Introduction and comparison of LTE inter-cell interference suppression technologies

　　With the continuous development of mobile communication technology, users have put forward higher requirements for the content and quality of mobile communication. In order to adapt to the trend of global wireless communication towards mobility, broadband and IP, and to compete with some emerging mobile communication technologies such as WiMAX and Wi-Fi, at the end of 2004, 3GPP proposed 3G Long Term Evolution (3G LTE) after HSDPA, HSUPA and other technical standards. The goal of 3G LTE is to obtain higher data rate, lower latency, improved system capacity and coverage, and lower cost. At the same time, since LTE adopts the access method of Orthogonal Frequency Division Multiple Access (OFDMA), the information of users in a cell is carried on different orthogonal carriers, so the interference comes from other cells. That is, the interference between cells. Therefore, the interference suppression between cells has become a problem to be solved urgently. This article introduces the main interference suppression methods currently used in LTE, and compares the advantages and disadvantages of different methods.

　　1 Introduction to LTE

　　LTE fills the huge technical gap between the third generation of mobile communications and the fourth generation of mobile communications. Its goal is to establish an evolvable wireless access architecture that can achieve high transmission rates, low latency, and packet optimization. The LTE system is expected to achieve a downlink transmission rate of 100 Mbit/s and an uplink transmission rate of 50 Mbit/s on a 20 MHz bandwidth, with a spectrum efficiency that is 2 to 4 times that of HSPA. It supports enhanced multimedia broadcast and multicast services and full-packet packet switching, with flexible bandwidth configuration, significantly improved transmission rates in edge cells, and enhanced system coverage. In order to achieve the above goals, the LTE system adopts a flat network structure that is close to a typical all-IP broadband network, and adopts advanced technologies such as multiple-input multiple-output (MIMO), orthogonal frequency division multiplexing (OFDM), hybrid automatic repeat request (HARQ), and adaptive modulation and coding (HARQ). The LTE system uses a single carrier frequency division multiple access (SC-FDMA) access method based on OFDM transmission technology for uplink. The downlink uses OFDMA access. The access method of OFDMA is different from that of code division multiple access (CDMA). It is impossible to eliminate the interference between cells by spreading the spectrum. The LTE system has high requirements for spectrum efficiency and cannot reduce the interference by using the traditional frequency reuse method with a higher reuse coefficient. Therefore, in LTE, great attention is paid to the interference suppression technology between cells. At present, the main methods of suppressing inter-cell interference discussed by 3GPP are divided into three types, namely, inter-cell interference randomization, inter-cell interference elimination and inter-cell interference coordination/avoidance.

　　2 Interference Suppression Technology between Cells

　　LTE's unique OFDMA access method enables user information in the cell to be carried on different orthogonal carriers, so all interference comes from other cells. For users in the center of the cell, they are relatively close to the base station, and the interference signals from the outer cells are far away, so their signal-to-noise ratio is relatively large: but for users at the edge of the cell, since users in adjacent cells occupying the same carrier resources have relatively large interference to them, and they are far away from the base station, their signal-to-noise ratio is relatively small, resulting in a high overall cell throughput, but poor service quality for users at the edge of the cell. Therefore, in LTE, inter-cell interference suppression technology is very important.

　　2.1 Interference Randomization

　　For the OFDMA access method, the number of interferences from external cells is limited, but the interference intensity is large, and the interference source changes quickly, making it difficult to estimate. Therefore, using mathematical statistics to estimate the interference becomes a relatively simple and feasible method. Interference randomization cannot reduce the energy of interference, but it can randomize the interference into "white noise" by scrambling the interference signal, thereby suppressing the interference between cells. Therefore, it is also called "interference whitening". Interference randomization methods mainly include cell-specific scrambling and cell-specific interleaving.

　　a) Cell-specific scrambling, that is, after channel coding, the interference signal is randomly scrambled. As shown in Figure 1, the transmission signals of cell A and cell B are scrambled respectively after channel coding and interleaving. Without scrambling, the decoder of the user equipment (UE) cannot distinguish whether the received signal is from this cell or from other cells. It may decode the signal of this cell or the signal of other cells, resulting in reduced performance. Cell-specific scrambling can distinguish the information of different cells through different scrambling codes, allowing the UE to decode only useful information to reduce interference. Scrambling does not affect bandwidth, but it can improve performance.

　　Figure 1 Cell-specific scrambling

　　b) Cell-specific interleaving, that is, after channel coding, the transmission signal is interleaved in different ways. As shown in Figure 2, for cell A and cell B, their interference signals are interleaved respectively after channel coding. The cell-specific interleaving pattern can be generated by a pseudo-random number method. The number of available interleaving patterns (interleaving seeds) is determined by the interleaving length. Different interleaving lengths correspond to different interleaving pattern numbers. The UE determines which interleaving pattern to use by checking the interleaving pattern number. Between cells with a long spatial distance, the interleaving seeds can be reused, similar to frequency division multiplexing in cellular systems. For the randomization of interference, cell-specific interleaving and cell-specific scrambling can achieve the same system performance.

　　Figure 2 Cell-specific interleaving

　　2.2 Interference Removal

　　The idea of ​​interference cancellation was first proposed in the CDMA system. The signal of the interfering cell can be demodulated and decoded, and then the interference from the cell can be reconstructed and cancelled. Although LTE adopts the OFDMA access method, it still introduces the concept of interference cancellation. There are two main methods for implementing inter-cell interference cancellation.

　　a) Using the multi-antenna spatial suppression method at the receiving end to perform interference cancellation, the related detection algorithm has been widely used in the research of multiple-input multiple-output (M1-MO).

　　b) Method based on detection/elimination. A typical example is the use of interleaved multiple access (IDMA) to eliminate interference between cells. IDMA can generate different interleaving patterns through a pseudo-random interleaver and assign them to different cells. The receiver uses different interleaving patterns to deinterleave, which can respectively decode the target signal and the interference signal, and then subtract the interference signal from the total received signal, thereby effectively improving the signal-to-noise ratio of the received signal.

　　In addition, in the downlink transmission of LTE, the information of interference signals can be obtained in different ways. When removing the interference between Node Bs, the information of interference signals can be obtained by detecting the interference control signal on the UE side; when removing the interference between sectors, the Node B directly uses its own control channel to send the information of interference signals to the UE. Obviously, the more interference signal information the receiver obtains, the better the interference removal performance.

　　The advantage of inter-cell interference cancellation is that there is no restriction on the frequency resources at the cell edge. Adjacent cells can use the same frequency resources even at the cell edge, which can achieve higher cell edge spectrum efficiency and total spectrum efficiency. The limitation is that the cells must be synchronized, and the target cell must know the pilot structure of the interfering cell to perform channel estimation on the interference signal. For users to perform inter-cell interference cancellation, the same frequency resources must be allocated to them.

　　2.3 Interference Coordination/Avoidance

　　For the 0FDMA access method, users in the center of the cell will not be interfered by users in the cell. The interference source from other cells is relatively far away, so they can achieve a relatively good reception effect. However, users at the edge of the cell will be seriously interfered by other cells.

　　The core idea of ​​interference coordination and avoidance is to impose some restrictions on the available resources of a cell through inter-cell coordination to reduce the interference of the cell to the adjacent cells, improve the signal-to-noise ratio of the adjacent cells on these resources, and improve the data rate and coverage at the cell edge. The industry has proposed many interference coordination/avoidance methods. This article will introduce a generally recognized soft frequency reuse scheme.

　　In this scheme, the subcarriers in each cell are divided into two groups. One group is called the primary subcarriers, and the other group is called the secondary subcarriers. The primary subcarriers can be used throughout the cell, while the secondary subcarriers can only be used in the center of the cell (see Figure 3). This way of allocating subcarriers can ensure that the subcarriers used at the borders of adjacent cells are uniform.

　　Figure 3 Schematic diagram of soft frequency reuse

　　Mutually orthogonal, users using the same frequency subcarrier are far enough apart. This effectively avoids or reduces the co-channel interference of users at the edge of adjacent cells. For users in the center of the cell, since they are close to the base station and receive less interference from external cells, they can use relatively low power for transmission, while the opposite is true for users at the edge of the cell. Therefore, in general, the maximum transmit power allowed by the main subcarrier is higher than the maximum transmit power allowed by the auxiliary subcarrier. When the power spectrum density is constant, more power allocated to the main subcarrier means that a wider bandwidth is allocated to the main subcarrier. The transmit power ratio of the auxiliary subcarrier to the main subcarrier can be adjusted between 0 and 1, and the corresponding effective frequency reuse coefficient varies from 3 to 1. By adjusting the power ratio of the auxiliary subcarrier to the main subcarrier, soft frequency reuse can adapt to changes in the service distribution of each cell. When high traffic occurs at the edge of the cell, the power ratio is set to a relatively small value to obtain a higher cell edge throughput; on the contrary, when the traffic is mainly concentrated inside the cell, a larger power ratio can be set.

　　2.4 Comparison of Several Interference Suppression Technologies

　　Comparing the above-mentioned several interference suppression schemes for LTE systems, we can see that interference randomization continues to use the mature scrambling technology of CDMA systems, which is relatively simple and feasible. However, the problem is that interference is treated as white noise, which may cause measurement errors due to different statistical characteristics. Interference cancellation technology can significantly improve the system performance at the cell edge and obtain higher spectrum efficiency, but it is not suitable for services with smaller bandwidths (such as VolP), and its implementation in OFDMA systems is also relatively complex. There is not much follow-up research on it. Interference coordination/avoidance is a popular technology currently under research. It is simple to implement and can be applied to services of various bandwidths. It also has a good effect on interference suppression and is suitable for OFDMA, a specific access mode. However, it brings about a loss in the overall cell throughput while improving the performance of users at the cell edge. The above three methods of interference suppression between cells can be combined and complemented with each other to obtain higher system gain.

　　3 Conclusion

　　LTE systems have high requirements for spectrum efficiency. The resulting inter-cell interference problem is an important issue affecting system performance. Interference randomization, interference elimination and interference coordination/avoidance as effective inter-cell interference suppression technologies will greatly improve the performance of 3G LTE systems, especially the performance of cell edge users.