Digital Communication

Communication Source Transmitter Receiver Destination Channel Main purpose of communication is to transfer information from a sender to a receiver via a channel or medium. Basic block diagram of a communication system: Source Transmitter Channel Receiver Destination

Brief Description Source: analog or digital Transmitter: transducer, amplifier, modulator, oscillator, power amp., antenna Channel: e.g. cable, optical fibre, free space Receiver: antenna, amplifier, demodulator, oscillator, power amplifier, transducer Destination: e.g. person, (loud) speaker, computer

Information Representation Communication system converts information into electrical electromagnetic/optical signals appropriate for the transmission medium. Analog systems convert analog message into signals that can propagate through the channel. Digital systems convert bits(digits, symbols) into signals Computers naturally generate information as characters/bits Most information can be converted into bits Analog signals converted to bits by sampling and quantizing (A/D conversion)

Why Digital Communications? Easy to regenerate the distorted signal Regenerative repeaters along the transmission path can detect a digital signal and retransmit a new, clean (noise free) signal These repeaters prevent accumulation of noise along the path This is not possible with analog communication systems Two-state signal representation The input to a digital system is in the form of a sequence of bits (binary or M_ary) Immunity to distortion and interference Digital communication is rugged in the sense that it is more immune to channel noise and distortion

Why Digital Communications? Hardware is more flexible Digital hardware implementation is flexible and permits the use of microprocessors, mini-processors, digital switching and VLSI Shorter design and production cycle Low cost The use of LSI and VLSI in the design of components and systems have resulted in lower cost Easier and more efficient to multiplex several digital signals Digital multiplexing techniques – Time & Code Division Multiple Access - are easier to implement than analog techniques such as Frequency Division Multiple Access

Why Digital Communications? Can combine different signal types – data, voice, text, etc. Data communication in computers is digital in nature whereas voice communication between people is analog in nature The two types of communication are difficult to combine over the same medium in the analog domain. Using digital techniques, it is possible to combine both format for transmission through a common medium Encryption and privacy techniques are easier to implement Better overall performance Digital communication is inherently more efficient than analog in realizing the exchange of SNR for bandwidth Digital signals can be coded to yield extremely low rates and high fidelity as well as privacy

Why Digital Communications? Disadvantages Requires reliable “synchronization” Requires A/D conversions at high rate Requires larger bandwidth Nongraceful degradation Performance Criteria Probability of error or Bit Error Rate

TYPES OF PULSE MODULATION

Pulse Amplitude Modulation Pulse-amplitude modulation is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses. Example: A two-bit modulator (PAM-4) will take two bits at a time and will map the signal amplitude to one of four possible levels, for example −3 volts, −1 volt, 1 volt, and 3 volts. Demodulation is performed by detecting the amplitude level of the carrier at every symbol period.

Types Of PAM There are two types of pulse amplitude modulation: 1.Single polarity PAM: In this a suitable fixed dc level is added to the signal to ensure that all the pulses are positive going. 2.Double polarity PAM: In this the pulses are both positive and negative going. Pulse-amplitude modulation is widely used in baseband transmission of digital data, with non-baseband applications having been largely replaced by pulse-code modulation, and, more recently, by pulse-position modulation.

Pulse Width Modulation Pulse-width modulation (PWM), or pulse-duration modulation (PDM), is a commonly used technique for controlling power to inertial electrical devices, made practical by modern electronic power switches. The PWM switching frequency has to be much faster than what would affect the load, which is to say the device that uses the power. The main advantage of PWM is that power loss in the switching devices is very low. PWM has also been used in certain communication systems where its duty cycle has been used to convey information over a communications channel.

Pulse Position Modulation Pulse-position modulation (PPM) is a form of signal modulation in which M message bits are encoded by transmitting a single pulse in one of possible time-shifts One of the key difficulties of implementing this technique is that the receiver must be properly synchronized to align the local clock with the beginning of each symbol. Therefore, it is often implemented differentially as differential pulse-position modulation, whereby each pulse position is encoded relative to the previous, such that the receiver must only measure the difference in the arrival time of successive pulses. It is possible to limit the propagation of errors to adjacent symbols, so that an error in measuring the differential delay of one pulse will affect only two symbols, instead of affecting all successive measurements.

Waveforms of PAM, PWM and PPM

Natural sampling in t- and f-domains

Flat topped sampling (PAM) in t- and f-domains

Relationship between PAM, quantised PAM and PCM signals

Quantization is a non linear transformation which maps elements from a continuous set to a finite set. It is also the second step required by A/D conversion. Sampler Quantizer Analog Signal - Continuous time - Continuous value Digital Signal - Discrete time - Discrete value - Discrete time - Continuous value

Uniform Quantization output w2(t) V -V V input w1(t) -V Region of operation For M=2n levels, step size :  = 2V /2n = V(2-n+1)

Quantization Error, e output w2(t) V -V V input w1(t) -V Error, e /2 -/2 input w1(t)

“Compressing-and-expanding” is called “companding.” Nonuniform quantizer Discrete samples Uniform Quantizer digital signals Compressor • • • • Channel • • • • output Decoder Expander received digital signals

DPCM Quantization error is accumulated.

Delta PCM transmitter and receiver

Spread Spectrum Concept Input fed into channel encoder Produces narrow bandwidth analog signal around central frequency Signal modulated using sequence of digits Spreading code/sequence Typically generated by pseudonoise/pseudorandom number generator Increases bandwidth significantly Spreads spectrum Receiver uses same sequence to demodulate signal Demodulated signal fed into channel decoder

Spread Spectrum System

Frequency Hopping Spread Spectrum (FHSS) Signal broadcast over seemingly random series of frequencies Receiver hops between frequencies in sync with transmitter Eavesdroppers hear unintelligible blips Jamming on one frequency affects only a few bits

Basic Operation Typically 2k carriers frequencies forming 2k channels Channel spacing corresponds with bandwidth of input Each channel used for fixed interval 300 ms in IEEE 802.11 Some number of bits transmitted using some encoding scheme May be fractions of bit (see later) Sequence dictated by spreading code

Frequency Hopping Example

Frequency Hopping Spread Spectrum System (Transmitter)

Frequency Hopping Spread Spectrum System (Receiver)

Slow and Fast FHSS Frequency shifted every Tc seconds Duration of signal element is Ts seconds Slow FHSS has Tc  Ts Fast FHSS has Tc < Ts Generally fast FHSS gives improved performance in noise (or jamming)

Amplitude Shift Keying (ASK) ASK is implemented by changing the amplitude of a carrier signal to reflect amplitude levels in the digital signal. For example: a digital “1” could not affect the signal, whereas a digital “0” would, by making it zero. The line encoding will determine the values of the analog waveform to reflect the digital data being carried.

Implementation of ASK

Frequency Shift Keying

Binary Phase Shift Keying

QPSK

Modem A modem [Modulator -Demodulator] is a device. Data communication means transmitting digital information form one computer to other computers through the comuunication channels. Communicate equipment used for long distance data transfer through telephone lines. A pair of modems are needed to communication

Modem

Communication between computers through the existing telephone cables.

Structure of data communication through modem Analog data Digital data

Working principle The digital data from the computer is converted into analog data by the modem and are transmitted over the telephone line.

The analog data received from the telephone line is converted to digital by the modem and is given to the computer.

Types of modems Two types of modem. External modem Internal modem

External modem External modem is separate device and is kept outside the computer. A 9 pin (DB9) or 25 pin serial cable connects the PC serial port to the modem. Thus CPU need not be opened during modem installation. External modems avoid hardware conflicts such as (conflict of I/O address lines and that of interrupt lines) the external modem setup is faster and easier than internal modems.

External modem

Internal modem Internal modem is a part of the computer and is connected inside the system.

Internal modem Internal modem that plugs directly into PCI or ISA expansion slot. Modem contain its own synchronous receiver/transmitter (UART). modulator converts digital data from computer into analog signal which is transmitted through RJ-11 jack. On receiver side receive the analog signal and pass them to demodulator it converts signal and sends to UART. It then converts serial bit data into parallel byte

Internal modem Controller is used to manage overall operation of modem NVRAM it stores modem parameters

Space division switching Each sample takes a different path through the switch, depending on its destination

Crossbar Simplest possible space-division switch Crosspoints can be turned on or off For multiplexed inputs, need a switching schedule (why?) Internally nonblocking but need N2 crosspoints time taken to set each crosspoint grows quadratically vulnerable to single faults (why?)

Time-space switching Precede each input trunk in a crossbar with a TSI Delay samples so that they arrive at the right time for the space division switch’s schedule

Time-space-time (TST) switching Allowed to flip samples both on input and output trunk Gives more flexibility => lowers call blocking probability