UNIT -IVDIGITAL BAND-PASS MODULATION TECHNIQUES

Topics covered fromChapter 1(Communication Systems-Simon Hykin)

1

Prof. Parjane M.A.

• Baseband Communication: Signals are transmitted without any shift in the range of frequency of the signal.

• Band-pass Communication: Uses modulation to shift the frequency spectrum of a (carrier) sinusoidal signal. Usually, one of the basic parameters (amplitude, frequency, or phase) of the carrier signal is varied in proportion to the baseband signal (information-bearing data stream).

• Why modulate signals?▫Convert signals to a form that is suitable for transmission▫Sharing the frequency band with other stations

• Three basic modulation schemes: Amplitude-shift keying (ASK) Phase-shift keying (PSK) Frequency-shift keying (FSK)

2

Amplitude Shift Keying (ASK)

• Pulse shaping can be employed to remove spectral spreading

• ASK demonstrates poor performance, as it is heavily affected by noise, fading, and interference

Baseband Data

ASK modulated

signal

1 10 0 0

Acos(t) Acos(t)

3

Frequency Shift Keying (FSK)

Example: The ITU-T V.21 modem standard uses FSK FSK can be expanded to a M-ary scheme, employing multiple frequencies as different states

Baseband Data

BFSK modulated

signal

1 10 0

where f0 =Acos(c-)t and f1 =Acos(c+)t

f0 f0 f1 f1

4

Phase Shift Keying (PSK)

Major drawback – rapid amplitude change between symbols due to phase discontinuity, which requires infinite bandwidth. Binary Phase Shift Keying (BPSK) demonstrates better performance than ASK and BFSKBPSK can be expanded to a M-ary scheme, employing multiple phases and amplitudes as different states

Baseband Data

BPSK modulated

signal

1 10 0

where s0 =-Acos(ct) and s1 =Acos(ct)

s0 s0 s1 s1

5

Types Of Reception For Passband Transmission•There are 2 types of methods for

detection of passband signals. Coherent (Synchronous) Detection: In this

method, the local carrier generated at the receiver in phase locked with the carrier at the transmitter.

Non coherent (Envelope) Detection: in this method the receiver carrier need not be phase locked with transmitter carrier.

6

Requirement of Passband Transmission Scheme•Maximum data transmission rate•Minimum probability of symbol error.•Minimum transmitted power.•Minimum channel bandwidth.•Maximum resistance to interfering

signals.•Minimum circuit complexity.

7

Advantages Of Passband Transmission Over Baseband Transmission•Long distance transmission.•Analog channels can be used for transmission.•Multiplexing techniques can be used for bandwidth

conservation.•Problems such as ISI and crosstalk are absent.•Passband transmission can be take place over wireless

channels also.•Large number of modulation techniques are available.

8

Drawbacks• Not suitable for short distance transmission.• Such Systems are complex in design.

Passband Transmission Model

• Message source emits the symbol at the rate of T symbols.• Encoder produces the vector Si made up of ‘N’ real elements. The

vector Si is unique for each set of ‘M’ symbols.• Modulator modulates carrier signal Si(t) of duration ‘T’ seconds

for every symbol mi. It is energy signal.• Channel is assumed to be linear & enough bandwidth to

accommodate the signal Si(t). Channel noise is white Gaussian of zero mean & psd of (N0/2).

• Detector demodulates the received signal & obtain an estimate of the signal vector.

• Decoder obtains the estimate of symbol back from the signal vector.

9

10

▫In digital comm., the usual practice is to assume that the carrier c(t) has unit energy measured over one symbol (bit) duration (Tb).

where

▫Decreasing the bit duration Tb has the effect of increasing the transmission bandwidth requirement of a binary modulated wave. (Fourier Transform property).

bc TA

2)2cos(

2)( cc

b

tfT

tm

11

•Band-Pass Assumption▫ The spectrum of a digital modulated wave s(t) is centered on

the carrier frequency fc

where b(t) is an incoming binary stream with bandwidth W.

▫ Assumption: fc>> BW,There will be no spectral overlap in the generation of s(t)

▫ The transmitted signal energy per bit can be approximated as:

)()()( tmtbts

)2cos()(2

)( tftbT

ts cb

Binary Phase Shift Keying (BPSK)

• In PSK, the phase of the carrier signal is switched between 2 (for BPSK) or more (for MPSK) in response to the baseband digital data

• With PSK the information is contained in the instantaneous phase of the modulated carrier

• Usually this phase is imposed and measured with respect to a fixed carrier of known phase – Coherent PSK

• For binary PSK, phase states of 0o and 180o are used• two phases represent two binary digits.

12

13

14

Generator of BPSK Signal

•binary data sequence converted into Bipolar NRZ signal.

•Balanced modulator is used for generating DSB (Double Side band) signals.

Note: produces the sum and difference frequencies but to cancel or balance out the carrier.

15

Bipolar NRZ level Encoder

Balanced Modulator

Binary data Sequence

b(t)BPSK Signal

Carrier Signal

Reception of BPSK Signal16

Pass band is centered around the 2f0.Recovered the carrier. Integrator integrates the signal over bit period. And synchronizer take care of starting and ending times of bit.

17

18