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 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 sourceis 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 asmentioned above. The point to remember is we should like to use as few binary digitsas possible to represent the signal. In such a way this efficient representation of thesource output results in little or no redundancy. This sequence of binary digits is calledinformation sequence.Source Encoding or Data Compression: the process of efficiently converting the outputof wither analog or digital source into a sequence of binary digits is known as sourceencoding.
DIGITAL COMMNICATIONChannel Encoder:The information sequence is passed through the channel encoder. The purpose of thechannel encoder is to introduced, in controlled manner, some redundancy in thebinary information sequence that can be used at the receiver to overcome the effectsof noise and interference encountered in the transmission on the signal through thechannel.e.g. take k bits of the information sequence and map that k bits to unique n bitsequence called code word. The amount of redundancy introduced is measured by theratio 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 thesequence into electric signals so that we can transmit them on channel (we will seechannel later). The digital modulator maps the binary sequences into signal waveforms , for example if we represent 1 by sin x and 0 by cos x then we will transmit sin xfor 1 and cos x for 0. ( a case similar to BPSK)
DIGITAL COMMNICATIONChannel:The communication channel is the physical medium that is used for transmitting signalsfrom transmitter to receiver. In wireless system, this channel consists of atmosphere ,for traditional telephony, this channel is wired , there are optical channels, under wateracoustic cahnenls etc.Digital Demodulator:The digital demodulator processes the channel corrupted transmitted waveform andreduces the waveform to the sequence of numbers that represents estimates of thetransmitted data symbols.
DIGITAL COMMNICATIONChannel Decoder:This sequence of numbers then passed through the channel decoder which attempts toreconstruct the original information sequence from the knowledge of the code used bythe channel encoder and the redundancy contained in the received dataSource DecoderAt the end, if an analog signal is desired then source decoder tries to decode thesequence from the knowledge of the encoding algorithm. And which results in theapproximate replica of the input at the transmitter end
DIGITAL COMMNICATIONChannel Decoder:This sequence of numbers then passed through the channel decoder which attempts toreconstruct the original information sequence from the knowledge of the code used bythe channel encoder and the redundancy contained in the received dataSource DecoderAt the end, if an analog signal is desired then source decoder tries to decode thesequence from the knowledge of the encoding algorithm. And which results in theapproximate replica of the input at the transmitter endOutput Transducer:Finally we get the desired signal in desired format analog or digital.
PULSE AMPLITUDE MODULATION (PAM) Analog pulseSample pulse The amplitude of a constant width, constant position pulse is varied according to theamplitude 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 theanalog signal at the time the signal is sampled. The maximum analog signal amplitude produces the widest pulse, and theminimum 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 variedaccording to the amplitude of the sample of the analog signal. The higher the amplitude of the sample, the farther to the right the pulseis positioned within the prescribed time slot. The highest amplitude sample produces a pulse to the far right, and thelowest 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 • 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.
PULSE CODE MODULATION (PCM) The 3 main processes in PCM: 1) Sampling 2) Quantization 3) Encoding
PCM - SAMPLING Process of taking samples of the information signals at Nyquist Rate : fs ≥ 2fmax fs – frequency sampling fm – 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 B by the equation: M= 2 M- Number of levels B – Bits/ samples •The more levels used means that an analogue signal can be describe more accurate.
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
DELTA MODULATION• Next form of pulse modulation• Transmits information only to indicate whether theanalog signal that is being encoded goes up or goesdown• The Encoder Outputs are highs or lows that“instruct” whether to go up or down, respectively• DM takes advantage of the fact that voice signalsdo not change abruptly
SLOPE OVERLOAD• When the analog input signal changes at afaster rate than the DAC can maintain.• The slope of the analog signal is greater thanthe delta modulator can maintain and is called slopeoverload.• Increasing the clock frequency reduces theprobability of slope overload occurring.• Another way to prevent slope overload is toincrease the magnitude of the minimum step size.
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 )
MULTIPLEXING (MUX)General multiplex scheme: the ν input lines-channels are multiplexed into a single fastline. The demultiplexer receives the multiplexed data stream and extracts the originalchannels to be transferred
COMPARE BETWEEN TDM & FDM The primary difference between FDM and TDM is how they divide thechannel. FDM divides the channel into two or more frequency ranges that donot overlap, while TDM divides and allocates certain time periods to eachchannel in an alternating manner. Due to this fact, we can say that for TDM, each signal uses all of thebandwidth some of the time, while for FDM, each signal uses a small portion ofthe bandwidth all of the time. TDM provides greater flexibility and efficiency, by dynamically allocatingmore time periods to the signals that need more of the bandwidth, whilereducing the time periods to those signals that do not need it. FDM lacks thistype of flexibility, as it cannot dynamically change the width of the allocatedfrequency.
COMPARE BETWEEN TDM & FDM The advantage of FDM over TDM is in latency. Latency is the time ittakes for the data to reach its destination. As TDM allocates time periods, only one channel can transmit at a giventime, and some data would often be delayed, though it’s often only inmilliseconds. Since channels in FDM can transmit at any time, their latencieswould be much lower compared to TDM. FDM is often used in applications where latency is of utmost priority, suchas those that require real-time information.FDM and TDM are often used in tandem, to create even more channels in agiven frequency range. The common practice is to divide the channel withFDM, so that you have a dedicated channel with a smaller frequency range.Each of the FDM channels is then occupied by multiple channels that aremultiplexed using TDM. This is what telecoms do to allow a huge number ofusers to use a certain frequency band.
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.