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  • 26/11/20071

    CDMA: An Introduction

    [email protected]

    http://mm.aueb.gr/~dourosmailto:[email protected]?subject= CDMA: An Introduction Presentation

  • 2

    Motivation

    Why do we need CDMA?Which are its basic principles?What do we earn by using it?

  • 3

    Outline

    Introduction CDMA Basics CDMA Deeper CDMA ProblemsCDMA BenefitsConclusions

  • 4

    Why Multiple Access?

    Goal: Multiple use of a shared medium.Multiplex channels in three dimensions:

    Time (t)

    Frequency (f)

    Code (c)

  • 5

    Frequency Division Multiple Access (FDMA)

    Cocktail Party Analogy: People create teams and discuss. There is a distance among them. (-) Requires guard band between channels(-) Waste of bandwidth if traffic is distributed unevenlyExample: broadcast radio

  • 6

    Time Division Multiple Access (TDMA)

    Cocktail Party Analogy: People have access to the same room but each of them waits for his turn to speak.(-) Precise synchronization necessary

  • 7

    Time and Frequency Division Multiple Access

    (-) Precise coordination requiredExample: GSM

  • 8

    Code Division Multiple Access (CDMA)

    Cocktail Party Analogy: All people are in the same room together. They can all be talking the same time!Example: UMTS

  • 9

    Spread Spectrum

    PN: Pseudo-Noise code sequence spread/ despread the signal. Modulation:

    FSK Frequency hopped (FH) multiple access)

    PSK Direct sequence (DS) multiple access)

    D mD

  • 10

    FHMA

    Bandwidth divides in non overlapping bands.Signal shifts from band to band in predefined intervals.Receiver is synchronized with the transmitter(+) less interferenceUse: Bluetooth

  • 11

    CDMA Basics (1)

    Each station is assigned a unique m-bit code (chip sequence) To send bit 1, station sends chip sequence.To send bit 0, station sends the complement.Example: 1 MHz band with 100 stations. FDMA

    Each station a 10kHz band

    Rate:10 kbps (Assume that you can send 1bit/Hz)CDMA

    Each station uses the whole 1MHz band 106 cps.

    If 10 kbps

  • 12

    CDMA Basics (2)

    Let or

    Compare any pair of these sequences-vectorsMultiply any pair of these sequences-vectorsTwo chips S,T are orthogonal IFF

    0 0S T S T

    = =

    0S T =

    1

    1 0m

    i ii

    S T S Tm =

    = =2 2

    1 1 1

    1 1 1 ( 1) 1m m m

    i i ii i i

    S S S S Sm m m= = =

    = = = = _ _

    1 1

    1 1 1( 1) ( ) 1m m

    iii i

    S S S S mm m m= =

    = = = =

  • 13

    CDMA Basics (3)

    Let A, B, C, D transmit correspondingly bit 1,0,1,_.

    Assume that:

    All stations are perfectly synchronous.

    All codes are pair wise orthogonal (arent they?).

    If two or more stations transmit simultaneously, the bipolar signals add up linearly.

    Receiver understands How can the receiver understand what bit station C send?

    _

    ( 1 1 3 3 1 1 1 1)S A B C= + + = + + + +

  • 14

    CDMA Basics (4)

    Lets compute the normalized inner product SC

    Right! By accident??

    Remember: All codes are pair wise orthogonal!

    1 ( 1 +1 -3 +3 +1 -1 -1 +1) (-1 +1 -1 +1 +1 +1 -1 -1)=8

    1 1= (1 1 3 3 1 1 1 1) 8 18 8

    S C =

    + + + + + = =

    ( ) 0 0 1 1S C A B C C A C B C C C

    = + + = + + = + + =

  • 15

    CDMA Basics (5)

    Reverse wayThink that each chip sequence arrives separatelyReceiver separately computes each inner product

    It keeps only the non-orthogonal pair, i.e. the right bit

    0 (1)

    0 (2)1 (3)

    0 0 1 1

    A C

    B CC CS

    =

    = == + + =

  • 16

    CDMA Deeper (1)

    More advanced analysisSender:

    Receiver:

    ( ) cos(2 ( ))( ) {0, }( ) { 1, 1}( ) cos(2 ( )) ( )

    d c

    c

    s t A f t tt

    c ts t A f t t c t

    = +

    + += +

    ( ) ( ) cos(2 ( )) ( ) ( )cos(2 ( )) ( )

    c

    c d

    s t c t A f t t c t c tA f t t s t

    = + == + =

  • 17

    CDMA Deeper (2)

    With orthogonal codes, we can safely decode the coding signals.Noise?R=R+N, N: m-digit noise vector and N=(a aa)Decode No problem if chipping codes are balanced (same )

    R' S=(R+N) S=S S +(orthogonal codes) S +N S= 0 ?1+ +

    R' S=(R+N) S=S S +(orthogonal codes) S +N S==1+0+(a a a a)(+1+1-1-1)=1+0+0=1

  • 18

    CDMA Deeper (3)

    How many codes can we construct with m chips?m (why?)If m=2k, Walsh-Hadamard codes can be constructed recursively!The set of codes of length 1 is For each code we have two codesCode Tree:

    0 { }C = < + >

    kc C< >_

    k+1{ >

    0

    1

    2

    { }{ , }{ , - - , - - , - - }

    CCC

    = < + >

    = < + + > < + >= < + + + + > < + + > < + + > < + + >

  • 19

    CDMA Deeper (4)

    Correlation: Determines similarity between two sets of data.

    Possible values1 sequences are similar0 no relationship between them. -1 one is the mirror of the other

    Cross correlation: Compare two sequences from different sourcesAuto correlation: Compare a sequence with itself after a time-intervalWalsh Codes: No cross correlation Low auto correlationPN sequences: Low cross correlation Low auto Correlation

  • 20

    CDMA Deeper (5)

    We cannot have more than m orthogonal codes.Let m + k stations and m chips Idea: Use PN Sequences.

    ?: the sum of the k random variables that are either 1 or -1.But PN Sequences = low cross correlation. ? should be 0. Experimental evaluation: For k=m=128, decoding is correct more than 80%.

    ( random codes) S+(m-1 orhogonal codes?

    ) =1+ +0R S S S k S = +

  • 21

    CDMA Problems (1)

    All stations are received with the same power level

    In reality users may be received with very different powers!Near-far ProblemSolutions:

    Empirical rule: Each MS transmits with the reverse power that it receives from the BS

    Power Control!Open LoopFast Closed Loop

  • 22

    CDMA Problems (2)

    Bad Properties of Walsh CodesPerfect Synchronization of all users required.Impossibleis nothing! Use a long enough known chip sequence.

    ButIn a multipath channel, delayed copies may be received, which are not orthogonal any longer. Self-Interference.

  • 23

    CDMA Problems (3)

    So far(-)tight synchronization(-)self-interference (-)Near-far problem(-)Higher complexity of sender/ receiverHow did Qualcomm convince people to use this stuff??

  • 24

    CDMA Benefits (1)

    Unlike FDMA and TDMA, CDMA does not rely on orthogonal frequency and time slots!

  • 25

    CDMA Benefits (2)

    In TDMA and FDMA systems

    Nothing to send time/frequency slot is wasted

    Dynamic allocation is very difficult

    In CDMA systems

    Nothing to send less interference

    Transmit ~half times doubles the capacity

  • 26

    CDMA Benefits (3)

    FDMA-TDMA use sectors to decrease the reuse distanceCDMA use sectors to increase capacity (triple it)!

  • 27

    CDMA Benefits (4)

    Why handoff?Types

    Hard

    Soft

  • 28

    CDMA Benefits (5)

    Break-Before-MakeEach MS communicates with only one BS each time(+)Reduced dropped calls

    Hard Handoff

  • 29

    CDMA Benefits (6)

    Each MS communicates with more than one BS each timeUse Signal Strength to decide where to connect.Make-Before-Break(+++) no dropped calls

    Soft Handoff

  • 30

    CDMA Benefits (7)

    CapacityTDMA-FDMA: bandwidth limitedCDMA: interference limitedCDMAs capacity is bigger.How?Long Story

  • 31

    Conclusions

    Back to the startWhy do we need CDMA?

    Introduction

    Which are its basic principles?

    CDMA Basics

    CDMA Deeper

    What do we earn by using it?

    CDMA Problems

    CDMA Benefits

  • 32

    !

    CDMA: An IntroductionMotivationOutline Why Multiple Access?Frequency Division Multiple Access (FDMA)Time Division Multiple Access (TDMA)Time and Frequency Division Multiple AccessCode Division Multiple Access (CDMA)Spread SpectrumFHMACDMA Basics (1)CDMA Basics (2)CDMA Basics (3)CDMA Basics (4)CDMA Basics (5)CDMA Deeper (1)CDMA Deeper (2)CDMA Deeper (3)CDMA Deeper (4)CDMA Deeper (5)CDMA Problems (1)CDMA Problems (2)CDMA Problems (3)CDMA Benefits (1)CDMA Benefits (2)CDMA Benefits (3)CDMA Benefits (4)CDMA Benefits (5)CDMA Benefits (6)CDMA Benefits (7)Conclusions!