Chapter 5 - Signal Encoding Techniques

Modulation - process of encoding source data onto a carrier signal in order to transmit data. Modulation techniques require variation to either the amplitude, frequency, or phase.

Carrier Signal - aka carrier wave or carrier, is a continuous and single-frequency signal that is able to be modulated with another data signal to transmit data.


Analog - Continuous. Ex - sinusoid.
Digital - Discrete and discontinuous. Series or sequence. Uses specific constant values, like 1 and 0. Ex - square wave.

4 Possible Encoding Combinations:

1. Digital data to digital signal
2. Analog data to digital signal
3. Digital data to analog signal
4. Analog data to analog signal

5.1/ Digital Data, Digital Signals
Data is the information, usually a binary one or zero, one wants to transmit. Each data element is a bit. Signal is the way that the data is transmitted. Signal is any quantity that can be changed over time or space. In electronics, voltage or current can be a signal.

A digital signal will be a series of discrete voltage fluctuations or pulses. The pulses are known as signal elements or symbols. A digital symbol is a pulse with constant amplitude. An analog symbol is a pulse with constant frequency, phase, and amplitude. Data can be transmitted by encoding data bits into symbols.

Unipolar - A signal is unipolar when all signal elements are either all positive or all negative.
Polar - Positive and negative values are both used in signal.

Bit - Binary data; zero or one.
Data rate - aka data signaling rate or bit rate, is the amount of bits that are transmitted per second. (Unit is bits/second)
The inverse (1/data rate) of the data rate is the amount of time it takes for one bit to be transmitted.

Modulation rate - the rate at which a carrier signal changes. (Units - baud or symbols/second)

Digital Signal Encoding Formats

1. NRZ-L (Nonreturn to Zero-Level)
   • 0 = high, 1 = low


2. NRZI (Nonreturn to Zero Inverted)
   • 0 = no transition at beginning of bit, 1 = transition at beginning of bit


3. Bipolar AMI (no net DC component)
   • 0 = no signal, 1 = positive or negative, with alternation for successive bits


4. Pseudoternary (opposite of Bipolar AMI)
   • 0 = positive or negative, with alternation for successive bits , 1 = no signal


5. Manchester
   • 0 = high to low transition in middle of bit, 1 = low to high transition in middle of bit


6. Differential Manchester (A transition will always occur in middle of bit)
   • 0 = transition at beginning of bit, 1 = no transition at beginning of bit

Factors that affect the receiving end of the signal:

1. SNR (signal-to-noise ratio) - Inversely related to bit error rate.
   
2. Data Rate - Directly related to BER (bit error rate).
3. Bandwidth - Directly related to data rate.

Bit error rate - aka bit error ratio, is the ratio of incorrect bits to the total number of bits.

Methods to compare encoding techniques:

1. Signal spectrum
   • less high-frequency components
   • no DC (direct-current) components
   • transmitted power mostly in middle of bandwidth instead of ends
     


2. Clocking
   • connection synchronization (determining beginning and end of bits)


3. Error detection
   • speed


4. Signal interference and noise immunity
   • expressed in BER


5. Cost and complexity
   • low cost for high availability

The information above is based on the textbook Data and Computer Communications by William Stallings.