2. History: In 1946, cartoonist Chester Gould tickled the fancies of readers of the comic strip Dick Tracy, with a two-way wrist radio. Today, a two-way wristwatch radio is not only feasible; it hardly rates a shrug from anyone who has kept pace with technology. The company’s wrist 3G cell phone far outstrips Dick Tracy’s phone, which was a mere two-way device that lacked the signaling and roaming capabilities of a cell phone, let alone Web browsing. The next version is even more futuristic.
3. It is difficult to remember when owning a cell phone was a matter of prestige; a tool for the affluent, but that was a scant two decades ago. Satellites have removed humanity’s last refuge from the reach of the telephone and users are embracing the service enthusiastically. Cellular and PCS (personal communication service) have demonstrated that the cost and lower quality of ubiquitous telephone service is more than outweighed by the convenience.
4. Users willingly tolerate quality that is often inferior to the PSTN in favor of the convenience, proving that toll-quality connections are not always needed. Cellular telephones became feasible only with solid-state transceivers and electronic switching systems. Meanwhile, conventional two-way mobile radio advanced rapidly. Police, taxicabs, utilities, farmers, and construction workers all rely heavily on conventional mobile and handheld radios. Citizens band radio has long been available to the public at low cost, but mobile and handheld radio connected to the PSTN was too costly for general use.
5. Digital radio, which on cerequired more bandwidth than its analog counterpart, now provides as much as a 20-to-1 bandwidth advantage in the same spectrum. CONVENTIONAL MOBILE TELEPHONE TECHNOLOGY The term mobile radio often is used synonymously with mobile telephone. Although the two services use the same technology and equipment, they differ in these ways: _ Mobile telephones use separate transmit and receive frequencies, enabling full-duplex operation. Mobile two-way radios operate either on the same frequency in a simplex mode or on different frequencies in a half-duplex mode.
6. Specialized mobile radio (SMR) is a hybrid service that has many of cellular’ characteristics, but a different network architecture and frequency assignment. The service is available in a trunked mode, which enables the mobile device to search for an available channel. Conventional mobile telephone service suffers from several drawbacks that led to its replacement. In its heyday, demand greatly outstripped the channels available. Also, a mobile telephone channel is a large party line with the disadvantages of limited access and lack of privacy.
7. When the signal drops off, the user must end the conversation and call on a different channel. PRIVATE MOBILE RADIO SERVICE Mobile radio operates in one of three modes—single-frequency simplex, twofrequency simplex, and duplex. Both of the simplex modes use push-to-talk operation. In a single-frequency mode, the mobile units and base unit send and receive on the same frequency. The base station usually uses separate transmit and receive antennas, but most mobile units use the same antenna to transmit and receive.
8. The base station often is mounted at a remote location to improve coverage and therefore must be remotely controlled. Mobile-to-mobile communication is easy to administer in single-frequency simplex operation because the mobiles can hear each other. In a duplex operation, the mobiles can hear only the base station unless the base retransmits the signal from mobile units.
9. Mobile Unit Signaling The simplest form of signaling a mobile unit is voice calling. The base station calls the mobile unit’s identification, and the mobile unit responds if the operator is within earshot. The simplest form of selective calling relies on the receiver’s squelch circuit. The squelch is the circuitry that deactivates the receiver’s audio in the absence of a received carrier. Busy channels can use a selective calling system similar to that used in mobile telephone can be employed
10. The digital codes operate through a standard known as binary interchange of information and signaling. Trunking Radio: Trunking radio employs multiple channels to improve service to a group of users. The best example of trunking radio is a cellular system, but many private and public safety radio systems also use trunking to improve service. The control channel can be the next idle channel in a sequence, or a talking channel may be designated as control.
11. Trunking systems must resolve the occasional conflict of two stations signaling simultaneously. Two methods are commonly used—polling and contention. Both systems work the same way that channel-sharing methods in data communications systems work. Mobile Radio Design Objectives: The design process is not precise because of the unpredictable nature of radio waves. Obstructions and fading cause the principal disturbances. The mobile radio designer is at a disadvantage because omnidirectional antennas are needed to reach roaming users.
12. As a result, no mobile radio system gives quality that is consistently as good as that provided by wired telephones. Wide Area Coverage: The major objective in mobile system design is to provide coverage that allows a mobile unit to move through a defined area without loss of communication. Cellular is one way of accomplishing this. Adequate Signal Strength Mobile units live in a hostile environment of high noise; most of which is man made, with the predominant source being auto ignition and charging systems.
13. A stationary user may experience a slow fade due to gradual changes in atmospheric reflection. A fade of about 10 cycles per second has the greatest adverse effect on the user and makes communication practically impossible. The range of frequencies that are subject to similar fading effects is called the coherence bandwidth.
14. Wave Propagation Radio waves propagate by one of three methods—the ground wave, the thropospheric wave, and the inospheric, or sky, wave. Each of these acts differently on different frequency ranges and has a significant effect on the propagation characteristics of the signal. The ground wave guides radio frequencies below about 30 MHz. The inospheric wave is most effective below VHF frequencies. The ionosphere reflects high frequencies in a frequency-selective manner.
15. CELLULAR TELEPHONES By the late 1960s all the technical elements for cellular were in place, but in the United States the technology languished through lengthy regulatory debates. Cellular caught on rapidly and exhausted the initial frequencies. The lower half of each band, called the A band, is designated for wireline carriers, which are defined roughly as local exchange companies. The upper half, or B band, is designated for non-wireline carriers, which are non-LEC common carriers.
16. Cellular Technology Cellular overcomes most of the disadvantages of conventional mobile telephone. Frequencies are not duplicated in adjacent cells, which reducess interference between base stations. It also allows the carrier to reuse frequencies within the coverage area with a buffer between cells that are operating on the same band of frequencies. The cells are smaller in urban and larger in rural areas. Mobile units are equipped with processor-driven logic units that respond to incoming calls and shift to radio channels under control of the base station. Each cell site is equipped with transmitters, receivers, and control apparatus.
17. Cell-Site Operation A cell site has one radio transmitter and two receivers per channel, the cell-site controller, an antenna system, and voice and data links to the MTSO. The cell shape is roughly hexagonal because that shape provides a practical way of covering an area without the gaps and overlaps of circular cells. The hand-off between cells is nearly instantaneous, and users are generally unaware that it has occurred. As many as 168 channels per cell can be provided, with the number of channels based on demand.
18. Mobile Telephone Switching Office The MTSO is essentially a digital end office with a special purpose generic program for cellular radio operation. The objective of most service providers is to offer cellular radio features that are essentially identical to wireline telephone features.
19. Mobile, Cellular, and PCS Radio Systems 353 The MTSO links to the cell-site controller with data circuits for control purposes and with four-wire voice circuits for communication channels. When mobile-to-mobile calls or calls from the local telephone system are placed, the MTSO pages the mobile unit by sending messages to all cell-site controllers. Mobile Units The cellular transceiver is a sophisticated device that can tune all channels in an area.
20. Roaming Every cellular operator provides a home area in which normal cellular rates apply.Roaming, which may carry a premium charge, enables a mobile unit to move outside its normal service area. Roaming service is enabled by use of SS7 between the MTSOs. The MTSO maintains home location register and visitor location register databases.
21. Digital Cellular The initial AMPS system used FDM analog modulation with channels spaced at 30 kHz. This provides space for 832 channels, 790 for voice and 42 for signaling, registration, and calling. Cell sites are equipped with some analog and some digital transceivers. In addition to dual-mode mobile phones, the industry offers dual-band phones that can communicate on either the cellular or the PCS bands. PCS, as discussed in the next section, operates in the 1900 MHz band.
22. PCS-1900 is the North American version of GSM. TIA has standardized TDMA as IS-54 and IS-136 and CDMA as IS-95. CDMA uses spread-spectrum techniques to multiply spectrum capacity. CDMA encodes 64 channels into 1.25 MHz of spectrum. PERSONAL COMMUNICATIONS SYSTEM (PCS) The frequency spectrum allocated to PCS in the United States is shown in Some C block licenses are split into multiple licenses. In Europe the service is called personal communication network (PCN).
23. PCS is similar in many ways to cellular, but with several notable exceptions including these: _ Its frequency spectrum is higher—in the 1900 MHz band. _ PCS frequency slots are 200 kHz wide and are divided into eight time slots. _ The cell size is smaller and power is lower. _ The frequency spectrum for PCS in the United States was auctioned off and divided into multiple segments to avoid some of the monopolistic characteristics of cellular. _ The system is intended to bundle other services such as caller ID, paging, and e-mail. Some service providers offer small-screen video.
