Evolution of Mobile Communication

The conversation established between two users with their handheld equipment at different locations is commonly referred as mobile communication. In the beginning, its focus was towards voice but later on, it also dealt with data. Currently cellular phone provide many services including E-mail, internet access, short message service, electronic address book, games and calculator. Further progress is being made to attract users towards commercial product. Subscribers can handle it anywhere due to cellular phones’ portability within frequency range of 825 to 845 MHz.

Advanced Mobile Phone System (AMPS) was the first generation digital cellular wireless network with a data rate of 19.2 Kbps. The popular Global System for Mobile communications (GSM) and Personal Communication Service (PCS) belong to the second generation wireless systems providing 9.6 Kbps data rate, with dedicated channels.

The worldwide development of wireless communication started in the year 1897 by means of radio due to the revolution in the fields like:

1. RF circuit fabrication
2. Large scale circuit integration
3. Digital circuit design
4. Miniaturization technologies

The impact of mobile communication development is personal communication services. Between 1960 and 1970, Bell Laboratories developed the cellular concepts. An exponential growth was observed in wireless communication. Wireless technologies penetration is witnessed more in our day to day lives as compared to other communications. Revolution in the communication field is due to personal as well as cell communication services. The graph below shows a comparison between mobile communication and other technologies.

Cellular mobile communication technology developed worldwide and emerged slowly. At the same time it has penetrated into the market with high demand for long time than other technologies. A significant growth can be seen in the graph.

Police radio systems used Amplitude Modulation (AM) systems in 1934 for transmission. Vehicular ignition noise was the major problem faced by the early cellular systems. In 1960’s, mobile users were not able to directly dial telephone numbers and majority of them were not linked through PSTN. In wireless communications, there is a worldwide increase in number of consumers. Later a better cellular mobile system using Code Division Multiple Access (CDMA) was developed by the Qualcomm, Inc which was then standardized by the respective Telecommunications Industry Association (TIA) and the system was named as Interim Standard (IS-95).

The IS-95 allowed many number of mobile users by Direct Sequence Spread Spectrum (DSSS) technique. The CDMA cellular phone systems were independent of interference problems and provided better call quality than the first generation (1G) AMPS cellular system. Some of the mobile standards of North America, Japan and Europe are listed below.

AMPS - Analog Mobile Phone System.

USDC - US Digital Cellular.

CDPD - Cellular Digital Packet Data.

IS-95 - Interim Standard-95.

JTACS - Japanese Total Access Cellular Systems.

PDC - Pacific Digital Cellular.

NIT - Nippon Telephone and Telegraph Company.

PHS - Personal Handy Phone System.

ETACS - European Total Access Cellular System.

GSM - Global System for Mobile.

CT2 - Cordless Telephone. (CT2)

DECT - Digital European Cordless Telephone.

In the examples of cellular, cordless and PCS systems each one of them has unique advantages and facilities with respect to mobile communication technology. Thus, the transition from analog mobile phones to digital mobile phones was made along a number of years and today digital cellular telephony is very popular worldwide due to its several technical advantages, including cellular coverage capability. Examples of the cellular radio communication.

1. Cellular telephone system.
2. Cordless Telephone (CT) system.
3. Paging system.

These examples are given below.

Cellular System

The cellular telephone system mainly helps to connect a Public Switched Telephone Network (PSTN) and any distant/near user provided the user is available within the corresponding radio range. (A basic cellular system is given below.) The mobile switching center or Mobile Telephone Switching Office (MTSO) connects the mobile units (called parties) to the PSTN. Every cell of the particular geographical area has its own base station with a transceiver, an antenna, and also a control circuitry. The base stations are capable of handling many full duplex cellular communications. The mobile switching center can handle at least 5000 telephonic conversation at a time and 100,000 cellular users/subscribers in a network. The cellular communication is made possible between mobile units and the base stations with the help of Common Air Interface (CM) which specifies four channels. They are:

1. Forward Control Channels (FCC)
2. Reverse Control Channels (RCC)
3. Forward Voice Channels (FVC) and
4. Reverse Voice Channels (RVC).

The control channels mentioned here are also termed as setup channels. They will have calls that are in progress but they usually send and receive data messages carrying call initiation and requests for services.

The Forward Control Channels (FCC) are also termed as "BEACONS.' since they continuously broadcast the traffic requests for the mobile units within the cellular system. As soon as the cell phone is switched on it scans the control channels searching for the strongest signal of a base station. When the call progresses the mobile switching center adjusts the power transmitted (PT) of the mobile unit and alters the channel of the mobile unit and also the base station so as 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 to another base station. Such a call continued process without termination is called as 'Hand off technique. As the mobile moves and the signal strength reduces when it is away from its base station of cell, the next base station of the neighboring cell where the mobile enters in will take charge of the call. A relay like process thus takes place within several base stations of the entire cellular system simply to sustain the call developed between two subscribers.

Whenever a mobile originates a call, a request signal will be sent through reverse control channel. By seeing this request the mobile unit will transmit its Mobile Identification Number (MIN), telephone number of its called subscriber, and the Electronic Serial Number (ESN). Then the MSC will check the proper validity of the signals sent by the mobile and responds to its request by connecting the called subscriber through PSTN.

The mobile communication establishes call, maintains it, and terminates as the call is over. It enables communication even though the distance between subscribers is large.

The cordless telephone systems are full duplex systems and it is intended to link a portable handset to the dedicated base station which in turn is connected to a particular dedicated telephone line. For this specific telephone number on Public Switched Telephone Network (PSTN) is used. The first generation (1G) cordless telephone systems came into existence in 1980's. But the distance the system covered was only few meters.

Cordless Telephone (CT) System

Later the second generation (2G) cordless systems the distance was not a problem and the subscribers used cordless systems in mobile environment also. The system was good only if the subscriber availability was within the coverage of base station.

The cordless system also work together with paging system such that the roaming subscriber can first be paged and he or she can respond to it with the help of cordless telephone. In the simple cordless system shown above it illustrates that the cordless handset is linked to PSTN through the base station (fixed port). The cordless handset has a wireless link with its dedicated base station. The cordless systems are divided into two namely Analog Cf and Digital CT. In the early days these cordless systems were analog (Analog CT). They provided analog voice transmissions and enabled mobility within a limited distances. But they had many demerits such as

1. Poor call qualities
2. Interference

These problems urged the need for digital cordless (Digital CT) systems. They provided better voice quality similar to wired telephone system. Some of the main criteria of CT2 system are

1. Voice signal is digitized through 32 kb/sec Adaptive Differential Pulse Code Modulation (ADPCM) technique.
2. Bit stream compression facility.
3. Final bit stream transmission at a rate of 72 kb/sec through Gaussian Frequency Shift Keying (GFSK).
4. Immune to errors.
5. Supports data transmissions effectively up to 32 kb/sec.
6. Traffic can be separated with the Time Division Duplex (TDD) access technique.

This CM standard does not provide for the mobility status and the later version CT2 + standard was used for this purpose. The paging systems are communication systems and they can transmit brief messages to subscribers. The message sent may be an alphanumeric message, numeric message or even a voice data. Paging systems also include news headlines, faxes and stock quotations. It may be sent to a particular paging subscriber through the paging system access number with a modem or a telephone keypad. Such a message is called as page.

In a technique called 'simulcasting' the wide paging systems sends a page from each base station simultaneously. The important performance metrics used in decision-making process under hand off situations (mobility management) are listed below.

1. Probability of call blocking
2. Probability of call dropping
3. Probability of call completion
4. Handoff delay
5. Rate of handoff
6. Probability of an incomplete handoff
7. Probability of handoff blocking
8. Interruption time duration
9. Handoff probability.

Strategies used to calculate the instant of handoff are:

1. Relative signal strength method.
2. Relative signal strength with hysteris method.
3. Relative signal strength with threshold method.
4. Prediction techniques.

In a wide area paging system a paging control center is available that connects the MIN to different paging terminals.

Wide Area Paging Systems

Thus paging systems enable communication with subscribers irrespective of their roaming state. But the system requires large transmitter powers in the order of kilowatts and uses only low data rates for providing proper coverage.

There are several functionalities possible with cellular mobile phones as shown above which includes the pager functions too. It is helpful in sending short messages which are highly used by subscribers. The short message or page is sent to a subscriber wherever he is, and it is the main advantage of these system in spite of low data rates and large transmitter power requirements.

Important Terminologies:

1. Cell: It is smallest geographical area considered for cellular mobile communication.
2. Base station (BS): Base station provides functionalities between mobile unit and Mobile Switching Center (MSC). The base station is located in each cell and it links the subscriber mobile unit with the MSC.
3. Cell Splitting: In high cellular traffic regions, a larger cell is divided into smaller cells to have complete radio coverage.
4. Handoff: When mobile unit moves from one cell to another cell the call in progress will be handed over form one base transceiver to the base transceiver of the new cell where the mobile unit enters so that the call in progress is not disturbed and such a process is called as “Handoff”.
5. Cell sectoring: A cell can be divided into many sectors. For example, from 3 sectors to 6 sectors in a hexagonal cell. The directional antenna should focus on each sector.
6. Umbrella cell pattern: A single large cell (Macro cell) consists of many small cells (Micro cells) and there will be interaction between the micro and macro cells.
7. Control Channel: They are used for necessary exchange of information related to setting up and establishing cell base stations and the mobile units.
8. Traffic Channels: They are used for carrying data or voice connections between different users.
9. Frequency Reuse: It is a concept followed in cellular communication for efficient spectrum utilization. The same carrier frequency is reused by many cells in a cellular cluster and it is known as 'frequency reuse scheme.
10. Fading: Fading is an effect in mobile radio propagation. It is common in multipath mobile signaling environment.
11. Mobile Telecommunication Switching Office/Mobile Switching Center (MTSO/MSO): It is the main unit that connects the base transceiver station and the Public Switched Telephone Network (PSTN) in mobile communication.

Trends in Cellular Radio and Personal Communications

With the help of digital signal processing, RF technology, network intelligence the personal wireless systems have developed worldwide and provide many number of services to subscribers in their unique way. The Personal Communication Services (PCS) initiated in the United Kingdom and the frequency spectrum allotted was in the range 1800 MHz. It focused on developing Personal Communication Networking (PCN).

The advantage of PCN is that the subscriber can receive or make a call irrespective of the roaming status. The Personal Communication Systems (PCS) includes several network features and provides more personalization, than the available cellular systems.

Then the indoor wireless networking got all the importance due to the better network connectivity within the building premises. One such standard is HIPERLAN compatible with indoor wireless standard and it was developed by European Telecommunications Standard Institute (ETSI).

An important worldwide standard known as Future Public Land Mobile Telephone System (FPLMTS) or International Mobile Telecommunication 2000 (INT-2003) emerged in the year 1995 and it was developed by International Telecommunications Union (ITU). This IMT-2000 is a third generation (3G) standard and some of its advantages are:

1. Global compatibility.
2. Integrate paging, cordless and the cellular mobile system and LEO satellites as a single mobile system.
3. Supports multi-function.

It is an excellent digital mobile radio system accepted worldwide. The satellite mobile systems incorporates good paging systems, data collection, global roaming and emergency communications. One such example is network of LEO satellites.

The fundamental technological developments has thus helped the wireless personal communication systems to grow rapidly and the demand it has is also high. The wireless networking will surely improve further to meet more requirements and additional features in wireless personal communication field.