams MIMO-enabled access points use spatial multiplexing to transmit different bits of a message over separate antennas Provides greater data throughput 81

172 MIMO Technology Multiple independent streams are transmitted simultaneously to increase the data rate 82

173 MIMO Performance gain is result of MIMO smart antenna technology Allows wireless access points to receive signals more reliably over greater distances than with standard antennas Example Distance from access point for a/g client communicating with a conventional access point drops from 54 Mbps to 48 Mbps or 36 Mbps Same client communicating with MIMO access point still able to operate at 54 Mbps 83

174 Channel Bonding Most straightforward way to increase capacity of network is to increase operating bandwidth However, conventional wireless technologies limited to transmit over one of several 20-MHz channels n networks employ technique called channel bonding to combine two adjacent 20-MHz channels into a single 40-MHz channel Technique more than doubles channel bandwidth 84

175 Channel Bonding Channel bonding most effective in 5-GHz frequency given greater number of available channels 2.4-GHz frequency has only 3 non-overlapping 20-MHz channels Thus, bonding two 20-MHz channels uses two thirds of total frequency capacity So, IEEE has rules on when a device can operate in 40MHz channels in 2.4GHz space to ensure optimal performance 5 GHz has larger number of channels available for bonding 85

176 Packet Aggregation In conventional wireless transmission methods Amount of channel access overhead required to transmit each packet is fixed, regardless of the size of the packet itself As data rates increase, time required to transmit each packet shrinks Overhead cost remains same 86

177 Packet Aggregation n technologies increase efficiency by aggregating multiple data packets into single transmission frame n networks can send multiple data packets with fixed overhead cost of just a single frame Packet aggregation is more beneficial for certain types of applications such as file transfers Real-time applications (e.g. voice) don t benefit from packet aggregation because its packets would need to be interspersed at regular intervals Combining packets into larger payload would introduce unnecessary latency 87

178 Comparison 88

179 Summary From 1999 until years changes in wireless LAN technology From 5.5 Mbps to Mbps and beyond How Parallelism of data streams Increased number of antennas Resolving interference through math and multiplexing Cramming more data within limited frequencies Better modulation techniques Future 89

180 End 90