Modern Multiplexing Schemes

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Modern Multiplexing Schemes
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Lesson Objectives
By the end of this lesson, you will be able to

• Describe the operation of QPSK multiplexing
• Describe the operation of QAM multiplexing
• Define adaptive modulation
• Describe the characteristics of DSSS
• Describe how DSSS multiplexing operates
• State the characteristics of CDMA

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Conceptual Building Blocks

• Cycle One complete wave
• Bit Binary Digit. A single element of
  information, 0 or 1
• bps bits per second
• Baud a measure of the number of signals sent
• Information can be transmitted using 3
  characteristics of a wave amplitude, frequency,
  phase

Cycle
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Basic Phase Shift Keying Concept
Possible phase shifts
¼
¾
1.0 00 ¼ 900 ½ 1800 ¾ 2700
4 possible states
½
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Phase Shift Concept
0o Shift
90o Shift
270o Shift
180o Shift
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Phase Shifts Applied
900
2700
1800
Each wave is shifted relative to the preceding
wave. The amount of shift communicates the
specific state/value
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Phase Shift Keying

• Four possible phase shifts (compared to the
  previous wave)
• 00 shift No shift
• 900 shift
• 1800 shift
• 2700 shift
• Four shifts ? 4 different states ? 4 bit
  combinations
• 00
• 01
• 10
• 11

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Quadrature Phase Shift Keying (QPSK)

• Convey information through phase changes
• One phase change per time period
• Each change can send 2 bits of information

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QPSK Example
Possible Wave forms
Stream of Signals Transmitted
A
B
D
C
C
D
2700
2700
900
1800
00
Phase Shift
Information Sent
00
10
01
11
11
0010011111
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QPSK Characteristics

• 2 bits per baud
• Robust, relatively simple can tolerate high
  noise levels
• Relatively slow rates, but good quality
• Satellite transmission applications
• Cable TV return paths
• V.22 standard
• 600 baud
• Uses 1200 Hz and 2400 Hz signals to transmit in
  both directions

1200 Hz
2400 Hz
Originating modem
Receiving modem
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Amplitude Modulation
A2
A1
A1
A2
1
1
0
0
2 possible states
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Quadrature Amplitude Modulation

• Builds upon the QPSK concept
• Combines Amplitude Modulation and Phase Shift
  Keying
• In our example - 2 possible states (AM) x 4
  possible states (PSK) 8 possible states
• 8 possible states ? 3 Bits
• Bit rate ? x Baud

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QAM Example
Bit value Amp. Phase shift
000 1 None
001 2 None
010 1 1/4
011 2 1/4
100 1 1/2
101 2 1/2
110 1 3/4
111 2 3/4
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QAM Expansion

• Number of amplitude levels and phase shifts is a
  direct function of line quality
• A Common version is 16 QAM 4 bits per baud
• Other versions 64 QAM, 256 QAM

• 12 phase angles
• 4 have 2 different amplitudes

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QAM Characteristics

• Higher spectral efficiency than QPSK ? more bps
• More bps ? greater vulnerability to noise,
  interference
• 16 QAM more stable than 64 QAM, more stable than
  256 QAM
• Used in standard NTSC Television broadcasts
• 64 QAM, 256 QAM used in digital cable TV and
  cable modems
• Typical cable modem network rates 20 Mbps (16
  QAM) through 40 Mbps (256 QAM) downstream
• Upstream 320 kbps (QPSK) through 10 Mbps (16 QAM)

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Comparison of Modulation Schemes
Increased states ? greater data rates AND
? greater vulnerability to errors
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Adaptive Modulation
Greater Distance ? Greater BER ? Less bps
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Frequency Division Multiplexing
Guardbands
Fq
76 kHz
68 kHz
64 kHz
60 kHz
56 kHz
72 kHz
Review

• Split the frequency band that can be transmitted
  over the media into narrow
• frequency sub-bands
• Assign a communications channel to each of these
  narrow sub-bands
• Modulate information signal with a carrier
  frequency
• All occupy different frequencies
• No interference between them due to guard bands
• Resulting signals are analog

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Discrete Multitone (DMT)

• QAM relatively high bit rate using one carrier
  frequency
• Combine QAM and FDM to achieve even greater
  capacity and efficiency
• The result Discrete Multitone (DMT)
• Developed by Amati Communications in collusion
  with Stanford University, originally for use in
  transmitting video over copper

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DMT Configuration
Voice
1
2
3
256

1.024 MHz
25 kHz
4 KHz
Frequency

• 256 frequency bands (channels), each 4.3125 kHz
  bandwidth (shown). Each channel is called a tone
• Each channel has different carrier frequency
• Each channel uses QAM for greater efficiency

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Digital Subscriber Line

• DSL - Most common application of DMT
• Originally developed as a possible means to offer
  video over copper, to compete with CATV
• Found niche as a way to achieve high speed
  Internet connections over copper wire
• Combines three distinct transmissions
• Voice Transmission
• Downstream data transmission
• Upstream data transmission

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DSL Concepts

• Dedicate one 4 KHz channel to voice (telephone)
• Dedicate remaining channels to high speed data
• Test each channel to determine its ability to
  handle data at an acceptable error rate
• As data is transmitted, allocate it to the
  appropriate channels based upon error rates
• Channels are monitored constantly
• Allocation of data to channels is dynamically
  adjusted, based upon errors

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Impact of Media on DSL

• DSL designed to operate on twisted copper media
• Affected by Attenuation Distortion
• Attenuation is different for each frequency
• Attenuation increases as frequency increases
• Load coils are a fix for attenuation distortion
• Equalizes loss to approximately 3.5 KHz
• Beyond 4 KHZ, loss is extreme

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DSL Typical Data Allocation
Attenuation increases as Frequency increases
Amplitude
Frequency
Attenuation ? Errors Decrease bpc as errors
increase
Bits per channel
Frequency
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DSL Deployment Today

• Copper loops gt 18,000 are loaded to compensate
  for attenuation distortion
• DSL only available in loops up to 18,000
• Data transmission capability decreases as
  distance increases
• Most common form of DSL is ADSL
• Data capability ranges from 600 kbps 7 Mbps
  downstream 128 kbps 1 Mbps upstream, depending
  upon cost and distance

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ADSL System
Voice Network
Voice Switch
DSLAM
Filter
Data Switch
DSL Modem
Data Network
DSLAM DSL Access Multiplexer
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Direct Sequence Spread Spectrum

• Converts a signal to a much wider bandwidth or
  data rate for transmission
• Specific spreading codes are used for each signal
  stream
• Codes pseudo random , pseudo noise
• Random 0s, and 1s appear to have no discernible
  pattern
• Pseudo there is a pattern that actually repeats
• Benefits
• Transmission is spread over broad bandwidth, so
  power is less at any frequency
• Immune to various kinds of noise
• Very difficult to intercept, decode, or jam

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DSSS Codes
Voice Signal
1 1 -1 -1 -1 1 1
1 -1 1 1 -1 1 -1
-1-1 1 -1 1 1 -1
Code Sequence
Voice Bit Stream 64 kbps Chipping Rate 1.25
million chips per second
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DSSS Process - 1
Establish a pseudo random code

1 Chip
Remember that over time this repeats
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DSSS Process - 2
Create a signal stream of information
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DSSS Process - 3
Multiply the signal stream by the pseudo random
code Using the exclusive or function (XOR)
Signal chip are the same Result is high
Signal chip are different Result is low
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DSSS Process 3 Continued
Signal
PN Code
Spread signal

• Frequency is higher than original voice signal
• Resulting sequence looks like noise
• Power (amplitude) in resultant signal is spread,
  so it
• appears to be less reduces interference
  from other DSSS
• All of original information is still contained in
  it

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DSSS Decoding - 1
Spread spectrum signal is received. Multiply it
by the same PN code, using the XOR process
Received signal
PN code
Original information
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DSSS Decoding - 2
How are unwanted signals handled?
Received signal
PN code
Decoded noise
Signal Format
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DSSS Multiple Stations

• Each station requires a different PN code
• Shifting a standard PN code creates a different
  code

Signal
PN Code
Spread signal
New PN Code
Decoded signal with new code noise
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Code Division Multiple Access (CDMA)

• Utilizes DSSS
• Originated in World War II as a code that would
  be difficult to intercept
• Actress Hedy Lamarr and George Antheil held
  original patent to original CDMA concept

• Commercial CDMA was invented by Irwin Jacobs who
  went on to
• found Qualcomm in 1985.
• 1989 Telecommunications Industry Assn agreed upon
  CDMA standard
• CDMA service began in 1996

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CDMA Operation
TRANSMITTING

• Voice signal generated by talking
• Signal is digitized through a PCM scheme
• Digitized signal multiplied by the PN code
• Resultant spread spectrum signal transmitted
  using QAM and/or QPSK techniques
• RECEIVING
• QAM, QPSK signal received and demodulated into
  spread spectrum
• Multiplied by PN code accepted as signal or
  rejected as noise
• Accepted signal converted to analog voice signal

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CDMA Systems

• Superior capacity 10 20 greater than other
  conventional cellular systems
• Power a critical consideration base stations
  control power for all cellular phones
• Low power levels keep system capacity high, and
  save batteries
• Vulnerable to channel pollution too many
  signals from base stations appear at subscribers
  phone, none are dominant
• Channel specific frequency
• Channel bandwidth 1.25 MHz
• 2 11 carrier frequencies/cell.
• 12 40 voice calls/channel

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What Weve Covered

• Describe the operation of QPSK multiplexing
• Describe the operation of QAM multiplexing
• Define adaptive modulation
• Describe the characteristics of DSSS
• Describe how DSSS multiplexing operates
• State the characteristics of CDMA