1 11 Frequency Reuse Techniques for Attaining both Coverage and High Spectral Efficiency in OFDMA Cellular Systems Zheng Xie and Bernhard Walke RWTH Aachen.

111

Frequency Reuse Techniques for Attaining both Coverage and

High Spectral Efficiency in OFDMA Cellular Systems

Zheng Xie and Bernhard Walke

RWTH Aachen UniversityAachen, Germany

WCNC 2010

222

Outline Introduction Previous Schemes

Soft Frequency Reuse (SFR) Incremental Frequency Reuse (IFR)

Enhanced Fractional Frequency Reuse (EFFR) Evaluation Conclusion

3

Frequency Reuse Factor Effective reuse of resources can highly enhance

the system capacity Frequency reuse factor (FRF) K defines frequency

reuse pattern

With a smaller frequency reuse factor (FRF), more available bandwidth can be obtained by each cell

4

Previous Frequency Reuse Schemes With the usage of FRF-1, the most user terminals (UTs) are

afflicted with heavy Inter-cell interference (ICI) Especially near the cell edge

The conventional method to figure out this problem is by increasing the FRF mitigate the ICI efficiently but decrease on available bandwidth

The most representative approaches improving cell-edge performance while retaining spectrum efficiency Soft Frequency Reuse (SFR) scheme Incremental Frequency Reuse (IFR) scheme

5

Soft Frequency Reuse (SFR) Scheme

CCU: cell-centre usersCEU: cell-edge users

6

Limitations of SFR How to define the borderline to divide cell area for CCUs

and CEUs is a key issue Generally, there are more CEUs than CCUs in a cell

since the outer surface area is much larger than the inner part CEUs have maximum one third of the entire bandwidth to utilize,

which results in lower spectrum efficiency

More ICI could happen even in a low traffic-load situation, while there are still subchannels in idle and underutilized in the system The resource allocation via the SFR scheme starts always from

the first subchannel up

7

Incremental Frequency Reuse (IFR) Scheme (1) The only difference between the IFR design and

the classical reuse-1 Classical reuse-1: allocate resources always from the

first subchannel IFR: start dispensing resources from different points

8

Incremental Frequency Reuse (IFR) Scheme (2) IFR scheme can overcome the low spectrum reuse

efficiency problem and the more ICI at low loading traffic problem

IFR scheme only performs better when just fewer traffic exists in a system When the loading factor is greater than 0.3, it is

inferior to the SFR scheme

9

Enhanced Fractional Frequency Reuse (EFFR) (1) Enhanced Fractional Frequency Reuse (EFFR) scheme

intends to retain the advantages of the both approaches Concept

Define 3 cell types for directly contiguous cells in a cellular system Reserves for each cell-type a part of the whole frequency band named

Primary Segment The Primary Segments among different type cells should be

orthogonal The Primary Segment of each cell will be further divided into

reuse-3 part: cannot be reused by directly neighboring cells reuse-1 part: is at the same time a part of the Secondary Segments

belonging to the other two cell-types

10

Enhanced Fractional Frequency Reuse (EFFR) (2)

11

Power Allocation and SINR Estimation Transmission Power Allocation

Any cell-type is not allowed to use the reuse-3 subchannels dedicated to the other two cell types The power allotted to the reuse-3 subchannels can be tripled

Signal-to-Interference-Ratio (SINR) Estimation A cell acts on the Secondary Segment as a guest, and occupying

secondary subchannels is actually reuse the primary subchannels belonging to the directly adjacent cells

Reuse on the Secondary Segment should conform to two rules: monitor before use resource reuse based on SINR estimation

Each cell listens on every secondary subchannel all the time Before occupation, a cell makes SINR evaluation and chooses resources

with best estimation values for reuse If all available secondary resources are either occupied or not good

enough to a link, this cell will give up scheduling resources

12

Resource Allocation The reuse-3 subchannels will be assigned to CEUs

If there are still resources remained after all CEUs are served, they will be continuing allotted to such CCUs with relatively poor SINR values

When the reuse-3 subchannels are exhausted, the remaining reuse-1 subchannels in the Primary Segment are allocated to residual unsatisfied users

If still resources are requested, available reuse-1 subchannels in the Secondary Segment will be scheduled to adequate users by applying interference-aware-operation

13

Distinctions between the EFFR Scheme and the Two Aforementioned Schemes

CEUs are very susceptible against ICI, the reuse-3 subchannels in the Primary Segment are exclusively available for the users

To reduce excessive ICI to the neighboring cells, packets will be sent on a reuse-1 subchannel in lower strength

Allocation of reuse-1 subchannels in the Secondary Segment is not blindly carried out, but in an interference-aware way according to SINR estimation

In the Primary Segment unsatisfied users, whether they are CCUs or CEUs, have the same chance to get resources in the Secondary Segment

IFR SFREFFR

14

Relevant Factors The following relevant factors play paramount

roles and could influence the system performance The ratio of the number of reuse-3 subchannels M to

reuse-1 subchannels N in the Primary Segment The power ratio of high power level to low power level Range definition for partition of CCUs and CEUs

This is the first work to present simulation results of the SFR scheme with varying range definitions

SINR threshold for reuse

15

Simulation Environment (1) Simulation Tool

The Open Wireless Network Simulator (OpenWNS)

16

Simulation Environment (2) For all simulations

We consider an OFDMA uplink cellular system in an omni-cell case

UTs are uniformly distributed within each hexagonal cell Each UT has a maximal transmission power of 200mW

Different cell specific power

17

Scenario with 15 UTs in each cellMean Overall Uplink Cell Capacity

18

Scenario with 15 UTs in each cellMean Weakest User Uplink Throughput

19

Scenario with 25 UTs in each cellMean Overall Uplink Cell Capacity

20

Scenario with 25 UTs in each cellMean Weakest User Uplink Throughput

21

Conclusion A novel frequency reuse technique named the EFFR

scheme for ICI mitigation in OFDMA networks is proposed

The EFFR scheme uses dedicated FRF-3 and higher transmission power for CEU allows CCU to occupy resources with FRF-1 and interference

awareness

With respect to the range definition for division CCU-zone and CEU-zone, the EFFR scheme can provide more flexibility and robustness than the SFR scheme