multiple access for wireless mobile communications

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Subject: wireless mobile communication.
Topics covered : FDMA, TDMA , Spread Spectrum Multiple Access,Space Division Multiple Access,Capacity of Cellular systems

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multiple access for wireless mobile communications

  1. 1. MULTIPLE ACCESS TECHNIQUES FOR WIRELESS COMMUNICATIONS UNIT - IV
  2. 2. CONTENTS <ul><li>FDMA </li></ul><ul><li>TDMA </li></ul><ul><li>SDMA </li></ul><ul><li>SSMA </li></ul>
  3. 3. Introduction <ul><li>many users at same time </li></ul><ul><li>share a finite amount of radio spectrum </li></ul><ul><li>high performance </li></ul><ul><li>duplexing generally required </li></ul><ul><li>frequency domain </li></ul><ul><li>time domain </li></ul>
  4. 4. Frequency division duplexing (FDD) <ul><li>two bands of frequencies for every user </li></ul><ul><li>forward band </li></ul><ul><li>reverse band </li></ul><ul><li>duplexer needed </li></ul><ul><li>frequency seperation between forward band and reverse band is constant </li></ul>frequency seperation reverse channel forward channel f
  5. 5. Time division duplexing (TDD) <ul><li>uses time for forward and reverse link </li></ul><ul><li>multiple users share a single radio channel </li></ul><ul><li>forward time slot </li></ul><ul><li>reverse time slot </li></ul><ul><li>no duplexer is required </li></ul>time seperation t forward channel reverse channel
  6. 6. Multiple Access Techniques <ul><li>Frequency division multiple access (FDMA) </li></ul><ul><li>Time division multiple access (TDMA) </li></ul><ul><li>Code division multiple access (CDMA) </li></ul><ul><li>Space division multiple access (SDMA) </li></ul><ul><li>grouped as: </li></ul><ul><li>narrowband systems </li></ul><ul><li>wideband systems </li></ul>
  7. 7. Narrowband systems <ul><li>large number of narrowband channels </li></ul><ul><li>usually FDD </li></ul><ul><li>Narrowband FDMA </li></ul><ul><li>Narrowband TDMA </li></ul><ul><li>FDMA/FDD </li></ul><ul><li>FDMA/TDD </li></ul><ul><li>TDMA/FDD </li></ul><ul><li>TDMA/TDD </li></ul>
  8. 8. Wideband systems <ul><li>large number of transmitters on one channel </li></ul><ul><li>TDMA techniques </li></ul><ul><li>CDMA techniques </li></ul><ul><li>FDD or TDD multiplexing techniques </li></ul><ul><li>TDMA/FDD </li></ul><ul><li>TDMA/TDD </li></ul><ul><li>CDMA/FDD </li></ul><ul><li>CDMA/TDD </li></ul>
  9. 9. Multiple Access Techniques in use Multiple Access Technique Advanced Mobile Phone System (AMPS) FDMA/FDD Global System for Mobile (GSM) TDMA/FDD US Digital Cellular (USDC) TDMA/FDD Digital European Cordless Telephone (DECT) FDMA/TDD US Narrowband Spread Spectrum (IS-95) CDMA/FDD Cellular System
  10. 10. Time-division Multiple Access
  11. 11. Frequency-division Multiple Acess
  12. 12. Frequency division multiple access FDMA <ul><li>one phone circuit per channel </li></ul><ul><li>idle time causes wasting of resources </li></ul><ul><li>simultaneously and continuously transmitting </li></ul><ul><li>usually implemented in narrowband systems </li></ul><ul><li>for example: in AMPS is a FDMA bandwidth of 30 kHz implemented </li></ul>
  13. 13. FDMA compared to TDMA <ul><li>fewer bits for synchronization </li></ul><ul><li>fewer bits for framing </li></ul><ul><li>higher cell site system costs </li></ul><ul><li>higher costs for duplexer used in base station and subscriber units </li></ul><ul><li>FDMA requires RF filtering to minimize adjacent channel interference </li></ul>
  14. 14. Nonlinear Effects in FDMA <ul><li>many channels - same antenna </li></ul><ul><li>for maximum power efficiency operate near saturation </li></ul><ul><li>near saturation power amplifiers are nonlinear </li></ul><ul><li>nonlinearities causes signal spreading </li></ul><ul><li>intermodulation frequencies </li></ul>
  15. 15. Nonlinear Effects in FDMA <ul><li>IM are undesired harmonics </li></ul><ul><li>interference with other channels in the FDMA system </li></ul><ul><li>decreases user C/I - decreases performance </li></ul><ul><li>interference outside the mobile radio band: adjacent-channel interference </li></ul><ul><li>RF filters needed - higher costs </li></ul>
  16. 16. Number of channels in a FDMA system <ul><li>N … number of channels </li></ul><ul><li>B t … total spectrum allocation </li></ul><ul><li>B guard … guard band </li></ul><ul><li>B c … channel bandwidth </li></ul>N= B t - B guard B c
  17. 17. Example: Advanced Mobile Phone System <ul><li>AMPS </li></ul><ul><li>FDMA/FDD </li></ul><ul><li>analog cellular system </li></ul><ul><li>12.5 MHz per simplex band - B t </li></ul><ul><li>B guard = 10 kHz ; B c = 30 kHz </li></ul>N= 12.5 E 6 - 2*(10 E 3) 30 E 3 = 416 channels
  18. 18. Time Division Multiple Access <ul><li>time slots </li></ul><ul><li>one user per slot </li></ul><ul><li>buffer and burst method </li></ul><ul><li>noncontinuous transmission </li></ul><ul><li>digital data </li></ul><ul><li>digital modulation </li></ul>
  19. 19. Repeating Frame Structure Slot 1 Slot 2 Slot 3 … Slot N Preamble Information Message Trail Bits One TDMA Frame Trail Bits Sync. Bits Information Data Guard Bits The frame is cyclically repeated over time.
  20. 20. Features of TDMA <ul><li>a single carrier frequency for several users </li></ul><ul><li>transmission in bursts </li></ul><ul><li>low battery consumption </li></ul><ul><li>handoff process much simpler </li></ul><ul><li>FDD : switch instead of duplexer </li></ul><ul><li>very high transmission rate </li></ul><ul><li>high synchronization overhead </li></ul><ul><li>guard slots necessary </li></ul>
  21. 21. Number of channels in a TDMA system <ul><li>N … number of channels </li></ul><ul><li>m … number of TDMA users per radio channel </li></ul><ul><li>B tot … total spectrum allocation </li></ul><ul><li>B guard … Guard Band </li></ul><ul><li>B c … channel bandwidth </li></ul>N= m*(B tot - 2*B guard) B c
  22. 22. Example: Global System for Mobile (GSM) <ul><li>TDMA/FDD </li></ul><ul><li>forward link at B tot = 25 MHz </li></ul><ul><li>radio channels of B c = 200 kHz </li></ul><ul><li>if m = 8 speech channels supported, and </li></ul><ul><li>if no guard band is assumed : </li></ul>N= 8*25E6 200E3 = 1000 simultaneous users
  23. 23. Efficiency of TDMA <ul><li>percentage of transmitted data that contain information </li></ul><ul><li>frame efficiency  f </li></ul><ul><li>usually end user efficiency <  f , </li></ul><ul><li>because of source and channel coding </li></ul><ul><li>How get  f ? </li></ul>
  24. 24. Repeating Frame Structure Slot 1 Slot 2 Slot 3 … Slot N Preamble Information Message Trail Bits One TDMA Frame Trail Bits Sync. Bits Information Data Guard Bits The frame is cyclically repeated over time.
  25. 25. Efficiency of TDMA <ul><li>b OH … number of overhead bits </li></ul><ul><li>N r … number of reference bursts per frame </li></ul><ul><li>b r … reference bits per reference burst </li></ul><ul><li>N t … number of traffic bursts per frame </li></ul><ul><li>b p … overhead bits per preamble in each slot </li></ul><ul><li>b g … equivalent bits in each guard time intervall </li></ul>b OH = N r *b r + N t *b p + N t *b g + N r *b g
  26. 26. Efficiency of TDMA <ul><li>b T … total number of bits per frame </li></ul><ul><li>T f … frame duration </li></ul><ul><li>R … channel bit rate </li></ul>b T = T f * R
  27. 27. Efficiency of TDMA <ul><li> f … frame efficiency </li></ul><ul><li>b OH … number of overhead bits per frame </li></ul><ul><li>b T … total number of bits per frame </li></ul> f = (1-b OH /b T )*100%
  28. 28. Space Division Multiple Access <ul><li>Controls radiated energy for each user in space </li></ul><ul><li>using spot beam antennas </li></ul><ul><li>base station tracks user when moving </li></ul><ul><li>cover areas with same frequency: </li></ul><ul><li>TDMA or CDMA systems </li></ul><ul><li>cover areas with same frequency: </li></ul><ul><li>FDMA systems </li></ul>
  29. 29. Space Division Multiple Access <ul><li>primitive applications are “Sectorized antennas” </li></ul><ul><li>in future adaptive antennas simultaneously steer energy in the direction of many users at once </li></ul>
  30. 30. Reverse link problems <ul><li>general problem </li></ul><ul><li>different propagation path from user to base </li></ul><ul><li>dynamic control of transmitting power from each user to the base station required </li></ul><ul><li>limits by battery consumption of subscriber units </li></ul><ul><li>possible solution is a filter for each user </li></ul>
  31. 31. Solution by SDMA systems <ul><li>adaptive antennas promise to mitigate reverse link problems </li></ul><ul><li>limiting case of infinitesimal beamwidth </li></ul><ul><li>limiting case of infinitely fast track ability </li></ul><ul><li>thereby unique channel that is free from interference </li></ul><ul><li>all user communicate at same time using the same channel </li></ul>
  32. 32. Disadvantage of SDMA <ul><li>perfect adaptive antenna system: infinitely large antenna needed </li></ul><ul><li>compromise needed </li></ul>
  33. 33. SPREAD SPECTRUM MULTIPLE ACCESS <ul><li>FHMA </li></ul><ul><li>CDMA </li></ul><ul><li>HYBRID SSMA </li></ul><ul><ul><li>(FCDMA = FDMA + CDMA) </li></ul></ul>
  34. 34. Spread Spectrum
  35. 35. Frequency Hoping Spread Spectrum
  36. 36. Frequency Hopping Spread Spectrum
  37. 37. Frequency Hopping Spread Spectrum <ul><li>Slow-frequency-hop spread spectrum </li></ul><ul><ul><li>The hopping duration is larger or equal to the symbol duration of the modulated signal </li></ul></ul><ul><ul><li>T c >= T s </li></ul></ul><ul><li>Fast-frequency-hop spread spectrum </li></ul><ul><ul><li>The hopping duration is smaller than the symbol duration of the modulated signal </li></ul></ul><ul><ul><li>T c < T s </li></ul></ul>
  38. 38. Slow Frequency-Hop SS
  39. 39. Fast Frequency-Hop SS
  40. 40. Code-Division Multiple Access (CDMA) <ul><li>CDMA is multiple access scheme that allows many users to share the same bandwidth </li></ul><ul><ul><li>3G (WCDMA), IS-95 </li></ul></ul><ul><li>Basic Principles of CDMA </li></ul><ul><ul><li>Each user is assigned a unique spreading code </li></ul></ul><ul><ul><li>The processing gain protects the useful signal and reduces interference between the different users </li></ul></ul><ul><ul><li>PG = (Bandwidth after spreading)/(Bandwidth before spreading) </li></ul></ul>
  41. 41. CDMA for Direct Sequence Spread Spectrum
  42. 42. CDMA Example
  43. 45. CAPACITY of CELLULAR SYSTEMS .
  44. 46. Spreading in Cellular CDMA Systems <ul><li>Cellular CDMA systems use two layers of spreading </li></ul><ul><li>Channelization codes (orthogonal codes) </li></ul><ul><ul><li>Provides orthogonality among users within the same cell </li></ul></ul><ul><li>Long PN sequences (scrambling code) </li></ul><ul><ul><li>Provides good randomness properties (low cross correlation) </li></ul></ul><ul><ul><li>Reduces interference from other cells </li></ul></ul>
  45. 47. Capacity of Cellular Systems <ul><li>channel capacity: maximum number of users in a fixed frequency band </li></ul><ul><li>radio capacity : value for spectrum efficiency </li></ul><ul><li>reverse channel interference </li></ul><ul><li>forward channel interference </li></ul><ul><li>How determine the radio capacity? </li></ul>
  46. 48. Co-Channel Reuse Ratio Q <ul><li>Q … co-channel reuse ratio </li></ul><ul><li>D … distance between two co-channel cells </li></ul><ul><li>R … cell radius </li></ul>Q=D/R
  47. 49. Forward channel interference <ul><li>cluster size of 4 </li></ul><ul><li>D 0 … distance serving station to user </li></ul><ul><li>DK … distance co-channel base station to user </li></ul>
  48. 50. Carrier-to-interference ratio C/I <ul><li>M closest co-channels cells cause first order interference </li></ul>C = I D 0 -n 0 M k=1 D K -n k <ul><li>n 0 … path loss exponent in the desired cell </li></ul><ul><li>n k … path loss exponent to the interfering base station </li></ul>
  49. 51. Carrier-to-interference ratio C/I <ul><li>Assumption: </li></ul><ul><li>just the 6 closest stations interfere </li></ul><ul><li>all these stations have the same distance D </li></ul><ul><li>all have similar path loss exponents to n 0 </li></ul>C I = D 0 -n 6*D -n
  50. 52. Worst Case Performance <ul><li>maximum interference at D 0 = R </li></ul><ul><li>(C/I) min for acceptable signal quality </li></ul><ul><li>following equation must hold: </li></ul>1/6 * (R/D) (C/I) min = > -n
  51. 53. Co-Channel reuse ratio Q <ul><li>D … distance of the 6 closest interfering base stations </li></ul><ul><li>R … cell radius </li></ul><ul><li>(C/I) min … minimum carrier-to-interference ratio </li></ul><ul><li>n … path loss exponent </li></ul>Q = D/R = (6*(C/I) min ) 1/n
  52. 54. Radio Capacity m <ul><li>Bt … total allocated spectrum for the system </li></ul><ul><li>Bc … channel bandwidth </li></ul><ul><li>N … number of cells in a complete frequency reuse cluster </li></ul>m = B t B c * N radio channels/cell
  53. 55. Radio Capacity m <ul><li>N is related to the co-channel factor Q by: </li></ul>Q = (3*N) 1/2 m= B t B c * (Q²/3) = B t Bc * 6 C I 3 n/2 ( * ( ) min ) 2/n
  54. 56. Radio Capacity m for n = 4 <ul><li>m … number of radio channels per cell </li></ul><ul><li>(C/I) min lower in digital systems compared to analog systems </li></ul><ul><li>lower (C/I) min imply more capacity </li></ul><ul><li>exact values in real world conditions measured </li></ul>m = B t B c * 2/3 * (C/I) min
  55. 57. Compare different Systems <ul><li>each digital wireless standard has different (C/I) min </li></ul><ul><li>to compare them an equivalent (C/I) needed </li></ul><ul><li>keep total spectrum allocation B t and number of rario channels per cell m constant to get (C/I) eq : </li></ul>
  56. 58. Compare different Systems <ul><li>B c … bandwidth of a particular system </li></ul><ul><li>(C/I) min … tolerable value for the same system </li></ul><ul><li>B c ’ … channel bandwidth for a different system </li></ul><ul><li>(C/I) eq … minimum C/I value for the different system </li></ul>C I = C I B c B c’ ( ) ( ) min )² eq * (
  57. 59. C/I in digital cellular systems <ul><li>R b … channel bit rate </li></ul><ul><li>E b … energy per bit </li></ul><ul><li>R c … rate of the channel code </li></ul><ul><li>E c … energy per code symbol </li></ul>C E b *R b E c *R c I I I = =
  58. 60. C/I in digital cellular systems <ul><li>combine last two equations: </li></ul>(C/I) (E c *R c )/I B c ’ (C/I) eq (E c ’*R c ’)/I’ B c = = ( )² <ul><li>The sign ‘ marks compared system parameters </li></ul>
  59. 61. C/I in digital cellular systems <ul><li>Relationship between R c and B c is always linear (R c /R c ’ = B c /B c ’ ) </li></ul><ul><li>assume that level I is the same for two different systems ( I’ = I ) : </li></ul>E c B c ’ E c ‘ B c = ( )³
  60. 62. Compare C/I between FDMA and TDMA <ul><li>Assume that multichannel FDMA system occupies same spectrum as a TDMA system </li></ul><ul><li>FDMA : C = E b * R b ; I = I 0 * B c </li></ul><ul><li>TDMA : C’ = E b * R b ’ ; I’ = I 0 * B c ’ </li></ul><ul><li>E b … Energy per bit </li></ul><ul><li>I 0 … interference power per Hertz </li></ul><ul><li>R b … channel bit rate </li></ul><ul><li>B c … channel bandwidth </li></ul>
  61. 63. Example <ul><li>A FDMA system has 3 channels , each with a bandwidth of 10kHz and a transmission rate of 10 kbps. </li></ul><ul><li>A TDMA system has 3 time slots, a channel bandwidth of 30kHz and a transmission rate of 30 kbps. </li></ul><ul><li>What’s the received carrier-to-interference ratio for a user ? </li></ul>
  62. 64. Example <ul><li>In TDMA system C’/I’ be measured in 333.3 ms per second - one time slot </li></ul>C’ = E b *R b ’ = 1/3*(E b *10 E 4 bits) = 3*R b *E b =3*C I’ = I0*Bc’ = I0*30kHz = 3*I <ul><li>In this example FDMA and TDMA have the same radio capacity (C/I leads to m) </li></ul>
  63. 65. Example <ul><li>Peak power of TDMA is 10logk higher then in FDMA ( k … time slots) </li></ul><ul><li>in practice TDMA have a 3-6 times better capacity </li></ul>
  64. 66. Capacity of SDMA systems <ul><li>one beam each user </li></ul><ul><li>base station tracks each user as it moves </li></ul><ul><li>adaptive antennas most powerful form </li></ul><ul><li>beam pattern G(  ) has maximum gain in the direction of desired user </li></ul><ul><li>beam is formed by N-element adaptive array antenna </li></ul>
  65. 67. Capacity of SDMA systems <ul><li>G(  ) steered in the horizontal  -plane through 360° </li></ul><ul><li>G(  ) has no variation in the elevation plane to account which are near to and far from the base station </li></ul><ul><li>following picture shows a 60 degree beamwidth with a 6 dB sideslope level </li></ul>
  66. 68. Capacity of SDMA systems
  67. 69. Capacity of SDMA systems <ul><li>reverse link received signal power, from desired mobiles, is P r;0 </li></ul><ul><li>interfering users i = 1,…,k-1 have received power P r;I </li></ul><ul><li>average total interference power I seen by a single desired user: </li></ul>
  68. 70. Capacity of SDMA <ul><li> i … direction of the i-th user in the horizontal plane </li></ul><ul><li>E … expectation operator </li></ul>I = E {  G(  i ) P r;I } K-1 i=1
  69. 71. Capacity of SDMA systems <ul><li>in case of perfect power control (received power from each user is the same) : </li></ul>P r;I = P c <ul><li>Average interference power seen by user 0: </li></ul>I = P c E {  G(  i ) } K-1 i=1
  70. 72. Capacity of SDMA systems <ul><li>users independently and identically distributed throughout the cell: </li></ul>I = P c *(k -1) * 1/D <ul><li>D … directivity of the antenna - given by max(G(  )) </li></ul><ul><li>D typ. 3dB …10dB </li></ul>
  71. 73. Capacity of SDMA systems <ul><li>Average bit error rate P b for user 0: </li></ul>P b = Q ( ) 3 D N K-1 <ul><li>D … directivity of the antenna </li></ul><ul><li>Q(x) … standard Q-function </li></ul><ul><li>N … spreading factor </li></ul><ul><li>K … number of users in a cell </li></ul>
  72. 74. Capacity of SDMA systems

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