IEEE C802.16m-09/0005
Project
IEEE 802.16 Broadband Wireless Access Working Group <http://ieee802.org/16>
Title
Coordinated scheduling for interference mitigation in IEEE 802.16m
Date
Submitted
2009-01-02
Source(s)
Olga Muñoz, Josep Vidal, Adrián Agustín
Dep. of Signal Theory and Comm.
Technical University of Catalonia (UPC)
Campus Nord, Jordi Girona 1-3
08034 Barcelona, SPAIN
Re:
Voice: +34 93 401 09 64
E-mail: olga.munoz@upc.edu
josep.vidal@upc.edu
adrian.agustin@upc.edu
Call for Comments on Project 802.16m System Description Document (SDD).
Contribution pertains to tgmsdd [12 December]
SDD section 20.3
Abstract
This contribution considers the usage of coordinated scheduling in IEEE 802.16m
Purpose
For IEEE 802.16m discussion and eventual adoption
Notice
Release
Patent
Policy
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Coordinated scheduling for interference mitigation in IEEE 802.16m
O.Muñoz, J.Vidal, A.Agustin
Dept. of Signal Theory and Communications
Technical University of Catalonia (UPC)
Introduction
IEEE 802.16m shall support the fractional frequency reuse (FFR) to allow different frequency reuse factors. If
users served from different BS experience a low level of interference, radio resources may be reused, applying a
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IEEE C802.16m-09/0005
high reuse factor and thus increasing the system spectral efficiency. On the other side, if the served users
experience large interference, orthogonal transmissions are better and therefore a lower frequency reuse factor
should be used.
In basic FFR concept, subcarriers across a whole frequency band are grouped into frequency partitions with
different reuse factors. Associated concepts to FFR are therefore the whole frequency band (or common
resource pool), the different partitions of the common resource pool (frequency partitions), and the set of
coordinated BSs which are going to use the same or different partitions over the common resource pool, using in
this way a different effective bandwidth depending on the reuse.
The use of a given frequency reuse factor must be synchronous for all the coordinated BSs, that is to say all the
BSs in the set are going to use the same frequency reuse factor at a given time. As the IEEE 802.16m frame
structure allows for the definition of time zones defined as an integer number (greater than 0) of consecutive
subframes, a mean to allow the use of the same frequency reuse factor at a given time is to define, in a given
frame, one or more zones, each one with a specific FFR. The time zones must be synchronous for the
coordinated BSs.
The FFR pattern may be fixed or dynamic. As the impact on the system throughput depending on the specific
FFR selected depends highly on the MS distribution and the channels conditions of the users to be served, a
dynamic FFR is preferable. To support dynamic FFR pattern the BSs shall be capable of exchanging FFR
information with each other (decentralized approach) or with some control element in the backhaul network as
an ASN gateway or some self-organization control entity (centralized approach).
Coordinated scheduling
The scheduling policy impacts on the suitability of the FFR, as it determines which users are to be served.
Therefore, it makes sense to consider the FFR as a result of the scheduling decision.
The proposal consists in allowing the control element in the network to be in charge of coordinating the
scheduling among the BSs. Depending on the interference and the scheduling metric considered, the scheduling
algorithm dynamically takes the decision on the reuse factor. Furthermore, the control element is allowed to take
the decision to move some of the resources (i.e. subchannels) from one sector to another, therefore making a
better use of the system resources to increase the system performance according to a given utility criteria.
Different levels of centralization can be considered:
-
The most centralized level is that one where the coordinator entity decides the scheduling of all the users
in the coordinated area. In this case, the selection of the users to be served is done directly by the
coordinator entity along with the FFR to be used and the number of subchannels assigned to each user.
The decision can be made based on the signal to noise ratio (SNR) and signal to noise ratio plus
interference (SNIR) in case of a frequency reuse factor of 1 of the users.
-
A lower level of centralization is possible when each BS decides the users to be served for both reuse 1
(where interference is expected from neighbors BS) and to reuse 1 over the number of coordinated
sectors (where no interference is expected). Then, the coordinated BSs reports to the coordinator entity
information regarding the result of this internal scheduling. This information can be a scheduling metric
or the SNR and SNIR of the selected users. Based on this information, the coordinator entity selects the
FFR to be used and the number of subchannels (or equivalently frequency partitions) each BS receives.
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IEEE C802.16m-09/0005
As an example consider, for instance, that the scheduling policy is to maximize the throughput in the
coordinated area. Consider the following scenario:
Number of coordinated sectors
3 (see Fig.1)
Cell radius
1 km
Spatial user distribution
Uniform within the coordinated area
BS transmission power
33 dBm (equal for the the three BSs)
Noise figure at the MS
0 dB
Noise power spectral density
-174 dBm/Hz
Total bandwidth
20 MHz
Carrier frequency
3.55 GHz
BS antenna gain
10.6 dB
Antenna pattern used for each Specified as [1]:
BS sector
   2

A     min 12 
 , Am 
  3dB 

A   is the antenna gain in dB in the direction  ,
3dB is the 3 dB bandwidth (corresponding to 70º)
Am =20dB is the maximum attenuation.
Finally, an SNR loss of 4 dB has been considered to account for the degradation w.r.t. the capacity due to nonideal modulation and coding.
Figure 2 shows the spectral efficiency of the coordinated area, in bits/s/Hz, versus the number of users in the
coordinated area. Several strategies are compared.

First, uncoordinated scheduling with a fixed reuse factor of 1 (solid line) is considered. Each BS
schedules its own users according to the scheduling strategy (in this example, the criteria has been the
maximization of the throughput in the serving area). As the BSs transmit in an uncoordinated manner
with a reuse factor of 1, the users will be interfered by the neighboring BSs.

Second, uncoordinated scheduling with a fixed reuse factor of 1/3 (-+-) where interference is avoided by
dividing the subchannels in three disjoints frequency partitions.

Finally, the previous strategies are compared to the performance achieved when the coordinator entity
selects the FFR to be used (-□-). According to the scheduling strategy (maximizing the throughput in the
coordinated area) the coordinator entity may move some frequency resources (arising to different
frequency partitions) from one BS to another.
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IEEE C802.16m-09/0005
24
23
Achievable Rate (bits/s/Hz)
22
Sect 1
Sect 3
Sect 2
21
20
19
18
17
16
Fixed reuse K=1 (interference)
Fixed reuse K=1/3 (orthogonal transmission)
Coordinated scheduling with flexible reuse factor
15
14
Fig. 1 Scenario
0
20
40
60
80
Number of users in the coordinated area
100
120
Fig. 2 Performance in bits/s/Hz
To support coordinated scheduling, the following issues are necessary:
-
A coordinator entity that can be a BS in the coordinated set or some control element in the backhaul
network.
-
A radio resource region common to all the coordinated BSs. The frequency partitions will be performed
over this radio resource region as a result of the scheduling policy.
-
Capability for the MS to perform interference measurement on this radio resource region (this is
something that is considered in 802.16m already, see section 20.1.1 of [2]).
-
Finally, the coordinated BSs must be capable to exchange interference information with the coordinator
entity.
Proposed Text for the support of coordinated scheduling
[section 20.3, line 11-12 of page 155, additional proposed text in blue]
ABS can schedule AMSs with high mutual interference potential on different subchannels or frequency
partitions, e.g. by exchanging scheduling constraints between coordinating ABSs.
AMSs with high mutual interference potential can be scheduled on different subchannels or frequency partitions
by using coordinated scheduling.
[section 20.3, line 12-13 of page 155, additional proposed text in blue]
The necessary interference prediction may be based on the interference measurement mechanisms defined in
20.1 and 20.2. The ABSs shall be capable of exchanging with each other, or reporting to some control element
in the backhaul network, interference statistics, FFR configuration parameters and/or scheduling metrics
associated to different FFRs.
The decision of the users to be served within each scheduling period shall be taken individually by the serving
ABS or by a control element in the backhaul network, according to the scheduling metrics and/or interference
statistics reported by the AMSs or the coordinating ABSs. The decision on the FFR and on the frequency
partition for each scheduling period shall be taken coordinately by the coordinating ABSs (decentralized mode)
or by the control element in the backhaul network (centralized mode), according to the scheduling metrics
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and/or interference statistics reported by the coordinating ABSs. The control element may decide to assign one,
several or all frequency resource units to one single ABS depending on the scheduling metrics reported. The
association of an AMS to a frequency partition can be taken by the AMS, the serving ABS or by the control
element in the backhaul network.
References
[1]
[2]
Gamini Senarath et al, "Multi-hop Relay System Evaluation Methodology (Channel Model and Performance
Metric)”, IEEE 802.16j-06/013
Shkumbin Hamiti, “The Draft IEEE 802.16m System Description Document”, IEEE 802.16m-08/003r6.
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