W-10 Wireless _ Mobile Communication

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					    Lecture-19 & 20
     Narrowband Digital Modulation
 Digital-to-Analog Modulation
 QAM (4-QAM, 8-QAM, 16-QAM, 32-QAM, 64-QAM, 128-QAM,
                 Narrowband Digital Modulation
• Narrowband
• Describes a channel in which the bandwidth of the message does not significantly
  exceed the channel's bandwidth.
• Narrowband describes telecommunication that carries voice information in a
  narrow band of frequencies.
• More specifically, the term has been used to describe a specific frequency range
  set aside by the U.S. FCC for mobile or radio services

• Digital Modulation
• Digital Modulation is the process of changing one of the characteristics of an
  analog signal (Amplitude, Frequency or Phase) based on the information in a
  digital signal (0s & 1s).
• When we transmit data from one computer to another across a public access
  phone line, e.g.
• The original data are digital, but because of telephone wires carry analog signal,
  the data must be converted.

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            Narrowband Digital Modulation (Cont’d)
• Carrier Signal
• In analog transmission, the sending device produces a high frequency signal that
  acts as a basis for the information signal.

• This base signal is called the carrier signal or carrier frequency.

• Digital information is modulated on the carrier signal by modifying one or more of
  its characteristics (amplitude, frequency or phase).

• This kind of modification is called Modulation (or Shift Keying) and the
  information signal is called Modulating Signal.

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              Narrowband Digital Modulation (Cont’d)
• Signal Element versus Data Element
•   In data communications, our goal is to send data elements.
•   A data element is the smallest entity that can represent a piece of information (bit).
•   In digital data communication, a signal element carries data elements.
•   A signal element is a shortest unit (timewise) of a digital signal. Or
•   Data elements are what we need to send; Signal element are we can send.
•   Data element are being carried; Signal elements are the carriers.

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             Narrowband Digital Modulation (Cont’d)
• Bit Rate
• The bit rate is the number of bits sent in 1s, denoted in bit per second (bps).

• Baud Rate
• Baud rate refers to the number of signal units per second that are required to
  represent those bits.
• The baud rate determine the bandwidth required to send the signal.
• Baud rate is less than or equal to the bit rate.

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           Narrowband Digital Modulation (Cont’d)
• To clarify the concept of bauds & bits,
• In transportation, a baud is similar to a car, a bit is similar to a passenger.
• A car can carry one or more passengers. If 1000 cars go from one point to
  another carrying only one passenger (the driver), then 1000 passengers are
  transported. However, if each car carry four passengers, then 4000 passengers
  are transported.
• Note that the number of cars, not the of passengers , determines the traffic and,
  therefore, the need for wider highway.
• Similarly, the number of bauds determines the required bandwidth, not the no;
  of bits.
• Example: An analog signal carries 4 bits per signal element. If 1000 signal
  elements are sent per second, find the bit rate.
• Solution
• Baud rate = Number of signal elements = 1000 baud per second
• Bit rate = Baud rate x Number of bits per signal element
•           = 1000 x 4 = 4000 bps

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                  Digital-to-Analog Modulation
• Three mechanism for modulating digital data to analog signal.: ASK, FSK & PSK.

• In addition, there is a fourth (and better) mechanism that combines changes in
  both Amplitude & Phase called Quadrature Amplitude Modulation (QAM).

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                      Amplitude Shift Keying (ASK)

• In ASK, the amplitude of the carrier signal is varied to represent binary 1 & 0.
• Both frequency & phase remain constant while the amplitude changes.

• Which voltage represent 0 and which represent 1 is left to the system designers.
• The peak amplitude of the signal during each bit duration is constant and its
  values depends on the bit (0 or 1).

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                 Amplitude Shift Keying (ASK) (Cont’d)

• There are several levels (kinds) of signal elements, each with a different
  amplitude, ASK is normally implemented using only two levels.

• This is referred to as Binary Amplitude Shift Keying or On-Off Keying (OOK).
• The peak amplitude of one signal level is 0, the other is the same as the
  amplitude of the carrier frequency.

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                      Frequency Shift Keying (FSK)

• In FSK, the frequency of the carrier signal is varied to represent binary 1 or 0.

• The frequency of the signal during each bit duration is constant and its value
  depends on the bit (0 or 1).
• Both the peak amplitude and phase remain constant.

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                 Frequency Shift Keying (FSK) (Cont’d)

• Considering two carrier frequencies f1and f2, the first carrier frequency if the
  data element is 0, and second if the data element is 1.
• This is the unrealistic example used only for demonstration purposes.
• Normally the carrier frequencies are very high and the difference between them
  is very small.

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            Gaussian Frequency Shift Keying (GFS)

 The FHSS Physical sublayer modulates the data stream using Gaussian
  Frequency Shift Keying (GFSK).
    Each symbol, a zero and a one, is represented by a different
    frequency (2 level GFSK)
    two symbols can be represented by four frequencies (4 level GFSK).
    A Gaussian filter smoothes the abrupt jumps between frequencies.

                            00         01               11       10

                           fc – fd2   fc - fd1         fc + fd1 fc + fd2

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                Modulation Techniques for FHSS

 Gaussian Frequency Shift Keying (GFSK).

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                         Phase Shift Keying (PSK)

• In PSK, the phase of the carrier is varied to represent two or more different signal
• Both peak amplitude and frequency remain constant.

• The 1 bit is representing by a phase starting at 00 and the o bit represented by a
  phase starting at 1800
• Each signal element represent one bit. So the baud rate is equal to bit rate.

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                            Binary Phase Shift Keying (BPSK)

       BPSK (also sometimes called PRK, Phase Reversal Keying, or 2PSK) is the simplest
        form of phase shift keying (PSK).
       In this instance we can transmit 1 bit for every phase shift.
       This is called Binary Phase Shift Keying (BPSK) in 802.11a


            1                    0          o
180                                         0               Change in Phase

                                                                   1 = 1800 Phase Change
                                                                   0 = No Phase Change

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          Differential Binary Phase Shift Keying (DPSK)

 In DPSK, the phase shift is with reference to the previous bit transmitted rather
  than to some constant reference signal
 Binary 0:signal burst with the same phase as the previous one

 Binary 1:signal burst of opposite phase to the preceding one

• DPSK can be significantly simpler to implement than ordinary PSK

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                Quadrature Phase Shift Keying (QPSK)
   QPSK is Digital Modulation Technique use Four Phases
   Also called 4-PSK or Four-level PSK
   Phase shifts separated by multiples of π/2 (90˚).
   Each signal element represents two bits rather than one.
              Bits    Phase                           900

              00        00                  10
              01       900

              10       1800
              11       2700                           2700

                                                    Constellation Diagram
 In QPSK the baud rate is half of the date rate.
 Constellation diagram is a representation of a signal modulated by a digital
  modulation scheme
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      Differential Quadrature Phase Shift Keying (DQPSK)

•   A Zero phase shift from the previous symbol is interpreted as a 00.
•   A 90 degree phase shift from the previous symbol is interpreted as a 01.
•   A 180 degree phase shift from the previous symbol is interpreted as a 11.
•   A 270 degree phase shift from the previous symbol is interpreted as a 10.

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                     Octal Phase Shift Keying (OPSK)
   OPSK is Digital Modulation Technique use Eight Phases
   Also called 8-PSK or Eight-level PSK
   Phase shifts separated by 45˚
   Each signal element represents three bits rather than one.
              Bits     Phase                   011                    001
              000        00                 1350                             450

              001       450                 100                                          000
              010       900
              011       1350
              100       1800                              110
              101       2350                              2700
              110       2700                         Constellation Diagram
              110       3150
 In 8-PSK the bit rate is three time of the baud rate.
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                 Quadrature Amplitude Modulation
 Combine ASK and PSK to obtain high data rate
 Apart from changing the Phase as in QPSK, we can also change Amplitude with
 Use two amplitude level or use two copies of carrier (one shifted by 180° means Two
 Which is equal to 4-PSK = 2 x 2 and this is called QAM
 The number of amplitude changes is equal to phase change.

                                1   1                     0       0   o
                          180                                         0

                                          Constellation Diagram

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        8-Quadrature Amplitude Modulation (8-QAM)

 Use two amplitude level or use two copies of carrier (one shifted by 90° means
  four phases).
 Which is equal to 8-PSK = 2 x 4 and this is called 8-QAM
 The number of amplitude changes is lower than phase change.



                                                   2700 11

                                                             Constellation Diagram
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               16-Quadrature Amplitude Modulation (QAM)

 Use two amplitude level or use two copies of carrier (one shifted by 45° means
  Eight phases).
 Which is equal to 16-QAM = 2 x 8
                                        QAM                     010
                                                        011                    001
             0110              0010                             900
      0111                                                                            450
0100         0101                0011                100
                                         0000        1800                                   00 000
  o                                         o
180                                          0

  1100        1111             1001     1000
      1101    1110         1010       1011                       110                    3150
                                                              Constellation Diagram

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    16-Quadrature Amplitude Modulation (QAM) (Cont’d)

 At telephone line the bandwidth for Modulation Technique   Baud Rate   Bit Rate
  data is 2400Hz
                                         ASK, FSK, PSK          N           N
 If we select 16-QAM, then data rate
  is 4x2400=9600bps                          4PSK               N          2N
 If we select 256-QAM then data
  rate is 8x2400=19200bps                    8PSK               N          3N
 It is possible to transmit at a much
  higher data rate then the                16-QAM               N          4N
  bandwidth of the channel, by using
  suitable modulation technique.           32-QAM               N          5N

                                                64-QAM          N          6N

                                                128-QAM         N          7N

                                                256-QAM         N          8N

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            Modulation Techniques for DSSS

Data Rate       Encoding                             Modulation
1       Barker Coding 11 chips encoding 1 bit           DBPSK
2       Barker Coding      11 chips encoding 1 bit      DQPSK
5.5     CCK Coding         8 chips encode 8 bits        DQPSK
11      CCK Coding         8 chips encode 4 bits        DQPSK

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Modulation Techniques for OFDM

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