Slide 1
Digital CommunicationContinuous of block codes, ISI, encoder
structureLecture - 9
Ir. Muhamad Asvial, MSc., PhDCenter for Information and Communication Engineering Research (CICER)
Electrical Engineering Department - University of IndonesiaE-mail: [email protected]
http://www.ee.ui.ac.id/cicer
Slide 2
Line coding and decoding
Slide 3
Signal element versus data element
Slide 4
Data Rate Vs. Signal Rate• Data rate: the number of data elements (bits) sent in 1s (bps).
It’s also called the bit rate• Signal rate: the number of signal elements sent in 1s (baud).
It’s also called the pulse rate, the modulation rate, or the baud rate.
• We wish to:– increase the data rate (increase the speed of transmission)– decrease the signal rate (decrease the bandwidth
requirement) – worst case, best case, and average case of r– N bit rate– c is a constant that depends on different line codes.– S = c * N / r baud
Slide 5
Other properties
• DC components• Transmission bandwidth• Power efficiency• Error detection and correction capability• Favorable power spectral density• Adequate timing content• Transparency
Slide 6
Line coding schemes
Slide 7
ISI Example
5T
0 t
Sequence of three pulses (1, 0, 1)sent at a rate 1/T
sequence sent 1 0 1
sequence received 1 1(!) 1
Signal received
Threshold
4T3T2TT0-T-2T-3T
Slide 8
Baseband binary data transmission system.
• ISI arises when the channel is dispersive • Frequency limited -> time unlimited -> ISI• Time limited -> bandwidth unlimited ->
bandpass channel -> time unlimited -> ISI
p(t)
Slide 9
ISI• First term : contribution of the i-th transmitted
bit.• Second term : ISI – residual effect of all other
transmitted bits.
• We wish to design transmit and receiver filters to minimize the ISI.
Slide 10
Sample rate vs. bandwidth• W is the bandwidth of P(f)• When 1/T > 2W, no function to satisfy Nyquist
condition.
P(f)
Slide 11
Sample rate vs. bandwidth• When 1/T = 2W, rectangular function satisfy
Nyquist condition
,
,0,
sincsin
otherwiseWfT
fPTt
tTttp
0 1 2 3 4 5 6-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Subcarrier Number k
Spe
ctra
Slide 12
Differential Coding• The response of a pulse is spread over more than one signaling
interval.• The response is partial in any signaling interval.• Detection :
– Major drawback : error propagation.• To avoid error propagation, need deferential coding (precoding).
Slide 13
Modified duobinary signaling
• Modified duobinary signaling– In duobinary signaling, H(f) is nonzero at the origin.– We can correct this deficiency by using the class
IV partial response.
Slide 14
Pulse Generation
Slide 15
Eye Diagram• The eye diagram is created by taking the time
domain signal and overlapping the traces for a certain number of symbols.
• The open part of the signal represents the time that we can safely sample the signal with fidelity
Slide 16
Vertical and Horizontal Eye Openings• The vertical eye opening or
noise margin is related to the SNR, and thus the BER– A large eye opening
corresponds to a low BER• The horizontal eye opening
relates the jitter and the sensitivity of the sampling instant to jitter– The red brace indicates the
range of sample instants with good eye opening
– At other sample instants, the eye opening is greatly reduced, as governed by the indicated slope
Slide 17
Jitter in Circuit design• Circuit design
Slide 18
Questions?
