Uplink Power Control Recommendations for IEEE 802.16m
Document Number: IEEE C802.16m-08/666r2
Date Submitted: 2008-07-13
Source: Ali Taha Koc, Shilpa Talwar, Apostolos Papathanassiou, ali.t.koc@intel.com, shilpa.talwar@intel.com
Rongzhen Yang, Nageen Himayat, Hujun Yin
Venue: IEEE 802.16m-08/024 Call for System Description Document (SDD) Comments and Contributions, on the
topic of “Power control”.
Base Contribution: None
Purpose: Discussion and approval of the proposal into the IEEE 802.16m System Description Document
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Uplink Power Control
IEEE 802.16m system requirements from SRD [1]
•
•
Average User Throughput
Cell edge Throughput
Our view, IEEE 802.16m requirements on uplink power control can be improved by
• Open Loop Power Control (OLPC)
–
–
–
•
Closed Loop Power Control (CLPC)
–
–
–
•
Slow power control
Provide balance between cell edge and sector throughput
Limit the level of IoT
Fast power control
Limit the MAC overhead of the signaling
Effective estimation of uplink interference
Coupling of CLPC and OLPC
–
Provides high efficiency with reduced signaling overhead
Open Loop Power Control
• Path loss and shadowing variation can be compensated
– Received signal level from different users does not have very large
dynamic range
• Average IoT (Interference over Thermal) levels can be controlled
• Using an Open Loop Power control saves on continuous
signaling of power control commands
• Using fractional open loop power control provides flexibility to
balance the cell edge vs. sector throughput
Closed Loop Power Control
• In a full frequency reuse system, inter-cell interference dominates
the system performance
– Controls the power of interference sources individually at the expense of
signaling overhead
– Requires estimating uplink interference per user
– Requires measurement of uplink received signal strength (for example,
HARQ ACK/NACK can be used)
• Track and compensate the effect of user’s fast fading channel
OLPC and CLPC Comparison
Open Loop
Power Control
Signaling Overhead Low
Power Control
Accuracy
Closed Loop
Power Control
High
• Only broadcast algorithm initial
parameters
• No signaling for power control
commands
• Broadcast algorithm initial
parameters
• Signaling power control
commands
Low
High
•Algorithm depends on the estimated
path loss value that is based on
reciprocity
• Limited information on MS make it
difficult to achieve better adaptation
• Accurate uplink signal
measurement and interference
estimation.
• Better control of interference by
BS information exchanging
Common Open Loop Schemes
• Full Power
User Thrpoughput CDF with different SNR Targets
1
– Bad for the cell edge users
– High SINR variation
0.9
SNR Target=0dB
SNR Target=8dB
SNR Target=15dB
SNR Target=23dB
Full Power
0.8
• SNR based
• Low SNR target: bad for the
center edge user (low
spectral efficiency)
• High SNR target: bad for the
cell edge users (high
interference)
0.6
F(x)
– Each user transmits enough
power to meet receive SNR
target at BS
– BS broadcasts the SNR target
– Tradeoff
0.7
0.5
0.4
0.3
0.2
0.1
0
0
0.5
1
1.5
User Throughput
2
2.5
3
x 10
6
Proposed Open Loop Scheme
• Takes interference into account
– Set the SNR target depending
on the DL SIR estimate
Users Throughput CDFs with SIR based power control
1
 = 0dB, 
 = 8dB, 
 = 8dB, 
 = 8dB, 
0.9
= 0.75
= 0.5
= 0.75
=1
0.8
dB
dB
SNRtarget
     SIRnearestBS
0.7
• Base station broadcasts gamma and
beta values
• Automatic ‘Soft reuse’
– Assign high SNR target for
cell-interior users
– Assign low SNR target for celledge users
F(x)
0.6
0.5
0.4
0.3
0.2
0.1
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Users Throughput
1.6
1.8
2
2.2
x 10
6
Justification of Proposed Method
We can write SNR as
SNR  (1  INR) * SINR
 (1  INR ) * SIR
in a interference limited system
We set SNR target as function of SIR (dB)
SNRdB  10 log 10(1  INR)  10 log 10( SIR )
SNRd B     * SIRd B
• By changing gamma, the scheme can control the IoT level
• By changing beta, the balance of cell edge versus sector throughput can
be adjusted
IoT Curves for Different Gamma
cdf of IoT
1
=0,=3/4
0.9
=8,=3/4
0.8
0.7
F(x)
0.6
0.5
0.4
0.3
0.2
0.1
0
-2
0
2
4
6
x
8
10
12
Open Loop and Closed Loop Coupling
Data Traffic
Fast control
Slow control
Fast
control
Slow control
Fast control
• Open Loop is going to be the default scheme
– Open Loop Power Control coarsely adjust uplink transmit power with
low signaling overhead
• When there is data packets, Closed loop will the main power
control scheme
– Closed Loop Power Control signals can be piggyback to the data traffic
for the fast control (for example, only a few bits combined with
resource allocation IE (DL/UL MAP IE) for refined power adjustment)
Minimize CLPC Signaling Overhead
by Piggyback
Proposed CL scheme
BS
1. Received signal strength and
interference measurement
2. CLPC algorithm decides the
offset
ACK/NACK for DL HARQ
CQICH
Uplink Data Burst
Resource Allocation IE with
Power control command
MS
Example for using existing loop – DL HARQ for CLPC
MAP
New
Data
MAP
Retrans
MAP
New
Data
MAP
Retrans
MAP
DL
UL
NACK
ACK
DL HARQ IE
NACK
Power Control Command
Bit Field (1~2 bits)
ACK
New
Data
Text Proposal to IEEE 802.16m SDD
Insert the following text into Physical Layer clause (Chapter xx in [IEEE 802.16m-08/003r1])
------------------------------- Text Start ------------------------------11.x.x.x Uplink Power Control
A power control algorithm is supported for in the uplink channels with both initial calibration and periodic
adjustment procedure. The parameters of power control algorithm are optimized on system-wide basis by
the BS, and broadcast periodically.
11.x.x.x.1 Open Loop Power Control
Uplink open loop power control compensates all or a fraction of the pathloss and shadowing. Uplink open
loop power control uses channel and interference knowledge to operate at optimum power control settings.
Mobile stations can derive their initial transmission power according to the path loss, interference
measurements obtained from the downlink preambles and pilots.
11.x.x.x.2 Closed Loop Power Control
Closed loop power control compensates for fast variations in channel and interference on a per subscriber
basis, while minimizing MAC signaling overhead.
Closed loop power control couples with open loop power control for efficient operations. Closed loop
power control is active with data transmission. Close loop power control measures uplink power using
uplink data and/or control channel transmissions and sends control command in unicast service control
channel (USCCH).
------------------------------- Text End -------------------------------
References
[1] IEEE 802.16m-07/002r4, “TGm System Requirements
Document (SRD)”
[2] IEEE 802.16m-08/004r1, “Project 802.16m Evaluation
Methodology Document (EMD)”
Backup
Uplink Power Control in Legacy System
•
Open Loop
–
–
–
•
Maintain same transmitted power density unless the maximum power reached
Normalized CNR values are determined in the standard
Full path loss compensation
Closed Loop
–
–
–
–
In the initial ranging phase, the mobile users derives its initial transmission power
according to the path loss measurement from the downlink pilot channel.
After that the mobile users adjusts its transmission power according closed-loop
power control messages signalled by the base station.
Signalling overhead is too much for the 802.16e system.
BS -> MS: Send Offset information
Pnew  Plast  (C N new  C N last )  (10 log 10 ( Rnew )  10 log 10 ( Rlast ))  offset
•
•
•
FPC (Fast Power Control) Message
Power Control IE
RNG-RSP for periodic ranging
IoT Curves
cdf of IoT
1
•
SNR Target=0dB
SNR Target=5dB
SNR Target=8dB
SNR Target=13dB
SNR Target=18dB
Full Power
0.9
0.8
0.7
•
•
F(x)
0.6
0.5
0.4
0.3
0.2
0.1
0
-10
0
10
20
30
x
40
50
60
Common
assumptions for IoT
= 7dB
SNR Target = 8dB
provides IoT target
Changing SNR target
has shifting affect on
IoT values
SINR Curves
cdf of SINR
1
0.9
SNR Target=0dB
SNR Target=5dB
SNR Target=8dB
SNR Target=13dB
SNR Target=18dB
Full Power
0.8
0.7
F(x)
0.6
0.5
0.4
0.3
0.2
0.1
0
-60
-40
-20
0
x
20
40
60
SINR Curves
cdf of SINR
1
•
0.9
0.8
Gamma=0,Alpha=3/4
Gamma=8,Alpha=1/2
Gamma=8,Alpha=3/4
Gamma=8,Alpha=1
–
0.7
F(x)
0.6
•
0.5
0.4
0.3
0.2
0.1
0
-30
-20
-10
Increasing Gamma
has minimal affect
on SINR
0
x
10
20
30
Increasing gamma
increase signal
strength and
interference at
same time
After finding the
optimum gamma,
alpha is important to
have cell edge and
center cell balance