LTE sites are deployed using 3 sectors. Each sector operates on the same carrier frequency. The nomenclature for this reuse pattern is 1x3x1. This frequency reuse nomenclature represents the number of sites in the reuse pattern, the number of sectors in each site, and the number of channels in the site.
For example, consider the 1x3x1 frequency reuse pattern.
- The first digit (i.e. 1x3x1) represents the number of sites used in the reuse pattern before a frequency is assigned again. For LTE, the first digit is always 1, meaning the frequencies are reused in every site.
- The second digit (i.e. 1x3x1) represents the number of sectors deployed in each site.
- The third digit (i.e. 1x3x1) represents the number of channels or carriers
distributed across the sectors of each site.
LTE is may be deployed as a transitional technology upgrade to existing older 2G/3G technology systems (e.g. GSM, CDMA, UMTS, etc.) within the same band. This is done by taking frequency spectrum away from the existing legacy technology (reducing its capacity) and assigning it to LTE. When this is done, the LTE channel bandwidth uses the spectrum made available.
As the system further transitions away from the legacy technology and toward LTE, the additional spectrum assigned to LTE is implemented as increasing the LTE channel bandwidth (up to 20 MHz) rather than adding another LTE channel. By following this approach, the LTE portion of the system provides the greatest capacity by avoiding duplication of overhead signaling.
For this reason, it is expected that LTE deployments will stay with their 1x3x1 reuse pattern. The available spectrum for use in a new start system (not within an existing 2G/3G system) will dictate the bandwidth of the LTE carrier.
Resource Block Group Reuse Sub-Pattern.
LTE down link traffic is allocated resource blocks as the traffic load requires. The resource blocks groups are managed with respect to adjacent sectors and sites to reduce interference and optimize system performance. In lightly loaded traffic situations, the LTE sector first draws resource blocks from predefined groups that consist of one third of the resource blocks available within the LTE channel. (The number of resource blocks varies with the bandwidth of the channel.)
The LTE system will perform the best under lightly loaded situations when the assignment of these predefined resource block groups follow a 1x3x3 reuse sub-pattern of their own. This reuse sub-pattern becomes less important as the traffic load requires a LTE sector to draw upon unused resource blocks initially assigned to adjacent sectors and sites.
Frequency Reuse Patterns.
Reuse patterns with 3 sectors per site are oriented so that each sector’s antenna azimuth is separated from its adjacent sectors by 120 degrees. Typically, LTE sectors use antennas with 65 to 70 degree horizontal beamwidths (i.e. antenna selection is at the discretion of the system designer).
These narrow horizontal beamwidth antennas have a higher gain than the antennas with 90 to 120 degree horizontal beamwidths. Along with this gain comes the necessity to keep the sites more closely aligned onto the 1x3x1 hexagon placement grid. The coverage of each sector provides minimal overlap with the coverage of the adjacent sector requiring the coverage gap to be filled in by a sector from an adjacent site.
The following figure provides three different examples of site placement. The image on the right is the optimum as the main beam of one sector is positioned in the location of the null of two sectors of an adjacent site.
1x3x1 Frequency Reuse Pattern.
This is the only reuse pattern currently planned for LTE sites. It consists of sites with 3 sectors all operating on the same carrier. To ensure best RF coverage and reduced C/I, sites placement and sector azimuth should follow the first pattern described in Section (1x3x3). In the pattern, the sectors of each site are oriented to point toward the face of the adjacent site hexagon (or to the null between the two sectors of the adjacent site).
1x3x3 Frequency Reuse Pattern.
This is not a true 3 frequency reuse pattern in that the same frequency broadband channel is used in each sector. This reuse pattern consists of sites with 3 sectors and 3 different Resource Block Group assignments. The following figure shows two different uniform configurations or layouts for this antenna pattern.
This figure indicates the Resource Block Group plan used for these configurations, with each color indicating a different Resource Block Group. In either layout, sector 1 of each site is using the first third of the Resource Block Groups, sector 2 of each site is using the second third of the Resource Block Groups, and sector 3 of each site is using the last third of the Resource Block Groups.
As can be seen from the figure above, one layout (i.e. interleaved pattern) for this reuse pattern has the sectors of each site oriented to point toward the face of the adjacent site hexagon. The other layout (i.e. non-interleaved pattern) for this reuse pattern has the sectors of each site oriented to point toward the apex of the adjacent site hexagon.
In the case where the sectors are pointing towards the face of the hexagon, it can be seen that each sector is pointing directly between two sectors at an adjacent site, essentially interleaving the coverage. This interleaving provides two benefits, better RF coverage and less potential for interference than the layout on the right, where the sector coverage is not interleaved. In addition, the interleaved pattern has fewer sectors which contend with each other to cover the areas between sites. This is illustrated in the following figure where some of these areas between sites are highlighted with red circles.
The interleaved pattern shows at most 3 sectors that contend to cover the area between sites, which is better than the non-interleaved pattern which shows as many as 6 sectors contending to cover the area between sites. In addition, the three sectors that contend to cover the area between sites in the left layout are all assigned different Resource Block Groups so there is less potential for interference.
This figure represents ideal site grid layouts. Adjustments may need to be made to the ideal plan (such as antenna down tilting, antenna orientation, Resource Block Group assignments, etc.) for deployments where the sites are not on an ideal grid. Coverage and interference studies need to be run to determine the best site layout and configuration for a system. Care must be taken in assigning the Resource Block Groups and configuring the sectors in order to minimize the potential for interference.