Improving PCS reliability through cell frequency management

Adding additional neighbors to each sector builds in a strategy for improving system reliability with no additional cost in hardware.

In the last few years, there has been an explosion of cell site growth in the wireless industry. Many PCS carrier’s system designs do not have the redundancy of their earlier cellular counterparts necessary to meet an aggressive buildout schedule. Some of this is due to the nature of the equipment design or the inherent weaknesses of the format. For example, GSM and TDMA typically use one RF carrier and multiplex several timeslots (talk slots, or logical channels) on one RF transmitter per sector. Failure of this single transmitter takes the sector out, opening up a coverage “hole.”

Many sites have been deployed without ac generators for backup or with backup battery capacity of only an hour. To meet the aggressive buildout, providers have deployed most PCS sites using the local telco’s T1 wire facilities instead of microwave. Wired T1s are much less reliable than microwave and inherently fail much more often. All of these factors can take a PCS site completely out in short order, often for hours (or even days) before it can be restored.

With its reduced coverage, as compared to 800MHz cellular, there are many more cell sites required for PCS. This inherently offers some opportunity for redundant coverage, but only if the RF frequency “neighbor list” is correctly implemented. This article discusses some ways to implement this backup coverage. GSM systems are the primary focus, but the underlying theory should apply to other technologies as well, including TDMA and some cellular systems.

Neighbor lists and implementations

A neighbor list is a group of parameters that are stored in the cellsite data base contained in the base station controller (BSC), which is typically located in the mobile switching center (MSC). The neighbor list consists of the channels (frequencies) assigned to the site (including its adjacent sectors) and the channels assigned to the neighboring cell sites.

These neighbor lists are downloaded into the BSC and are used by the BSC to control mobile handoffs between adjacent sectors and neighboring cell sites. An example will help to clarify this datafill. Imagine a simple system based on highway coverage, using three two-sector sites. This is a typical configuration where coverage is of prime importance, and capacity is not the driving factor. The hypothetical cell sites are part of the RF buildout designed to cover Interstate 95. Cell sites are located at the towns of Four Oaks, Benson and Dunn as shown in Figure 1 on page 38.

The distance between these sites is covered by deployment of directional antennas pointing up and down the interstate. On a typical PCS1900 GSM system, a channel consists of a control time slot and seven talk time slots. Convention has it that the alpha sector points north and the beta sector points south. At Four Oaks, the north sector (alpha) is assigned channel 620. The south sector (beta) is assigned channel 630 (see Figure 1 and Table 1 on page 38).

To the south of Four Oaks lies the town of Benson. Its cell site has channel 640 loaded in the alpha sector and channel 650 loaded in the beta sector. Continuing south on I-95, we reach the town of Dunn, with its cell site loaded with channel 660 (alpha sector) and channel 670 (beta sector).

This buildout continues up and down I-95 with cell sites located about every five miles north and south of this part of the system.

Cell site mobile handovers

Before we can look at a failure mode, it is necessary to look at the basic operation of the system and its intracell and intercell handovers. Suppose a car is traveling south on I-95 and approaches the town of Four Oaks. At some point north of town, the mobile originates a call. The call is established on channel 620 (alpha). As the mobile passes by the site, the intracell handover occurs according to the data in the site’s neighbor list. Typical implementation has channel 630 loaded, so the mobile is handed off from alpha (channel 620) to beta (channel 630). The beta sector has its neighbor list loaded with channel 620 (intracell neighbor) and channel 640 (intercell neighbor), which is the alpha sector of the site at Benson. These handovers continue in the same fashion, and as the car travels south on I-95, the mobile is handed off from channel 640 to 650, from 650 to 660, 660 to 670 and so on.

One last point. Coverage from Four Oaks overlaps the Benson site, and coverage from Dunn also overlaps the Benson site. The coverage at the town of Benson from these two sites is weak, but useable, even without the Benson site on the air, as shown in Figure 2 on page 38.

Here is where the problem begins.

Failure modes and coverage

Suppose the site at Benson goes off the air due to a cut leased T1 from the local Telco, which occurred when a water line was installed between the local Central Office and the Benson site. (See Figure 3 on page 39).

The cut T1 renders the site at Benson unavailable, as call traffic cannot reach the MSC for processing. The site shuts down automatically when it loses communcations with the MSC.

Our hypothetical user passes by the Four Oaks site, which is loaded with the now-absent Benson alpha sector. Although adequate coverage to maintain a call exists between Four Oaks and Dunn, without Benson on the air, the sites do not know about each other because they are not on each other’s neighbor lists. Hence, as the mobile caller travels down I-95, Four Oaks cannot hand off to Dunn. Somewhere between the two, where Four Oaks’ coverage ends, the call is dropped.

The neighbor lists, as initially implemented by the original RF engineering for these sites, are listed in Table 2 above. These are typical basic neighbor implementations found in many carriers’ systems.

In the second example (see Figure 4 on page 39), the beta transmitter at the Benson site fails. This is a typical GSM failure that takes a sector down. If a call is up, it passes from the Four Oaks beta sector to the Benson alpha sector as normal.

However, as the mobile caller passes Benson, a problem begins. Because the beta sector is off the air, the alpha sector of Benson has nowhere to hand off to, even though coverage is available from the Dunn alpha sector. The site simply does not know that the other sector (Dunn channel 660) is available because it is not loaded into its neighbor list.

Table 2 shows that the only neighbors loaded into the Benson alpha sector are Four Oaks channel 630 to the north and its adjacent sector to the south on the same tower, channel 650, which currently is off the air.

The mobile “drags” the coverage from Benson alpha around the site to the south side and the call drops shortly after the mobile passes by.

Neighbor list additions

What would have happened in the previous two examples if the neighbor lists in Table 3, above, had been implemented instead of the neighbor lists in Table 2. The new neighbor list has three more entries per sector for neighbors (five instead of two).

The list has more neighbors because it is looking ahead one sector in each direction past its immediate neighbor. Compare the Table 2 typical implementation with the Table 3 implementation, looking at the Benson alpha sector as an example.

In the basic implementation in Table 2, Benson alpha only has its immediate neighbors loaded, that is, Four Oaks beta sector at the site to the north, channel 630, and Benson beta channel 650 to the south, located at the same site. In the improved implementation, the site knows to look ahead. The system parameters are set to use the strongest available neighbor, so under normal system operation, the neighbor lists cause handovers to proceed exactly as in Table 2.

To the north, Benson alpha not only looks at Four Oaks beta, but knows that Four Oaks alpha and Smithfield beta exist as well. Likewise, to the south, in addition to its own beta sector, it also knows that Dunn alpha and beta and Godwin alpha exist.

What will happen this time when the T1 is cut to the site at Benson? The site at Benson is off the air. A southbound caller initiates a call and is on the alpha sector of Four Oaks. As he passes the site, he is handed off from Four Oaks alpha to beta (intracell handover). The mobile continues south on I-95. The site at Benson is not on the air, but this time the neighbor list makes the call available for handover to the alpha sector at Dunn. Although call quality is not as good as normal, the call does stay up and is not dropped as the mobile moves from Four Oaks beta to Dunn alpha. The hole in coverage with the failure of the Benson site has been reduced to a weak area, all done by neighbor list data fill.

In the second scenario, repeated from above, the Benson beta transmitter has failed. As the vehicle travels south from Four Oaks toward Benson, all is well. When the caller goes past the Benson site, instead of “dragging” the alpha sector around the south side of the site, the neighbor list indicates that Dunn alpha exists, and the call is handed off to Dunn alpha instead of being dropped. Again, proper neighbor list loading has prevented dropped calls by filling in a coverage hole caused by equipment failure.

Summary

Clearly, in many locations, system redundancy can be built in by using existing coverage that is not normally used when all sites are on the air. When all sites are operating normally, handovers occur in the simplistic fashion shown in Table 2. However, adding additional neighbors to each sector builds in a strategy for improving system reliability with no additional cost in hardware.

In these cases, call quality will most likely be degraded, but call processing will continue, with handovers being made, and will not necessarily drop if the sites are appropriately spaced.

Not implementing additional neighbors will definitely cause dropped calls as the mobile drives into a failure-induced hole and the sector is “dragged” out.

This strategy is easily implemented with a little additional time and planning when the RF engineering of the system is done. Having fewer dropped calls and apparent holes keeps customers happier, reducing churn and increasing revenues.