For the 2nd part of this post series (Check here the 1st part), we continue analyzing the Huawei LTE Superband set of features.
In this post our focus will be on the Seamless Intra-Band Carrier Joining for FDD, specifically in the feature Zero Guard Band Between Contiguous Intra-Band Carriers.
This LTE Superband feature is getting more valuable every day as spectrum resources become increasingly limited, so taking advantage of the last minimum resource is of great value to improve spectrum utilization. The feature joins spectrum portions to utilize the spectrum that used to serve as guard bands, increasing the number of available resource blocks (RBs) and improving spectrum utilization.
LTE Superband: Zero Guard Band Between Contiguous Intra-Band Carriers
As per LTE standard, guard bands exists on each side of LTE carrier, let’s take as an example a 20MHz carrier.
For each LTE carrier size, this is the reserved amount of bandwidth for guard band.
This LTE guard band is “empty” space which belongs to the operator but not used due to the nature of the LTE working principle. This feature helps to use that guard band in specific scenarios increasing the spectrum usage efficiency.
When an operator has more than 20 MHz contiguous spectrum in a band, Zero Guard Band Between Contiguous Intra-Band Carriers eliminates guard bands between carriers through seamless joining of contiguous carriers. This increases the number of available RBs and improves spectrum utilization.
The joined schemes available are:
| Spectrum available for the operator | Carrier Bandwidth used | Carrier Bandwidth with Joining Feature |
|---|---|---|
| 21.2 MHz to 21.8 MHz | 20 MHz | 20 MHz+3 MHz |
| 21.8 MHz to 23 MHz | 20 MHz | 20 MHz+5 MHz |
| 23 MHz to 24 MHz | 20 MHz or 20 MHz+3 MHz | 20 MHz+5 MHz |
| 24 MHz to 25.9 MHz | 20 MHz or 20 MHz+5 MHz | 20 MHz+10 MHz |
In these feature there are 2 kinds of band joins:
Overlapping joining
Below figure shows how the LTE Superband feature will cause that some PRBs overlaps for the 2 LTE carriers to be deployed. In the example, the operator has 25 MHz contiguous spectrum. Before this feature is active, the operator deploys its LTE network with a total of 125 available RBs on a 20 MHz carrier and a 5 MHz carrier. This feature eliminates guard bands between carriers. In this case, the 1 MHz guard band of the 20 MHz carrier is spared to form a 6 MHz contiguous spectrum portion with the 5 MHz carrier. Then, the operator can use this portion to deploy a new 10 MHz carrier, which overlaps the 20 MHz carrier.
The operator can now deploy its LTE network on a 20 MHz carrier and a 10 MHz carrier using the 25 MHz contiguous spectrum. Zero guard band is present between them and some RBs on the carriers overlap. For this example, there are 129 available RBs on the entire spectrum. The two carriers shares dynamically the overlapping RBs.
To reduce performance loss caused by RB puncturing, eNodeBs adjust resource management algorithms for related physical channels. The adjustment prevents allocating punctured RBs to channels.
After two carriers join in overlapping mode, the remaining guard bands of the carriers compreses to further improve spectrum utilization. The specific compressed guard bands include the guard band on the low-frequency side of the lower-frequency carrier and the guard band on the high-frequency side of the higher-frequency carrier.
The guard bands are compressed by adjusting the cell center frequencies. For example, the center frequency of the 20 MHz cell reduces and that of the 10 MHz cell increases. In this way, the guard bands on the two ends of the contiguous spectrum can also be utilized to offer more available RBs.
Power and load balancing requires special attention
The power of two cells joined in overlapping mode is calculated based on the total number of available RBs in the cells. The total power cannot exceed the configured power for 2 standard cells (20MHz + 10MHz)
As this LTE Superband feature increases carrier bandwidths or has new carriers deployed, there are more available RBs. As a result, the distribution of UE services on the network changes. So optimizing the load balancing algorithms for this scenario is very important in order to maintain the correct distribution of users among the total available bandwidth.
Non-Overlapping joining
In the next figure, we can compare the difference of spectrum usage with the feature OFF and ON with no PRB overlaping. Basically the overlaping occurs only between the guard bands of both carriers.
This LTE Superband feature utilizes guard bands between standard-bandwidth carriers. Therefore, the number of available RBs in the cells and LTE downlink capacity increase. In general, the downlink throughput rate increases thanks to the additional PRBs.
Because no RBs overlap in non-overlapping joining, resource management algorithms for physical channels do not need to be adjusted.
Considerations before deployment this LTE Superband feature
- Interference increase which means that access success rate and handover success rate decreases, service drop increases. RF indicators such as SINR, CQI, RSRQ, PMI deteriorates.
- PCI (Physical cell identity) planning should consider the joined band, this intensifies the PCI reuse.
- With overlapping joining, available RBs are asymmetrically located at the two ends of each LTE standard bandwidth. In the single-user scenario, the PUSCH cannot occupy all the available RBs, and consequently the uplink single-user peak rate decreases.
