The implementation problem of SFR in LTE

杨学志

Soft frequency reuse (SFR) has been the de-facto standard of inter cell interference coordination (ICIC), one of the most fundamental feature of LTE. In the SFR scheme, only a part of the band is used on the cell edge with full power while the whole band is available in the cell center with limited power. The cell edge bands of adjacent cells are othorgonal to avoid severe interference. The essence of SFR is to increse the SINR at the cell edge at the cost of available bandwidth. It was proved that reuse 3 can achieve the best compromise between the SINR and available bandwidth, to get the highest cell edge capacity.

However, in the current popular implementation of SFR in LTE system, the gain of SFR is senario dependent. In some cases, a 30%~100% cell edge capacity gain can be observed, while negative gain can still be seen in lot of cases. Let's have a close look at the LTE specification to find the reason.

In the LTE R8, there are common reference signal (CRS) and PDCCH in the downlink. These two channels are transmitted at full power. In the SFR implementation in LTE, only the power on PDSCH are reduced on the cell center band, while the power of CRS and PDCCH can not be reduced, leading to interference. In a synchronized network, the CRS interference will cause channel estimation error which is later introduced to data in the demodulation process. The PDCCH interference will cause scheduling signaling errors. In a asynchronize network, the CRS and PDCCH will interference the PDSCH of adjacent cells directly. The result of CRS and PDCCH interference is that, the available bandwidth at cell edge is only 1/3 while the SINR gain is not that much to compensate the bandwidth loss.

The CRS and PDCCH interference has been noticed by the 3GPP community, the ongoing CSI-RS and ePDCCH projects are trying to solve the problem. Even if these endeavor can succeed, it is sth. beyond R11. The LTE are still required to be compatible with R8 in a farely long period. So it is crucial to solve the problem in R8. Is it possible?

The answer is YES.

The SFR can either be implemented on 3 parts of one carrier, or on 3 carriers. They are essentially identical. However, the one carrier implementation will face the CRS and PDCCH interference problem due to the limitation of LTE R8. If you use 3 carriers, one as the cell edge carrier and whole as the cell center carrier, the power on CRS and PDCCH can be equally reduced with the PDSCH in cell center. Then the problem is solved simply and completely. At the same time, SFR is also implemented on the PDCCH channel, solving its capacity problem.

One problem with the multiple carrier solution may be the peak rate, if an operator do not have enough spectrum. For example, if an operator has 20MHz spectrum, it can be divided into four 5MHz carriers. Actually, the peak rate is mainly used for ads, it rarely happens in practical cases. Then the multiple carrier solution do not impair the advertisement if it can improve the actual user experience. You can still use the high peak data rate in your ads, if the user can not actually experience it.