U.S. patent application number 11/371707 was filed with the patent office on 2007-09-13 for wireless communication handoffs within a macrocell.
Invention is credited to Patrick Li, Vic Pan, David A. Rossetti.
Application Number | 20070213067 11/371707 |
Document ID | / |
Family ID | 38270881 |
Filed Date | 2007-09-13 |
United States Patent
Application |
20070213067 |
Kind Code |
A1 |
Li; Patrick ; et
al. |
September 13, 2007 |
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.
Inventors: |
Li; Patrick; (Beijing,
CN) ; Pan; Vic; (Bedminster, NJ) ; Rossetti;
David A.; (Randolph, NJ) |
Correspondence
Address: |
CARLSON, GASKEY & OLDS, P.C.
400 W MAPLE RD
SUITE 350
BIRMINGHAM
MI
48009
US
|
Family ID: |
38270881 |
Appl. No.: |
11/371707 |
Filed: |
March 9, 2006 |
Current U.S.
Class: |
455/444 |
Current CPC
Class: |
H04W 24/02 20130101;
H04W 36/04 20130101; H04W 28/06 20130101 |
Class at
Publication: |
455/444 |
International
Class: |
H04Q 7/20 20060101
H04Q007/20 |
Claims
1. A method of communicating, comprising: using a common cell
definition code for a plurality of microcells within a single
macrocell to facilitate a handoff between the macrocell and at
least one of the microcells.
2. The method of claim 1, comprising using the common cell
definition code to facilitate a handoff from the macrocell to at
least one of the microcells.
3. The method of claim 2, comprising using a second common cell
definition code to facilitate a handoff from at least one of the
microcells to the macrocell.
4. The method of claim 1, wherein the microcell definition code
comprises at least one of a pseudo random noise offset or a
scrambling code.
5. The method of claim 1, wherein each of the microcells
corresponds to a building and comprising using a first common cell
definition code as an indicator that a mobile station is entering a
building; and using a second common cell definition code as an
indicator that a mobile station is exiting a building.
6. The method of claim 1, comprising at least one of maintaining a
neighbor list having only one identifier of the macrocell at each
of a plurality of base stations serving the plurality of
microcells, respectively; or maintaining a neighbor list at a base
station serving the macrocell having the common cell definition
code for each of the plurality of microcells within the macrocell
as the only entry identifying the entire plurality of
microcells.
7. The method of claim 1, comprising determining if a mobile
station currently communicating with a base station serving the
macrocell detects the common cell definition code; taking a power
measurement from the mobile station at each of the plurality of
microcells; and identifying one of the microcells to which the
mobile station should be handed off based upon the taken power
measurement.
8. The method of claim 7, comprising determining whether the mobile
station detects a corresponding pilot signal strength that exceeds
a selected threshold before taking the power measurement.
9. The method of claim 7, comprising receiving a power measurement
from the mobile station at only the identified microcell to which
the mobile station should be handed off.
10. The method of claim 7, wherein the macrocell and the identified
microcell use the same frequency for communicating with the mobile
station.
11. The method of claim 7, wherein the plurality of microcells is a
first plurality; there is at least one second plurality of
microcells within the macrocell having a second, different common
cell definition code; and comprising taking the power measurement
at only the plurality of microcells corresponding to the common
cell definition code detected by the mobile station.
12. The method of claim 7, comprising maintaining a neighbor list
for the base station serving the macrocell that does not include
the common cell definition code.
13. The method of claim 1, comprising determining if a mobile
station currently communicating with a base station serving the
macrocell detects the common cell definition code; locating a
reverse pilot signal from the mobile station on a frequency used by
the macrocell and the mobile station; and identifying one of the
microcells to which the mobile station should be handed off based
upon the located reverse pilot signal.
14. The method of claim 13, comprising transmitting the common cell
definition code near an interface between the microcell and the
macrocell on the frequency used by the macrocell.
15. The method of claim 1, comprising determining if a mobile
station within one of the microcells detects the common cell
definition code; and obtaining a radio frequency measurement report
associated with the common cell definition code from the mobile
station; and handing off the mobile station from the one of the
microcells to a frequency of the macrocell responsive to the
obtained radio frequency measurement report.
16. The method of claim 15, comprising maintaining a neighbor list
for a base station serving the microcell that does not include the
macrocell; and mapping at least one cell definition code from the
base station serving the microcell to at least one cell definition
code of the macrocell for initiating the handoff.
17. The method of claim 1, comprising detecting the common cell
definition code when approaching an interface between at least one
of the microcells and the macrocell; and reporting the detected
common cell definition code.
18. The method of claim 17, comprising at least one of detecting
the common cell definition code when approaching the macrocell from
within at least one of the microcells; or detecting the common cell
definition code when approaching at least one of the microcells
from within the macrocell.
19. The method of claim 17, comprising responding to a request for
at least one of a power measurement or a radio frequency
measurement associated with the detected common cell definition
code.
20. The method of claim 1, wherein the plurality of microcells
comprise in-building wireless service coverage areas.
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 include a
BTS within a building to provide wireless service coverage within
that building. 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. Another reason for including in-building
BTS equipment is to relieve the macrocell BTS from having to handle
traffic from users within such a building, which can increase
capacity of the overall system.
[0004] It is common for mobile stations to communicate with at
least one BTS when the mobile station is within the coverage area
of the corresponding cell or sector. As a mobile station moves, it
is often necessary to handoff between cells or sectors. One
technique for doing so is known as a hard handoff where a mobile
station stops communicating with one BTS before communicating with
a next BTS. Another technique is known as a soft handoff where a
mobile station communicates with multiple BTSs at one time during a
handoff procedure.
[0005] 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. Such handoffs are highly desirable from a
quality of service perspective. As a mobile subscriber enters a
building, for example, a handoff between the macrocell BTS and the
in-building BTS ensures that the mobile subscriber has continuous
coverage inside and outside of the building. In many situations,
handoffs may occur between the macrocell and the in-building
microcell if a mobile subscriber is near a window, for example,
where the RF signaling of the macrocell may be favorable for
communications for that mobile station.
[0006] 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.
[0007] 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. In some cases, when the
macrocell BTS and the microcell BTS are not on each other's
neighbor list, a dropped call will typically result as the mobile
station moves from the RF coverage of one cell to the other.
[0008] For such situations, the microcell or in-building system
must be designed to "overpower" the outside macrocell system to
provide inside RF coverage throughout the microcell (e.g.,
throughout the interior of a building), thereby making handoff
unnecessary. 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.
[0009] Neighbor list resource exhaustion can occur in situations
where there are a plurality of in-building microcells within a
single macrocell. The BTSs for each microcell or in-building system
are typically placed on the neighbor list of the macrocell. Having
the BTSs on the neighbor list facilitates the desirable handoff
through building portals and provides call continuity over entire
floor areas in the upper levels of a building, for example. Because
every in-building BTS must be on the macrocell's neighbor list,
problems arise where there are more than a few such BTSs. Typical
neighbor list resources place a hard limit on the number of
microcell BTSs that can be supported. Typical neighbor list
resource limits provide for approximately 20 BTS listings on the
neighbor list. Many of these are needed to support outside
communications. Adding a plurality of in-building BTSs to a
neighbor list becomes problematic.
[0010] One proposed solution is to increase the resources used for
the neighbor list (e.g., provide for a larger neighbor list size).
While this alleviates a hard limit on the number of in-building
systems that can be covered, there still are situations in urban
areas where many buildings should be covered. 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.
[0011] Another problem that arises has to do with PN offset or
scrambling code assignments. A known approach for hard handoffs
between a macrocell and in-building microcells includes using a set
of pilot beacons, one for every outside carrier that is not used
inside, for entering the building. Another set of pilot beacons,
one for every inside carrier not used outside, is used for leaving
the building. These pilot beacons transmit PN offsets or scrambling
codes on frequencies visible for mobile stations operating on
frequencies prior to handoff. Because mobile stations typically
tune to only one frequency at a time, the pilot beacons are
operating on the serving frequency before a hard handoff. The PN
offsets or scramblings codes trigger the hard handoff to the new
frequency.
[0012] As more in-building BTSs are deployed, PN offset or
scrambling code planning becomes much more complicated. New PN
offsets or scrambling code assignments are needed for each
in-building BTS and sector. These PN offsets or scrambling code
assignments need to be coordinated with the macrocell for entering
and leaving the building on each possible carrier. 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. The resources for PN offset or
scrambling code assignment are fixed by applicable standards. As
the plurality of in-building BTS increases, finding the assignments
becomes increasingly problematic.
[0013] Traffic capacity loss can arise when there are in-building
BTSs within a macrocell. When there are many such buildings within
a macrocell coverage area, the macrocell may be in greatly
increased soft handoff with the various floors of all of the
buildings within the macrocell coverage area. This can result in a
significant loss in traffic capacity as channel elements become
tied up in relatively excessive soft handoff. One compelling
financial incentive to deploy a dedicated BTS within a building is
to provide traffic capacity exhaust relief for the serving
macrocell. If there is excessive soft handoff between the macrocell
and such in building BTSs, the incentive for adding the dedicated
BTS equipment is diminished.
[0014] In the case of hard handoffs, the problems with exhausting
neighbor list resources and PN offset or scrambling code assignment
still exist. Arguably, there is no BTS traffic capacity loss nor
pilot pollution when hard handoffs are employed between a macrocell
and in-building cells within the macrocell coverage area.
Conventional wisdom favors soft handoffs as they are considered
generally more reliable than hard handoffs. Some believe that all
hard handoffs are unreliable. In the case of a handoff between a
macrocell and an in-building microcell, the scenario allows for
more reliable hard handoffs. Walking speeds of individual
subscribers combined with wall or ceiling mounted antennas that
radiate over much smaller areas make macrocell handoff experience
(i.e., handoffs between macrocells) not applicable. Some believe
that appropriate RF coverage design can facilitate reliable hard
handoffs between a macrocell and an in-building cell.
[0015] It is desirable to be able to include any number of
microcells such as in-building microcells within a macrocell
coverage area to meet the needs of a particular situation. This
invention addresses that need and avoids the shortcomings
associated with pilot pollution, traffic capacity loss, neighbor
list resource exhaustion and PN offset or scrambling code
assignment described above.
SUMMARY OF THE INVENTION
[0016] This invention includes a unique strategy using a common
cell definition code for triggering handoffs between a macrocell
and any of a plurality of microcells within the macrocell coverage
area.
[0017] An exemplary method of communicating includes using a common
cell definition code for a plurality of microcells within a single
macrocell to facilitate handoff between the macrocell and at least
one of the microcells.
[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.
[0019] An example implementation includes a PN offset or a
scrambling code as the common cell definition code.
[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.
[0021] One example includes a mobile station locating technique to
facilitate handoff between the macrocell and a corresponding
microcell. In one example, whenever a mobile station currently
communicating with the macrocell detects the common cell definition
code, a power measurement from the mobile station is taken at each
of the plurality of microcells. The microcell that receives the
power measurement from the mobile station is identified as the one
to which handoff should be completed. This example technique is
useful where soft handoffs are used and the same frequencies are
used in the microcell and the macrocell.
[0022] Another example includes hard handoffs between the macrocell
and the microcells. In this example, a long code mask is used for
locating a reverse pilot signal from the mobile station on a
frequency used by the macrocell and the mobile station. The
microcell to which handoff should be completed is identified based
upon the located reverse pilot signal.
[0023] The various features and advantages of this invention will
become apparent to those skilled in the art from the following
detailed description. The drawings that accompany the detailed
description can be briefly described as follows.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 schematically illustrates selected portions of a
wireless communication system that is useful with an embodiment of
this invention.
[0025] FIG. 2 is a flowchart diagram summarizing one example
approach.
[0026] FIG. 3 is a flowchart diagram summarizing another example
approach.
DETAILED DESCRIPTION
[0027] This invention includes a unique strategy for triggering
handoff between a macrocell and any one of a plurality of
microcells such as in-building cells within the macrocell coverage
area. A disclosed example includes using a common cell definition
code for each of a plurality of microcells within a microcell
coverage area. One common cell definition code is used for
triggering handoffs from the macrocell to a microcell. Another
common cell definition code is used for triggering all handoffs
from a microcell to the macrocell. Techniques for locating the
mobile station provide sufficient identity of the microcell
involved in a handoff as a mobile station enters the microcell
coverage area.
[0028] FIG. 1 schematically shows selected portions of a wireless
communication system 20. A wireless network 22 includes known
elements and operates in a known fashion to facilitate wireless
communications. In the illustration, a base station transceiver
(BTS) 24 and an associated radio tower provides coverage for a
macrocell schematically shown at 26.
[0029] The illustrated example is an urban area where there are a
plurality of relatively large buildings within the macrocell 26. At
least some of those buildings have a dedicated BTS to provide radio
frequency (RF) coverage within the building. Each such building is
considered a microcell within the macrocell 26 for discussion
purposes. One example microcell BTS 30 is included within a
building 32 to provide RF coverage within that building. At least
one beacon 34 is strategically positioned relative to a portal 36
that provides egress between the interior of the building (e.g., a
lobby) and the outside.
[0030] Another example BTS 40 is included within a building 42. A
beacon 44 is strategically positioned relative to a portal 46. The
example building 42 is relatively larger than the building 32 and
includes a separate BTS 48 to provide coverage within the upper
levels of the building, for example, while the BTS 40 provides
adequate coverage in the lower levels of the building, for example.
In the case of the microcell of the building 42, handoffs will
occur between the macrocell 26 and the BTS 40, for example, and
handoffs may occur between the BTSs 40 and 48 as the mobile
subscriber moves within the building 42.
[0031] The illustration includes another BTS 50 associated with a
building 52. At least one beacon 54 is strategically positioned
relative to the portal 56 to provide wireless signaling in that
area in a manner to be described.
[0032] One feature of the illustrated example is that a common cell
definition code is used for triggering a handoff between the
macrocell 26 and any one of the microcells associated with the
buildings 32, 42 and 52, respectively. Using a common cell
definition code in this example provides the same cell definition
code for each in-building microcell within the macrocell 26. In one
example, the cell definition code is a pseudo random noise offset
(PN offset). PN offsets are known in CDMA systems. Another example
includes using a scrambling code as the cell definition code.
Scrambling codes are known in UMTS systems, for example.
[0033] One example includes using a common cell definition code to
trigger a handoff as a mobile station enters a building or
microcell within the macrocell 26. Another common cell definition
code is used to trigger a handoff as a mobile station exits a
building or microcell. In such an example, only one macrocell
neighbor list entry is required to cover all in-building or
microcell BTSs for facilitating handoffs between the macrocell and
any one of the in-building BTSs. Additionally, only two cell
definition code (e.g., a PN offset or a scrambling code)
assignments are needed. This example represents a significant
simplification for managing handoffs between a macrocell and any
one of a plurality of microcells within the macrocell coverage
area.
[0034] With handoffs entering the building, an additional mobile
station locate function is included for identifying the in-building
BTS to which the mobile station should be handed off. The common
cell definition code provides an indication to initiate a handoff
but, because it is shared among several microcells, it cannot
specifically identify the microcell to which handoff is needed.
Once the appropriate building, microcell or BTS is identified, the
handoff can be completed.
[0035] As an example, consider a mobile subscriber 60 entering the
building 32. The mobile subscriber 60 is using a mobile station 62,
which is communicating with the macrocell BTS 24 while the
subscriber 60 is outside the building 32. As the mobile subscriber
60 enters the building 32, it is desirable to handoff from the BTS
24 to the BTS 30.
[0036] FIG. 2 includes a flowchart diagram 70 summarizing one
example approach for facilitating such a handoff. At 72, the mobile
station 62 is communicating in the macrocell 26. The beacon 34
transmits a signal that provides the common cell definition code
for each of the in-building BTSs 30, 40, 50 for example. As the
mobile station 62 approaches the portal 36 and enters the building
32, the mobile station detects the common cell definition code
transmitted by the beacon 34. In one example CDMA system, the cell
definition code comprises a PN offset. In one example UMTS system,
the cell definition code comprises a scrambling code.
[0037] In FIG. 2, the mobile station detects the common cell
definition code at 74. This is reported to the macrocell BTS 24 on
the frequency currently used by the mobile station 62. The BTS 24
or another portion of the network 22 such as the corresponding
mobile switching center (MSC) uses the detection of the common cell
definition code as an initiation of a handoff. To complete the
handoff, the MSC will need to identify which of the buildings the
mobile station 62 is entering. The common cell definition code does
not identify which of the microcells should be involved in the
handoff. The process for locating the corresponding microcell is
schematically shown at 76 in FIG. 2.
[0038] Two possibilities exist for completing such a handoff. In
the case of a soft handoff, the mobile station 62 is communicating
on a frequency with the BTS 24 prior to handoff. The same frequency
will be supported within the microcell so that soft handoff is
possible. In such a case, the example of FIG. 2 includes proceeding
to the step at 78 where each microcell BTS 30, 40, 50 requests a
power measurement from the mobile station. A MSC can command each
BTS associated with each microcell to request such a power
measurement from the mobile station 62. In reality, only the
microcell of the building into which the mobile station 62 is
moving or has moved will receive the power measurement from the
mobile station 62. That microcell is identified at 80 as the
microcell to which handoff should be made. The process in FIG. 2
continues at 82 by initiating the handoff between the macrocell and
the identified microcell.
[0039] In one example, initiating the handoff does not occur until
a mobile station detects the cell definition code (e.g., PN offset)
from the in-building BTS 30 or the beacon 34. One example includes
requiring that the mobile station concurrently detect a pilot
signal strength above a selected threshold. The mobile station
reports the detected pilot signal strength to the macrocell BTS 24,
which reports that to the MSC as appropriate. Once that occurs, the
system knows that the mobile station is transitioning from outside
to an inside of a building.
[0040] In situations where hard handoffs are used, the example of
FIG. 2 includes a locate function step at 84. In this example, a
tunable locate radio is included within each microcell. The BTS 30,
for example, includes a tunable locate radio 64 while the BTS 40
includes a tunable locate radio 66. In one example, the tunable
locate radio is realized by adding additional software to the radio
components of the BTS. Those skilled in the art who have the
benefit of this invention will realize how to arrange a locate
radio to meet their particular needs.
[0041] Because a hard handoff is being used, the frequency on which
the mobile station is communicating with the macrocell BTS 24 is
not being used by the corresponding in-building BTS. The locate
radio tunes to the macrocell uplink frequency and looks for the
reverse pilot from the mobile station. Tuning to the macrocell
uplink carrier and looking for the reverse pilot is possible in the
case of 3G mobile stations. In the case of CDMA mobile stations, a
specified long code mask is used for this process. For 2G mobile
stations, the locate radio uses a multi-path detection option to
search for the long code mask.
[0042] Once the locate radio detects the reverse pilot, it has
located the mobile station. The corresponding BTS is identified as
the microcell involved in the handoff at 86. Once the appropriate
microcell is identified, handoff is initiated at 82.
[0043] Now consider the mobile subscriber 67 using a mobile station
68 within the building 42. As the mobile subscriber 67 approaches
the outside of the building 42, handoff will be necessary between
the BTS 40, for example, and the BTS 24. In this example, a
dedicated, common cell definition code such as a PN offset or a
scrambling code is used for all situations when a mobile station is
leaving one of the buildings 32, 42 or 52. In this example, the
beacon 44 "illuminates" the area (e.g., a lobby) near the portal 46
by transmitting the common cell definition code.
[0044] As the mobile subscriber 67 approaches the portal 46, the
mobile station 68 detects the common cell definition code dedicated
to triggering handoffs from a microcell to the macrocell 26. FIG. 3
includes a flowchart diagram 90 that summarizes this scenario. The
mobile station is communicating in the microcell of the building 42
at 92. At 94, the mobile station detects the common cell definition
code as the mobile station 68 approaches the outside of the
building 42.
[0045] In a hard handoff case, the mobile station detects the
common cell definition code used to trigger a handoff from a
microcell to the macrocell 26 on the inside frequency and reports a
power measurement back to the MSC through the BTS 40. The MSC
already knows the identity of the BTS 40 and, because the
corresponding microcell is in a known position relative to the
macrocell 26, the MSC already knows which sector within the
macrocell 26 to which handoff should be completed. An RF
measurement report from the mobile station 68 triggers the MSC to
instruct the mobile station 68 to switch to one of the available
macrocell frequencies using standard BTS and mobile hard handoff
mechanisms. This occurs at 96 in FIG. 3.
[0046] In this example, only one cell definition code such as a PN
offset assignment is needed to exit a building and the same cell
definition code can be used for leaving all buildings to which that
assignment is made. One advantage to this example is that no
additional PN offset planning or scrambling code assignments are
needed and all in-building BTSs can be configured with a single,
identical cell definition code for triggering a handoff from any
one of the microcells to the macrocell.
[0047] A soft handoff example occurs in a similar fashion. The
mobile station 68 detects the cell definition code as the mobile
subscriber 67 approaches the portal 46. The MSC of the network 22
receives the RF power measurement associated with the cell
definition code. The MSC already knows that the mobile station is
in an area where the mobile station 68 is about to leave the
building 42 because the MSC knows, for example, the location of the
BTS 40. The MSC in one example maps the cell definition code (e.g.,
PN offset) on the in-building BTS 40 to those of the macrocell BTS
24 to initiate the soft handoff.
[0048] In soft handoff cases, it is desirable to limit soft
handoffs from the in-building BTS to the macrocell to only
scenarios when the mobile station is actually about to leave or
leaving the building. Avoiding soft handoffs while an individual 98
is using a mobile station 100 in upper levels of the building 52,
for example, is desirable. The in-building BTS should carry as much
of the inside traffic as possible to maximize macrocell traffic
capacity. Soft handoffs involving a mobile station on an upper
level of a building diminishes the macrocell traffic capacity.
[0049] One example includes preventing such soft handoffs when the
same frequency is used inside and outside by not placing the
macrocell on the in-building BTS neighbor list. If the macrocell is
not on the in-building BTS neighbor list, a handoff from the
microcell to the macrocell will not occur under most circumstances.
In a building lobby or other access point, however, soft handoff to
the outside macrocell from the inside system is desirable. The
dedicated cell definition code for triggering a handoff from all
inside systems to the macrocell can be selectively transmitted on
the inside carrier frequency only in areas near a building access
point (e.g., in a lobby). A mobile station within the appropriate
distance of one of the beacons 34, 44 or 54, for example, will
detect the appropriate cell definition code to trigger a handoff to
the macrocell 26. When the MSC receives the RF power measurement
associated with such a detected cell definition code, a
determination is made that the mobile station is near an access
point and not in an upper building level, for example. The MSC
responds by mapping PN offsets, for example, on the in-building BTS
to those of the macrocell to initiate the soft handoff.
[0050] This example approach provides the significant advantage of
facilitating a soft handoff only when desired and maximizing the
capacity of the macrocell, which is a desired result associated
with introducing the in-building BTS.
[0051] One example includes not placing the cell definition code
used for triggering a handoff as the mobile station enters a
building on the macrocell neighbor list so that no soft handoffs
from the outside system to the inside system are possible other
than near access points or portals of a corresponding building. In
such an example, no macrocell neighbor list entries are used for
facilitating handoffs from the macrocell to one of the in-building
microcells.
[0052] Although the illustrated example includes a single cell
definition code for all of the plurality of buildings shown, there
will be situations where some macrocells include a large enough
number of buildings so that buildings may be grouped into distinct
pluralities. For such situations, there may be a first plurality of
buildings having a first common cell definition code for triggering
handoffs between the macrocell and corresponding BTSs within those
buildings. A second plurality of buildings may have a second,
different common cell definition code for triggering handoffs
between the macrocell and corresponding BTSs. In other words, some
example implementations of this invention use a common cell
definition code for a plurality of buildings within a macrocell but
not necessarily all buildings within the macrocell. Even in such
situations, the advantages of reduced neighbor list size, enhanced
traffic capacity, reduced pilot pollution and simplified PN offset
or scrambling code assignment can all be realized.
[0053] The disclosed example provides a scalable solution for
supporting seamless handoffs to large numbers of in-building BTSs
deployed within the coverage area of a single macrocell sector. The
disclosed example also greatly simplifies planning PN offsets or
scrambling codes when adding in-building BTSs into an existing
network. Installation is also simplified because fewer pilot
beacons are needed for hard handoff scenarios.
[0054] The preceding description is exemplary rather than limiting
in nature. Variations and modifications to the disclosed examples
may become apparent to those skilled in the art that do not
necessarily depart from the essence of this invention. The scope of
legal protection given to this invention can only be determined by
studying the following claims.
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