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TOPIC:  MOBILE COMMUNICATIONS

         MOBILE COMMUNICATIONS.

ABSTRACT:

The most modern telephone is the cellular telephone, or commonly Called a cell phone. A cellular telephone is designed to give the user maximum freedom of movement while using a telephone.

Mobile communications is a hot topic. The number of mobile communication devices users is growing very fast. The number of mobiles (cellular phones) is now exceeding the number of fixed lines in many countries (Finland, Japan etc.).

Cellular/mobile phones are everywhere and their utility is growing. A cell phone is a radio telephone, that may be used wherever "cell" coverage is provided. The role of cellular phones has risen with improvement in services, reduction in service costs and the ever increasing services available through cell phones.

This paper covers  the basic architecture of Cellular Communications and concentrates mainly on GSM.

Content:

1.      Introduction & problems of Cellular Communication.

2.      Basic Cellular Architecture

3.      Frequency Reuse and Cell design

4.      Hand off procedures

5.      GSM Specifications

6.      GSM Subscribers services

7.      Conclusions

Introduction & problems of Cellular Communications:

Ubiquity is a pinnacle that the cellular communication sector has hoped to reach for the past five years. To reach this goal, a series of networks must be built that allow consumers to use their phone anytime, anywhere. The truth is ubiquity is far from becoming a reality. Across the world cellular carriers can't seem to agree on a single air interface for wireless operation. But, despite battles on the standards front, the wireless community has pushed forward in its efforts to build mobile networks and phones that deliver worldwide coverage. To make this happen, they have focused their attention on developing multimode systems that can support CDMA, TDMA, GSM, GPRS, wideband CDMA (W-CDMA), and a host of other air interfaces in the same box.

Global systems for Mobile Communications:

Cellular Mobile Networks are very complex, however  there are a few fundamental features of a cellular system  that differentiates it from other radio systems such as

broad TV, radio and wireless LANS:

• Geographic Sharing (Frequency Re-use)

• Cell Splitting

• Handoff

• Mobility

This tutorial gives a brief introduction into the main components of a cellular network and explains the cellular concepts as listed above.

• MS – Mobile Station. Usually referred to as a mobile phone or cell phone.

• BTS – Base Transceiver Station. Usually referred to as a Base Station or Cell Site.

Transmits and receives calls to the Mobile Station. Controlled by a BSC.

• BSC – Base Station Controller. Each BSC controls a number of BTS’s and is

responsible for switching calls to the correct BTS (i.e. the one the subscriber is currently

closest to). Also responsible for call handovers. Controlled by the MSC.

• MSC – Mobile Switching Centre. The switching gateway between the mobile network

and the PSTN. Responsible for call control and subscriber management. A network may

have multiple MSC’s depending on the size of the network.

• VLR – Visitor Location Register. A database containing the location and information

relating to visiting subscribers (i.e. subscribers roaming from another network).

• HLR – Home Location Register. A database containing the location and information

relating to network subscribers (so the BSC knows where the subscriber is and can send

calls to the correct BTS).

• EIR – Equipment Identity Register. A database that keeps track of stolen phones so that

calls cannot be made to or from a stolen mobile.

• AuC – Authentication Centre. Authenticates that the subscriber is who they says they

are, by sending a secret number to the mobile and verifying the mobile’s response.

• PSTN – Public Switched Telephone Network. The worldwide network of copper cable

and switches that allows people to call one another from anywhere in the world.

Early mobile telephone systems operated similar to radio stations, with one high powered

transceiver covering large geographic areas.

Capacity problem is overcome by geographic sharing (frequency re-use). By

replacing high powered transceivers with many low powered transceivers, the

same channels can be re-used in a consistent pattern, as long as there is enough

distance between the re-used frequencies to avoid interference.

In an Attempt to further increase network capacity, the idea of cell splitting was conceived.

Cell splitting involves segmenting the base station into sections, with a separate antenna for each section.

Standard Cell Sectored Cell:

Instead of an omni directional antenna radiation pattern (360o), the cell is split into three

sectors, with each antenna radiating a 120o coverage pattern. Each sector acts like its own

base station, with a particular group of frequencies assigned to it. The fact that the antenna  beam is more directional means the mobile phone usually gets a stronger, cleaner signal. A  majority of cellular systems use cell splitting.

The ultimate purpose of cellular telephony is mobility. The problem with

mobility is that as a subscriber moves further away from the base station during a

call, the signal between the mobile phone and the base station gets weaker while

interference from adjoining cells increases.

Handoff was introduced to solve the problem of subscribers getting too far away

from the transmitting base station.

Handoff occurs when the mobile network automatically transfers a call from a

radio channel in one base station to another radio channel in an adjacent base station as

the subscriber crosses into the adjacent base stations cell area.

The procedure for handoff :

As the subscriber approaches the cell border, the call signal drops to a minimum threshold, at which point the mobile informs the network of this fact.

• The network then must find an unused channel on the appropriate adjoining base station

and sends the mobile the details to switch to the new channel (channel number ect).

• The mobile phone then switches to the new channel, without the subscriber even noticing.

However, some problems still occur:

• When the mobile needs to handoff and the adjoining basestation is at full capacity, there

are no free channels to switch to, so the call is dropped.

• The call is sometimes lost between handoff as the adjoining base station doesn’t lock into  the call before the first base station hands it off.

To reduce these problems, other handoff procedures were implemented:

• Mobile Assisted Handoff (MAHO)

• Soft Handoff

Mobile Assisted Handoff:

Used in current GSM systems, MAHO makes use of the mobile phone to help in

collecting statistics regarding the most suitable base station to hand over to. The mobile

phone makes signal strength measurements from the surrounding base stations and sends

the measurements back to the network. The network then uses these figures, as well as

others (such as the spare channels available in each base station), to make a more

informed decision about which base station to hand the call over to.

Soft Handover:

Used in current CDMA systems, soft handover is similar to MAHO except that in this

configuration the first base station does not let go of a call until there is confirmation from the second base station that they have received the call. When the confirmation comes from the second base station that the handoff is complete, the first base station releases the call. This has the effect of greatly reducing the amount of dropped calls, reducing the Bit Error Rate (BER), and improving call quality.

Cellular Telecommunication Systems are designed with a tradeoff between

efficient spectrum use and complexity and cost of the system. In a bid to extract the most from the limited spectrum bandwidth allocated to them and service as many customers as possible, network operators use smaller low powered base stations to take advantage of frequency reuse, resulting in an escalation in base station costs. Cell splitting also increases the need to handoff calls, increasing switching subsystem complexity.

GSM Specifications:

Specifications for different personal communication services (PCS) systems vary  among the different PCS networks. Listed below is a description of the  specifications and characteristics for GSM.

•frequency band—The frequency range specified for GSM is 1,850 to 1,990 MHz (mobile station to base station).

•duplex distance—The duplex distance is 80 MHz. Duplex distance is the distance between the uplink and downlink frequencies. A channel  has two frequencies, 80 MHz apart.

•channel separation—The separation between adjacent carrier  frequencies. In GSM, this is 200 kHz.

•modulation—Modulation is the process of sending a signal by  changing the characteristics of a carrier frequency. This is done in GSM  via Gaussian minimum shift keying (GMSK).

•transmission rate—GSM is a digital system with an over-the-air bit  rate of 270 kbps.

•access method—GSM utilizes the time division multiple access  (TDMA) concept. TDMA is a technique in which several different calls

may share the same carrier. Each call is assigned a particular time slot.

•speech coder—GSM uses linear predictive coding (LPC). The purpose of LPC is to reduce the bit rate. The LPC provides parameters for a filter that mimics the vocal tract. The signal passes through this filter, leaving behind a residual signal. Speech is encoded at 13 kbps.

GSM is a combination of FDMA &  TDMA.The total No. of channels are 124 and each channel is 200kHz.Both 935-960MHz uplink and 890-915 downlink have allocated 25MHz for a total of 50MHz  and 45 for duplex. In TDMA with in a  200kHz, 8 time slots of 4.615ms frame duration and burst duration of 0.557ms .

GSM Subscriber Services:

There are two basic types of services offered through GSM: telephony (also referred to as teleservices) and data (also referred to as bearer services).

Telephony services are mainly voice services that provide subscribers with the complete capability (including necessary terminal equipment) to communicate with other subscribers. Data services provide the capacity necessary to transmit appropriate data signals between two access points creating an interface to the network. In addition to normal telephony and emergency calling, the following  subscriber services are supported by GSM:

•dual-tone multifrequency (DTMF)—DTMF is a tone signaling scheme often used for various control purposes via the telephone  network, such as remote control of an

answering machine. GSM supports full-originating DTMF.

•facsimile group III—GSM supports CCITT Group 3 facsimile. As standard fax machines are designed to be connected to a telephone using analog signals, a special fax converter connected to the exchange is used in the GSM system. This enables a GSM–connected fax to communicate with any analog fax in the network.

•short message services—A convenient facility of the GSM network is the short message service. A message consisting of a maximum of 160 alphanumeric characters

can be sent to or from a mobile station. This service can be viewed as an advanced form of alphanumeric paging with a number of advantages. If the subscriber's mobile unit is

powered off or has left the coverage area, the message is stored and offered back to the subscriber when the mobile is powered on or has reentered the coverage area of the network. This function ensures that the message will be received.

•cell broadcast—A variation of the short message service is the cell broadcast facility. A message of a maximum of 93 characters can be broadcast to all mobile subscribers in a certain geographic area. Typical applications include traffic congestion warnings and reports on accidents.

•voice mail—This service is actually an answering machine within the network, which is controlled by the subscriber. Calls can be forwarded to the subscriber's voice-mail box and the subscriber checks for messages via a personal security code.

•fax mail—With this service, the subscriber can receive fax messages at any fax machine. The messages are stored in a service center from which they can be retrieved by the subscriber via a personal security code to the desired fax number.

GSM supports a comprehensive set of supplementary services that can complement and support both telephony and data services. Supplementary  services are defined by GSM and are characterized as revenue-generating features. A partial listing of supplementary services follows.

•call forwarding—This service gives the subscriber the ability to forward incoming calls to another number if the called mobile unit is not reachable, if it is busy, if there is no reply, or if call forwarding is allowed unconditionally.

•barring of outgoing calls—This service makes it possible for a mobile subscriber to prevent all outgoing calls.

•barring of incoming calls—This function allows the subscriber to prevent incoming calls. The following two conditions for incoming call barring exist: baring of all incoming calls and barring of incoming calls when roaming outside the home PLMN.

•advice of charge (AoC)—The AoC service provides the mobile subscriber with an estimate of the call charges. There are two types of AoC information: one that provides

the subscriber with an estimate of the bill and one that can be used for immediate charging purposes. AoC for data calls is provided on the basis of time measurements.

•call hold—This service enables the subscriber to interrupt an ongoing call and then subsequently reestablish the call. The call hold service is only applicable to normal telephony.

•call waiting—This service enables the mobile subscriber to be notified of an incoming call during a conversation. The subscriber can answer, reject, or ignore the incoming call. Call waiting is applicable to all GSM telecommunications services using a circuit-switched connection.

•multiparty service—The multiparty service enables a mobile subscriber to establish a multiparty conversation—that is, a simultaneous conversation between three and six subscribers. This service is only applicable to normal telephony.

•calling line identification presentation/restriction—These services supply the called party with the integrated services digital network (ISDN) number of the calling party. The restriction service enables the calling party to restrict the presentation. The restriction

overrides the presentation.

•closed user groups (CUGs)—CUGs are generally comparable to a PBX. They are a group of subscribers who are capable of only calling themselves and certain numbers.

Conclusion:

The cellular & mobile beyond any doubt, has emerged as the most powerful medium effecting our lives in many way no other medium has ever. It has brought the world and its people closer than imagined before. But what makes it an indispensable part of our lives is the enormous potential that it provides for growth and success in our fields.

Now cellular with advanced  concepts like picture messages and internet access are to give a roaming net, there by increasing our knowledge, be it a fun, education or employment internet with cellular could change our lives and enhance our performance in what ever duties we have been given to handle.