So far I've always assumed that LTE-Advanced Cooperative Multi Point (CoMP) transmission would be similar to what we have in UMTS for voice calls. Here, several base stations transmit a signal in the downlink direction for the mobile at the same time. The mobile device then tries to decode all signals simultaneously to improve reception conditions. With the introduction of HSPA for packet based data transmission this was no longer possible. Here the central scheduler in the central Radio Network Controler was replaced by individual packet schedulers in the base stations. As a consequence fast coordination between the schedulers of the different base stations was not possible due to the delay and limited transmission capacity of the backhaul link.
But I thought time has moved on, technology has improved and some way has been found for schedulers to communicate over the backhaul link to synchronize transmissions to a mobile device. Actually, that is not the case and the CoMP scenarios that have been studied in 3GPP TR 36.819 work quite differently. In fact, all except one scenario is based on fiber links that transmit the fully processed RF signal which is only converted from optical to an electromagnetic signal at a remote radio head. Here's a summary of the two modes and four approches discussed in the study for 3GPP Release 11:
In the Joint Processing (JP) mode, the downlink data for a mobile device transmitted from several locations simultaneously (Joint Transmission). A simpler alternative is Dynamic Point Selection (DPS) where data is also available at several locations but only sent from one location at any one time.
Another CoMP mode is Coordinated Scheduling / Beamforming (CS/CB). Here the downlink data for a mobile device is only available and transmitted from one point. The scheduling and optionally beamforming decisions are made among all cells in the CoMP set. Locations from which the transmission is performed can be changed semi-statically.
Scenario 1, homogeneous network intra-site CoMP: A single eNodeB base station site is usually comprised of 3 or more cells, each being responsible for a 120 degrees sector. In this scenario the eNodeB controls each of the three cell schedulers. This way it is possible to schedule a joint transmission by several cells of the eNodeB or to blank out the resource blocks in one cell that are used in another cell for a subscriber located in the area between two cells to reduce interference. This CoMP method is easy to implement as no external communication to other entities is required. At the same time this is also the major downside as there is no coordination with other eNodeBs. This means that data rates for mobile devices that are located between two cells of two different eNodeBs cannot be improved this way.
Scenario 2, high power TX remote radio heads: Due to the inevitable delay on the backhaul link between different eNodeBs its not possible to define a CoMP scheme to synchronize the scheduler. To improve on scenario 1, it was thus decided to study the use of many (9 or more) Remote Radio Heads (RRH) distributed over an area that is today covered by several independent eNodeBs. The RRHs are connected to a single eNodeB over fiber optic links that transport a fully generated RF signal that the RRH only converts from an optical into an electromagnetic signal that is then transmitted over the antenna. While this CoMP approach can coordinate transmission points in a much larger area than the first approach, its practical implementation is difficult as a fiber infrastructure must be put in place to connect the RRHs with the central eNodeB. A traditional copper based infrastructure is insufficient for this purpose due to the very high data rates required by the RF signal and the length of the cabling.
Scenario 3 and 4, heterogeneous networks: Another CoMP approach is to have several low power transmitters in the area of a macro cell to cover hotspots such as parts of buildings, different locations in shopping malls, etc.). The idea of this approach is to have a general coverage via a macro cell and offload localized traffic via local transmitters with a very limited range, reducing the interference elsewhere. This can be done in two ways. The localized transmissions could have their own cell IDs and thus act as independent cells from a mobile device point of view. From a network point of view, those cells, however would be little more than RRHs with a lower power output instead of a high power output as in scenario 2. Another option would be to use RRHs as defined above with a low power output without a separate cell ID which would make the local signal indistinguishable from the macrocell coverage for the mobile device. Again, fiber optical cabling would be required to connect the low powered transmitter to a central eNodeB.
Overall, the 3GPP CoMP study comes to the conclusion that data rates could be improved between 25 to 50% for mobiles at cell edges with neighboring interference which is a significant enhancement. Except for scenario 1, however, fiber cable installations are required which makes it unlikely that CoMP scenarios 2,3 and 4 are likely to be implemented on a broad scale in the next 5 years.