Seize highest coverage in 4G with this feature 1/2

Telecom operators can reach highest coverage in 4G without expansion or deployment of new sites but just enabling a special feature known as: Extended cell range.

Let’s have a look on how the feature work, benefits and impacts of deploying it and at last but not least how to deploy it in Huawei networks. This post comes in 2 parts in order to analyze with details the possible scenarios to implement the feature.

Extended cell range

In LTE (Long Term Evolution) networks, the concept of extended cell radius refers to techniques used to extend the coverage area of a cell beyond what is achievable with traditional macrocells. Mobile operator usually implement one of the following options to reach highest coverage in 4G:

Deploy lower frequency bands: Using lower frequency bands (e.g., 700 MHz) propagate further and penetrate obstacles better than higher frequency bands

highest coverage in 4G based on frequency

Enhanced Antenna Systems: Deploying advanced antenna systems, such as MIMO (Multiple Input Multiple Output) and beamforming, can improve coverage and capacity in LTE networks. These technologies can focus signal strength in specific directions, potentially reaching further distances.
Cell Range Expansion (CRE): This technique involves adjusting network parameters to increase the coverage area of a cell. It optimizes parameters such as cell selection thresholds, handover parameters, and power levels. This allows mobile devices to maintain connections at greater distances from the cell site.

Use Cases:

Rural Areas: Extended cell radius techniques are particularly useful in rural areas where deploying additional cell sites may not be cost-effective due to low population density.
Remote Locations: In remote locations, such as islands or mountainous regions, extending the cell radius can significantly improve connectivity without the need for extensive infrastructure investments.

Basically deploying new infrastructure in rural areas is not very cost-effective, mainly due to low traffic and the huge investment requirements. A feature like “Extended cell range” is a great solution to reach highest coverage in 4G at a low cost.

This feature basically works for 3 types of scenarios which depends on the cell radius expected:

Function	Scenario
Cell radius less than 15 km	The cell radius is less than or equal to 15 km.
Cell radius greater than 15 km but lower than 100 km	The cell radius is in the range of (15 km, 100 km].
Cell radius greater than 100 km	The cell radius is in the range of (100 km, 200 km].

The working principle of the feature is mainly based on the random access procedure, because it is based on the Time Advanced reported during this procedure that UEs can receive the right preamble to access the cell.

How highest coverage in 4G relates to Random Access Preamble Formats

For Huawei equipment, FDD supports random access preamble formats 0 to 3. For TDD, it supports random access preamble formats 0 to 4. A preamble format determines the cell coverage and the time domain resources that can be used by a physical random access channel (PRACH).

In a random access procedure, the eNodeB detects the preamble sequences transmitted from a UE to estimates the round-trip delay (RTD), and completes the synchronization for the UE.

The RTD has a direct relationship with the cell radius, higher the cell radius higher the RTD. Extended cell range uses different random access preamble formats to adapt to different RTDs. In order to reach highest coverage in 4G, it is important to define the preamble format that allows higher radius.

Preamble Format	Cyclic Prefix (CP) Length T_CP (µs)	Cell Radius (R) (km)	Maximum RTD (µs)
0	103.1	R ≤ 14.5	96.7
1	684.4	R ≤ 77.3	515.6
2	203.1	R ≤ 29.5	196.8
3	684.4	R ≤ 100	666.7

FDD

In FDD scenarios, the default and recommended settings are as follows:

If cell radius is less than or equal to 15 km, the recommended preamble format is 0.
If cell radius is greater than 15 km, the recommended preamble format is 1, 2, or 3.
When the cell radius is greater than 100 km, the recommended preamble format is 3.

Preamble Format	CP Length T_CP (µs)	Cell Radius (R) (km)	Maximum RTD (µs)	Permissible Multipath Delay (µs)
0	103.1	R ≤ 14.5	96.7	6.4
1	684.4	R ≤ 77.3	515.6	168.3
2	203.1	R ≤ 29.5	196.8	6.3
3	684.4	R ≤ 100	666.7	17.3
4	14.6	R≤ 1.4 km	9.3	5.2

TDD

In TDD scenarios, the default and recommended settings are as follow

If the cell radius is less than or equal to 1.4 km, the recommended preamble format is 4.
If cell radius is greater than 1.4 km and less than or equal to 15 km, the recommended preamble format is 0.
When the cell radius is greater than 15 km, the recommended preamble format is 1, 2, or 3.

In this post we will analyze the deployment of the Extended Cell Radius feature for scenario 3 where cell radius is above 100 km. In the part 2 we will analyze the scenario 1 and 2.

Highest coverage in 4G with cell radius greater than 100 Km (Only FDD)

This configuration is great for rural and remote areas where users can benefit from LTE coverage without incurring in high deployment costs for the mobile operator.

To use extended cell range, operators need to obtain the actual coverage radius of a cell. It is recommended that this function be enabled when the maximum coverage distance of a cell is greater than 100 km.

In this scenario the cell radius can reach up to 200 km.

Enabling this function bring impacts in some existing functionalities for LTE. Mobile operators must be careful to check this impact before activating the function:

Coverage limitation in the uplink for TTI Bundling and PUSCH IRC.
Closed-loop MIMO cannot work properly beyond 100 km.
VirtualGrid (SuperBand – Multi Carrier Unified Scheduling) cannot be built for cells using this functionality.

Some features must be disabled in order to activate this functionality since they are mutually exclusive. Some of the most common features impacted are:

UL Comp
eMTC
3D Beamforming
High speed mobility
NB-IoT
LTE+NR Spectrum Sharing

Activation MML

The Cell MO allows to modify the CellRadius configuration, in this case it is set to 120 Km. The parameter CellRadiusStartLocation indicates the start location of the cell coverage. This parameter is used to calculate the hysteresis in uplink timing when the Extended Cell Access Radius Beyond 100km feature takes effect.

MOD CELL: LocalCellId=0, PreambleFmt=3, CellRadius=120000, CellRadiusStartLocation=20000;
MOD BBP: CN=0, SRN=0, SN=4, TYPE=UBBP, WM=FDD, SRT=FDD_ENHANCE;