Global System for Mobile Communications

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INTRODUCTION TO GSM

What is GSM and GSM 1800?

The Global System for Mobile Communications (GSM: originally from Groupe Spécial Mobile) is the most popular standard for mobile phones in the world.It is one of the leading digital cellular systems. Since it is launched in 1992, it has simply taken this world by storm. GSM uses narrow band TDMA, which allows 8 simultaneous calls on the same radio frequency.

GSM service is used by over 2 billion people across more than 212 countries and territories. The ubiquity of the GSM standard makes international roaming very common between mobile phone operators, enabling subscribers to use their phones in many parts of the world. GSM differs significantly from its predecessors in that both signaling and speech channels are Digital call quality, which means that it is considered a second generation (2G) mobile phone system. This fact has also meant that data communication was built into the system from the 3rd Generation Partnership Project (3GPP).

From the point of view of the consumers, the key advantage of GSM systems has been higher digital voice quality and low cost alternatives to making calls such as text messaging. As the GSM standard continued to develop, it retained backward compatibility with the original GSM phones; for example, packet data capabilities were added in the Release '97 version of the standard, by means of GPRS. Higher speed data transmission has also been introduced with EDGE in the Release '99 version of the standard. The GSM logo is used to identify compatible handsets and equipment.

The GSM logo is used to identify compatible handsets and equipment

GSM 1800 is the name given to the GSM (Global System for Mobile Communications) network operating at 1800 MHz.

GSM networks operating at this frequency are present in Europe (including the UK), the Asia-Pacific region and Australia. It should not be confused with GSM 1900 (or American GSM) which serves the North American continent.

In the UK, Orange and One2One operate a GSM 1800 network. One2One provide network services to other companies in order for them to offer a "virtual" network. Virgin Mobile is one of the larger companies operating this type of network which runs GSM 1800 as it is actually using the One2One service.

If we buy a cell phone then we should be aware that if it is capable of operating only on GSM 1800 then in the UK, we will not be able to transfer our service provider to Vodafone or Cell net. Similarly, if we purchase a single band GSM 900 phone, then we will not be able to use it on the Orange or One2One networks.

Most dual band phones will include GSM 1800, and offer increased portability particularly when travelling outside of the UK. In particular, if we intend to use our phone outside of the UK, we should strongly consider either a dual band or tri band phone.

EVOLUTION OF CELLULAR NETWORKS

“The wireless telegraph is not difficult to understand. The ordinary telegraph is like a very long cat. You pull the tail in New York and it meows in Los Angeles. The wireless is the same, only without the cat.” - Albert Einstein.

SECOND GENERATION (2G) CELLULAR NETWORKS

Most of today's ubiquitous cellular networks use what is commonly called second generation or 2G technologies. Unlike first generation cellular systems that relied exclusively on FDMA/FDD and analog FM, second generation standards use digital modulation formats and TDMA/FDD and CDMA/FDD multiple access techniques. The most popular second generation standards include three TDMA standards and one CDMA standards.

1. Global System Mobile (GSM), which supports 8 time slotted users for each 200 KHz radio channel and has been deployed widely in the cellular and PCS bands by service providers in Europe, Asia, Australia, South America, and some parts of the US (in the PCS spectrum band only)

2. Interim Standard, also known as North American Digital Cellular (NADC) or US DIGITAL CELLULAR (USDC), which supports three time slotted users for each 30 KHz radio channel and is a popular choice for carriers in North America, South America and Australia (in both the cellular and PCS bands).

3. Pacific Digital Cellular

4. The popular 2G CDMA standard Interim Standard 95 Code Division Multiple Access (IS-95) also known as cdmaOne, which supports up to 64 users that are orthogonally coded and simultaneously transmitted on each 1.25 MHz channel. CDMA is widely deployed by carriers in North America (in both cellular and PCS bands), as well as in Korea, Japan, China, South America, and Australia.

2G systems were first introduced in the early 1990s, and evolved from the first generation of analog mobile phone systems (e.g. AMPS, ETACS, and JTACS). Today, many wireless service providers use both first generation and second generation equipment in major markets and often provide customers with subscriber units that can support multiple frequency bands and multiple air interface standards. For example, in many countries tri-mode phones are able to automatically sense and adapt to whichever standard is being used in a particular market.

In many countries, 2G wireless networks are designed and deployed for conventional mobile telephone service, as a high capacity replacement for, or in competition with existing older first generation cellular telephone systems. Since all 2G technologies offer at least a three-times increase in spectrum efficiency (and thus at least a 3X increase in overall system capacity) as compared to first generation analog technologies, the need to meet a rapidly growing customer base justifies the gradual, on going changes out of analog to digital 2G technologies in any growing wireless networks.

In mid 2001, several major carriers such as AT & T wireless and Cingular in the US and NTT in Japan announced their decisions to eventually abandon the IS-136 and PDC standards. Simultaneously, international wireless carrier Nextel announced its decision to upgrade its iDen air interface standard to support up to five times the number of current users based on a data compression methodology using Internet protocol (IP) packet data.

2.5G MOBILE RADIO NETWORKS

Since the mid 1990s, the 2G digital standards have been widely deployed by wireless carriers for cellular and PCS. 2G technologies use circuit-switched data modems that limit data users to a signal circuit-switched voice channel. Data transmissions in 2G are generally limited to the data throughout rate of an individual user. Each of the 2G standards specify different coding schemes and error protection algorithms for data transmission versus voice transmissions, but the data throughput rate for computer data is approximately the same as the throughput rate for speech coded voice data in all 2G standards. From inspection it can be seen that all 2G networks, as originally developed, only support single user data rates on the order of 10 kbps, which is too slow for rapid email and Internet browsing applications. The original GSM, CDMA, and IS-136 standards originally supported 9.6 kbps transmission rates for data messages.

These new standards represent 2.5G technology and allow existing 2G equipment to be modified and supplemented with new base station add-ons and subscriber unit software upgrades to support higher data rates transmission for web browsing, e-mail traffic mobile commerce (m-commerce), and location-based mobile services. The 2.5G technologies also support a popular new web browsing format language, called wireless Application protocol (WAP).

EVOLUTION FOR 2.5G TDMA STANDARDS

Three different upgrade paths have been developed for GSM carrier, and two of these solutions also support IS-136. These options provide significant improvements in Internet access speed over today's GSM and IS-136 technology and support the creation of new Internet-ready cell phones.

HSCSD for 2.5G GSM

High Speed Circuit Switched Data is a circuit switched technique that allows a single mobile subscriber to use consecutive user time slots in the GSM standard. That is, instead of limiting each user to only to only one specific time slots in the GSM TDMA standard, HSCSD allows individual data users to commandeer consecutive time slots in order to obtain higher speed data access on the GSM network. HSCSD relaxes the error control coding algorithms and increase the available application data rate to 14,400 bps, as compared to the original 9,600 bps in the GSM specification. By using up to four consecutive time slots, HSCSD is able to provide a raw transmission rate of up to 57.6 kbps to individual users. HSCSD is ideal dedicated streaming Internet access or real-time interactive web sessions and simply requires the service provider to implement a software changes at existing GSM base stations.

GPRS for 2.5G GSM and IS-136

General Packet Radio Service is a packet based data network, which is well suited for non-real time Internet usage, including the retrieval of email, faxes, and asymmetric web browsing, where the user downloads much more data than it uploads on the Internet. GPRS supports multi-user network sharing of individual radio channels and time slots. Thus, GPRS can support many more users than HSCSD, but in a burst manner. Similar to the Cellular digital packet data (CDPD) standard developed for North American AMPS systems in early 1990s the GPRS standard provides a packet network on dedicated GSM or IS-136 radio channels. GPRS retains the original modulation formats specified in the original 2G TDMA standards, but uses a completely redefined air interface in order to better handle packet data access.

When all 8 time slots of a GSM radio channel are dedicated to GPRS, an individual user is able to achieve as much as 171.2 kbps (eight time slots multiplied by 21.4kbps of raw uncoded data throughout). Applications are required to provide their own error correction schemes as part of the carried data payload in GPRS. Implementation of GPRS merely requires the GSM operate to install new routers and Internet gateways at the base station, along with new software that redefines the base station air interface standard for GPRS channels and time slots- no new base station RF hardware is required.

It is worth noting that GPRS was originally designed to provide a packet data access overlay solely for GSM networks, but at the North America IS_136 operates. GPRS was extended to include both TDMA standards. As of late 2001, GPRS has been installed in markets serving over 100 million subscribers, and is poised to be popular near-team packet data solution for 2G TDMA-based technologies. The dedicated peak 21.4 kbps per channel data rate specified by GPRS works well with both GSM and IS-136 and has successfully been implemented.

EGDE for 2.5G GSM and IS-136

EDGE, which stands for Enhanced Data rates for GSM (or global) Evolution is a more advanced upgrade to the GSM standards, and requires the addition of new hardware and software at existing base stations. EDGE was developed from the desire of both GSM and IS-136 operates to have a common technology path for eventual 3G high speed data access.

EDGE introduces a new digital modulation format, 8-PSK (octal phase shift keying), which is used in addition to GSM's standard GMSK modulation. EDGE allows for 9 different (autonomously and rapidly selectable) air interface formats, known as multiple modulation and coding schemes (MCS), with varying degrees of error control protection. Because of the higher data rates and relaxed error control covering in many of the selectable air interface formats, the coverage range is smaller in EDGE than in HSDRC or GPRS. EGDE is sometimes referred to as Enhanced GPRS, or EGPRS. The adaptive capability of EDGE to select the “best” air interface is called incremental redundancy, whereby packets are transmitted first with maximum error protection and maximum data rate throughput, and then subsequent packets are transmitted with less error protection (usually using punctured convolution codes) and less throughput, until the link has an unacceptable outage or delay.

IS-95B for 2.5G CDMA

Unlike the several GSM and IS -136 evolution paths to high speed data access, CDMA (often called cdmaOne)has a single upgrade path for eventual 3G operation. The interim data solution for CDMA is called IS-95B is already being deployed worldwide, and provides high speed packet and circuit switched data access on a common CDMA radio channel by dedicating multiple orthogonal user channels (Walsh functions) for specific users and specific purposes. Each IS-95 CDMA radio channel supports up to 64 different users channels. The original IS-95 throughput rate specification of 9600 bps was not implemented in practice, but was improved to the current rate of 14,400bps as specified in IS-95A. The 2.5G CDMA solution, IS-95, supports medium data rate (MDR) service by allowing a dedicated user to command up to eight different user Walsh codes simultaneously and in parallel foe an instantaneous throughput of 115.2 kbps per user (8*14.4 kbps). However only about 64 kbps of practical throughput is available to a single user in IS-95B due to the slotting techniques of the air interface

IS-95B also specifies hard handoff produces that allow subscriber units to search different radio channels in the network without instruction from the switch so that subscriber units can rapidly tune to different base stations to maintain link quality. Prior to IS-95B, the link quality experienced by each subscriber had to be reported back to the switch through the serving base station several times per second, and at the appropriate moment, the switch would initiate a soft-handoff between the subscriber and candidate base stations. The new hard handoff capability of IS-95B is more efficient for multiple channel systems now being used in today's more congested CDMA markets.

THIRD GENERATION (3G) WIRELESS NETWORKS

3G systems promise unparalleled wireless access in ways that have never been possible before Multi-megabit Internet access, communications using Voice over Internet Protocol (VoIP), voice activated calls, unparalleled networks capacity, and ubiquitous “always-on” access are just some of the advantages being touted by 3G developers. Companies developing 3G equipment envision users having the ability to receive live music, conduct interactive web sessions, and have simultaneous voice and data access with multiple parties at the same time using a single mobile handset, whether driving, walking, or standing still in an office setting.

The International Telecommunications Union (ITU) formulated a plan to implement a global frequency band in the 2000 MHz range that would support a wireless communication standard for all countries throughout the world. This plan, called International Mobile Telephone 2000 (IMT-2000), has been successful in helping to cultivate active debate and technical analysis for new high speed mobile telephone solutions when compared to 2G. However, the hope foe a single worldwide standard has not materialized, as the worldwide user community remains spilt between two camps: GSM/IS-136/PDC and CDMA.

The eventual 3G evolution for 2G CDMA systems leads to cdma2000, several variants of CDMA 2000 are currently being developed, but they all are based on the fundamentals of IS-95 and IS-95B technologies. The eventual 3G evolution for GSM, IS-136, and PDC systems leads to Wideband CDMA (W-CDMA), also called Universal Mobile Telecommunication Service (UMTS). W-CDMA is based on the network fundamentals of GSM, as well as the merged versions of GSM and IS-136 through EDGE. It is fair to say that these two major 3G technology camps, cdma2000 and W-CDMA, will remain popular throughout the early part of the 21st century.

3G W-CDMA (UMTS)

The Universal Mobile Telecommunication System (UMTS) is an air interface standard that has evolved since late 1996 under the auspices of the European Telecommunication Standards Institute (ETSI). UMTS was submitted by ETSI to ITU's IMT-2000 body in 1998 for consideration as a world standard. At that time, UMTS was known as UMTS Terrestrial Radio Access (UTRA), and was designed to provide a high capacity upgrade path for GSM. Around the turn of the century, several other competing wideband CDMA (W-CDMA) proposals agreed to merge into a single W-CDMA standard, and this resulting W-CDMA standard is now called UMTS. UMTS, or W-CDMA, assures backward compatibility with second generation GSM, IS-136, and PDC TDMA technologies, as well we all 2.5G TDMA technologies. The network structure and bit level packaging of GSM data is retained by W-CDMA, with additional capacity and bandwidth provided by a new CDMA air interface.

The 3G W-CDMA air interface standard had been designed for “always-on” packet-based wireless service, so that computers, entertainment devices, and telephones may all share the same wireless network and be connected to the Internet, anytime , anywhere. W-CDMA will support packet data rates up to 2.048 Mbps per user (if the user is stationary), thereby allowing high quality data, multimedia, streaming audio, streaming video, and broadcast-type services to consumers.

Future versions of W-CDMA will support stationary user data rates in excess of 8Mbps. W-CDMA provides public and private network features, as well as videoconferencing and virtual home entertainment (VHE). W-CDMA requires a minimum spectrum allocation of 5 MHz, which is an important distinction from the other 3G standards. With W-CDMA, data rates from as low as 8 kbps to as high as 2 Mbps will be carried simultaneously on a single W-CDMA 5MHz radio channel, and each channel will be able to support between 100 and 350 simultaneous voice calls at once, depending on antenna sectoring, propagation conditions, user velocity, and antenna polarizations. W-CDMA employs variable/selectable direct sequence spread spectrum chip rate that can exceed 16 Mega chips per second per user. A common reel of thumb is that W-CDMA will provide at least a six times increase in special in spectral efficiency over GSM when compared on a system wide basis.

3G cdma2000

The cdma2000 vision provides a seamless and evolutionary high data rate upgrade path for current users of 2G and 2.5G CDMA technology using a building block approach that centers on the original 2G CDMA channel bandwidth of 1.25MHz per radio channel. The improvements in cdma2000 1X over 2G and 2.5G CDMA systems are gained through the use of rapidly adaptable base band signaling rates and chipping rates for each user (provided through incremental redundancy) and multi-level keying with in the same gross framework of the original cdmaOne standard. No additional RF equipment is needed to enhance performance- the changes are all made in software or in base band hardware. To upgrade from 2G CDMA to cdma2000 1X, a wireless carrier merely needs to purchase new backbone software and new channel cards at the base station, without having to change out RF system components at the base station.

Cdma2000 1*EV is an evolutionary advancement for CDMA originally developed by Qualcomm, Inc, as a proprietary high data rate (HDR) packet standard to be overlaid upon existing IS-95, IS-95B, and cdma2000 networks, Qualcomm later modifies its HDR standard to be compatible with W-CDMA as well, and in august 2001, ITU recognized cdma2000 1*EV as part of IMT-2000. cdma2000 1*EV provides CDMA carriers with the option of installing radio channels with data only (cdma2000 1*EV-DO) or with data and voice (cdma2000 1*EV-DV). Using cdma2000 1*EV technology, individual 1.25MHz channels may be installed in CDMA base stations to provide specific high speed packet data access within selected cells. The cdma2000 1*EV-DO option dedicates the radio channel strictly to data users, and supports greater than 2.4 Mbps of instantaneous high-speed packet throughput per user on a particular CDMA channel, although actual user data rates are typically much lower and are highly dependent upon the number of users, the propagation conditions, and vehicle speed. Typical users may experience throughputs on the order of several hundred kilobits per second, which I is sufficient to support web browsing, email access, and m-commerce applications. Cdma2000 1*EV-DV supports both voice and data users, and can offer usable data rates up to 144 kilobits per second with about twice as many voice channels as IS-95B.

The ultimate 3G solution for CDMA relies multicarrier techniques that gang adjacent cdmaOne radio channels together for increased bandwidth. The cdma2000 3*RTT standard uses three adjacent 1.25 MHz radio channels that are used together actual throughput depends upon cell loading, vehicle speed, and propagation conditions. Three non-adjacent radio channels may be operated simultaneously an din parallel as individual 1.25 MHz channels (in which case no new RF hardware is required at the base station), or adjacent channels may be combined into a single 3.75 MHz super channel (in which case new RF hardware is required at the base station). With peak user data rates in excess of 2 Mbps, it is clear that cdma2000 3X has a very similar user data rate throughput goal when compared to W-CDMA (UMTS). Advocates of cdma2000 claim their standard gives a wireless service provider a much more seamless and less expensive upgrade path when compared to W-CDMA, since cdma20 cdma2000 allows the same spectrum, bandwidth, RF equipment, and air interface framework to be used at each base station as the 3G upgrades are introduced over time.

3G TD-SCDMA

IN China, GSM is the most popular wireless air interface standard, and the wireless subscriber growth in China is unmatched anywhere in the world. The China Academy of Telecommunication Technology (CATT) and Siemens Corporation jointly submitted an IMT-2000 3G standard proposal in 1998, based on Time Division Synchronous Code Division Multiple Access (TD-SCDMA). This proposal was adopted by ITU as one of the 3G options in late 1999.

TD-SCDMA relies on the existing core GSM infrastructure and allows a 3G network to evolve through the addition of high data rate equipment at each GSM base station. TD-SCDMA combines TDMA and TDD techniques to provide a data-only overlay in an existing GSM network. Up to 384 kbps of packet data is provided to data users in TD-SCDMA [TD-SCDMA]. The radio channels in TD-SCDMA are 1.6 MHz in bandwidth and rely on smart antennas, spatial filtering, and joint detection techniques to yield several times more spectrum efficiency than GSM. A 5 millisecond frame is used in TD-SCDMA, and this frame is subdivided into seven time slots which are flexibly assigned to either a single high data rate user or several slower users. By using TDD, different time slots within a single frame on single carrier frequency are used to provide both forward channel and reverse channel transmissions. For the case of asynchronous traffic demand, such as when a user downloads a file, the forward link will require more bandwidth than the reverse link, and thus more time slots will be dedicated to providing forward link traffic. TD-SCDMA proponents claim that the TDD feature allows this 3G standard to be very easily and inexpensively added to existing GSM systems.

WLAN

Gfeller at the IBM laboratories in Switzerland first introduced the idea of WLAN in 1970s. In 1987 WLAN started as 802.4L standard and 802.11 in 1889. Currently we have 802.11 a,b,e,g,h,i,j,k standards. Channels are specified in 802.11b standard for 2400-2483.5 MHz. This shows a typical wireless LAN configuration, which includes an Access Point and PC cards in notebook or palmtop computers.

Architecture of WLAN

WLAN uses CSMA/CD (Carrier sense multiple access/collision avoidance) mechanism. The receiver reads the peak voltage on cable and compares it with a threshold. It supports three physical layers DSSS, FHSS, DFIR. It works on two frequency hopping techniques i.e. DSSS and FHSS. The protocol layers of IEEE 802.11 are shown below.

Where PLCP is physical layer convergence protocol and PMD is physical medium dependent. LLC provide an interface to higher layers and perform flow and error control MAC layer gives access mechanism. MAC Management provides roaming in ESS and power management and security. PLCP provides carrier sensing assessment. PMD gives modulation and coding. Physical Layer Management tunes channel. Station Management: interacts with MAC and Physical layer.

Distributed Coordination Function (DCF)

It uses CSMA/CA algorithm based on Inter frame Space (IFS).

1) If the medium is idle, the station waits to see if the medium is idle for a time equal to IFS. If so, it may transmit immediately.

2) If the medium is busy, the station defers transmission and continues to monitor the medium.

3) Once the current transmission is over, the station delays another IFS. If the medium remains idle for this period, then it back off the random amount of time and again sense the medium. If the medium is still idle, it may transmit. During the back off time, if the medium becomes busy, the back off timer is halted and resumes when the medium is idle. Additive increase, Multiplicative decrease is a good algorithm for this purpose.

MAC Management

Beacons are sent periodically (every 100ms) by AP to establish time sync. (TSF) and maintain connectivity or associations. Beacons contains BSS-ID used to identify the AP and network, traffic indication map (for sleep mode), power management, roaming. RSS measurements are based on the beacon message. AP and mobile devices form “associations” and mobile device “registers” with AP. After registering can mobiles send/receive DATA.

This shows the handover principle of WLAN, when two signals appear in the same coverage, the receiver will switch its frequency to select the strongest signal. When the WLAN is constructed, it is necessary to consider the signal superposition phenomenon.

Handover of Frequency

Wireless network standards have been aimed at specific market regions such as North America, Europe and Asia. In general wireless networks can be divided into WLAN, Wireless personal area networks (WPAN), Wireless metropolitan area networks (WMAN) and wireless wide area networks (WWAN), including cellular and satellite networks. They are summarized in table below.

NETWORK

STANDARD

DATA RATE

BAND

Cellular Network

GSM (2G)

HSCSD(2G+)

GPRS (2.5G)

EDGE (2.75 G)

UMTS (3G)

9.6 kbps

14-42 kbps

14-128 kbps

128-384 kbps

Up to 2 Mbps

900/1800/1900 MHz

1900-2025 MHz

WLAN

IEEE 802.11 b

IEEE 802.11 a

1, 2, 5.5, 11 Mbps

1-54 Mbps

2.4 GHz

5 GHz

BLUETOOTH

IEEE 802.15.1

721 kbps (BT 1.1)

2-20 Mbps (BT 2.0)

2.4 GHz

WPAN

IEEE 802.15.3

11-55 Mbps

2.4 GHz

WMAN

IEEE 802.16a

75 Mbps

2-11 GHz

WiMAX

IEEE 802.16c

134 Mbps

10-66 GHz

WWAN

IEEE 802.20

2.25 - 18 Mbps

<3.5 GHz

HIPERLAN

HIPERLAN 1

HIPERLAN 2

HIPERLAN 3

HIPERLAN 4

23.5 Mbps

1-54 Mbps

25-100 Mbps

Up to 155 Mbps

5 GHz

5 GHz

40.5 - 43.5 GHz

17 GHz

PRINCIPLE OF GSM

The principle of GSM is frequency reuse. The same frequency sets are used and reused systematically throughout a carrier's coverage area. Frequency reuse distinguishes cellular from conventional mobile telephone service, where only a few frequencies are used over a larger area, with many customers competing to use the same channels.

gsm family radio band spectrum

The frequency spectrum is very congested, with only narrow slots of bandwidth allocated for cellular communications. The following list shows the number of frequencies and spectrum allocated for GSM, Extended GSM 900 (EGSM), GSM 1800 (DCS1800) and PCS1900.

A single Absolute Radio Frequency Channel Number (ARFCN) or RF carrier is actually a pair of frequencies, one used in each direction (transmit and receive). This allows information to be passed in both directions. For GSM900 and EGSM900 the paired frequencies are separated by 45 MHz, for DCS1800 the separation is 95 MHz and for PCS1900 separation is 80 MHz.

For each cell in a GSM network at least one ARFCN must be allocated, and more may be allocated to provide greater capacity.

The RF carrier in GSM can support up to 8 Time Division Multiple Access (TDMA) timeslots. Although this is possible that each RF carrier is capable of supporting up to 8 simultaneous telephone calls, but network signaling and messaging may reduce the overall number from eight timeslots per RF carrier to six or seven timeslots per RF carrier, therefore reducing the no. of mobiles that can be supported.

Frequency Range

GSM 900

Receive (uplink) 890 - 915 MHz
Transmit (downlink) 935 - 960 MHz
124 absolute radio frequency channels (ARFCN)

EGSM 900

Receive (uplink) 880 - 915 MHz
Transmit (downlink) 925 - 960 MHz
174 absolute radio frequency channels (ARFCN)

GSM 1800 (DCS 1800)

Receive (uplink) 1710 - 1785 MHz
Transmit (downlink) 1805 - 1880 MHz
374 absolute radio frequency channels (ARFCN)

GSM 1900 (PCS 1900)

Receive (uplink) 1850 - 1930 MHz
Transmit (downlink) 1930 - 1990 MHz
299 absolute radio frequency channels (ARFCN)

ARFCN

Bandwidth = 200 KHz
8 TDMA timeslots

CELL CONCEPT

Radio Spectrum is very limited that's why we have only 10-25MHz band dedicated to wireless communication. Such narrow bandwidth allows 100-400 channels of reasonable quality, which is not rational and commercially not profitable to develop network for such small number of mobile subscribers. Genius idea lead to the division of whole geographical area to relatively small cells, and each cell may reuse the same frequencies by reducing power of transmission. Each cell has its own antenna (base station), and all base stations are interconnected using microwave or cable communication.

The cells are normally represented by a hexagon, but in practice they are irregular in shape, because:

The terrain makes the structure inappropriate - there may be radio shadows in the cell if the hexagonal shape is used. Obstacles to propagation such as hills or large buildings are placed randomly.
Base stations sites cannot always be put at the cell centre. The best site such as a hill top or a place to put a mast might not be in the centre. Base stations are installed where possible, usually not the ideal location
each cell will not have the same traffic level
power loss along the path is never in a simple analytical form (e.g. inverse square law)

CELL SIZE

The number of cells in any geographic area is determined by the number of MS subscribers who will be operating in that area, and the geographic layout of the area (hills, lakes, buildings etc).

LARGE CELLS

The maximum cell size for GSM is approximately 70 km in diameter, but this is dependent on the terrain the cell is covering and the power class of the MS. In GSM, the MS can be transmitting anything up to 8 Watts; obviously, the higher the power output of the MS the larger the cell size. If the cell site is on top of a hill, with no obstructions for miles, then the radio waves will travel much further than if the cell site was in the middle of a city, with many high-rise buildings blocking the path of the radio waves. Generally large cells are employed in:

Remote areas.
Coastal regions.
Areas with few subscribers.
Large areas which need to be covered with the minimum number of cell sites.

SMALL CELLS

Small cells are used where there is a requirement to support a large number of MSs, in a small geographic region, or where a low transmission power may be required to reduce the effects of interference. Small cells currently cover 200 m and upwards. Small cells are employed in:

Urban areas.
Low transmission power required.
High number of MSs.

The Trade Off -Large vs Small

There is no right answer when choosing the type of cell to use. Network providers would like to use large cells to reduce installation and maintenance cost, but realizes that to provide a quality service to their customers, they have to consider many factors, such as terrain, transmission power required, and number of MSs etc. This inevitably leads to a mixture of both large and small cells.

CELL HIERARCHY

As capacity needs increase, various solutions have to be implemented to provide local extra capacity.

Micro cells provide coverage to one or several streets as well as indoor coverage improvement.
Pico cells provide specific service in given buildings, shopping malls, conference halls.

CELL SPLITTING /SECTORIZATION

At the advent of GSM, subscribers were very few and the radio resources available in each cell were sufficient to cope with the call requests. The cells were called omni-directional and each cell had a single transmitting antenna radiating at 360 degrees. As subscribers' no. grew, some dense urban cells became congested and the need of extra radio resources appeared. The solution was to add extra sites to provide extra channels even if the radio coverage was good enough. This is called splitting/sectorization.

Sectorization splits a single cell into a no. of cells, each cell has transmit and receive antennas and behaves as an independent cell. The size of the cell is determined by the signal strength necessary at the edge of the cell. Each cell uses special directional antennas to ensure that the radio propagation from one cell is concentrated in a particular direction.

This has a no. of advantages:

Firstly, as we are now concentrating all the energy from the cell in a smaller area 60, 120, 180 degrees instead of 360 degrees, we get a much stronger signal, which is beneficial in locations such as “in-building coverage”.

Secondly, we can use the same frequencies in a much closer re-use pattern, thus allowing more cells in our geographic region which allows us to support more MSs.

For capacity limited areas, the BTS manages a given maximum no. of subscribers. To determine the no. of sites necessary to provide the service is simply divide the amount of subscribers located in the area by the no. of subscribers managed by one site.

4 SITE/3 CELL

The sectorization of sites typically occurs in densely populated areas, or where a high demand of MSs is anticipated, such as conference centers/business premises.

A typical re-use pattern used in GSM planning is the 4 site/3 cell. For example, the network provider has 36 frequencies available, and wishes to use the 4 site/3 cell re-use pattern he may split the frequencies up as follows:

Cell

A1

Cell A2

Cell

A3

Cell

B1

Cell B2

Cell

B3

Cell

C1

Cell

C2

Cell

C3

Cell

D1

Cell

D2

Cell

D3

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

In this configuration each cell has a total of 3 carriers and each site has a total of 9 carriers. If the provider wished to recon to a 3 site/3 cell then the result would be:

Cell

A1

Cell

A2

Cell

A3

Cell

B1

Cell

B2

Cell

B3

Cell

C1

Cell

C2

Cell

C3

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

The table shows that each cell now has 4 carriers and each site has 12 carriers. This has the benefit of supporting more subscribers in the same geographic region, but problems could arise with co-channel and adjacent channel interference.

NETWORK COMPONENTS AND ARCHITECTURE

COMPONENTS

GSM networks are made up of Mobile services Switching Centers (MSC), Base Station Systems (BSS) and Mobile Stations (MS). These three entities can be broken down further into smaller entities; such as, within the BSS we have Base Station Controllers, Base Transceiver Stations and Transcoders. These smaller network elements, as they are referred to, will be discussed later in the course. For now we will use the three major entities.

With the MSC, BSS and MS we can make calls, receive calls, perform billing etc, as any normal PSTN network would be able to do. The only problem for the MS is that not all the calls made or received are from other MSs. Therefore, it is also necessary to connect the GSM network to the PSTN.

Mobile Stations within the cellular network are located in “cells”, these cells are provided by the BSSs. Each BSS can provide one or more cells, dependent on the manufacturers' equipment.

ARCHITECTURE

In GSM, each network component is designed to communicate over an interface specified by the GSM standards. The principle component groups of a GSM network are:

The Mobile Station (MS)

This consists of the mobile telephone, fax machine etc. This is the part of the network that the subscriber will see.

The Base Station System (BSS)

This is the part of the network which provides the radio interconnection from the MS to the land-based switching equipment.

The Network Switching System(NSS)

This consists of the Mobile services Switching Centre (MSC) and its associated system-control databases and processors together with the required interfaces. This is the part which provides for interconnection between the GSM network and the Public Switched Telephone Network (PSTN).

Mobile Station (MS)

The MS consists of two parts:

The Mobile Equipment (ME)
Subscriber Identity module (SIM).

The ME is the hardware used by the subscriber to access the network. The hardware has an identity number associated with it, which is unique for that particular device and permanently stored in it. This identity number, called the International Mobile Equipment Identity (IMEI) enables the network operator to identify mobile equipment which may be causing problems on the system.

The SIM is a card which plugs into the ME. This card identifies the MS subscriber and also provides other information regarding the service that subscriber should receive. The subscriber is identified by an identity number called the International Mobile Subscriber Identity (IMSI).

Mobile Equipment may be purchased from any store but the SIM must be obtained from the GSM network provider. Without the SIM inserted, the ME will only be able to make emergency calls.

By making a distinction between the subscriber identity and the ME identity, GSM can route calls and perform billing based on the identity of the ‘subscriber' rather than the equipment or its location.

Mobile Equipment (ME)

The ME is the only part of the GSM network which the subscriber will really see. There are three main types of ME, these are listed below:

Vehicle Mounted

These devices are mounted in a vehicle and the antenna is physically mounted on the outside of the vehicle.

Portable Mobile Unit

This equipment can be handheld when in operation, but the antenna is not connected to the handset of the unit.

Handportable Unit

This equipment comprises of a small telephone handset not much bigger than a calculator. The antenna is connected to the handset.

The ME is capable of operating at a certain maximum power output dependent on its type and use.

These mobile types have distinct features which must be known by the network, for example their maximum transmission power and the services they support. The ME is therefore identified by means of a class mark. The class mark is sent by the ME in its initial message.

Subscriber Identity Module(SIM)

The SIM as mentioned previously is a “smart card” which plugs into the mobile equipment (ME) and contains information about the MS subscriber hence the name Subscriber Identity Module.

SIM TYPES

The SIM contains several pieces of information:

International Mobile Subscriber Identity (IMSI)

This number identifies the MS subscriber. It is only transmitted over the air during initialization.

Temporary Mobile Subscriber Identity (TMSI)

This number identifies the subscriber, it is periodically changed by the system management to protect the subscriber from being identified by someone attempting to monitor the radio interface.

Location Area Identity (LAI)

Identifies the current location of the subscriber.

Subscriber Authentication Key (Ki)

This is used to authenticate the SIM card.

Mobile Station International Services Digital Network (MSISDN)

This is the telephone number of the mobile subscriber. It is comprised of a country code, a network code and a subscriber number.

SIM PROTECTION

The SIM card, and the high degree of inbuilt system security, provides protection of the subscriber's information and protection of networks against fraudulent access. SIM cards are designed to be difficult to duplicate. The SIM can be protected by use of Personal Identity Number (PIN) password, similar to bank/credit charge cards, to prevent unauthorized use of the card. The SIM is capable of storing additional information such as accumulated call charges. This information will be accessible to the customer via handset/keyboard key entry.

Base Station System (BSS)

The GSM Base Station System is the equipment located at a cell site. It comprises a combination of digital and RF equipment. The BSS provides the link between the MS and the MSC.

The BSS communicates with the MS over the digital air interface and with the MSC via 2 Mbit/s links.

The BSS consists of three major hardware components:

The Base Transceiver Station (BTS)

The BTS contains the RF components that provide the air interface for a particular cell. This is the part of the GSM network which communicates with the MS. The antenna is included as part of the BTS.

The Base Station Controller (BSC)

The BSC as its name implies provides the control for the BSS. The BSC communicates directly with the MSC. The BSC may control single or multiple BTSs.

The Transcoder (XCDR)

The Transcoder is used to compact the signals from the MS so that they are more efficiently sent over the terrestrial interfaces. Although the transcoder is considered to be a part of the BSS, it is very often located closer to the MSC. It is used to reduce the rate at which the traffic (voice/data) is transmitted over the air interface. Although the transcoder is part of the BSS, it is often found physically closer to the NSS to allow more efficient use of the terrestrial links.

Network Switching System (NSS)

The Network Switching System includes the main switching functions of the GSM network. It also contains the databases required for subscriber data and mobility management. Its main function is to manage communications between the GSM network and other telecommunications networks.

The components of the Network Switching System are listed below:

Mobile Services Switching Centre - MSC
Home Location Register - HLR
Visitor Location Register - VLR
Equipment Identity Register - EIR
Authentication Centre - AUC
InterWorking Function - IWF
Echo Canceller - EC

In addition to the more traditional elements of a cellular telephone system, GSM has Location Register network entities. These entities are the Home Location Register (HLR), Visitor Location Register (VLR), and the Equipment Identity Register (EIR). The location registers are database-oriented processing nodes which address the problems of managing subscriber data and keeping track of a MSs location as it roams around the network.

Functionally, the Interworking Function and the Echo Cancellers may be considered as parts of the MSC, since their activities are inextricably linked with those of the switch as it connects speech and data calls to and from the MSs.

FORM SPEECH TO RADIO TRANSMISSION

From speech to radio signal, several operations are performed. The reverse transformations are performed on the receiver side. Main operations are the following:

Digitizing

Speech blocks are first digitized to obtain digital blocks: 20 ms = 260 bits.

Source Coding

It use low bit rate code for air interface.

Channel Coding

Channel coding uses codes enabling detection and correction of signal errors. The result is a flow of code words (456 bits long).

Interleaving and Burst Formatting

This spreads the bits of several code words to expand data of the same block in different bursts. The result is a succession of blocks, one block for each channel burst.

Ciphering

Ciphering modifies the contents of these blocks through a “secret recipe” known only by the mobile telephone and the Base Transceiver Station, thus protecting data from eavesdropping.

Modulation

Modulation transforms the binary signal into an analog signal at the right frequency and moment using Gaussian Minimum Shift Keying (GMSK).

Transmission

Transmission amplifies and radiates the resulting signal as radio waves via air antenna.

Diversity

Diversity is different techniques used to provide reception quality.

Demodulation

From the radio waves captured by the antenna, the portion of the received signal which is of interest to the receiver is demodulated.

Deciphering

Deciphering reverses the encryption “secret recipe”.

Burst de-Formatting

Burst de-formatting and de-interleaving puts the bits of the different burst back in order to rebuild the code words.

Channel Decoding

It reconstructs the source information from the output of the demodulator using added redundancy to detect or correct possible errors.

Speech Decoding

Speech decoding operates as suitable filters receiving the voice parameters, and then transforms them out as analog speech.

GSM Features

COMPATIBILITY

The rapid development of analogue cellular networks during the 1980s resulted in many different cellular systems which were incompatible with one another. The need for a common standard for mobile telecommunications was therefore obvious, and so an executive body was set up to co-ordinate the complicated task of specifying the new standardized network.

GSM has been specified and developed by many European countries working in co-operation with each other. The result is a cellular system which has been implemented throughout Europe and many parts of the world. An additional advantage resulting from this is that there is a large market for GSM equipment.

NIOSE ROBUST

In cellular telephone systems, such as AMPs, TACs or NMT the MS communicates with the cell site by means of analogue radio signals. Although this technique can provide an excellent audio quality (it is widely used for stereo radio broadcasting, for example), it is vulnerable to noise, as anyone who has tried to receive broadcast stereo with a poor aerial will testify!

The noise which interferes with the current system may be produced by any of the following sources:

A powerful or nearby external source (a vehicle ignition system or a lightning bolt, perhaps);
Another transmission on the same frequency (co-channel interference);
Another transmission “breaking through” from a nearby frequency (adjacent channel interference);
Background radio noise intruding because the required signal is too weak to exclude it.

In order to combat the problems caused by noise, GSM uses digital technology instead of analogue. By using digital signals, we can manipulate the data and include sophisticated error protection, detection and correction software. The overall result is that the signals passed across the GSM air interface withstand more errors (that is, we can locate and correct more errors than current analogue systems). This leads to better frequency re-use patterns and more capacity.

Flexibility and Increased Capacity

With an analogue air interface, every connection between an MS and a cell site requires a separate RF carrier, which in turn requires a separate set of RF hardware. In order to expand the capacity of a cell site by a given number of channels, an equivalent quantity of hardware must be added. This makes system expansion time consuming, expensive and labor intensive.

Easily RF Cond GSM equipment is fully controlled by its software. Network re-configurations can be made quickly and easily with a minimum of manual intervention required. Also, since one carrier can support 8 users, expansion can be made with less equipment.

Half Rate An enhancement soon to be realized is the half rate speech channel, where mobiles will use new speech algorithms requiring half as much data to be sent over the air interface. By implementing half rate, one carrier will be able to support 16 users, effectively doubling the capacity of the network. However, this is the optimum since the mobile, as well as the BTS, will need to be modified to support half rate.

International Roaming GSM networks also offer the flexibility of international roaming. This allows the mobile user to travel to foreign countries and still use their mobiles on the foreign network. If necessary, the user may leave their mobile equipment at home and carry only the SIM card, making use of a hired mobile or any available equipment.

Better frequency re-use GSMs use of a digital air interface makes it more resilient to interference from users on the same or nearby frequencies and so cells can be packed closer together, which means more carriers in a given area to give better frequency re-use.

Multi-band operation Multi-band networks and mobiles are available where a user can make use of both the 900 MHz network and the 1800/1900 networks. The mobile must be capable of operation in dual frequency bands; however, to the user it will be transparent. This enables network operators to add in capacity and reduce network interference by using cells operating in different frequency bands. The operator will be required to show that they have made efficient use of their existing frequencies before they will be granted access to frequencies in another band. This means using techniques like sectorization, micro cells and frequency hopping.

GSM is highly software dependent and, although this makes it very complex, it also provides for a high degree of flexibility.

IMPROVED SECURITY AND CONFIDENTIALITY

With some of the 1G systems, it has been estimated that up to 20% of cellular phone calls are stolen. When specifying the GSM system, extensive measures have been taken to increase the security with regard to both Call theft and Equipment theft.

With GSM, both the Mobile Equipment (ME) and Mobile Subscriber (MS) are identified. The ME has a unique no. coded into it when it is manufactured. This can be checked against a database every time the mobile makes a call to validate the actual equipment. The subscriber is authenticated by the SIM-Card.GSM also offers the capability to encrypt all signaling over the air interface. With the authentication processes for both the ME and MS, together with the encryption and the digital encoding of the air interface signals, it makes it very difficult for the casual “hacker” to listen-in to personal calls. GSM air interface also supports Frequency Hopping; this entails each “burst” of information being transmitted to/from the MS/base site on a different frequency, again making it very difficult for an observer (hacker) to follow/listen to a specific call.

GSM SERVICES

GSM has the potential to offer a greatly enhanced range of services compared to existing analogue cellular systems.

The services available to a subscriber will be determined by three factors:

1. The level of service provided by the network provider.

2. The level of service purchased by the subscriber.

3. The capabilities of the subscriber's mobile equipment.

speech Services

The following services involve the transmission of speech information and would make up the basic service offered by a network provider:

Telephony

Provides for normal MS originated/terminated voice calls.

Emergency Calls (with/without SIM Card inserted in MS)

The number 112 has been agreed as the international emergency call number. This should place you in contact with the emergency services (Police, Fire, and Ambulance) whichever country you are in.

Short Message Service (SMS) Point To Point

SMS allows the point-to-point transmission of a short message to/from MS, using their IMSI.A short message is an alphanumeric string that can be up to 160 characters long.

Two different types of short messages are defined:

Short message MT/PP (Mobile Terminated / Point to Point)
Short message MO/PP (Mobile Originated / Point to Point)

Point-to-point massages may be sent or received when the MS is engaged on a call (voice or data), or in an idle mode. However, massages which overlap the boundary of such a call, or during a handover, may be lost, in which case they will be sent again. An acknowledgment indicates that the GSM Network has successfully transferred the massage to the mobile telephone or the SC.

Optionally the (service center) SC may offer final delivery notification to the originator. This delivery report indicates whether this particular massage has been correctly received at the receiving station or not, to the extant that the SC is able to establish this. It does not indicate whether the massage has been read. If the delivery report is negative, it includes the failure cause. The delivery report is sent to the originator as soon as the information is available.

A Macro-cell Propagation Model

The Okumara-Hata model is the most commonly used model for macro-cell coverage planning in GSM. It is used for the frequency ranges 150-1000MHz and 1500-2000 MHz. The range of calculation is from 1 to 20 km.

The loss between the transmitting and receiving stations is given as:

L = A + Blog f − 13.82loghbts − a(hm)(44.9 − 6.55loghb)logd + Lother (2.7)

where f is the frequency (MHz), h is the BTS antenna height (m), a(h) is a function of the

MS antenna height, d is the distance between the BS and MS (km), L other is the attenuation due to land usage classes, and a(hm) is given by:

a(hm) = (1.1log fc − 0.7)hm − (1.56log fc − 0.8).

For a small or medium-sized city:

a(hm) = 8.25(log1.54hm)2 − 1.1, for fc ≤ 200MHz (2.8)

For a large city:

a(hm) = 3.2(log11.75hm)2 − 4.97, for fc ≥ 400 MHz (2.9)

The value of the constants A and B varies with frequencies as shown below:

A = 69.55 and B = 26.16 for 150−1000MHz

A = 46.3 and B = 33.9 for 1000−2000MHz.

The attenuation will vary with the type of terrain. This may include losses in an urban environment where small cells are predominant. Then there are foliage losses when forests are present in the landscape. Similarly, the effects of other natural aspects such as water bodies, hills, mountains, glaciers, etc., and the change in behavior in different seasons have to be taken into account.

A Micro-cell Propagation Model

The most commonly used micro-cellular propagation model is the Walfish-Ikegami model.

This is basically used for micro-cells in urban environments. It can be used for the frequency range 800-2000 MHz, for heights up to 50m (i.e. the height of building + height of the BTS antenna) for a distance of up to 5 km. This model talks about two conditions: line-of-sight (LOS) and no-line-of-sight (NLOS).

The path loss formula for the LOS condition is:

P = 42.6 + 26 log d + 20log f.

For the NLOS condition, the path loss is given as:

P = 32.4 + 20 log f + 20 log d + Lrds + Lms.

The parameters in the equations above for the model can be understood from below.

The values of the rooftop-to-street diffraction loss are dependent upon the street orientation, street width and the frequency of operation. The multi-screen diffraction losses are dependent upon the distance and frequency.

Note:

Walfish-Ikegami model can be used also for macro-cells. However, some radio planning engineers do use other models - such as ray tracing - for the micro-cellular environment.

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