Modulation-Why?

1. Low frequency signal has less energy, which means it can travel less distance.

2. Practibility of antenna

Modulation-Why?

3. Multiplexing

4. Operating Range

Modulation

1. Amplitude shift keying

2. Frequency shift keying

3. Phase shift keying

Digital data has to be converted into analog for sending over analog channel.

Amplitude shift keying

Digital data converted into analog by changing amplitude property of analog signal.

Digit 1 represented by high amplitude and digit 0 represented as low amplitude.

Frequency shift keying

Digital data converted into analog by changing frequency of analog signal.

Digit 1 represented by high frequency and digit 0 represented as low freqeuncy.

Phase shift keying

Digital data converted into analog by changing phase of analog signal.

In BPSK only two phase are used to represent 0 and 1.

Phase Change

Phase Change

Spread Spectrum

Problem with conventional wireless communication

Sender Receiver

In conventional wireless communication a fixed frequency is used and this frequency does not changed over time.for example when you listen radio and tune it for 93.5FM. This frequency will be same always.

Problem with conventional wireless communication

1. Interference

2. Interception

When a signal has a constant frequency that signal subject to interference. This occurs when another signal is transmitted on, or very near, the frequency of the desired signal. Catastrophic interference can be accidental (as in amateur-radio communications) or it can be deliberate (as in wartime)

A constant-frequency signal is easy to intercept, and is therefore not well suited to applications in which information must be kept confidential between the source (transmitting party) and destination (receiving party).

What is spread spectrum?

Signal is sent using many frequency.

Spread Spectrum Spread Spectrum modulation techniques are defined as being those techniques in which: - The bandwidth of the transmitted signal is much greater than the bandwidth of the original message, and - The bandwidth of the transmitted signal is determined by the message to be transmitted and by an additional signal known as the Spreading Code.

Advantage of spread spectrum

1. Reduced interference

2. Immunity to Jamming

Types of spread spectrum

1. Frequency Hopping Spread Spectrum

2. Direct sequence Spread Spectrum

Frequency Hopping Spread Spectrum

Sender don’t use single frequency to transmit data. Multiple frequency is used for transmission.

Sender send data using frequency f1 for 625 micro second and then change frequency.

Different-2 sender use different-2 frequency patter.

FHSS process

1. Frequency of carrier is periodically modified following a specified sequence of frequency.

2. This sequence is known as hopping sequence or spreading code.

3. The amount of time spent on each frequency or hop is known as dwell time.

4. Following frequency hopping sequence, message is modulated.

FHSS Example

F 8

F 7

F 6

F 5

F 4

F 3

F 2

F 1

Freq

uenc

y

Time

Let’s say sender A want to send some data. Hopping sequence for A is F1,F5,F3,F8

FHSS Example

F 8

F 7

F 6

F 5

F 4

F 3

F 2

F 1

Freq

uenc

y

Time

Let’s say sender A want to send some data. Hopping sequence for A is F1,F5,F3,F8

FHSS Example

F 8

F 7

F 6

F 5

F 4

F 3

F 2

F 1

Freq

uenc

y

Time

Let’s say sender A want to send some data. Hopping sequence for A is F1,F5,F3,F8

FHSS Example

F 8

F 7

F 6

F 5

F 4

F 3

F 2

F 1

Freq

uenc

y

Time

Let’s say sender A want to send some data. Hopping sequence for A is F1,F5,F3,F8

FHSS Example

F 8

F 7

F 6

F 5

F 4

F 3

F 2

F 1

Freq

uenc

y

Time

Let’s say sender A want to send some data. Hopping sequence for A is F1,F5,F3,F8

FHSS

Let’s say sender A want to send some data. Hopping sequence for A is F1,F5,F3,F8

At the time of transmission sender first modulate their signal using frequency F1. Once dwell time is completed then it modulate their signal using frequency F5, then F3 and F8.

Direct Sequence Spread Spectrum

Every user assigned a spreading code. This secret code is used to encode the signal.

This code is multiplied with original message and resultant message is then transmitted.

Receiver use same spreading code to decode the message to retrieve original message.

Direct Sequence Spread Spectrum

For the duration of every message bit, the carrier is modulated following a specific sequence of bits (known as chips). The process is known as “chipping” and results in the substitution of every message bit by (same) sequence of chips.

Spreading code example (100101)0 is represented as -1Spreading code is now (1,-1,-1,1,-1,1)

Low-Bandwidth Signal:

High-Bandwidth Spreading Code:

...repeated...

Direct Sequence Spread Spectrum

Low-Bandwidth Signal:

High-Bandwidth Spreading Code:

Mix is a simple multiply

… and transmit.

Direct Sequence Spread Spectrum

To Decode / Receive, take the signal:

Multiply by the same Spreading Code:

… to get ...

… which you should recognise as...

Direct Sequence Spread Spectrum

To Decode / Receive, take the signal:

Multiply by the same Spreading Code:

… to get ...

Direct Sequence Spread Spectrum

1

0

1

1 1 1 0 1 0 1 1 1 1 1 0 1 0 1 1

Chip period

One bit period (symbol period)

Data

Coded Signal

Input to the modulator (phase modulation)

Represent bit 1 with +1Represent bit 0 with -1

Direct Sequence Spread Spectrum-CDMA

FHSS (Frequency Hopping Spread Spectrum) I

• Discrete changes of carrier frequency– sequence of frequency changes determined via pseudo random number

sequence

• Two versions– Fast Hopping:

several frequencies per user bit– Slow Hopping:

several user bits per frequency

• Advantages– frequency selective fading and interference limited to short period– simple implementation– uses only small portion of spectrum at any time

• Disadvantages– not as robust as DSSS– simpler to detect

FHSS (Frequency Hopping Spread Spectrum) II

user data

slowhopping(3 bits/hop)

fasthopping(3 hops/bit)

0 1

tb

0 1 1 t

f

f1

f2

f3

t

td

f

f1

f2

f3

t

td

tb: bit period td: dwell time

FHSS (Frequency Hopping Spread Spectrum) III

modulatoruser data

hoppingsequence

modulator

narrowbandsignal

spreadtransmitsignal

transmitter

receivedsignal

receiver

demodulatordata

frequencysynthesizer

hoppingsequence

demodulator

frequencysynthesizer

narrowbandsignal

DSSS (Direct Sequence Spread Spectrum) II

Xuser data

chippingsequence

modulator

radiocarrier

spreadspectrumsignal

transmitsignal

transmitter

demodulator

receivedsignal

radiocarrier

X

chippingsequence

lowpassfilteredsignal

receiver

integrator

products

decisiondata

sampledsums

correlator