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Dynamic TDD Frame Offset in Local Area Environment

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AALTO UNIVERSITY

SCHOOL OF SCIENCE AND TECHNOLOGY

Faculty of Electronics, Communications and Automation

Department of Communications and Networking

Espoo, Finland, 08 March 2011

Dynamic TDD Frame Offset in Local Area Environment

Student: Shahrukh Bin Ali (80540W)

Supervisor: Professor Olav Tirkkonen

Instructor: M.Sc. Chia-Hao Yu

Place: Nokia Research Center, Nokia Group, Finland

Methods: Literature studies, MATLAB simulations

ALI 80540W - MS Thesis Seminar

- Background:
- TDD and Crossed-Slot Interference
- Problem Setting
- Thesis Objective

- Parameters
- Results
- Observation Summary

- Parameters
- Results
- Observation Summary

ALI 80540W - MS Thesis Seminar

- Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) networks are expected to deliver higher data rates with lower latency and improved indoor coverage, in particular with the utilization of Femto cells1.
- Uncoordinated Femto cell deployments imply high interference multiple-user scenarios are likely to be very common, especially if closed subscriber groups (CSGs) are in use.
- Many techniques like frequency hopping, Single Antenna Interference Cancellation (SAIC), Successive Interference Cancellation (SIC) exists to boost user date rates by mitigating interference.
- In LTE systems based on Time Division Duplex (TDD) scheme, the temporal relationship between the TDD frames of the cells can be utilized to minimize interference.

1V. Chandrasekhar, J. Andrews, and A. Gatherer, “Femtocell networks: a survey,” Communications Magazine, IEEE, vol. 46, no. 9, pp. 59 –67, 2008.

ALI 80540W - MS Thesis Seminar

- One notable feature of TDD systems is the presence of crossed-slot interference in case the network is not fully synchronized, i.e. the downlink (DL) transmissions of one cell coincide with the uplink (UL) transmissions (or vice versa) of another cell in the neighborhood. The time slots in which crossed-slot interference occur are also called crossed slots.
- Quantitative analysis of this type of interference has already been conducted in some published works2,3 and its magnitude with respect to the co-channel and adjacent channel interference has been compared. While these works attempt to mitigate crossed-slot interference, in our study we attempt to utilize it when the non-crossed-slot interference is higher.
- In our work, we employ dynamic (variable) TDD frame offset parameter4 to control TDD related interference by introducing crossed-slots and use it to maximize certain system utility functions in symmetrical traffic situation when there is very low density of UEs associated per AP.

2H. Holma, G. Povey, and A. Toskala, “Evaluation of interference between uplink and downlink in UTRA/TDD ,” in Vehicular Technology Conference, 1999. VTC 1999 - Fall. IEEE VTS 50th, 1999.

3C.-J. Chen and L.-C. Wang, “Suppressing opposite-direction interference in TDD/CDMA systems with asymmetric traffic by antenna beamforming,” Vehicular Technology, IEEE Transactions on, vol. 53, no. 4, pp. 956 – 967, 2004.

4H. Haas, S. McLaughlin, and G. Povey, “A novel interference resolving algorithm for the TDD TD-CDMA mode in UMTS,” in Personal, Indoor and Mobile Radio Communications, 2000. PIMRC 2000. The 11th IEEE International Symposium on, 2000.

ALI 80540W - MS Thesis Seminar

- Problem Setting

Cell 1

Cell 2

1

2

4

3

Cell 1

Cell 2

ALI 80540W - MS Thesis Seminar

- Problem Setting

Cell 1

Cell 2

1

2

4

3

Cell 1

Cell 2

ALI 80540W - MS Thesis Seminar

- Thesis Objective: To investigate the impact of dynamic offset between the frames in a TDD system (on signal interference level) within neighboring cells with very low density of users in local area scenario.
- More specifically, to explore the degree to which we could exploit the TDD frame offset to improve the throughput by lowering interference level through admission of crossed-slots and maximization of system level utility.

ALI 80540W - MS Thesis Seminar

- Background:
- TDD and Crossed-Slot Interference
- Problem Setting
- Thesis Objective

- Parameters
- Results
- Observation Summary

- Parameters
- Results
- Observation Summary

ALI 80540W - MS Thesis Seminar

- Dynamic frame offset between the three cells is defined by ‘m’ and ‘n’.
- Maximum value of ‘m’ and ‘n’ is 1 frame length. Minimum value of ‘m’ and ‘n’ is 0.
- The granularity of frame offset is given by the steps in which the values of ‘m’ and ‘n’ are varied uniformly between 0 and 1 as shown in the table.

ALI 80540W - MS Thesis Seminar

- In the preliminary investigation of three cells we calculate throughput normalized to system bandwidth:
where is the received signal-to-interference and noise ratio or SINR in linear scale.

[bps/Hz]

ALI 80540W - MS Thesis Seminar

- For investigation in local area scenario we calculate throughput as:

[bps/Hz]

where, represents the upper bound for throughput, is the SINR, represents an implementation margin and H(·) is an unit step function which forces throughputs to be zero for less than . We have used = 5.8 corresponding to the rate coded 64-QAM, = 1.5848 (2 dB) and = 0.398 (- 4 dB).

The Shannon Capacity plot represents the upper bound for the throughput values for respective SINR. Individual MCS plot represents the throughput values for the respective MCS schemes employed. They reach individual saturation points at different SINR value. The envelope of these saturation values has been profiled by the Ideal-Envelope plot as shown. The Modelled plot of the above equation shows its legitimacy for estimating throughputs based on SINR values in local area environment.

ALI 80540W - MS Thesis Seminar

- We consider three utility functions (sum-rate, proportional fair and max-min) as the objective functions for maximization of system level utility (fairness) in order to determine the optimal frame offset through exhaustive search.

ALI 80540W - MS Thesis Seminar

- Background:
- TDD and Crossed-Slot Interference
- Problem Setting
- Thesis Objective

- Parameters
- Results
- Observation Summary

- Parameters
- Results
- Observation Summary

ALI 80540W - MS Thesis Seminar

- Parameters for preliminary simulation.

ALI 80540W - MS Thesis Seminar

- Comparing the distribution of system utility with and without offset for the three utility functions reveal significant gain in system level utility through optimization of frame offset.
- We here consider 20-ary frame offset.

ALI 80540W - MS Thesis Seminar

- Comparing the distributions of total throughput (DL+UL) per UE with and without offset for the three utility functions reveal significant gain in throughput through optimization of frame offset.
- The CDF plots of total throughput per UE for the three utility functions almost exactly overlap and exhibit almost equal gain.

ALI 80540W - MS Thesis Seminar

- Plot of histogram of the distribution of optimum offset for the three utility functions reveal that offset combination indexes 1, 11, 201 and 211 have been selected as optimal for most number of drops.
- These four combination indexes signify 2-ary frame offsets between the cells, which is a subset of the 20-ary frame offsets.

ALI 80540W - MS Thesis Seminar

- Hence, we conclude that almost equal gain in performance could be achieved through frame offset granularity of 2-ary compared to 20-ary frame offset or frame offset of finer granularity.

ALI 80540W - MS Thesis Seminar

- Here, we observe the variation of crossed-slot area based on the relative positions of the nodes in a two cell scenario.

ALI 80540W - MS Thesis Seminar

(

(

(

(

(

(

(

(

)

)

)

)

)

)

)

)

1 + d2-n

d6-n +1

1 + d2-n

d4-n +1

1 + d1-n

d5-n +1

1 + d1-n

d4-n +1

1 + d1-n

d3-n +1

1 + d2-n

d5-n +1

1 + d2-n

d3-n +1

1 + d1-n

d6-n +1

> 1

n = path loss exponent

d-n = is a measure of signal strength of a particular radio link

- We derive this criterion and suggest that when this criterion is fulfilled 2-ary frame offset should be applied to maximize sum-rate system utility.

ALI 80540W - MS Thesis Seminar

Using crossed-slot criterion

- We verify here that optimization of frame offset through maximization of sum-rate utility using crossed-slot criterion yields equal performance compared to that of exhaustive search.

UE 2 fixed

Using exhaustive search

ALI 80540W - MS Thesis Seminar

- We now propose a majority voting scheme intending to manifest self-organization based on the derived crossed-slot criterion. We intend to maximize sum-rate system utility with optimized 2-ary frame offset.

- STEPS (for every simulation drop):
- Each cell forms Pairs with each of the other cells in turn .
- Each cell determines the required frame offset as per the X-slot criterion for each pair.
- Each cell combines the frame offsets it has determined in step-2 and generates a X_SLOT_REPORT of the optimum frame state of the system by considering itself as the reference point.
- Each X_SLOT_REPORT from each cell is referred to cell 1 as the reference.
- The referred-X_SLOT_REPORT from each cell is compared with each other and a final optimum frame state of the system is determined based on MAJORITY DECISION.

ALI 80540W - MS Thesis Seminar

ALI 80540W - MS Thesis Seminar

ALI 80540W - MS Thesis Seminar

- We verify here that majority voting scheme yields similar gain in system utility and throughput compared to exhaustive search using sum-rate system utility.

ALI 80540W - MS Thesis Seminar

- 2-mary frame offset provides essentially the optimal system performance compared to other granularity of frame offsets.
- Significant gain in system utility (fairness) and throughput is achieved through optimization of TDD frame offset.
- Analytical expression in the form of crossed-slot criterion exists for sum-rate system utility for optimization of 2-ary frame offset for a two cell scenario.
- Majority Voting Scheme could provide a self-organization through optimization of 2-ary frame offset with maximized sum-rate utility.

ALI 80540W - MS Thesis Seminar

- Background:
- TDD and Crossed-Slot Interference
- Problem Setting
- Thesis Objective

- Parameters
- Results
- Observation Summary

- Parameters
- Results
- Observation Summary

ALI 80540W - MS Thesis Seminar

- We chose a standard WINNER A1 Indoor Office Scenario in this study.

ALI 80540W - MS Thesis Seminar

- There is significant gain in DL and UL throughput. Mean DL and UL throughput for Sumrate system utility is increased by 27% and 36% respectively. Mean DL and UL throughput for PF system utility is increased by 23% and 39% respectively. However, system Max-min utility function yields zero gain and equals the performance of no offset case (overlapping plots). In addition, there is about 10% reduction in outage for DL throughput for frame offset with Sum-rate and PF utility maximization.

ALI 80540W - MS Thesis Seminar

- Plots of distribution of system utility functions exhibit gains except for system utility function Max-min, which overlaps with the system utility in case of no frame offset.
- Note: for PF system utility function, distribution of geometric mean is plotted instead of sum of logarithms as almost all snapshots have at least one throughput (DL or UL) value equal to zero.

ALI 80540W - MS Thesis Seminar

- Significant gain in mean throughput and system utility (fairness) is achieved through optimization of 2-ary frame offset.
- System utility functions Sum-Rate and PF yield almost equal performance gain in throughput level.
- Gain in sum-rate utility is more prominent than proportional-fair utility.
- System utility function Max-min yields zero gain or equal performance compared to no offset case.

ALI 80540W - MS Thesis Seminar

- Background:
- TDD and Crossed-Slot Interference
- Problem Setting
- Thesis Objective

- Parameters
- Results
- Observation Summary

- Parameters
- Results
- Observation Summary

ALI 80540W - MS Thesis Seminar

- 2-ary TDD frame offset provides the optimal performance compared to frame offsets of finer granularity.
- In local area scenario with very low density of UEs associated per AP having symmetrical DL/UL traffic, application of dynamic (2-ary) TDD frame offset with maximization of sum-rate and proportional-fairness system utility significantly improves mean throughput by lowering interference level for average performing users.
- Optimization with max-min system utility yields zero performance improvement for average users.
- There is increase in system utility (fairness) with the application of dynamic 2-ary frame offset with gain in sum-rate utility more prominent than proportional-fair utility. However, both achieves almost equal gain in mean throughput.
- It would be fruitful to asses the scenario with 2 or more UEs per AP, and check whether further granularity in frame offset can be advantageous (M-ary frame offset when there is M/2 UEs per AP). This may manifest the possibility of slot level frame offsetting (within a frame) in the form of fractional dynamic frame offset with different traffic symmetry to maximize system performance.
- It would be also fruitful to check the effectiveness of majority voting scheme in local area scenario.

ALI 80540W - MS Thesis Seminar

ALI 80540W - MS Thesis Seminar