1. IEEE 802.16m DL Interference Mitigation
Document Number:
IEEE C802.16m-08/626
Date Submitted:
Source:
Shirish Nagaraj, Phil Fleming, Fan Wang
Motorola
*<
Venue:
TGm – Call for contributions on Project m System Description Document – IEEE m-08/024
(interference mitigation)
Base Contribution:
Abstract:
Proposal for 16m downlink interference mitigation.
Purpose:
Adoption of proposed text/content for m System Description Document
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2. DL Interference Mitigation
Background & Problem
Latency-sensitive traffic (VoIP, video etc) capacity is determined by the air-interface delay outage.
Coverage improvements even for other classes of traffic are highly desirable, especially to allow dynamic fractional frequency reuse
Loading conditions in real deployments not known a-priori
Objective is to improve coverage and reduce outage with minimal impact to system capacity:
Interference Overload Feedback: feedback from MS to interfering cells to indicate interference overload
Enables slow but dynamic adaptation of Fractional Frequency Reuse (FFR) patterns using feedback from all users
Adapts to actual cell and traffic distributions as opposed to static power/bandwidth partitioning

3. Adaptive Interference Mitigation
Interference mitigation by fractional frequency reuse (FFR)
Partition total bandwidth into sub-bands
Reduce power spectral density (PSD) on specific sub-bands to achieve interference relief to users in other cells
Allocate different cells a priority ordering of sub-band usage –
high priority indicates that the PSD will not be reduced, and low priority indicated PSD can be reduced on that sub-band
A cell will schedule its bad geometry users to a high priority (Priority I) sub-band, and good users on the lowest priority (low PSD) sub-band
Power distribution on sub-bands is not fixed a-priori
Users have a periodic (slow) signaling opportunity to indicate interference overload on their sub-band
Signal is sent over the air through the UL Interference Overload Feedback information
Cells respond to the composite Interference Overload Feedback signal and reduce their PSD on specific sub-bands according to received power of received signal
Achieves adaptation in FFR patterns autonomously, without having to fix power distributions

4. Adaptive FFR using Interference Overload Feedback
Users in Priority I sub-band in all cells transmit Interference Overload Feedback indicating sub-band used
In response to received Interference Overload Feedback indicating sub-band 2, Cell A will reduce PSD by a larger value compared to Cell C
Default power configuration is equal PSD for all sub-bands
Cells re-adjust their PSD levels after the reception of every Interference Overload Feedback signaling interval

5. Interference Relief using Interference Overload Feedback
Slow adaptation of fractional frequency reuse patterns
Depending on an aggregate metric of Interference Overload Feedback signal, a BS can decide how much power to reduce on a given sub-band to allow for “soft FFR”
Signaling is fed back at a slow basis, and allows FFR to adapt to network-specific loading conditions
Other cells know the sub-band request by the position/modulation of the Interference Overload Feedback
Users will automatically reach their dominant interfering cells (immediate neighbors)
Requires a pool of common resources allocated for Interference Overload Feedback signaling
Users can send the Interference Overload Feedback in a CDM or FDM fashion
All the Interference Overload Feedback signals corresponding to the same sub-band request will incoherently combine at each base-station receiver
Cells will mute or reduce transmit PSD on those requested sub-channels for the a duration following the request till the next Interference Overload Feedback opportunity
How much power to reduce depends on the total power of the received (composite) Interference Overload Feedback signal, and the sub-band priority for that cell
Power of Interference Overload Feedback signal can be boosted to reach strongest interfering cell

6. Standards Support Required for Adaptive Interference Mitigation
Ability for MS to transmit an Interference Overload Feedback signal on UL
Can use a system-wide common pool of bandwidth for such signaling (UL common control channel)
Signaling opportunities in time also defined system-wide, with a given periodicity
Same Interference Overload Feedback signal is transmitted by all users corresponding to a given sub-band given incoherent combining gains at the cell site
Signaling can be a CDM waveform modulated with on-off keying, with a specific signature sequence for each sub-band
Users not experiencing outage (or lower than expected data rates), will not transmit any energy on Interference Overload Feedback signal
Sub-bands defined can be logical (no need to be physically contiguous)
See C80216m-08_275 on UL Interference Overload Feedback