Overview of Cellular Systems

Introduction 


A communication system transmits information from one place to another

whether separated by a few kilometers or transoceanic distance.


The information is carried by electromagnetic waves whose frequency can vary from a few megahertz to several hundred terahertz.


In 1897, Guglielmo Marconi demonstrated the concept of wireless telegraphy. It was built on Maxwell and Hertz work to send and receive the Morse code.


Marconi's technology was based on longwave, spark transmitter technology that needs high power transmitters. His invention was well received at that time as it was the most perfect invention at that time, as anything better than it was scarcely conceivable at that time.


Beginning with this invention the field of wireless communication has experienced various advanced developments. However, it can be said that The Mobile Radio Industry has grown up during the last ten years. A number of factors are responsible for explosive growth in wireless

technology. It includes digital and RF circuit fabrication improvement, new

large-scale circuit integration, and another set of technologies that make

portable radio equipment are cheaper, smaller, and more reliable.


The digital switching techniques have facilitated the large-scale deployment

of affordable, easy-to-use radio communication networks. It is expected to

grow in the near future.


Types of Wireless Communication Systems:


The users are familiar with a number of mobile radio communication systems that are used in

day-to-day life. The pagers, hand-held walkie-talkies, cordless telephones, remote controllers of television, cellular telephones are all types of wireless communication systems.

The cost, performance, complexity, and services offered by these systems varies.


Some Definitions of Terms Used in Wireless Communication System


The term mobile is used to classify a radio terminal that can be moved during operation.

e.g. a cellular telephone can be moved while talking.

The term portable describes a radio terminal that is handheld and is used at walking speed.

e.g. cordless phone at home, a walkie-talkie, etc.

The term subscriber describes a mobile or portable user. This is because in mobile communication systems each user pays a subscription fee in order to use the system. Each device is referred to as a subscriber unit. In wireless communication systems, a group of users is called mobiles. These mobiles communicate to the fixed base stations that are connected to a power source and fixed backbone.

Definitions of Elements of Wireless Communication System:


1. Base station:

It is a fixed station in a mobile radio system. The base station is used for radio communication with the

mobile stations. The base stations are located at the center or on the edge of a coverage region. The base stations comprise radio channels, transmitter, and receiver antennas that are mounted on a tower.


2. Forward channel:

The forward channel is the radio channel used for transmitting information from the base station to the

mobile.


3. Reverse channel:

The reverse channel is the radio channel used for transmitting information from the mobile to the base station.

 

4. Control channel:

Control channel is the radio channel used for transmission of call set up, call request, call initiation,

and other beacon or control purposes.


5. Mobile station (MS): 

A mobile station is recommended for use during motion at unspecified locations in the cellular radio

service. The mobile stations can be hand-held personal units or installed in vehicles.


6. Mobile Switching:

A mobile switching center coordinates with the calls that are routed. In a cellular phone, it connects the

Center base stations and the mobiles to the PSTN. It is also called as mobile telephone switching office (MTSO).


7. Subscriber:

A user who pays subscription charges in order to use Mobile is called a subscriber.


8. Hand-off:

The process of transferring a mobile station from one channel to another is called a hand-off.


9. Page:

A message that is broadcast over the complete service area by many base stations at the same time is called a page.


10. Roamer:

 A roamer is a mobile station that operates in a service area other than that from which the service is

subscribed.


11. Transceiver:

 A transceiver is a device capable of transmitting and receiving radio signals simultaneously.


Types of Mobile Radio Transmission Systems

The types of mobile radio transmission systems are :

(i) Simplex systems

(ii) Half-duplex systems

(iii) Full-duplex systems


(i) Simplex Systems:

Simplex systems are those systems that provide one-way communication i.e. communication is possible in one direction only e.g. a paging system where messages are received, but not acknowledged.


(ii) Half-duplex Systems:

Half-duplex communication systems are systems that allow two-way communication with the help of some radio channel for both transmission and reception. The user can either transmit information or receive information at a given time. Some of the features of such systems are "push-to-talk" and

release-to-listen".


(iii) Full-Duplex Systems:

Full-duplex systems are communication systems that allow simultaneously two-way communication between a subscriber and a base station. The transmission and reception is on two different channels (FDD frequency division duplex) or at adjacent time slots on a single radio channel (Time division duplex TDD) for communication to and from the user.


Frequency Division Duplexing (FDD) and Time Division Duplexing(TDD)


Frequency Division Duplexing (FDD) allows transmission of radio channels simultaneously for the base station and the subscriber so that both can continuously transmit and at the same time receive signals. Separate transmit and receive antennas are used at the base station to incorporate the two separate channels. However, at the subscriber unit, a single antenna is used for transmission to

and reception from the base station.


A duplexer is used inside the subscriber unit to allow the same antenna to be used for transmitting and receiving data simultaneously, To support frequency division duplexing it is essential to separate the

transmit and receive frequencies by 5% of the RF frequency. This is done so that the duplexer can provide isolation.


A pair of simplex channels with a fixed and known frequency separation is used in FDD to define a particular radio channel in the system. This radio channel is used to transmit information from the base station to the mobile user. It is called a forward channel. The radio channel that transmits information from the mobile user to the base station is called a reverse channel. The AMPS standard of U.S. has forward channel frequency 45 MHz greater than that of the reverse channel.

The half-duplex and full-duplex systems use transceivers for radio communication. FDD is used widely in analog mobile radio systems.


Time-division duplexing (TDD) shares a single radio channel in time so that a part of the time can be used to transmit information from the base station to the mobile and the remaining time is used to transmit from the mobile to the base station Information bursts can be stored if the data transmission rate in the channelis greater than the data rate of the user.


Time-division duplexing (TDD) is possible only with digital modulation and digital transmission formats. TDD is very sensitive to timing. Hence, it is used for small area wireless applications where physical coverage distances are smaller e.g. Conventional cellular telephone systems.


Paging Systems:


They are systems that send small messages to a subscriber. The message can be numeric, an alphanumeric message, or a voice message depending on the type of service. The paging systems are used to inform a subscriber that he needs to call a specific telephone number or travel to a known place to receive further instructions. However, modern paging systems can be used to send news

headlines, stock quotations, faxes, etc.


wide area paging system
wide area paging system


The paging control centre dispatches pages received from the PSTN throughout several cities simultaneously. A message is sent to a paging subscriber through the paging system access number i.e. a toll-free number with a telephone keypad or a modem. This message that is issued is called a page.

Then the paging system transmits the page throughout the service area with the help of base stations on the radio carrier. There are different paging systems depending upon the coverage area and

system complexity.The simple paging systems are limited to a range of 2 to 5 km or confined

within individual buildings. The wide area paging systems can provide worldwide coverage. The transmission system is complicated while the receivers are simple and cheap.


As shown in image the wide-area paging systems comprise a number of telephone lines, base station transmitters, large radio towers that can simultaneously broadcast a page from each base station.

The process of simultaneously broadcasting a page from each base station is called simulcasting. Simulcast transmitters can be located within the same service area or in different cities or different countries.


The paging systems provide reliable communication to their subscribers irrespective of their location i.e. whether they are driving on highway or flying in an aeroplane or inside a building. For obtaining reliable communication large transmitter power and low data rates are required for maximum coverage from each base station.


Why are Paging Transmitters Located on Tall Buildings?


The paging systems are designed to provide reliable communication to their subscribers irrespective of the subscribers location i.e. whether they are inside a building or driving on a highway or flying in an aeroplane.

The buildings may attenuate the radio signals by 20 or 30 dB. This makes the location of base stations difficult for the paging companies. Hence, the paging transmitters are located generally in tall buildings in the centre of the city. Simulcasting is also used with additional base stations that are located on the perimeter of the city to cover the complete area. To maximize the signal-to-noise ratio at each paging receiver, small RF bandwidths are used so that low data rates are obtained.


Cordless Telephone Systems:


Cordless Telephone Systems are full-duplex communication systems. They use a radio to connect a handset to a base station, which is then connected to a telephone line with a particular telephone number on the public switched telephone network (PSTN).


In the earlier systems i.e. in first-generation cordless systems the portable unit communicates only to the dedicated base unit and over a distance of few tens of meters. e.g. home use.


In the second generation, cordless telephones allow the subscribers to use their handsets at outdoor locations within the urban centres. They provide coverage ranges up to a few hundred meters.


cordless telephone system
Cordless telephone system

With the advancements in technology, modern cordless telephones are combined with paging receivers. This is done so that the subscriber can first be paged and then he can respond to the page with the help of a cordless telephone.

If a user travels outside the range of the base station then it is not possible to retain the call.

Limited range and mobility are obtained by cordless telephone systems.


Cellular Telephone Systems:


Within the corresponding radio range, the cellular telephone system helps us to connect to the Public Switched Telephone Network (PSTN) and any near/far user.

Below image shows a basic cellular system. It consists of mobile stations, base stations and a mobile switching centre (MSC) or mobile telephone switching office (MTSO).


Cellular systems
Cellular systems


The Mobile Switching Centre (MSC) or the Mobile Telephone Switching Office(MTSO) connects the mobile units (called parties) to the PSTN in a cellular system. Each mobile communicates through radio with one of the base stations and can be handed off to a number of base stations throughout the call.


Every cell of a particular geographical area has its base station with a transceiver, an antenna, and control circuitry. It can be a portable handheld unit or mounted in a vehicle. (The towers represent base stations that provide radio access between the mobile users and the mobile switching centre (MSC)).


The base stations are capable of handling many full-duplex communications as they have towers that support multiple transmitting and receiving antennas.The base station is a bridge between the mobile users and the MSC.


The MSC coordinates the activities of all the base stations and connects the cellular system to the PSTN. It can handle at least 5000 telephonic conversations at a time and 1,00,000 cellular subscribers in a network. It also accommodates all billing and system maintenance functions.


Cellular communication is made possible between the mobile units and the base stations with the help of the Common Air Interface (CAI).

It has four different channels. They are:

(i) Forward Voice Channels (FVC): They are used for voice transmission from the base station to mobiles.

(ii) Reverse Voice Channels (RVC): They are used for voice transmission from mobiles to the base station.

(iii) Forward Control Channels (FCC): They are responsible for initiating the mobile calls.


(iv) Reverse Control Channels (RCC): They are responsible for initiating the mobile calls.


The control channels are also called setup channels as they are responsible for initiating a call and moving it to an unused voice channel. The control channels will have calls that are in progress but they usually send and receive data messages carrying call initiation and requests for service.

The Forward Control Channels (FCC) is called "BEACONS" as they continuously broadcast the traffic requests for the mobile units within the cellular system.


How a Cellular Telephone Call is Made?

As soon as the cell phone is switched on it scans the control channels searching for the strongest signal of a base station. For each cellular system, the control channels are standardized and identical

throughout the different markets within the country or continent. Each mobile scans the same channels when it is idle.


Call initiated by telephone user :

When a telephone user does a call to a mobile user the mobile switching centre (MSC) dispatches the request to all the base stations in the system.


The subscriber's telephone number i.e. mobile identification number (MIN) is broadcast as a paging message on all the forward control channels of the cellular system.Then the mobile receives the paging message sent by the base station that it observes and responds by identifying itself over the reverse control channel.


The base station receives the acknowledgment sent by mobile and informs the MSC about the handshake.Then the MSC instructs the base station to move the call on to an unused voice channel within the cell.


The base station signals the frequencies to modify to unused forward and reverse voice channels. Also, an alert (data message) is sent on the forward voice channel to tell the mobile telephone to ring and the other user to answer the ring. All these events occur very fast and are not noticeable by

the subscriber.


When the call progresses the mobile switching center (MSC) adjusts the power transmitted (Pr) of the mobile unit and the base stations to maintain the call quality even though the mobile unit is non-stationary.


The call-in progress continues irrespective of the frequency changes from one base station to another base station. Such a call continued process without termination is called a "hand-off”.


As the mobile moves and the signal strength reduces when it is away from its base station of the cell, the next base station of the neighboring cell where the mobile enters will take charge of the call. A relay a process thus takes place within several base stations of the complete cellular system to maintain the call developed between two subscribers.


Call initiated by a mobile user:

Whenever a mobile originates a call to a telephone user a request signal is sent on the reverse control channel. By seeing this request the mobile unit will transmit its, Mobile Identification Number (MIN), Electronic Serial Number (ESN) and the telephone number of the called party. The mobile unit also transmits a SCM (Station Class Mark) that indicates the maximum transmitter power level for a user.


The base station receives this information and sends it to the mobile switching center (MSC). Then the MSC will check the proper validation of the signals sent by the mobile and responds to its request by connecting the called subscriber through PSTN.


It instructs the base station and mobile user to move to an unused forward and reverse voice channel pair to allow the conversation to begin. All cellular systems provide a facility called Roaming. It allows the users to operate in service areas other than that from which the service is subscribed.


When a mobile enters another city or area it is registered as a roamer in that Mobile communication establishes call, maintains it, and terminates as The call is over. It enables communication even though the distance between the subscriber is large.


Advantages of Wireless Communications:


(i) Mobility: The primary advantage of wireless communication is to offer the user freedom to move about connected with the network. Wireless technology enables the industries to shift towards an increasingly mobile workforce whether they are in meetings or working on a factory floor or

conducting research.


(ii) Increased reliability: The most common source of network problems is failure or damage of network cables because of environmental conditions or erosion of metallic conductors using wireless technology not only eliminates these types of cable failures, but also increase the reliability of the network.


(iii) Easy installation: They can be easily installed without worrying about providing network connectivity through cables.


(iv) Lower cost: The need to install cables is eliminated using wireless communication and results in significant cost savings.


(v) Rapid disaster recovery: Accidents can happen because of earthquakes, fire, floods at any location and at any time. Any organization that is not prepared to recover from such natural disasters can find itself quickly out of business. As a computer network is a routine part of business, the network

can be easily set up after a disaster. However, a documented disaster recovery is essential.

 

Disadvantages of Wireless Communications:


(i) Radio signal interference: Signals from other wireless devices can disrupt its radio transmission or a wireless device can itself be a source of interference for other wireless devices, e.g. Commonly used wireless devices like cordless phones, elevator motors, microwave ovens, etc. transmit radio

signals that can interface with wireless LAN operation. They can cause errors. Also Bluetooth and WLAN devices both operate in the same radio frequency, potentially resulting in interference between these devices.


(ii) Security: A wireless communication device transmits radio signals over a wide-open area, Hence, security is a major concern. It is possible for an intruder with a notebook computer and wireless N/C to intercept the signals from a nearby wireless network. Because of more of business, network traffic

can contain sensitive information, it becomes a serious issue for many users Some wireless technologies can provide added levels of security with authorization features earlier to gaining access to the network. Network administrators can limit access for approved wireless devices only.


(iii) Health hazards: Biological damage can be produced as a result of High power levels of RF energy

However, we do not know how much levels of RF energy can lead to adverse health effects. The continuous radiations at lower levels can be harmful to sensitive body organs. The RF energy is emitted by radio transmitters in wireless communication systems. Typically these systems emit low levels of RF as they are used. Although research is carried to solve these problems there is no clear information about the biological effects of this type of radiation.


Wireless Communication Applications:

The wireless communication systems are used for the following applications:

(1) Paging and messaging

(2) Cordless Telephone Systems

(3) Wireless local loop

(4) Wireless PAN

(6) Wireless LAN

(6) Cellular phones

(7) Security systems

(8) Television remote control

(9) Wi-Fi

(10) Wireless keyboards

(11) Remote access to medical records.

(12) Bluetooth


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