Lecture 1

References In no particular order Modern Digital and Analog Communication Systems, B. P. Lathi, 3 rd edition, 1998 Communication Systems Engineering, J. G. Proakis, M. Salehi, 2 nd edition, 2002 Communication Systems, S. Haykin, 4 th edition, 2001 Any

Course Contents Communication Systems I (SC4104) covers: Signal analysis (revision from signal and systems) Fourier transform (revision from signal and systems) Power spectrum (also revision) Principles of modulation AM FM Pulse Modulation Comparison between Analog and Digital transmission

Course Contents Time Division Multiplexing Frequency Division Multiplexing Random Signals and Noise (will be discussed again for digital communication) Transmission systems for cable, radio, satellite and optical systems (just a very brief introduction for later courses)

Assessment Final exam (probably 70%) Test (probably 30%) H.W, CW, … etc (probably 0%, but you will not pass if you don’t do them!!!!!) Laboratory sessions (another subject!!)

What is Telecommunication Telecommunication is the process of transferring a usually unknown message from a source to a destination separated by some distance (tele) The data/signal sent from the source should be delivered to the destination accurately enough to be identified by the destination

Communication System Source of information generates messages Transducer converts them to suitable electrical signal (baseband or message signal) Transmitter modifies the baseband signal for efficient transmission over the channel

Communication System The signal is sent from the transmitter via the channel (coaxial cable, fiber, wireless, …) The channel distorts the signal and adds noise to it Receiver recovers the baseband signal by undoing what the transmitter did and trying to undo what the channel did The output transducer converts the baseband signal to its original form and it is delivered to the destination

The Source We always assume the source have something to send This can be: Voice (continuous analog signal varying with time) Image (2D signal, converted to 1 or 3 analog time varying signals) Video (continuous 2D signal, converted to 1 or 3 analog time varying signals) Text and symbols Transducers convert these into electrical signals which are fed to the transmitter To understand why we need the transmitter (and receiver), we need to see the channel first

The Channel Plays key role in choosing the elements of the system Causes attenuation that increases with the distance Its attenuation is frequency dependent hence, it distorts the signal Usually modeled as a low pass filter Also random signals, e.g. from other transmitters (interference) or other natural or human sources (noise), are added to the signal

Sample Signal

Frequency Domain

Channel Response

Freq. Domain After the Channel

Input and Output Signals

Channel with twice the bandwidth

Channel with 4 times the BW

Ten times the original BW

Noise Noise power is 1/10 that of signal power (SNR = 10dB)

So, the channel Attenuates the signal -> limits the distance Acts as a low pass filter -> distorts the signal Adds noise -> adds a random signal

What Does The Transmitter Do? Transmitter Modifies the message to generate a transmission signal The objective is to avoid/reduce effects of the bad characteristics of the channel Possible modifications include: Amplification Band limiting (usually for digital signals “another course”) Signal shaping Modulation E.g. if channel has high attenuation in 0-1 MHz, avoid this bandwidth if possible

What Does The Receiver Do? Receiver should undo the changes made by the transmitter and the channel Possible modifications made by the receiver: Amplification Demodulation Signal shaping Filtering The ultimate goal is to make the combined response of the transmitter, channel and receiver = 1, i.e. transparent system

Modulation The message from the source, typically has a bandwidth that starts from (or near) 0 Hz Such a signal is called a baseband signal These frequencies are not always suitable for transmission in the channel To solve this, the transmitter modifies the signal to move it to a higher frequency more suitable for the channel This process is known as modulation

Modulation Modulation requires the existence of a high frequency carrier This carrier is usually a cosine and/or sine signal in the form: The modulation process alters the amplitude (A), frequency (f c ) or phase (  ) of the carrier under the control of the input message This leads to Amplitude Modulation (AM), Frequency Modulation (FM) or Phase Modulation (PM)

Demodulation The receiver monitors the amplitude, frequency or phase of the received signal Based on the changes in these values, it recovers the message transmitted

Benefits of Modulation Ease of radiation Simultaneous transmission of several signals Avoid bad bands in the channel Can make the signal more resistant to noise

Frequencies for Various Wired Channels

Frequency Bands for Wireless Channels

Required Course Tools To study communication systems we need knowledge of the following: Analysis of signals and systems Fourier transforms (signal BW, channel response) Laplace transform (channel response) Convolution (output signal) Background about filters Characteristics of trigonometric functions Probability and statistics (study of noise)

Revision What is Fourier transform? For periodic signals (x(t)) the signal is given by: Where  is any suitable constant Fourier transform is discrete (multiples of fundamental frequency f 0 = 1/T 0 )

Revision For aperiodic signals (x(t)), the Fourier spectrum (X(f)) is continuous For x(t) to have a Fourier transform, Dirichlet conditions are met: x(t) is absolutely integrable in time Finite number of maxima and minima within a finite time interval Number of discontinuities within a finite time interval is finite All applicable for real signals

Revision The energy across 1 Ohm of an aperiodic signal is given by (power = 0): |G(f)| 2 is the energy spectral density (energy unit/Hz) Periodic signals have infinite energy, so we calculate the energy in one cycle and average it over the cycle duration to give the power (energy =  ):