analysis of hybrid adaptive/non-adaptive multi-user ofdma systems with imperfect channel knowledge

Analysis of hybrid adaptive/non-adaptive multi-user OFDMA systems with imperfect channel knowledge

Alexander Kühne

18.03.10 | Alexander Kühne, Technische Universität Darmstadt, Communications Engineering Lab

Motivation (I)

1

OFDMA Multiple access scheme for future radio systems

Offers the possibility to allocate time-frequency resources to different users

f

t

Adaptive OFDMA: Adaptive subcarrier allocation and

modulationAdvantages:Exploitation of multi-user diversityGood performance with perfect channel

knowledgeDisadvantages:– Instantaneous channel knowledge required

at the transmitter

Non-adaptive OFDMA: Fixed subcarrier allocation and

modulationAdvantages:Exploitation of frequency diversityNo instantaneous channel knowledge

at transmitter requiredDisadvantages:– No optimal channel exploitation

possible


Motivation (II)

2

Combine both access schemes to a hybrid OFDMA system

t

f

- Resource for non-adaptive transmission

- Resource for adaptive transmission

Frequency multiplexing

Problems:

User specific imperfect channel knowledge

How to decide which user is served adaptively or non-adaptively?How to allocate the resources?How to select the applied modulation schemes?


Outline

Assumptions

Hybrid OFDMA

3

Problem formulation

SNR threshold problem

User serving problem

Considering overhead

Performance evaluation

Conclusions


Assumptions

Single cell scenario: One BS with nT transmit antennas and

U MSs with nR receive antennas each

TDD-OFDMA with N subcarriers

Orthogonal Space Time Block Coding (OSTBC) or Transmit Antenna Selection (TAS) at the transmitter and Maximum Ratio Combining (MRC) at the receiver

Different user demands Du

4

System assumptions

Channel modelResource unit consisting of Q subcarriers in frequency and MT OFDMA symbols in time

Temporally correlated block fading

Resource unit based Channel Quality Information (CQI):

Imperfect CQI due to time delays and estimation errors

Q

MTt

f


Problem formulation hybrid OFDMA

5

Preprocessing:

Impairment parameters

SNR threshold vector

User serving vector

User demand vector



6

Problem formulation:

Preprocessing:



6


Preprocessing:



Two types of adaptive/non-adaptive resource allocation

7

Non-Adaptive First (NAF) allocationFirst, the resource units of the non-adaptive users are allocated following an round robin approach

Second, the remaining resource units are allocated following the WPFS policy

t

f

Non-adaptive user

adaptive users


Two types of adaptive/non-adaptive resource allocation

7

Non-Adaptive First (NAF) allocationFirst, the resource units of the non-adaptive users are allocated following an round robin approach

Second, the remaining resource units are allocated following the WPFS policy

Adaptive First (AF) allocation

First, all resource units are allocated to the adaptive users applying WPFS

Second, the worst of these selected resource units are re-allocated to non-adaptive users t

f

Non-adaptive user

adaptive users



8

Goal: To optimally adjust the modulation scheme SNR thresholds for each possible in dependency of the impairment parameters and the different user demands

Adjustment of the weighting factors applying WPFS to fulfill user demands

Analysis of the distribution of the SNR values of allocated resource units

Derivation of analytical expressions of the average user data rate and bit error rate (BER) using the CQI error models together with SNR distributions

Maximization of the user data rate subject to the target BER using the analytical expressions by adjusting the SNR thresholds


Adjustment of the weighting applying WPFS

9

Assumptions:Each user demands Du resource units with

Users having the same demand are grouped in demand groups with i=1,..,G

Resource units for adaptive users are allocated following WPFS policy:

Question: How to adjust p such that Du is fulfilled for each user?No direct relation between pu and Du

Different antenna techniques (OSTBC-TAS) and adaptive/non-adaptive resource allocation schemes (NAF-AF)

Solution:


Weighting for WPFS

10

Example with


Maximizing user data rate

For both OSTBC and TAS as well as NAF and AF analytical expressions for the average user data rate and BER of an adaptively served user u can be derived:

- SNR thresholds

- Channel estimation error variance

- Correlation coefficient (time delay)- User demand vector11

- Number of TX/RX antennas

- User serving vector


Assumptions: Parameters are known at the BS

12

Solution:

Non-adaptive users: Problem reduces to a one-dimensional search for the proper modulation scheme

Adaptive users: Lagrange multiplier approach to determine SNR threshold vector

Maximizing user data rate by means of user-wise SNR threshold optimization



13

For each possible user serving realization , the maximum achievable data rate of adaptively and non-adaptively served users subject to the target BER are determinable

Problem: Find that which maximizes the system data rate while fulfilling the minimum user data rate requirement for NAF and AF

Assumption:

There are 2U possible user serving realizations


Algorithms

Exhaustive Search (ES) algorithm:

14

Check all 2U possible solutions

Unpractical for a large number of users

Reduced Complexity (RedCom) algorithm:

Notice: User data rate and BER of adaptively served users do not depend on itself, but on the number of adaptive users in the different demand groups

Not all 2U combinations have to be tested but only the tuples

Example:


Algorithms

Exhaustive Search (ES) algorithm:

14

Check all 2U possible solutions

Unpractical for a large number of users

Reduced Complexity (RedCom) algorithm:

Notice: User data rate and BER of adaptively served users do not depend on itself, but on the number of adaptive users in the different demand groups

Not all 2U combinations have to be tested but only the tuples

Example: for G=U:

for G=1:Further complexity reduction possible exploiting monotonic behavior of data rate with respect to number of adaptive users


Complexity

15


Considering overhead

16

So far, no pilot and signaling overhead has been taken into account

To achieve realistic results and for a fair comparison, it is important to incorporate the overhead in the overall system performance

Pilot and signaling overhead effects both downlink and uplink

Introduction of frame structure to identify the amount of required pilot and signaling overhead in hybrid systems

Introduction of an effective data rate taken into account the overhead in both up- and downlink


Considering overhead – Superframe structure

17

LSF – superframe length

MT – time frame size in OFDMA symbols

TS – ODFMA symbol duration

for NAF LSF ≥ 1

for AF LSF = 1


Considering overhead – Effective user data rate

18

For a given user set of one can formulate the effective user data rate of adaptively or non-adaptively served users as a weighted sum of uplink and downlink data rates:

The effective system data rate can be also maximized using the RedCom algorithm


Performance evaluation – Simulation Parameters

19

Bandwidth for DL and UL each 10 MHz

Number U of users 15

Number N of subcarriers 240

Frequency block size Q 8

Number Nru of resource units 30

Time frame size MT in OFDMA symbols 28

Number nT of transmit antennas 2

Number nR of receive antennas each 2

Multiple antennas scheme TAS-MRC

Carrier frequency 2 GHz

Target bit error rate BERT 10-3

User demand D in resource units [2, 2, …., 2]


Performance evaluation – Neglecting overhead (I)

20


Performance evaluation – Neglecting overhead (II)

21


Performance evaluation – Neglecting overhead (III)

22


Performance evaluation – Considering overhead (I)

23


Performance evaluation – Considering overhead (II)

24


Conclusions

Analytical expressions for the average user data rate and BER of a hybrid OFDMA system

25

for two different adaptive/non-adaptive resource allocation schemes NAF and AF

applying OSTBC and TAS in combination with MRC

for different user demands

assuming imperfect CQI

Maximization of system data rate subject to target BER and minimum user data rate by solving the SNR threshold and the user serving problem

Consideration of pilot and signaling overhead

Hybrid OFDMA systems outperform conventional pure adaptive or pure non-adaptive OFDMA systems for increasing user-dependent imperfect CQI even when considering overhead

analysis of hybrid adaptive/non-adaptive multi-user ofdma systems with imperfect channel knowledge

Documents

transmitternonadaptive

adaptive subcarrier

timefrequency resources

fixed subcarrier allocation

mt ofdma symbols

access schemes

future radio systemsoffers

time delays