New generation 4G systems like LTE and WIMAX is designed to support frequency reuse-1 mechanisms to enable a universal frequency reuse pattern providing operators with best achievable use of their valuable spectrum. Ever increasing interest is shown by companies and operators to find mechanisms to allow reuse1 deployments. This article will cover one of the key enabling technologies i.e. ICIC and its application in allowing universal use of Reuse 1 networks.

OFDMA based 4G networks can support reuse one deployment through the use of;

• Ability to power control and vary the coding rate of control channel;

• Fractional power control with coordination with controller based overload control messages in UL;

• Support of very low code rates;

• Incremental redundancy based HARQ;

• DL and UL ICIC further enhance single cell frequency reuse;

• Multi-Antenna techniques.

One of the key goals is to increase spectral efficiency and overall SINR of the system. However, users at the cell edge are particularly susceptible to increased interference resulting in reduced throughput due to higher transmit powers required and inter-cell interference. Therefore Interference coordination as methods to reduce inter cell interference is gaining momentum and industry attention.

Figure 1: Intercell interference scenarios

Fractional Frequency Reuse

: In its simplest form is Fractional frequency re-use (FFR) implements a reuse scheme-n (n > 1) system. A reuse-n system partitions a geographical area into n regions, each of which is exclusively allocated to a band in such a way that cells physically close to each other are assigned with different bands to avoid dominant ICI. Cells that are sufficiently far from each other may reuse the same band, and how frequently the reuse is practiced is dictated by the reuse factor n. For instance, n = 3 if the same band is reused every three cells. However, segmenting frequency re-use suffers from reduced spectral efficiency.

Therefore FFR in 4G systems(LTE and WIMAX) tries to define a sweet spot where the cell center of neighboring cells share the same band, while their cell edge are separate on orthogonal bands. Besides, the cell-center and cell-edge bands in neighboring cells are non-overlapping. The colour on the spectrum is shown to match the colour of the geographical area.

Figure 2: Illustration of Fractional Frequency Reuse

Possible ICIC implementation in LTE

ICIC in downlink

Two common implementations that will be supported for Downlink ICIC are static and semi-static. Static ICIC is initially planned which also includes the need for some level of system planning. Semi-static ICIC utilizes an event triggered message (RNTP) over the X2 interface and reduces the system planning impact. Relative Narrowband Tx Power (RNTP) is transmitted when the Tx power exceeds a specified threshold. The frequency of the RNTP transmission is limited to no more than 200 ms to prevent overload of messaging.

DL Static ICIC implementation: Figure 3 shows the concept of DL ICIC using inverted Reuse scheme. Some restricted PRBs (also called non-preferred frequency zones) are defined per each cell and the base stations transmit at a certain constant nominal power across the entire bandwidth except for those restricted PRBs. The restricted PRBs can be transmitted at a lower power (e.g. 10 dB lower than the nonrestricted PRBs) which results in a soft fractional frequency reuse scheme or there may not be any transmissions at all in the restricted PRBs which results in a hard fractional frequency reuse scheme.

Each cell can then schedule its cell edge users in the restricted PRBs of its closest neighbor and hence they can realise an improved SINR as the neighboring cell is transmitting with a lower power or not transmitting at all on those PRBs. The classification of users is done in two groups: cell inner users and cell edge users. Reporting of the users can be done based on some metrics such as path loss for UL or CQI reporting for DL.

Figure 3: Downlink ICIC static implementation

ICIC in uplink

Two implementations also exist for Uplink ICIC. Static ICIC includes the need for some level of system planning. Semi-static ICIC utilizes an event triggered message (HII and OI) over the X2 interface and reduces the system planning impact. High Interference Indicator (HII) and Overload Indicator (OI) are event triggered messages. HII is sent indicating the PRBs and subbands where the serving cell intends on scheduling cell edge UEs and thereby causing high interference. OI is sent indicating low, medium, or high interference levels. Care needs to be taken when utilizing ICIC in a multi-vendor eNB environment, as the behavior of the eNB is implementation specific when receiving ICIC related indicators.

Figure 4: Uplink ICIC implementation

DL Static ICIC implementation: In the uplink, the main idea is to concentrate all interference in specific portions of the bandwidth, known as the Trash Heap. Figure 4 shows the concept of the Trash Heap which consists in designating a portion of the bandwidth in each cell to bear the brunt of the interference from neighboring cells. As for DL, the users are classified in two groups, inner cell and cell edge users, using a any algorithm based on CQI reports for DL and path loss estimation for UL. And as for DL, the soft fractional frequency reuse concept is applied and included in the concept of Trash Heap. For cell edge mobiles, the uplink scheduler will assign resources in the Trash Heap of the mobile’s strongest neighbouring cell, which is identified by event triggered reporting. If the scheduler needs to assign the mobile outside the Trash Heap, it does so with a reduced transmit power spectral density (PSD) level, which is implemented through an absolute power control command in the UL scheduling grant. The idea here is to concentrate the bulk of the inter-cell interference in a small portion of the total bandwidth, thereby preventing the majority of the users from getting impacted by this interference which is now localized to certain sub-carriers.

Possible ICIC implementation in WiMAX

WiMAX Release 1.0 allows reuse 1 deployments, however most of the commercial systems are deployed in reuse3. The initial implementation of WiMAX in reuse 1 will be realized by FFR based techniques. Specific implementation of FFR techniques is presented in WIMAX 1.5 and WiMAX 2.0 papers.

The effect of inter-cell interference in reuse 1 systems of WiMAx results in MAP and traffic performance degradation especially in cell edge areas. Interference mitigation technique is needed to alleviate the cell edge performance degradation. Inter-Cell Interference Coordination (ICIC) may be used as the key technique for making reuse 1 work better.
ICIC implementation in WiMAX is possible with signaling information exchanges within Base stations over R8 interfaces.

Possible implementation of ICIC in downlink:

The WiMAX systems can implement DL ICIC to solve DL inter-cell interference problem in reuse 1 systems.

• It can automatically adjusts both frequency usage and Tx power pattern considering interference pattern from neighboring cells
  
• Edge user throughput is increased by coordinating inter-cell interference which is needed to exchange scheduling information among BSs
  

Possible implementation of ICIC in uplink:

• Automatically controls MS Tx power based on load information exchanged among BSs
  
• It is needed to exchange scheduling information and measurement reports form MS
  

Figure 5: ICIC implementation in WiMAX

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