Wireless communication handoffs within a macrocell

Abstract

Handoffs within a wireless communication system (20) include using a common cell definition code for each of a plurality of microcell BTSs (30, 40, 50) to facilitate handoffs between a macrocell (26) and any one of the microcells. In a disclosed example, a common cell definition code such as a PN offset or a scrambling code is used to trigger a handoff from the macrocell (26) to any one of the microcell BTSs (30, 40, 50). A mobile station locate feature identifies which of the BTSs is involved in the handoff. Another common cell definition code is used in one example to trigger all handoffs from any one of the microcells (30, 40, 50) to the macrocell (26). Soft handoff and hard handoff examples are disclosed.

Description

FIELD OF THE INVENTION [0001] This invention generally relates to communication. More particularly, this invention relates to wireless communications. DESCRIPTION OF THE RELATED ART [0002] Wireless communication systems are well known and in widespread use. Many systems are arranged to provide wireless service or coverage within geographically or otherwise distinct areas. Most systems include a plurality of base station transceivers (BTS) that are situated to provide coverage over particular areas. The area of coverage for each BTS is commonly referred to as a cell. As known, many cells are divided into several sectors to increase wireless coverage within the cell. [0003] In some situations, a cell can be considered a macrocell because there are other BTS units within the macrocell region that are intended to serve a particular portion of the macrocell. These smaller areas within a macrocell can be referred to as microcells. One example use of a BTS to establish a microcell is to in...

AI Technical Summary

Benefits of technology

[0018] One example includes using the common cell definition code to facilitate a handoff from the macrocell to at least one of the microcells. This is useful, for example, when a mobile subscriber is entering a building that has a dedicated BTS to provide in-building coverage. A second common cell definition code facilitates a handoff from any of the microcells to the macrocell. This is useful as a mobile subscriber exits a building.

[0020] One advantage to such an arrangement is that it greatly reduces the number of cell definition codes required for a neighbor list. In one example, a macrocell maintains only one such code on the neighbor list to facilitate handoffs between it and any one of the plurality of microcells having that code.

Problems solved by technology

One reason for doing so is that outside BTS equipment may not be able to provide adequate radio frequency (RF) coverage to provide reliable wireless service throughout the interior of a building.

Including BTS equipment to establish microcells such as within buildings introduces challenges and complexities associated with managing handoffs between the macrocell and the microcell BTSs, for example.

The problems associated with handoffs between a macrocell and microcells within the macrocell include pilot pollution, neighbor list resource exhaustion, pseudo random noise offset (PN offset) assignment, scrambling code assignment and traffic capacity loss.

Pilot pollution arises from the RF of the outside macrocell undesirably leaking inside a building or otherwise within an area considered a microcell served by a separate BTS.

The outside macrocell RF in this regard essentially pollutes the inside as the RF leakage interferes with the inside RF.

At the same time, making an in-building or microcell system too powerful results in RF from the inside system leaking outside of the microcell.

This can cause problems with macrocell performance, also.

Because every in-building BTS must be on the macrocell's neighbor list, problems arise where there are more than a few such BTSs.

Adding a plurality of in-building BTSs to a neighbor list becomes problematic.

A drawback to increasing the neighbor list size is that it slows down a mobile station's measurement time of handoff candidates and degrades handoff performance with macrocells, in general.

Another drawback to increasing the neighbor list size is that it increases paging channel occupancy.

Therefore, simply increasing the size of a neighbor list is not an adequate solution because there are limits on a manageable size and performance problems are introduced as the size of the list increases.

Another problem that arises has to do with PN offset or scrambling code assignments.

As more in-building BTSs are deployed, PN offset or scrambling code planning becomes much more complicated.

The limits on available PN offset or scrambling code assignment also place a hard limit on the number of buildings or other microcells that can be covered by dedicated BTS equipment.

As the plurality of in-building BTS increases, finding the assignments becomes increasingly problematic.

Traffic capacity loss can arise when there are in-building BTSs within a macrocell.

This can result in a significant loss in traffic capacity as channel elements become tied up in relatively excessive soft handoff.

If there is excessive soft handoff between the macrocell and such in building BTSs, the incentive for adding the dedicated BTS equipment is diminished.

In the case of hard handoffs, the problems with exhausting neighbor list resources and PN offset or scrambling code assignment still exist.

Some believe that all hard handoffs are unreliable.

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