Basics of Fourier Transform

Fourier Transform is a mathematical tool that is widely used in Signal processing applications.

Fourier Transform is explained in this VIDEO in just SIX minutes.

https://youtu.be/Og6JDPxQBFM

TRF Vs Superheterodyne Receiver

RF to AF

RF to IF to AF

Audio Frequency Vs Radio frequency

Generally,radio frequencies has more power than audio frequencies.The signal with more power penetrates more deeply.The first known frequencies from the 20's and 30's were in the millions of hertz range. 

 So why do most of the instruments built today only use low audio frequencies when original frequencies were much higher? 

 The FCC (Federal Communication Commission) is the reason why. In 1935 Congress created the FCC and by 1936 they began to regulate the airwaves. Before 1935 there were no regulations to prevent Dr. Rife from using his high frequencies output through a ray tube. The frequencies output through the ray tube would travel about 12 miles in each direction from his laboratory, and would. Dr.Rife said his equipment would “raise the devil” with all the radios. 


In 1936, Philip Hoyland built the first audio frequency instrument. (right)  In these early designs, a radio frequency was used to "carry" the new lower audio frequencies into the body. 

In the 1950s, Rife's research partners John Crane and John Marsh continued to build this lower audio frequency instrument, using simple off-the-shelf  technology of the day. From this time on, Dr. Rife’s original high frequencies were no longer used in any of the equipment, nor were his RF carrier waves used.   Most of today’s modern frequency generators are patterned after these early 50's style instruments. 

The RF carrier frequency gives the same harmonics and penetration as the earlier ray tube instruments.Hence we do use RF carrier wave.

Example:

When u hear a song ,it is audio frequency.These audio frequency are brought to your radio by radio frequency.The audio frequencies are "piggy backed" onto the radio frequencies.   

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Satellite Communication

     In satellite communication, signal transferring between the sender and receiver is done with the help of satellite. In this process, the signal which is basically a beam of modulated microwaves is sent towards the satellite. Then the satellite amplifies the signal and sent it back to the receiver’s antenna present on the earth’s surface. So, all the signal transferring is happening in space. Thus this type of communication is known as space communication.

Two satellites which are commonly used in satellite communication are Active and passive satellites.

Passive satellites: It is just a plastic balloon having a metal coated over it. This sphere reflects the coming microwave signals coming from one part of the earth to other part. This is also known as passive sphere. Our earth also has a passive satellite i.e. moon.

Active satellites: It basically does the work of amplifying the microwave signals coming. In active satellites an antenna system, transmitter, power supply and a receiver is used. These satellites are also called as transpoder. The transmitters fitted on the earth generate the microwaves. These rays are received by the transponders attached to the satellite. Then after amplifying, these signals are transmitted back to earth. This sending can be done at the same time or after some delay. These amplified signals are stored in the memory of the satellites, when earth properly faces the satellite. Then the satellite starts sending the signals to earth. Some active satellites also have programming and recording features. Then these recording can be easily played and watched. The first active satellite was launched by Russia in 1957. The signals coming from the satellite when reach the earth, are of very low intensity. Their amplification is done by the receivers themselves. After amplification these become available for further use.

Microwave communication is possible only if the position of satellite becomes stationary with respect to the position of earth. So, these types of satellites are known as geostationary satellites.

What are the requirements for a satellite to be geostationary?

1. Its revolutionary direction must be same as that of the earth, i.e. from west to east.

2. The time period of satellite’s revolution must be same to the time period of the rotation of earth along its polar axis, which is equal to 24 hours.

3. The equatorial plane of earth must be co planar with the orbit plane of the satellites revolution.

The name given to the orbit of the geo-stationary satellites is synchronous orbit. Due to this geo-stationary satellites are also called as geo-synchronous satellites. Geo-synchronous orbit is at a height of nearly 36000km from the surface of earth.

These orbits are capable of giving a successful communication link between two stations present on the earth. These satellites can handle communication up to large distances. But it is impossible for a single geo-stationary satellite to cover the whole earth and provide a communication link. Due to curvature of earth the stations will be out of sight  after covering some distance. If we want to cover the whole earth then we have to put three satellites onto the geosynchronous orbit. These satellites can cover the earth if all are inclined at an angle of 120^o to each other.

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Cellular Repeater

A cellular repeater may be considered as a form of bi-directional amplifier. They will receive a signal from the local base-station and then re-broadcast it locally within the users premises - house, office, etc. The signal transmitted by the cellular handset or user equipment is packed up by the repeater and rebroadcast to the base-station.

Typically a cellular repeater will utilise an external directional antenna to communicate with the base station. There will then be a down lead to the repeater unit itself which will contain the antenna for radiating the signal within the premises.

Many simple cellular repeaters only allow use by one cell phone, but others allow the use of multiple phones.

In order to reduce cellular interference and congestion, the cellular repeater will transmit and receive on the same frequency. 

Repeater

Radio Communication

Sound and radio waves are different phenomena. Sound consists of pressure variations in matter, such as air or water. Sound will not travel through a vacuum. Radio waves, like visible light, infrared, ultraviolet, X-rays and gamma rays, are electromagnetic waves that do travel through a vacuum. When you turn on a radio you hear sounds because the transmitter at the radio station has converted the sound waves into electromagnetic waves, which are then encoded onto an electromagnetic wave in the radio frequency range (generally in the range of 500-1600 kHz for AM stations, or 86-107 MHz for FM stations). Radio electromagnetic waves are used because they can travel very large distances through the atmosphere without being greatly attenuated due to scattering or absorption. Your radio receives the radio waves, decodes this information, and uses a speaker to change it back into a sound wave. An animated illustration of this process is given below (mouse-over the images for animations).

• A sound wave is produced with a frequency of 5 Hz - 20 kHz.                                                                                                    
• The sound wave is equivalent to a pressure wave traveling through the air.

• A microphone converts the sound wave into an electrical signal.

•    The electrical wave traveling through the microphone wire is analogous to the original sound wave. 
• The electrical wave is used to encode or modulate a high-frequency "carrier" radio wave. The carrier wave itself does not include any of the sound information until it has been modulated.
• The carrier wave can either be amplitude modulated by the electrical signal, or frequency modulated.
• 
• The signal is transmitted by a radio broadcast tower.
• 
• Your radio contains an antennato detect the transmitted signal, a tuner to pick out the desired frequency, a demodulator to extract the original sound wave from the transmitted signal, and an amplifier which sends the signal to the speakers. The speakers convert the electrical signal into physical vibrations (sound).

Distributed Antenna System (DAS)

The concept of a Distributed Antenna System, DAS has many advantages in some applications. A Distributed antenna system, DAS is a network of antennas spaced apart from each other, but connected to a common source. In this way the DAS is able to provide wireless or radio coverage within a given area.

The idea of a distributed antenna system is being adopted increasingly as it enables a number of advantages to be gained. However this is at the cost of a larger more complicated system. Nevertheless, distributed antenna systems are being used in a variety of areas to enable the right coverage to be gained for several applications.

Although the concept of distributed antenna systems has been known about for many years, it is with the increased deployment of wireless systems within buildings and other difficult coverage areas that the idea of distributed antenna systems has come to the fore.

Advantages of using a distributed antenna system

• Better defined coverage
• Fewer coverage holes
• Same coverage using a lower overall power
• Lowers health risk as a result of using lower overall power levels
• Individual antennas do not need to be as high as a single antenna for the equivalent coverage

Disadvantages of using a distributed antenna system

• Higher cost as a result of additional infrastructure required
• Possible greater visual impact in some applications as a result of greater number of antennas, although they are likely to be much lower in height.

Basic concept of a distributed antenna system

The basic idea behind the distributed antenna system is to utilise several different antennas over the required coverage area. Using this approach the overall power required is less because these more localised antennas can be placed more effectively for a small area, rather than having a single, larger antenna that is a compromise for the wider coverage needed. By adopting a distributed antenna system approach, this helps overcome the shadowing and penetration losses because a line of sight link is available more frequently. As a result the levels of absorption are lower and this means the overall power levels can be reduced.

www.RadioElectronics.com

10-Gigabit Ethernet (10GBASE-T)

10-Gigabit Ethernet (10GBASE-T), being standardized in IEEE 802.3a, is a telecommunication technology that offers data speeds up to 10 billion bits per second. Built on the Ethernet technology used in most of today's local area networks (LANs), 10-Gigabit Ethernet is described as a "disruptive" technology that offers a more efficient and less expensive approach to moving data on backbone connections between networks while also providing a consistent technology end-to-end. Using optical fiber, 10-Gigabit Ethernet can replace existing networks that use ATM switches and SONET multiplexers on an OC-48 SONET ring with a simpler network of 10-Gigabit Ethernet switches and at the same time improve the data rate from 2.5 Gbps to 10 Gbps.

10-Gigabit Ethernet is expected to be used to interconnect local area networks (LANs), wide area networks (WANs), and metropolitan area networks (MANs). 10-Gigabit Ethernet uses the familiar IEEE 802.3 Ethernet media access control (MAC) protocol and its frame format and size. Like Fast Ethernet and Gigabit Ethernet, 10-Gigabit Ethernet uses full-duplex transmission, which makes possible a considerable distance range. On multimode fiber, 10-Gigabit Ethernet will support distances up to 300 meters; on single mode fiber, it will support distances up to 40 kilometers. Smaller Gigabit Ethernet networks can feed into a 10-Gigabit Ethernet network.

http://searchnetworking.techtarget.com/definition/10-Gigabit-Ethernet

IEEE 802 Standards

IEEE 802 refers to a family of IEEE standards dealing with local area networks and metropolitan area networks.

Name	Description	Note
IEEE 802.1	Bridging (networking) and Network Management
IEEE 802.2	Logic Link Control	inactive
IEEE 802.3	Ethernet
IEEE 802.4	Token bus	disbanded
IEEE 802.5	Defines the MAC layer for a Token Ring	inactive
IEEE 802.6	Metropolitan Area Network (Distributed Queue Dual Bus)	disbanded
IEEE 802.7	Broadband Local Area Network using Coaxial Cable	disbanded
IEEE 802.8	Fiber Optic TAG	disbanded
IEEE 802.9	Integrated Services LAN	disbanded
IEEE 802.10	Interoperable LAN Security	disbanded
IEEE 802.11 a/b/g/n	Wireless LAN (WLAN) & Mesh (Wi-Fi certification)
IEEE 802.12	100BaseVG	disbanded
IEEE 802.13	Unused
IEEE 802.14	Cable modems	disbanded
IEEE 802.15	Wireless Personal area Network
IEEE 802.15.1	Bluetooth certification
IEEE 802.15.2	IEEE 802.15 and IEEE 802.11 coexistence
IEEE 802.15.3	High-Rate wireless PAN
IEEE 802.15.4	Low-Rate wireless PAN (e.g., ZigBee, WirelessHART, MiWi, etc.)
IEEE 802.15.5	Mesh networking for WPAN
IEEE 802.15.6	Body area network
IEEE 802.16	Broadband Wireless Access (WiMAX certification)
IEEE 802.16.1	Local Multipoint Distribution Service
IEEE 802.17	Resilient packet ring
IEEE 802.18	Radio Regulatory TAG
IEEE 802.19	Coexistence TAG
IEEE 802.20	Mobile Broadband Wireless Access
IEEE 802.21	Media Independent Handoff
IEEE 802.22	Wireless Regional Area Network
IEEE 802.23	Emergency Services Working Group
IEEE 802.24	Smart Grid TAG	New (November, 2012)
IEEE 802.25	Omni-Range Area Network

NETWORK BRIDGING

Network bridging describes the action taken by network equipment to allow two or more communication networks, or two or more network segments creating an aggregate network. Bridging is distinct from routing which allows the networks to communicate independently as separate networks. A network bridge is a network device that connects more than one network segment.

Microwave Transmission

• Microwave transmission refers to the technology of transmitting information or energy by the use of radio waves whose wavelengths are conveniently measured in small numbers of centimetre. 
• Microwave radio spectrum ranges across frequencies of roughly 1.0 gigahertz (GHz) to 30 GHz. These correspond to wavelengths from 30 centimeters down to 1.0 cm.
• Microwaves are widely used for point-to-point communications because their small wavelength allows conveniently-sized antennas to direct them in narrow beams, which can be pointed directly at the receiving antenna. This allows nearby microwave equipment to use the same frequencies without interfering with each other, as lower frequency radio waves do. 
• Another advantage is that the high frequency of microwaves gives the microwave band a very large information-carrying capacity; the microwave band has a bandwidth 30 times that of all the rest of the radio spectrum below it.
•  A disadvantage is that microwaves are limited to line of sight propagation; they cannot pass around hills or mountains as lower frequency radio waves can.

Applications:

• Microwave radio transmission is commonly used in point-to-point communication systems on the surface of the Earth, in satellite communications, and in deep space radio communications. 
• Other parts of the microwave radio band are used for radars, radio navigation systems, sensor systems, and radio astronomy.

MIMO SYSTEM

To multiply throughput of a radio link, multiple antennas (and multiple RF chains accordingly) are put at both the transmitter and the receiver. This system is referred to as Multiple Input Multiple Output (MIMO). A MIMO system with similar count of antennas at both the transmitter and the receiver in a point-to-point (PTP) link is able to multiply the system throughput linearly with every additional antenna. For example, a 2x2 MIMO will double the throughput.

                                             Fig: Multiple Input Multiple Output (MIMO), 2x2
                                            Two antennas at both the transmitter and the receiver.

         MIMO often employs Spatial Multiplexing (SM) to enable signal (coded and modulated data stream) to be transmitted across different spatial domains. Meanwhile, Mobile WiMAX supports multiple MIMO which will maximize spectral efficiency (increase throughput) without shrinking the coverage area. The dynamic switching between these modes based on channel conditions is called Adaptive MIMO Switching (AMS). If combined with AAS (Adaptive Antenna System), MIMO can further boost WiMAX performance.  

What are the major differences between AM and FM?

	AM	FM
Origin:	AM method of audio transmission was first successfully carried out in the mid 1870s.	FM radio was developed in the United states mainly by Edwin Armstrong in the 1930s.
Modulating differences:	In AM, a radio wave known as the "carrier" or "carrier wave" is modulated in amplitude by the signal that is to be transmitted.	In FM, a radio wave known as the "carrier" or "carrier wave" is modulated in frequency by the signal that is to be transmitted.
Importance:	It is used in both analog and digital communication and telemetry.	It is used in both analog and digital communication and telemetry.
Pros and cons:	AM has poorer sound quality compared to FM, but is cheaper and can be transmitted over long distances. It has a smaller bandwidth so it can have more stations available in any frequency range.	FM is less prone to interference than AM. However, FM signals are impacted by physical barriers. FM has greater sound quality due to higher bandwidth.
Stands for:	AM stands for Amplitude Modulation	FM stands for Frequency Modulation
Range:	AM radio ranges from 535 to 1705 kilohertz (OR) Up to 1200 Bits per second	FM radio ranges in a higher spectrum from 88 to 108 megahertz. (OR) 1200 to 2400 bits per second
Bandwidth Requirements:	Twice the highest modulating frequency. In AM radio broadcasting, the modulating signal has bandwidth of 15kHz, and hence the bandwidth of an amplitude-modulated signal is 30kHz	Twice the sum of the modulating signal frequency and the frequency deviation. If the frequency deviation is 75kHz and the modulating signal frequency is 15kHz, the bandwidth required is 180kHz

  

What is the difference between Amplitude modulation and frequency modulation?

1. In case of frequency modulation the change in amplitude may be due to noise. If we make use of amplitude limiters in FM receivers then we can completely vanish this noise effect.
2. FM waves are waves having constant amplitude. These are independent of the modulation. So, due to this the power transmission of these waves is also constant. The power transmission of FM waves is better than that of the AM signals.
3. In FM signals, all the transmitted power can be used, but in AM wave the transmission carriers contain most of the power. So, complete use of power is not possible.
4. In FM wave’s noise can be controlled by increasing the deviation up to some amount. This is impossible in case of AM waves.
5. VHF and UHF are the bands of FM broadcasting. In these bands noise effect is very less. But on the other hand bands of AM broadcasting such as MF and HF has higher effects.
6. Co-channel interference can be reduced by using some space wave in FM broadcasting.

The major disadvantages of FM are:
1. Complex apparatus is used to transmit and receive the FM wave.
2. FM waves needs 10 times larger channel width than that of the AM waves.
3. The reception area of FM waves is less than that of AM waves. Due to this wide area communication using the Fm waves is not possible.

CRYPTOGRAPHY

Cryptography or cryptology is  from the Greek words κρυπτός, "hidden, secret"; and γράφειν, graphein, "writing", or -λογία, -logia, "study", respectively) is the practice and study of techniques for secure communication in the presence of third parties (called adversaries). More generally, it is about constructing and analyzing protocols that overcome the influence of adversaries and which are related to various aspects in information security such as data confidentiality, data integrity, authentication, and non-repudiation.Modern cryptography intersects the disciplines of mathematics, computer science, and electrical engineering. Applications of cryptography include ATM cards, computer passwords, and electronic commerce.
Cryptography prior to the modern age was effectively synonymous with encryption, the conversion of information from a readable state to apparent nonsense. The originator of an encrypted message shared the decoding technique needed to recover the original information only with intended recipients, thereby precluding unwanted persons to do the same.