# CoMP-Aware BBU Placements for 5G Radio Access Networks over Optical Aggregation Networks
## 4 CoMP Aware BBU Placement Algorithm  
As stated earlier, the contribution of this paper is divided into two parts. First, we optimize the placement of the BBU pool. Second, we reconfigure the CoMP inter BBU into CoMP intra-BBU. Thus, we construct a heuristic algorithm for the reconfiguration phase. This is illustrated in Figure 2. The heuristic algorithm has following steps:  
1. Based on the optimization problem explained in Section 3, we use outputted BBUs locations and the routing as the initial solution. 2. We distribute the users uniformly on the given area. 3. We classify the users to CoMP users and non-CoMP users based on their location with respect to the cell's center. Given that the cell's radius equals 500 m if the distance between the user and the cell center is less than 80 % from cell radius, the user is considered as a non-CoMP user. Otherwise, the user is considered as a CoMP user (i.e., the user is near the cell edge). 4. We find the cell edge with the highest number of CoMP users; then, we change the serving BBU according to the algorithm explained below. Assume two coordinated RRHs: _RRH a_ and _RRH b_. If _RRH a_ is served by the _BBU pool a_ while _RRH b_ is served by the _BBU pool b_. The objective is to re-allocate the two RRHs so that they are served by the same BBU to undergo intra-CoMP instead of inter-CoMP. Choosing the RRH that will change its serving BBU will be performed based on the following aspects: (i) maximizing the multiplexing gain (Case 1 and Case 2), (ii) minimizing the number of active fiber links (Case 3 and Case 4), and (iii) minimizing the end-to-end delay between the RRHs and the BBUs (Case 5 and Case 6). This is considered in the following six cases. In the following, we will use the terminology _path 1_ as the path between _RRH a_ and _BBU pool b_. On the other hand, _path 2_ is the path between _RRH b_ and _BBU pool a_. * **Case 1**:
* As shown in Algorithm 1, if the delay of _path 1_ is less than the maximum allowable delay **AND*
* if the delay of _path 2_ is less than the maximum allowable delay, then the following is the case:
* Condition 1: Condition of BBU Computational Capacity: choose the serving BBU (i.e., _BBU pool a_ or _BBU pool b_) for the two coordinated RRHs (i.e., _RRH a_ or _RRH b_) based on the BBU that accommodates more RRHs and then end the process. If two BBUs accommodate the same number of RRHs, then the following is the case:
* Condition 2: Condition of the number of active links: choose the serving BBU (i.e., _BBU pool a_ or _BBU pool b_) for the two coordinated RRHs (i.e., _RRH a_ or _RRH
\(b\)) based on the path (i.e., _path 1_ or _path 2_) with the minimum number of new activated fiber links and then end the process. If the two paths have the same path delay, then the following is the case: * Condition 3: Condition of the link Delay: choose the serving BBU (i.e., _BBU pool \(a\)_ or _BBU pool \(b\)_) for the two coordinated RRHs (i.e., _RRH a_ or _RRH b_) based on the path (i.e., _path 1_ or _path 2_) with minimum path delay then end. If the two paths have the same path delay, then the reconfiguration is unavailable and the RRHs will undergo inter-CoMP. * If the delay of _path 1_**ONLY** less than the maximum allowable delay, then choose _BBU pool \(b\)_ as the serving BBU for the two coordinated RRHs (BBU pool in _path 1_). Then end \(a\) is the serving BBU for the two coordinated RRHs (BBU pool in _path 2_). Then, end the process. * Otherwise, the reconfiguration is unavailable and the RRHs will undergo inter-CoMP. The pesudo-code for case 1 is shown below. Figure 2: CoMP-aware BBU placement algorithm. ```
0: Optimum results of BBU Placement Output: Reconfigurable placement RRH-BBU
1: Initialization
2:\(U_{Inter}\) : Number of inter-BBU CoMP users between two given RRHs. 3:\(Path1\) : Shortest path from RRH a to BBU pool b. 4:\(Path2\) : Shortest path from RRH b to BBU pool a. 5:\(BBU_{CapA}\) : Number of RRHs accommodated by BBU a. 6:\(BBU_{CapB}\) : Number of RRHs accommodated by BBU b.