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CDMA

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CDMA

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  1. Multiple Access Techniques Multiple Access Techniques ? Multiple access schemes are used to allow many Multiple access schemes are used to allow many mobile users to share simultaneously a finite mobile users to share simultaneously a finite amount of radio spectrum. amount of radio spectrum. The sharing of spectrum is required to achieve high capacity high capacity by simultaneously allocating the by simultaneously allocating the available bandwidth (or the available amount of available bandwidth (or the available amount of channels) to multiple users. channels) to multiple users. For high quality communications, this must be done without severe degradation in the done without severe degradation in the performance of the system. performance of the system. ? The sharing of spectrum is required to achieve , this must be ? For high quality communications 1 1

  2. Difference between multiplexing and multiple access Difference between multiplexing and multiple access 2 2

  3. Multiple Access Techniques Multiple Access Techniques Multiple Access Techniques Multiple Access Techniques SDMA FDMA FDMA TDMA TDMA CDMA CDMA SDMA 3 3

  4. Multiple Access (MA) Technologies Multiple Access (MA) Technologies used in Different Wireless Systems used in Different Wireless Systems Cellular Systems Cellular Systems AMPS ( Advanced Mobile AMPS ( Advanced Mobile Phone system ) Phone system ) GSM ( Global System for GSM ( Global System for Mobile ) Mobile ) US DC ( U. S Digital US DC ( U. S Digital Cellular ) Cellular ) JDC ( Japanese Digital JDC ( Japanese Digital Cellular ) Cellular ) MA Technique MA Technique FDMA / FDD FDMA / FDD TDMA / FDD TDMA / FDD TDMA / FDD TDMA / FDD TDMA / FDD TDMA / FDD 4 4

  5. … …Multiple Access (MA) Technologies Multiple Access (MA) Technologies used in Different Wireless Systems used in Different Wireless Systems Cellular Systems Cellular Systems DECT ( Digital DECT ( Digital European Cordless European Cordless Telephone ) Telephone ) IS – 95 ( U.S Narrowband IS – 95 ( U.S Narrowband Spread Spectrum ) Spread Spectrum ) MA Technique MA Technique FDMA / FDD FDMA / FDD CDMA / FDD CDMA / FDD 5 5

  6. Frequency Division Frequency Division Multiple Access (FDMA) Multiple Access (FDMA) code code C C1 1 C C2 2 C Cn n frequency frequency time time C C1 1 frequency frequency C C2 2 C Cn n 6 6

  7. Principles Of Operation Principles Of Operation ? Each user is allocated a unique frequency Each user is allocated a unique frequency band or channel. These channels are band or channel. These channels are assigned on demand to users who request assigned on demand to users who request service. service. In FDD, the channel has two frequencies – forward channel & reverse channel. forward channel & reverse channel. ? In FDD, the channel has two frequencies – 7 7

  8. … …Principles Of Operation Principles Of Operation ? During the period of the call, no other user During the period of the call, no other user can share the same frequency band. can share the same frequency band. If the FDMA channel is not in use, then it sits idle and cannot be used by other users sits idle and cannot be used by other users to increase or share capacity. This is a to increase or share capacity. This is a wasted resource. wasted resource. ? If the FDMA channel is not in use, then it 8 8

  9. Properties of FDMA Properties of FDMA ? The bandwidth of FDMA channels is narrow The bandwidth of FDMA channels is narrow (30 KHz) since it supports only one call/ (30 KHz) since it supports only one call/ carrier. carrier. FDMA systems have higher cost ? FDMA systems have higher cost ?Costly band pass filters to eliminate spurious radiation ? Duplexers in both T/R increase subscriber costs 9 9

  10. Number Of Channel Supported Number Of Channel Supported By FDMA System By FDMA System g B g B − tB B 2B t g = N B c → B GuardBand → g B ChannelBandwidth c 10 10

  11. Example Example In the US, each cellular carrier is allocated 416 In the US, each cellular carrier is allocated 416 channels, channels, = = = B B 12.5MHz 10KHz t g B 30KHz c     × − × 6 3 (12.5 10 ) 2(10 10 ) 30 10 = = N 416 × 3 11 11

  12. Time Division Time Division Multiple Access (TDMA) Multiple Access (TDMA) code code C C1 1 C Cn n frequency frequency time time C C1 1 time time C C2 2 C Cn n 12 12

  13. Principles Of Operation Principles Of Operation ? TDMA systems divide the radio spectrum TDMA systems divide the radio spectrum into time slots and each user is allowed to into time slots and each user is allowed to either transmit or receive in each time slots. either transmit or receive in each time slots. Each user occupies a cyclically repeating time slots. TDMA can allow different number time slots. TDMA can allow different number of time slots for separate user. of time slots for separate user. ? Each user occupies a cyclically repeating 13 13

  14. TDMA Frame Structure TDMA Frame Structure Preamble Preamble Information Information message message Trail Bits Trail Bits Slot 1 Slot 1 Slot 2 Slot 2 Slot N Slot N Trail Bit Trail Bit Sync Bit Sync Bit Information Information Bit Bit Guard Bits Guard Bits 14 14

  15. Components of 1 TDMA Frame Components of 1 TDMA Frame ? Preamble Preamble  information for base station and subscriber information for base station and subscriber identification identification Guard times     Synchronization of Synchronization of receivers between a different slots and receivers between a different slots and frames frames    Address and synchronization Address and synchronization ? Guard times 15 15

  16. Principles Of Operation Principles Of Operation TDMA shares the single carrier frequency TDMA shares the single carrier frequency with several users, where each user with several users, where each user makes use of non-overlapping timeslots. makes use of non-overlapping timeslots. Data Transmission for user of TDMA Data Transmission for user of TDMA system is discrete bursts system is discrete bursts The result is low battery consumption. The result is low battery consumption. Handoff process is simpler, since it is Handoff process is simpler, since it is able to listen for other base stations able to listen for other base stations during idle time slots. during idle time slots. ? ? • • 16 16

  17. … …Principles Of Operation Principles Of Operation ? Since different slots are used for T and R, Since different slots are used for T and R, duplexers are not required. duplexers are not required. Equalization is required, since transmission rates are generally very high as compared to rates are generally very high as compared to FDMA channels. FDMA channels. ? Equalization is required, since transmission 17 17

  18. Efficiency of TDMA Efficiency of TDMA Frame Efficiency : Frame Efficiency : No.ofbits/ framecontainingtransmitteddata TotalNumberofbits/ frame η = f = − /b ) 100 × (1 b (b OH b ) − T = × 100 T OH b T 18 18

  19. Frame efficiency parameters Frame efficiency parameters Total Number of bits per frame = =T R × Tb f fT =Frame duration R=Channel bit rate b =Number of overhead bits /frame OH × + × + × + × =N b N b N b N b r r t p t g r g 19 19

  20. … …Frame efficiency parameters Frame efficiency parameters = = = = = N N b b b Number of reference bits per frame Number of traffic bits per frame Number of overhead bits per reference burst Number of overhead bits per preamble in each slots Number of equivalent bits r t r p in each guard time interval g 20 20

  21. Number of channels in TDMA System Number of channels in TDMA System m(B -2B ) tot guard N= B c = m Maximum number of TDMA users supported on each radio channel B Guard band to present user at the edge of the band = guard from 'bleeding over' to an adjacent radio service 21 21

  22. Example Example GSM System uses a TDMA / FDD system. GSM System uses a TDMA / FDD system. The GSM System uses a frame structure The GSM System uses a frame structure where each frame consist of 8 time slots, and where each frame consist of 8 time slots, and each time slot contains 156.25 bits, and data is each time slot contains 156.25 bits, and data is transmitted at 270.833 kbps in the channel. transmitted at 270.833 kbps in the channel. Find: …… Find: …… 22 22

  23. … …Example Example Time duration of a bit Time duration of a bit Time duration of a slot Time duration of a slot Time duration of a frame and Time duration of a frame and How long must a user occupying a single How long must a user occupying a single slot must wait between two simultaneous slot must wait between two simultaneous transmissions? transmissions? 1. 1. 2. 2. 3. 3. 4. 4. 23 23

  24. Solution Solution Time duration of a bit Time duration of a bit 1 =T = bit-rate • 1 = = µ 3.692 s b × 3 270.833 10 Time duration of a slot Time duration of a slot • = = × = µ ms T 156.25 T 0.577 s slot b 24 24

  25. … …Solution Solution • Time duration of a frame Time duration of a frame 8 T = × = 4.615ms slot • A user has to wait 4.615 ms before next A user has to wait 4.615 ms before next transmission transmission 25 25

  26. Example Example If a normal GSM timeslot consists of 6 trailing bits, 8.25 guard bits, 26 training bits, and 2 bits, 8.25 guard bits, 26 training bits, and 2 traffic bursts of 58 bits of data, find the frame traffic bursts of 58 bits of data, find the frame efficiency efficiency If a normal GSM timeslot consists of 6 trailing Solution Solution ?Time slots have 6 + 8.25 + 26 + 2(58) = Time slots have 6 + 8.25 + 26 + 2(58) = 156.25 bits. 156.25 bits. A frame has 8 * 156.25 = 1250 bits / frame. ?A frame has 8 * 156.25 = 1250 bits / frame. 26 26

  27. … …Example Example The number of overhead bits per frame is The number of overhead bits per frame is given by given by ? b bOH Frame efficiency = (1250 – 322 ) / 1250 = 74.24 % = 74.24 % OH = 8(6) + 8(8.25) + 8(26) = 322 bits = 8(6) + 8(8.25) + 8(26) = 322 bits ? Frame efficiency = (1250 – 322 ) / 1250 27 27

  28. Spread Spectrum Multiple Access Spread Spectrum Multiple Access Technologies (SSMA) Technologies (SSMA) ? SSMA technologies uses techniques which SSMA technologies uses techniques which has a transmission bandwidth that is much has a transmission bandwidth that is much greater than maximum required RF greater than maximum required RF bandwidth. bandwidth. This is achieved by pseudo noise (PN) sequence that contents a narrowband signal sequence that contents a narrowband signal to a wideband noise-like signal before to a wideband noise-like signal before transmission. transmission. ? This is achieved by pseudo noise (PN) 28 28

  29. … …Spread Spectrum Multiple Access Spread Spectrum Multiple Access Technologies (SSMA) Technologies (SSMA) ? SSMA provides immunity to multiple SSMA provides immunity to multiple interference and has robust multiple access interference and has robust multiple access capability. capability. 29 29

  30. Types Of Spread Types Of Spread Spectrum Techniques Spectrum Techniques ? Frequency Hopped Multiple Access Frequency Hopped Multiple Access ( FHMA ) ( FHMA ) ? Direct Sequence Multiple Access ( CDMA ) Direct Sequence Multiple Access ( CDMA ) 30 30

  31. Direct Sequence Spread Direct Sequence Spread Spectrum (DS-SS) Spectrum (DS-SS) code code C C1 1 C C2 2 C C3 3 C Cn n frequency frequency time time 31 31

  32. Direct Sequence Spread Direct Sequence Spread Spectrum (DS-SS) Spectrum (DS-SS) 1 S (t) τ τ τ τ1 1 m (t) r(t) 1 Σ Σ Σ Σ PN (t) π +ϕ cos(2 f t ) 1 c 1 τ τ τ τk k m (t) 2 PN (t) π +ϕ cos(2 f t ) K c k 32 32

  33. Principles of operation-transmitter Principles of operation-transmitter ? The narrowband message signal The narrowband message signal m mi i(t) is multiplied by a pseudo noise (t) is multiplied by a pseudo noise code sequence that has a chip rate >> code sequence that has a chip rate >> data rate of message. data rate of message. All users use the same carrier frequency and may transmit frequency and may transmit simultaneously. The kth transmitted simultaneously. The kth transmitted signal is given by: signal is given by: S (t) (2E /T )1/2m (t)p (t)cos(2 f t = ? All users use the same carrier π +ϕ ) k s s k k c k 33 33

  34. CDMA Receiver CDMA Receiver k iZ (t) > > > > < < < < ∫ ∫ ∫ ∫(.)dt (.)dt r(t) m (t) k π +ϕ cos(2 f t ) PN (t) c k K 34 34

  35. Principles of operation-receiver Principles of operation-receiver At the receiver, the received signal is At the receiver, the received signal is correlated with the appropriate signature correlated with the appropriate signature sequence to produce desire variable. sequence to produce desire variable. +τ ∫ iT 1 = −τ π −τ +ϕ 1 i Z (t) r(t)p (t )cos[2 f (t ) ]dt 1 1 c 1 1 (i 1)T − +τ 1 35 35

  36. Message Signal Message Signal ? m(t) is a time sequence of non-overlapping m(t) is a time sequence of non-overlapping pulses of duration T, each of which has an pulses of duration T, each of which has an amplitude (+/-) 1. amplitude (+/-) 1. The PN waveform consists of N pulses or chips for message symbol period T. chips for message symbol period T. NT NTC C = T = T where T where TC C is the chip period. is the chip period. ? The PN waveform consists of N pulses or 36 36

  37. Example: Example: Assume N=4 Assume N=4 1 1 -1 -1 PN Wave for N =4 1 1 -1 -1 37 37

  38. Correlator output for first user Correlator output for first user +τ ∫ iT 1 = −τ π −τ +ϕ 1 i Z (t) r(t)p (t )cos[2 f (t ) ]dt 1 1 c 1 1 (i 1)T − +τ 1 •The multiplied signal will be p The multiplied signal will be p2 2(t) = 1 for the correct signal and will yield the dispersed correct signal and will yield the dispersed signal and can be demodulated to yield the signal and can be demodulated to yield the message signal m message signal mi i(t). (t). = (t) = 1 for the π +ϕ 1/2 S (t) (2E /T ) m (t)p (t)cos(2 f t ) 1 s s 1 1 c 1 38 38

  39. Probability of bit error Probability of bit error Probability of bit error Probability of bit error P Pe e = Q {1/ [(K –1)/3N + (N = Q {1/ [(K –1)/3N + (N0 0/2E K K = Number of users = Number of users N N = Number of chips/ symbol = Number of chips/ symbol /2Eb b)] )]1/2 1/2} }   ∝ ∝ ∝ ∝ Now when, E Now when, Eb b/N /No o P Pe e = Q{[3N/(K-1)] = Q{[3N/(K-1)]1/2 1/2 } } 39 39

  40. Important Advantages of CDMA Important Advantages of CDMA ? Many users of CDMA use the same Many users of CDMA use the same frequency. Either TDD or FDD may be used. frequency. Either TDD or FDD may be used. Multipath fading may be substantially reduced because of large signal bandwidth. reduced because of large signal bandwidth. There is no absolute limit on the number of users in CDMA. The system performance users in CDMA. The system performance gradually degrades for all users as the gradually degrades for all users as the number of users is increased. number of users is increased. ? Multipath fading may be substantially ? There is no absolute limit on the number of 40 40

  41. Drawbacks of CDMA Drawbacks of CDMA ? Self-jamming is a problem in a CDMA Self-jamming is a problem in a CDMA system. Self-jamming arise because the PN system. Self-jamming arise because the PN sequence are not exactly orthogonal, non- sequence are not exactly orthogonal, non- zero contributions from other users in the zero contributions from other users in the system arise system arise The near- far problem occurs at a CDMA receiver if an undesired user has high receiver if an undesired user has high detected power as compared to the desired detected power as compared to the desired user. user. ? The near- far problem occurs at a CDMA 41 41

  42. Capacity of Cellular Systems Capacity of Cellular Systems ? Channel capacity for a radio system is Channel capacity for a radio system is defined as the maximum number of defined as the maximum number of channels or users that can be provided in a channels or users that can be provided in a fixed frequency band fixed frequency band  efficiency of wireless system. efficiency of wireless system.    spectrum spectrum 42 42

  43. … …Capacity of Cellular Systems Capacity of Cellular Systems For a Cellular System For a Cellular System m = Radio Capacity Matrix = Bt t / (B = Total allocated spectrum for the system system ? B BC C = Channel bandwidth = Channel bandwidth = Number of cells in frequency reuse pattern pattern / (BC C * N) * N) ? m = Radio Capacity Matrix = B ? B Bt t = Total allocated spectrum for the ? N N = Number of cells in frequency reuse 43 43

  44. Channel capacity design Channel capacity design CELLA CELLA CELLA CELLA CELLA CELLA 44 44

  45. … …Channel capacity design Channel capacity design for given C/I ratio for given C/I ratio ? Carrier to Interference ratio Carrier to Interference ratio n o D CI 6 D D =Distance from desired base station to mobile − = − × n o ? For maximum interference D For maximum interference D0 0 = R ( ( min I 6 = R ) ) ( ( ) − n C 1 R = I 6 D min ) C − 1 n = Q 45 45

  46. … …Channel capacity design Channel capacity design for given C/I ratio for given C/I ratio = Q Co- Channel reuse ratio ( min I { } ) −µ n C × = 6 = (3 N) × 0.5 Also,Q { } ( ) 2/n C × 6 2 (Q) I = = − Therefore,N eqn2 min 3 3 46 46

  47. … …Channel capacity design Channel capacity design for given C/I ratio for given C/I ratio B = substituting, m t ( )       2/n       ( ) C × B 6 1/3 I c min B = = Whenn 4,m radio channels/ cells t ) ( )       (1/2)       ( C × B 2/3 I c min ( )min C = Typical Values of 18dBforAnalogFM and12dBfor Digital I 47 47

  48. Equation of C/I for digital Equation of C/I for digital cellular system cellular system ( ) × (E R ) CI = b b I × (E R ) = c c I = = = = R E R E Channel bit rate Energy per bit Rate of channel code Energy per code symbol b b c c 48 48

  49. Comparison of FDMA and Comparison of FDMA and TDMA systems - FDMA TDMA systems - FDMA The total bandwidth Bt t is divided into M channels, each with Bandwidth B channels, each with Bandwidth Bc c. The radio capacity for FDMA is given by radio capacity for FDMA is given by M m C 23 I C E R I I B where I Interference power /Hz = is divided into M ? The total bandwidth B . The = ( ) 0.5 = b b = o c o 49 49

  50. TDMA TDMA ? Assume FDMA occupies the same Assume FDMA occupies the same spectrum as a single channel TDMA. spectrum as a single channel TDMA. C E R I I B where R Transmission rate of TDMA system R Transmission rate of FDMA system E Energy per bit = ′ = b ′ = b ′ ′ o c b ′ = = b b 50 50

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