Cdma me

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