UNIT-4
DIGITAL MODULATION TECHNIQUES
(30-04-20 & 01-05-20)
By:
Atifa Aqueel
Guest Teacher
Electronics Engg. Section
University Women’s Polytechnic, AMU
LINE CODING
• The binary data such as ‘1s’ and ‘0s’ produced by PCM encoder may be represented in various signaling format for transmission over a channel.
• These signaling formats are known as line codes.
• A line code is the code used for data transmission of a digital signal over a transmission line.
• This process of coding is chosen so as to avoid overlap and distortion of signal such as inter-symbol interference.
• In other words, we can say that line coding is a process that
converts digital data into digital signals.
• Line coding techniques can be broadly classified into three categories:
1. Unipolar (0,A) 2. Polar (-A, +A)
3. Bipolar (_A, 0, +A)
Properties of Line coding
1. Transmission Bandwidth: as small as possible
2. As the coding is done to make more bits transmit on a single signal, the bandwidth used is much reduced
3. Power efficiency as small as possible for given bandwidth and probability of error.
4. Error detection and correction capability. Ex. Bipolar 5. Favorable power spectral density. DC=0
6. Self synchronization between transmitter and receiver 7. Transparency: Prevent long runs of 0s and 1s.
DC Component
• Over one cycle period of a waveform, if all the positive voltages are cancelled by negative voltages then DC component of the waveform is zero.
• In line coding, the signal with non-zero DC component can be treated as distorted one and it can create errors in received signal.
• The signal with DC component cannot pass through a transformer.
Self-synchronization
• 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 misinterpret the signals. The following figure represents the synchronization problem.
Unipolar Signalling
• Unipolar signalling is also called as On-Off Keying or simply OOK.
• In unipolar encoding technique only two voltage levels are used.
• The presence of pulse represents a 1 and the absence of pulse represents a 0.
• Unfortunately, DC component is present in the encoded signal and there is loss of synchronization for long sequence of 0’s and 1’s.
• It is simple but obsolete.
• There are two variations in Unipolar signalling −
• Non Return to Zero NRZ
• Return to Zero RZ
Unipolar Non-Return to Zero (NRZ)
• In this type of unipolar signaling, a High in data is represented by a positive pulse called as Mark, which has a duration Tb equal to the symbol bit
duration. A Low in data input has no pulse.
• The following figure clearly depicts this.
• 𝑥𝑥 𝑡𝑡 = 𝐴𝐴 𝑓𝑓𝑓𝑓𝑓𝑓 0 ≤ 𝑡𝑡 < 𝑇𝑇𝑏𝑏 𝑥𝑥 𝑡𝑡 = 0 𝑓𝑓𝑓𝑓𝑓𝑓 0 ≤ 𝑡𝑡 < 𝑇𝑇𝑏𝑏
A
Tb
Unipolar Return to Zero (RZ)
• In this type of unipolar signaling, a High in data, though represented by a Mark pulse, its duration Tb is less than the symbol bit duration. Half of the bit duration remains high but it immediately returns to zero and shows the absence of pulse during the remaining half of the bit duration.
• 𝑥𝑥 𝑡𝑡 = 𝐴𝐴 𝑓𝑓𝑓𝑓𝑓𝑓 0 ≤ 𝑡𝑡 < 𝑇𝑇𝑏𝑏/2 𝑥𝑥 𝑡𝑡 = 0 𝑓𝑓𝑓𝑓𝑓𝑓 0 ≤ 𝑡𝑡 < 𝑇𝑇𝑏𝑏
• It is clearly understood with the help of the following figure.
Polar Signaling
• There are two methods of Polar Signaling. They are:
1. Polar NRZ 2. Polar RZ
• Polar NRZ
• In this type of Polar signaling, a High in data is represented by a positive pulse, while a Low in data is represented by a
negative pulse.
• 𝑥𝑥 𝑡𝑡 = +𝐴𝐴 /2 𝑓𝑓𝑓𝑓𝑓𝑓 0 ≤ 𝑡𝑡 < 𝑇𝑇
𝑏𝑏𝑥𝑥 𝑡𝑡 = −𝐴𝐴/2 𝑓𝑓𝑓𝑓𝑓𝑓 0 ≤ 𝑡𝑡 < 𝑇𝑇
𝑏𝑏• The following figure depicts this well.
+A/2 or -A/2 or
Polar RZ
• In this type of Polar signaling, a High in data, though represented by a Mark pulse, its duration Tb is less than the symbol bit duration.
• Half of the bit duration remains high but it immediately returns to zero and shows the absence of pulse during the remaining half of the bit duration.
• However, for a Low input, a negative pulse represents the data, and the zero level remains same for the other half of the bit duration. The following figure depicts this clearly.
• For symbol ‘1’ x t = +𝐴𝐴2 𝑓𝑓𝑓𝑓𝑓𝑓 0 ≤ 𝑡𝑡 < 𝑇𝑇𝑏𝑏/2
0 𝑓𝑓𝑓𝑓𝑓𝑓 𝑇𝑇𝑏𝑏/2 ≤ 𝑡𝑡 < 𝑇𝑇𝑏𝑏 (Half Interval)
• For symbol ‘0’ x t = −𝐴𝐴2 𝑓𝑓𝑓𝑓𝑓𝑓 0 ≤ 𝑡𝑡 < 𝑇𝑇𝑏𝑏/2 0 𝑓𝑓𝑓𝑓𝑓𝑓 𝑇𝑇𝑏𝑏/2 ≤ 𝑡𝑡 < 𝑇𝑇𝑏𝑏
(Half Interval)
+A/2 -A/2
BIPOLAR NRZ
• This is an encoding technique which has three voltage levels namely +, - and 0. Such a signal is called as duo-binary signal.
• In this format, the successive one’s are represented by pulses with alternate polarity and zero’s are represented by no pulses.
• This coding technique is also known as Alternate Mark Inversion (AMI).
SPLIT PHASE MANCHESTER FORMAT
• In this format, if symbol ‘1’ is transmitted,then
• For symbol ‘1’ x t = +𝐴𝐴2 𝑓𝑓𝑓𝑓𝑓𝑓 0 ≤ 𝑡𝑡 < 𝑇𝑇𝑏𝑏/2
−𝐴𝐴2 𝑓𝑓𝑓𝑓𝑓𝑓 𝑇𝑇𝑏𝑏/2 ≤ 𝑡𝑡 < 𝑇𝑇𝑏𝑏
• if symbol ‘0’ is transmitted,then
• For symbol ‘0’ x t = −𝐴𝐴2 𝑓𝑓𝑓𝑓𝑓𝑓 0 ≤ 𝑡𝑡 < 𝑇𝑇𝑏𝑏/2 +𝐴𝐴2 𝑓𝑓𝑓𝑓𝑓𝑓 𝑇𝑇𝑏𝑏/2 ≤ 𝑡𝑡 < 𝑇𝑇𝑏𝑏
+A/2 -A/2
Numerical
Numerical
Q. The binary data 101100110101 is transmitted over a baseband channel.
Draw the waveform for the transmitted data using following formats.
1. Unipolar RZ 2. Unipolar NRZ 3. Bipolar RZ
4. Split Phase Manchester
Signal Element Vs Data Element
• A data element is the smallest entity that can represent a piece of information. This is the bit.
• In digital data communications, a signal element carries data elements.
• A signal element is the shortest unit (time wise) of a digital signal.
• In other words, data elements are what we need to send; signal elements are what we can send.
• Data elements are being carried; signal elements are the carriers.
• We define a ratio r which is the number of data elements carried by each signal element. The shows several situations with different values of r.
• In part a of the figure, one data element is carried by one signal element (r
= 1).
• In part b of the figure, we need two signal elements (two transitions) to carry each data element (r =1/2).
• In part c of the figure, a signal element carries two data elements (r = 2).
• In part d, a group of 4 bits is being carried by a group of three signal elements (r = 4/3).
• For every line coding scheme r value should be defined.
Data Rate (Bit Rate) vs Signal Rate (Baud Rate):
• The data rate or bit rate defines the number of data elements (bits) sent in 1s. The unit is bits per second (bps).
• The signal rate is the number of signal elements sent in 1s. The unit is the baud.
• The data rate is sometimes called the bit rate; the signal rate is sometimes called the symbol rate, the modulation rate, or the baud rate.
• A signal or symbol rate typically consists of fixed number of bits depending on what the signal is defined as (for e.g. 2bit, 4bit, 8bit, 9bit etc.)
• The baud rate is measured in signals per second.
• If r is the number of bits per signal then
• Baud Rate (S) = Bit Rate (N)/r (no. of bits per signal)
• Take an example where an ASCII code is transmitted over a channel every 1 second. The binary equivalent of ‘R’ is ‘01010010’. So in this case the baud rate is 1 (1 symbol per second) and bit rate is 8 i.e 8 bits are
transmitted per second.
• One goal in data communications is to increase the data rate while decreasing the signal rate. Increasing the data rate increases the speed of transmission; decreasing the signal rate decreases the bandwidth requirement.
QUES 1. An analog signal carries four bits in each signal unit. If 1000 signal units are transmitted per second. Find the baud rate and bit rate.
Solution: 𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵 𝑅𝑅𝐵𝐵𝑡𝑡𝑅𝑅 = 𝐵𝐵𝐵𝐵𝑡𝑡 𝑅𝑅𝐵𝐵𝑡𝑡𝑅𝑅/𝑓𝑓
𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵 𝑅𝑅𝐵𝐵𝑡𝑡𝑅𝑅 = 1000 𝑠𝑠𝐵𝐵𝑠𝑠𝑠𝑠𝐵𝐵𝑠𝑠𝑠𝑠/sec Bit rate = 4× 1000 = 4000 𝑏𝑏𝐵𝐵𝑡𝑡𝑠𝑠/𝑠𝑠𝑅𝑅𝑠𝑠
