Modulation is the technique of embedment of a signal into the carrier frequency (wave). There may be analog (includes FM, AM) or digital modulation. In digital modulation Binary values 0 and 1 is embedded in the carrier frequency by changing its frequency, amplitude or phase. In this way digital modulation may be ASK (Amplitude shift keying), FSK (Frequency shift keying) or PSK (Phase shift keying).
In GSM, GMSK ("Gaussian Minimum shift keying", also known as "Gaussian filtered Minimum Shift Keying") Modulation technique is used which is a type of FSK that continuously uses Phase modulation.
Advantages of GMSK over other techniques->
- Being able to carry digital modulation and efficiently using spectrum.
- Sidebands do not extend outwards from main carrier like other phase shift keying and thus, it avoids interference.
- No discontinuities as change in frequency occur at "carrier zero" crossing points.
- Being continuous frequency shift keying modulation scheme.
It uses the frequency hopping technique, which involves switching the call frequency many times per second for security.
The unique factor of MSK (Maximum Shift Keying) is that frequency difference between the logical 1 and 0 states always equals to half of data rate, expressed in modulation index terms and always being equal to 0.5.
Signal using MSK modulation
Sidebands extend can be easily traced in the plot drawn for the spectrum of an MSK signal. Therefore before applying it to the carrier modulating signal is passed through a low pass filter (Gaussian filter). Before filtering it must have narrow bandwidth, sharp cut-off and impulse response should have no overshoot. Finally MSK signal gets converted to GMSK modulation.
Spectral density of MSK and GMSK signals
Generation of MSK Modulation
There are mainly two ways of to generate MSK modulation.
Description of First way- The filtering of Modulating signal is done using Gaussian filter and then it is applied to frequency Modulator to set modulation index to 0.5. It is the simplest way. Its limitation lies in the fact that Modulation index must exactly equal to 0.5, which is practically not suitable as drifts are experienced by the components (which is not possible to be set exactly).
Generating GMSK using Gaussian filter & VCO
Description of second way- Quadrature Modulator (or I-Q Modulator) is one of the ways. Here the phase of a signal lies in quadrature or 90 degrees to other one. It uses one signal, known as to be in-phase and another being quadrature to the first one. Use of Modulator of this type the Modulator index is exactly maintained to 0.5 without any further setting. Thus it is much easier to use and also it provides the required level of performance.
Block diagram of I-Q modulator used to create GMSK
For Demodulation the above techniques can be used in reverse.
BASIC IMPLEMENTATION REQUIREMENTS
HARDWARE AND SOFTWARE REQUIREMENTS-
The three systems of GSM are--
- Switching system (SS)
- Base station system (BSS)
- Operation and support system (OSS)
The Switching System- It is responsible for all call processing and other functions with following described units.
GSM Network Elements
Home Location Register (HLR)- It is the most important database of an operator storing every important details of subscribed customer. Whenever a customer buys a new connection his registration is made in HLR.
Mobile services switching centre (MSC)- It performs telephony switching functions of system including controlling calls to and from other telephone and data systems.
Visitor Location registers (VLR)- When a cell phone roams into a new MSC area, VLR connected to that MSC requests for data about mobile station from HLR. Later VLR will not need to interrogate HLR whenever customer makes any call.
Authentication centre (AUC)- It verifies each caller's identity and ensures confidentiality of each call.
Equipment Identity Register (EIR)- This database stores information of Mobile devices thus preventing calla from unauthorised or stolen ones.
Base Station System- It consists of
- Base Station Controllers (BSC)- It is the switch serving as physical links between MSC and BTS also providing functions like handover, cell configuration data, and control of Radio frequency (RF) power levels in BTS.
- Base Transceiver Stations (BTS)- It is the radio equipment which serves each cell in Network. A group of BTS are controlled by BSC.
Operation and Support System (OMC)- This entity (connected to all equipment in switching System and to BSC) helps the operator to govern the whole system and offer customer cost-effective support and maintenance activities need for GSM network.
Message centre (MXE)- It Handles SMS, voice mail, fax mail, email, and cell broadcast.
Mobile Service Node (MSN)- It governs Mobile intelligent network (IN) services.
Gateway Mobile Services Switching Centre (GMSC)- It interconnects two networks. MSC then known as GMSC.
GSM Interworking Unit (GIWU)- It consists of Hardwires and Software both and acts as an interface to various network for data communication.
Problems in Implementation of Technology:-
- Implementation of GSM technology requires a number of equipment. Many equipment need to be installed like tall towers. These towers are fixed at proper area in line of sight. Also satellites functionality also requires attention.
- Installation of Several equipment and devices require huge amount of money. Thus GSM architecture is very costly.
- Maintenance of equipment used in GSM is practically not affordable time and again.
- There are several protocols applied in GSM Technology, resulting problem in implementation.
- There is possibility of breaking into GSM networks that use cipher text-only cryptanalysis encrypted communication. These attacks are based on security flaws of the GSM protocols, and work whenever the mobile phone supports A5/2.
- The available radio frequencies need to be re-used closer together and continuously within the network so that spectral efficiency can be unlimited.
- There is huge consumption of power in GSM implementation, which is not convenient.
- GSM is also not secure as the sent signal or message is not so much hard to be decoded. So the third party may avail of it somehow using some technology.
ABOUT THE TECHNOLOGY
GSM (Global System for Mobile communications: originally from Groupe Special Mobile) is the most widely used digital cellular technology for transmitting mobile voice and data services in the world.
It uses SIM (SubscriberIdentityModule) cards, which acts as digital identity, and is tied to the cellular network by searching for cells towards the towers (bts) in the nearby location.
GSM is a second-generation (2G) cell phone system used by over 3 billion people across more than 212 countries and territories. In over twenty years of development, GSM has been continuously upgraded to provide better signal, speech quality and thus fulfil the mass demands. It also facilitates to roam, switch carriers and the operators without replacement of handsets.
The GSM technology originally originated by the European Conference of Postal and Telecommunications Administrations (CEPT) in 1982 when it firstly created Group Special Mobile (GSM) for the purpose of designing a pan-European mobile technology. European Telecommunications Standardization Institute (ETSI) later in 1991 adopted the GSM standard. GSM technology was firstly commercially lunched in 1991 in Finland, although it has been for testing purposes since 1980.
Newer versions of the standard are much compatible with the original GSM system. For example,Release '97,the General Packet Radio Service (GPRS) added the packet data capabilities to GSM technology. Release '99 introduced higher speed data transmission usingEnhanced Data Rates for GSM Evolution(EDGE).
It uses variation of Time Division Multiple Access (TDMA) and is the most widely used of the three digital wireless telephone technologies (TDMA, GSM, and CDMA). Data is digitized and compressed and, sent down a channel with two other streams of user data, each in its own time slot. Most 2G GSM networks operates in the 900MHz or 1800MHz bands. It operates on four different frequencies worldwide. GSM calls are either based on data or voice. Voice calls use audio codecs called half-rate, full-rate and enhanced full-rate. Data calls can turn the cell phone into a modem operating at 9600 bps having ability to carry 64 kbps to 120 Mbps of data rates.
GSM technology even keeps people in touch while roaming also as it home operators are having roaming collaboration with foreign operators.
IMPORTANCE OF TECHNOLOGY
GSM is the most popular, reliable and the fast growing wireless technology for mobile phones in the world. It has approximately covered the 89% share market of the global mobile telecom in more than 219 countries and territories with more than 3.8 billion people as per May 2009 study. Its signalling and speech channels being digital differs it from its and so pronounced as a second generation (2G) mobile phone system. This has also meant that data communication was easy to build into the system.
It also provides the integrated high-speed data, fax, paging, voice mail and short messages services (SMS) capabilities and thus offering communication to be secure, private and fraud prevented.
Many operators provide access to ISDN services to transmit data fast. Several offer GPRS facility to connect to high speed data communication channel (9600 bps).
GSM market share has grown at exponent rate over recent years. However the target to achieve 1 billion customers took 12 years (Feb 2004) but later it was a surprising fact that it crossed the 2 billion mark only in another 2.5 years (By June 2006) and 3 billion GSM customers in another less than 2 years. Now the Telecoms and Media hope to surpass 5 billion targets by 2013.
GSM is the legacy network of the evolution to the third generation (3G) technologies Universal Mobile Telecommunication System, (UMTS) also known as WCDMA, and High Speed Packet Access (HSPA).
Maximum Mobile phones manufacturer develop their products based on GSM technology and all mobile companies provide their customers GSM networks.
The Social Impact of Mobile Telephony
Mobile telephony is undoubted one of the most superb developments telecommunications industry. The number of subscribers is rapidly increasing in tremendous way. More and more people are being connected to the telecom.
GSM technology has greatly affected the social life. The various operators have been proving a number of offers to fascinate their customers. Due to aspects like reliability, security, voice and data transmission facilities GSM is much popular among the users. People are shifting to wireless communication from their fixed phones.
Popularity Reasons of GSM Technology
- Improved spectrum efficiency.
- High speech quality.
- International roaming.
- Better signal quality.
- Offer voice, data and fax.
- Offer secure communication and privacy.
- Low-cost mobile sets and base stations (BSs).
- Flexibility SIM.
- High share market capture.
- Low terminal and service cost.
- ISDN compatibility.
- Compatibility with Integrated Services Digital Network (ISDN) for data transmission.
- Support for new services like VAS.
Backbone for 3G and other evaluating technologies
It is the recent report that use of GSM mobile technology poses Health Hazards. Its regular and continuous use for hours may create heath problems like irritations, allergies, headache, ears pain etc.
It is fact proved by scientists that radio waves of frequencies that it utilises can create electric fieldsin human cells and brain. Long term exposure may have adverse health effects too.
Many research institutions controlled by government and independent bodies are continuously studying over it. Also research results are being revived by ICNIRP (International Commission on Non-Ionizing Radiation Protection) and the WHO (World Health Organisation) at global level.
Use of cellular phone is highly restricted to be turned off while in flight as the radio frequencies may interfere. The same case happens at hospitals as the radio signal may create interference with electronic medical devices at short rang or cause uneasiness among patients.
While driving use of cell phone is advised to avoid and in petrol pumps as the radio waves used in GSM technology may cause interference.