System integration of the Flex paging protocol Part 2–System design recommendations.
Flex, a Motorola trademark, is short for flexible, wide-area, synchronous paging protocol.
Integration of the Flex paging protocol onto existing paging channels will present system design challenges for systems not yet optimized for the higher data rate of 6,400 bits-per-second (bps), but which are slated for operation at that rate. The most significant constraint facing designers is control of coverage overlap areas. However, until such time that the system may be optimized for 6,400bps, carriers will enjoy several benefits in paging with 3200 4-level Flex over 2400 POCSAG. The main benefits include relaxation of the coverage overlap increase in throughput. In many cases, there is good reason to introduce the Flex paging protocol, at the operator’s earliest convenience, onto paging channels currently performing, even marginally, at 2400 POCSAG.
Meeting the simulcast delay spread (SDS) constraint for 6400 Flex will require a certain amount of system design review. Yet, a system design review is long overdue for many paging systems, regardless of plans to introduce Flex. Periodic system evaluation should be an intrinsic part of a carrier’s strategic plan to provide adequate service. This implies proactive–rather than reactive–system planning, design, monitoring and maintenance. A recommended plan for evaluating system performance and implementing design changes follows, with individual parts presented in a step-by-step order.
Step One — System performance check: Use all available information for an indication of overall system performance and to reveal localized pockets of less-than-adequate service. Trouble-report databases, bit-error-rate mapping studies and field technicians all are good sources of information. Pay particular attention to areas that exhibit poor or marginal POCSAG 2400 reception, because these areas will provide a good starting place for evaluating Flex performance. Launch a periodic system monitoring and evaluation program, if one does not already exist.
Step Two — Configuration check: Check and re-check all configuration operating characteristics and settings throughout all components of the paging signal flow chain. A systematic approach starts at the terminal and ends at the paging transmitter power amplifiers. Of particular interest are terminal software patches, controller software versions, exciter software versions, equalization delay settings, deviation settings and rise-time settings. If the system is composed of multiple frequencies, be sure the channel frequencies are programmed properly into the controllers and the exciters. A transmitter keying on one channel with another channel’s data destroys reception for a large area of coverage. Configuration log books at each site are particularly helpful in determining and maintaining system status.
Step Three — Transmission site check: Each transmitter site should be checked on a routine basis for proper installation and operation. Every aspect and detail of physical installation should be inspected, from cabinet grounding to proper antenna installation. It is advisable to draft a site-visit checklist and to require its completion upon each visit.
Step Four — System performance baseline: Once configuration and hardware checks are completed, a thorough system baseline should be documented to pinpoint trouble areas for simulcast testing and to serve as a reference point for comparing performance results after system changes have been implemented. Use a variety of analysis tools for characterizing the system (e.g., bit-error-rate mapping, received signal strength indication [RSSI] mapping and paging probability counts), and thoroughly document the testing process so that tests may be duplicated as closely as possible after implementing the system design changes.
Step Five — Configuration recheck: If any system changes or component replacements have been made during or after the system performance baseline, then verify all affected components for proper configuration.
Step Six — Simulcast testing: Based on the system performance baseline results, localized performance problem areas that have adequate signal strength should be tested to find out why a problem exists. Usually, this is an indication of an SDS problem. A good initial test is to take page probability counts in areas with marginal POCSAG 2400 performance. First, look for page count improvements at 3200 4-level Flex compared with POCSAG 2400. Next, compare 3200 4-level Flex with 6400 Flex for degradation at the higher data rate. If both of these conditions hold true, then the probable cause is that the SDS constraint has been exceeded. The degree of paging improvement between 6400 Flex and POCSAG 2400 and between POCSAG 2400 and 3200 4-level Flex indicates the severity of the delay spread.
Presuming that the reception problem is SDS-related, it may be possible to determine which distant sites are causing the problem by keying up all transmitters in the area one at a time while halting paging traffic and taking a relative measurement of the received signal strength of each transmitter. If it is found that one or more distant sites (i.e., farther than 14 miles away from nearest site) is being received with signal strength within 10dB of the nearest site, then it is likely that that site is contributing to the SDS problem. After establishing which sites are likely contributors, they could be taken off-line so a page count test can be rerun to compare results with the original page count test for signs of improvement. This process may identify which distant transmitters are affecting the test area. However, it is important to note that a correlation may only be drawn between “culprit” distant sites and the particular test area where the measurements have been taken. Therefore, the most efficient step is to take measurements in as many problem areas as practical and, in that way, identify culprit sites that are creating problems in multiple areas.
Step Seven — Propagation and coverage analysis: Based on simulcast and baseline test results, site selection, antenna placement and antenna selection may be simulated for optimizing system design. Whether this analysis is done in-house or contracted out, ensure that the analysis gives consideration to delay-spread issues, and not simply to RSSI level. It is strongly suggested that high-gain, omnidirectional antennas be considered for replacement with lower-gain, downtilt or panel-type antennas. In addition, reconsider whether to use antenna sites on mountains or other high elevations, because they are often found to be major contributors to SDS problems.
Note: It is in the carrier’s best interest to take an active role in system analysis and design because no other party has a more vested interest in the system’s performance nor a more intimate knowledge of the customer base for which service is being provided.
Step Eight — Implementation of system design changes: Sectionalize planned system design changes both geographically and conceptually. That is, categorize and confine design changes by geographic conditions and by the proposed type of solution. Also, instead of sweeping the entire system with broad changes, try less-costly solutions first in small sections of the coverage region. It may be found that taking mountaintop or other high-elevation sites off-line and implementing relatively inexpensive antenna changes will result in an acceptable level of service. Conduct pilot tests in areas with lower use to verify the effectiveness of the design changes to reduce the chance of causing an adverse effect on area paging. If possible, implement changes in steps that may help to characterize the degree of improvement that each provides.
Step Nine — System baseline: After system design changes have been completed, once again document the system baseline, using all of the characterization tools available. Building-penetration issues that did not show up in the initial studies now may become apparent. These types of issues probably will need individual attention.
Step Ten — Continued system monitoring: Take the time to establish a system performance monitoring schedule, and stick to it. Included in the schedule should be site visits, configuration checks, simulcast tests and baseline tests.
Small deviation offsets for Flex Whether or not to implement a deviation offset scheme into a paging system is a controversial issue. In my opinion, an offset scheme is advisable because, if properly implemented, deviation offsets would not degrade system performance but would provide the potential to improve it. Furthermore, if zero-beating is considered to be problematic in a particular system, then any offset, even a small one, would be preferable to no offset at all.
An important control feature for system optimization is the ability to switch to deviation offsets on a protocol basis. This feature allows the system to have different deviation offsets programmed for different protocols that have conflicting optimum settings, such as POCSAG and Flex. However, if a deviation offset scheme is to be implemented on a mixed protocol channel, and the offsets cannot be adjusted according to which protocol is in use at any given time, then an offset regimen appropriate for Flex should be enacted. The reason is simply that the POCSAG protocol is tolerant to Flex-type offsets even though they are not the optimum choice for POCSAG, whereas Flex is intolerant to POCSAG-type offsets. A good regimen to try initially would be -30Hz, 0Hz and +30Hz implemented in such a way that the overlap areas experience deviation differentials of 30Hz or 60Hz. (This recommendation is based upon laboratory testing with a static environment.)
Deviation for the majority protocol From a system optimization viewpoint, it would be preferable to deviate at the recommended levels for each protocol. However, for systems currently unable to adjust deviation to different levels based on protocol, it is recommended to set deviation levels to match those that are appropriate for the paging protocol occupying the majority of air time. That is, if the majority of traffic is POCSAG, then set deviation levels to 4,500Hz. It should be mentioned that some national carriers use 4,500Hz deviation exclusively, and other national carriers use 4,800Hz deviation exclusively, with a mixture of protocols on their channels. To date, there has been no reported significant degradation associated with either option. Therefore, unless degradation is experienced by a particular deviation setting, reconfiguring a large system from one deviation setting to the other probably would not be worth the effort.
Conclusion The Flex protocol is not only an attractive alternative to POCSAG, but to many paging systems with unprecedented growth, it is a necessity for increasing throughput. In the near term, paging systems may require redesigning to provide adequate 6400 Flex performance. However, a key feature of Flex is its ability to perform at any of the programmable Flex speeds. Thus, if the system slated for introduction of Flex protocol is not currently capable of acceptable 6400 performance, then Flex may still be introduced, if only at a slower data rate. System redesign may be accomplished concurrently with Flex migration or at some later date.
There is little doubt that, eventually, all Flex system infrastructure providers will be producing paging equipment capable of deviation and offset changes on a protocol basis, thereby allowing systems to be optimized for more than a single protocol. Although such features are necessary for complete system optimization, the system improvements enjoyed as a consequence may not be readily apparent until such time as the basic system design has been optimized.
The biggest challenge facing the introduction of 6400 Flex is meeting the SDS constraint while at the same time providing adequate signal density to support the increased data rate. In rural and suburban areas, the system may require little or no redesign. However, in the urban or the mountainous environment, a significant amount of redesign effort may be necessary. The system performance that will be achieved will be a function of the effort and expertise invested in the system redesign phase.
Acknowledgments A special thanks to Jack Gleeson, Subscriber Development Engineering, Motorola, for an unbiased sharing of deviation offset test results and information, and to Michael J. McCabe, Glenayre Electronics, for assistance in a technical review of this article.
Flex integration series
“System Integration of the Flex Paging Protocol.”
Part 1–System design constraints, June 1996.
Part 2–System design recommendations, July 1996.
Back issues for the past two years are available for purchase from Intertec Publishing customer service. Call 800-441-0294 or 913-341-1300.
Article photocopies are unavailable from the publisher, but they can be ordered from UMI Information Store, 800-248-0360 or 415-433-5500 ext. 282; fax 415-433-0100.