Spread spectrum

A collective class of signaling techniques are employed before transmitting a signal to provide a secure communication, known as the Spread Spectrum Modulation. The main advantage of spread spectrum communication technique is to prevent “interference” whether it is intentional or unintentional.

The signals modulated with these techniques are hard to interfere and cannot be jammed. An intruder with no official access is never allowed to crack them. Hence, these techniques are used for military purposes. These spread spectrum signals transmit at low power density and has a wide spread of signals.

Pseudo-Noise Sequence

A coded sequence of 1s and 0s with certain auto-correlation properties, called as Pseudo-Noise coding sequence is used in spread spectrum techniques. It is a maximum-length sequence, which is a type of cyclic code.

Narrow-band and Spread-spectrum Signals

Both the Narrow band and Spread spectrum signals can be understood easily by observing their frequency spectrum as shown in the following figures.

Narrow-band Signals

The Narrow-band signals have the signal strength concentrated as shown in the following frequency spectrum figure.

Following are some of its features −

• Band of signals occupy a narrow range of frequencies.
• Power density is high.
• Spread of energy is low and concentrated.

Though the features are good, these signals are prone to interference.

Spread Spectrum Signals

The spread spectrum signals have the signal strength distributed as shown in the following frequency spectrum figure.

Following are some of its features −

• Band of signals occupy a wide range of frequencies.
• Power density is very low.
• Energy is wide spread.

With these features, the spread spectrum signals are highly resistant to interference or jamming. Since multiple users can share the same spread spectrum bandwidth without interfering with one another, these can be called as multiple access techniques.

FHSS and DSSS / CDMA

Spread spectrum multiple access techniques uses signals which have a transmission bandwidth of a magnitude greater than the minimum required RF bandwidth.

These are of two types.

• Frequency Hopped Spread Spectrum (FHSS)
• Direct Sequence Spread Spectrum (DSSS)

Frequency Hopped Spread Spectrum (FHSS)

This is frequency hopping technique, where the users are made to change the frequencies of usage, from one to another in a specified time interval, hence called as frequency hopping. For example, a frequency was allotted to sender 1 for a particular period of time. Now, after a while, sender 1 hops to the other frequency and sender 2 uses the first frequency, which was previously used by sender 1. This is called as frequency reuse.

The frequencies of the data are hopped from one to another in order to provide a secure transmission. The amount of time spent on each frequency hop is called as Dwell time.

Direct Sequence Spread Spectrum (DSSS)

Whenever a user wants to send data using this DSSS technique, each and every bit of the user data is multiplied by a secret code, called as chipping code. This chipping code is nothing but the spreading code which is multiplied with the original message and transmitted. The receiver uses the same code to retrieve the original message.

Comparison between FHSS and DSSS/CDMA

Both the spread spectrum techniques are popular for their characteristics. To have a clear understanding, let us take a look at their comparisons.

FHSS	DSSS / CDMA
Multiple frequencies are used	Single frequency is used
Hard to find the user’s frequency at any instant of time	User frequency, once allotted is always the same
Frequency reuse is allowed	Frequency reuse is not allowed
Sender need not wait	Sender has to wait if the spectrum is busy
Power strength of the signal is high	Power strength of the signal is low
Stronger and penetrates through the obstacles	It is weaker compared to FHSS
It is never affected by interference	It can be affected by interference
It is cheaper	It is expensive
This is the commonly used technique	This technique is not frequently used

Advantages of Spread Spectrum

Following are the advantages of spread spectrum −

• Cross-talk elimination
• Better output with data integrity
• Reduced effect of multipath fading
• Better security
• Reduction in noise
• Co-existence with other systems
• Longer operative distances
• Hard to detect
• Not easy to demodulate/decode
• Difficult to jam the signals

Although spread spectrum techniques were originally designed for military uses, they are now being used widely for commercial purpose.

How satellite Works

A satellite is an object that revolves around another object. For example, earth is a satellite of The Sun, and moon is a satellite of earth.

A communication satellite is a microwave repeater station in a space that is used for telecommunication, radio and television signals. A communication satellite processes the data coming from one earth station and it converts the data into another form and send it to the second earth station.

How a Satellite Works

Two stations on earth want to communicate through radio broadcast but are too far away to use conventional means. The two stations can use a relay station for their communication. One earth station transmits the signal to the satellite.

Uplink frequency is the frequency at which ground station is communicating with satellite. The satellite transponder converts the signal and sends it down to the second earth station, and this is called Downlink frequency. The second earth station also communicates with the first one in the same way.

Advantages of Satellite

The advantages of Satellite Communications are as follows −

• The Coverage area is very high than that of terrestrial systems.
• The transmission cost is independent of the coverage area.
• Higher bandwidths are possible.

Disadvantages of Satellite

The disadvantages of Satellite Communications are as follows −

• Launching satellites into orbits is a costly process.
• The bandwidths are gradually used up.
• High propagation delay for satellite systems than the conventional terrestrial systems.

Satellite Communication Basics

The process of satellite communication begins at an earth station. Here an installation is designed to transmit and receive signals from a satellite in orbit around the earth. Earth stations send information to satellites in the form of high powered, high frequency (GHz range) signals.

The satellites receive and retransmit the signals back to earth where they are received by other earth stations in the coverage area of the satellite. Satellite's footprint is the area which receives a signal of useful strength from the satellite.

The transmission system from the earth station to the satellite through a channel is called the uplink. The system from the satellite to the earth station through the channel is called the downlink.

Satellite Frequency Bands

The satellite frequency bands which are commonly used for communication are the Cband, Ku-band, and Ka-band. C-band and Ku-band are the commonly used frequency spectrums by today's satellites.

It is important to note that there is an inverse relationship between frequency and wavelength i.e. when frequency increases, wavelength decreases this helps to understand the relationship between antenna diameter and transmission frequency. Larger antennas (satellite dishes) are necessary to gather the signal with increasing wavelength.

Earth Orbits

A satellite when launched into space, needs to be placed in certain orbit to provide a particular way for its revolution, so as to maintain accessibility and serve its purpose whether scientific, military or commercial. Such orbits which are assigned to satellites, with respect to earth are called as Earth Orbits. The satellites in these orbits are Earth Orbit Satellites.

The important kinds of Earth Orbits are −

• Geo-synchronous Earth Orbit
• Geo-stationary Earth Orbit
• Medium Earth Orbit
• Low Earth Orbit

Geo-synchronous Earth Orbit (GEO) Satellites

A Geo-synchronous Earth orbit Satellite is one which is placed at an altitude of 22,300 miles above the Earth. This orbit is synchronized with a side real day(i.e., 23hours 56minutes). This orbit can have inclination and eccentricity. It may not be circular. This orbit can be tilted at the poles of the earth. But it appears stationary when observed from the Earth.

The same geo-synchronous orbit, if it is circular and in the plane of equator, it is called as geo-stationary orbit. These Satellites are placed at 35,900kms (same as geosynchronous) above the Earth’s Equator and they keep on rotating with respect to earth’s direction (west to east). These satellites are considered stationary with respect to earth and hence the name implies.

Geo-Stationary Earth Orbit Satellites are used for weather forecasting, satellite TV, satellite radio and other types of global communications.

The above figure shows the difference between Geo-synchronous and Geo- Stationary orbits. The Axis of rotation indicates the movement of Earth.

The main point to note here is that every Geo-Stationary orbit is a Geo-Synchronous orbit. But every Geo-Synchronous orbit is NOT a Geo-stationary orbit.

Medium Earth Orbit (MEO) Satellites

Medium earth orbit (MEO) satellite networks will orbit at distances of about 8000 miles from earth's surface. Signals transmitted from a MEO satellite travel a shorter distance. This translates to improved signal strength at the receiving end. This shows that smaller, more lightweight receiving terminals can be used at the receiving end.

Since the signal is travelling a shorter distance to and from the satellite, there is less transmission delay. Transmission delay can be defined as the time it takes for a signal to travel up to a satellite and back down to a receiving station.

For real-time communications, the shorter the transmission delay, the better will be the communication system. As an example, if a GEO satellite requires 0.25 seconds for a round trip, then MEO satellite requires less than 0.1 seconds to complete the same trip. MEOs operates in the frequency range of 2 GHz and above.

Low Earth Orbit (LEO) Satellites

The LEO satellites are mainly classified into three categories namely, little LEOs, big LEOs, and Mega-LEOs. LEOs will orbit at a distance of 500 to 1000 miles above the earth's surface.

This relatively short distance reduces transmission delay to only 0.05 seconds. This further reduces the need for sensitive and bulky receiving equipment. Little LEOs will operate in the 800 MHz (0.8 GHz) range. Big LEOs will operate in the 2 GHz or above range, and Mega-LEOs operates in the 20-30 GHz range.

The higher frequencies associated with Mega-LEOs translates into more information carrying capacity and yields to the capability of real-time, low delay video transmission scheme.

High Altitude Long Endurance (HALE) Platforms

Experimental HALE platforms are basically highly efficient and lightweight airplanes carrying communications equipment. This will act as very low earth orbit geosynchronous satellites.

These crafts will be powered by a combination of battery and solar power or high efficiency turbine engines. HALE platforms will offer transmission delays of less than 0.001 seconds at an altitude of only 70,000 feet, and even better signal strength for very lightweight hand-held receiving devices.

Orbital Slots

Here there may arise a question that with more than 200 satellites up there in geosynchronous orbit, how do we keep them from running into each other or from attempting to use the same location in space? To answer this problem, international regulatory bodies like the International Telecommunications Union (ITU) and national government organizations like the Federal Communications Commission (FCC) designate the locations on the geosynchronous orbit where the communications satellites can be located.

These locations are specified in degrees of longitude and are called as orbital slots. The FCC and ITU have progressively reduced the required spacing down to only 2 degrees for C-band and Ku-band satellites due to the huge demand for orbital slots.

Daily updates 6 July 2019

National News

1. India's first Design Development Centre launched in Gujarat

i. India's first Design Development Center 'Fashionova' was launched in the Textile city Surat.  
ii. The centre will promote the city in the field of the fashion design sector. 
iii. It will also fulfill the business needs from co-working space, technicians, expert advises to cognitive workshops and exposure to the industry.

Static/Current Takeaways Important For ESIC:

• Gujarat CM: Vijay Rupani; Governor: Om Prakash Kohli; Capital: Gandhinagar.

2. Longest electrified railway tunnel in India commissioned

i. The South Central Railway has commissioned the longest electrified tunnel of India. 
ii. The tunnel which is between Cherlopalli and Rapuru stations is 6.6 km long, is built at a total cost of ₹460 crore. 
iii. The horse shoe shaped tunnel is of 6.5 metres height with ballastless track and LED lighting provided at 10 metre intervals inside.

3. Army Chief releases Kargil Tribute Song

i. Army Chief General Bipin Rawat has released a Kargil Tribute Song to honour, salute and pay homage to the Kargil martyrs and war veterans. 
ii. The song has been composed by eminent Hindi lyricist Sammer Anjaan and sung by Shatadru Kabir.
iii. The song was released as part of the celebration of the 20th anniversary of Kargil Vijay Diwas to be celebrated on 26th of July.

Agreements

4. Cabinet approves India-Maldives MoU for passenger, cargo services

i. The Union Cabinet has approved the pact signed between India and Maldives to start passenger and cargo services via sea route. 
ii. The MoU will pave way for ferry services between Maldives and India. The proposed ferry service would contribute in a big way to promote people to people contact and to boost bilateral trade.

Static/Current Takeaways Important For EPFO/LIC ADO Mains:

• Capital of Maldives: Male; Currency: Maldivian Rufiyaa.
• President: Ibrahim Mohamed Solih.

5. CCMB and CDFD sign MoU to improve genetic disease diagnostics

i. An MoU has been signed between CSIR-Centre for Cellular and Molecular Biology (CCMB) and Centre for DNA Fingerprinting and Diagnostics (CDFD). 
ii. The aim of the MoU is to enable better ways of diagnosis and treatment of genetic disorders.

6. Reliance Jio ties up with Facebook for 'Digital Udaan'

i. Reliance Jio ties up with Facebook for digital literacy initiative 'Digital Udaan'. 
ii. The initiative will have audio-visual training in 10 regional languages. The initiative is announced for the first-time Internet users in the country. 
iii. The programme is being launched in nearly 200 different locations across 13 states.

Static/Current Takeaways Important For ESIC:

• Facebook Headquarters: California, US; CEO: Mark Zuckerberg.

State News

7. Tripura gets Rs 358 crore from GoI for road projects

i. The GoI has sanctioned an additional amount of Rs 358 crore for the construction of new roads in rural areas of Tripura.

Static/Current Takeaways Important For ESIC:

• Capital of Tripura: Agartala, CM: Biplab Kumar Deb, Governor: Kaptan Singh Solanki.

8. Hong Kong govt grants over $9 million for cyclone-hit Odisha

i. The Hong Kong government has approved over $9 million for relief and rehabilitation work in cyclone-hit Odisha. 
ii. The grants will benefit around 45,100 victims of "Fani cyclone" and will be used to provide hygiene kits, kitchen kits, water, education, household items and shelter kits.

Static/Current Takeaways Important For RRB NTPC/IBPS RRB Mains:

• CM of Odisha: Naveen Patnaik; Governor: Ganeshi Lal.

Banking News

9. Punjab & Sind Bank sets up Cen-MARG

i. Punjab & Sind Bank has set up a centralised hub "Centralised MSME & Retail Group". It will process retail and MSME loans for the better efficiency of branches in business acquisition. 
ii. The Cen-MARG has been envisioned to ensure qualitative improvement in credit appraisal, improved turnaround time, uniform documentation, and efficient monitoring.

Static/Current Takeaways Important For RRB NTPC/IBPS RRB Mains:

• MD & CEO of Punjab & Sind Bank: S. Harisankar.

Ranks & Reports

10. India ranked 86th in the Henley Passport Index 2019

i. The Henley Passport Index 2019 have ranked the Indian passport on the 86th position with a mobility score of 58. 
ii. The score points out that Indian passport holders can access 58 countries around the world without a prior visa. Japan and Singapore holds the top spot with a score of 189.
iii. The index includes 199 passports and 227 travel destinations including micro-states and territories. 

Miscellaneous

11. Nasscom, NSDC launch Global Cloud Computing challenge

i. Nasscom partners with National Skill Development Corporation to organise ‘World Skills India’s International Cloud Computing Challenge 2019’. 
ii. Ten countries including India are participating in the competition, along with New Zealand, Belgium, Japan, China, South Korea, Russia, Oman and Ireland.
iii. “WorldSkills India” is an NSDC initiative launched in 2011.

Static/Current Takeaways Important For SBI PO/Clerk Mains:

• President of NASSCOM: Debjani Ghosh.
• Chairman of NSDC: A M Naik; MD & CEO: Manish Kumar.

12. Iraq's Babylon listed as World Heritage Site by UNESCO

i. UNESCO's World Heritage Committee has listed Iraq's Babylon as a World Heritage Site. 
ii. Babylon is developed as a walled city of mudbrick temples and towers, constituting the Tower of Babel, and the Ishtar Gate. 
iii. The city was the centre of the ancient Babylonian empire more than 4,000 years ago.

Static/Current Takeaways Important For SBI PO/Clerk Mains:

• Capital of Iraq: Baghdad; Currency: Iraqi dinar.
• Headquarters of UNESCO: Paris, France; Director General: Audrey Azoulay.

Important Days

13. International Day of Cooperatives: 6 July

i. The United Nations observes first Saturday of July every year as International Day of Cooperatives.  
ii. The day is celebrated to increase the awareness of cooperatives. The event underscores the contributions of the cooperative movement in resolving the major problems addressed by the United Nations.
iii. Theme for 2019: COOPS 4 DECENT WORK.
Static/Current Takeaways Important For EPFO/LIC ADO Mains:

• United Nations Headquarters: New York, USA; Founded: 24 October 1945.
• Secretary General of the United Nations: Antonio Guterres.