Handover

When the mobile station moves from one cell (referring to the base station or the coverage area of the base station) to another cell, the channel switching that needs to be performed in order to maintain the uninterrupted communication of the mobile user is called handover.

There are three criteria for determining whether a mobile station needs to be handed off:

(1) It is determined by the carrier level of the received signal. When the signal carrier level is lower than the threshold level (eg -100dBm), it will switch.

(2) Determined according to the carrier/interference ratio of the received signal. When the load/dry ratio is lower than the given value, it will switch.

⑶ According to the distance between the mobile station and the base station. When the distance is greater than the given value, it will switch.

In fact, it is difficult to measure the carrier/interference ratio of the received signal during a call; and when using distance judgment, it is sometimes difficult to guarantee the distance accuracy. Therefore, the first one is generally used.

Handover can be technically divided into hard handover and soft handover:

Hard handover: Before a new connection is established, the old connection is terminated first. For example the GSM system.

Soft handover: refers to maintaining the old connection and establishing a new connection at the same time. For example CDMA system.

Handover can be divided from the nature of the cell:

Handover between base stations of the same switching center

Handover between the same BSC

Handover between different BSCs

Handover of base stations between different switching centers,

Handover between micro cells and macro cells

The handover of different sectors in the same base station,

Handover between different operators.

Handover Control

There are three main types of process control:

① Mobile station controlled handover

The mobile station continuously monitors the signal strength and quality of the current base station and candidate base stations during several handovers. When a certain handover criterion is met, the mobile station selects the best candidate base station with available traffic channels and sends a handover request. .

Small systems such as DECT are often used, and it is easy to cause switching conflicts in large systems.

② Network-controlled handover

The base station monitors the strength and quality of the signal from the mobile station, and when the signal falls below a certain threshold, the network starts arranging a handover to another base station.

Disadvantage: If the MS loses contact, the signal will be interrupted.

The first generation of analog systems used this approach

The switching time is long, up to 10S.

③ Mobile-assisted handover

The network requires the mobile station to measure the signals of its surrounding base stations and report the results to the old base station. The network decides when to handover and which base station to switch to based on the test results.

The second generation systems GSM and CDMA all use this method.

Features: Fast time, switching process 1s~2s, signal interruption <1s.

The problem of high-speed mobile handover of micro cells

In a small cell, high-speed mobile users only have to switch in a short time, which puts too much pressure on the system.

An umbrella-shaped cell structure combining macro cells and micro cells, and the use of macro cell channels during handover can solve the above problems.

Handover Guidelines:

Criterion 1: Relative Signal Criterion.

Choose the base station with the strongest received signal at any time.

The disadvantage of this criterion is that when the signal strength of the original base station still meets the requirements, it will cause too many unnecessary handovers.

Criterion 2: Relative signal strength criterion with threshold specification.

The mobile user is only allowed to perform handover when the signal of the current base station is weak enough (below a certain threshold) and the signal of the new base station is stronger than that of the current base station.

Criterion 3: Relative signal strength criterion with hysteresis margin.

The mobile user is only allowed to perform handover when the signal strength of the new base station is much stronger than that of the original base station (that is, greater than the hysteresis margin).

This technology can prevent the mobile station from repeatedly switching back and forth between the two base stations due to signal fluctuations, that is, the "ping-pong effect".

Criterion 4: Relative signal strength criterion with hysteresis margin and threshold specification.

Mobile users are only allowed to handoff when the signal level of the current base station is below a specified threshold and the signal strength of the new base station is higher than the current base station by a given hysteresis margin.