mobile computing and wireless communication

MOBILE COMPUTING

& WIRELESS COMMUNICATION

Jay Nagar+91-9601957620www.jaynagarblog.wordpress.com

Introduction

• Wireless Comes of Age

• The Cellular Revolution

• The Global Cellular Network

• Broadband

• The Trouble With Wireless

Wireless Comes of Age

1. Transmission Fundamentals

• Transmission Fundamentals

• Provides a basic overview of transmission topics.

• Some data communications concepts

• Signaling techniques.

• Analog and Digital data transmission.

• Channel capacity,

• Transmission media, and the concept of multiplexing.

Outline

• Signals for Conveying Information

– Time Domain Concepts

– Frequency Domain Concepts

– Relationship between Data Rate and Bandwidth

• Analog and Digital Data Transmission

– Analog and Digital Data

– Analog and Digital Signaling

– Analog and Digital Transmission

• Channel Capacity

– Nyquist Bandwidth

– Shannon Capacity Formula

Outline

• Transmission Media

– Terrestrial Microwave

– Satellite Microwave

– Broadcast Radio

– Infrared

• Multiplexing

Signals for Conveying Information

• We are concerned with electromagnetic signals.

• An electromagnetic signal is a function of time, but it can alsobe expressed as a function of frequency;

• Time Domain Concepts:

• An electromagnetic signal can be either analog or digital.

• Analog signal is one in which the signal intensity varies in asmooth fashion over time. In other words, there are no breaksor discontinuities in the signal.

• Digital signal is one in which the signal intensity maintains aconstant level for some period of time and then changes toanother constant level.


• The effect of varying each of the three parameters.In part (a) of the figure, the frequency is 1 Hz; thusthe period is T = 1 second


• Part (b) has the same frequency and phase but apeak amplitude of 0.5. peak amplitude of 0.5.


• In part (c) we have f = 2, which is equivalent to T = 1/2.


• Part (d) shows the effect of a phase shift.


• There is a simple relationship between the two sinewaves, one in time and one in space.

• The wavelength of a signal is the distanceoccupied by a single cycle, or, put another way,the distance between two points of correspondingphase of two consecutive cycles.


• Frequency Domain Concepts

• Relationship between Data Rate andBandwidth

– There is a direct relationship between theinformation-carrying capacity of a signal and itsbandwidth: The greater the bandwidth, the higherthe information-carrying capacity.

Analog and Digital Data Transmission

• The terms analog and digital correspond, roughly, tocontinuous and discrete, respectively.

• These two terms are used frequently in datacommunications in at least three contexts: data, signals,and transmission.

• Data as entities that convey meaning, or information.

• Signals are electric or electromagnetic representationsof data.

• Transmission is the communication of data by thepropagation and processing of signals.

Analog and Digital Data

• Analog data take on continuous values in someinterval For example, voice and video arecontinuously varying patterns of intensity.

• Digital data take on discrete values; examples aretext and integers.

• The most familiar example of analog data is audio,which, in the form of sound waves, can beperceived directly by human beings.

Analog and Digital Signaling• In communications system, data are propagated from one point to

another by means of electromagnetic signals.

• An analog signal is a continuously varying electromagneticwave that may be propagated over a variety of media, dependingon frequency; examples are copper wire media, such as twistedpair and coaxial cable wireless media…

• A digital signal is a sequence of voltage pulses that may betransmitted over a copper wire medium; for example, a constantpositive voltage level may represent binary 0 and a constantnegative voltage level may represent binary l.

Analog and Digital Signaling

• Digital data can also be represented by analogsignals by use of a modem (modulator-demodulator). The modem converts a series ofbinary (two-valued) voltage pulses into an analogsignal by modulating a carrier frequency.


• Analog signals: Represent data withcontinuously varying electromagnetic wave


• Digital signals: Represent data withsequence of voltage pulses

Analog and Digital Transmission


• Analog transmission is a means of transmittinganalog signals without regard to their content; thesignals may represent analog data (e.g., voice) ordigital data (e.g., data that pass through a modem).

• To achieve longer distances, the analogtransmission system includes amplifiers that boostthe energy in the signal.


• Digital transmission, in contrast, is concernedwith the content of the signal. We havementioned that a digital signal can be propagatedonly a limited distance.

• To achieve greater distances, repeaters are used. Arepeater receives the digital signal, recovers thepattern of ones and zeros, and retransmits a newsignal. Thus, the attenuation is overcome.

Channel Capacity

• A variety of impairments can distort or corrupt asignal. A common impairment is noise.

• Noise is something that degrades signal quality.

• The maximum rate at which data can be transmittedover a given communication path, or channel, undergiven conditions is referred to as the channelcapacity.

Channel Capacity

• There are four concepts here that we are trying to relateto one another:

1. Data rate: This is the rate, in bits per second (bps), at whichdata can be communicated.

2. Bandwidth: This is the bandwidth of the transmitted signal asconstrained by the transmitter and the nature of the transmissionmedium, expressed in cycles per second, or Hertz.

3. Noise: For this discussion, we are concerned with the averagelevel of noise over the communications path.

4. Error rate: This is the rate at which errors occur, where an erroris the reception of a 1 when a 0 was transmitted or the receptionof a 0 when a 1 was transmitted.

Channel Capacity - NyquistBandwidth

• let us consider the case of a channel that is noisefree.

• Nyquist, states that if the rate of signal transmission is2B, then a signal with frequencies no greater than Bis sufficient to carry the signal rate.

• C = 2B log2 M

– M is the number of discrete signal elements or voltagelevels.

– B is bandwidth

– For Example : M = 8, a value used with some modems, abandwidth of B = 3100 Hz yields

Shannon Capacity

• Nyquist's formula indicates that, all other thingsbeing equal, doubling the bandwidth doubles thedata rate.

• Consider the relationship among data rate, noise,and error rate.

Shannon Capacity

• The key parameter involved in this reasoning is thesignal-to-noise ratio (SNR, or S/N).

• This ratio is measured at a receiver.

• A high SNR will mean a high-quality signal.

• Shannon's result is that, the maximum channel capacity,in bits per second, obeys the equation.

C = B log2(1 + SNR)

Transmission Media

• Physical path between transmitter and receiver.

• Can be classified as guided or unguided. In both cases,communication is in the form of electromagnetic waves.

• In Guided media, the waves are guided along a solidmedium, such as copper twisted pair, copper coaxialcable, or optical fiber.

• Unguided media, which provide a means oftransmitting electromagnetic signals but do notguide them; The atmosphere and outer space areexamples

Transmission Media

• There are basically two types of configurations forwireless transmission: directional andomnidirectional.

• For the directional configuration, the transmittingantenna puts out a focused electromagnetic beam;the transmitting and receiving antennas musttherefore be carefully aligned.

• In the omnidirectional case, the transmitted signalspreads out in all directions and can be received bymany antennas.

Transmission Media

• Three general ranges of frequencies are categories.

• Frequencies in the range of about 1 GHz (gigahertz =109 Hz) to 100 GHz are referred to as microwavefrequencies. microwave is quite suitable for point-to-point transmission.

• Frequencies in the range 30 MHz to 1 GHz are suitablefor omnidirectional applications. We refer to this range asthe radio range.

• Another important frequency range, for localapplications, is the infrared portion

• of the spectrum. This covers, roughly, from 3 X 1011 to 2X 1014 Hz.

Multiplexing

• To make efficient use of the transmission system, itis desirable to carry multiple signals on a singlemedium. This is referred to as multiplexing.

Multiplexing

• Two techniques for multiplexing intelecommunications networks are in common use:

• Frequency division multiplexing (FDM)

• Time division multiplexing (TDM).

Multiplexing - FDM

• A number of signals can be carried simultaneously ifeach signal is modulated onto a different carrierfrequency and the carrier frequencies aresufficiently separated so that the bandwidths ofthe signals do not overlap.

• the channels are separated by guard bands, which are unused portions of the spectrum.

Multiplexing - TDM

• Multiple digital signals can be carried on a singletransmission path by interleaving portions of eachsignal in time.

• The interleaving can be at the bit level or in blocksof bytes or larger quantities.

Synchronous TDM System -Transmitter

Synchronous TDM System -Receiver

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