1. CSCD 433/533 Wireless Networks
Lecture 8
Physical Layer, and b,g,a,n Differences
Winter 2019

2. Topics Physical of 802.11 Spread Spectrum in General
Differences between b,g,a and n
Frequency ranges
Speeds
DSSS Spread Spectrum, b
OFDM and n

3. Physical Layer Services
Physical Layer defines electrical and physical specifications for devices … defines relationship between a device and a transmission medium
Physical layer performs the following services:
Establishment and termination of a connection to a communications medium.
Participation in sharing among multiple users.
For example, contention resolution and flow control.
Modulation or conversion between representation of digital data in user equipment and the corresponding signals transmitted over a communications channel

4. Frequencies
Many techniques have helped to increase throughput as versions evolved over time
First look at frequency bands where is allowed to operate
Then, start examining flavors of
We will start with slowest and end with fastest
Note, we will not cover ac
Will point to some good references

5. Frequency Band ISM: Industry, Science, Medical
unlicensed frequency spectrum: 900Mhz, 2.4Ghz, 5.1Ghz, 5.7Ghz 5

6. IEEE 802.11 Frequency Band Wavelength and 802.11b/g 802.11a 802.11 ac
Early Wireless LANS 6

7. Comparisons of 802.11 Physical Layer
3 Older Versions of Frequency Bands
802.11a GHz
802.11b – 2.5 GHz g – 2.5 GHz
Newest ones
802.11n – 2.5 GHz, 5 GHz
802.11ac GHz

8. Physical Channels
The early 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. 802.11b/g Channels Channel Width = 22 MHz
Channels – 12 – 14, not sanctioned by FCC
2400 – Each channel spaced 5 MHz apart
Only non-overlapping channels are 1, 6 and 11 9

10. 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
25 MHz
Channel 1
2.412 GHz
Channel 11
2.462 GHz
Channel 6
2.437 GHz 10

11. Spread Spectrum

12. 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. 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. 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 to Interference
Stop using device,
Amplify its power or
Move it

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

16. Radio Communications Spread Spectrum continued
On receiver side, spread signal is transformed back to narrow-band and noise is removed
Spread spectrum is a requirement for unlicensed devices
Minimize interference between unlicensed devices

17. Spread Spectrum Defined
Spread spectrum is a method that spreads a narrowband communication signal over a wider range of frequencies for transmission then de-spreads it into 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 …. 17

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

19. Hedy Lamarr
Austrian actress Hedy Lamarr became a pioneer in the field of wireless communications after emigration to United States
With co-inventor George Anthiel, developed "Secret Communications System" to help combat Nazis in World War II
By manipulating radio frequencies at irregular intervals between transmission and reception, invention formed an unbreakable code to prevent classified messages from being intercepted by enemy personnel
Patented in 1941

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

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
802.11a, g and n use this technology
Allows for even faster throughput than DSSS

22. 802.11 Spread Spectrum and Examples

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

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 codes, independent of data
3. Receiver uses same code to recover the data

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 years
Concerned with 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

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

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

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

29. General Model of Spread Spectrum System

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

31. DSSS and HR/DSSS

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
Which means that the chips operate at a higher frequency than the original signal
Each information bit is modulated by this sequence

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 using the same modulation 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

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

36. Figure DSSS example

37. Direct Sequence Spread Spectrum Example
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. 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
Way to share medium among several users
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. CDMA Example
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. DSSS
Chipping Stream
Two costs to increased chipping ratio
Direct cost of more expensive RF components that operate at higher frequencies
Amount of bandwidth required, need more

41. DSSS Encoding DSSS 802.11 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. 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. Orthogonal Frequency Division Multiplexing
OFDM
Orthogonal Frequency Division Multiplexing

44. Intro to Orthogonal Frequency Division Multiplexing (OFDM)
802.11a and g/n based on OFDM
While DSSS spread spectrum achieved data rates up to 11 Mbps, in b by upgrading modulation methods, it had its limits
Symbols could only be sent so fast before symbol interference occurred
Signal multi-path interference created slower data rates
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. 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. 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. 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. OFDM Subchannels in Time and Frequency Domains

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 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

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

54. Harmonics of Fundamental Signal
Harmonics are voltages or currents that operate at a frequency that is an integer (whole-number) multiple of the fundamental frequency.
So given a 50Hz fundamental waveform, a 2nd harmonic frequency would be 100Hz (2 x 50Hz), a 3rd harmonic would be 150Hz (3 x 50Hz)
Harmonics are multiples of a fundamental frequency and can therefore be expressed as:
2ƒ, 3ƒ, 4ƒ
Which as the previous slide shows, are all orthogonal to each other

55. OFDM
Shows parallel nature of subcarriers and orthogonality

56. 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 m2.pdf

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

58. 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
Randomly boosts or cancels out parts of signal !!

59. Multipath Fading

60. 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
802.11a has 4 error-correcting subchannels

61. 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

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

63. 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

64. 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

65. Benefits of OFDM OFDM meets this challenge by
Dividing transmissions among multiple subcarriers
Symbol transmission time is multiplied by number of subcarriers
For example
802.11a, 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 fewer symbols per channel

66. OFDM vs. Single Channel

67. 802.11a

68. Intro to a
802.11a 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

69. ISM vs. U-NII

70. 802.11a OFDM 802.11a 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

71. 802.11a Channels

72. 802.11a Advantage Disadvantage
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

73. 802.11b vs a Path Loss
Free Space Path Loss in dB for 2.4 and 5 GHz Spectrums
Distance (miles) GHz 5 GHz
Loss = 32.4 X 20Log(MHz) X 20Log(distance)

74. Range and Data Rate

75. 802.11g June 2003, a third modulation standard ratified 802.11g
Works in 2.4 GHz band (like b)
Maximum data rate of 54 Mbit/s
802.11g hardware works with b hardware
Older networks, b node significantly reduces the speed of an g network

76. 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

77. 802.11g Rates, Transmission, Modulation
Data Rate Mbps Trans Type Modulation
OFDM QAM
OFDM QAM
DSSS QPSK1
OFDM BPSK
DSSS CCK

78. 802.11n

79. 802.11n
802.11n is long anticipated update to WiFi standards a/b/g
4x increase in throughput
Improvement in range
802.11n ratified by IEEE 2009

80. 802.11n Features 802.11n 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

81. 802.11n Features
802.11n standard operates in 2.4-GHz, the 5- GHz 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

82. 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 ...

83. MIMO 802.11n 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

84. MIMO
802.11n 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

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

86. 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

87. 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
802.11n 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

88. 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

89. 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

90. Packet Aggregation
802.11n technologies increase efficiency by aggregating multiple data packets into single transmission frame
802.11n 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

91. Comparison

92. Summary
From 1999 until 2016 … 17 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 …

93. End