EE578 Assignment #5 Abdul-Aziz.M Al-Yami November 8 th 2010

 Multiple Access Schemes  Code Division Multiple Access (CDMA)  CDMA Protocols  Direct Sequence Code Division Multiple Access (DS-CDMA)  Matlab Results

 For radio systems there are two resources: frequency and time.  Division by frequency, so that each pair of communicators is allocated part of the spectrum for all of the time, results in Frequency Division Multiple Access (FDMA).  Division by time, so that each pair of communicators is allocated all (or at least a large part) of the spectrum for part of the time results in Time Division Multiple Access (TDMA).  In Code Division Multiple Access (CDMA), every communicator will be allocated the entire spectrum all of the time. CDMA uses codes to identify connections.

 A digital method for simultaneously transmitting signals over a shared portion of the spectrum by coding each distinct signal with a unique code.  CDMA is a wireless communications technology that uses the principle of spread spectrum communication.  Advantages  Multiple access capability  Protection against multipath interference  Privacy  Interference rejection  Ant jamming capability  Low probability of interception

CDMA Protocols Concept behind the Protocols Averaging Systems Avoidance Systems Modulation Method DS FH TH Chirp SSHybrid

o Characteristics: o All users use same frequency and may transmit simultaneously. o Narrowband message signal multiplied by wideband spreading signal, or codeword o Each user has its own pseudo-codeword (orthogonal to others). o Receivers detect only the desired codeword. All others appearas noise. o Receivers must know transmitter’s codeword.

 Signal transmission consists of the following steps:  1. A pseudo-random code is generated, different for each channel and each successive connection.  2. The Information data modulates the pseudo-random code (the Information data is “spread”).  3. The resulting signal modulates a carrier.  4. The modulated carrier is amplified and broadcast.  Signal reception consists of the following steps:  1. The carrier is received and amplified.  2. The received signal is mixed with a local carrier to recover the spread digital signal.  3. A pseudo-random code is generated, matching the anticipated signal.  4. The receiver acquires the received code and phase locks its own code to it.  5. The received signal is correlated with the generated code, extracting the Information data.

sr = ; % symbol rate ml = 2; % number of modulation levels br = sr * ml; % bit rate nd = 100; % number of symbol ebn0 = 3; % Eb/No %************************** Filter initialization ************************** irfn = 21; % number of filter taps IPOINT = 8; % number of oversample alfs = 0.5; % roll off factor [xh] = hrollfcoef(irfn,IPOINT,sr,alfs,1); % T FILTER FUNCTION [xh2] = hrollfcoef(irfn,IPOINT,sr,alfs,0); sr = ; % symbol rate ml = 2; % number of modulation levels br = sr * ml; % bit rate nd = 100; % number of symbol ebn0 = 3; % Eb/No %************************** Filter initialization ************************** irfn = 21; % number of filter taps IPOINT = 8; % number of oversample alfs = 0.5; % roll off factor [xh] = hrollfcoef(irfn,IPOINT,sr,alfs,1); % T FILTER FUNCTION [xh2] = hrollfcoef(irfn,IPOINT,sr,alfs,0);  AWGN

Transmitter

Receiver

 Noe2=0

 Advantages: o Increased capacity o Improved voice quality o Eliminating the audible effects of multipath fading o Enhanced privacy and security o Reduced average transmitted power o Reduced interference to other electronic devices  Disadvantages: o Wide bandwidth per user required o Precision code synchronization needed

AWGNRayleigh

AWGNRayleigh

M-sequence Gold SequenceOrthogonal Gold Sequence