## 5 Simulation Results  
In this section, 2-bit feedback SPC is compared to 1-bit feedback SPC. The proposed CoMP scheme is compared to the conventional scheme. The simulation results are based on the OFDM system. The simulation parameters are as follows: the number of carriers is 256 and the length of the cyclic prefix (CP) is 64. The modulation order is quadrature phase shift keying (QPSK). All channels are the Rayleigh fading channels of 7-multi paths. The conventional coding is applied with the code rate of 1/2 and constraint length of 3. The path loss is applied to the signal according to distance between the base station and mobile [5]. Figure 7 shows the BER performance of the constructive SPC and MRT (full-bit SPC) scheme. The constructive 2-bit SPC scheme has higher BER performance than the constructive 1-bit SPC scheme in all SNR environments. In addition, the constructive 2-bit SPC scheme achieves a BER performance similar to that of the MRT scheme with only 2-bit feedback information. Therefore, the proposed CoMP scheme uses the constructive 2-bit SPC scheme to improve the diversity gain of the mobile and reduce implementation complexity. Figure 8 shows the BER performance when the interference is mitigated by the destructive SPC and MRT (full-bit SPC) schemes. The destructive 2-bit SPC scheme mitigates interference better than the 1-bit destructive SPC scheme. And, the destructive 2-bit SPC scheme has lower performance than the MRT scheme. However, in the MRT scheme, the implementation complexity is high because all feedback information on the channel phase experienced by the interference signal is used. Therefore, the proposed CoMP scheme uses the destructive 2-bit SPC scheme to mitigate the interference signal. Figure 9 shows the BER performance of the proposed and conventional schemes. In this figure, \(-3\), \(-6\) and \(-12\) dB mean the power in proportion to ICI compared with the signal power. The conventional system refers to the system without interference mitigation, and the conventional scheme refers to the scheme proposed in [18; 19]. When the mobile is located between two adjacent pico cells (case 2), the proposed and conventional schemes improve the diversity gain and mitigate the interference by applying the constructive and destructive SPC schemes. However, because the conventional schemes use a relay base station or relay mobile, system performance is reduced by propagating errors to the receiver. The proposed scheme does not use a relay base station or relay mobile and the pre-coding is applied directly in the base station in the pico cell, so there is no performance degradation due to error propagation. Figure 7: BER performance of the constructive SPC and MRT schemes. Figure 8: BER performance of the destructive SPC and MRT schemes. Figure 9: BER performance of the proposed and conventional schemes according to the mobile’s location. In case of the conventional system case 2, the BER \(10^{-2}\) is not satisfied in all SNR environments regardless of the ratio of interference signal to the desired signal. In the conventional scheme in [18], when the interference signal has the lowest power, it satisfies BER \(10^{-2}\) at approximately SNR 9 dB. However, the remaining case of the conventional scheme in [19] does not satisfy the BER performance in all SNR environments. On the other hand, the proposed scheme satisfies BER \(10^{-2}\) at approximately 3, 7 and 18 dBs. Therefore, the proposed scheme has better BER performance than the conventional schemes. Figure 10 shows the BER performance when there are no interference signals (case 1) and the mobile is located between three pico cells (case 3). In case 1, because there are no interference signals, the proposed scheme has the same BER performance as that of conventional systems. In case 3, because the interference signal is transmitted from the other two base stations, the conventional scheme has low BER performance.