Today, we will learn about CoMP, an inter-cell cooperation technology in LTE-A, since we learned about ICIC and eICIC in the previous posts. At an early stage of LTE/LTE-A, offering high speed is the most important marketing point for operators. However, as LTE subscribers and traffic grow, satisfying users with high Quality of Experience (QoE), for example, by improving user throughputs at cell edge areas where data transmission speed drops drastically becomes far more important than just supporting the highest speed.  

 

Increased radio network capacity can be achieved by improving spectral efficiency. Spectral efficiency (bit/sec/Hz) is the transmission rate measured in bps per Hz. The higher spectral efficiency, the more data can be transmitted with the same amount of bandwidth. By default, LTE networks provide broadband radio links by obtaining higher spectral efficiency through using at least 2x2 MIMO antennas. At cell centers, installing more antennas at a base station improves spectral efficiency, leading to higher UE throughputs. At cell edge areas, however, only insignificant throughput improvement can be expected. So, we should find another way to gain the same effect.    

 

■ Definition of CoMP   

Coordinated Multi-Point (CoMP) is a new inter-cell cooperation technology specifically aiming to enhance throughputs of UEs at cell edge. CoMP mitigates inter-cell interference and increases throughputs of a UE at cell edge by allowing not only the UE's serving cell, but also other cell(s) to communicate with the UE, through cooperation with one another.

 

Traditionally, a UE accesses only one cell (serving cell) for communication. But, a CoMP-enabled UE can communicate with more than one cell located in different points, and this group of cells works as a virtual MIMO system. Cells that are in charge of directly or indirectly transmitting data to UE are called "CoMP cooperating cells" ("CoMP cooperating set" in 3GPP terms^*), and specifically those actually responsible for transmitting data to UE are called "CoMP transmission cell(s)" ("CoMP transmission points" in 3GPP terms^*).

 

In summary, CoMP is an inter-cell cooperation technology that enables more than one transmission cell to communicate with a UE to achieve better throughputs at cell edge areas by reducing inter-cell interference. CoMP cooperating cells share channel information of a UE, and based on the information, transmission cell(s) are decided.

 

■ Why CoMP? – Problems with ICIC and eICIC  

As discussed in the previous posts, ICIC (defined in Release 8) reduces inter-cell interference by allocating different frequency resources (RBs or sub-carriers) to UEs at cell edge. On the other hand, eICIC (defined in Release 10) does the same task in time domain, by allocating different time resources (subframes) through cooperation between a macro cell and small cells in a HetNet.

 

ICIC and eICIC, both aiming to reduce inter-cell interference, can help UEs at cell edge to communicate, but neither can actually improve their throughputs. That's because they restrict radio resource usage in frequency domain (ICIC) and time domain (eICIC) to mitigate interference. And interference information between neighbor cells is shared on a relatively long term basis. As a result, fast-changing channel conditions of UE (e.g. when UE is traveling fast, or entering a shadowing area) are not reflected in inter-cell cooperation promptly in time, inevitably impeding dynamic allocation of resources.

 

​CoMP, recognized as the most advanced inter-cell cooperation technology so far, was first standardized in Release 11, and further standardization is still taking place in Release 12. It uses radio resources not just in frequency/time domain, but also in spatial domain, to enhance spectral efficiency. That is, it performs beamforming using a smart antenna, or works as a virtual MIMO system. With CoMP, cooperating cells can share UE's channel information every time scheduling is performed, and hence UE's instantaneous channel conditions can be reflected in time. This sharing makes joint scheduling possible. CoMP can be used either in a homogeneous or heterogeneous network (HetNet), and features various types of inter-cell cooperation: CS, CB JT, and DPS (see CoMP Types below).

 

■ Channel Information used in CoMP  

​Channels are transmission routes for data, i.e. between Tx antenna and Rx antenna across air. If base stations know UE's channel information beforehand, they can transmit precoded data so that UE can get better reception. For this purpose, UEs measure their channels, and report the resulting Channel State Information (CSI) to their base stations. 

 

​Base stations give their UEs an instruction on how and which cell's CSI are to be measured by sending a CSI-RS (CSI Reference Signal) configuration message. Upon this instruction, UEs measure CSI and report to their serving cells. In general, CSI information includes Channel Quality Indicator (CQI), Precoding Matrix Indicator (PMI), and Rank Indicator (RI). 

 

CQI: An indicator of channel quality. Displayed as a highest modulation and coding rate (MCR) value that satisfies the condition of 'channel block error rate (BLER) < 0.1'. It is set as a value ranging 0 ~ 15 (4 bits). The better channel quality, the higher MCR is used. Subband CQIs indicate the quality for specific frequency ranges (subrange) while wideband CQIs indicate that for the entire channel bandwidth.

 

PMI: Base stations deliver more than one data stream (layer) through Tx antenna. Precoding matrix shows how individual data streams (layers) are mapped to antennas. To calculate precoding matrix, UEs obtain channel information by measuring the channel quality of each DL antenna. Because providing feedback on all channel information results in significantly increased overheads, generally a code book is pre-configured at base stations and UEs. Using this code book, UEs send the index of a corresponding precoding matrix only. Base stations, by referring the reported precoding matrix, calculate its own precoding matrix, and use the optimal value from it.

 

RI: Indicates the number of data stream(s) being delivered in DL. For instance, with 2 X 2 MIMO, this value is 1 in case of transmit diversity MIMO where two antennas at a base station are sending the same data stream, and it is 2 in case of spatial multiplexing MIMO where the antennas are sending different data streams.

 

■ CoMP Types (CoMP Categories in 3GPP Terms^*)

Specific CoMP types can be categorized in many ways depending on the criteria used for categorization - whether backhaul is ideal or non-ideal, whether CoMP between eNBs is supported or not, whether MIMO antennas support one user or multiple users, whether it is to be applied to DL or UL, etc.
This post will discuss DL CoMP. CoMP is designed to reduce inter-cell interference and enhance throughputs of cell-edge UEs. When cell(s) send data to UEs, they can use one of the following CoMP types depending on the extent of coordination among cells and traffic load. Although different types of CoMP can be used together, we will explain the specific types one by one below for easier understanding.

 

Coordinated Scheduling/Coordinated Beamforming (CS/CB)  

As an effort to minimize interference among cell-edge UEs, CS and CB CoMP select one of the cooperating cells as a transmission cell, and use it in communicating with UE.

 

1. Coordinated Scheduling (CS)

The basic idea of CS CoMP is pretty similar to ICIC in that it reduces inter-cell interference by allocating different frequency resources (RBs or sub-carriers) to cell-edge UEs. But from technical perspective, CS CoMP is a more advanced technology that requires a much shorter operation period, more complicated signal processing and more elaborate algorithm, compared to ICIC. In ICIC, cooperating cells share interference information of each cell, but in CS CoMP they can share channel information of each user.

• First, cooperation periods in CS CoMP are a lot shorter than in ICIC. In ICIC, each cooperation period is tens ~ hundreds of msecs long. So, once ICIC coordination results are updated, schedulings are based on the result for a long time. On the other hand, in CS CoMP, with a cooperation period as short as 1 msec, new CS coordination results are applied every time scheduling is performed. So, resources can be dynamically allocated even with instantaneous changes of UE's channel condition.  
• Second, in CS CoMP, cooperating cells share greater amount of more elaborate information, compared to those in ICIC. In ICIC, pretty simple information like interference level by radio block is shared (see ICIC) while user-detailed channel information (CQI, PMI, RI, SINR, etc.) between UEs and their cooperating cells is shared in CS CoMP.

Figure 1. Coordinated Scheduling (CS)

 

In Figure 1, A1 and B1 at cell edge, each with a different frequency resource allocated (f3 and f2), can avoid interference, and hence have improved throughputs. Both UEs do receive signals from the other UE. These signals do not cause interference with the other's, but may cause degraded reception of their own signals.

 

2. Coordinated Beamforming (CB)

CB CoMP allocates different spatial resources (beam patterns) to UEs at cell edge by using smart antenna technology. Without CS, A1 and B1 may end up being allocated the same frequency resource (f3 in Figure 2). CB CoMP allows Cell A and Cell B to cooperate with each other, and allocate different spatial resources (beam pattern 1, beam pattern 2) to A1 and B1 at cell edge. These two cells can prevent interference by allocating main beam to their own UE, and null beam to the other neighbor UE.

 

Figure 2. Coordinated Beamforming (CB)

 

Generally, CB is more often used with CS, than alone. Figure 3 shows a case where CS and CB are used together. Cell A and Cell B cooperate with each other to allocate different frequency resources (f3, f2) and different spatial resources (beam pattern 1, beam pattern 2) to A1 and B1, respectively. This cooperation is pretty effective because, CS alone can easily take care of interference issues, and besides CB can even ensure better reception quality. If used with CB, CS can achieve better cell-edge throughputs because CB helps A1 and B1 to avoid signals sent to the other, and better receive those destined for themselves.

 

Figure 3. CS/CB 

 

Joint Processing (JP): Joint Transmission/Dynamic Point Selection (JT/DPS)
In JT/DPS CoMP, multiple cells are selected among cooperating cells as transmission cells for better reception of UEs at cell edge.

 

 

 

3. Joint Transmission (JT)

4. Dynamic Point Selection (DPS)