; wireless-system-basics-1-Lec2 Part1.pdf
Documents
Resources
Learning Center
Upload
Plans & pricing Sign in
Sign Out
Your Federal Quarterly Tax Payments are due April 15th Get Help Now >>

wireless-system-basics-1-Lec2 Part1.pdf

VIEWS: 4 PAGES: 56

  • pg 1
									Simplified System Diagrams for Typical RF Tx & Rx




                                             Prof. C. Patrick Yue, ECE, UCSB
Frequency Terminology




   RF – Radio Frequency
      Modulated by baseband (BB) signal
      BB data are digital
   LO – Local Oscillator
      Produced by a PLL
   IF – Intermediate Frequency
      Not used in Direct Conversion transceivers
      Channel selection i performed at IF
      Ch      l l ti is       f      d t
      Often times, the analog-to digital conversion is done at IF
                                                        Prof. C. Patrick Yue, ECE, UCSB
Modulation




             Prof. C. Patrick Yue, ECE, UCSB
Time and Frequency Domain of Analog Modulation




                                           Prof. C. Patrick Yue, ECE, UCSB
Time and Frequency Domain Plot of Digital Modulation




                                             Prof. C. Patrick Yue, ECE, UCSB
Constellation of BPSK and QPSK




                                 Prof. C. Patrick Yue, ECE, UCSB
Transition between Symbols




   Different variations of QPSK is employed to minimized the transitions
        Reduces the channel capacity, e.g. for p/4-QPSK, it achieves ~1.62 bit/Hz < the limit of 2
        R d       th h        l      it       f    /4 QPSK       hi      1 62 bit/H th li it f
        bit/Hz
   A major objective in digital modulation is to ensure that the RF trajectory from one phase state
   to another does not go through the origin.
        The transition is slow so that if the trajectory goes through the origin, the amplitude of the
        carrier will be below the noise floor for a considerable time and it will not be possible to
        recover its frequency (increases BER).
                                                                                    Prof. C. Patrick Yue, ECE, UCSB
Channel Capacity of Different Modulation Scheme




   Bandwidth, Capacity and Eb/No requirements for symbol error
   rate of 10^-5 (what is the equivalent BER?)
     S       htt //          d i       / ti l / i t bl A ti l jht l? ti l ID 208802825
     Source: http://www.commsdesign.com/article/printableArticle.jhtml?articleID=208802825



                                                                                     Prof. C. Patrick Yue, ECE, UCSB
Modulation Scheme and BER




                            Prof. C. Patrick Yue, ECE, UCSB
Trade-Offs in Modulation Scheme
                             vs.
   Bandwidth (Throughput) vs Power Consumption
   Constant amplitude, aka constant envelop, modulation is more power
   efficient but less throughput
      U d in systems where power is scared
      Used i    t     h          i       d
         Cellular
         Space communication
                             information,
   Modulation with amplitude information like quadrature amplitude
   modulation (QAM) less power efficient, but provides more throughput
      Used in systems where spectrum is at a premiere
                            DSL giga bit
         Wireline examples: DSL, giga-bit Ethernet
         Wireless examples: WLAN (IEEE 802.11)




                                                              Prof. C. Patrick Yue, ECE, UCSB
      Amplitude Modulation (Transmitter)
                                             Transmitter Output


       0
                 x(t)                 y(t)



                        2cos(2πfot)




           Vary the amplitude of a sine wave at carrier frequency fo
           according to a baseband modulation signal
           DC component of baseband modulation signal
           influences transmit signal and receiver possibilities
           - DC value greater than signal amplitude shown above
                 Allows simple envelope detector for receiver
                 Creates spurious tone at carrier frequency (wasted power)
M.H. Perrott                                                           MIT OCW
      Frequency Domain View of AM Transmitter
                       X(f)
          avg(x(t))
                                                                 Transmitter Output
                                                  f                    Y(f)
                        0
                      x(t)                 y(t)
                                                                                           f
                                                           -fo          0             fo

                             2cos(2πfot)
               1                                  1

                                                             f
               -fo               0                    fo


          Baseband signal is assumed to have a nonzero DC
          component in above diagram
           - Causes impulse to appear at DC in baseband signal
           - Transmitter output has an impulse at the carrier frequency
                       For coherent detection, does not provide key information
                       about information in baseband signal, and therefore is a
                       waste of power
M.H. Perrott                                                                                   MIT OCW
     Impact of Having Zero DC Value for Baseband Signal
                       X(f)
          avg(x(t))
            =0
                                                                 Transmitter Output
                                                  f                    Y(f)
                        0
                      x(t)                 y(t)
                                                                                           f
                                                           -fo          0             fo

                             2cos(2πfot)
               1                                  1

                                                             f
               -fo               0                    fo



          Impulse in DC portion of baseband signal is now gone
           - Transmitter output now is now free from having an
                   impulse at the carrier frequency (for ideal
                   implementation)



M.H. Perrott                                                                                   MIT OCW
      Frequency Domain View of AM Receiver (Coherent)
                      X(f)

                                                                    Transmitter Output
                                                 f                        Y(f)
                       0
                     x(t)                 y(t)
                                                                                                          f
                                                          -fo               0                    fo

                            2cos(2πfot)                                                                                   Z(f)
               1                                 1                                              Lowpass                          Receiver Output
                                                            f
               -fo              0                    fo
                                                                                                                                                   f
                                                                                       -2fo           -fo                  0        fo     2fo
                                                                                                Lowpass
                                                                         y(t)            z(t)                      r(t)



                                                                                2cos(2πfot)
                                                                1                                     1

                                                                                                               f
                                                                -fo                0                      fo

M.H. Perrott                                                                                                                                     MIT OCW
      Impact of 90 Degree Phase Misalignment
                      X(f)

                                                                    Transmitter Output
                                                 f                        Y(f)
                       0
                     x(t)                 y(t)
                                                                                                        f
                                                          -fo               0                    fo

                            2cos(2πfot)                                                                                 Z(f)
               1                                 1                                              Lowpass                        Receiver Output
                                                            f                                                                        =0
               -fo              0                    fo
                                                                                                                                         2fo
                                                                                                                                                f
                                                                                       -2fo           -fo                0        fo
                                                                                                Lowpass
                                                                         y(t)            z(t)                    r(t)



                                                                                2sin(2πfot)
                                                                j
                                                                                                        fo
                                                                                                             f
                                                                -fo                0
                                                                                                   -j
M.H. Perrott                                                                                                                                   MIT OCW
      Quadrature Modulation

                        1                1
                                                              Transmitter Output
                                                   f
     It(f)               -fo         0        fo                     Yi(f)
      1                                                   1                  1
                      it(t)

                  f                                                                    f
                                                              -fo     0           fo
             0        2cos(2πfot)                      y(t)
     Qt(f)
                       2sin(2πfot)
      1                                                   j         Yq(f)
                      qt(t)
                                                                                  fo
                  f                                                                    f
             0           j                                    -fo     0
                                              fo                             -j
                                                   f
                          -fo        0
                                         -j

                 Takes advantage of coherent receiver’s sensitivity to
                 phase alignment with transmitter local oscillator
                 - We essentially have two orthogonal transmission
                   channels (I and Q) available to us
                 - Transmit two independent baseband signals (I and Q)
                      onto two sine waves in quadrature at transmitter
M.H. Perrott                                                                               MIT OCW
      Accompanying Receiver

                         1                1                                                   1                  1
                                                               Transmitter Output                                                 Receiver Output
                                                    f                                                                      f
     It(f)                -fo         0        fo                     Yi(f)
                                                                                               -fo        0           fo                 Ir(f)
      1                                                    1                  1                                        Lowpass            2
                       it(t)                                                                                                     ir(t)

                   f                                                                    f                                                           f
                                                               -fo     0           fo
             0         2cos(2πfot)                      y(t)                                y(t)         2cos(2πfot)                         0
     Qt(f)                                                                                                                               Qr(f)
                        2sin(2πfot)                                                                      2sin(2πfot) Lowpass
      1                                                    j         Yq(f)                                                                2
                       qt(t)                                                                                                     qr(t)
                                                                                   fo
                   f                                                                    f                                                           f
             0            j                                    -fo     0                       j                                             0
                                               fo                             -j                                      fo
                                                    f                                                                      f
                           -fo        0                                                            -fo    0
                                          -j                                                                     -j

             Demodulate using two sine waves in quadrature at
             receiver
                 - Must align receiver LO signals in frequency and phase to
                   transmitter LO signals
                      Proper alignment allows I and Q signals to be recovered as
                      shown
M.H. Perrott                                                                                                                                     MIT OCW
      Impact of 90 Degree Phase Misalignment

                        1                1                                                    j
                                                              Transmitter Output                                     f1        Receiver Output
                                                   f                                                                      f
     It(f)               -fo         0        fo                     Yi(f)                        -f1    0                            Ir(f)
      1                                                   1                  1                                  -j                     2
                      it(t)                                                                                                   ir(t)

                  f                                                                    f                                                         f
                                                              -fo     0           fo
             0        2cos(2πfot)                      y(t)                                y(t)         2sin(2πfot)                        0
     Qt(f)                                                                                                                            Qr(f)
                       2sin(2πfot)                                                                      2cos(2πfot)
      1                                                   j         Yq(f)                                                              2
                      qt(t)                                                                                                   qr(t)
                                                                                  fo
                  f                                                                    f                                                         f
             0           j                                    -fo     0                      1                   1                         0
                                              fo                             -j
                                                   f                                                                      f
                          -fo        0                                                        -fo        0           fo
                                         -j

                 I and Q channels are swapped at receiver if its LO
                 signal is 90 degrees out of phase with transmitter
                 - However, no information is lost!
                 - Can use baseband signal processing to extract I/Q
                      signals despite phase offset between transmitter and
                      receiver
M.H. Perrott                                                                                                                                  MIT OCW
      Simplified View
               Baseband Input                                                      Receiver Output
                   It(f)                                                              Ir(f)
                    1                                          Lowpass                 2
                                   it(t)                                   ir(t)

                               f                                                              f
                           0           2cos(2πfot)                                        0
                                                     2cos(2πfot)
                  Qt(f)                2sin(2πfot)   2sin(2πfot)                     Qr(f)
                   1                                             Lowpass              2
                                   qt(t)                                   qr(t)

                               f                                                              f
                           0                                                              0
          For discussion to follow, assume that
           - Transmitter and receiver phases are aligned
           - Lowpass filters in receiver are ideal
           - Transmit and receive I/Q signals are the same except for
                 scale factor                                      add AWGN Channel and Rayleigh Fading
                                                                   Channel in Simulink
          In reality
           - RF channel adds distortion, causes fading
           - Signal processing in baseband DSP used to correct
                 problems
M.H. Perrott                                                                                          MIT OCW
      Analog Modulation
               Baseband Input                                                       Receiver Output

                                                                Lowpass
                                    it(t)                                   ir(t)
                                t                                                                     t
                                        2cos(2πf1t)   2cos(2πf1t)
                                        2sin(2πf1t)   2sin(2πf1t)
                                                                  Lowpass
                                    qt(t)                                   qr(t)
                                t                                                                     t



          I/Q signals take on a continuous range of values (as
          viewed in the time domain)
          Used for AM/FM radios, television (non-HDTV), and
          the first cell phones
          Newer systems typically employ digital modulation
          instead


M.H. Perrott                                                                                              MIT OCW
      Digital Modulation

           Baseband Input                                                       Receiver Output
                                                            Lowpass
                                it(t)                                   ir(t)

                            t                                                                     t
                                    2cos(2πf1t)   2cos(2πf1t)
                                    2sin(2πf1t)   2sin(2πf1t)
                                                              Lowpass
                                qt(t)                                   qr(t)

                            t                                                                     t
                                                             Decision
                                                            Boundaries                 Sample
                                                                                        Times

          I/Q signals take on discrete values at discrete time
          instants corresponding to digital data
           - Receiver samples I/Q channels
                   Uses decision boundaries to evaluate value of data at
                   each time instant
          I/Q signals may be binary or multi-bit
           - Multi-bit shown above
M.H. Perrott                                                                                      MIT OCW
      Advantages of Digital Modulation

          Allows information to be “packetized”
           - Can compress information in time and efficiently send
             as packets through network
           - In contrast, analog modulation requires “circuit-
               switched” connections that are continuously available
                  Inefficient use of radio channel if there is “dead time” in
                  information flow
          Allows error correction to be achieved
           - Less sensitivity to radio channel imperfections
          Enables compression of information
           - More efficient use of channel
          Supports a wide variety of information content
           - Voice, text and email messages, video can all be
               represented as digital bit streams

M.H. Perrott                                                                    MIT OCW
     Add summary of modulation schemes




Constant Envelope Modulation
      The Issue of Power Efficiency
           Baseband                           Power Amp
             Input                                            Transmitter
                      Baseband to RF
                        Modulation                              Output




               Variable-Envelope Modulation          Constant-Envelope Modulation




      Power amp dominates power consumption for many wireless
      systems
       - Linear power amps more power consuming than nonlinear ones
      Constant-envelope modulation allows nonlinear power amp
       - Lower power consumption possible
M.H. Perrott                                                                        MIT OCW
      Simplified Implementation for Constant-Envelope
                Baseband to RF Modulation   Power Amp
     Baseband
       Input       Transmit                                 Transmitter
                     Filter                                   Output


                                                   Constant-Envelope Modulation




       Constant-envelope modulation limited to phase and
       frequency modulation methods
       Can achieve both phase and frequency modulation with
       ideal VCO
        - Use as model for analysis purposes
        - Note: phase modulation nearly impossible with practical VCO
M.H. Perrott                                                                      MIT OCW
      Example Constellation Diagram for Phase Modulation
                                          Q

                                       000
                                 100                001


                  Decision 101                            011
                 Boundaries                                     I



                                 111                010
                                             110

                                        Decision
                                       Boundaries

          I/Q signals must always combine such that amplitude
          remains constant
           - Limits constellation points to a circle in I/Q plane
           - Draw decision boundaries about different phase regions
M.H. Perrott                                                        MIT OCW
      Transitioning Between Constellation Points
                                           Q

                                        000
                                  100                001


                   Decision 101                            011
                  Boundaries                                     I



                                  111                010
                                              110

                                         Decision
                                        Boundaries

         Constant-envelope requirement forces transitions to
         allows occur along circle that constellation points sit on
          - I/Q filtering cannot be done independently!
          - Significantly impacts output spectrum
M.H. Perrott                                                         MIT OCW
    Constellation Diagram
                                        Q
                              00   01       11   10

                                                      00

                  Decision                            01
                 Boundaries                                I
                                                      11

                                                      10



                                    Decision
                                   Boundaries

          We can view I/Q values at sample instants on a two-
          dimensional coordinate system
          Decision boundaries mark up regions corresponding
          to different data values
          Gray coding used to minimize number of bit errors
          that occur if wrong decision is made due to noise
M.H. Perrott                                                  MIT OCW
      Impact of Noise on Constellation Diagram
                                         Q
                               00   01       11   10

                                                       00

                   Decision                            01
                  Boundaries                                I
                                                       11

                                                       10



                                     Decision
                                    Boundaries


          Sampled data values no longer land in exact same
          location across all sample instants
          Decision boundaries remain fixed
          Significant noise causes bit errors to be made

M.H. Perrott                                                    MIT OCW
      Transition Behavior Between Constellation Points
                                        Q
                              00   01       11   10

                                                      00

                  Decision                            01
                 Boundaries                                I
                                                      11

                                                      10



                                    Decision
                                   Boundaries


       Constellation diagrams provide us with a snapshot of I/Q
       signals at sample instants
       Transition behavior between sample points depends on
       modulation scheme and transmit filter

M.H. Perrott                                                   MIT OCW
      Choosing an Appropriate Transmit Filter
                data(t)                       p(t)                x(t)
           Td


                                  t
                                          *   Td
                                                          t                      t



                       Sdata(f)                 |P(f)|2                  Sx(f)




                                      f        0 1/Td
                                                              f     0 1/Td
                                                                                      f
                   0

        Transmit filter, p(t), convolved with data symbols that are
        viewed as impulses
         - Example so far: p(t) is a square pulse
        Output spectrum of transmitter corresponds to square of
        transmit filter (assuming data has white spectrum)
         - Want good spectral efficiency (i.e. narrow spectrum)
M.H. Perrott                                                                         MIT OCW
      Highest Spectral Efficiency with Brick-wall Lowpass
                data(t)                         p(t)                  x(t)
           Td


                                  t
                                          *   -Td 0 Td
                                                              t                      t



                       Sdata(f)                    |P(f)|2                   Sx(f)




                                      f           0 1/(2Td)
                                                                  f     0 1/(2Td)
                                                                                          f
                   0

        Use a sinc function for transmit filter
         - Corresponds to ideal lowpass in frequency domain
        Issues
         - Nonrealizable in practice
         - Sampling offset causes significant intersymbol interference
M.H. Perrott                                                                             MIT OCW
      Requirement for Transmit Filter to Avoid ISI
               data(t)           p(t)                      x(t)
                   Td                   Td                        Td


                         t
                             *               t                         t




          Time samples of transmit filter (spaced Td apart) must
          be nonzero at only one sample time instant
           - Sinc function satisfies this criterion if we have no offset
                in the sample times
          Intersymbol interference (ISI) occurs otherwise
          Example: look at result of convolving p(t) with 4
          impulses
           - With zero sampling offset, x(kT ) correspond to
                                                 d
                associated impulse areas

M.H. Perrott                                                           MIT OCW
      Derive Nyquist Condition for Avoiding ISI (Step 1)
               data(t)               p(t)                  x(t)
                   Td                       Td                     Td


                             t
                                 *               t                      t




         impulse train               p(t)                 p(kTd)
                  Td
                                            Td                     Td


                         t                       t                      t




         Consider multiplying p(t) by impulse train with period Td
          - Resulting signal must be a single impulse in order to
               avoid ISI (same argument as in previous slide)

M.H. Perrott                                                            MIT OCW
      Derive Nyquist Condition for Avoiding ISI (Step 2)

         impulse train             P(f)   1/Td               F{p(kTd)}   1/Td
                1/Td



                         f
                             *                   f                              f
                                    0


         impulse train             p(t)                        p(kTd)
                 Td
                                          Td                             Td


                         t                       t                              t


         In frequency domain, the Fourier transform of sampled
         p(t) must be flat to avoid ISI
          - We see this in two ways for above example
                  Fourier transform of an impulse is flat
                  Convolution of P(f) with impulse train in frequency is flat
M.H. Perrott                                                                    MIT OCW
      A More Practical Transmit Filter

          Raised-cosine filter is quite popular in many applications
                         p(t)                                    P(f)


                                                             1-α 1+α
                                                             2Td 2Td
                                       t                                    f
                                                   -1/Td    0      1/Td
               -Td   0   Td




         Transition band in frequency set by “rolloff” factor, α


               Rolloff factor = 0: P(f) becomes a brick-wall filter
               Rolloff factor = 1: P(f) looks nearly like a triangle
               Rolloff factor = 0.5: shown above
M.H. Perrott                                                              MIT OCW
      Raised-Cosine Filter Satisfies Nyquist Condition
               Nyquist Condition                Nyquist Condition
               Observed in Time               Observed in Frequency
                            p(t)                            P(f)




                                      t                                   f
                                                -1/Td   0     1/Td
                 -Td    0   Td


          In time
           - p(kT ) = 0 for all k not equal to 0
                    d
          In frequency
           - Fourier transform of p(kT ) is flat
           - Alternatively: Addition of shifted P(f) centered about k/T
                                          d

                                                                      d
               leads to flat Fourier transform (as shown above)

M.H. Perrott                                                          MIT OCW
      Spectral Efficiency With Raised-Cosine Filter
               data(t)                         p(t)                  x(t)
         Td

                                                                                    t
                                 t
                                         *   -Td 0 Td
                                                             t



                      Sdata(f)                    |P(f)|2                   Sx(f)




                                     f           0 1/(2Td)
                                                                 f     0 1/(2Td)
                                                                                          f
                  0

         More efficient than when p(t) is a square pulse
         Less efficient than brick-wall lowpass
          - But implementation is much more practical
         Note: Raised-cosine P(f) often “split” between
         transmitter and receiver
M.H. Perrott                                                                            MIT OCW
      Receiver Filter: ISI Versus Noise Performance

                Raised-Cosine
                    Filter
                                                                 Lowpass
               It(t)           it(t)                     ir(t)
                                                                             Ir(t)
                           f
      Baseband                     2cos(2πf1t)   2cos(2πf1t)               Receiver
        Input                      2sin(2πf1t)   2sin(2πf1t)                Output
                                                                 Lowpass
               Qt(t)           qt(t)                    qr(t)
                                                                            Qr(t)
                           f



          Conflicting requirements for receiver lowpass
           - Low bandwidth desirable to remove receiver noise and
             to reject high frequency components of mixer output
           - High bandwidth desirable to minimize ISI at receiver
                  output



M.H. Perrott                                                                          MIT OCW
    Split Raised-Cosine Filter Between Transmitter/Receiver
                                                                            Additional
                Raised-Cosine                               Raised-Cosine   Lowpass
                    Filter                                      Filter       Filtering
               It(t)           it(t)                     ir(t)
                                                                                   Ir(t)
                           f                                        f       f
      Baseband                     2cos(2πf1t)   2cos(2πf1t)                    Receiver
        Input                      2sin(2πf1t)   2sin(2πf1t)                     Output
               Qt(t)           qt(t)                    qr(t)
                                                                                   Qr(t)
                           f                                        f       f


          We know that passing data through raised-cosine
          filter does not cause additional ISI to be produced
           - Implement P(f) as cascade of two filters corresponding
                  to square root of P(f)

                       Place one in transmitter, the other in receiver
          Use additional lowpass filtering in receiver to further
          reduce high frequency noise and mixer products
M.H. Perrott                                                                          MIT OCW
      Modeling The Impact of VCO Phase Modulation

       Recall unmodulated VCO model                                                            Phase
                                                                            Sout(f)            Noise
               Phase/Frequency                                                                         Spurious
               modulation Signal                  Overall
                                                phase noise                                             Noise
                       SΦmod(f)
                                                Φtn(t)                                                   f
                                                                                         fo
                                      Φmod(t)            Φout                         out(t)
                                  f                             2cos(2πfot+Φout(t))
                      0
       Relationship between sine wave output and instantaneous
       phase

       Impact of modulation
        - Same as examined with VCO/PLL modeling, but now we
           consider Φout(t) as sum of modulation and noise components



M.H. Perrott                                                                                                      MIT OCW
      Relationship Between Sine Wave Output and its Phase

          Key relationship (note we have dropped the factor of 2)



          Using a familiar trigonometric identity




          Approximation given |Φtn(t)| << 1




M.H. Perrott                                                  MIT OCW
      Relationship Between Output and Phase Spectra

          Approximation from previous slide



          Autocorrelation (assume modulation signal
          independent of noise)



          Output spectral density (Fourier transform of
          autocorrelation)



           - Where * represents convolution and
M.H. Perrott                                              MIT OCW
      Impact of Phase Modulation on the Output Spectrum
                                                                                                  Phase
                                                                               Sout(f)            Noise
               Phase/Frequency                                                                            Spurious
               modulation Signal                     Overall
                                                   phase noise                                             Noise
                       SΦmod(f)
                                                   Φtn(t)                                                   f
                                                                                            fo
                                         Φmod(t)            Φout                         out(t)
                                  f                                2cos(2πfot+Φout(t))
                      0

                                                                               Sout(f)
               Phase/Frequency
               modulation Signal                     Overall
                                                   phase noise
                          SΦmod(f)
                                                   Φtn(t)                                                   f
                                                                                            fo
                                         Φmod(t)            Φout                         out(t)
                                     f                             2cos(2πfot+Φout(t))
                      0

          Spectrum of output is distorted compared to SΦmod(f)
          Spurs converted to phase noise
M.H. Perrott                                                                                                         MIT OCW
      I/Q Model for Phase Modulation



          Applying trigonometric identity


          Can view as I/Q modulation
          - I/Q components are coupled and related nonlinearly to
               Φmod(t)
                                            Sa(f)

                                    it(t)
      SΦmod(f)       cos(Φmod(t))
                                                    f                             Sy(f)
                                            0
           Φmod(t)                                      cos(2πf2t)   y(t)
                                            Sb(f)       sin(2πf2t)
                 f                                                                             f
      0                             qt(t)                                   -fo   0       fo
                     sin(Φmod(t))
                                                    f
                                            0
M.H. Perrott                                                                              MIT OCW
Multiple Access Techniques
      The Issue of Multiple Access

          Want to allow communication between many different
          users
          Freespace spectrum is a shared resource
           - Must be partitioned between users
          Can partition in either time, frequency, or through
          “orthogonal coding” (or nearly orthogonal coding) of
          data signals




M.H. Perrott                                                 MIT OCW
      Frequency-Division Multiple Access (FDMA)
                       Channel   Channel   Channel
                          1         2        N


                                                     f


         Place users into different frequency channels
         Two different methods of dealing with transmit/receive of
         a given user
          - Frequency-division duplexing
          - Time-division duplexing



M.H. Perrott                                                  MIT OCW
      Frequency-Division Duplexing

                                       Duplexer             Antenna
                  Transmitter           TX    RX
                                TX

                                                        f
                                RX   Transmit Receive
                   Receiver            Band    Band


        Separate frequency channels into transmit and receive
        bands
        Allows simultaneous transmission and reception
         - Isolation of receiver from transmitter achieved with duplexer
         - Cannot communicate directly between users, only between
               handsets and base station
        Advantage: isolates users
        Disadvantage: deplexer has high insertion loss (i.e.
        attenuates signals passing through it)
M.H. Perrott                                                          MIT OCW
      Time-Division Duplexing

                                     Switch      Antenna
               Transmitter
                             TX



                Receiver     RX

                                  switch
                                  control
          Use any desired frequency channel for transmitter and
          receiver
          Send transmit and receive signals at different times
          Allows communication directly between users (not
          necessarily desirable)
          Advantage: switch has low insertion loss relative to
          duplexer
          Disadvantage: receiver more sensitive to transmitted
          signals from other users
M.H. Perrott                                                 MIT OCW
      Time-Division Multiple Access (TDMA)
               Time Slot Time Slot Time Slot     Time Slot Time Slot
                  N         1         2             N         1

                                                                       t


                                    Time Frame


          Place users into different time slots
           - A given time slot repeats according to time frame period
          Often combined with FDMA
           - Allows many users to occupy the same frequency
               channel




M.H. Perrott                                                               MIT OCW
    Channel Partitioning Using (Nearly) “Orthogonal Coding”
                      Uncorrelated Signals                 Correlated Signals

                  1            x(t)          y(t)      1           x(t)         y(t)
               A                                    B
                 -1                                   -1
                  1            c(t)                    1           c(t)
               B                                    B
                 -1                                   -1
                  1                                    1
                 -1                                   -1

                  1            x(t)          y(t)      1           x(t)         y(t)
               -A                                   -B
                 -1                                   -1
                  1            c(t)                    1           c(t)
               B                                    B
                 -1                                   -1
                  1                                    1
                 -1                                   -1


          Consider two correlation cases
           - Two independent random Bernoulli sequences
               Result is a random Bernoulli sequence
           - Same Bernoulli sequence
                      Result is 1 or -1, depending on relative polarity
M.H. Perrott                                                                           MIT OCW
      Code-Division Multiple Access (CDMA)
                                                          Separate         Transmit Signals
                     Td                                 Transmitters           Combine
                                                      x1(t)       y1(t)      in Freespace

                                                      PN1(t)                        y(t)


                                                      x2(t)        y2(t)

                                                      PN2(t)


                             Tc

         Assign a unique code sequence to each transmitter
         Data values are encoded in transmitter output stream by
         varying the polarity of the transmitter code sequence
          - Each pulse in data sequence has period T           d
               Individual pulses represent binary data values
          - Each pulse in code sequence has period T           c
               Individual pulses are called “chips”
M.H. Perrott                                                                           MIT OCW
    Receiver Selects Desired Transmitter Through Its Code
                                                   Separate                Transmit Signals
                                                 Transmitters                  Combine
                                               x1(t)       y1(t)             in Freespace

                                               PN1(t)


                                               x2(t)             y2(t)

                                               PN2(t)




                                                                                PN1(t)
                                                              Lowpass
                                                       r(t)              x(t)       y(t)


                                                               Receiver
                                                         (Desired Channel = 1)



      Receiver correlates its input with desired transmitter code
          - Data from desired transmitter restored
          - Data from other transmitter(s) remains randomized
M.H. Perrott                                                                               MIT OCW
      Frequency Domain View of Chip Vs Data Sequences
               Tc   data(t)                             p(t)                                     x(t)
          1                                            1                              1



          -1
                                     t
                                             *             Tc
                                                                       t
                                                                                      -1
                                                                                                                t


                    data(t)                                p(t)                                  x(t)
          1                                           1                               1
                       Td


          -1
                                     t
                                             *             Td
                                                                       t
                                                                                      -1
                                                                                                                t



                          Sdata(f)                           |P(f)|2                                    Sx(f)
                                                      Td                                        Td



                                                 Tc                                        Tc
                                         f                                        f                                        f
                      0                                     0 1/Td         1/Tc                      0 1/Td         1/Tc

        Data and chip sequences operate on different time scales
         - Associated spectra have different width and height
M.H. Perrott                                                                                                         MIT OCW
      Frequency Domain View of CDMA
                Sx1(f)                                     Sy1(f)
          Td
                             Transmitter 1
                             x1(t)        y1(t)
                                                  Tc
                              f                                            f                              Sx(f)
               0 1/Td                PN1(t)            0            1/Tc
                                                                                      PN1(t)        Td       Sx1(f)
                Sx2(f)                                     Sy2(f)
          Td                                                                                                      Lowpass
                                                                               y(t)
                             Transmitter 2                                                                                       r(t)
                             x2(t)        y2(t)                                                Tc
                                                  Tc
                                                                                                                             f
                                                                                                         0 1/Td       1/Tc
                              f                                            f
               0 1/Td                PN2(t)            0            1/Tc

          CDMA transmitters broaden data spectra by encoding it
          onto chip sequences
          CDMA receiver correlates with desired transmitter code
           - Spectra of desired channel reverts to its original width
           - Spectra of undesired channel remains broad
                         Can be “mostly” filtered out by lowpass
M.H. Perrott                                                                                                           MIT OCW

								
To top