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## Khurram Masood 200806100

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**KhurramMasood200806100**Code Division Multiple Access (CDMA) Transmission Technology Chapter 5 of Hiroshi Harada Book**Outline**• Introduction • Type of CDMA • Averaging systems • Avoidance systems • Spreading code • M-seuence • Gold sequence • Ortogonal Gold sequence • Simulation and results Prepared By Ibrahim AL-OBIDA**Type of Multiplexing:**1. Frequency-Division Multiple Access (FDMA). 2. Time-Division Multiple Access (TDMA). 3. Code-division Multiple-Access (CDMA) Prepared By Ibrahim AL-OBIDA**Code Division Multiple Access (CDMA)**• CDMA: • 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 Prepared By Ibrahim AL-OBIDA**Code Division Multiple Access (CDMA)**• There are different ways to spread the bandwidth of the signal: • Direct sequence • Frequency hopping • Time hopping • Chirp spread spectrum • Hybrid systems Prepared By Ibrahim AL-OBIDA**Direct Sequence**Features: • All users use same frequency and may transmit simultaneously • Narrowband message signal multiplied by wideband spreading signal, or codeword • Each user has its own pseudo-codeword (orthogonal to others). • Receivers detect only the desired codeword. All others appear as noise. • Receivers must know transmitter’s codeword. Prepared By Ibrahim AL-OBIDA**Direct Sequence**Prepared By Ibrahim AL-OBIDA**Direct Sequence**Pseudo-Noise Spreading Prepared By Ibrahim AL-OBIDA**Direct Sequence**Direct Sequence Spread Spectrum System Prepared By Ibrahim AL-OBIDA**Direct Sequence**Processing Gain: = is the processing gain fc is Chipping Frequency (the bit rate of the PN code). fi is Information Frequency (the bit rate of the digital data). Prepared By Ibrahim AL-OBIDA**Direct Sequence**Advantages: • Increased capacity • Improved voice quality • Eliminating the audible effects of multipath fading • Enhanced privacy and security • Reduced average transmitted power • Reduced interference to other electronic devices Disadvantages: • Wide bandwidth per user required • Precision code synchronization needed Prepared By Ibrahim AL-OBIDA**Frequency Hopping Spread Spectrum (FHSS)**• signal is broadcast over seemingly random series of frequencies • receiver hops between frequencies in sync with transmitter • jamming on one frequency affects only a few bits**Slow and Fast FHSS**• commonly use multiple FSK (MFSK) • have frequency shifted every Tc seconds • duration of signal element is Ts seconds • Slow FHSS has Tc Ts • Fast FHSS has Tc < Ts • FHSS quite resistant to noise or jamming • with fast FHSS giving better performance**Linear Feedback Shift RegisterImplementation of PN Generator**• Output is periodic with max-period N=2n-1; • LFSR can always give a period N sequence -> resulting in m-sequences. • Different Ai allow generation of different m-sequences**Properties of M-Sequences**• Property 1: • Has 2n-1 ones and 2n-1-1 zeros • Property 2: • For a window of length n slid along output for N (=2n-1) shifts, each n-tuple appears once, except for the all zeros Sequence • Property 3: • Sequence contains one run of ones of length n • One run of zeros of length n-1 • One run of ones and one run of zeros of length n-2 • Two runs of ones and two runs of zeros of length n-3 • 2n-3 runs of ones and 2n-3 runs of zeros of length 1**Advantages of Cross Correlation**• The cross correlation between an m-sequence and noise is low • This property is useful to the receiver in filtering out Noise • The cross correlation between two different msequences is low • This property is useful for CDMA applications • Enables a receiver to discriminate among spread spectrum signals generated by different m-sequences**Gold Sequences**• Gold sequences constructed by the XOR of two m-sequences with the same clocking • Codes have well-defined cross correlation Properties • Only simple circuitry needed to generate large number of unique codes • In following example two shift registers generate the two m-sequences and these are then bitwise XORed**Orthogonal Codes**• Orthogonal codes • All pairwise cross correlations are zero • Fixed- and variable-length codes used in CDMA Systems • For CDMA application, each mobile user uses one sequence in the set as a spreading code • Provides zero cross correlation among all users**BER performance of DS CDMA with orthogonal Gold sequence in**AWGN**BER performance of DS CDMA with m-sequence in Rayleigh**fading**BER performance of DS CDMA with orthogonal Gold sequence in**Rayleigh fading