In cellular communication Inter-Cell interference has always been a major concern.
In Second generation communication the operators divided the allocated bandwidth and cells
in the vicinity used different frequency to operate.
The technique is popularly known as frequency reuse.
But with the evolution of communication the users became data hungry and Operators had
to allocate more resources to the Users or provide them with more bandwidth.
To overcome this, a frequency reuse factor of 1 is used in 4G i.e all cells uses same
frequency . As a result at the cell edge users may face Inter cell interference.
So what did the Operators came up with to mitigate this issue?
Hello Everyone!
Welcome back to the Fundamentals of LTE.
Today we will see how using Multipoint Co-ordination the user experience at the cell-edge is improved
by exchanging cell information among different base stations.
The prime motivation of Coordinated multipoint as described by 3GPP are
To improve the coverage To improve the cell edge throughput and
To increase the overall system throughput.
There are primarily 2 types of CoMP.
1. Intrasite CoMP - Where Multiple sectors of One eNB co-operates.
The Advantage of Intra-site CoMP is that significant amount of information can be exchanged since
the communication is within the cells of same eNodeB and doesn't involve the backhaul.
2. Intersite CoMP.
This involves coordination of multiple eNodeBs.
It puts additional burden on the backhaul as lot of information is exchanged.
Now there are Two ways by which Coordinated multipoint operates.
The first method is called JOINT PROCESSING. In this method for downlink transmission,
data is transmitted to the UE simultaneously from a number of different eNBs.
The aim is to improve the received signal quality and strength.
In Uplink the PUSCH is received at multiple cells.
The basic concept behind this format is to utilise antennas at different sites.
By coordinating between the different eNBs a virtual antenna array is formed.
The signals received by the eNBs are then combined and processed to produce the final
signal.
This technique allows for signals that are very low in strength, or masked by interference
to be received with few errors.
In both UL and DL the Joint Processing method places a high demand onto the backhaul network.
A lot of data needs to be exchanged between the co-ordinated eNodeBs.
Depending upon the number of eNodeBs involved this data may double or triple.
The Second Method is called Co-ordinated scheduling or Beamforming
In this case data to a single UE is transmitted from one eNB.
For Downlink, transmit beamforming weights are generated.
Based on it the scheduling decisions as well as any beams are coordinated to reduce the
unnecessary interference from other cells.
As a result, the cell edge user throughput can be improved due to the increase in received SINR
In Uplink the UE transmits the Physical Uplink Shared Channel using a Resource Block to a Single cell.
The scheduling decision is based on the coordination among the cells.
This format provides a much reduced load in the backhaul network because only the scheduling
data needs to be transferred between the different eNBs that are coordinating with each other.
The major challenges faced in Co-ordinated Multipoint Transmission reception are -
Efficient feedback compression, Reduced feedback delay,
Efficient channel prediction at the precoder, Flexible formation of cooperation clusters,
Handling of outer interference within the cluster,
Efficient multi-user selection, Flexible networking behind Coordinated Multipoint,
Integration of Coordinated Multipoint into higher layers and
Reduce cost of base station synchronization and low-phase noise transmitters.
Now, One of the key requirements for LTE is that it should be able to provide a very low
level of latency.
The additional processing required for multiple site reception and transmission could add
significantly to any delays.
This could result from the need for the additional processing as well as the communication between
the different sites.
To overcome this, it is anticipated that the different sites may be connected together
in a form of centralized RAN, or C-RAN.
So friends, We have seen here that Coordinated multipoint enhances the overall system performance
through dynamic coordination among multiple geographically separated eNBs or cells from
same eNBs for transmission and reception.
In our next video we will see how throughput is calculated.
You can post your doubts in the comment section.
Happy Learning.