## 4 Proposed Scheme
When the number of pico cells and base station antennas in the macro cell is increased, the received signal is as follows,  
\[Y_{Case2}=\underbrace{\left(H_{SB,1}+\sum_{k=1}^{M}\Big{[}H_{LB,k}\cdot C_{2} ^{s}\Big{]}\right)\cdot S_{1}}_{a2}+\underbrace{\left(\sum_{i=2}^{N}\Big{[}H_ {SB,i}+H_{LB,i}\cdot C_{2}^{d}\Big{]}\right)\cdot S_{2}}_{b2}+N. \tag{12}\]  
In Equation (12), the \(k\) means the index of the base station antenna in the macro cell. Case 3: Figure 6 shows the case 3 that the mobile located within the intersection of three pico cells. When the mobile is located within the intersection of the three pico cells, the mobile receives the signal distorted by the interference signal transmitted from the base station of the pico cell 2 and pico cell 3. The received signal is as follows,  
Figure 3:0 Figure 3:1  
The \(S_{2}\) is the interference signal transmitted from the base station of pico cells 2 and 3. \(H_{SB,3}\) is the channel coefficient between the base station of the pico cell 3 and the mobile. \(a3\) denotes the desired signal. Because \(b3\) is not a desired signal, it is regarded as noise by the mobile. When the number of the pico cells is increased, the received signal is as follows,  
Figure 3:2  
In case 3, the proposed CoMP scheme forms the virtual MISO system in cooperation with the base stations of the macro cell for improving diversity gain of the mobile. The base station of the macro cell transmits the signal with the constructive 2-bit feedback SPC scheme, in order to improve the diversity gain of the mobile by raising the SNR of the desired signal. Additionally, the base stations of pico cells 2 and 3 transmit the signal with the destructive 2-bit feedback SPC scheme in order to mitigate the effect of the interference, by reducing the SNR of the interference signal. After the constructive and destructive SPC scheme are applied, the received signal is as follows,  
Figure 3:3 Figure 3:4  
In this case, the pre-coding vectors \(C_{2}^{s}\) and \(C_{2}^{d}\) facilitate that the signal is constructively and destructively superimposed respectively. \(a4\) can be larger than \(a3\) and \(b4\) can be smaller than \(b3\). Therefore, the SNR of the desired signal is increased, and the SNR of the interference signal is reduced. When the number of pico cells and base station antennas in the macro cell is increased, the received signal is as follows,  
Figure 3:5  
Also, the schemes in [18; 19] mitigate the interference but exhibit several disadvantages. These schemes use the relay; as such, a complicated relay selection scheme is needed. Because the schemes apply the pre-coding not only to the base station but also to the relay, a large number of feedback bits are required. The schemes in [18; 19] can mitigate the interference only when the channel conditions between the base station and relay are good. Additionally, the interference mitigation methods of [18; 19] can reduce the transmission rate due to the use of the relay, but the interference mitigation scheme proposed in this paper cannot reduce the transmission rate. As a result, the proposed CoMP scheme can increase the diversity gain of the mobile, without reducing the transmission rate, and can efficiently mitigate interference. The proposed CoMP scheme uses constructive and destructive SPC schemes adaptively, according to the location of the mobile. This CoMP scheme improves the diversity gain of the mobile by increasing the SNR of the desired signal through the constructive 2-bit feedback SPC scheme. Additionally, when the mobile is located in the cell edge and is affected by the interference, the effect of the interference is mitigated by reducing the SNR of the interference signal using the destructive 2-bit feedback SPC scheme. Therefore, the proposed CoMP scheme improves the reliability of the HetNet system by improving the reception performance of the mobile.