Interference mitigation in Femtocell
Document Number:
IEEE C802.16m-08/1427
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, typically overlapped
with macro-cells to improve the indoor coverage by providing the
coverage up to 30m.
– Using existing wired broadband connection as backhaul (cable,
DSL, Ethernet)
• Scenarios to be considered
– Open access
• Femto serves any MS within the coverage
– Closed access
• Femto only serves the MS subscribed with it
Interference scenarios in overlapped networks
• Macro<->Femto
– Frequency shared by macro and Femto
• Mutual co-channel interference
– Dedicated frequency for Femtocell
• No interference
• Femto<->Femto
– Co-channel interference if spectrum reusing
Macro-cell BS
Femtocell
Femtocell
Femtocell
Downlink Interference of femto -> macro
macro MS interfered by Femtocell
•
Outdoor macro users may experience interference by nearby indoor
Femtocell
– Femto installed near houses boundary, penetration/leakage through windows will
cause interference to outdoor macro users near houses
– In order to cover whole house, femtocell has to enhance/boost its tx power,
which would increase the interference to outdoor nearby users.
•
Indoor macro users may be interfered by Femtocell
– In closed access scenario
– DL signal from Femtocell is much stronger than that from macro BS for indoor
users, which may prevent indoor macro MS from decoding signals from macro
BS
– There might be a dead zone around Femto BS within which macro MS can not
connect to macrocell
Femto DL strongly interferes
indoor macro MS from
connecting macro BS
Femto BS
Leakage from indoor
Femto interfere with nearby
outdoor DL signal
macro BS
Macro MS
Femto MS
macro MS
Uplink Interference of macro to femto
• Indoor macro MS may interfere with indoor femto MS
– High tx power of indoor macro MS, due to big path loss and wall
loss, especially at cell edge
– Low tx power of indoor Femto MS, due to short distance
– High UL signal strength from indoor macro MS may prevent
indoor MS from connecting to Femtocell in UL
Femtocell experiences
interference from strong
macro MS UL signal
Femto BS
macro BS
Macro
MS
Femto MS
Solutions to interference mitigation
• Interference detection & mitigation
– Femtocell BS should have the capability to estimate the surrounding
potential interference based on scanning the neighboring Femtocells and
macro-cell
– Femtocell BS estimates and reports the interference from all neighboring
cells to NCMS or Femto gateway
– The NCMS shall coordinate with macro BS and neighboring Femto BSs to
mitigate the interference based on the scanning results from Femto BS as
well as MS
• Interference mitigation approaches may include
– Assigning appropriate frame partitions or subframes different from neighbors
– Adjustment of transmitting power
• It could be performed
– Once a Femto BS initiates itself
– Periodically to adapt to the changing environment
• Femto BS may perform periodical interference detection when the attached MSs
are in idle/sleep unavailable intervals
Proposed Text For SDD
17 Support for Femtocell
17.x Interference mitigation
Femto BS shall have the capability to measure the surrounding
interference by scanning the neighboring Femtocells and
macro-cell. MS shall also have such capability and report the
measurement results. The measurement may be performed
periodically. The interference mitigation approaches shall be
performed by Femtocells and macro-cells under the
coordination.