Code Division Multiple Access (CDMA) Transmission Technology EE 578 Assignment #5 Mohammad Alkhodary 200806080

Outline Introduction Direct Sequence Spread Spectrum (DSSS) Frequency Hopping Spread Spectrum (FHSS) Time Hopping Spread Spectrum (THSS) Hybrid Systems Basic Principle of Spread Spectrum Systems Code Division Multiple Access M-Sequence Gold , Orthogonal Gold Sequence Simulation Result

Introduction Multiple-access capability is primarily achieved by means of coding. Each user is assigned a unique code sequence that he uses to encode his information signal. The receiver. knowing the code sequences of the user, decodes the received signal after reception and recovers the original data. Because the bandwidth of the code signal is chosen to be much larger than the bandwidth of the information signal, the encoding process enlarges (spreads) the spectrum modulation. (SS) The resulting encoded signal is also called an SS signal.

Multiple Access FDMA TDMA CDMA Frequency Division Multiple Access Time Division Multiple Access CDMA Code Division Multiple Access TDMA CDMA time FDMA freq code

Spread Spectrum Advantages? Hide a signal below the noise floor Resistance to narrowband jamming and interference Mitigate performance degradation due to intersymbol and narrowband interference In conjunction with RAKE receiver, SS can provide coherent combining of different multipath components Allow multiple users to share the same signal bandwidth Wide bandwidth of SS signals is useful for location and timing acquisition

Spread Spectrum Properties Signal occupies a bandwidth much larger than is needed for the information signal Spread spectrum modulation is done using a spreading code independent of the data in the signal Despreading at the receiver is done by correlating the received signal with a synchronized copy of the spreading code

Spread Spectrum System: DSSS Vs. FHSS

Direct Sequence Spread Spectrum (DSSS) General Block Diagram Input Binary data dt with symbol rate Rs=1/Ts (=Rb for BPSK) Pseudo-noise code: pnt with chip rate Rc=1/Tc (an integer of Rs) Spreading Transmitted baseband signal: txb=dt * pnt

Spreading Processing gain

Despreading

DSSS Example C=AxB A=CxB Processing Gain

CDMA for DSSS Multiple Access n users each using different orthogonal PN sequence Modulate each users data stream Using qPSK Multiply by spreading code of user

Seven Channel CDMA Encoding and Decoding

CDMA Power Control

Frequency Hopping Spread Spectrum (FHSS) Signal broadcast over seemingly random series of frequencies Receiver hops between frequencies in sync with transmitter Eavesdroppers hear unintelligible blips Jamming on one frequency affects only a few bits

Basic Operation Typically 2k carriers frequencies forming 2k channels Channel spacing corresponds with bandwidth of input Each channel used for fixed interval

FHSS System Transmitter Receiver

Slow and Fast Frequency Hopping Frequency shifted every Tc seconds Duration of signal element is Ts seconds Slow FHSS has Tc  Ts Fast FHSS has Tc < Ts Generally fast FHSS gives improved performance in noise (or jamming)

Fast Hopping versus Slow Hopping

Fast Hopping versus Slow Hopping

Hybrid DS/FH Spread Spectrum System

Maximal Length Sequence (m-sequence)

Gold Sequence

IMT-2000: Gold Sequence Mutually Orthogonal Gold Codes (256) Generator Polynomials:

CDMA M-Sequence performance in AWGN Simulation Result CDMA M-Sequence performance in AWGN

CDMA Gold Sequence performance in AWGN Simulation Result CDMA Gold Sequence performance in AWGN

CDMA Orthogonal Sequence Gold performance in AWGN Chanel Simulation Result CDMA Orthogonal Sequence Gold performance in AWGN Chanel

CDMA M-Sequence performance in Fading Channel Simulation Result CDMA M-Sequence performance in Fading Channel

CDMA Gold Sequence performance in Fading Channel Simulation Result CDMA Gold Sequence performance in Fading Channel

CDMA Orthogonal Sequence Gold performance in Fading Channel Simulation Result CDMA Orthogonal Sequence Gold performance in Fading Channel