3G Soft Handover Operation & 3G Handover Types.

Following is the list of 3G handover types.

Intra-frequency handover.
- Between cells with same carrier.
- soft/hard handover.
Inter-frequency handover.
- Between cells with different carrier.
Inter-system/inter-RAT handover.
- Between 3G and 2G cells.

Types of Handovers in 3G Networks.

In 3G networks, there are three different types of handovers: intra-frequency handover, inter-frequency handover, and inter-RAT handover.

1. Intra-Frequency Handover:

This is the most common type of handover in 3G, involving handovers between 3G cells within the same 5MHz carrier.

Both soft handover (where the UE is connected to multiple cells simultaneously) and hard handover (a complete switch from one cell to another) can occur, although hard handover is rare. This type of handover will be discussed in detail in this article.

2. Inter-Frequency Handover:

This involves a handover from a 3G cell on one carrier to a 3G cell on a different 5MHz carrier. It is always a hard handover since it involves switching between different frequencies.

3. Inter-RAT Handover:

Inter-RAT stands for inter Radio Access Technology. It involves a hard handover between a 3G cell and a 2G cell. This type of handover is crucial for maintaining call continuity when 3G coverage is limited.

The details of inter-RAT handover will be discussed in the next article.

3G Neighbour List.

Different 3G handover types result in different 3G neighbour list types.
- Intra-frequency handover: intra-frequency NB list.
- Inter-frequency handover: inter-frequency NB list.
- Inter-system/inter-RAT handover: inter-system NB list.
These different NB lists are only communicated to the UE on a need-to-know basis.
- Intra-frequency NB list.
  - Always needed in 3G operation
  - Sent to UE at start of session.
- Inter-frequency NB list.
  - Only needed when handover to second carrier is considered.
  - Sent to UE when Compressed Mode is entered.
- Inter-system NB list.
  - Only needed when handover to 2G is considered.
  - Sent to UE when Compressed Mode is entered.
Neighbour list transmitted to UE with RRC Measurement Control message.

3G Neighbour List Types and Their Importance.

In 3G networks, different handover types necessitate different neighbour list types: intra-frequency, inter-frequency, and inter-RAT neighbour lists.

1. Intra-Frequency Neighbour List:

Required for intra-frequency handovers, this list contains 3G neighbours on the same 5MHz carrier. It is essential for 3G operation and must be available to the UE from the start of the session.

2. Inter-Frequency Neighbour List:

Needed for inter-frequency handovers, this list includes 3G neighbours on different carriers. This list is only communicated to the UE when a handover to a second carrier is considered. It becomes relevant once the UE enters compressed mode.

3. Inter-RAT Neighbour List:

Required for inter-RAT handovers between 3G and 2G cells, this list contains the 2G neighbours for the relevant 3G cell. This list is only needed when a handover to 2G is considered. It is sent to the UE once compressed mode is entered.

Compressed Mode:

Compressed mode is crucial for both inter-frequency and inter-RAT handovers. It allows the UE to measure other frequencies and technologies without losing connection to its current serving cell.

Neighbour Lists Transmission:

RRC Message Measurement Control: This message is used in connected mode to send all types of neighbour lists to the UE. It will be analyzed in detail in this and the next chapter.

1. Intra-frequency Soft Handover.

Handover between cells with same carrier.
Soft handover.
- UE connected simultaneously to multiple cells.
- l_ur interface needed if several RNCs involved.
Soft handover area:
- Overlapping cells with same quality.

Intra-Frequency Soft Handover in 3G Networks

The figure above illustrates intra-frequency soft handover, the most common type of handover in 3G networks. Both cells (green and blue) in the figure use the same 3G carrier. The UE moves from the leftmost position to the middle and further to the right while in connected mode.

Initial Position (Leftmost):

In the initial position, the UE is connected to the green cell with one radio link, as only this cell is dominant.

Middle Position (Overlapping Area):

Soft Handover: As the UE moves to the middle position, both the green and blue cells provide similar signal quality (Ec/No). Here, the UE is connected to both cells simultaneously, a state known as soft handover. This overlapping area with comparable quality from both cells is called the soft handover area.
Redundancy: During soft handover, both radio links carry the same signalling (RRC and NAS) and user data in both uplink and downlink. While this redundancy consumes additional network capacity, it results in the so-called Soft Handover Gain, achieving a 2-3 dB reduction in transmission power.

Final Position (Rightmost):

Connection: As the UE moves further to the right, the blue cell becomes dominant, and the green cell’s signal weakens. In this position, the UE is connected to the blue cell with a single radio link, exiting the soft handover state.

Requirements for Soft Handover:

I_ur Interface: Soft handover, where the UE connects simultaneously to multiple cells, requires the implementation of the I_ur interface between the involved RNCs. Without the I_ur interface, a hard handover (a complete switch from one cell to another without simultaneous connections) would be used instead.

Intra-frequency soft handover area.

Active Set (AS): all cells the UE is connected to in connected mode.
Typically Active Set is limited to maximum three cells.
Several possibilities for soft handover area:
- 2-cell soft handover area.
- 3-cell soft handover area.
Soft handover causes redundancy, but results in reduction of transmitted power (SHO Gain).

Intra-Frequency Soft Handover in a Three-Cell Scenario

The figure above illustrates the intra-frequency soft handover area for a scenario involving three cells. This scenario includes both 2-cell and 3-cell soft handover areas.

2-Cell Soft Handover Areas:

Indicated in Blue: These areas represent regions where the UE connects to two cells simultaneously. The overlapping signal quality in these regions allows the UE to maintain connections to both cells.

3-Cell Soft Handover Area:

Indicated in Red: This central area is where the UE connects to three cells at the same time. The overlapping signal quality from all three cells in this region allows for simultaneous connections to all of them.

Redundancy and Soft Handover Gain:

Redundancy: Soft handover creates redundancy by maintaining multiple radio links for the same UE, which increases network capacity usage.
Soft Handover Gain: Despite the redundancy, this process results in soft handover gain, which reduces transmission power by 2-3 dB, improving overall efficiency.

Active Set (AS):

The set of cells to which the UE is connected in connected mode is known as the Active Set (AS). The maximum number of cells the UE can connect to is determined by the parameter Maximum Active Set Size. Typically, a maximum of three cells are allowed in the active set.

Intra Frequency Soft Handover Gain.

Left graph.
- SHO gain as function of # active set cells with equal Ec/No.
- Optimal point: (maximum) three cells in active set.
Right graph.
- Only two cells in active set.
- SHO gain as function of delta Ec/No between two active set cells.
- The smaller the difference between the active set cells, the higher the SHO gain.

Soft Handover Gain Analysis.

The figure on the left side illustrates the Soft Handover Gain as a function of the active set count, assuming that all cells in the active set have equal Ec/No (signal quality). It is evident that as the number of cells in the active set increases, the Soft Handover Gain also rises. However, this additional gain diminishes with each added cell. Consequently, it is recommended to limit the active set to no more than three simultaneous cells. Adding more than three cells results in a capacity loss that outweighs the additional Soft Handover Gain.

The figure on the right side shows the Soft Handover Gain as a function of the quality difference between the active set cells, assuming only two cells are present in the active set. The smaller the quality difference between the cells, the higher the resulting Soft Handover Gain. If the quality difference exceeds 4 dB, it is advisable to remove the weaker cell from the active set, as it no longer contributes to the Soft Handover Gain.

Intra-frequency Hard Handover.

Handover between cells with same carrier.
UE connected to maximum 1 cell at a time.
Connection release old cell and connection setup new cell performed simultaneously.
Occurs if several RNCs involved without lur interface.

The figure above depicts the less common scenario of intra-frequency hard handover, where both cells belong to the same carrier. In this instance, the lur interface between the different RNCs involved is not implemented, which means that soft handover is not possible. Consequently, the UE can only be connected to one cell at a time. The hard handover in this scenario involves a synchronized switch from a radio link with the green cell to a radio link with the blue cell. To date, the I have not observed this type of handover in any commercial network.

2. Inter-System / Inter-RAT Handover.

The figure above illustrates an inter-RAT (Radio Access Technology) handover, where the transition occurs from the green 3G cell to the blue 2G cell. Due to the use of entirely different technologies and frequency bands, only a hard handover is feasible in this scenario. Inter-RAT handover is a crucial mechanism for maintaining call continuity when 3G coverage is limited, particularly during the initial rollout of 3G networks. This type of handover ensures that calls are not dropped when a UE moves out of the 3G coverage area into a region covered by 2G.

You can study more about this handover here in detail.

3. Inter-Frequency Handover.

Handover between cells with different carrier.
Always hard handover.
Different types: load based, service based, UE triggered.

The figure above illustrates the inter-frequency handover, a type of handover utilized in more advanced 3G networks with multiple carriers. In this scenario, both 3G cells operate on different 5MHz carriers and are often co-located to enhance capacity in hotspot areas.

This handover is always a hard handover due to the differing frequencies. Various inter-frequency handover types exist, including load-based handover, service-based handover, and UE-triggered handover.