Cellular network emulationfor specialized mobile radio
Advances in switching and computer control allow smaller SMR operators to transcend technical barriers to providing subscriber cellular-like service, thereby expanding revenue and market opportunities.
The specialized mobile radio (SMR) industry has gone through a technical evolution, although one not as abrupt as developments in the cellular and personal communications services (PCS) industries. Although it has been a “given” that SMR serves different markets than PCS and cellular, technical advances in switching and computer control provide an opportunity for SMR to provide similar service. The following information explains a new method to do an old job and how to become a trendsetter in the SMR industry.
Since the first installation of cellular switching equipment and the inception of the cellular telephone industry in the early and mid-1980s, SMR operators have been asking for a cellular-like operational system for their industry. To date, only one large SMR carrier, Nextel Communications, McLean, VA, has been successful in implementing this type of operational capability on a large geographic basis. A smaller carrier, Hawaiian Wireless, Honolulu, operates a modified Ericsson cellular infrastructure and handsets that use SMR frequencies. The Nextel system uses the Motorola Iden digital format signaling and protocol. A new system design, the CellSMR, a cellular emulation switching system, is available to all SMR carriers. It offers: * cellular-like networking, switching and call-handling. * low-cost base station equipment as well as personal units. * frequency reuse and frequency agility, permitting all SMR channels to be programmed into each base station RF unit and portable or mobile unit.
This system is a joint development of A Communications, Dallas, and Tecom, Taiwan, working together to provide a cellular type of operational efficiency in the SMR frequency bands. The system provides frequency agility and spreads the channel availability from the top to the bottom of the SMR allotted frequency spectrum. The joint engineering, including exclusive modifications to Tecom’s switch design, has been accomplished and the equipment is now available for shipment and installation.
As shown in Figure 1 at the left, this new communications concept uses proven switching technology to emulate the cellular AMPS switching protocol. The AMPS format is embedded in the SMR transmit-receive unit and is used with a low-power-level transmitter combining package. Coupled to a feed-forward linear RF power amplifier and a computer-controlled digital switch, a frequency-agile, two-channel trunked SMR system, with full-duplex, cellular-quality interconnect and a standard dispatch SMR operation, is provided.
The earliest cellular emulation switching subscriber-based systems were launched in China, Taiwan and Guatemala and are now producing revenue. At several U.S.-based installations, beta testing is being performed by Lubbock Communications, Lubbock, TX, Sioux Falls Two-Way Radio, Sioux Falls, SD, and Flagstaff Communications, Flagstaff, AZ, among others.
The site equipment has been FCC type-accepted, and 18 U.S. patent claims have been granted. A patent for the technology was issued in December 1997. Normal installation time on a five-channel system, once uncrated, is less than four hours, including all connectors.
This system is designed for and aimed specifically at the small SMR operators who are continually losing customers to the new competition and technology that has developed during the past 15 years. Without the resources of the “deep pocket” competition and unless the small operators adopt i nnovative strategies, they face the same pattern of declining revenues and business consolidations that have affected some small two-way radio dealerships.
The new technology uses the cellular AMPS signaling protocol evolved by “downbanding” the RF units and compressing the 30kHz channel spacing to 25kHz, matching the SMR spectrum frequency requirement. All cellular software benefits are retained, such as subscriber equipment electronic serial number (ESN) assignment, which permits an almost unlimited subscriber database capability and establishes user codes for national roaming. Furthermore, the proprietary nature and controlled distribution of the system equipment precludes the cellular cloning fraud problem so prevalent in the cellular AMPS signaling.
The system also provides for full-duplex or half-duplex operation, digital audio processing and diversity receivers with power level control that allows adjustable cell coverage and frequency reuse. As with cellular subscriber equipment, the subscriber unit automatically logs into the control channel when the subscriber roams into a new cell with total user transparency. When the subscriber moves out of the coverage area, the ser-vice light goes out, identical to regular cellular.
Low-power-level combining is a design feature combining repeater exciters operating at less than 1W output power level, then amplifying for full required system output power. A 12-channel combiner weighs less than 5 pounds and is less than 2 inches thick; however, it covers the entire SMR frequency band with no retuning required on the system.
The smart “feed-forward” linear RF power amplifiers automatically raise the power level to balance the base station output signal to the subscriber input level. This in-out balance is important, and SMR operators will be required to change the current prevailing attitude, “more power answers all needs.” The in-out balance allows frequency reuse, and an increase in subscribing mobiles, without interference from excessive RF “pollution.”
The demand for portability in personal communications keeps increasing. Hand-held units will continue to replace vehicle-mounted mobile units and full-duplex operation is crucial to the future of SMR operators. Just as important is the ability to adjust to any SMR frequency in the system database similar to cellular, and the cellular emulation switching subscriber equipment provides this ability.
The heart of the system is the computer-controlled digital switch. Intelligent switching between high and low channels, in conjunction with the system’s switching software, provides the SMR operator with more flexibility and opportunity than has been available in the history of SMR. The system may be set up and remotely controlled via modems. Each cell site has a digital cell switch that handles all calls, whether dispatch or interconnect (PSTN), retains all call accounting for billing purposes, assigns the repeater frequency from the look-up frequency table and monitors call progress and status. The cell switch will also monitor the subscriber talk power level for hand-off when available. System-to-system networking with hand-off capability for exact cellular emulation will be available in the second phase equipment by the end of the first quarter of 1998. This feature is being designed for addition to existing equipment.
The system monitors channel activity and determines if a channel is busy or available for use. If a co-channel licensee keys up on a channel assigned to a remote control unit (RCU) or is detected as busy, the control computer immediately goes to a look-up table of frequencies and tells the RCU to go to another channel. This collision detection and avoidance allows low-use channels to be shared by the emulation system and traditional SMR systems. Spectrum usage efficiency is provided by a “dynamic frequency assignment feature.”
Some of the advanced features available in the cellular emulation switching system include: * frequency-agile RCUs that do not require contiguous spectrum because they are capable of operating on any available channel assigned by the control computer. * automatic log-in to the control channel for automatic roaming capability. * subscriber ESN with millions of identification number assignments. * full-duplex interconnect calls, for both portable and mobile units. * diversity receivers for more sensitive receive performance. * simplex PTT dispatch service transmissions that access trunked group member cells only. * small space requirements_one rack for each 20 channels. * easy installation_simply “plug and play” with no tuning required. * detailed call accounting billing. * local and/or remote system control, programming and management. * economical site maintenance and operating cost. Overall, the cellular AMPS signaling protocol assures an ongoing compatibility with new cellular product developments. Additionally, 24 total inputs are included in the 20-channel configuration for future new technology, specifically dynamic carrier multiple access (DCMA) technology under development by ComSpace, Irving, TX (formerly Unique Technologies International).
Conclusion Implementation of the cellular emulation switching system can expand the business opportunities for SMR operators. Small SMR operators can upgrade to the system to effectively compete with large wireless service providers. Spectrum-starved area SMRs can effectively form cooperatives and negotiate low-use channels, pooling frequencies for greater efficiency and resultant income. The system may also effectively benefit an SMR operator in purchasing SMR channels because contiguous channel spacing is not required. Local area incumbents can work together to compete with the new digital-based giant competition. Additionally, networking capability via use of microwave can efficiently extend coverage to any network geography required.
Additional online information about this system can be found at www.cellsmr.com
Tucker is president of UltraTek, Corinth, MS, which provides consulting and marketing services for the switching system described in the article.
System operators considering the potential for specialized mobile radio (SMR) to offer cellular-like service may wish to keep abreast of both technological and service issues in the cellular industry.
For service at cellular frequencies, four standards are now in use in the United States: the analog advanced mobile phone system (AMPS); and three digital standards, Global System for Mobile Communications (GSM) time-division multiple-access (TDMA) and code-division multiple-access (CDMA). The digital standards offer advantages in terms of spectrum efficiency and call privacy or security.
The weight of research and application in the cellular industry now seems to favor CDMA as the dominant digital technology. A report issued in the fourth quarter of 1997 by the Telecommunications Research and Action Center (TRAC), a Washington-based consumer research group, found that although analog systems offer the greatest availability, “all three of the digital technologies provide significant advantages over analog when it comes to services.”
The report selected CDMA as nominally superior to the other digital services in categories such as signal security, enhanced services and reception, and “a clear winner” in comparisons of power needs and system reliability. The report also recommended that consumers may prefer to use dual-mode phones capable of switching between analog and digital services, particularly if inter-city travel or roaming is required, until digital systems are more widely established in the country.
According to a report by Dataquest, the number of cellular subscribers using CDMA systems will surpass those using TDMA by next year, with a concurrent decline in AMPS use, and a moderate growth in GSM use. Within three years, CDMA is projected to have a plurality of subscribers. The TRAC report also notes that CDMA offers advantages in land use and tower siting issues, requiring about one-third fewer cell sites than TDMA and a deployment ratio of about 1:3 with analog sites.
Ericsson, an established TDMA proponent, has also explored the CDMA technology by offering a wideband CDMA interface that uses a 5MHz radio channel instead of the 1.25MHz channel specified for IS-95-based CDMA systems.
These comparative differences and advantages are generally lost on consumers, however. A 1997 survey commissioned by Ameritech Cellular Services, Hoffman Estates, IL, demonstrated that 86% of consumers are unable to differentiate wireless phone service types. Only 8% or respondents were familiar with the terms TDMA, CDMA or GSM, and virtually none knew what the acronyms represented. Similar confusion surrounds personal communications services (PCS), with only 3% of consumers knowing what PCS means.
“These different wireless technologies are not what’s important to customers; what really matters to them is the quality of their service_that their calls go through when they want and need them to do so,” said Herb Hribar, president of Ameritech Cellular Services.
Enhanced services is one of the driving factors in the cellular industry. Ameritech, for example, launched CDMA service in Chicago in July, promoting features such as caller ID, voice mail, and numeric and text messaging services.
Another enhanced service that will affect wireless adoption over the next few years is data transfer, particularly access to the Internet. Unwired Planet, Redwood Shores, CA, for example, is promoting access to the World Wide Web with a platform that connects mobile phones and other wireless devices. CDMA, TDMA and GSM are now deploying data communications protocols that are compatible with this type of access.
Clearly, issues that will affect the success of SMR as a cellular-type provider will include call security, siting, signal clarity and enhanced services, whether those services are integrated or bundled for the customer. If SMR operators choose to pursue the cellular customer market, the technology issue will not be nearly as important as the perceived needs of the users.
