PHY-Mode Selection and
Multi User Diversity in
OFDM based Transmission Systems
M. Stemick, S. Olonbayar, H. Rohling
Hamburg University of Technology
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Frequency
OFDM-FDMA System (Single Cell)
Time
1) Frequency Selectivity
2) Additional Path Loss + Shadowing
10
|H|2 [dB]
2 [dB]
|H|
|H|[dB]
0
-10
-20
user1
user2
user3
-30
-40
0
4
8
12
Bandwidth [MHz]
Bandwidth
[MHz]
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20
Bandwidth [MHz]
2
Define User Capacity
User Capacity: Number of users per cell @ fixed data rate
Goal: maximize user capacity
maximize number of users at a fixed data rate
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Cell Models
• Same data rate for all users
• Perfect channel knowledge
and synchronisation
• Downlink situation
• Time-invariant channel
Single cell with N users at the
same distance from base station
This scenario is based on a frequency selective channel model
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Channel Model
Channel variation over bandwidth:
WSSUS channel parameters:
10
Maximum delay
3,2 µs
0
Exp.
Number of multipathes
30
OFDM symbol duration
16 µs
Bandwidth
20 MHz
Subcarriers
256
2 [dB]
|H|
|H|[dB]
Power delay profile
-10
-20
user1
user2
user3
-30
-40
0
4
8
12
Bandwidth [MHz]
Frequency
16
20
Fading of subcarriers varies strongly between users
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Subcarrier Selection Algorithm
Selection criteria: maximize Z
Nu
K
Selection
parameter
Z  H i , j xi , j
2
i 1 j 1
user
1
xi , j  
0
allocation
no allocation
subc.
Nu
x
i 1
i, j
 1 for all j
subcarrier
under the following constraints:
user
Solution of optimization problem by Hungarian Algorithm
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Simulation Results
random select.
Subcarrierwise selection, QPSK, R=1/2
adapt. select.
best subc. select.
4 users
8 users
16 users
Increasing number of users
→ Adaptive subcarrier allocation yields a high diversity gain
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Additional Path Loss and Shadowing
New situation in the cell:
• Users are uniformly distributed
• Path loss: GPL  d    2.6
• Shadowing: Log normal distribution
(  SH  4dB )
• Cell radius: R = 100m
Consequence:
Path loss and shadowing cause high variation in average
receive power between users
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Received SNR For Different Users
Received SNR for different users
High variance of receive power:
SNR [dB]
• Different number of subcarriers
per user to fulfil QoS
• Additional PHY-Mode selection
Frequency [MHz]
Higher complexity of subcarrier allocation
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Modified Allocation Algorithm
start
1) Select one subcarrier for each user by
Hungarian Algorithm
2) Determine SNR for each selected subcarrier
and choose a suitable PHY-Mode
3) Repeat steps 1) and 2) for every user,
until demands for data rate are satisfied
Select one
subcarrier for
every user
Choose PHY
for selected
subcarriers
Rate
achieved?
no
yes
end
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Allocation Example
Selecting subcarriers based on
H i, j
2
Hi
users
subcarriers
1
2
3
1
2
3
1
11
7
5
1
11
7
5
2
8
12
7
3
4
6
3
4
6
9
4
9
4
9
6
7
5
5
5
7
11
6
12
8
7
5
8
1
2
3
1
11
7
5
9
3
4
6
9
6
7
4
9
6
7
5
7
11
9
9
7
10
6
12
8
11
11
8
6
9
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11
13
9
9
7
10
6
9
10
9
9
7
10
10
12
6
8
3rd iteration
2nd iteration
1st iteration
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Subcarrier Allocation & Data Rate Development
Target Data Rate: 3Mbps
User Index
1
2
256 QAM
3
128 QAM
4
64 QAM
5
6
32 QAM
7
16 QAM
8
QPSK
9
10
1
2
3
4
5
6
7
8
9
10
Number of Subcarriers
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Simulation Results
Percentage of satisfied users for different data rates
• Subcarriers allocated by Hungarian Algorithm
• Subcarrierwise PHY-Mode selection
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20 users
38 users
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Comparison Between Considered Models
Comparison between uniform und circular user distribution
data rate: 3.0 Mbps cell radius: 100m
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Conclusions
• Adaption to frequency selectivity achieves a high diversity
gain of maximum 7 dB
• Simultaneous adaption to path loss and frequency
selectivity achieves high user capacity
• OFDM-FDMA + PHY-Mode selection allows flexible
adaption to various QoS demands and system loads
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Thank you for your attention
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