will cause severe degradation of system performance and cannot be mitigated by increasing the transmit power of desired signals. At the same time, in view of the analysis, single cell wireless resources usage efficiency is low. To improve system spectrum efficiency, advanced multi-cell joint RRM and cooperative multi-point transmission schemes should be adopted in the C-RAN system.
Cooperative Radio Resource Management for multi-cells
The multi-cell RRM problem has been addressed in various academic studies. Many uses various optimization techniques in trying to determine the optimal resource scheduling and the power control solutions to maximize the total throughput of all cells with some specific constraints. To reduce the complexity incurred in the C-RAN network architecture and the scheduling process, the joint processing/scheduling should be limited to a number of cells within a “cluster”. The complexity of scheduling among the eNBs clusters is determined by the velocity of mobile users and the number of UEs and RRHs in the cluster. Thus, choosing an optimal clustering approach will require balancing among the performance gain, the requirement of backhaul capacity and the complexity of scheduling.
As shown in Fig. 5-3, UEs will be served by one of the available clusters which are formed in a static or semi-static way based on the feedback or measurements reports of UEs. In this scenario, a subset of cells within a cluster will cooperate in transmission to the UEs associated with the cluster. To further reduce the complexity, it is possible to limit the number of cells cooperating in joint transmission to a UE at each scheduling instant. The cells in actual transmission to a UE are called active cells for the UE. The active cells can be defined from the UE perspective based on the signal strength (normally cells with strong signal strength are chosen among cells within the supercell). The activation/de-activation of a cell can be done by a super eNB, which is the control entity in cell clustering and can adjust the sets scope based on the UE feedback.
Cell cluster 1 Cell cluster 2 Cell cluster 3
Fig. 5-3 The UE assisted network controlled cell clustering
Cooperative Transmission / Reception
Cooperative transmission / reception (CT/CR) is well accepted as a promising technique to increase cell average spectrum efficiency and cell-edge user throughput. Although CT/CR naturally increases system complexity, it has potentially significant performance benefits, making it worth a more detailed consideration. To be specific, the cooperative transmission / reception is characterized into two classes, as shown in Fig.5-4:
Joint processing/transmission (JP)
The JP scheme incurs a large system overhead: UE data distribution and joint processing across multiple transmission points (TPs); and channel state information (CSI) is required for all the TP-UE pairs.
Coordinated scheduling and/or Coordinated Beam-Forming (CBF)
With a “minimum” cooperation overhead, to improve the cell edge-user throughput via coordinated beam-forming: No need for UE data sharing across multiple TPs; Each TP only needs CSI between itself and the involved UEs (no need for CSI between other TPs and UEs).
Fig. 5-4 JP scheme and CBF scheme Technical Challenges
Cooperative transmission / reception (CT/CR) has great potentials in reducing interference and improving spectrum efficiency of system. However, this technology has many problems that need to be further studied before it can be applied to the practical networks. There are many challenges listed as follows:
Advanced joint processing schemes
DL channel state information (CSI) feedback mechanism
5.3 Large Scale Baseband Pool and Its Interconnection
Centralized Baseband Pool
There are many distributed BS products using RRH+BBU architecture in market. Some TEM‟s products have realized dynamic allocation of carrier processing within one BBU to adapt to dynamic workloads among different RRH connected to it. This architecture can be viewed as the first step of centralized baseband pool concept, but in general a single BBU has limited processing capability, typically only supporting about 10 macro BSs‟ carriers. It‟s not yet capable of supporting dynamic resource allocation across different BBU, thus hard to resolve the dynamic network load in a larger area. In the current RRH+BBU architecture, the RRH is usually connected to a particular BBU by a fixed link, and it can only transmits its baseband signal and O&M signaling to the BBU it‟s connected to. This makes it difficult for another BBU to obtain any uplink baseband data from that RRH. Similarly, any other BBU has difficulty sending downlink baseband data to this RRH. Because of this limitation, the processing resources of different BBUs can hardly be shared: the idle BBU‟s processing resources are wasted and it cannot be used to help the BBU with a heavy workload.
The centralized baseband pool should provide a high bandwidth, low latency switch matrix with an appropriate protocol to support the high speed, low latency and low cost interconnection among multiple BBUs. In a medium sized dense urban network coverage (approximately 25 sq.
km in area), with an average distance between BS of 500m, a centralized baseband pool that can cover the whole area needs to support about 100 BS. For a typical TD-SCDMA system with 3 sectors per macro BS and 3 carriers/sectors, it means that the centralized baseband pool needs to support 900 TD-SCDMA carriers. Imagine if the centralized Baseband pool coverage is even larger, such as 15 km X 15 km, then the baseband pool would need to support up to 1000 macro BSs‟ carriers. Because of the limitation in the high-speed differential signal transmission, the traditional BBU architecture cannot scale up to support such capacity by simply expanding the backplane dimensions.
Infinite Band technology can provide significant switching bandwidth (20Gbps-40Gpbs/port) and very low switching latency. It is widely used in supercomputers. However, the cost per port is very high (20,000RMB) and as such does not meet the C-RAN cost requirement. Inspired by the data center network‟s distributed inter-connect architecture, the centralized BBU pool in C-RAN can also use a distributed optic interconnection to combine multiple BBU into a scalable baseband pool. Based on that, the RRHs‟ signal can be routed to any one of BBUs in the pool.
Thus load balance according to dynamic network load among BBUs can be achieved, and system power consumption can be reduced. It also makes the deployment of multi-point MIMO technology and interference mitigation algorithms easier, which can improve radio system capacity.
Dynamic carrier scheduling
The dynamic carrier scheduling of resources within baseband pools enhances redundancy of the BBU and increases overall operational reliability of the baseband pool. When a baseband card or a carrier processing unit fails, the work load can be promptly redistributed to other available resources within the pool, and restore the normal operation. In addition, for areas that have strong dynamic network load, the operator can deploy fewer baseband resources to meet the demands of different sites that have opposite peak loads at different times. For example, operator can use the same BBU pool with multiple RRHs to cover both residential areas and office areas. Then dynamically allocates baseband resources to ensure basic coverage for both areas. Remaining baseband resources can be dynamically allocated to cover the business area during working hours and the residential area during after working hours. This will increase the overall carrier resource utilization.
Large-scale BBU Inter-connection
A large scale baseband inter-connect solution should be able to support 10-1000 macro BS, with the following requirements:
Inter-connection between BBUs must satisfy the wireless signal‟s requirements of low latency, high speed, and high reliability. The requirements are similar to the CPRI/Ir/OBRI interface, and should support real-time transmission of 2.5/6.144/10Gbps rate.
Dynamic carrier scheduling among BBUs to achieve efficient load balance within the system and failure protection without service interruption.
Support multipoint collaboration (CoMP). It needs to consider the data flow between different BBUs to support collaboration radio.
Fault-tolerance. Fiber inter connection should support 1+1 failure protection, BBU frame and baseband processing board N +1 protection to achieve high system robustness.
High scalability: it can extend the system capability smoothly without services interruption.