Mobility Support in Femtocell
Document Number:
IEEE C802.16m-08/1431r1
Date Submitted:
2008-10-31
Source:
Kaibin Zhang, Gang Shen, Jimin Liu
Voice:
+86-21-50554550-8261
Alcatel Shanghai Bell Co., Ltd.
E-mail:
kaibin.zhang@alcatel-sbell.com.cn
Venue:
IEEE 802.16m-08/040-Call for Comments and Contributions on Project 802.16m System Description Document (SDD), on the topic of
TGm SDD Femtocells
Base Contribution:
N/A
Purpose:
To be discussed and adopted into the 802.16m SDD by TGm.
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Introduction
• Femtocell
– Femtocell, tiny, low-power cellular base stations, operated in
licensed frequencies.
– A femtocell installed in homes or SOHO, improves the indoor
coverage by providing the coverage up to 30m.
– Typically using existing wired broadband connection as backhaul
(cable, DSL, Ethernet)
• Scenarios to be considered
– Femtocell overlapped with macro-cell - Full coverage
• Frequency shared by macro and femto
– High spectrum efficiency, potential interference
• Dedicated frequency for femtocell
– No co-channel interference, but less efficiency
– Discrete femtocells w/o macro overlapping
• Seldom using, not considered in this contribution
Mobility issues to be considered
• Handover scenarios related to Femtocell
– Macro->Femto, Femto->macro, Femto<->Femto
• Technical issues
– Neighboring notification & scanning
• Long time and larger signaling overhead if broadcasting neighboring list due
to the large number of neighboring Femtos
• Number of Femtos is greater than the upper limit of size of neighbor list
– Ineffectual HO attempts
• A Femto only support very few users, other handover attempts will be rejected,
which introduces much signaling load over air interface
– Paging
• Put hundreds of Femto BS in a paging group may cause much overhead
Proposed solutions
• The neighboring list of a macro cell is classified into two parts, the
neighboring macro-cells and neighboring/ overlapped Femtocells
• The neighboring macro-cells list shall be sent to MS via broadcast
– Same as usual, no impact on solutions w/o Femto
• The neighboring Femtos list shall be sent to MS via unicast
– The neighboring Femtos may only include the Femtocells close to the MS,
so that the overhead can be reduced.
– The neighboring Femto list may be determined based on the network
topology
– When the MS moves within the coverage of macro-cell, the neighboring
Femto list may be updated by macro BS or other network control entity
• The Femtocells not supporting more users shall not be included in the
neighboring Femtocell list
– Ineffectual handover attempts are avoided
Proposed solutions
• If an MS performs handover from a macro-cell to a
Femtocell, the context of the MS shall be kept at macro
BS, instead of removed.
– Speed up the handover from Femtocell back to macro-cell
• Authentication, ranging, registration may be omitted
• Two methods to support paging in Femtocells
– Femto BS performs paging as a normal macro BS
• Signaling overhead over backhauling
– MS may get the paging messages from the macro BS directly.
Femto BS is not included into the paging group
• Simple application and less overhead
Proposed Text For SDD
17 Support for Femtocell
17.x Mobility
The major handover process shall follow the description in section
10.3, except the following:
– The neighboring macro-cell list shall be sent out via broadcast
while the neighboring Femtocells list may be sent to an MS via
unicast. The neighboring Femto list may only include the
Femtocells close to the MS. The Femtocells w/o the capability to
support more users shall be excluded from the neighboring
Femtocell list.
– For an MS performing handover from a macro-cell to a Femtocell,
the context of MS shall be kept at macro BS, instead of removed.
– Femto BS may not be included into the paging group and not be
involved in the paging process, such that an MS in a Femtocell
may synchronize to and get the paging messages from the
overlapped macro BS directly once the idle mode of this MS is
initiated.