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).

Telecom Made Simple

Related Posts with Thumbnails