Prof. Ahmed El-Mahdy, Communications Department The German University in Cairo
Text Books [1] Couch, Digital and Analog Communication Systems, 7 th edition, Prentice Hall, 2007. [2] Simon Haykin, Communication Systems, 4 th edition, John Wiley & Sons, 2001.
Instructor: Prof. Ahmed El-Mahdy Office : C3.319 Office Hours: Monday (12 am-5pm) Email: ahmed.elmahdy@guc.edu.eg
Grading Quizzes (3 Quizzes best 2) 20% (NO compensation Quizzes will be held) Assignments (2 Assignments best 2) 10% Project 5% Midterm Exam 25% Final Exam 40%
Course Contents No. 1 2 3 4 5 6 7 Subject Introduction Hilbert Transform Linear CW Modulations (AM, DSB, SSB, & VSB) Angle Modulations (FM & PM) Noise in AM receivers. Noise in FM receivers. Channel Estimation & Equalization
Lecture 1 - Introduction & Revision - Hilbert Transform
Remember: Elements of Communication System The transmitter is located at some point in space The receiver is located at some other point separate from the transmitter The channel is the physical medium that connects them
Transmitter The purpose of the transmitter is to convert the message signal produced by the source of information into a form suitable for transmission over the channel. One of the process performed by the transmitter is the MODULATION. MODULATION is the process of transformation of baseband signal m(t) into a passband signal s(t) using a carrier signal c(t).
Receiver The purpose of the receiver is to recover the message signal through demodulation. DEMODULATION is the inverse process of modulation i.e transformation of passband signal s(t) into the original baseband signal m(t). Some other tasks of the receiver are: - Noise reduction - Interference suppression.
Channel
Fundamental Concepts There are TWO basic modes of communication: (1)Broadcasting: - Use of a single powerful transmitter and several receivers (ex. broadcasting). - Signals flow only in one direction. (2) Point to Point: - The communication process takes place over a link between a single transmitter and a receiver. - Signals flow in two direction.
Main Resourses of Communication Systems Transmitted Power: is the average power of the transmitted signal. Channel Bandwidth: is the range of frequencies allocated to the channel. Performance measure In Analog communications: Performance is measured by output signal to noise ratio defined by: SNR Signal power Noisepowerin signal Bandwidth
Remember : Effect of multiplying by exponential y( t) x( t) e j 2 f ct Y( f ) X ( f fc)
Remember : Effect of multiplying by cos( 2 f c t) y( t) x( t)cos(2 f t) Y f ) X ( f f ) X ( f f ) c 1 2 ( c c
Analog Signals - Used in: - Radio broadcasting - TV - Microwave Communications. Processing of analog signals: is simple in concept but difficult to build because of requirements on linearity and system adjustment. For example: voice communication requires non-linear distortion. Analog Signals more affected by noise than digital (noise is part of the signal).
Example: Analog Signals
Signal Frequencies around 6MHz
Baseband and bandpass Signals Baseband Signal: Has its spectrum centered around the origin f=0 and is zero elsewhere. W(f) - f m 0 +f m baseband Signal f
Band Pass Signals: A carrier frequency is used to shift the frequency spectrum of transmitted signals. Baseband signals w(t) may be transformed into bandpass signals through multiplication by a carrier (Modulation): w t cos w t W f f W f f c 2 1 c c W(f) W(f) - f m 0 +f m f - f c 0 +f c f
Remember: Definition of Bandwidth Bandwidth is a term used to describe a positive frequency range over which the signal has significant content. Absolute Bandwidth:
Remember Bandwidth - Baseband Absolute Bandwidth=W
Remember Bandwidth - Bandpass Absolute Bandwidth=2W
The Reasons for Bandpass Modulation 1. To decrease size of Antenna For efficient radiation of electromagnetic energy, the size of the radiation antenna is proportional to the wavelength of the radiated signal. Size of Antenna c f, c 3 10 8 m/s As f increases decreases and then Anntena size decreases
2. Simultaneous Transmission of Several Signals Consider the case of several radio stations broadcasting audio baseband signals directly, without any modulation. They would interfere with each other because the spectra of all the signals occupy more or less the same bandwidth. We can use various audio signals to modulate different carrier frequencies, thus translating each signal to a different frequency range. If the various carriers are chosen sufficiently far apart in frequency, the spectra of the modulating signals will not overlap and thus will not interfere with each other. 3. Better control of filtering operation
Complex Envelope Representation Any bandpass signal s(t) (modulated signal) can be represented by: Where: s( t) Re C( t) e j c t 1 Re {.} represents the real part of any complex number c is the angular frequency of the carrier signal C(t) is called the complex envelope
Other Representations
Relationship Between the Three Forms
Types of Continuous Wave Modulations Digital Modulations Analog Modulations - ASK - FSK -PSK -QAM - AM (DSB, SSB,..) - FM - PM
Analog Modulations Angle Modulation Linear Modulation schemes FM PM AM: DSB, SSB, and VSB
Parameters of Analog Modulation
Hilbert Transform 1 x( t ) d
Hilbert Transform (From Signals & Systems)
Examples on Hilbert Transform