Discussion on key technologies of TD-LTE co-frequency networking

The key technology of TD-LTE is composed of some loaded signals. Due to the special way of the signals, how to carry out the same-frequency networking and how to operate it are placed in a very important position. This includes some resource blocks and some PRB situations. For the same-frequency networking, Datang has proposed several ways and mechanisms to reduce the same-frequency interference to reduce the interference into the cell, so as to realize the ultimate same-frequency networking method.

The following centralized co-frequency networking methods were considered during the design: scrambling, frequency hopping transmission, power control, inter-cell interference coordination, and the beamforming method proposed by Datang.

First, let’s look at scrambling. It is performed by randomizing the data using a random sequence after encoding. It has a relatively small impact on the system, but the gain it provides in reducing interference is not very obvious.

The second is frequency hopping, which is used to randomize the interference between cells. Currently, in LTE, except for CH, all other downlink physical control channel resources are subject to the cell ID, which can fully realize such frequency hopping transmission. POSCH uses frequency hopping transmission between pointers to reduce interference. For the sub-pin tip method, the frequency hopping transmission method uses a fixed frequency for frequency hopping transmission. The frequency hopping transmission method within the sub-pin is relatively simpler than the frequency hopping transmission within the sub-pin. The frequency hopping transmission between sub-pins is more complicated and the effect may be worse. Generally speaking, the gain of reducing interference by frequency hopping transmission is about 1 to 2 DB from the current simulation situation.

The third method is beamforming. First, it improves the user's interference ratio. At the same time, beamforming can also be used to reduce the intensity desired by the interfered user. When we do beamforming, we adjust the position of the beam and the position of the zero line to achieve the purpose of reducing interference.

The fourth mechanism to reduce inter-cell interference is inter-cell interference coordination. At present, the interference coordination method is mainly to coordinate and control the resources between cells, the different resources between two different cells, video resources and even power resources. This includes static inter-cell interference coordination and semi-static inter-cell interference coordination. For static cell interference coordination, it is fixed, setting the frequency resources that my edge users can use. This is a fixed setting. There is no need to further strengthen this part in the standard and write it into the standard. It can be achieved through the parameters and configurations of each system manufacturer. For semi-static inter-cell interference coordination, it requires the cell base station to exchange inter-cell PRB usage information, power allocation information, and overload indication information through the X2 interface.

The last is power control. Power control also has inter-cell power control and intra-cell power space. The inter-cell system periodically tells the neighboring cell the LT information of the cell, controls the LT of the cell to reduce the LT between cells, and achieves the purpose of reducing interference between cells. The power control within the cell is very obvious. In TD-LTE, we reduce the terminal's transmission power to reduce interference to other cells, so that the LT of the cell remains below a certain level.

For the above-mentioned controls to reduce inter-cell interference, the gain of adding frequency and hopping to reduce inter-cell interference is not very obvious, that is, 1 to 2 DB. For the methods of inter-cell interference coordination and power control to reduce interference, this method is effective in reducing inter-cell interference when the cell load is very light. If the cell load is heavy, information exchange is required. The timeliness of this information exchange cannot be guaranteed when the cell load is heavy. In this case, coordination is very difficult. At the same time, these two methods of inter-cell coordination also bring certain system complexity. Therefore, this method also has room for improvement. Beamforming can reduce inter-cell interference obviously, but we still need to improve it. We have proposed two enhanced solutions to the same-frequency networking problem. The first solution is BR-OFDMA, that is, cells can share the same video resources, but different cells use different repetition codes for the same video resources to reduce interference between cells.

The same video resource is calculated with a repetition code of 8 to obtain the corresponding video operation diagram. This repetition code can be adjusted according to the interference situation. For example, if the interference is serious and multiple cells are repeated at a certain point, the interference factor can be adjusted from 4 to 8 or 8 to 16. In this way, recognition and frequency division can be achieved and then code division can be performed to match different interference scenarios. This method can perform joint detection of users in multiple cells in the contact segment to fully utilize interference signals to offset interference, and can also further reduce interference between cells. This method is mainly for the inter-cell coordination we mentioned earlier. This method cannot effectively reduce interference when the cell load is heavy. This is a method we use.

In the case of heavy cell interference, this advantage can be greatly improved. I just talked about the first method. The second aspect is the enhanced beam attribute. The already supported beam assignment technology can effectively improve the system throughput and the throughput of edge users. The subsequent introduction of the multi-cell cooperative transmission mechanism and the beam attribute into the cell coordination method can improve the cell interference.

There are several ways to reduce cell interference. The first is interference avoidance. The idea of ​​interference avoidance is mainly for different beam attributes. It can transmit the same resources, but the same beam is not suitable for the same resources. The same resources are allocated to different directions. Try to use video resources in different directions, and achieve the goal of reducing interference with spatial resources. This is an interference avoidance method.

Interference suppression takes into account the joint scheduling of multiple cells. When the cell is in operation, it is necessary to consider the interference users in other cells and avoid interference to other cells through adjacent lines. This is a way to avoid interference suppression. The last way is signal enhancement. This way is to enhance the signal strength of edge users through the interaction and coordination of information from multiple surrounding adjacent cells.