King Fahd University of Petroleum & Minerals Electrical Engineering DepartmentEE 578 Simulation of Wireless SystemsCode Division Multiple Access Transmission Technology Assignment # 5 Ali Al-Saihati ID# 200350130
OUTLINE • Introduction. • CDMA Transmission Scheme. • Direct Sequence SS • Frequency Hopping. • Time Hopping • Chirp SS. • Hybrid Systems. • Code Generation. • BER Performance.
Introduction • The transmitted signals are coded using unique codes. • It is also called spread spectrum modulation because encoding enlarges the signal spectrum. • Originally used in military radar and communication systems because of low probability of detection and jamming resistance. • SS criteria: 1) Bt > Bi. 2) Br should be determined by pther functions. • The receiver correlates the received signal with a generated code to extract the information.
SS Properties • 1) Multiple access capability. • 2) Multipath interference protection. • 3) Privacy. • 4) Interference rejection. • 5) Antijamming capability. • 6) Low probability of interference (LPI)
CDMA Transmission Scheme • Could be explained by two concepts: protocol & modulation method. • Protocol method: has 2 protocols 1) Averaging systems: average the interference over long duration. 2) Avoidance systems: avoid interference. • Modulation method: 5 protocols: DS – FH – TH – Chirp SS- Hybrid.
Direct Sequence SS • Digital code signal modulates information signal. • Digital or analog signals could be used yet digital signals are used. • For spreading: Rcc > Rci • PSK modulation is used for code modulation. • Receiver must know the code and synchronize it.
DSSS Properties • Multiple access: Computes cross correlation. • Multipath interference: delayed signals > 2T cosidered interference signals using coherent demodulation. • Narrowband interference: power decreases by a factor of Gp. • LPI: low power- hard to detect.
Frequency Hopping • Carrier frequency changes with time. • Occupies a small BW at a time. • Generally, FM & FSK modulations are used for signal modulation. • Fast hopping: Rh> Rd, 1 bit with different carrier frequencies. • Slow hopping: Rh < Rd, multiple bits within one frequency.
FH Properties • Multiple access: different for FFH & SFH. • Multipath interference: averages the frequency responses at the receiver. • Narrowband interference: power decreases by a factor of Gp. • LPI: frequency transmission not known- hard to detect.
Time Hopping • Signaling rate is high for the data signal. • Time is divided into frame that contains M slots. • Transmission frequency needed for 1 user increases by M.
TH Properties • Multiple access: uses error correcting codes for multi transmission. • Multipath interference: signals overlap- no advantage. • Narrowband interference: power decreases by a factor of Gp. • LPI: difficult to detect.
Chirp SS • Not adapted to CDMA. • Spreads the BW using linear frequency modulation of the carrier.
Hybrid Systems • Utilize advantages from different modulation techniques. • DS/FH has good multipath property. • Transmitters and receivers are complex.
Generation of a Spreading Code • Codes Requirements: periodic, constant length easy to distinguish from other codes and time shifted. • Autocorrelation and cross-correlation are applied for code evaluation. • Autocorrelation and cross-correlation are given by:
Code Generation by Linear Feedback Registers • Each block in a register is a memory. • Can be linear or nonlinear determined by a function. • LFSR is represented by generator polynomial.
M Sequence • Codes are generated by using LSR. • Maximum period is Nc= 2n-1 • Autocorrelation period: • If n ≠ 0 mod 4, there exists 3 values of cross correlation: [-1, -t(n), t(n)-2]
Gold Sequence • Developed by gold. • Sequences with different codes that have the same correlation values are needed. • Generated by using 2 M sequence and XOR the results. • Cross correlation values = [-1, -t(n), t(n)-2]
Orthogonal Gold Sequence • There are issues in gold sequence: 1) 0 to 1 proportion not always balanced. 2) cross correlation value not 0 for synchronized environment. 3) special clocks are needed due to the odd number of code length. • Orthogonal gold sequence is inserted in the gold sequence as a chip to give zero cross correlation value during synchronization. • The rest have the same characteristics as gold sequence.