Yesterday’s networks may provide tomorrow’s profits For a network upgrade, choose a protocol that increases existing speed by several factors, that has subscriber units with quality equal to or better than existing protocol units and with sufficient avail

How well your one-way system is optimized is a subjective matter, even though the problems involved with optimizing are well-known. If your batching algorithms and RF network optimization provide adequate paging airtime in the busy hour, fine. Or maybe that was yesterday. “Adequate” airtime may not exist today, and given the paging industry’s projected growth, it most certainly will not be here tomorrow. The paging business is highly competitive. Subscriber-unit growth is critical to success. As more people take advantage of the convenience of pagers and messaging devices, the old business equation applies: the greater the revenue, the greater the profits. The problem that most operations managers face is how to get more revenue out of existing channels.

The radio spectrum is a finite resource. Each paging system channel has a specific limit to the number of pagers it can support. This capacity is defined by the baud rate of the transport medium, the busy hour call average (BHCA) and other factors such as message length. As the number of paging calls increases, available airtime is consumed until the channel reaches its limit. Hence, management, investment and often regulatory decisions must be made to maintain continued growth.

Since the early ’80s, POCSAG baud rates have increased to a point where the protocol’s over-the-air integrity was challenged. When pushed to 2,400 baud, not only was the POCSAG code stressed, but many existing transmission networks began to experience synchronization difficulties. The networking solution was overcome by the new store-and-forward simulcast technologies. However, that still left the trusted, but outdated and stressed, POCSAG protocol.

For the industry to continue to grow and to meet the demand for improved one-way communications, a new universal data transport protocol was needed. Where one-way POCSAG was the high-speed code for the ’80s, a new approach was needed to meet the challenges of the next century. Industry analysts at MTA-EMCI, Washington, project the growth of global paging to escalate from its present 70 million subscribers to 137 million subscribers by the year 2000. Combining existing POCSAG technology with whatever new technology meets tomorrow’s needs obviously makes the most sense.

Since 1989, several new paging transport protocols have been introduced. First on the scene was the European Radio Messaging System (ERMES). This was followed in 1993 by Motorola’s Flex family of transport protocols. Philips, with the assistance of the software company Envoy, developed and introduced the advanced paging operators code (APOC), an advanced version of POCSAG, in 1994. All of these enhanced-code formats use four-level modulation, or a combination of both two- and four-level modulation. To make more efficient use of existing POCSAG channels, increasing both revenue and profits, a four-level modulated format is the answer.

Available 4-level protocols ERMES is a 6,250-baud, four-level, modulated protocol. It was commissioned in the mid-1980s by the European Union (EU). A robust code, it provides improved spectral efficiency. Perhaps its greatest contribution is its standard of network interfaces. To conform with its roaming specification, ERMES requires compliance to a block of 16 defined VHF channels. Each member nation or service provider is allocated as many as four channels. Compliance to the block of 16 VHF channels often places impossible demands on spectrally stressed urban centers. Although not associated with the code’s performance, beta tests in Germany ran into unforeseen cable and television interference (TVI) problems. The German Bundespost has yet to resolve these spectrum problems with its EU neighbors. To date, France is the most successful ERMES implementer, with three regional systems operating in and around Paris.

Although Europe is considered the stronghold of ERMES, the fortress is beginning to show signs of cracking. The German experience with TVI and cable interference has weakened the resolve of the German government to follow the EU doctrine of excluding other protocols. Because of these problems and other public pressures, the German government recently issued a new paging license to DFR, a German communications company. The license provides DFR with the right to test and, with Bundespost approval, implement other transport protocols. DFR publicly supported the one-way Flex protocol and, with Bundespost approval, will begin commercial service using the 3,200bps Flex in Spring 1996.

ERMES, which is not compatible with POCSAG, would require a complete trade-out of existing infrastructure equipment and associated pages. This action would be expensive and time-consuming, and it would place existing customer revenues at risk. Hence, ERMES does not qualify as an integrated-channel, POCSAG growth solution.

As a four-level protocol, Philips’ APOC was a late arrival. It offers many features, including spectrum efficiency, a robust code and an effective transmission speed of 6,400 baud. Possibly its most attractive feature is its compatibility with POCSAG, which allows it to cohabit the same channel with POCSAG. By using a data dictionary embedded within the pager software, APOC reduces the over-the-air data required to complete a given message. It is claimed that, depending on the amount of message commonality, the data dictionary feature can increase the throughput to 380kb/hr.

APOC’s late arrival has undoubtedly placed it last behind the Flex and ERMES protocols in the race to become the successor to POCSAG. It is reported that an APOC system beta test was conducted in Great Britain in 1995. However, little international media attention was given to the beta test and, to date, no commercial APOC system build-out has been reported. In the global arena, little interest in APOC has been shown. Further, APOC and ERMES lost to the Flex protocol when Japan selected a version of the Flex technology called Flex Time Diversity as its national high-speed paging protocol.

Since the Flex protocol was announced in 1993, this family of transport protocols has clearly become the global leader. Motorola has invested heavily in the ongoing development of this transport protocol family and is committed to maintaining its evolution as the wireless world evolves. The January 1996 announcement by the Chinese Ministry of Posts and Telecommunications of China’s adoption of the Flex transport protocol secures it as the global de facto high-speed transport standard. The Flex protocol was designed to be backward-compatible with POCSAG. Messages in both the Flex and POCSAG protocols can be batched in a terminal and broadcast using the same transmitter network. One-way Flex is a robust protocol that delivers messages even in high-fade areas. Its flexible baud rate allows delivery at 1,600bps, 3,200bps and 6,400bps. In the lower delivery speeds of 1,600bps and 3,200bps, it can operate as either a two- or four-level modulated code. This feature allows carriers to immediately start a progressive migration toward increased capacity, higher speeds and increased profits without a heavy financial outlay. The Flex family of protocols is not frequency-dependent, and the protocols provide a graceful and financially manageable migration path to high-speed paging.

What also sets the Flex family of protocols apart from the other contenders is its highly publicized two-way paging technology. The two-way Flex protocols, ReFlex and InFlexion (which provide data and voice features), have reportedly sent the ERMES Memorandum of Understanding Committee, known as the MoU, back to the think tank.

To date, our company has signed 30 manufacturing licenses with other companies for Flex technology. These companies will produce pagers, infrastructure and test equipment. Multiple vendors ensure both a stable technology and an adequate supply of products. Among those that have signed manufacturing licenses are NEC, Ericsson and Glenayre.

Following its commitment to make access to the Flex technology simpler, our company announced in September 1995 the availability of Flex protocol chip sets. Since the September announcement, Texas Instruments, Motorola Semiconductor and Taiwan’s Industrial Technology Research Institute (ITRI) have manufactured and marketed the chip set product. The Flex protocol chip sets will make the technology easily available to anyone wanting a high-speed wireless protocol. Chip sets will provide the wireless transport solution to the personal computer (PC) market and other consumer electronic manufacturers whose technology is moving us ever closer toward the wireless world.

One distinct advantage to Flex and its chip set technology is that it is open-licensable and controllable. Control and future developments of the Flex standard will rest with the industry leader, while the distribution of the chip sets will be available from multiple sources at competitive pricing. The industry needs to avoid the random and diverse development that occurred with POCSAG after the British Post Office gave up control and development of the code. One singular Flex protocol standard will reduce pager complexity, manufacturing costs and retail pager prices.

Choosing a transport protocol Multiple and often confusing offerings in transport protocols not only make engineering decisions more complex, they tend to dilute the cost benefit associated with the volume production of subscriber products. To assist you in making a choice, here are some guidelines. First, the new protocol has to offer a speed increase greater than your existing code by several factors. Second, the quality of the pagers or messaging units must be equal to or better than existing POCSAG units. Remember, with any new technology, there are increased risks. Many service providers have suffered business reversals when new pagers failed to meet their customers’ expectations. Third, and most importantly, is the availability of subscriber products. Are the pagers manufactured in your country? Do the vendor’s local or multinational growth plans have sufficient capacity to avoid shortages and delays in delivery? A secure and stable international supply of paging products can be a critical factor in controlling costs and securing tomorrow’s profits. Adding capacity to existing channels involves vision and planning. However, growth and additional profits can be found in yesterday’s networks.

Last month we talked about using preamplifiers ahead of a spectrum analyzer in order to boost the weak signal performance of the instrument. Such amplifiers need not cost a fortune! for example, a gallium-arsenide field-effect transistor (GaAs FET) wideband preamplifier covering the frequency range from 1MHz to more than 1GHz and featuring adjustable gain from -3dB to +20dB and a noise figure of 1dB-2dB can be purchased for less than $100.

In an emergency, a simple cable television (CATV) preamplifer can be used, even though the system impedance is 75ohms instead of 50ohms. The cost of these preamplifiers is $25 to $30. You might be surprised at what you can see with the preamp ahead of the spectrum analyzer.

Increasing the dynamic range There are many times when you might need to view a signal in the presence of a much stronger signal. For example, you might like to measure the second harmonic of a transmitter signal. Obviously, if the second harmonic is present, so is the carrier, and the carrier is going to be at a much higher level. The dynamic range of the spectrum analyzer does not extend to the normal difference between the carrier and the second or third harmonic.

This problem can be solved by the use of a filter to reduce the level of the carrier at the input to the spectrum analyzer while allowing the second or third harmonic to pass virtually unattenuated. In reality, the dynamic range of the spectrum analyzer is not extended; the difference between the two input signals to be viewed is simply compressed.

It is important to remember never to exceed the maximum input level specification of the spectrum analyzer. Do not confuse the maximum input level specification with the 1dB-compression point or third-order intercept point specification. The maximum input level specification is the maximum input level that will not damage the attenuator or mixer. The 1dB-compression point or third-order intercept point refers to distortion caused by excessive input levels. The third-order intercept point ususally occurs about 10dB-15dB above the 1dB-compression point. If an analyzer specifies the 1dB-compression point as 0dBm, then input levels higher than 0dBm should not be present at the output of the RF attenuator. If the RF attenuation is set for 20dB, then the input level (at the input connector) may be as high as +20dBm without exceeding the 1dB-compression point. It is always best to allow an extra few decibels of safety margin. For example, if the 1dB-compression point is 0dBm, then the input level should be kept several decibels below this point for best performance.

Refer to the block diagram in Figure 1 above left. A power pad is used to limit the transmitter output to a level that the spectrum analyzer input can safely handle. Suppose that the spectrum analyzer has a maximum input rating of +30dBm and a 1dB-compression point of 0dBm. Further suppose that the transmitter output is 100W or 50dBm. If the power pad between the transmitter output and the spectrum analyzer input is rated at 20dB attenuation, then the RF level at the spectrum analyzer input will be equal to the rated maximum input level. A 30dB power pad would provide a 10dB margin of safety.

Suppose that the dynamic range of the spectrum analyzer is 70dB. This means that we can measure the level of the two signals on the spectrum analyzer as long as the difference in level between the two signals is less than 70dB. Also, the larger signal should be kept a few decibels below the 1dB-compression point of the spectrum analyzer.

As an example, suppose we wish to measure the level of the second harmonic of the transmitter relative to the carrier. There are several things that we must consider in setting up to make this measurement. One is the 1dB-compression point of the spectrum analyzer. Another is the dynamic range of the analzyer.

Assuming a 30dB power pad is used, the setup shown in Figure 1 will limit the input to the spectrum analyzer to +20dB. This is 10dB below the maximum input level, so we are safe here. Suppose the second harmonic level is -80dBc (80dB below carrier level). To set the spectrum analyzer reference level to the carrier would require a reference level of +20dBm, but the input level without RF attenuation should not exceed 0dBm to remain below the 1dB-compression point. Therefore, the RF attenuation should be 30dB to be on the safe side. This brings the input level down to -10dBm at a reference level of +20dBm. Since the reference level is +20dBm and the dynamic range of the spectrum analyzer is 70dB, then the minimum level that falls within the dynamic range of the spectrum analyzer must be above the -50dBm level. Since the second harmonic is at the -60dBm level, it would not fall within the range of the instrument.

To get around this problem, we can insert a filter into the line between the power pad and the spectrum analyzer in order to reduce the level of the carrier while causing minimal insertion loss to the second harmonic. (See Figure 2 on page 50.) In order to keep our measurements as precise as possible, it is necessary to know the degree of insertion loss caused by the notch filter to the second harmonic signal. The filter can be either a cavity type or a small LC filter in a metal “can.” These are fairly small and more convenient than large cavities to use for such testing purposes. Usually, only 20dB-30dB of rejection is needed at the carrier frequency, so the small LC “canned” filters with tunable notches serve the purpose very well. Photo 1 on page 50 shows such a filter with BNC input/output connectors and the tuning adjustment to tune the notch within a broad range of frequencies.

Figures 3 and 4 on page 50 show the setup for determining the insertion loss of the filter to the second harmonic frequency. First, the signal generator and spectrum analyzer are set to the carrier frequency and the filter is tuned for maximum rejection of the carrier frequency as shown on the spectrum analyzer. Next, the signal generator is tuned to the second harmonic frequency (along with the specturm analyzer) and the generator level set to 0dBm. The insertion loss is now displayed on the spectrum analyzer. For example, if the spectrum analyzer display indicates -3dBm, then the insertion loss of the filter at the second harmonic frequency is 3dB. Make a note of this figure.

Now, using the setup shown in Figure 1, adjust the spectrum analyzer to produce a reference level of +20dBm with an RF attenuation of 30dB. This will keep the input level below the compression point.

Next, insert the notch filter (see Figure 2) and tune the notch filter for maximum rejection of the carrier. Now, increase the sensitivity of the spectrum analyzer by reducing the reference level and observe the level of the second harmonic signal. Suppose the second harmonic level is -60dBm and the insertion loss of the filter at the second harmonic frequency (as measured previously) was 3dB. Then, the actual level of the second harmonic is -60dBm -(-3dB) = -57dBm. Since the carrier level is +20dBm, the second harmonic is at a level of -77dBc (77 decibels below carrier).

The degree of accuracy obtained from the measurements above will be affected by the degree of shielding of cables, etc. The higher the attenuation of the power pad, the more likely it is that some stray RF paths will find a way around the attenuator, thus bypassing the test setup and invalidating the results. Use well-shielded interconnecting cables and keep the transmitter as far from the spectrum analyzer as possible. With reasonable care, you should be able to achieve very accurate results.

Until next time–stay tuned!