Cell sites have traditionally existed as macrocells on towers that cover areas up to several miles in diameter. Macrocells are typically public cells, serving all wireless-phone users. IS–136 DCCH TDMA technology enables the use of much smaller cells called microcells. Microcells provide customized service within the coverage of existing macrocells. Microcells typically provide WOS features to specific phones within a private building or campus environment.

Hierarchical Cell Coverage

The combined coverage of both macrocells and microcells is called hierarchical cell coverage, with the microcells creating a second level of coverage under the existing level. Although macrocells are usually public and microcells are usually private, they can reverse roles.

For example, a public macrocell can also provide private WOS services to offices within its coverage area. Conversely, a microcell can provide public coverage to fill in geographic gaps due to topography or to enhance coverage in high-density areas.

Figure 8 shows a private-system microcell within a public macrocell.

Figure 8. Macrocell/Microcell Configuration

Hierarchical Cell Structures

In a PCS environment, a geographic area might be covered by a mix of macrocells and microcells as well as public and private systems. A PCS phone must therefore assess the most suitable control channel on which to provide service, even if the signal strength of a neighboring cell is not the highest signal being received by the phone, but is of a sufficient level to provide quality service. PCS uses hierarchical cell structures (HCS) to accomplish this by identifying neighboring cells as preferred, regular, or non-preferred.

• preferred neighbor cell—A preferred cell has the highest preference. The phone reselects it even if its signal strength is lower than the serving cell. The main criterion here is that the preferred neighbor cell must have signal strength sufficient to provide quality service.

• regular neighbor cell—A regular cell has the second-highest preference. The phone reselects it if the cell's signal strength is greater than the serving cell (plus a hysteresis value) and there is no eligible preferred cell available.

• non-preferred neighbor cell—A non-preferred cell has the lowest preference. The phone reselects it only if the signal strength of the serving cell becomes insufficient to provide service and the signal strength of the non-preferred neighbor is greater than the serving cell (plus a hysteresis value).

Operating Principle

HCSs enable the DCCH to identify and designate neighboring cells as preferred, regular, or non-preferred. A PCS phone uses that hierarchical information to reselect a particular neighbor cell over another based on the type of relationship defined between the cell it is using (serving cell) and the adjacent neighbor cell. Each neighbor cell's designation dictates which type of algorithm the phone uses when it considers the cell as a reselection candidate.

For example, when a low-power microcell is providing capacity in a dense-traffic area that is also served by a high-power macrocell, the HCS allows the phone to give preference to the weaker microcell. Without the multitier environment, the phones would have difficulty capturing microcells, and the cellular system would require highly specific parameter settings.

Figure 9 shows reselection based on HCS cell-type designation.

Figure 9. Reselection Based on HCS Cell Type