Wireless Cable (Wireless Networks)

Wireless Cable
Wireless Cable is the common term assigned to a radio frequency-based alternative to the cable TV distribution system. An example of wireless cable technologies is multichannel multipoint distribution system (MMDS) or local multichannel distribution system (LMDS). By 1998, there were over 10 million wireless customers throughout the world and over 1.1 million in the United States.

Wireless cable system can simultaneously supply local television channels and high-speed data services. MMDS has a high-speed standardized air interface allowing mass deployment of cable television service by the new unregulated telephone companies. Cable television providers, who have access to most homes, can now provide telephone service.

In 1996, some analog MMDS systems began upgrading to digital service. Through the use of digital video compression, digital transmission allows 5 or 6 times the video channel capacity. In addition to video programming, wireless cable can provide telephone service and data services.

Figure 1 shows that the major component of a wireless cable system is the head-end equipment. The head-end equipment is equivalent to a telephone central office. The head-end building has a satellite connection for cable channels and video players for video on demand. The head-end is linked to base stations (BS) which transmit radio frequency signals for reception. An antenna and receiver in the home converts the microwave radio signals into the standard television channels for use in the home. Like traditional cable systems a set-top box decodes the signal for input to the television. Low frequency wireless cable systems can reach up to approximately 70 miles.


Figure 1: Wireless Cable System
Wireless cable is one of the most economical technologies available for the delivery of pay television service. Wireless cable systems do not require extensive networks of cables and amplifiers, bringing the offered price generally lower than a traditional cable service. To the customer, a wireless cable system operates in the same manner as a traditional cable system. Because wireless signals are transmitted over the air rather than through underground or above-ground cable networks, wireless systems may be less susceptible to outages, offer better signal quality and be less expensive to operate and maintain than traditional cable systems. In conventional coaxial cable distribution networks, the television signal quality declines in strength as it travels along the cables and must be boosted by amplifiers thus introducing distortion into the television signal.

To add security for wireless cable systems, so unauthorized users do not gain access to the system (stealing service), signals from video sources are scrambled with a code. The user must have the code to successfully view the video signals. Like traditional cable systems, wireless cable systems employ “addressable” subscriber authorization technology, which enables the system operator to control centrally the programming available to each individual subscriber, such as a pay-per-view selection

There are two primary methods of providing a communication path back from the end customer to the network operator: a telephone line and wireless. Wireless cable systems have typically only provided wireless downlink service (radio transmission from the system to the customer). Some of the new wireless cable systems now dedicate some of their radio channel capacity to uplink channels (from the customer to the system). Uplink channels allow the customer to select programming sources (such as pay per view) or may allow two-way Internet access.

Wireless Piconets (Wireless Networks)

Wireless Piconets
Wireless piconets are very small networks that connect 5 to 10 devices or allow access to each other. Wireless piconets are primarily personal networks that allow accessories such as printers and headsets and other devices to interconnect without the need for wires. These communication systems allow for the connections to be dynamically added and removed between devices.

A particular form of wireless piconet is called Bluetooth. The Bluetooth system is an industry standard wireless data network. Information and the specifications for Bluetooth can be found at www.Bluetooth.com.

Figure 1 shows Bluetooth devices that have created temporary connections. In this diagram, the personal digital assistant (PDA) device is synchronizing (deleting, changing, and adding) addresses with a laptop computer. The laptop computer is also connected to the Internet through a Bluetooth enabled access node. A mobile phone is also synchronizing its phone book listing with the laptop computer. However, because it is out of direct range of communicating with the laptop, it communicates through the access node. The mobile phone is also communicating with a wireless headset.


Figure 1: Bluetooth Wireless Piconet

Residential Cordless (Wireless Networks)

Residential Cordless
Cordless systems are short-range wireless telephone systems that are primarily used in residential applications. Cordless telephones regularly use radio transmitters that have a maximum power level below 10 milliWatts (0.01 Watts). This limits their usable range to 100 meters or less.

The earliest generation of home cordless telephones used a single radio channel that used amplitude modulation. These first generation cordless phones were susceptible to electrical noise (static) from various types of electronic equipment such as florescent lights. The noise encountered when using these phones sometimes created a consumer impression that cordless telephone quality was below standard wired telephone quality. Improved versions of cordless phones that used FM modulation to overcome the electrical noise resulted. As cordless phones became more popular, interference from nearby phones became a problem. In apartment buildings where there were many users of cordless phones in close proximity, the ability to initiate and receive calls could be difficult as radio channels became busy with many users. This led to the development of cordless phones that used multiple radio channels. As voice privacy became more of an issue, cordless phones began to use scrambled voice. Some of these voice privacy systems were analog while a majority of cordless phones that offer voice privacy use digital transmission.

Figure 1 shows the evolution of cordless telephones. Until the mid 1990’s, most cordless telephones were limited to use in a small radio coverage area of their base station that was usually located in the home. That home base station was normally connected to the telephone line of the owner (either residential or a single office telephone line) and they were not intended to serve the general public. To add more value to the use of cordless phones, cordless telephones evolved to allow access to base stations in public locations. Cordless telephones could then be used in the home and in areas that were served by public base stations. The next evolution for cordless telephones was the combination of other types of wireless products and services into the cordless phone. This included the combination of wireless office and cellular telephones into a cordless phone.


Figure 1: Evolution of Cordless Telephone Systems
Most home cordless telephones used frequencies in unlicensed radio frequency bands. Because so many homes operate cordless phones, each manufacturer must build-in circuitry to minimize the interference caused by other cordless devices. The original cordless phones use a very crowded frequency band (around 27 and 49 MHz) utilizing analog radio wave modulation. Recently, cordless telephones have been developed that operate in the 902-928 MHz unlicensed industrial, scientific, and medical (ISM) frequency band.

Residential cordless telephones must automatically coordinate their radio channel access as they operate independently of any type of network control. To coordinate radio channel access and avoid interference to other cordless handsets installed in the vicinity, cordless phones perform radio channel scanning and interference detecting prior to transmitting a signal.

Because cordless telephone systems do not as a rule have a dedicated control channel to provide information, the cordless handset and base station continuously scan all of the available channels (typically 10 to 25 channels). Figure 2 shows the basic cordless telephone coordination process. This diagram shows that when the cordless phone or base station desires to transmit, the unit will choose an unused radio channel and begin to transmit a pilot tone or digital code with a unique identification code to indicate a request for service. The other cordless device (base station or cordless phone) will detect this request for service when it is scanning and its receiver will stop scanning and transmit an acknowledgement to the request for service. After both devices have communicated, conversation can begin. When another nearby base station detects the request for service, it will determine that the message is not intended for it and will not process the call and scanning will continue.


Figure 2: Cordless Telephone System

Wireless PBX | Telecommunications

Wireless PBX
Wireless PBX (wireless office) telephone systems are used in a business environment to provide similar features as a private branch exchange (PBX) with the ability of mobility throughout the office area. The wireless office commonly begins with a specialized wireless private branch exchange (WPBX) that has been adapted for wireless. While more complex than a home cordless telephone, it is not typically as complex as a complete cellular telephone system.

The WPBX telephone radio coverage area is usually within one or more company buildings or on a campus. The more popular WPBX systems use unlicensed frequencies with a protocol available only to the manufacturer of the WPBX. Ordinarily, WPBX telephones cannot be used outside the established campus.

These private WPBX systems use small wall mounted antennas, and like cellular, the space is divided to provide adequate capacity for the expected usage. WPBX telephones, like the one shown in Figure 1, have become commonplace in many hospitals and warehouse environments where the staff is primarily walking around to do their job.


Figure 1: Wireless Office Telephone System
Recent hybrids have been developed whereby the telephone handset has two technologies built into the operation of the phone. When the telephone is inside the WPBX coverage area (preferred) it acts as a private phone; when outside the WPBX coverage area, the phone has the ability to send and receive calls on the public cellular system, incurring airtime charges as any other cellular user.

Satellite (Wireless Networks)

Satellite
Satellite communication systems use of orbiting satellites to relay communications signals from one satellite station to one or several other users. Satellite communication can be divided into categories of fixed satellite service, positioning systems, and mobile satellite communication systems.

There are three basic types of satellite systems: geosynchronous earth orbit (GEO), medium earth orbit (MEO), and low earth orbit (LEO). GEO satellites hover at approximately 22,300 miles above the surface of the earth. GEO satellites revolve along with the earth once a day; they appear stationary with respect to the earth. The high-gain antennas used to receive signals from 22 thousand miles away (usually called “dish” antennas) are pointed directly toward the satellite. MEO satellites are located closer to the earth than GEO satellites and do not as a rule require high-gain antennas. This is important as MEO satellites revolve around the earth several times per day and fixed antennas cannot be used. The newest satellite technology being deployed is LEO satellites. LEO satellites are located approximately 450 miles above the surface of the earth. Because these satellites are relatively close to the earth, portable phones with smaller antennas can be used.

Figure 1 shows the different types of satellite communication systems. The GEO satellite system is primarily used for television broadcast services, as their satellites appear stationary above the Earth. MEO and LEO systems are used for mobile communications as they are located much closer to the Earth. However, these satellites continuously move relative to the surface of the Earth.


Figure 1: Satellite Systems
Mobile satellite telephone service allows customers to use specialized satellite mobile telephones to communicate in any part of the world to the PSTN through the use of communication satellites. Commercial communication satellite services began in the mid-1960’s with the establishment of Intelsat, a multinational organization with well over 130 member nations today. An organization known as the Communications Satellite Corporation (COMSAT) also was established in the early-1960’s and became the United States’ representative in Intelsat. These first commercial applications of satellites provided international telephone and television program transmission, primarily between the United States and Europe.

Aircraft Telephones (Wireless Networks)

Aircraft Telephones
Aircraft telephones allow people on an airplane to initiate telephone calls with the public telephone system through connection via land based radio or satellite transmission systems. Recently, some aircraft telephone systems have been upgraded to allow calls to be received on the airplane.

Aircraft telephone systems are ordinarily a hybrid wireless system that is a terrestrial wireless system (land-based) combined with satellite service. The terrestrial system is used to connect telephone calls when the aircraft is above land and is within distance of a ground transmitter. For the terrestrial-based system, the phone handset in the airplane is connected to a transmitter in the plane’s belly that connects the call down to one of the ground antennas located strategically throughout the country. The call is routed to a ground switching station that connects the call to the receiving party.

The satellite system is used mainly over the water, where calls are out of reach of the ground antennas. For the satellite-based system, the phone handset on the plane is connected to an antenna on the top of the plane that connects the signal up to an orbiting satellite. The call is then sent down to earth by the satellite frequencies to its satellite earth station, then to one of the main ground switching stations that routes the call to the PSTN.

Aircraft phone systems normally have handsets in a common area or handsets that are located in the back of passenger seats. If the handset is located in the seat, some aircraft phone systems allow incoming calls. For someone to reach you on an aviation telephone system, the person on the aircraft must first get an telephone access number and temporary identification code by registering with the aviation telephone operator. The person placing the call from the ground dials the access number and enters the temporary identification code and the call will be routed to the aviation telephone.

Figure 1 shows a public aircraft telephone system. This diagram shows that aircraft may be served by terrestrial (land-based) systems or satellite communication systems. In either case, the aircraft communicates with a gateway that links the radio system to the public telephone system.


Figure 1: Public Aircraft Telephone System

Wireless Local Loop (WLL)

Wireless Local Loop (WLL)
Wireless local loop (WLL) service refers to the distribution of telephone service from the nearest telephone central office to individual customers via a wireless link. In some cases, it is referred to as “the last mile” in a telephone network. This term is a bit misleading, though, because the coverage area of a WLL system may extend many miles from the central office.

Competitive local exchange carriers (CLEC) are competitors to the incumbent local exchange carriers (ILECS) and are likely to use WLL systems to rapidly deploy competing systems. If CLECs do not use wireless systems, they must either pay the existing phone company for access to the local loop (resale) or dig and install their own wire to the local customers. Many countries, that do not have large wired networks such as the United States, are using wireless local loop as their primary phone system.

Figure 1 shows a wireless local loop system. In this diagram, a central office switch is connected via a fiberoptic cable to radio transmitters located in a residential neighborhoods. Each house that desires to have dial tone service from the WLL service provider has a radio receiver mounted outside with a dial tone converter box. The dial tone converter box changes the radio signal into the dial tone that can be used in standard telephone devices such as answering machines and fax machines. It is also possible for the customer to have one or more wireless (cordless) telephones to use in the house and to use around the residential area where the WLL transmitters are located.


Figure 1: Wireless Local Loop
The most basic service offered by wireless local loop (WLL) system is to provide standard dial tone service known as plain old telephone service (POTS). In addition to the basic services, WLL systems typically offer advanced features such as high-speed data, residential area cordless service, and in some cases, video services. To add value to WLL systems, WLL service providers will likely integrate and bundle standard phone service with other services such as cellular, paging, high speed Internet, or cable service.

WLL systems can provide for single or multiple-line units that connect to one or more standard telephones. The telephone interface devices may include battery back up for use during power outages. Most wireless local loop (WLL) systems provide for both voice and data services. The available data rates for WLL systems vary from 9.6 kbps to over several hundred kbps. WLL systems can be provided on cellular and PCS, private mobile radio, unlicensed cordless, and proprietary wideband systems that operate the 3.4 GHz range.

Wireless Data (Wireless Networks)

Wireless Data
Wireless data systems transfer of digital signals between two data devices via a wireless communication path. Most wireless data services are dedicated to specific types of applications. Vertical wireless data applications (vertical) are very specific solutions, and have continued to win over mass market “horizontal” offerings. Vertical solutions include applications such as utility meter reading or mobile dispatch. Horizontal solutions have mass-market appeal such as wireless e-mail.

The growth of the Internet has also enabled low-cost, standardized access to wireless data networks that is accelerating the growth of the wireless data marketplace. In 2001, almost all the new mobile telephones had wireless Internet access capability.

Figure 1 shows a basic wireless data system. In this example, many types of wireless data devices communicate through a public wireless data system. In the core of the system, there is a switching system. The switching system commonly routes the data between the wireless device and a computer system (such as a company computer). In this diagram, there are more receivers than transmitters. This is required to allow low-power mobile data transmitters to reach the system. Base station transmitters can provide up to 500 Watts effective radiated power (ERP) while portable mobile data devices can usually provide less than 1 Watt of transmitted power.


Figure 1: Wireless Data System
Wireless data get the attention it deserves when a mass-market wireless data application (often called the “Killer App”) is embraced by the public. Here are a few successful vertical wireless data applications:

Wireless data for the electric power, waste water, and natural gas industries. New competition in the utility industry demands the benefits of a wireless data solution for timely customer-focused improvements.

Wireless data for field service personnel. Field service organizations use wireless data to close the gap on a geographic distance to improve customer service, technician productivity, and increased revenues.

Companies with mobile sales forces have increased their productivity and efficiency of personnel by filling out much of their paperwork “on-line”. Sales force access to corporate databases has proven paramount in the new paradigm of doing business the 21st century style.

Paging (Wireless Networks)

Paging
Paging is a method of delivering a message, via a public or private communications system or radio signal, to a person whose exact whereabouts are unknown. Users as a rule carry a small paging receiver that displays a numeric or alphanumeric message displayed on an electronic readout or it could be sent and received as a voice message or other data.

Commercial paging service began in 1949 with the allocation of frequencies exclusively dedicated to one-way signaling services. Subscribers used AM receivers, listened for an operator to announce their number, and then called the service to receive their messages. Selective addressing (the ability to choose one individual pager from the group) was introduced in the mid 1950’s and FM was first used in an experimental paging system in 1960. Pagers with alphanumeric displays made their debut in the early 1990’s. In addition to complete messages that can be sent and stored in these pagers, a number of other services such as stock market and sports score reporting have been developed.

There are 4 basic types of messaging services offered by paging systems: tone, numeric, text (alpha), and voice. Two types of paging systems can deliver these messaging services: one-way and two-way paging. One-way paging systems only allow the sending of messages from the system to the pager. Two-way paging systems allow the confirmation and response of a message from the pager to the system as well.

One-way paging is a process where paging messages (signals) are sent from a radio tower to a pager without a return verification signal. In its simplest form, a one-way paging system can serve up to several hundred thousand numeric paging customers.

Figure 1 shows a one-way paging system. In this diagram, a high-power transmitter broadcasts a paging message to a relatively large geographic area. All pagers that operate on this system listen to all the pages sent, paying close attention for their specific address message. Paging messages are received and processed by a paging center. The paging center receives pages from the local telephone company or it may receive messages from a satellite network. After it receives these messages, they are sent after processing to the high-power paging transmitter by an encoder. The encoder converts the pagers telephone number or identification code entered by the caller to the necessary tones or digital signal to be sent by the paging transmitter.


Figure 1: One-Way Paging System
Two-way paging systems allow the paging device to acknowledge and sometimes respond to messages sent by a nearby paging tower. The two-way pager’s low-power transmitter necessitates many receiving antennas being located close together to receive the low-power signal.

Figure 2 shows a high-power transmitter (200-500 Watts) which broadcasts a paging message to a relatively large geographic area and several receiving antennas. The reason for having multiple receiving antennas is that the transmit power level of pagers are much lower than the transmit power level of the paging radio tower. The receiving antennas are very sensitive, capable of receiving the signal from pagers transmitting only 1 watt.


Figure 2: Two-Way Paging System
The number of required receivers for a two-way paging system is dependent on the available transmittal power from the paging and how fast the information is to be transferred. The higher the data transmission rate results in a higher number of required receivers.

The main advantage of two-way paging systems is their ability to require pagers to register their location within the paging system. This allows the paging system to direct pages for a specific pager only to the area near where the pager last registered. This frees up the paging capacity of channels in other geographic areas so paging messages can be sent to other pagers. This is a type of frequency reuse based on geographically separated systems.

Broadcast Television (Wireless Networks)

Broadcast Television
Television broadcasting is the transmission of video and audio to a geographic area that is intended for general reception by the public, funded by commercials or government agencies. Television broadcasters transmit at high power levels from several hundred foot high towers. A high-power television broadcast station can reach over 50 miles.

The standard television system used in the Americas is the National Television Standards Committee (NTSC) system. The first version of this system used 6 MHz RF channels to provide black and white television. The NTSC standard was later modified to allow color television signals to co-exist on the same type of video channel. The television system used in Europe and other parts of the world is phase alternating line (PAL).

The PAL television system was developed in the 1980’s to provide a common television standard in Europe. The PAL system uses 7 or 8 MHz wide radio channels.

Several enhancements have been added to this basic television broadcasting system, including audio stereo sound, additional audio programming channels, very low data rate digital transfer (closed captioning), and ghost canceling.

The NTSC and PAL enhancements are minor when compared to the technological improvements represented by HDTV proposed to provide significantly higher resolution audio and video, as well as data services. A consortium called the Grand Alliance has produced a standard called Grand Alliance HDTV for digital television. The FCC plans to introduce HDTV initially by allowing broadcasters to offer a simulcast of their regular programming, transmitted on UHF television assignments. The period of simulcast will continue for up to 15 years as old broadcast facilities and receivers are phased out. Receivers for the HDTV system will also include the capability to receive and display regular analog broadcasts.

Figure 1 shows a television broadcast system. This television system consists of a television production studio, a high-power transmitter, a communications link between the studio and the transmitter, and network feeds for programming. The production studio controls and mixes the sources of information including videotapes, video studio, computer created images (such as captions), and other video sources. A high-power transmitter broadcasts a single television channel. The television studio is connected to the transmitter by a high bandwidth communications link that can pass video and control signals. This communications link may be a wired (coax) line or a microwave link. Many television stations receive their video source from a television network. This allows a single video source to be relayed to many television transmitters.


Figure 1: Television Broadcast System

Broadcast Radio (Wireless Networks)

Broadcast Radio
Radio broadcasting is the transmission of audio material (called a program) to a geographic area that is intended for general reception by the public, funded by airtime sold between programs.

Amplitude modulation (AM) radio broadcast services have been available for the past 100 years. Most AM radio broadcast systems use relatively low radio frequencies and very narrow radio channel bandwidth to efficiently deliver audio information over large geographic areas. Unfortunately, low frequency used for AM transmission often result in signals that sometimes skip long distances (hundreds of kilometers). This has the potential for interference in distant cities. Amplitude modulation is also easily subject to electrical noise and signal distortion. Recent advancements in AM modulation can allow channel coding for stereo and more reliable (less distorted) radio signals.

To overcome some of the limitations of AM, frequency modulation (FM) was developed. FM transmission is less susceptible to noise and distortion. Unfortunately, most FM broadcast systems use a wider radio channel than AM systems. FM broadcast channels can be up to 20 times the bandwidth of a single AM broadcast channel. The latest advancements in FM broadcasting include conversion from analog to digital and the ability to simultaneously send some additional information (sub-channels) with their audio broadcasts.

The current technology used for FM radio channel broadcast uses less bandwidth than is authorized for transmission. With some modifications to the transmitter, it has been possible for FM broadcast stations to simultaneously send some additional information (sub-channels) with their audio broadcasts. These sub-channels can contain audio or digital information. Sub-channels can be used for data transmission and paging services.

Figure 1 shows a typical radio broadcast system. The radio broadcast system consists of a production studio, a high-power AM or FM transmitter, a communications link between the studio and the transmitter, and network feeds for programming. Radio broadcasting involves the use of various types of information sources called “program sources.” These program sources come from compact discs, tape recordings, soundproof audio studios, remote location sites (such as a van), or other network sources. The production studio controls and mixes the sources of information including audio compact discs, audio studio, audiotape, and other audio sources. A high-power transmitter broadcasts a single radio channel. The studio is connected to the transmitter by a coaxial cable, special leased telephone line (extra high quality), or dedicated radio link. Many radio broadcast stations receive their programming source from a radio broadcast network. This allows a single audio source to be relayed to many radio broadcast transmitters. The diagram also shows how a sub-channel is combined to provide a private audio broadcast service.


Figure 1: Radio Broadcast System
Two separate technologies are being tested to bring digital audio and data services to conventional radio broadcasts. The first incorporates digital data into the conventional FM broadcast by adding the digital data signal to the existing audio signal before FM modulation. The second is a fully digital transmission that is transmitted in addition to the conventional FM. This separate signal is added to the conventional FM signal after the FM modulation. Unlike high definition television (HDTV), these systems do not replace the analog service; they provide additional services and are completely compatible with conventional AM or FM broadcasts. The additional services are available only to those users with a receiver capable of accessing the digital data.

The entry of digital transmission into commercial broadcasting represents a revolution in the types of services that will be available to the public in the near future. Compare the possibilities to the many digital satellite features or the digital programming available with CD players. Imagine pressing one button on the car radio to request only news stations, or your preferred music category.

Digital audio broadcasting (DAB) transmits voice and other information using digital radio transmission. The DAB signal is normally shared with additional digital information on a single digital radio channel.

Cellular and Personal Communication Service (PCS) - (Wireless Networks)

Mobile telephones connect people to the public switched telephone system (PSTN) or to other mobile telephones. Mobile telephone service includes cellular, PCS, specialized and enhanced mobile radio, air-to-ground, marine, and railroad telephone services.

The first mobile telephone system in the United States began in St. Louis, Missouri in 1946. By 1947, more than 25 cities in the United States had mobile telephone service available. The systems used a single high-power transmitter for the base station in the center of a metropolitan area. Coverage was provided for 50 miles or more from the transmitter. These initial systems used a human operator at the base station to manually connect the mobile user with the landline network. In most of these systems, service was very poor because too many customers (called subscribers) shared each radio channel (called loading). It was not uncommon to have busy channels over 50% of the time. Despite this poor service, it revolutionized the definition of telephone service and priority was given to police and ambulance service. The waiting list for mobile phones in some cities was more than 7 years. This type of system was improved many times and the last upgrade, called improved mobile telephone service (IMTS), was introduced in the mid 1960’s. While there may still be some original systems in operation throughout the United States, new equipment for these systems is not currently being produced. It has been replaced with cellular systems.

Cellular and Personal Communication Service (PCS)
Cellular and PCS mobile telephone systems allow mobile telephones to communicate with each other or to the public telephone system through an interconnected network of radio towers. In early mobile radio-telephone systems, one high-power transmitter served a large geographic area with a limited number of radio channels. Because each radio channel requires a certain frequency bandwidth (radio spectrum) and there is a very limited amount of radio spectrum available, this dramatically limited the number of radio channels that kept the serving capacity of such systems low. For example, in 1976, New York City had only 12 radio channels to support 545 customers and a two-year long waiting list of typically 3,700.

When linked together to cover an entire metro area, the radio coverage areas (called cells) form a cellular structure resembling that of a honeycomb. The cellular systems are designed to have overlap at each cell boarder to enable a “hand-off” (also called a “handover”) from one cell to the next. As a customer (called a subscriber) moves through a cellular or PCS system, the mobile switching center (MSC) coordinates and transfers calls from one cell to another and maintains call continuity.

Figure 1 shows a mobile telephone system. The wireless network connects mobile radios to each other or the public switched telephone network (PSTN) by using radio towers (base stations) that are connected to a mobile switching center (MSC). The mobile switching center can transfer calls to the PSTN.


Figure 1: Mobile Telephone System
When a cellular system is first established, it can effectively serve only a limited number of callers. When that limit is exceeded, callers experience too many system busy signals (known as blocking) and their calls cannot be completed. More callers can be served by adding more cells with smaller coverage areas - that is, by cell splitting. The increased number of smaller cells provides more available radio channels in a given area because it allows radio channels to be reused at closer geographical distances.

There are two basic types of systems: analog and digital. Analog systems typically use FM modulation to transfer voice information and digital systems use some form of phase modulation to transfer digital voice and data information. Although analog systems are capable of providing many of the services that digital systems offer, digital systems offer added flexibility as many of the features can be created by software changes. The trend at the end of the 1990’s was for analog systems to convert to digital systems.

To allow the conversion from analog systems to digital systems, some cellular technologies allow for the use of dual-mode or multi-mode mobile telephones. These telephones are capable of operating on an analog or digital radio channel, depending on availability. Most dual-mode phones prefer to use digital radio channels in the event both are available. This allows them to take advantage of the new features such as short messaging and digital voice quality.

Cellular systems have several key differences that include the radio channel bandwidth, access technology type (FDMA, TDMA, CDMA), data signaling rates of their control channel(s), and power levels. Analog cellular systems have very narrow radio channels that vary from 10 kHz to 30 kHz. Digital systems channel bandwidth ranges from 30 kHz to 1.25 MHz. Access technologies determine how mobile telephones obtain service and how they share each radio channel. The data signaling rates determine how fast messages can be sent on control channels. The RF power level of mobile telephones and how the power level is controlled typically determines how far away the mobile telephone can operate from the base station (radio tower).

Wireless Networks - Technologies

Key enabling technologies for wireless communication include digital modulation, data compression, and digital signal processing.

Digital Modulation
Digital modulation is the process of modifying the amplitude, frequency, or phase of a carrier signal using the discrete states (On and Off) of a digital signal.

When modulating a carrier signal using a digital information signal, this causes rapid changes to the carrier wave. These rapid changes result in the creation of other signals that are usually undesirable. As a result, digital modulation usually includes a process of adjusting the maximum rate of change of the input signal (rounding the digital signal edges) and filtering out some of the unwanted signals that are created during the transition.

Figure 1 shows different forms of digital modulation. This diagram shows ASK modulation that turns the carrier signal on and off with the digital signal. FSK modulation shifts the frequency of the carrier signal according to the on and off levels of the digital information signal. The phase shift modulator changes the phase of the carrier signal in accordance with the digital information signal. This diagram also shows that advanced forms of modulation such as QAM can combine amplitude and phase of digital signals.


Figure 1: Digital Modulation
Data Compression
Data compression is a process that is used encoding information so that fewer data bits of information are required to represent a given amount of data. Compression allows the transmission of more data over a given amount of time and circuit capacity. It also reduces the amount of memory required for data storage.

Access Multiplexing
Access multiplexing is a process used by a communications system to coordinate and allow more than one user to access the communication channels within the system. There are four basic access multiplexing technologies used in wireless systems: frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access, (CDMA), and space division multiple access (SDMA). Other forms of access multiplexing (such as voice activity multiplexing) use the fundamentals of these access-multiplexing technologies to operate.

FDMA systems use a process of allowing mobile radios to share radio frequency allocation by dividing up that allocation into separate radio channels where each radio device can communicate on a single radio channel during communication. TDMA systems allow several users to share a single radio channel by dividing the channel into time slots. When a mobile radio communicates with a TDMA system, it is assigned a specific time position on the radio channel. By allow several users to use different time positions (time slots) on a single radio channel, TDMA systems increase their ability to serve multiple users with a limited number of radio channels. Code division multiple access (CDMA), a form of spread spectrum communication. CDMA is a method of spreading information signals (typically digital signals) so the frequency bandwidth of the radio channel is much larger than the original information bandwidth.

Some systems coordinate system access on the same radio channels that are used for communication and other systems use a separate (dedicated) control channel. When using a control channel to coordinate access to the system, it is called an access control channel. The access control channel coordinates the random requests for service that is received from users (mobile radios) in the system. The control channel may also transfer identification information that allows the system to determine if the user is authorized to receive access to the system.

Figure 2 shows the common types of channel-multiplexing technologies used in wireless systems. This diagram shows that FDMA systems have multiple communication channels and each user on the system occupies an entire channel. TDMA systems dynamically assign users to one or more time slots on each radio channel. CDMA systems assign users a unique spreading code to minimize the interference receive and cause with other users. SDMA systems focus radio energy to the geographic area where specific users are operating.


Figure 2: Channel Multiplexing

Wireless Networks - Market Growth

In 2001, approximately 1 in 8 people in the world were using mobile telephones. The growth of some vertical wireless data markets is over 80% per year.

Mobile Telephone Service
By 2001, there were 781 million mobile telephone subscribers in the world. Figure 1 shows the recent trend in subscribership to mobile telephone services. Some of the key drivers for continual growth include lower monthly cost of service and pre-paid wireless services. Pre-paid wireless service allows customers with bad or damaged credit to forego the normal credit check required with wireless service and pay for their service before they use it. Many of the new wireless subscribers have credit challenges.


Figure 1: Mobile Telephone Wireless Growth.
Source: GSM MOU

Data Networks
With the demand for high data rate communications solutions, paralleling interest in the Internet, (fueled by easy-to-use application software, its wide array of text, graphics, video and audio content), wireless data market growth has increased substantially. The availability of Internet services over wireless radio channels will be a critical factor in determining overall market growth.

To date, most wireless data applications are non-human in nature. These include applications such as monitoring wireless parking meters, vending machines, and environmental concerns among others. Human access includes the ability to access data available on the Internet, private intranets, new services, and e-mail. The Internet, for example, is being used by businesses for building interactive branding via communication with customers, advertising products and services, publishing product specifications; and acting as a source for point-of-sale applications.

Market growth for wide area wireless data communications services is in the early stages, primarily because wireless data is not yet capable of providing high data transfer rates at a cost comparable to fiber optic cable or wired connectivity. However, the overall market growth of the wireless data market is up. In 1997, there was over 21% growth for circuit switched data (primarily cellular data) and over 89% growth for packet data (ARDIS, RAM, CDPD, and Ricochet).

Wireless Networks - Radios

Wireless networks are composed of radios, radio towers or base stations, interconnection systems, and network management and information systems.

Radios
Radios may be fixed in location (such as a television) or may be mobile (such as a cellular telephone). Some radios may only communicate in one direction (typically a receiver) or may have two-way capability. When a single radio has both a transmitter and receiver contained in the same unit, it is called a transceiver.

Figure 1 shows a block diagram of a mobile radio transceiver. In this diagram, sound is converted to an electrical signal by a microphone. The audio signal is processed (filtered and adjusted) and is sent to a modulator. The modulator creates a modulated RF signal using the audio signal. The modulated signal is supplied to an RF amplifier that increases the level of the RF signal and supplies it to the antenna for radio transmission. This mobile radio simultaneously receives another RF signal on a different frequency to allow the listening of the other person while talking. The received RF signal is then boosted by the receiver to a level acceptable for the demodulator assembly. The demodulator extracts the audio signal and the audio signal is amplified so it can create sound from the speaker.


Figure 1: Mobile Radio Block Diagram
Radio Towers and Transmitter Equipment
Radio towers are poles, guided towers, or free standing constructed grids that raise one or more antennas to a height that increases the range of a transmitted signal. Radio towers can vary in height from about 20 feet to more than 300 feet. A single radio tower may host several antenna systems that include paging, microwave, or cellular systems. Radio towers are located strategically around the city to provide radio signal coverage to specific areas. At the base of the towers are electronic control rooms that contain the components to operate the radio portion of the communications system.

Radio towers and their associated radio equipment (e.g., base station) may include one or more antennas, transmitters, receivers (for two-way systems), system controllers, communication links, and power supplies. Transmitters provide the high level RF power that is supplied to the antenna. For broadcast systems, the amount of transmitter power can exceed 50,000 Watts. Receivers boost and demodulate incoming RF signals from mobile radios. If a base station contains receivers, it is typical to use one or more different antennas for the receivers. Controllers coordinate the overall operation of the base station and coordinate the alarm monitoring of electronic assemblies. Communication links allow a command location (such as a television studio or a telephone switching center) to control and exchange information with the base station. Base station radio equipment requires power supplies. Most base stations contain primary and backup power supplies. A battery typically maintains operation when primary power is interrupted. A generator may also be included to allow operation during extended power outages.

Figure 2 shows a typical radio base station block diagram that is used in a mobile telephone system. This diagram shows that the base station holds the radio transceiver (transmitter and receiver assemblies) that is part of the radio tower (cell site). This diagram also shows that one antenna is used for transmitting and two antennas are used for receiving (for improved reception). This base station also contains a backup battery that is maintained at full charge so radio communications will not be interrupted in the event AC power is lost.



Figure 2: Radio Tower and Base Station Equipment
Switching Facilities
Switching facilities are typically used in two-way mobile communication systems to allow the connection of mobile radios to other radios in the system or to the public telephone network. When used in a cellular system, the switching system is typically called a mobile switching center (MSC). The MSC, just like a local telephone company, processes requests for service from mobile radios (subscribers) and routes the calls to other destinations.

Figure 3 illustrates a wireless switching system basic functional components. These include: communication line interfaces, a switch, a customer database, system and communication controllers, primary and backup (batteries) power, and the software to interface and control the radio tower’s and base station (BS) it is connected to.


Figure 3: Wireless Switching System Block Diagram
Interconnection to Other Networks
Wireless systems may be connected to other networks. Broadcast wireless systems are connected to media sources (such as audio or video programs) via satellite links while cellular networks may be interconnected to the public telephone network. Interconnection involves the physical and software connection of network equipment or communications systems to the facilities of another network such as the public telephone network. Government agencies such as the Federal Communications Commission (FCC) or Department of Communications (DOC) regulate interconnection of wireless systems to the public telephone networks to ensure reliable operation.

Customer Databases
Customer databases are computer storage devices (typically a computer hard disk) that hold service authorization and feature preferences of customers. For wireless systems that allow the customer to operate in other territories, a home (local) database is used. Each wireless subscriber has a real-time user profile in the database that is typically called the home location register (HLR). The HLR identifies the current location of the mobile radio, the most likely place for the mobile to be, or the last location the subscriber was active. The MSC system controller uses this information to route calls to the appropriate radio tower for call completion. If the wireless user is not in a predetermined “home” range of the MSC, the mobile will register back through to the home signaling system to its home location register (HLR) for profile information.

When customers use the wireless services of systems outside of their home area, their information is transferred to a database in that system called the visitor location register (VLR). The VLR is part of a wireless network (typically cellular or PCS) that holds the subscription and other information about visiting subscribers that are authorized to use the wireless network.

System Security
In some wireless networks, access to system services requires validation of the customer’s identity. These systems may use an authentication center (AUC) to store and process secret data to stop fraudulent calls or prohibit access to other paid for subscription services.

Wireless phones transmit some of their identification information over the public airwaves when they attempt to access the system. Thieves may try and intercept this information and copy (clone) the identification information that would allow them to make phone calls that would be billed to the other telephone. To prevent this unauthorized duplication of identification information, an authentication process can be used that uses secret keys to validate access information.

During the authentication process, code keys are created from secret codes that are stored in both the mobile radio and in the system. Along with basic identification information, these keys are exchanged during each system access attempt. The secret codes are not transmitted. Because the system and the mobile radio have the secret keys, both the mobile phone and the system can validate that the code information is correct. If the codes do not match, the system should not allow the call to be processed. New codes are created during each access attempt to prevent copying of the codes and immediately attempting access.

Wireless Networks: Radio Frequency (RF)

Radio Frequency (RF)
The radio frequency spectrum is divided into frequency bands that are authorized for use in specific geographic regions. Globally, the International Telecommunications Union (ITU) specifies the typical use for radio frequency bands. Within each country, government agencies create and enforce the rules for which specific types of systems and services are used in specific frequency bands and which companies will be able (will be licensed) to own and operate these systems.

Licensing
The national government is responsible for dividing the available frequency bands for licensing to users and regulates what the frequencies may be used for. The legal right-to-use of this public resource is controlled by rules and licensing of very specific frequencies, a range of frequencies or a block of sub-divided channels at a given frequency or frequency range.

For example, the frequencies allocated for FM radio must be used for the purpose licensed; that is a combination of music or news and public information. FM radio stations are not licensed to broadcast a secret “Morse-code” to a following of undercover militia! Neither can a “Paging Service” use one or all of their frequency channels to broadcast radio. However, with the recent deregulation of telecommunications services, wireless service providers are now permitted to offer many new types of services provided they can fulfill their basic licensing requirements.

To prevent unwanted interference from radio devices, the reckless use of transmitting energy or information on our public airwaves according to publicly published rules or licenses will violate federal law. Such transmissions are subject to prosecution or suspension of the radio operator’s license.

Frequency Allocation Charting
There are thousands of wireless applications that are assigned to many different frequency bands. The selection of the assigned frequency bands is determined by a variety of factors including the radio propagation characteristics and the availability of radio channel frequencies at the time.

Because most of the frequencies have already been assigned to licensees, a new assignment of frequencies typically requires existing licensees or users to stop using a band. These users are typically shifted to another band. This process is called re-allocation.

Historically, major re-allocations are done in the higher frequencies to avoid congestion. This has advantages and disadvantages. The radio frequency (RF) devices employed within the newer systems are subject to more loss based on distance. This requires closer distances, increasing the total number radio sites to cover the same area previously covered by radio devices at a lower frequency. However, the higher frequencies tend to penetrate buildings more readily and the antennas involved are physically smaller - both important attributes for systems that seek to reach 100% of the available population.

RF Channels and Bandwidth
An RF channel is a communication link that use radio signals to transfer information between two (or more) points. To transfer this information, a radio wave (typically called a radio carrier) is modulated (modified) within an authorized frequency band to carry the information. The modulation of the radio wave forces the radio frequency to shift above and below the reference (center) frequency. Typically, the more the modification of frequency, the more information can be carried on the radio wave. This results in RF channels typically defined by their frequency and bandwidth allocation.

Bandwidth allocation is the frequency width of a radio channel in Hertz (high and low limits) that can be modulated to transfer information. The amount type of information being sent determines the amount of bandwidth used and the method of modulation used to impose the information on the radio signal.

A government regulation agency (the FCC in the United States) defines a total frequency range (upper and lower frequency limits) that a radio service provider can use to transmit information. In some systems (such as AM or FM radio station broadcasting), this is a single radio channel. For other systems (such as cellular, PCS, or PCN), this is a range of frequencies that can be sub divided into smaller radio channels as determined by the radio carrier. When the allocated frequency range is further subdivided into smaller allowable bands, these subdivided areas are referred to as channels.

Mobility and Fixed Wireless
Most applications use wireless to allow mobile service. However, many fixed applications of wireless are practical. There is a general data transmission rate tradeoff between mobile and fixed wireless systems. Mobile wireless systems have a relatively low data transfer rate (typically below 28 kbps) while fixed wireless systems can have data transfer rates that exceed 45 Mbps. The primary advantage of fixed wireless service is the ability to focus radio transmissions to a particular direction or region. This typically reduces interference to and from other radios and increases the capacity (data transfer rate) available to the fixed wireless device. The basic types of fixed wireless systems in use include wireless computer networks, competing wireless television systems, and wireless local telephone service.