Line Coding and Binary Keying Modulation
Some Characteristics of Line Coding Signal level and Data Level 1 Amplitude Time (a) Two signal level and two data level (b) Three signal level and two data level
Bit Rate and Pulse rate If a pulse carries only 1 bit, the pulse rate and the bit rate are the same. If the pulse carries more than 1 bit, then the bit rate is greater than the pulse rate. In general we have the following formula, in which L is the number of data levels of the signal..
DC component Amplitude 1 Time (a) Signal with DC components 1 Amplitude Time (a) Signal with DC components (b) Signal without DC components
Synchronization In order to correctly interpret the signals received from the sender, the receiver's bit intervals must correspond exactly to the sender's bit intervals. If the receiver clock is faster or slower, the bit intervals are not matched and the receiver might interpret the signals differently than the sender intended. A self-synchronizing digital signal includes timing information in the data being transmitted. This can be achieved if there are transitions in the signal that alert the receiver to the beginning, middle, or end of the pulse. If the receiver's clock is out of synchronization, these alerting points can reset the clock.
1 Amplitude Time Sent Received
Line Coding Polar Bipolar Unipolar
Unipolar Time Amplitude (a) Logic 0 1 (b) Logic 1 (c) Timing diagram
Differential Manchester Polar RZ NRZ Polar Manchester Differential Manchester
Nonreturn to Zero-Level(NRZ-L) Time Amplitude (a) Logic 0 1 (b) Logic 1 (c) Timing diagram
Nonreturn to Zero-Invert(NRZ-I) Time Amplitude 1 Logic 0 - No Transition from current state Logic 1- Transition from current state to different state
This transition is used for synchronization Return to Zero (RZ) Time 1 Amplitude (b) Logic 1 (a) Logic 0 (c) Timing diagram This transition is used for synchronization
Manchester Encoding Time Amplitude (b) Logic 1 (a) Logic 0 (a) Logic 0 (c) Timing Diagram
Differential Manchester Encoding Time Amplitude 1 Logic 0 - Transition from current state to different state Logic 1- Continuation of current state i.e. no transition
Logic 1- Levels are positive and negative alternately Bipolar Time Amplitude 1 Logic 0 – Level Zero Logic 1- Levels are positive and negative alternately
Two binary, one quaternary (2B1Q) 00 Time Amplitude 1 3 -1 -3 01 10 11
Multiline transmission, three level (MLT-3) Time Amplitude 1
Block Coding 010111010101 ... 01011110 m Bit Division 0101 1101 1110 m Bits 0101 1101 1110 m Bit to n Bit Substitution n Bits 01101 10101 011011 Line coding Digital signal
4B/5B encoding With a 4-bit block, we can have 16 (=24) different groups. With a 5-bit code, there are 32 (=25) possible codes. This means that we can map some of the 5-bit groups to the 4-bit groups. Some of the 5-bit codes are not used. We can apply a strategy or a policy to choose only the 5-bit codes that help us in synchronization and error detection. 0000 0001 0010 1000 1111 4-bit Blocks 00000 00011 00010 10000 11011 5-bit Blocks 11111
To achieve synchronization, we can use the 5-bit codes in such a way that, for example, we do not have more than three consecutive 0s or 1s. Block coding can definitely help in error detection. Because only a subset of the 5-bit codes is used, if one or more of the bits in the block is changed in such a way that one of the unused codes is received, the receiver can easily detect the error. The selection of the 5-bit code is such that each code contains no more than one leading 0 and no more than two trailing 0s. Therefore, when these 5-bit codes are sent
Data Sequence Encoded Sequence 0000 11110 Q (Quit) 00000 0001 01001 I (Idle) 11111 0010 10100 H (Halt) 00100 0011 10101 J ( start delimiter) 11000 0100 01010 K ( start delimiter) 10001 0101 01011 T (end delimiter) 01101 0110 01110 S(Set) 11001 0111 10010 R(Reset) 00111 1000 10011 1001 1010 10110 1011 10111 1100 11010 1101 11011 1110 11100 1111 11101
Digital to Analog Modulation Types of methods Amplitude shift keying (ASK): some audio modem, optical fiber, printer Frequency shift keying (FSK): Computer Modem, HF and VHF radio Amplitude shift keying (PSK): Mobile, Satellite (microwave) etc Digital to Analog Modulation FSK QAM PSK ASK Bit rate is the number of bits transmitted per second. Baud rate is the number of signal elements transmitted per second. Baud rate is less than or equal to the bit rate
ASK The carrier with unity amplitude 1 Baud 1 Bit 1 Bit rate: 6 Baud rate: 6 1 Sec Time Amplitude The carrier with unity amplitude Mathematically ASK waveform can be written as
The bandwidth which needs to be transmitted depends on the requirements of the application. Where there is a need to recover recognizably square pulses at the receiver (to operate a printer), it may be necessary to include at least the third harmonic sideband pair This leads to the criterion
Example The standard symbol speed of 66 words per minute used by many automatic teletype machines. This corresponds to 50 baud (symbol rate) per second which need an f0=25 Hz. Thus its 3rd harmonic BW is 75Hz or 120 Hz with guard band. But where all that is needed is to be able to decide whether a one or a zero was received and an exact bit shape is not required.
FSK Where s2(t) is the complement of s1(t) so that s2(t)=1- s1(t) 1 Baud 1 Bit 1 Bit rate: 6 Baud rate: 6 1 Sec Time Amplitude Where s2(t) is the complement of s1(t) so that s2(t)=1- s1(t)
PSK PSK has advantages over both ASK and FSK. It has the same BW as ASK which is of course less than that of FSK. It is less susceptible to noise corruption than FSK and of course very much better than ASK BW reduced by multilevel scheme.
Which is same spectrum as ASK
Constellation Diagram Dibits (2 Bits) 10 Dibit Phase 00 0o 01 90o 11 Tribit Phase 000 0o 001 45o 010 90o 011 135o 100 180o 101 225o 110 270o 111 315o 000 Constellation Diagram 001 010 011 100 101 110 111
4PSK 1 Baud 2 Bits 00 11 01 10 Bit rate: 12 Baud rate: 6 1 Sec Time Amplitude
4QAM 00 01 10 11 000 001 010 011 100 101 110 111 4-QAM 1 Amplitude, 4 Phases 8-QAM 2 Amplitudes, 4 Phases
8QAM 000 001 011 010 110 111 101 100 1 Baud 3 Bits Bit rate: 24 Baud rate: 08 1 Sec Time Amplitude
16 QAM 3 Amplitudes 12 Phases 2 Amplitudes 8 Phases 4 Amplitudes