DIGITAL MODULATION TECHNIQUES

WHAT IS DIGITAL COMMUNICATION?

Digital communications broadly refers to the transmission of information using digital messages or bit streams.

There are notable advantages to transmitting data using discrete messages.

Errors caused by noise and interference can be detected and corrected systematically.

Digital communications also make the networking of heterogeneous systems possible, with the Internet

being the most obvious such example.

Digital communications broadly refers to the transmission of information using digital messages or bit streams.

There are notable advantages to transmitting data using discrete messages.

Errors caused by noise and interference can be detected and corrected systematically.

Digital communications also make the networking of heterogeneous systems possible, with the Internet

being the most obvious such example.

DIGITAL COMMUNICATION

DIGITAL COMMNICATION

•Information Source and Input Transducer:

The source of information can be analog or digital, e.g. analog: audio or video signal, digital: like teletype signal. In digital communication the signal produced by this source is converted into digital signal consists of 1 s and 0 s. For this we need source encoder.′ ′

Source EncoderIn digital communication we convert the signal from source into digital signal as mentioned above. The point to remember is we should like to use as few binary digits as possible to represent the signal. In such a way this efficient representation of the source output results in little or no redundancy. This sequence of binary digits is called information sequence.Source Encoding or Data Compression: the process of efficiently converting the output of wither analog or digital source into a sequence of binary digits is known as source encoding.

DIGITAL COMMNICATION

Channel Encoder:The information sequence is passed through the channel encoder. The purpose of the channel encoder is to introduced, in controlled manner, some redundancy in the binary information sequence that can be used at the receiver to overcome the effects of noise and interference encountered in the transmission on the signal through the channel.e.g. take k bits of the information sequence and map that k bits to unique n bit sequence called code word. The amount of redundancy introduced is measured by the ratio n/k and the reciprocal of this ratio (k/n) is known as rate of code or code rate.

Digital Modulator:The binary sequence is passed to digital modulator which in turns convert the sequence into electric signals so that we can transmit them on channel (we will see channel later). The digital modulator maps the binary sequences into signal wave forms , for example if we represent 1 by sin x and 0 by cos x then we will transmit sin x for 1 and cos x for 0. ( a case similar to BPSK)

DIGITAL COMMNICATION

Channel:The communication channel is the physical medium that is used for transmitting signals from transmitter to receiver. In wireless system, this channel consists of atmosphere , for traditional telephony, this channel is wired , there are optical channels, under water acoustic cahnenls etc.

Digital Demodulator:The digital demodulator processes the channel corrupted transmitted waveform and reduces the waveform to the sequence of numbers that represents estimates of the transmitted data symbols.

DIGITAL COMMNICATION

Channel Decoder:This sequence of numbers then passed through the channel decoder which attempts to reconstruct the original information sequence from the knowledge of the code used by the channel encoder and the redundancy contained in the received data

Source DecoderAt the end, if an analog signal is desired then source decoder tries to decode the sequence from the knowledge of the encoding algorithm. And which results in the approximate replica of the input at the transmitter end

DIGITAL COMMNICATION

Channel Decoder:This sequence of numbers then passed through the channel decoder which attempts to reconstruct the original information sequence from the knowledge of the code used by the channel encoder and the redundancy contained in the received data

Source DecoderAt the end, if an analog signal is desired then source decoder tries to decode the sequence from the knowledge of the encoding algorithm. And which results in the approximate replica of the input at the transmitter end

Output Transducer:Finally we get the desired signal in desired format analog or digital.

PULSE MODULATION

PULSE MODULATION

SAMPLING

SAMPLING THEOREM

3 ANALOG PULSE MODULATION :

PULSE AMPLITUDE MODULATION (PAM)

Analog pulse

Sample pulse

The amplitude of a constant width, constant position pulse is varied according to the amplitude of the sample of the analog signal.

the amplitude of a pulse coincides with the amplitude of the analog signal.

The amplitude of a constant width, constant position pulse is varied according to the amplitude of the sample of the analog signal.

the amplitude of a pulse coincides with the amplitude of the analog signal.

PULSE WIDTH MODULATION (PWM)

A constant amplitude pulse is varied proportional to the amplitude of the analog signal at the time the signal is sampled. The maximum analog signal amplitude produces the widest pulse, and the minimum analog signal amplitude produces the narrowest pulse. All pulses have the same amplitude.

PULSE POSITION MODULATION (PPM)

The position of a constant-width pulse within prescribed time slot is varied according to the amplitude of the sample of the analog signal. The higher the amplitude of the sample, the farther to the right the pulse is positioned within the prescribed time slot. The highest amplitude sample produces a pulse to the far right, and the lowest amplitude sample produces a pulse to the far left.

The position of a constant-width pulse within prescribed time slot is varied according to the amplitude of the sample of the analog signal. The higher the amplitude of the sample, the farther to the right the pulse is positioned within the prescribed time slot. The highest amplitude sample produces a pulse to the far right, and the lowest amplitude sample produces a pulse to the far left.

DIGITAL PULSE MODULATION (DPM)

• In DPM, a code used to represent the amplitude of the samples that has been divided into various levels.

• There are 2 types of DPM:– Pulse Code Modulation– Delta Modulation

PULSE CODE MODULATION (PCM)

• PCM is a form of modulation, which uses coded group of pulses to represent certain values of the information signal.

• The analog signal is sampled and then converted to a serial n-bit binary code for transmission.

• Each code has the same number of bits and requires the same length of time or transmission.

PCM BLOCK DIAGRAM

Low Pass FilterLow Pass Filter

DecoderDecoder ExpanderExpanderSamplerSampler QuantiserQuantiser EncoderEncoder

Low Pass FilterLow Pass Filter

Analogue SignalAnalogue Signal

Analogue SignalAnalogue Signal

PCM BLOCK DIAGRAM

• PCM is a form of modulation, which uses coded group of pulses to represent certain values of the information signal.

• The information signal is limited to a certain maximum freq and sampled and changed to PAM.

• The PAM signal is then quantise by the quantiser and then changed into the binary code by the encoder.

• Then the PCM signal is sent through the cable.

• PCM has superior signal to signal characteristics for a given bandwidth.

PCM BLOCK DIAGRAM

PCM TRANSMITTER

PCM RECEIVER

PULSE CODE MODULATION (PCM)

The 3 main processes in PCM:

1) Sampling2) Quantization3) Encoding

PCM - SAMPLING

Process of taking samples of the information signals at Nyquist Rate :

fs ≥ 2fmaxfs – frequency samplingfm – modulating frequency

Minimum freq sampling, fs = 2fm

PCM - QUANTIZATION

• The amplitude of the samples are then divided into respective levels. The number of levels for the samples depend on the number of bits used to code the signal.

• The relationship between the number of bits (B) is given by the equation: M= 2

M- Number of levelsB – Bits/ samples

•The more levels used means that an analogue signal can be describe more accurate.

B

PCM - ENCODING

• In this process, the samples that has been divided into various levels is coded into respective codes where the samples that have the same number of level are coded into the same code.

• The number of bits depends on the number of level used to quantise the samples.

B = log2 M

PCM

DELTA MODULATION

DELTA MODULATION

• Next form of pulse modulation• Transmits information only to indicate whether the analog signal that is being encoded goes up or goes down• The Encoder Outputs are highs or lows that “instruct” whether to go up or down, respectively• DM takes advantage of the fact that voice signals do not change abruptly

• Next form of pulse modulation• Transmits information only to indicate whether the analog signal that is being encoded goes up or goes down• The Encoder Outputs are highs or lows that “instruct” whether to go up or down, respectively• DM takes advantage of the fact that voice signals do not change abruptly

DELTA MODULATION

There are two problems associated with delta modulation that do

not occur with conventional PCM: slope overload and granular noise.

DELTA MODULATION

DELTA MODULATION (Quantization Errors)

SLOPE OVERLOAD

• When the analog input signal changes at a faster rate than the DAC can maintain. • The slope of the analog signal is greater than the delta modulator can maintain and is called slope overload. • Increasing the clock frequency reduces the probability of slope overload occurring. • Another way to prevent slope overload is to increase the magnitude of the minimum step size.

SLOPE OVERLOAD

GRANULAR NOISE

• When the original analog input signal has a relatively constant amplitude, the reconstructed signal has variations that were not present in the original signal.

DELTA MODULATION (Quantization Errors)

DELTA SIGMA MODULATION

The modulation which has an integrator can relieve the draw back of delta modulation (differentiator)

Beneficial effects of using integrator: 1. Pre-emphasize the low-frequency content 2. Increase correlation between adjacent samples (reduce the variance of the error signal at the quantizer input ) 3. Simplify receiver design

Because the transmitter has an integrator , the receiver consists simply of a low-pass filter. (The differentiator in the conventional DM receiver is cancelled by the integrator )

DELTA SIGMA MODULATION

DELTA SIGMA MODULATION

• is a measurement of the effect of quantization errors introduced by analog-to-digital conversion at the ADC.

Refer to page 421 - 422

Signal-to-Quantization-Noise Ratio (SQNR or SNqR)

MODEM (Modulation & Demodulation)

MODEM CONNECTION

PCMODEM

MODEM

PC

DTE

DCE

DTE

DCE

110011110011

MODEM BLOCK DIAGRAM

MODEM BLOCK DIAGRAM

MODEM BLOCK DIAGRAM

COMMON MODEM USED:

MULTIPLEXING

MULTIPLEXING (MUX)

General multiplex scheme: the ν input lines-channels are multiplexed into a single fast line. The demultiplexer receives the multiplexed data stream and extracts the original channels to be transferred

MULTIPLEXING (MUX)

MULTIPLEXING (MUX)

FREQUENCY DIVISION MULTIPLEXING

FDM

TIME DIVISION MULTIPLEXING

TDM

WAVELENGTH DIVISION MULTIPLEXING

WDM

CDMA (CODE DIVISION MULTIPLE ACCESS)

CDMA

COMPARE BETWEEN TDM & FDM

COMPARE BETWEEN TDM & FDM

The primary difference between FDM and TDM is how they divide the channel. FDM divides the channel into two or more frequency ranges that do not overlap, while TDM divides and allocates certain time periods to each channel in an alternating manner.

Due to this fact, we can say that for TDM, each signal uses all of the bandwidth some of the time, while for FDM, each signal uses a small portion of the bandwidth all of the time.

TDM provides greater flexibility and efficiency, by dynamically allocating more time periods to the signals that need more of the bandwidth, while reducing the time periods to those signals that do not need it. FDM lacks this type of flexibility, as it cannot dynamically change the width of the allocated frequency.

COMPARE BETWEEN TDM & FDM

The advantage of FDM over TDM is in latency. Latency is the time it takes for the data to reach its destination.

As TDM allocates time periods, only one channel can transmit at a given time, and some data would often be delayed, though it’s often only in milliseconds. Since channels in FDM can transmit at any time, their latencies would be much lower compared to TDM.

FDM is often used in applications where latency is of utmost priority, such as those that require real-time information.FDM and TDM are often used in tandem, to create even more channels in a given frequency range. The common practice is to divide the channel with FDM, so that you have a dedicated channel with a smaller frequency range. Each of the FDM channels is then occupied by multiple channels that are multiplexed using TDM. This is what telecoms do to allow a huge number of users to use a certain frequency band.

COMPARE BETWEEN MUX & MULTIPLE ACCESS

INFORMATION CAPACITY

•Is a measure of how much information can be propagated through a communications system and is a function of bandwidth and transmission time.

•Information capacity represents the number of independent symbols that can carried through a system in a given unit of time.

•The most basic digital symbol used to represent information is the bit.

BIT, BIT RATE, BAUD, BANDWIDTH

BANDWIDTH

SHANNON’S LIMIT & M-ary ENCODING

DIGITAL MODULATION TECHNIQUES

Amplitude Shift Keying (ASK)

Frequency Shift Keying (FSK)

Phase Shift Keying (PSK)