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# Modulation & Multiple Access Schemes

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### Modulation & Multiple Access Schemes

1. 1. 4S Communica(ons Believes  In  Engineering4S Communications Privileged and Confidential www.4scommunications.com
2. 2. Modula2on  Schemes4S Communications Privileged and Confidential www.4scommunications.com
3. 3. What  is  Modula2on? Modulaon  is   a  process   of  mixing  a   signal   with   a  sinusoid  to   produce  a   new   signal.   This   new   signal,   possibly,   will   have   certain   beneﬁts   of   an   un-­‐modulated   signal,   especially   during   transmission.   If   we   look   at   a   general  func?on  for  a  sinusoid: Information Signal Modulated Signalwe   can   see   that   this   sinusoid   has   3   parameters   that   can   be   altered,   to   Modulatoraﬀect  the  shape  of  the  graph.  The  ﬁrst  term,  A,  is  called  the  magnitude,  or  amplitude  of  the  sinusoid.  The  next   term,    is  known  as  the  frequency,  and  the   last   term,     is   known   as   the   phase   angle.   All   3   parameters   can   be  altered  to  transmit  data. Carrier SignalThe   sinusoidal   signal   that   is   used   in   the   modula?on   is   known   as   the  carrier   signal,   or   simply   "the   carrier".   The   signal   that   is   used   in  modula?ng   the   carrier   signal(or   sinusoidal   signal)   is   known   as   the   "data  signal"   or   the   "message   signal".   It   is   important   to   no?ce   that   a   simple  sinusoidal  carrier  contains  no  informa?on  of  its  own.In  other  words  we  can  say  that  modula?on  is  used  because  the  some  data  signals   are  not   always  suitable   for  direct  transmission,  but   the  modulated  signal  may  be  more  suitable.4S Communications Privileged and Confidential www.4scommunications.com
4. 4. Types  of  Modula2on There  are  3  basic  types  of  modula6on: 1.  AM  (Amplitude  Modula6on:  a  type  of  modula6on  where  the  amplitude  of  the  carrier   signal  is  modulated  (changed)  in  propor6on  to  the  message  signal  while  the  frequency  and   phase  are  kept  constant) 2.  FM  (Frequency  Modula6on:  a  type  of  modula6on  where  the  frequency  of  the  carrier   signal  is  modulated  (changed)  in  propor6on  to  the  message  signal  while  the  amplitude  and   phase  are  kept  constant) 3.  PM  (Phase  Modula6on:  a  type  of  modula6on  where  the  phase  of  the  carrier  signal  is   varied  accordance  to  the  low  frequency  of  the  message  signal  is  known  as  phase   modula6on)4S Communications Privileged and Confidential www.4scommunications.com
5. 5. Amplitude  Modula2on Amplitude  modula6on  (AM)  occurs   Carrier#Signal# Actual#Signal# when  the  amplitude  of  a  carrier  wave   2# is  modulated,  to  correspond  to  a   1.5# 1# informa6on  signal. 0.5# 0# !0.5# 0# 20# 40# 60# 80# 100# 120# 140# 160# 180# 200# Amplitude  modula6on  requires  a  high   !1# !1.5# frequency  constant  carrier  and  a  low   !2# frequency  modula6on  (informa6on)   3.5% Amplitude%Modula8on% signal. 2.5% 1.5% 0.5% Normalized  equa6on  for  amplitude   !0.5% 0% 20% 40% 60% 80% 100% 120% 140% 160% 180% 200% modula6on: !1.5% !2.5% !3.5%4S Communications Privileged and Confidential www.4scommunications.com
6. 6. Frequency  Modula2on Carrier#Signal# Actual#Signal# 2#  Frequency   modula6on   (AM)   occurs   1.5# 1# when   the   frequency   of   a   carrier   0.5# wave  is  modulated,  to  correspond  to   0# !0.5# 0# 20# 40# 60# 80# 100# 120# 140# 160# 180# 200# a  informa6on  signal. !1# !1.5#  In   general   the   frequency   of   the   !2# Frequency#Modula5on# carrier   wave   is   varied   in   accordance   2# with  the  amplitude   and  phase  of  the   1.5# 1# input   signal,   the   amplitude   of   the   0.5# 0# carrier  remaining  unchanged. !0.5# 0# 50# 100# 150# 200# !1# !1.5# !2#4S Communications Privileged and Confidential www.4scommunications.com
7. 7. Phase  Modula2on Carrier#Signal# Actual#Signal#  Phase   modula6on   (AM)   occurs   2# 1.5# when   the   phase   of   a   carrier   wave   1# 0.5# is   modulated,   to   correspond   to   a   0# !0.5# 0# 20# 40# 60# 80# 100# 120# 140# 160# 180# 200# informa6on  signal. !1# !1.5#  In   general   the   phase   of   a   carrier   !2# Phase#Modula5on# wave   is   varied   by   an   amount   2# propor6onal   to   the   instantaneous   1.5# 1# amplitude   of   the   modula6ng   0.5# (informa6on)  signal 0# 0# 20# 40# 60# 80# 100# 120# 140# 160# 180# 200# !0.5# !1# !1.5# !2#4S Communications Privileged and Confidential www.4scommunications.com
8. 8. Phase  shiB  keying  (PSK) In  PSK,  we  change  the  phase  of  the  sinusoidal  carrier  to  indicate  informa6on.  Phase  in  this   context  is  the  star6ng  angle  at  which  the  sinusoid  starts.  To  transmit  0,  we  shiO  the  phase  of   the  sinusoid  by  180°.  Phase  shiO  represents  the  change  in  the  state  of  the  informa6on  in  this   case. 1 -14S Communications Privileged and Confidential www.4scommunications.com
9. 9. Binary  phase  shiB  keying  (BPSK)  BPSK  (also  called  2PSK)  is  the  simplest  form  of  phase  shiO  keying  (PSK).  It  uses  two  phases   which   are   separated   by   180°   and   so   can   also   be   termed   2-­‐PSK.   It   does   not   par6cularly   maTer   exactly   where   the   constella6on   points   are   posi6oned,   and   in   this   ﬁgure   they   are   shown   on  the  real   axis,  at   0°   and   180°.   This  modula6on   is   the  most   robust   of  all   the   PSKs   since   it   takes   the   highest   level   of   noise   or   distor6on   to   make   the   demodulator   reach   an   incorrect  decision.   It   is,   however,   only   able   to  modulate   at   1   bit/symbol   and   so   is  unsuitable   for  high  data-­‐rate  applica6ons  when  bandwidth  is  limited. Informaon  Bit:  0  1  0  1  0  0  1  0  1  0  1  0    Modula6ng  angle  is  deﬁned  as:                                  (for  BPSK:  M=        ); 0 1 Where  M  =  Modula6on  Order           Constellaon  Diagram For  BPSK:  n  =  1(implies  you  can  only  transmit  1  bit)4S Communications Privileged and Confidential www.4scommunications.com
10. 10. Quadrature  phase  shiB  keying  (QPSK)  Quadrature  Phase  ShiO  Keying  (QPSK)  is  a  form  of  Phase  ShiO  Keying  in  which  two  bits  are   modulated  at  once,  selec6ng  one  of   four  possible  carrier  phase  shiOs  (0,  90,  180,  or  270   degrees).  QPSK  allows  the  signal  to  carry  twice  as  much  informa6on  as  ordinary  PSK  using   the  same  bandwidth.  Using  QPSK  we  can  transmit  2  bits/symbol. 10  Modula6ng  angle  is  deﬁned  as:                                  (for  QPSK:  M=      ) 11 00 01 Where  M  =  Modula6on  Order           Constellaon  Diagram For  BPSK:  n  =  2  (implies  you  can  only  transmit  2  bit) Informa6on  Bit  Example:  01  01  00  10  00  10  11  10  10  10  104S Communications Privileged and Confidential www.4scommunications.com
11. 11. Other  Modula2ons  Used  8-­‐PSK  16QAM  64QAM4S Communications Privileged and Confidential www.4scommunications.com
12. 12. Mul2ple  Access  Schemes4S Communications Privileged and Confidential www.4scommunications.com
13. 13. Introduc2on  to  Mul2ple  Access  Diﬀerent  Mul6plex  Schemes  FDMA  TDMA  OFDMA4S Communications Privileged and Confidential www.4scommunications.com
14. 14. Frequency  Division  Mul2ple  Access  (FDMA) • In  FDMA  users  are  assigned  speciﬁc  frequency   bands.  Once  assigned,  the  user  has  the  sole   right  of  using  the  frequency  band  for  the  en6re   dura6on  of  a  call. • Typically  many  users  are  supported,  due  to  the   rela6vely  narrow  spectrum  alloca6on  per  user.   The  Uplink  or  Downlink  receiver  must  use   ﬁltering  to  mi6gate  interference  from  other   users.  4S Communications Privileged and Confidential www.4scommunications.com
15. 15. Time  Division  Mul2ple  Access  (TDMA) • Each  user  is  allowed  to  transmit  only  within  speciﬁed   6me  intervals  (Time  Slots).  Diﬀerent  users  transmit  in   diﬀerent  Time  Slots.   • When  users  transmit,  they  occupy  the  whole  frequency   bandwidth  (separa6on  among  users  is  performed  in  the   6me  domain).   • Like  FDMA,  ﬁltering  is  required  for  both  the  Uplink  and   Downlink  receiver  to  separate  adjacent  carriers.  4S Communications Privileged and Confidential www.4scommunications.com
16. 16. Orthogonal  Frequency  Division  Mul2plexing  (OFDM) • OFDM  uses   a  large  number  of   closely  spaced  narrowband  carriers.  In  a  conven6onal  FDM   system,  the  frequency  spacing  between  carriers  is  chosen  with  a  suﬃcient  guard  band  to   ensure   that   interference   is   minimized   and   can   be   cost   eﬀec6vely   ﬁltered.   In   OFDM,   however,  the  carriers  are  packed  much  closer  together.  This  increases  spectral  eﬃciency  by   u6lizing  a  carrier  spacing  that  is  the  inverse  of  the  symbol  or  modula6on  rate. • High   data   rates   are   achieved   in   OFDM   by   alloca6ng   a   single   data   stream   in   a   parallel   manner  across  mul6ple  subcarriers.  4S Communications Privileged and Confidential www.4scommunications.com
17. 17. FDMA  vs  OFDM FDMA Carrier1 Carrier2 Carrier3 Carrier4 Carrier5 OFDM Spectral Efﬁciency4S Communications Privileged and Confidential www.4scommunications.com
18. 18. OFDM  to  OFDMA • Orthogonal Frequency Division Multiple Access (OFDMA) is a form of OFDM. The description of OFDM up to this point has defined that all subcarriers are assigned to a single user during each subframe, where a subframe is some number of OFDM symbols. In OFDMA, however, multiple users can be assigned subcarriers during the same subframe. Sub Carriers OFDM OFDMA Frequency Frequency Sub Carriers Subframe3 Subframe1 Subframe2 Subframe1 Subframe2 Subframe3 Time Time4S Communications Privileged and Confidential www.4scommunications.com
19. 19. Example  on  how  sub  carriers  are  assigned: • For  example,  consider  a  system  consis6ng  of  18  subcarriers.   • In  OFDM  all  18  subcarriers  would  be  assigned  to  a  single  user  during  each  subframe.   • In  OFDMA,  based  on  both  user  demand  and  channel  condi6ons,  diﬀerent  users  can  be  given   diﬀerent  groups  of  subcarriers  during  each  subframe.   • For  example,  3  users  may  be  assigned  6  subcarriers  each  during  one  subframe.  During  the   next  subframe,  user  1  may  be  assigned  12  subcarriers,  user  2  may  be  assigned  6,  and  user  3   may  be  assigned  0.4S Communications Privileged and Confidential www.4scommunications.com
20. 20. Ques2ons???4S Communications Privileged and Confidential www.4scommunications.com