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Code Division Multiple Access(CDMA)
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Code Division Multiple Access(CDMA) What is CDMA
Introduction to Spread Spectrum Communications
Direct Sequence Spread Spectrum (DSSS) CDMA Algorithm
Advantages of CDMA
CDMA Vs TDMA Vs FDMA
Sites and Books
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What is CDMA CDMA : Code Division Multiple Access is a channel
access method
CDMA employs spreadspectrum (SS) technology and aspecial coding scheme (where each transmitter is assigned a
code) to allow multiple users to be multiplexed over the
same physical channel
A conflict-free protocol that allows overlap transmission,
both in frequency division and time division techniques
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Multiple Access Schemes
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Introduction to spread spectrum Spectrum , refers to the range of frequencies over which
electromagnetic radio frequencies signals can be sent Spread Spectrum, refers Spreading the bandwidth to transmit data.
There are three ways to spread the bandwidth of the signal: Frequency hopping : The signal is rapidly switched between
different frequencies within the hopping bandwidth pseudo-randomly, and the receiver knows before hand where to find thesignal at any given time.
Time hopping : The signal is transmitted in short bursts pseudo-randomly, and the receiver knows beforehand when to expect the
burst. Direct sequence : The digital data is directly coded at a much higher
frequency. The code is generated pseudo-randomly, the receiverknows how to generate the same code, and correlates the received
signal with that code to extract the data.
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Direct Sequence Spread Spectrum (DSSS) Chippingsequence Code given to user DSSS takes user bit stream and XOR with chipping
sequence. Chip- is smaller pulses of Chipping sequence Spreading factor : s = tb/tc tb = User bit time
tc = Chip duration Example : Chip sequences
11,110,1110,11101,1110010,10110111000 called Bankerscode
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Direct Sequence Spread Spectrum (DSSS)
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CDMA Algorithm Each station is assigned m bit unique code called chip
sequence say 8 bit Consider 4 stations A : 0 0 0 1 1 0 1 1 ( 1 1 1 +1 +1 1 +1 +1) B : 0 0 1 0 1 1 1 0 ( 1 1 +1 1 +1 +1 +1 1) C : 0 1 0 1 1 1 0 0 ( 1 +1 1 +1 +1 +1 1 1) D : 0 1 0 0 0 0 1 0 ( 1 +1 1 1 1 1 +1 1)
(Binary chips) (Bipolar chip) All chip sequence are pairwise orthogonal Orthogonal chip sequences are generated using Walsh
method
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CDMA Algorithm cont..- - 1 - C S1 = ( 1 +1 1 +1 +1 +1 1 1)
- 1 1 - B+C S2 = ( 2 0 0 0 +2 +2 0 2)
1 0 - - A+B S3 = ( 0 0 2 +2 0 2 0 +2)1 0 1 - A+B+C S4 = ( 1 +1 3 +3 +1 1 1 +1)
1 1 1 1 A+B+C+D S5 = ( 4 0 2 0 +2 0 +2 2)
1 1 0 1 A+B+C+D S6 = ( 2 2 0 2 0 2 +4 0 )
Examples of transmission
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CDMA Algorithm cont.. To Transmit bit 1 by B and Bit 1 C
S2 = ( 2 0 0 0 +2 +2 0 2)
S2 = ( 1 1 +1 1 +1 +1 +1 1) +( 1 +1 1 +1 +1 +1 1 1)
To recover the signal
Suppose S = transmitted chip sequence
Receiver have to extract bit sent by station C then
S.C = (A + B+C).C = A.C +B.C + C.C = 0 + 0 + 1
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CDMA Algorithm cont..Code
Time
FreqABC
D
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CDMA Algorithm cont..S4.C = ( 1 +1 3 +3 +1 1 1 +1) .
( 1 +1 1 +1 +1 +1 1 1)
= ( +1+1 +3 +3 +1 1+1 1)/8 = 1
S1 . C = (1+1+1+1+1+1+1)/8 = 1
S2 . C = (2+0+0+2+2+0+2)/8 = 1
S3 . C = (0+0+2+2+0-2+0-2)/8 = 0
S4 . C = (1+1+3+3+1-1+1-1)/8 = 1
S5 . C = (4+0+2+0+2+0-2+2)/8 = 1
S6 . C = (2-2+0-2+0-2-4+0)/8 = -1
Recovery of station C signal
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Advantages of CDMA No of users increased
Soft Handover
Flexible Used in military applications as provides security for data
Universal frequency reuse
Fast and accurate power control
Rake receiver
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CDMA VS TDMA VS FDMACDMA TDMA FDMA
Idea Spread spectrumusing codes
Time slots Frequency bands
Format Flexibility Flexible Flexible Rigid
Multiple Fading Rake Receiver Better than FDMA Different frequencies somore
Capacity 108 32 6
Handoff Soft Hard Hard
Signal separation Code and Specialreceivers
Synchronization intime domain
Filtering in frequencydomain
Implementation
complexity
Most Complex Complex Simple
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References http://www.telecomspace.com/cdma.html
http://en.wikipedia.org/wiki/Code_division_multiple_access
http://www.complextoreal.com/CDMA.pdf http://www.bee.net/mhendry/vrml/library/cdma/cdma.htm
http://www.umtsworld.com/technology/cdmabasics.htm
Computer Networks by Tenenbaum
Mobile Communications by J Schiller
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ThankYou