The integration of paging and cellular technologies: Combining two popular wireless communications devices involves technical challenges including limited dimensions and insulating a low-noise paging receiver from a strong cellphone transmit signal and ra
Paging and cellular technologies are a lot like milk and cookies: Each fills a need on its own, but together they provide a much higher level of user satisfaction.
Backed by research that showed 30% of cellular users carry pagers, Motorola was convinced that there was a market for a product that integrated the two popular forms of personal communications. Many individuals_lawyers, business executives, real estate agents, students of all ages_often carry both devices. “Double” wireless users like the ease and discreet nature of paging while enjoying the immediacy and personal security provided by cellular. A product that combined the two, we concluded, was a natural next step in advanced communications.
Integrating cellular and paging technologies into a single unit was a novel idea, but the actual task proved far more complicated than simply affixing a pager to the back of a cellular phone. To develop this paging-cellular integrated product, Motorola initially formed a research and development team consisting of engineers from both its cellular phone group and Derivative Technologies Division (DTD) (a branch of Motorola’s Paging Group created to develop new uses for the company’s existing paging technology).
Originally, the design called for the incorporation of the pager into the cellular phone, enabling the pager’s numeric message to appear on the phone’s display screen. To receive a page, however, the user would have to keep the phone on all the time, causing an accelerated decrease in the battery’s power. Because batteries deteriorate after each charge, the life span of the battery would be greatly reduced. Another significant problem with this design was that if you happened to be on the phone, you would be unable to receive a page.
After more tests, we developed a product that enabled the two units to be powered by the same battery, yet function independently by means of the use of two numbers. Pagers take only small amounts of energy from their power center, and the integrated device could function when used with a single, cellular phone battery. The design team then decided to place the user interface buttons of the pager and its display screen on the back of the battery. From an engineering standpoint, before the product could be brought to market, several design aspects had to be considered. With all the cellular phones on the market, could a paging product be designed that could successfully interface with all the various phone platforms? The engineering team determined that the Motorola MicroTAC and retail pocket personal cellphone platform, which consists of analog and digital time-division, multiple-access (TDMA) phones in North and South America, had the greatest potential for success with a single pager design and form factor. If the pager could fit all of the flip phone products in the installed base, the engineering team believed it had a huge potential market.
After the design platform was chosen, the engineering team encountered several mechanical and electrical design challenges. Mechanically, the engineers had to resolve the issue of how a numeric pager and a cellular phone battery could fit into a housing with the same dimensions as a cellphone’s battery.
Initially, this task seemed insurmountable, but the team was able to fit a two-board numeric pager and 500mAh nickel-metal hydride (NiMH) battery into the housing. To achieve this, the design team had to make the two-piece housing extremely thin, relying on the strength of the Lexan material and complex internal support structures to keep the product from being damaged in a mobile, high-impact environment. In addition, the NiMH cell pack was redesigned and repackaged into a thin, removable cell pack.
Not only did the mechanical design have to withstand the most rugged environments, but it had to fit all variations of the cellphone models, chargers and vehicular accessories. This was not an easy feat, considering the many critical design dimensions and tight tolerances that had to be met for a perfect fit. After several design revisions, the mechanical team produced a rugged design that fit all the cellphone models and accessories and included a small, removable battery pack.
The electrical engineering team also faced many performance challenges. The biggest was to insulate the low-noise paging receiver from the strong cellular phone transmit signal. The engineer-ing team started with a proven pager design, modified it and then added some enhancements.
Immediately, there were problems with the transmitter signal level. It caused interference so severe that pages could not be received. The phone transmit frequency that was in the passband of the paging receiver’s initial filtering stages was hindering the normal operation of several stages of the low-noise receiver. This interference was eliminated with a combination of shielding, additional signal filtering and a new pager antenna design. An added plus: The mechanical design that sandwiched the battery pack between the phone and the paging receiver helped to increase the distance between the phone’s circuitry and the pager’s receiver, minimizing the radiated board-to-board interference. After this modification, the sensitive paging receiver operated normally regardless of whether the phone was off, in standby or transmitting during a call.
In addition to the United States, we wanted to market the pager on an international level, creating new electrical challenges. Typically, the international paging bandsplits are much lower in frequency than those in the United States. This, along with new types of digital and analog cellular phone systems that are prevalent worldwide, generated a new set of issues relating to pager performance. In the international market, there was no longer the issue of the phone’s transmit signal interfering with the receiver because the frequency ranges were separated by a larger margin.
Instead, the problem was radiated noise from the phone’s circuit boards inundating the pager. The complex digital signal processing (DSP) circuitry in the phones runs at extremely high speeds and generates a large amount of noise in the processor’s harmonics. At high levels in the paging receiver’s bandsplit, this noise must be eliminated by physical separation and shielding. The mechanical design and layout of the new pager and battery pack, coupled with extensive shielding in cellular phones and the pager, minimized the board-to-board radiated interference.
The design team’s creation is now called the RSVP pager. The pager is being marketed as a cellphone accessory that provides complete, cost-effective wireless accessibility. Because the pager has its own number and works independently of the cellphone, users can keep the phone turned off, extend battery life and still receive pages. By just giving the pager number, users can use the device as a call screener, choosing to return the call either immediately or waiting to make calls from a more cost-effective location. Many cellular phone users turn to pagers because of recurrent dead batteries and expensive cellular air time, so cost-cutting characteristics are beneficial. The future challenges for the pager include the issue of longer battery life and an inevitable move to a smaller, lighter version of the pager because of mark-et demands. And with the explosive growth of alphanumeric paging, do not be surprised if word messages are soon transmitted.
