chapter 1 · see slide number one. chapter 1 fundamentals of wireless communications these slides...

Chapter 1

Acknowledgment: This material is based on the slides formatted by Dr Sunilkumar S. Manvi and Dr Mahabaleshwar S. Kakkasageri, the authors of the textbook: Wireless and Mobile Networks, concepts and protocols. Most of the slides are used without any change. Some additional slides are taken and/or inspired by material from various paper and / or electronic resources. Some comments and/or additive information are directly enclosed in the original slides. Such comments are usually written with a different color. I apologize for any involuntary omission to other references if any.

These slides are based on the slides formatted by Dr Sunilkumar S. manvi and Dr Mahabaleshwar S. Kakkasageri, the authors of the textbook: Wireless and Mobile Networks, concepts and protocols. See slide number one.

Chapter 1

Fundamentals of Wireless

Communications


Learning objectives

To introduce the concept of wireless media

To know the frequency spectrum for wireless communications

To review the principles of coding and modulation schemes

To understand the limitations of wireless communication

To study the wireless transmission and switching technologies

To study the diversity techniques for wireless systems

To discuss the performance improvement techniques for wireless communication systems.

To know the generations of cellular systems


DIGITAL COMMUNICATIONS


Digital communications

Digital communication refers to the transmission of a sequence of digital messages (a bit stream) or a digitized analog signal.

Digital messages are either represented by a sequence of pulses by means of a line code, or by a limited set of analog wave forms using a digital modulation method.

Digital modulation

1 0 1 1 0

1 0 1 1 0

Digital modulation

Digital modulation

digital message / digitized analog signal


Digital communications (Contd..)

Transmitting data in digital form allows

greater signal processing capability,

errors caused by random processes can be detected and corrected,

digital signals can also be sampled instead of continuously monitored,

multiplexing of multiple digital signals is much simpler to the multiplexing of analog signals.


Recall: FDM vs TDM

frequency

time1 time

frequency

Frequency Division Multiplexing (FDM) Time Division Multiplexing (TDM)

User 1

User 2

User 3

f3

f2

f1

f

t0 t1 t2 t3


WIRELESS COMMUNICATION SYSTEM


Wireless communication system

Source coding: digitization; channel encoder: error detection and error correction


Recall

Channel capacity: the maximum rate at which data can be transmitted over the channel under given conditions.

Bandwidth: the bandwidth of the transmitted signal as constrained by the transmitter and the nature of the transmission medium.

Shannon capacity defines maximum possible data rate for systems with noise: C = B log2(1 + S/N) (in bps).

B the bandwidth (in Hz, sometimes bps is ambiguously used).

S/N the signal-to-noise ratio.

Nyquist formulation: C = 2B log2 M (the limitation on data rate is simply the bandwidth of the signal )


Wireless communication limitations

Bandwidth

Mobile phones and PDAs have smaller memory, and it is hard to read large documents on them and may require information of lesser bandwidth.

Larger computing devices connected in wireless environments may require more bandwidth information since there is no constraint on the screen and the memory.

Available LAN device bandwidth is maximum 55Mbps, whereas users demand is 10 Gbps. This imbalance forces to have a clever wireless networking environments.


Wireless communication limitations (Contd..)

Frequency spectrum

The frequency spectrum is limited and finite.

The number of users who can be connected to a wireless network at a given time are limited.

Dynamic channel allocation schemes can be used to optimize the frequency usage of the given wireless communication area.

Power

The power density from a wireless antenna decreases rapidly as one moves away from the antenna.

A mobile node transmitter power in a wireless local environment ranges from 230 mW to 2 W in 1850-1880 MHz range (uplink) that covers a distance of 8 km.


WIRELESS MEDIA


Wireless and Radio

Radio may be defined as

A method of communicating over a distance by modulating electromagnetic waves by means of an intelligence bearing-signal and radiating these modulated waves by means of transmitter and a receiver.

A device or pertaining to a device, that transmits or receives electromagnetic waves in the frequency bands that are between 10kHz and 3000 GHz.


Voice over radio

Early 20th century, there were several experiments transmitting the voice data over the radio.

1914 : First voice over radio transmission.

1920 : Mobile receivers installed in police cars in Detroit, USA.

1930 : Mobile transmitters developed; radio equipment occupied most of police car trunk.

1935 : Frequency modulation (FM) demonstrated by Armstrong.

1940s : Majority of police systems converted to FM.


Birth of mobile telephony

Mobile telephone started gaining importance in 1940s. Developments of mobile telephony in chronological order are as follows.

1946 - First interconnection of mobile users to public switched telephone network (PSTN).

1960s - Improved Mobile Telephone Service (IMTS) introduced; supports full-duplex.


Wireless and Infrared IR waves are longer than visible light waves and shorter than radio waves. It cannot pass through walls or ceilings, but it can bounce off flat surfaces and pass through open door ways.


Uses of infrared wireless IR used in PANs, specific wireless LAN, impractical in mobile; used only to implement fixed wireless networks.


IR connected devices


TECHNOLOGIES IN DIGITAL WIRELESS COMMUNICATION

(Coding, Modulation, Physical channel creation)


Source CODING


Source CODING (Contd..)

Source coding: Source coding deals with the time and amplitude discretization of the analog source signal.

There are various techniques of coding.

We just consider Pulse Code Modulation (PCM), a basic

form of waveform coding. ‏


Pulse Code Modulation (PCM)

PCM encoding process


Transmission of digitized data

We are in the case of (analog data, digital signals).

More correctly: process of converting analog data into digital data.

Two things might happen:

The digital data can be transmitted using digital signals. So we are in the case (analog data, digital data, digital signal).

The digital data can be transmitted using an analog signal. So we are in the case (analog data, digital data, analog signal). This might be dictated by the nature of the transmission medium (electromagnetic waves in the air).

We are in the case of a digital transmission because the digitized voice data can be treated as digital data.


Digitizing analog data and transmission

Digitizer Modulator


Channel CODING


Cyclic redundancy check

Cyclic redundancy check (CRC) is probably the most reliable scheme for error detection.

Let k = Block of binary data, an n = length of CRC sequence, which is appended to the k-bit data block.

The n-bit CRC determined in such a way that the resulting k + n bit sequence is exactly divisible by some perdetermined bit string (generator polynomial) which is n + 1 bit long.


Example

Consider the data to be transmitted D is 101110 using

CRC-error checking method. Suppose the generator polynomial G is chosen as 1001. Find the final value that the transmitter sends and show the error detection process at the receiver.


CRC: at transmitter


CRC: at receiver


Block coding

Block coding is an error correction technique in which the data is grouped into k-bit blocks.

To each of the k bits for a block, (n − k) redundant bits called parity bits are appended.

The resulting n-bit sequence for transmission is called (n, k) block code, for every n bits transmitted, only k bits are information-bearing. The code rate is defined as k/n.


Modulation


WIRELESS MODULATION SCHEMES

There are various schemes of wireless modulation. We just consider the following of them.

Binary phase shift keying (BPSK)

Quadrature Phase Shift Keying (QPSK)

Spread spectrum

Direct-sequence spread spectrum

Frequency hopping spread spectrum


Binary Phase Shift Keying (BPSK)

Changing the bit changes the sign of the transmitted signal is known as binary phase shift keying.


Quadrature Phase Shift Keying (QPSK)

In QPSK modulation scheme, four different phase angles are used to indicate the phase shifts based on binary stream, thereby creating four symbols: π/4, 3π/4, -3π/4, and -π/4.

The amplitude is constant.

The QPSK corresponds to the Phase modulation with 4 symbols.


Spread spectrum

Spreading the data across the frequency spectrum makes signal resistant to noise, interference, jamming and eavesdropping.

There are two types of spread spectrum techniques

Direct Sequence Spread Spectrum (DSSS)

Frequency Hopping Spread Spectrum (FHSS)


Direct-sequence spread spectrum (DSSS)

• Each bit in the original signal is represented by multiple bits in the transmitted signal, using a Pseudo Random Noise Code (PN-code).

• One technique is to use an eXclusive OR (XOR)


Frequency hopping spread spectrum (FHSS)

The large bandwidth is effectively split into frequency channels.

The hop set (channel hopping sequence) is not arbitrary, but determined by the use of a pseudo random sequence.

The receiver can reproduce the identical hop set and so decode the signal.


Frequency hopping spread spectrum (FHSS)


Orthogonal Frequency Division Multiplexing (OFDM)

OFDM is a multi carrier modulation (MCM) scheme, in which many parallel data streams are transmitted at the same time over a channel, with each transmitting only a small part of the total data rate. Similar to FDM. However, in the case of OFDM, all the

subchannels are dedicated to a single source.

DMT (discrete multitone) is a similar system used in copper-based DSL (Digital Subscriber Line) systems to overcome transmission problems.

With OFDM, a high-speed digital message is divided into a large number of separate carrier waves.


FDM vs OFDM

frequency

time1 time

frequency

Frequency Division Multiplexing (FDM) Orthogonal Frequency Division Multiplexing (OFDM)

User 1

User 2

User 3

f3

f2

f1

f3

f2

f1

Same

user


Diversity techniques

Diversity technique is a method for improving the reliability of a message signal by utilizing two or more communication channels with different characteristics.

Diversity plays an important role in combating fading, co-channel interference, avoiding error bursts and it may exploit the multipath propagation resulting in a diversity gain.


Types of diversity

There are several different kinds of diversity which are commonly employed in wireless communication systems as follows.

Time diversity

Frequency diversity

Space diversity


Time diversity


Time diversity (Contd..)

frequency

time1

Without interleaving

With interleaving

S1(t) S2(t) S3(t) S4(t)


Frequency diversity


Space diversity


Multiple-Input and Multiple-Output (MIMO)

Multiple-input and multiple-output, or MIMO, is the use of multiple antennas at both the transmitter and receiver to improve communication performance.

It offers significant increases in data throughput and link range

without additional bandwidth or transmit power.


WIRELESS COMMUNICATION CHANNEL SPECIFICATIONS


Wireless communication channel specifications

Wireless communication channel specification involves allocating the finite resource in two steps:

Finite resource is allocated for the two directions of transmission

Portion allocated for each direction is used to create multiple channels for that direction.

These two steps of subdividing the finite resource are referred to as:

Duplexing methods

Multiple access methods


Duplexing methods

Frequency Division Duplexing (FDD)

FDD is employed in radio systems to provide an uplink and downlink radio channel between the sender and the receiver.

In FDD, the total available bandwidth is allocated separately for the forward and reverse directions of transmission.


Time Division Duplexing (TDD)

Two directions of the transmission are created by alternating the transmission in time.

The maximum bit rate supportable for a given bandwidth W is allocated between the two directions.


The multiple access methods

The multiple access methods refer to the method of creating multiple channels for each transmission direction. There are three main types of multiple access methods:

Frequency Division Multiple Access (FDMA)

Time Division Multiple Access (TDMA)

Code Division Multiple Access (CDMA)


Frequency Division Multiple Access (FDMA)

Frequency Division Multiple Access or FDMA is a channel access method used in multiple access protocols as a channelization protocol.

FDMA gives users an individual allocation of one or several frequency bands, or channels, so that receivers can discriminate among them by tuning to the desired channel.

Multiple Access systems coordinate access between multiple users.


Illustration of FDMA

frequency

time

f1

f2

f3

f4

f5

f6

f7 User 1

User 1

User 1

User 1

User 2

User 3

User 3


Time Division Multiple Access (TDMA)

Time division multiple access (TDMA) is a channel access method for shared medium networks.

It allows several users to share the same frequency channel by dividing the signal into different time slots.

The users transmit in rapid succession, one after the other, each using his own time slot.

This allows multiple stations to share the same transmission medium (e.g. radio frequency channel) while using only a part

of its channel capacity.


Illustration of TDMA frequency

time

f

User 1 User 3 User 1 User 1 User 1 User 2 User 3

t1 t2 t3 t4 t5 t6 t7


Code Division Multiple Access (CDMA)

CDMA (or spread spectrum) is a form of multiplexing where the transmitter encodes the signal using a pseudo-random sequence which the receiver also knows and can use to decode the received signal.

Each different random sequence corresponds to a different communication channel.


Illustration of CDMA using FHSS

time

frequency

f1

f2

f3

User 1 User 2 User 3

t1 t2 t3 t4 t5 t6 t7 t8 t9


TYPES OF WIRELESS COMMUNICATION SYSTEMS


Traditional Communications Systems

FM Mobile

AM Aircraft Communications

Paging Systems

Amateur Radio (Ham Radio)

Personal Communications Systems (PCS)

Global System for Mobile Communications (GSM)


• First Generation (1G)

• Second Generation (2G)

• 2.5G Communication Systems

• 3G Communication Systems

• Fourth generation systems

• Long-Term Evolution (LTE)

Cellular communication systems


FM Mobile

Typical mobile FM transmitters emit 5 to 150 Watts while their permanent base stations often transmit at 150 Watts with an omni-directional footprint. The spectrum profile of narrowband FM spans about 5 kHz.

Before the emergence of the personal mobile wireless (cellular) phones in the 1990’s, public safety applications, such as police, fire and forest services, used narrowband FM technology.

These applications still exist in the 50, 150 and 450 MHz FM bands.


AM aircraft communications

Using the VHF (Very High Frequency) frequencies in the 118-136 MHz region, authorities allocated 25 kHz-wide channels for a higher voice quality AM for aircraft communications.

Being exceedingly mobile, aircraft interferer’s are also difficult to pin down since any one aircraft is only in the area for tens of seconds.

But again, their ground transmitters can be a constant source of relatively high signal powers. The spectrum profile again reflects the voice nature of this application.


Paging Systems

Paging is one way communication where a person informs the base center the message to be sent to the destination.

Paging systems typically use FSK modulation format which exhibits a spectrum profile with two separated peaks, each representing one of the two frequencies which shift according to the digital “one” or “zero” being transmitted.


Amateur radio (Ham radio)

Scattered throughout the frequency spectrum are a number of allocated frequency bands dedicated to “Ham” radio operators.

While their transmitters largely use AM modulation, they are also authorized to run experimental transmissions in other formats. Their emitted powers can be quite high since they intend to transmit to others around the earth.

Hams often use large, steerable directional arrays of HF (high frequency) antennas to increase their directional power, so their interfering power can be quite high.


Personal Communications Systems (PCS)

The PCS in the 1800-1900 MHz frequency band. PCS was supposed to be a more comprehensive specification than the earlier cellular specification at 800 MHz.

Customers often prefer PCS systems because of their inexpensive nature and the ability to install them without a tedious licensing process.

They are popular for point-to-point and point-to-multipoint data link applications such as intra company data bridges.


Global System for Mobile communications (GSM)

GSM is assigned two frequency bands at 900 and 1800 MHz. Each band supports 124 channels at 200 kHz spacing and each is broken into 8 time slots operating in a TDMA mode.

Short Message Service which allows you to send and receive 126 character text mes-sages.

Allows data transmission and reception across GSM networks at speeds up to 9,600 bps currently.

More capacity, ensuring rapid call set-up. Handsets also smaller and more robust.


Cellular communication systems

Cellular communication systems went through many generations of development.

First Generation (1G)

AMPS (Advanced Mobile Phone Service) was the first standard. Based on FDMA, it provided wireless voice transmission and minimal data services.

Due to frequency reuse issues, one transmission tower (commonly called a cell) can only support 118 calls at any one

time.


Second generation (2G)

Second generation digital systems are classified by their multiple access techniques as either FDMA, TDMA or CDMA.

2G standard is the GSM system, developed to bring the inherent advantages of digital cellular network technology to the consumer market and provide a common standard.


2.5G communication systems

The cdma2000 (1X) is part to Qualcomm’s strategy to smooth the transition for CDMA WSPs to its new 3G technology, cdma2000 (3X). GPRS provides a packet switched data delivery mechanism to GSM systems, with higher bandwidths than GSM itself.

EDGE offers 384 kbps suitable for mobile multimedia communications whereas earlier GPRS was offering 144 kbps.


3G communication systems

Third Generation (3G) cellular systems were developed with the aim of offering high-speed data connectivity to mobile customers.

3G systems are defined by the International Telecommunications Union initiative IMT-2000, as being capable of supporting high-speed data rates in the range of 144 Kbps to >2 Mbps, depending on the

conditions and mobile speed.


Fourth generation systems

With UMTS paving the way for an explosion of multimedia services on the mobile Internet, researching on 4G has started, which will demand increasingly sophisticated systems and ever-faster radio links to support them.

A primary area of 4G research includes bandwidth efficient multiple access technology. This will result in improved quality of service, increased data throughput and spectral efficiency, adaptive modulation coding and the design of novel algorithms.


chapter 1 · see slide number one. chapter 1 fundamentals of wireless communications these slides...

Documents