14-11-2012, 01:17 PM
digital communication
[attachment=39385]
Introduction
Line coding involves converting a sequence of 1s and 0s to a time-domain signal (a sequence of pulses) suitable for transmission over a channel. The following primary factors should be considered when choosing or designing a line code.
1. Self-synchronization. Timing information should be built into the time-domain signal so that the timing information can be extracted for clock synchronization. A long string of consecutive 1s and 0s should not cause a problem in clock recovery.
2. Transmission power and bandwidth efficiency. The transmitted power should be as small as possible, and the transmission bandwidth needs to be sufficiently small compared to the channel bandwidth so that inter-symbol interference will not be a problem.
3. Favorable Power Spectral Density. The spectrum of the time-domain signal should be suitable for the transmission channel. For example, if a channel is ac coupled, it is desirable to have zero power spectral density near dc to avoid dc
wandering in the pulse stream.
4. Low probability of error. When the received signal is corrupted by noise, the receiver can easily recover the un-coded signal with low error probability.
5. Error detection and correction capability. The line code should have error detection capability, and preferably have error correction capability.
6. Transparency. It should be possible to transmit every signal sequence correctly regardless of the patterns of 1s and 0s. If the data are coded so that the coded signal is received faithfully, the code is transparent. Given a sequence of pulses, there are two possible waveform formats that we can use to send a pulse of duration Tb seconds over a channel. The duty cycle of the pulse can be used to define these two waveform formats.
Manchester (Split-phase, Twinned-Binary) Coding
Manchester coding was developed by Manchester University. In positive logic, a 1 is represented by +A volts over a half-pulse period followed by -A volts over a half-pulse period. A 0 is represented by -A volts over a half-pulse period followed by +A volts over a half-pulse period. This is shown in Figure 20.1 (g). Other names in use for Manchester coding are split-phase and twinned-binary coding. Sometimes it is called bi-phase-level (Bi- φ-L)
A Manchester signal can be generated by multiplying a polar NRZ signal by a synchronized square-wave clock having a period Tb [4]. It can also be generated by exclusive-ORing a polar NRZ signal with a synchronized but inverted square-wave clock having a period Tb.
Miller (delay modulation) Coding
A transition occurs at the mid-point of each symbol interval for a 1. For a 1 followed by a 1, no transition occurs at the symbol interval. No transition occurs at the mid-point of each symbol interval for a 0. For a 0 followed by a 0, a transition occurs at the symbol interval. For a 0 followed by a 1 or a 1 followed by a 0, no transition occurs at the symbol interval. This is shown in Figure 20.1 (h). Miller coding is also called delay modulation.
[attachment=39385]
Introduction
Line coding involves converting a sequence of 1s and 0s to a time-domain signal (a sequence of pulses) suitable for transmission over a channel. The following primary factors should be considered when choosing or designing a line code.
1. Self-synchronization. Timing information should be built into the time-domain signal so that the timing information can be extracted for clock synchronization. A long string of consecutive 1s and 0s should not cause a problem in clock recovery.
2. Transmission power and bandwidth efficiency. The transmitted power should be as small as possible, and the transmission bandwidth needs to be sufficiently small compared to the channel bandwidth so that inter-symbol interference will not be a problem.
3. Favorable Power Spectral Density. The spectrum of the time-domain signal should be suitable for the transmission channel. For example, if a channel is ac coupled, it is desirable to have zero power spectral density near dc to avoid dc
wandering in the pulse stream.
4. Low probability of error. When the received signal is corrupted by noise, the receiver can easily recover the un-coded signal with low error probability.
5. Error detection and correction capability. The line code should have error detection capability, and preferably have error correction capability.
6. Transparency. It should be possible to transmit every signal sequence correctly regardless of the patterns of 1s and 0s. If the data are coded so that the coded signal is received faithfully, the code is transparent. Given a sequence of pulses, there are two possible waveform formats that we can use to send a pulse of duration Tb seconds over a channel. The duty cycle of the pulse can be used to define these two waveform formats.
Manchester (Split-phase, Twinned-Binary) Coding
Manchester coding was developed by Manchester University. In positive logic, a 1 is represented by +A volts over a half-pulse period followed by -A volts over a half-pulse period. A 0 is represented by -A volts over a half-pulse period followed by +A volts over a half-pulse period. This is shown in Figure 20.1 (g). Other names in use for Manchester coding are split-phase and twinned-binary coding. Sometimes it is called bi-phase-level (Bi- φ-L)
A Manchester signal can be generated by multiplying a polar NRZ signal by a synchronized square-wave clock having a period Tb [4]. It can also be generated by exclusive-ORing a polar NRZ signal with a synchronized but inverted square-wave clock having a period Tb.
Miller (delay modulation) Coding
A transition occurs at the mid-point of each symbol interval for a 1. For a 1 followed by a 1, no transition occurs at the symbol interval. No transition occurs at the mid-point of each symbol interval for a 0. For a 0 followed by a 0, a transition occurs at the symbol interval. For a 0 followed by a 1 or a 1 followed by a 0, no transition occurs at the symbol interval. This is shown in Figure 20.1 (h). Miller coding is also called delay modulation.