UNITI REFERENCE B P LATHI BASE BAND CARRIER
UNIT-I REFERENCE: B. P. LATHI
BASE BAND CARRIER COMMUNICATION Base band signal and Bandwidth requirement Carrier Communication Need for Modulation
What is baseband signal ? Term ‘base band’ is used to designate band of frequencies of the signal delivered by source or input transducer: ü Speech Signals: 0 to 5 k. Hz ü Audio signals: 0 to 20 KHz ü Video signals: 0 to 5 MHz ü In telephony: baseband is audio band(0 -3. 5 k. Hz) ü In television: baseband is video band(0 -4. 3 MHz)
Base band signal transmission Base band signal can be digital or analog Digital msg are constructed with finite no of symbols: text, numbers, ‘M’ symbols : M-ary msg Analog message has data whose values vary over continuous range: can have infinite possible values Eg: Speech
Bandwidth requirement BW of a channel is range of frequencies over which an information signal is transmitted. Specifically, it is the difference between the upper and lower frequency limits of the signal üSpeech Signals: 0 to 5 k. Hz: BW- 5 k. Hz üAudio signals: 0 to 20 KHz : BW- 20 KHz üVideo signals: 0 to 5 MHz : BW- 5 MHz üTelephony: baseband is audio band(0 -3. 5 k. Hz): BW 3. 5 k. Hz
Carrier Communication that uses modulation to shift frequency spectrum of a signal One of the basic parameters of sinusoidal carrier of high frequency is varied in proportion to the base band signal m(t), resulting in AM, FM, PM Used to transmit analog as well as digital baseband signals
Need for Modulation Reduction in height of Antenna Efficient transmission Multiplexing Frequency Assignment Improve SNR
Reduction in height of Antenna For efficient radiation: Preferable size of antenna is where, , is wavelength of the signal to be radiated Eg: Audio (DC to 20 KHz) assume, designing antenna for mid frequency i. e. 10 KHz Length of antenna required:
Modulation for efficient transmission Ground wave propagation is possible only up to 2 MHz Sky propagation possible for frequencies in the range of 2 to 30 MHz Line of sight propagation used beyond 30 MHz Preferred frequencies for satellite communication are around 3 to 6 GHz Hence, for efficient transmission, it would be necessary to shift message spectrum into the pass band, by choosing appropriate carrier frequency and modulation technique. i. e. Modulation results in frequency translation
Modulation for Multiplexing Several message signals can be transmitted on a given channel Each message signal is assigned an appropriate slot in pass band of the channel E. g. AM broadcast
Modulation for frequency assignment If each message signal is being broadcast by different station Each station can be assigned suitable carrier Therefore, corresponding program material can be received by tuning to the station desired.
Modulation to improve SNR Certain modulation schemes (notably FM & PM) have the feature to permit improved signal to noise ratio (SNR) at the receiver o/p
What is Modulation ? A process that causes shift in the range of frequencies in a signal
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