Cellular sytem : basic concept



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CELLULAR SYTEM : BASIC CONCEPT

There are a variety of wireless communication systems for transmitting voice, video, and data in local or wide areas. There are point-to-point wireless bridges, wireless local area networks, multidirectional wireless cellular systems, and satellite communication systems.

A cell in a cellular system is a roughly circular area with a central transmitter/receiver base station(although the base station may be located off-center to conform to local topology). The station is raised up on a tower or placed on top of a building. Some are located on church steeples. The station has a 360-degree omnidirectional antenna (except when directional transmissions are required) that is tuned to create a cellular area of a specific size. Cells are usually pictured as hexagonal in shape and arranged in a honeycomb pattern. Cell size varies depending on the area. In a city, there are many small cells, while rural area may have very large cells.

When a user turns a phone on, its phone number and serial number are broadcast within the local cell. The base station picks up these signals and informs the switching office that the particular device is located within its area. This information is recorded by the switching office for future reference. An actual call takes place when the user enters a phone number and hits the Send button. The cellular system selects a channel for the user to use during the duration of the call.



Overview:

The first generation of cellular systems used analog radio technology. Analog cellular systems consist of three basic elements: a mobile telephone (mobile radio), cell sites, and a mobile switching center (MSC). Figure 2.1 shows a basic cellular system in which a geographic service area such as a city is divided into smaller radio coverage area cells. A mobile telephone communicates by radio signals to the cell site within a radio coverage area. The cell site s base station (BS) converts these radio signals for transfer to the MSC via wired (landline) or wireless (microwave) communications links. The MSC routes the call to another mobile telephone in the system or the appropriate landline facility. These three elements are integrated to form a ubiquitous coverage radio system that can connect to the public switched telephone network (PSTN).



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Figure 1a). Basic Cellular System block diagram 1b)cross sectional view



Cell systems for frequency re-use:

The method that is employed is to enable the frequencies to be re-used. Any radio transmitter will only have a certain coverage area. Beyond this the signal level will fall to a limited below which it cannot be used and will not cause significant interference to users associated with a different radio transmitter. This means that it is possible to re-use a channel once outside the range of the radio transmitter. The same is also true in the reverse direction for the receiver, where it will only be able to receive signals over a given range. In this way it is possible to arrange split up an area into several smaller regions, each covered by a different transmitter / receiver station.

These regions are conveniently known as cells, and give rise to the name of a "cellular" technology used today. Diagrammatically these cells are often shown as hexagonal shapes that conveniently fit together. In reality this is not the case. They have irregular boundaries because of the terrain over which they travel. Hills, buildings and other objects all cause the signal to be attenuated and diminish differently in each direction.

Interference and System Capacity:

  • Sources of interference

    • another mobile in the same cell

    • a call in progress in the neighboring cell

    • other base stations operating in the same frequency band

    • noncellular system leaks energy into the cellular frequency band

  • Two major cellular interference

    • co-channel interference

    • adjacent channel interference


Co-channel Interference and System Capacity:

  • Frequency reuse - there are several cells that use the same set of frequencies

    • co-channel cells

    • co-channel interference

  • To reduce co-channel interference, co-channel cell must be separated by a minimum distance.

  • When the size of the cell is approximately the same

    • co-channel interference is independent of the transmitted power

    • co-channel interference is a function of

  • Increasing the ratio Q=D/R, the interference is reduced.

  • Q is called the co-channel reuse ratio. For a hexagonal geometry





  • From Analog to Digital Systems:

Analog cellular services were introduced by AT&T in the 1970s and became widespread in the 1980s. The primary analog service in the United States is called AMPS (Advanced Mobile Phone Service). There are similar systems around the world that go by different names. The equivalent system in England is called TACS (Total Access Communications System).

The AMPS system is a circuit-oriented communication system that operates in the 824-MHz to 894-MHz frequency range. This range is divided into a pool of 832 full-duplex channel pairs (1 send, 1 receive). Any one of these channels may be assigned to a user. A channel is like physical circuit, except that it occupies a specific radiofrequency range and has a bandwidth of 30 kHz. The circuit remains dedicated to a subscriber call until it is disconnected, even if voice or data is not being transmitted.



Cellular systems are described in multiple generations, with third- and fourth-generation (3G and 4G) systems just emerging:

  • 1G systems    These are the analog systems such as AMPS that grew rapidly in the 1980s and are still available today. Many metropolitan areas have a mix of 1G and 2G systems, as well as emerging 3G systems. The systems use frequency division multiplexing to divide the bandwidth into specific frequencies that are assigned to individual calls.

  • 2G systems    These second-generation systems are digital, and use either TDMA (Time Division Multiple Access) or CDMA (Code Division Multiple Access) access methods. The European GSM (Global System for Mobile communications) is a 2G digital system with its own TDMA access methods. The 2G digital services began appearing in the late 1980s, providing expanded capacity and unique services such as caller ID, call forwarding, and short messaging. A critical feature was seamless roaming, which lets subscribers move across provider boundaries.

  • 3G systems    3G has become an umbrella term to describe cellular data communications with a target data rate of 2 Mbits/sec. The ITU originally attempted to define 3G in its IMT-2000 (International Mobile Communications-2000) specification, which specified global wireless frequency ranges, data rates, and availability dates. However, a global standard was difficult to implement due to different frequency allocations around the world and conflicting input. So, three operating modes were specified. According to Nokia, a 3G device will be a personal, mobile, multimedia communications device that supports speech, color pictures, and video, and various kinds of information content. Nokia's Web site (http://www.Nokia.com) provides interesting information about 3G systems. There is some doubt that 3G systems will ever be able to deliver the bandwidth to support these features because bandwidth is shared. However, 3G systems will certainly support more phone calls per cell.

  • 4G Systems    On the horizon are 4G systems that may become available even before 3G matures (3G is a confusing mix of standards). While 3G is important in boosting the number of wireless calls, 4G will offer true high-speed data services. 4G data rates will be in the 2-Mbit/sec to 156-Mbit/sec range.


1G - First Generation networks:


1G - First Generation mobile phone networks were the earliest cellular systems to develop, and they relied on a network of distributed transceivers to communicate with the mobile phones. First Generation phones were also analogue, used for voice calls only, and their signals were transmitted by the method of frequency modulation. These systems typically allocated one 25 MHz frequency band for the signals to be sent from the cell base station to the handset, and a second different 25 MHz band for signals being returned from the handset to the base station. These bands were then split into a number of communications channels, each of which would be used by a particular caller.

In the case of AMPS, the first 1G system to start operating in the USA (in July 1978), each channel was separated from the adjacent channels by a spacing of 30 kHz, which was not particularly efficient in terms of the available radio spectrum, and this placed a limitation on the number of calls that could be made at any one time. However, the system was a multiple access one, because a second caller could use the same channel, once the first caller had hung up. Such a system is called "frequency division multiple access" (FDMA).

In addition, because the cell transmitter's power output is restricted and designed to cover a specific area, it is possible to use the same frequencies in other cells that are far enough away for there to be no interference - this system is called frequency re-use, and enables the network capacity to be increased. The cellular structure of the network is also responsible for another feature of cell phone communications, i.e. that it is necessary for some sort of handover to take place when the mobile phone passes from one cell area to another, and this requires that the pair of frequencies used by the phone are changed at the time of handover.

NMT 450, the Nordic Mobile Telephone System using the 450 MHz band, was the first cell phone network to start operating in Europe (i.e. Scandinavia) in 1981. Later, in 1985, the United Kingdom began operations with its TACS (Total Access Communications System). With the introduction of 2G networks, the 1G phones were destined to become obsolete, as they were not adaptable to the new 2G standards and also had other drawbacks, such as their poor security due to the lack of encryption, and the fact that anyone with a receiver tuned to the right frequency could overhear the conversation.

Nowadays, everybody has a mobile phone. This article will be looking at the the mobile phone's history - & its future - in order to discover more about the now-essential telecommunications device.

Ever since the launch of 3G mobile telephone technology, people have been discussing 4-G. 4-G technology will signify the future of mobile telephones, producing the most advanced handsets & best services to date. In actual fact, one of the next services to be developed is thought to be the live streaming of radio and television shows to 3G handsets is & businesses including Disney & Real recently announced that they'll be offering services like these.

This second-generation system, widely deployed in the United States, Canada, and South America, goes by many names, including North American TDMA, IS-136, and D-AMPS (Digital AMPS). For the sake of clarity, we will refer to it as North American TDMA, as well as simply TDMA, when the context makes it clear. TDMA has been used in North America since 1992 and was the first digital technology to be commercially deployed there. As its name indicates, it is based on Time Division Multiple Access. In TDMA the resources are shared in time, combined with frequency-division multiplexing (that is, when multiple frequencies are used). As a result, TDMA offers multiple digital channels using different time slots on a shared frequency carrier. Each mobile station is assigned both a specific frequency and a time slot during which it can communicate with the base station, as shown in Figure 3.4.



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Figure 2: Time Division Multiple Access.

The TDMA transmitter is active during the assigned time slot and inactive during other time slots, which allows for power-saving terminal designs, among other advantages. North American TDMA supports three time slots, at 30 kHz each, further divided into three or six channels to maximize air interface utilization. A sequence of time-division multiplexed time slots in TDMA makes up frames, which are 40 ms long. The TDMA traffic channel total bit rate is 48.6 Kbps. Control overhead and number of users per channel, which is greater than one, decrease the effective throughput of a channel available for user traffic to 13 Kbps. TDMA is a dual-band technology, which means it can be deployed in 800-MHz and 1900-MHz frequency bands. In regions where both AMPS and TDMA are deployed, TDMA phones are often designed to operate in dual mode, analog and digital, in order to offer customers the ability to utilize coverage of the existing analog infrastructure.


Global System for Mobile Communications (GSM)


There are still some analog cellular systems in operations in Europe, but their number is declining, and some regional networks are being completely shut down or converted to Global System for Mobile Communications. The GSM cellular system initiative was initiated in 1982 by the Conference of European Posts and Telecommunications Administrations (CEPT) and is currently governed by European Telecommunications Standards Institute (ETSI), which in turn has delegated GSM specifications maintenance and evolution to 3GPP (reviewed in part in Chapter 1). The intent behind GSM introduction was to have a common approach to the creation of digital systems across. In hindsight, this was a smart political decision, which contributed to the worldwide success of European cellular infrastructure providers and equipment manufacturers.

Let's look at some details of the GSM air interface technology. The GSM standard, similarly to North American TDMA, is based on the use of two simultaneous multiplexing technologies, TDMA and FDMA. Each radio frequency (RF) channel in GSM supports eight time slots (compared to three for North American TDMA) grouped into TDMA frames, which are in turn grouped into multiframes consisting of 26 TDMA frames carrying traffic and control channels. Multiframes are built into superframes and hyperframes. This yields an 8-to-1 capacity increase over NMT or TACS in the same RF spectrum. The allocation of the time slots is essentially static on a short-term basis; for instance, the eighth time slot of a given RF channel is assigned to the same user each time it comes around, whether or not the user has voice or data to send.

The GSM system, emphasizing not only physical properties but also service definitions (unlike some 1G systems), supports three major types of services: bearer services, tele-services, and supplementary services. GSM bearer services allow for transparent or acknowledged user data transfer and define access attributes, information transfer attributes, and general attributes with specific roles. Access attributes define access channel properties and parameters such as bit rate; transfer attributes define data transfer mode (bidirectional, unidirectional), information type (speech or data), and call setup mode; general attributes define network-specific services such as QoS and internetworking options. Tele-services are what GSM subscribers actually use. They are based on the foundation provided by bearer services and govern user-to-user communications for voice or data applications. Examples of tele-services include Group 3 Fax, telephony, Short Message Service (SMS), and circuit data IP and X.25 communications. GSM supplementary services provide additional value-added features such as call waiting, call forwarding, call barring, and conference calling used by wireless operators to further differentiate their offerings.

GSM or GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS is a digital cellphone technology that is based on TDMA (Time Division Multiple Access) and is predominantly used in European countries and it is also used in other countries of the world including India. GSM was developed in the 1980s and was distributed to be used in around 7 European countries in 1992. Nowadays, GSM is used in Asia, Europe, Australia, North America, and Chile. GSM operates on 1.9GHz PCS bands in USA and 900MHz and 1.8GHz bands in Europe. Global System for Mobile Communications defines the entire mobile system and is not just used as a radio interface but also TDMA and CDMA which is Code Division Multiple Access. In the year 2000, world had more than 250 million GSM users which represents more than half of world’s population of cellphone users. GSM use is increasing day by day due to its reliability and easy connectivity services. WAV and AIFF files are used for audio coding of the GSM standard used in IP telephony

Cellular Standards

Keeping track of the analog and digital cellular standards can be difficult. Table W-2 lists the most common standards.



Common
Reference Name

Standard

Category

Frequency Band(s)

Comments

Analog cellular

TIA/EIA-553

FDMA analog cellular

800 MHz

The AMPS standard. Does not support N-AMPS.

Analog cellular (enhanced)

IS-91

FDMA analog cellular

800 MHz

Same as above, but also includes N-AMPS and authentication support.

Narrowband-AMPS (N-AMPS)

IS-88

FDMA analog cellular

800 MHz

Divides one FDMA channel into three smaller channels. Meant for PDA and messaging.

Local AMPS

IS-94

FDMA analog cellular

800 MHz

A low-power cellular system designed for local (in-building) use.

TDMA digital cellular, also
called D-AMPS (digital-AMPS)

IS-54

TDMA digital cellular

800 MHz

Same as AMPS, except uses digital TDMA to divide each channel into three time-slotted channels. Does not directly support data.

TDMA digital cellular (enhanced)

IS-136

TDMA digital cellular

800 MHz

An enhancement to above (TIA/EIA/IS-54) that supports circuit- switched data at 9,600 bits/sec.

CDMA digital cellular

IS-95a

CDMA digital cellular

800 MHz
1,900 MHz

Uses spread spectrum radio and code division multiplexing to put up to
20 conversations on a single band. Data rate is 16 Kbits/sec.

GSM

 

TDMA

900 MHz

GSM was designed by the European community as a digital system to replace analog system.

Table W-2: Wireless mobile standards

Multiple Access Schemes for Cellular Systems

- a summary or tutorial about the basics of the different multiple access schemes including FDMA, TDMA, CDMA and OFDMA, used within cellular technology to allow multiple users to access the cellular system. These include FDMA, TDMA, CDMA and OFDMA.


In any cellular system or cellular technology, it is necessary to have a scheme that enables several multiple users to gain access to it and use it simultaneously. As cellular technology has progressed different multiple access schemes have been used. They form the very core of the way in which the radio technology of the cellular system works.

There are four main multiple access schemes that are used in cellular systems ranging from the very first analogue cellular technologies to those cellular technologies that are being developed for use in the future. The multiple access schemes are known as FDMA, TDMA, CDMA and OFDMA.

Requirements for a multiple access scheme

In any cellular system it is necessary for it to be able have a scheme whereby it can handle multiple users at any given time. There are many ways of doing this, and as cellular technology has advanced, different techniques have been used.

There are a number of requirements that any multiple access scheme must be able to meet:

  • Ability to handle several users without mutual interference.

  • Ability to be able to maximize the spectrum efficiency

  • Must be robust, enabling ease of handover between cells.




FDMA - Frequency Division Multiple Access


FDMA is the most straightforward of the multiple access schemes that have been used. As a subscriber comes onto the system, or swaps from one cell to the next, the network allocates a channel or frequency to each one. In this way the different subscribers are allocated a different slot and access to the network. As different frequencies are used, the system is naturally termed Frequency Division Multiple Access. This scheme was used by all analogue systems.

TDMA - Time Division Multiple Access

The second system came about with the transition to digital schemes for cellular technology. Here digital data could be split up in time and sent as bursts when required. As speech was digitised it could be sent in short data bursts, any small delay caused by sending the data in bursts would be short and not noticed. In this way it became possible to organise the system so that a given number of slots were available on a give transmission. Each subscriber would then be allocated a different time slot in which they could transmit or receive data. As different time slots are used for each subscriber to gain access to the system, it is known as time division multiple access. Obviously this only allows a certain number of users access to the system. Beyond this another channel may be used, so systems that use TDMA may also have elements of FDMA operation as well.


CDMA - Code Division Multiple Access

CDMA uses one of the aspects associated with the use of direct sequence spread spectrum. It can be seen from the article in the cellular telecoms area of this site that when extracting the required data from a DSSS signal it was necessary to have the correct spreading or chip code, and all other data from sources using different orthogonal chip codes would be rejected. It is therefore possible to allocate different users different codes, and use this as the means by which different users are given access to the system.

The scheme has been likened to being in a room filled with people all speaking different languages. Even though the noise level is very high, it is still possible to understand someone speaking in your own language. With CDMA different spreading or chip codes are used. When generating a direct sequence spread spectrum, the data to be transmitted is multiplied with spreading or chip code. This widens the spectrum of the signal, but it can only be decided in the receiver if it is again multiplied with the same spreading code. All signals that use different spreading codes are not seen, and are discarded in the process. Thus in the presence of a variety of signals it is possible to receive only the required one.

In this way the base station allocates different codes to different users and when it receives the signal it will use one code to receive the signal from one mobile, and another spreading code to receive the signal from a second mobile. In this way the same frequency channel can be used to serve a number of different mobiles.




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