CDMACODE DIVISION MULTIPLE ACCESS
BY:SURABHI TANKKAR
CDMA (Code Division Multiple Access):
Spread Spectrum approach Pseudo-noise codes Correlation calculations Greater capacity & data rates
CDMA
Comparison
Spread Spectrum communications is distinguished by three key elements:
Bandwidth Code Synchronous reception
Spread Spectrum
3 ways to spread the bw of the signal: Frequency hopping Time hopping Direct sequence
Spread spectrum
General Model of Spread Spectrum Digital communication System
Frequency Hoping Spread Spectrum
Each bit in original signal is represented by multiple bits in the transmitted signal
Spreading code spreads signal across a wider frequency band ◦ Spread is in direct proportion to number of bits used
One technique combines digital information stream with the spreading code bit stream using exclusive-OR
Direct Sequence Spread Spectrum (DSSS
Pseudo-Noise Spreading◦ Bit rate of PN is much higher. (chip rate)
Spread Spectrum Generation
SS modulation is applied on top of a conventional modulation.
One can demonstrate that all other signals not receiving the SS code will stay as they are, unspread.
Spectrum of DS/SS
Secure Communication◦ The signal can be detected by authorized persons
who know the PN code.◦ The signal power is small due to spreading (hide
signal inside the noise)◦ Difficult to jam since it is wideband
Multiple Access◦ Individual users have independent, uncorrelated
spreading codes
Properties of DS/SS
Known as CDMAOne Chip rate at 1.25Mbps Convolutional codes, Viterbi Decoding Downlink (Base station to mobile):
◦ Walsh code 64-bit for channel separation◦ M-sequence 215 for cell separation
Uplink (Mobile to base station):◦ M-sequence 241 for channel and user separation
2G: IS-95A (1995)
Increased data rate for internet applications◦ Up to 115 kbps (8 times that of 2G)
Support web browser format language◦ Wireless Application Protocol (WAP)
2.5G: IS-95B (1998)
Ability to receive live music, interactive web sessions, voice and data with multimedia features
Global Standard IMT-2000◦ CDMA 2000, proposed by TIA◦ W-CDMA, proposed by ARIB/ETSI
Issued by ITU (International Telecommunication Union)
3G Technology
Excellent voice quality Data rate
◦ 144 kbps in high mobility◦ 384 kbps in limited mobility◦ 2 Mbps in door
Frequency Band 1885-2025 MHz Convolutional Codes Turbo Codes for high data rates
3G Specficiations
CDMA 1xEV-DO◦ peak data rate 2.4 Mbps◦ supports mp3 transfer and video conferencing
CDMA 1xEV-DV◦ Integrated voice and high-speed data multimedia
service up to 3.1 Mbps Channel Bandwidth:
◦ 1.25, 5, 10, 15 or 20 MHz Chip rate at 3.6864 Mbps Modulation Scheme
◦ QPSK in downlink ◦ BPSK in uplink
3G: CDMA2000 (2000)
Downlink ◦ Variable length orthogonal Walsh sequences for
channel separation◦ M-sequences 3x215 for cell separation (different
phase shifts) Uplink
◦ Variable length orthogonal Walsh sequences for channel separation
◦ M-sequences 241 for user separation (different phase shifts)
3G: CDMA2000 Spreading Codes
Stands for “wideband” CDMA Channel Bandwidth:
◦ 5, 10 or 20 MHz Chip rate at 4.096 Mbps Modulation Scheme
◦ QPSK in downlink◦ BPSK in uplink
3G: W-CDMA (2000)
High data rates => 100 Mbps Multimedia data up to ~102 Mbps IP-oriented network => Internet MIMO using Multiple Antennas Spreading code
◦ Non-binary sequences Multiple Access Technique
◦ MC-CDMA
4G Technology (2010)
MC-CDMA
Greater capacity TDMA and FDMA have a fixed number of
slots Frequencies can be reused in all the cells in
CDMA. No hard limit to the number of users. Resistance to multipath fading
Advantages of CDMA over TDMA and FDMA
GPS (Global Positioning System)◦ Determine time, location and velocity of a person◦ Consists of 24 satellites to measure the exact
location◦ Each satellite uses the same frequency band with
DS/SS. Military Applications
Other Applications of cdma
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