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III BUSINESS 102: OTHER THINGS ARE IMPORTANT TOO The Wireless Data Handbook, Fourth Edition. James F. DeRose Copyright © 1999 John Wiley & Sons, Inc. ISBNs: 0-471-31651-2 (Hardback); 0-471-22458-8 (Electronic) 12 COVERAGE VERSUS CAPACITY 12.1 INTRODUCTION Paraphrasing the late Tip ONeill: All coverage is local. With 1214-in. full wavelengths it is quite possible to encounter an ARDIS, BSWD, CDPD, or circuit switched cellular dead spot simply by walking to another position in a room. This annoying fact does not mean that all carriers operating near the same frequency band offer essentially the same coverage choices. Design trade-offs are made between area coverage and its close cousin, building penetration, versus subscriber capacity. These trade-offs flow from fundamental decisions on channels allocated for data versus control, channel reuse philosophy, base station quantity and location, transmit power levels, FM capture exploitation, bit rates, and message redundancyto name just a few. Each of these decisions can cloud the certainty of coverage comparisons. ARDIS configures minimal infrastructure for maximum in-building penetration at the cost of subscriber capacity. For roughly equivalent geography, BSWDs technical choices predispose it toward maximum subscriber potential at the cost of some penetration. CDPD is not closely comparable to either ARDIS or BSWD because, in a full-blown implementation, it would deploy many more base stations, each with a dedicated channel, thereby achieving both high capacity and good penetration. But carriers are hesitant about the cost of a CDPD deployment of that scope. Bell Atlantic Mobile has stated 1 : CDPD will be offered in markets where it is expected to be commercially viable, although equipment will not be installed at all cell sites in those markets. This uncertainty does not mean that coverage is simply unknowable without comprehensive field tests. A great deal can be accomplished, without the special 171 The Wireless Data Handbook, Fourth Edition. James F. DeRose Copyright © 1999 John Wiley & Sons, Inc. ISBNs: 0-471-31651-2 (Hardback); 0-471-22458-8 (Electronic) knowledge of a radio engineer, by analysis of site location licenses, judicious use of vendor coverage maps, and ZIP code predictors. Not all carriers make these tools available, and there are often quality variations in those that do exist. The base station license and vendor coverage map techniques are most effective when judging relatively wide geography such as an entire county. At their weakest these simple methods permit the user to confine field testing to likely trouble spots. At their best they are often powerful enough to be excellent predictors of coverage, eliminating much tiresome, labor-intensive work. Ultimately, some field tests are required to prove contract coverage in specific areas. For a prospective user these tests need not be laden with instrumentation. Far simpler techniques can give the thoughtful reviewer valuable insight into coverage and retransmission performance that tends to vary with message length. Since 1994 JFD Associates has conducted coverage and building penetration tests at roughly six-month intervals on ARDIS, BSWD, and, later, CDPD. There are also public domain summary reports from users such as Pitney-Bowes and Schindler/Millar elevator, as well as carrier reports from BSWD. Illustrative extracts from these sources are used in this chapter. 12.2 KEY COVERAGE PHILOSOPHIES 12.2.1 ARDIS ARDIS employs each channel in a given area in single-frequency reuse (SFR) mode. As the user moves from one single channel cell to another, the device remains tuned to the same frequency. Higher level logic determines which base station will work with the device. Its 90% coverage areas are deliberately designed not to overlap, as illustrated in Figure 12-1. Since ARDIS employs SFR, the area of reduced coverage probability between two adjacent cells can be exploited. Extending beyond the solid 90% line are areas of ever-decreasing coverage. The 70% curve is portrayed as a dotted line. Where two of these 70% zones overlap, the effective coverage is 90% since the user can be handled by either of two base stations. The explanation is straightforward: When in an overlap zone there is a 30% probability that the user cannot be heard by cell A; the same 30% probability extends to cell B. But the probability that the user will not be heard by either cell A or B is 0.30 2 = 9%. The probability that the user will be heard by at least one base station is thus 91%. Note that there are scattered areas in which the probability of success is very high (shown here as triple-coverage zones). The proper siting of base stations can cause these areas to become quite rich, thus improving building penetration. As an illustration, the typical user transmission in Chicago is heard by 8 to 10 base stations. 2 Clearly not all these base stations hear with anything like a 70% probability, but there is always some finite chance that a message that would be missed by BSWD or CDPD will be picked up by ARDIS. 172 COVERAGE VERSUS CAPACITY When ARDIS adds a new channel, a new base station is typically installed at the same physical site. Stamford, Connecticut, has such an overlay. When the device modem is powered on, it listens first for an RD-LAP channel. If found, it begins operation there. Lacking RD-LAP, it will switch to MDC4800. As the device moves around, or as channel loads/conditions vary, it is quite possible to receive sequential packets from the same message on different channels, some RD-LAP, some MDC. These layers of channels are currently highest in metropolitan New York with a total of eight 25-kHz-wide channels. 12.2.2 BSWD BSWD has a cellularlike base station siting plan, though on a smaller scale. Omnidirectional cells are grouped in clusters; no two cells in the same cluster employ the same frequencies. The initial goal was to provide a 90% or greater probability of street-level coverage. With the drive toward two-way paging, the goal has evolved to coverage comparable to one-way paging, nationwide. 3 BSWDs base station density does not approach that of cellular. As an example, in March 1998 BSWD had only 26 base stations in the entire state of Connecticut. There are other helper sites along the Massachusetts, New York, and Rhode Island borders. Since Connecticut is very small, about 5000 square miles, the average cell radius for just the 26 Connecticut base stations is ~7.8 miles. This is not the same Figure 12-1 ARDIS base station siting (representative). 12.2 KEY COVERAGE PHILOSOPHIES 173 design criteria as, say, metropolitan voice cellular, which sometimes operates with 1/2-mile-radius cells and microcells. Naturally the sites are not uniformly distributed. BSWD has 12 of the 26 located just in Fairfield County to improve Interactive Pager (I@P) coverage for the Greater New York metropolitan area. Fairfield County is ~435 square miles, so the average cell radius drops to ~3.4 miles in this densely populated sector. BSWD has stated that it is able to reuse (each channel) easily four times. 4 This requires a broad geographic spread employing perhaps 30 base stations. While the reuse figure has been disputed, in this analysis BSWDs claim is accepted as correct. Thus, say, Los Angeles County has ~4 channels per cell and 7 cells per cluster. The 90% coverage lines of each cell in the cluster overlap to ensure smooth hand-off. An overly simplified representation of BSWDs base station siting is shown in Figure 12-2; the principal hand-off areas from the central base station are designated H. 12.2.3 ARDIS Versus BSWD: Representative 90% Coverage Contours The resulting 90% area contours are depicted in Figure 12-3. The key point is that for approximately the same geographic area coverage contours are always somewhat different. There will be locations in which BSWD will operate and ARDIS will not, and vice-versa. Note that in this example BSWD requires more cells to achieve roughly the same 90% area coverage. Figure 12-2 BSWD base station siting (representative). 174 COVERAGE VERSUS CAPACITY 12.2.4 Improving Building Penetration with More Base Stations BSWD is clearly on a path to enrich its infrastructure for two-way paging building penetration. One must be wary about carrier base station counts; they seldom correspond to geographic locations. ARDIS counts each new frequency deployed at the same physical location as an additional base station. BSWD counts two transceivers at the same location as two base stations. Nevertheless, BSWDs August 1998 claim of 1900 installed base stations 5 probably brings it to a parity position with ARDIS in unique site locationsand BSWD is growing its infrastructure more rapidly than ARDIS. Further, BSWD is focused on ~160 fewer cities, each of which costs ARDIS a base station. In selected areas, BSWD likely has more actual base stations than ARDIS. An interesting example is Fairfield County, Connecticut. BSWD has 12 physical locations; ARDIS has only 9. As noted in Section 12.2.3, BSWD probably requires Figure 12-3 Comparative areas of 90% coverage. 12.2 KEY COVERAGE PHILOSOPHIES 175 more base stations to achieve parity in an equivalent area. Field tests at 48 street-level locations in Fairfield County during January 1998 show that ARDIS and BSWD have equivalent, and very good, street-level coverage. Additional tests revealed that ARDIS had ~6% coverage edge in totally enclosed buildings. Continued enhancements in BSWD infrastructure would tend to offset any building penetration shortfall. Very large additions might permit it to outperform ARDIS and gain more capacity as well. But one must be mindful of the rules of plane geometry. If BSWD drove its infrastructure to 18 locations, doubling the number of ARDIS sites, the average cell diameter would only fall from 3.4 to 2.8 miles. This is a significant reductionand would surely be pleasantly noted by BSWD usersbut this major investment would not result in microcells. 12.2.5 CDPD CDPD, using the same physical locations (and, e.g., antennas, T1 lines) as voice cellular, follows the voice cochannel reuse rules. In metropolitan areas where CDPD build-out is most complete, this means a trisectored layout. Hand-offs occur in the zones marked H, as shown in Figure 12-4. The hand-off threshold varies somewhat by carrier; the right user device is key. In early BAM dedicated channel implementations, using the PCSI PAL phone, it was possible to be in the central hand-off area and be unable to transmit or receive. Putting the phone in diag mode, one could watch the hunt: channel 721, 555, 729, 721, 555, . . .unable to decide which channel was the best! Now BAM has time and dBm selection Figure 12-4 CDPD trisectored base station siting (representative). 176 COVERAGE VERSUS CAPACITY thresholds. Every 90 seconds the device listens for alternatives. If the other channels that can be heard are not 8 dBm better than the current channel, no switching occurs. 12.2.6 Other Coverage Considerations 12.2.6.1 Transmit Power Levels 12.2.6.1.1 ARDIS Pre-ARDIS, the KDT-800 device used in IBMs Field Service system had a transmit power of 4 watts. Much of this energy was wasted because the hand-held brick had internal antennas (this was a dual-diversity receive unit). As the infrastructure was enriched, succeeding internal antenna devices such as the KDT-840 had their output power reduced to 3 watts. This level remains the upper limit for vehicular devices employing, say, Motorolas mobile radio modem (MRM) line of modems. With the coming of frequency-agile, hand-held devices a design trade-off was made. The power-hungry synthesizers, which replaced simple crystal cut oscillators, drew so much current at the 3-watt transmission level as to make battery life unacceptably short. Regrettably for some users who treated their devices carelessly, the antenna would have to be external. The trade-off was a reduction in output power to 1.5 watts and longer battery life. This is the usual output power level for ARDIS hand held devices. The Interactive Pager (I@P) returned to an internal antenna located inside its flip-top lid. The transmit power level was also eased down to 1 watt. Performance is good with the lid open. It has been reported that IBM Field Service people who normally work with the lid closed encounter reduced coverage as compared to the old KDT-800. 12.2.6.1.2 BSWD The Ericsson infrastructure employed in the original RAM deployment had poor receive sensitivity, which limited the optimum radius of the base station to ~5 miles (115 dBm laboratory capability). 6 In 1997, to prepare for the coming of the I@P, BSWD began to retrofit all existing base stations to improve the receive sensitivity to 121 dBm. This pushed their effective radius to ~7 miles. Because of these early restrictions, the original external modems for portable users had a transmit power of 2 watts AND an external antenna. The version of the I@P built for BSWD also has 2 watts transmit power. The higher transmit power requirement forced Ericsson, then subsequent vendors, into excellent battery-saving techniques. They are among the best available today. During one 14-day test I transmitted 157 messages each on both an ARDIS and a BSWD I@P. The ARDIS unit was an early version, and there was actually a bit of extra traffic on it, but that does not justify the very different results. I was forced to replace ARDIS I@P batteries three times during the 14 days; I never changed the BSWD I@P batteries. ( Note : In some heavy usage CDPD tests external modem batteries must be changed every 23 hours ). 12.2 KEY COVERAGE PHILOSOPHIES 177 All BSWD I@P tests were performed with lids open. A lids-closed test might have revealed shortcomings, but it is a moot point since, in August 1998, BSWD announced the availability of the no-lid RIM I@P950. 12.2.6.1.3 CDPD After early consideration of 256 discrete power levels, CDPD settled on the same range as its voice cellular counterpart. Except for expensive, multipurpose vehicular units, CDPD modems generally operate with a maximum transmission power of .6 watts, identical to hand-held voice phones. This is generally satisfactory in richly endowed urban areas with small-diameter cells. In the period 19951996, before some urban areas were optimized for portable phones, CDPD performance was very poor. The lack of power is still evident along many interstates that are really designed for 3-watt vehicular devices. Very large, very troublesome coverage holes often appear in this situation. Figure 12-5 ARDIS RF coverage contours: Naples, Florida. 178 COVERAGE VERSUS CAPACITY 12.2.6.2 External Antennas Another technique to extend the coverage area of packet modems is to use external antennas without an increase in transmit power. The use of an external mag mount antenna in my office improves CDPD signal strength by about 20 dBm. Naturally, these improvements are terrain dependent. In the flat geography of Florida the effects can be quite pronounced. Figure 12-5 shows the improvement in the 90% ARDIS coverage area when a mag mount antenna is employed with a PM100D class modem. The 90% coverage line extends an additional 5 miles from the base station center. Note, also, that coverage does not suddenly vanish after the 90% contour curve is reached. Some success will be had at even greater distances. This technique is often employed with over-the-road trucks having multiprotocol modems that feature satellite connections. Extending the terrestrial coverage markedly lowers the higher cost satellite bills. 12.2.6.3 Repeaters In some countries bidirectional amplifiersor repeatersare used to extend coverage into areas without base stations. This technique is used in Germanys DataTAC system, for example. The use of repeaters in the United States tends to be limited to frequencies below 450 MHz. If selected carriers did gain FCC approval for repeaters in the 800/900-MHz bands, coverage might well be extended into rural areas. 12.3 ESTIMATING COVERAGE WITHOUT FIELD TESTS 12.3.1 License Examinations For years most carriers filed a radio station license with the FCC. Reference copies are available to all U.S. citizens for a nominal fee. The copious information on these licenses includes a number of facts that can help a coverage analysis, including the base station: 1. Address (street, city, county, state) 2. Latitude/longitude 3. Output power (watts) 4. Effective radiated power (watts) 5. Antenna height (meters) Recently, BSWD license facts have become more difficult to check. BSWD administratively canceled its licenses as a part of the 900-MHz auction process by a letter to the FCC on September 26, 1996. 7 However, even purged records are still public information. Much of the data can still be recovered. A listing of base station sites for ARDIS and BSWD in Connecticut is contained in Appendix I. With position 12.3 ESTIMATING COVERAGE WITHOUT FIELD TESTS 179