King Fahd University of Petroleum & Minerals Electrical Engineering Department

EE 578 Simulation of Wireless Systems

Code Division Multiple Access Transmission Technology

Assignment # 5

Ali Al-SaihatiID# 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 Properties1) Multiple access capability.

2) Multipath interference protection.

3) Privacy.

4) Interference rejection.

5) Antijamming capability.

6) Low probability of interference (LPI)

CDMA Transmission SchemeCould 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 SSDigital 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 PropertiesMultiple 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 HoppingCarrier 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 PropertiesMultiple 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 HoppingSignaling 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 PropertiesMultiple 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 SSNot 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 CodeCodes 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 RegistersEach block in a register is a memory.

Can be linear or nonlinear determined by a function.

LFSR is represented by generator polynomial.

M SequenceCodes 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 SequenceDeveloped 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 SequenceThere 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.

BER Performance in AWGN

BER Performance in Fading

Thank You