INTRODUCTION TO SPREAD SPECTRUM ETI 2526 Monday, April 9, 2018 - - PowerPoint PPT Presentation

INTRODUCTION TO SPREAD SPECTRUM

ETI 2526 Monday, April 9, 2018

CONCERNS IN COMMUNICATION SYSTEM

• 1. Most focus in communication system study is

focused on efficient use of signal energy and bandwidth.

• 2. There are however circumstances where it is

necessary for the communication system to:

(a) resist external interference; (b) operate with low spectral density (c) provide Multiple Access without external control (d) Make it difficult for unauthorized receivers to decode the signal.

3. Spread Spectrum is one of the mostly applied technique to achieve this objective.

WHEN IS A COMMUNICATION SYSTEM CONSIDERED AS SPREAD SPECTRUM?

A system is considered to be spread spectrum if is satisfies these two conditions:

a) The bandwidth of the transmitted signal is much greater than the message bandwidth, and b) The transmitted bandwidth is determined by some function that is independent of the message

ADVANTAGES OF SPREAD SPECTRUM Spread spectrum systems do not combat noise unlike systems like PCM and FM, they however have the following advantages:

a) Has anti-jam capability, especially narrow-band jamming b) Provides interference rejection c) Has multiple Access Capability d) Protects agains multipath interference e) Has lower probability of intercept (or is use for covert

• peration)

f) Provides secure Communication g) Can be used for Ranging

CLASSIFICATION OF SPREAD SPECTRUM SYSTEMS

• Spread Spectrum system are classified according to

the modulation system used as: a) Direct sequence (pseudo-noise) where data is scrambled using user specific pseudo noise code b) Frequency Hopping where the signal is spread by changing the frequency over the transmitted time. c) Time hopping where data is divided into frames with each frame being split further into time

• intervals. Each data burst is then hopped over

frames by utilizing code sequences.

DIRECT SEQUENCE/PSEUDO NOISE TRANSMITTER

• In Direct Sequence(also called a Pseudo-noise), the

signal is generated by adding a binary message with the

• utput of a pseudo-noise generator as shown below:

Binary Adder Balanced Modulator Carrier 𝑔

𝑑

Pseudo-noise Generator Clock Message Output

DIRECT SEQUENCE/PSEUDO NOISE RECEIVER 1.The receiver of the Pseudo noise signal performs three distinct functions, i.e

• 1. detect the presence of signal,
• 2. de-spread it and
• 3. demodulate the message.

2.The de-spreader multiplies the incoming signal with a locally generated PN sequence. 3.When the PN sequence and the incoming signal are aligned, the output from the de-spreader is the

• riginal message which can be filtered and

demodulated to produce the original.

De-spreader Narrowband Filter Message Demodulation PN Code Generator Local Clock Reconstruction 𝑡(𝑢) 𝑛(𝑢)

DIRECT SEQUENCE/PSEUDO NOISE RECEIVER (2)

Multiplies the incoming signal with a locally generated PN sequence. Original message

FREQUENCY HOPPING SPREAD SPECTRUM

1. Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels. 2. A pseudorandom sequence known to both transmitter and receiver is used by the frequency synthesizer.

TIME HOPPING SPREAD SPECTRUM

• A time hopping

system is a spread spectrum system in which the period or the duty cycle of a pulsed RF carrier are varied in a pseudorandom manner under the control of a coded sequence.

HYBRID DS/FH SPREAD SPECTRUM SYSTEM

Frequency Synthesiser PN Code Generator Data RF Oscillator ~ Hybrid DS/FH Output Baseband DS Output

HISTORY OF SPREAD SPECTRUM 1941: First publications. 1949: C. Shannon and R. Pierce develop basic ideas of CDMA. 1950s: First applications as military Anti-jam applications 1980s: Investigations for cellular use 80s. 1993: IS-95 standard. 1995: First commercial introduction. 1997/1998: Applied as 3G technology – WCDMA and UMTS

CDMA PRINCIPLE

Message signal Pseudo Code + Each bit of sequence 1 is replaced by the code sequence Output Rate of Change Known as bitrate Rate of Change Known as chiprate

CDMA SIGNAL SPECTRUM

Message Signal Output Signal Bandwidth = 2KHz Bandwidth = 16KHz The spectrum has spread from 2 Hz to 16 Hz, i.e by a factor of 8.

THE SPREADING FACTOR

• 1. Multiplication with the

code sequence which is

• f a higher bit rate, results

in a much wider spectrum.

• 2. The ratio of the code rate

to the information bit rate is called the spreading factor of the spread spectrum system.

• 3. In CDMA (IS-95), the

spreading factor is 64.

GENERSATIONS OF MOBILE DATA STANDARDS

High Speed Uplink Packet Access High Speed Downlink Packet Access

SAFARICOM WCDMA

SAFARICOM HSDPA

CDMA

• 1. CDMA uses unique

spreading codes to spread the baseband data before transmission.

• 2. The signal is transmitted in

a channel (BTS to BSC).

• 3. The receiver then uses a

correlator to de-spread the wanted signal

• 4. The output of the correlator

is passed through a narrow bandpass filter.

• 5. Unwanted signals are not

de-spread and do not pass through the filter.

BASIC CDMA COMMUNICATION SYSTEM

INTERLEAVER

• Interleaving is used to facilitate error correction when a burst

error affecting consecutive bits occurs.

• Interleaved signals can therefore be easily recovered when

fast fading of signals occurs over the air while non- interleaved are not.

EVOLUTION OF CDMA/1

• 1. The CDMA history can be directly linked back to

the 1940s when this form of transmission was first envisaged. 2.In the 1950s, it started to be used for covert military transmissions in view of the facts that the transmissions look like noise, it is difficult to decipher without the knowledge of the right codes, and furthermore it is difficult to jam. 3.With the revolution in cellular telecommunications that occurred in the 1980s a then little know company named Qualcomm working on CDMA as a basis for a cellular telecommunications multiple access scheme - CDMA - code division multiple access..

• 4. Qualcomm was joined by US network operators Nynex

and Ameritech to develop the first experimental CDMA system.

• 5. With the support of the Cellular Telecommunications

Industry Association (CTIA) and the Telecommunications Industry Association (TIA) a standards group was set up. This group then published the standard for the first CDMA system in the form of IS-95 in 1995.

• 6. This development lead on to the CDMA2000 series of

standards.

• 7. Later, it became necessary to evolve the GSM standard

so that it could carry data and provide significant improvements in terms of spectrum use efficiency.

• 8. Accordingly CDMA, in the form of Wideband CDMA

(WCDMA) was adopted for the GSM standard

EVOLUTION OF CDMA/2