Impact of Sub-Band Correlation on SFR and Comparison of FFR and SFR

Abstract

In soft frequency reuse (SFR) when the user is classified as a cell-center user based on signal-to-interference-plus-noise-ratio in a sub-band, the user retains its sub-band. On the other hand, if the user is classified as a cell-edge user, a new sub-band is allocated. We analyze the impact of correlation between the cell-center sub-band and the cell-edge sub-band for a user when the SFR technique is used in a cellular orthogonal frequency division multiple access (OFDMA) system. The coverage probability (CP) and the average rate are derived for the following two cases: 1) when the sub-bands are independent and 2) when the sub-bands are completely correlated. We show that correlation significantly decreases the edge CP and the average rate of the SFR technique, and as the power control factor increases, the impact of correlation decreases. Fractional frequency reuse (FFR) and SFR techniques are compared and it is shown that the impact of correlation on the FFR is significantly lower. Furthermore, it is also shown that the FFR provides better edge CP and average rate when compared with the SFR in single-input single-output (SISO) networks, thereby suggesting that the FFR should be preferred over SFR in cellular SISO OFDMA systems. However, for single-input multiple-output networks, the SFR provides a better average rate when compared with the FFR, especially when the sub-band correlation is not significant. Finally, the impact of log-normal shadowing has been carefully studied.