When a mobile station moves from one cell (referring to the coverage of the base station or the base station) to another cell, the channel handover required to maintain the uninterrupted communication of mobile users is called handover.
switch(also known asHandover、Over zone switching；English: handover, handoff) refers to the process in which a mobile station moves from one base station coverage area to another during a call, or switches to another voice channel due to external interference.
Handover
In cellular mobile communication networks, handoff is to ensure that mobile users can achieve uninterrupted communication in the mobile state;Handover is also to maintain an acceptable communication quality between the mobile station and the network and prevent communication interruption, which is an essential measure to adapt to the characteristics of mobile fading channels.In particular, the handover initiated by the network is a powerful measure to balance the traffic of cells in the service area and reduce the call loss rate of high user cells.Handover can optimize the use of wireless resources (frequency, time slot, code);It can also reduce the power consumption of the mobile station and limit the global interference level in time.
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There are many reasons for switching
1. When a call is in progress or there is a data connection (e.gGPRSWhen a mobile phone enters another cell from one cell, it is necessary to make a handover in order to avoid a call drop or data disconnection.
2. In a cell, when the ability to connect to a new call reaches the upper limit, and the cell phone that initiated the new call covers another cell, it is wise to transfer the call to that cell.This is not only conducive to load balancing, but also good news for those mobile phones that can only access the first cell.
3. In a non code division multiple access system, the channel used by a mobile phone may interfere with the channel used by a mobile phone in a similar cell. In this case, switching the communication channel of the mobile phone to different channels in the same cell or different channels in adjacent cells will reduce the possibility of interference.
4. Similarly, in the non code division multiple access system, when the user's behavior changes, for example, when a high-speed user connected to a large umbrella type cell suddenly stops, the call will usually be transferred to a macro cell or a micro cell, which will enable the umbrella type cell to have more ability to deal with fast moving users,And reduce interference.And vice versa (macro cell to macro cell).
5. In the code division multiple access system, soft handover (see below) can be used to reduce interference to adjacent cells (see near far problem), even when there is a good connection.
6. The most basic handover (soft handover) is when a mobile phone in a call is transferred from the current cell (called the source cell and the currently used channel) to another available cell (called the destination cell) and channel (the source cell and the destination cell can be different cells, or different sectors of the same cell).This handover process can be called inter cell handover;Another special example is that the source cell and the destination cell are the same, or even the same sector, but the transfer is only carried out between the used channels. This handover is called intra cell handover.
Judgment conditions
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Handover
In mobile communication systems, handover can generally be judged according to RF signal strength, carrier interference ratio, relative position between mobile station and base station, and bit error rate in digital systems.In practical application, one or more of them can be selected as parameters.The switching modes are as follows:
Relative signal strength
The handover decision is based on the average value of the signals received from the base station.The mobile station continuously monitors the signal strength of each cell. When the signal strength of a neighboring cell's BTS exceeds that of the current BTS, it initiates a handover.The disadvantage of this method is that when the service base station can provide the required service quality, many unnecessary handovers are performed
Relative signal strength with threshold
The mobile station continuously monitors the signal strength of each cell. When the signal strength of a neighboring cell's BTS exceeds that of the current cell's BTS, and the signal strength of the current cell's BTS is below a certain threshold, the mobile station initiates handover.In this method, the threshold value needs to be properly selected.If the selected threshold value is higher than the signal strength of two base stations, the effect similar to the relative signal strength will appear.If the selected threshold value is lower than this signal strength, the mobile station will delay the handover until the signal strength of the serving base station passes this threshold value.If the selected threshold value is much lower than the signal strength, it will cause excessive delay.This will reduce the quality of the communication link and even cause call interruption.
Relative signal strength with hysteresis
The mobile station also continuously monitors the signal strength of each cell. When the signal strength of the BTS in an adjacent cell is greater than the signal strength of the BTS in the current cell for more than a lag range, the mobile station initiates a handover.This scheme can effectively avoid the "ping pong effect" caused by signal fluctuation, but when the signal strength of the serving base station is strong enough, it also generates unnecessary handovers.
Relative signal strength with hysteresis and threshold
In this mode, when the base station of an adjacent cellsignal intensity When the BTS in the current cell exceeds a lag range and the signal strength of the BTS in the current cell is lower than a threshold, the handover is initiated.When the signal strength of the current base station can provide the required quality requirements, this method can further reduce unnecessary handovers.
Relative signal strength with lag time
In this mode, when the signal strength of a neighboring cell base station is higher than that of the current cell base station, and it remains higher than that of the current cell base station for a period of time, the handover starts.This method reduces the number of frequent handovers.
If the handover scheme is designed based on the handover times and handover delay, the ideal handover scheme requirements are: as few handover times as possible to reduce the signaling burden;The delay should be as short as possible to avoid slow handover.Obviously, the number of handovers and the handover delay cannot be minimized at the same time, so a compromise scheme should be considered.
Business system
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Single business system
Handover
Although the third generationMobile communication systemIt has developed rapidly, but the second generation mobile communication system is the most widely used in the world today. Its business is relatively single, mainly voice business, so its handover is also aimed at single business systems.Compared with the second generation system, the most obvious feature of the third generation system is that it supports high-speed data services and can carry multiple services;The use of microcellular technology has greatly increased the system capacity.Its handover strategy has also changed accordingly.
For a single service system, there are the following basic handover strategies:
1. There is no priority policy.The BTS does not distinguish between new call requests and handover call requests. If there is no free channel, it will clear it.
2. Queuing priority strategy.If there is no idle channel, the handover call can queue up for channel release, and the queuing channel release can be varied.
3. Measurement based priority strategy.Handover requests are queued according to the speed of calls approaching the receiver, rather than allocating channels to these calls according to the principle of first in, first served.
4. Reserved channel strategy.In order to reduce the failure rate of handoff calls, each cell reserves a fixed number of channels for handoff calls. If the number of free channels is greater than the number of reserved channels, the arrival of new calls will not cause call loss, while the arrival of handoff calls will not cause call loss as long as there are free channels.
5. Half speed strategy.Define some channels for each cell. When the cell channel is busy, you can temporarily divide these channels into two, that is, half speed channels. One is used for ongoing calls, and the other is used for handoff calls.To enable the handover call to continue, thereby reducing the forced outage rate.When a half rate channel is released, it can be combined with other idle half rate channels to form a full rate channel. However, the implementation of this method requires that the air interface structure can support the function of dividing the full rate channel into two sub rate channels.
Multi service system
Handover
For multi service handover, these strategies will not meet the requirements of different services. It is necessary to integrate the advantages of single service handover strategies to form a multi service handover strategy.1. Give different priority to different calls
New calls of different services have the same priority, with the lowest priority;Handover calls have higher priority than new calls. Because data services are non real-time services, their handover calls are given second priority;Due to the real-time requirements and high delay requirements of voice services, the priority of handover calls is the highest.
When a call arrives, first determine its service type, that is, whether it is voice or data service, then determine whether it is a new call or a handover call, and give the corresponding priority level.New calls of all kinds of services have the same priority and the lowest level, and receive services according to the first come, first served (FIFO) rule.Handover calls have higher priority than new calls, and handover calls of different services have different priorities.Handover call of data service has second priority;Voice handover calls have the highest priority, and also receive services according to FIFO rules.
2. Origin borrowing scheme
The biggest difference between the multi service handover problem and the single service handover problem is that there are different services in the system, different services have different priorities, different services use different amounts of various resources, and various services have different sensitivity to time delay. Therefore, it can be considered that when necessary, the service channel of the former call can be temporarily lent to the latter to switch calls,For the former, the signaling channel is used to keep contact with the base station. When the service channel is sufficient, the service communication can be carried out.
As mentioned earlier, the third generation mobile communication system greatly improves the system capacity by reducing the cell size.However, this has also brought some problems. Users move through more and more cells, resulting in an increase in the handover rate and signaling load. When there are high-speed mobile users, it is easy to drop the line.In order to solve this problem, we introduce hierarchical cellular structure into wireless network, and use soft handover technology and vertical handover strategy at the same time.
Basic classification
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When a handover is triggered, a new channel will be established, communication will be transferred to the new link, and the original channel will be released.The handover process can be classified according to the establishment path of the new link (the release of the old link occurs before, during or after the establishment of the new link).
Hard switching
Handover
The main feature of hard handover is that the mobile station only occupies one wireless channel at a time in the case of hard handover. It must first interrupt the connection with the original base station, tune to a new frequency, and then contact the new base station within a specified time. During the handover process, communication may be interrupted for a short time.Hard handover is mainly the handover between base stations and sectors with different frequencies.In hard handover, in order to minimize the interruption time, new links should be established in advance in the network.One of the main advantages of hard handover is that at the same time, the mobile station only occupies one wireless channel.The disadvantage of hard handover is that there will be short-term transmission interruption in the communication process, so hard handover will affect the call quality to a certain extent.In addition, if the interruption time is affected by factors such as interference or unreasonable handover parameter settings, the handover will fail and call drops will occur;When the hard handover area is narrow, the "ping-pong effect" of handoff between the new base station and the original base station will appear, affecting the transmission of the traffic channel.Hard handover is mainly used in GSM system.
Soft switching
In the process of soft handover, the two links and the corresponding two data streams are activated at the same time for a relatively long time. The connection with the original base station is not disconnected until the new base station is entered and the transmission quality of the new base station is measured to meet the index requirements.Soft handover is the handover between different base stations at the same frequency.Whether from the perspective of mobile station or network, the two links transmit the same data stream, ensuring that communication will not be interrupted.In soft handoff, the mobile station will not break the connection with the original base station until it has obtained the link with the new base station, so there is no interruption in the handover process, which will not affect the call quality;Soft handover is at the junction of base stations with the same frequency. The mobile station communicates with multiple base stations at the same time, playing the role of path diversity of the forward traffic channel and the reverse traffic channel, which can greatly reduce the call drop caused by handover.In addition, diversity technology and reverse power control are used in both mobile station and base station in soft handover, which can improve the performance of the system.However, soft handoff also has some shortcomings, such as more channel resources to be occupied, complex signaling leading to heavier system load, increased downlink interference, increased equipment investment and complexity of the system backplane.Soft handover is mainly used in CDMA systems.
Softer switching
In CDMA system, the soft handover between different sectors of the same cell in a sectorized cell by a mobile station is called softer handover.It is actually the switching between pilots on the same channel board.This handover is completed by the BSC without notifying the MSC.
Relay switching
Relay switching process Relay switching is a switching scheme based on smart antenna.Relay handover is to use accurate positioning technology to determine whether the mobile station has moved to the adjacent area of adjacent base stations that can be switched based on the location of the distance and direction of the mobile station.If the mobile station enters this handover zone, the RNC (wireless network controller) informs the base station to prepare for handover, so as to achieve fast, reliable and efficient handover.This not only saves channel resources, simplifies signaling, reduces system load, but also adapts to handover between cells with different frequencies.In the third generation mobile communication standard, TD-SCDMA adopts relay handover.The necessary condition for relay handover is that the network should be ready to obtain the location information of the mobile station, including the signal arrival direction of the mobile station and the distance between the mobile station and the base station.
Vertical switch
The above mentioned switching modes can be classified as horizontal switching according to the direction of switching, while there is also a switching mode, namely vertical switching.Horizontal handover and vertical handover can be summarized as follows: the handover of a mobile station between base stations (sectors, channels) in the same system is called horizontal handover, while the handover of a mobile station between base stations (sectors, channels) in different systems is called vertical handover.
In the mobile communication system, a hierarchical cell structure is formed by introducing microcells under the macro cell to solve the "blind spots" and "hot spots" in the network. At the same time, different levels of cells are used to provide communication capabilities for different mobile states of users.The macro cell is mainly used for mobile terminals that move at a high speed or cannot be served by the micro cell due to the lack of channels, so it can reduce the handover rate and increase the system capacity at the same time.When the mobile terminal is talking, it maintains the connection with the macro cell and the adjacent micro cell with the strongest signal.And continuously measure the signal strength of the macro cell and the adjacent micro cell, report to the base station system controller, and the base station system controller adjusts the connection between the mobile station and its own adjacent micro cell.When the mobile terminal moves at a low speed and switches, the base station gives priority to switching the mobile terminal to the microcell with the strongest signal according to the signal strength measured by the mobile terminal. Since the mobile terminal always keeps connected to the microcell with the strongest signal, the switching speed is very fast, and the connection between the mobile terminal and the microcell is adjusted after the handover is completed.Of course, when the mobile terminal moves rapidly and switches, the base station will give priority to switching the mobile terminal to the macro cell according to the speed of the mobile terminal, so that the mobile terminal will not switch when it continuously passes through the micro cell, reducing the occurrence of handover;When the speed of the mobile terminal decreases to a certain extent, the base station switches the mobile terminal to the microcell with the strongest signal to ensure that users get the best communication quality and improve the system capacity.
practical application
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Soft handover in WCDMA
Handover
WCDMA handover strategy: The WCDMA soft handover algorithm uses the same pilot channel's Ec/Io as the handover measurement value. It has four crucial pilot sets as follows: activation set: the pilot corresponding to the forward traffic channel assigned to the mobile station.
Candidate set: the pilot set that is not currently in the activation set, but has sufficient strength to indicate that the forward traffic channel of the base station corresponding to the pilot can be successfully demodulated.
Proximity set: currently not in the activation set or candidate pilot set, but can enter the pilot set of the candidate set.
Residual set: In the current system, pilots other than the above three pilot sets.
Wireless link addition and release process:
⑴ The pilot signal strength of Cell 2 gradually increases. When the pilot strength Ec/Io of Cell 2 reaches (the best pilot strength Ec/Io - (report threshold - increase lag threshold)) and maintains the △ T time, the selection set is not full at this time, and Cell 2 is now added to the candidate set.This action is also called wireless link increase.
⑵ The pilot signal strength of cell 3 gradually increases and starts to exceed the pilot signal strength of the earliest cell 1. The pilot (best candidate pilot) strength Ec/Io of cell 3 reaches (the weakest pilot Ec/Io+replacement lag threshold) and maintains the △ T time, and the number of selected sets is full at this time (assuming that the maximum number of candidate sets set by the system at this time is two),Cell 3 (the strongest signal in the candidate set) is now added to the candidate set as a substitute cell 2 (the weakest signal in the candidate set), and cell 1 is simultaneously removed from the candidate set.This action is also called wireless link increase and release.
⑶ At this time, the pilot signal strength of cell 3 in the candidate set gradually weakens. When the pilot signal strength Ec/Io of cell 3 weakens to (the best pilot Ec/Io - (report threshold+delete lag threshold)) and maintains the △ T time, cell 3 (the weakest signal in the candidate set) is now moved out of the candidate set.This action is also called wireless link release.
The report threshold is the threshold for adding or deleting cells in the candidate set during soft handover;△ T is the time left for action triggering;Pilot Ec/Io refers to the pilot strength after measurement;The best candidate pilot refers to the pilot with the strongest signal in the candidate set.
There are three types of hysteresis thresholds:
To increase the delay threshold is to increase the delay threshold of the wireless link;
If you want to delete the hysteresis threshold, you need to delete the hysteresis threshold of the wireless link;
Replacing the hysteresis threshold is to increase and release the hysteresis threshold of a wireless connection at the same time.
Soft Handover in CDMA2000
Handover strategy in CDMA2000.The mobile station constantly searches the strength of each pilot of the active class, candidate class, adjacent class, and residual class, and maintains each class according to the pilot strength. When the mobile station is close to the switching area, the mobile station starts the following operation process:
⑴ The pilot p2 strength exceeds the T_ADD, but has not reached the dynamic threshold. The mobile station moves the pilot to the candidate set.
⑵ The pilot p2 strength exceeds [(SOFT_SLOP/8) × 10 × log10 (PS1)+ADD_IN TERCEPT/2].The mobile station sends the pilot strength measurement message.
⑶ The mobile station receives the extended switching instruction message DROP_INTERCEPT/2, moves p2 into the activation set, and starts macro diversity.Then send the handover completion message.
⑷ The strength of pilot p1 drops below the dynamic threshold [(SOFT_SLOPE/8) × 10 × log10 (PS2)+DROP_INTERCEPT/2], and the mobile station starts to start the transmission switching timer.DROP_INTERCEPT/2 is a parameter to calculate the dynamic threshold when pilot p1 is removed, which corresponds to ADD_INTERCEPT/2.
(5) When the handover down timer times out, the mobile station sends the pilot strength measurement message to the base station.
(6) The mobile station receives the switching instruction message and moves p1 into the candidate class.Then send the handover completion message.
(7) The strength decrease of pilot p1 is lower than T_DROP.The mobile station starts sending and switching timer.
(8) The switch down timer timed out, and the mobile station moved p1 from the candidate class to the adjacent set.
This is the whole process of the mobile station entering and leaving the handover area. It can be seen that the key to the mobile station's handover is to continuously and accurately measure the strength of each pilot under complex wireless channel conditions, and support the macro diversity in the handover area.
Relay Handover in TD-SCDMA
Relay handover is to use accurate positioning technology to determine whether the mobile station has moved to the adjacent area of the adjacent base station that can be switched based on the location of the distance and direction of the mobile station and the direction and distance of the mobile station as auxiliary information.If the mobile station enters this handover area, the RNC notifies the base station to prepare for handover, so as to achieve fast, reliable and efficient handover.This not only saves channel resources, simplifies signaling, reduces system load, but also adapts to handover between cells with different frequencies.
The necessary condition for relay handover is that the network should be ready to obtain the location information of the mobile station, including the direction of arrival (DOA) of the mobile station signal and the distance between the mobile station and the base station.In the TD-SCDMA system, due to the use of smart antenna and uplink synchronization technology, it is easier for the system to obtain the DOA of the mobile station, thus obtaining the location information of the mobile station.The specific process is:
(1) Using smart antenna and baseband digital signal processing technology, the antenna can be shaped adaptively according to the DOA of each mobile station.For each mobile station, it seems that there is always a high gain antenna automatically tracking it. The base station can determine the DOA of the mobile station according to the calculation result of the smart antenna, so as to obtain the direction information of the mobile station.
⑵ Using the uplink synchronization technology, the system can obtain the time offset of the mobile station signal transmission, and then calculate the distance between the mobile station and the base station.
⑶ After the first two steps, the system can accurately obtain the location information of the mobile station.
Therefore, uplink synchronization, smart antenna, digital signal processing and other technologies are the key technical basis for TD-SCDMA mobile communication system to achieve relay handover.The execution process of relay switching is shown in the figure.
⑴ The mobile station communicates with nodeB1 normally.
⑵ When the mobile station needs handover and the network finds the handover target cell through the measurement of the mobile station on the candidate cell, the network sends the handover command to the mobile station, and the mobile station establishes uplink synchronization with the target cell.Then the mobile station establishes signaling connection with NodeB2 while maintaining signaling and service connection with NodeB1.
⑶ After the signaling between the mobile station and NodeB2 is established, the mobile station will delete the service connection with NodeB1.
⑷ The mobile station attempts to establish a service connection with NodeB2. At this time, the service and signaling connections between the mobile station and NodeB1 are all disconnected, while the signaling and service are only maintained with NodeB2?By analyzing the differences in handover technologies in WCDMA, CDMA2000, and TD-SCDMA, we can see that in the measurement process, both soft handover and hard handover are handover and measured without knowing the exact location of the mobile station. Therefore, it is necessary to measure all adjacent cells, and then make handover judgment and target cell selection according to the given handover algorithm and criteria.The relay handover is a handover measurement when the exact location of the mobile station is known. Therefore, it is not necessary to measure all adjacent cells. It only needs to measure a few cells on the side close to the mobile station that are consistent with the mobile station's moving direction, and then make handover judgment and select target cells according to the given handover algorithm and criteria,You can achieve high-quality handoff.
Basic principles
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⑴ It depends on the carrier level of the received signal.When the signal carrier level is lower than the threshold level (for example - 100dBm), switching is performed.
(2) Determine according to the carrier/interference ratio of the received signal.When the load/dry ratio is lower than the given value, switching will be carried out.
⑶ Determine according to the distance between the mobile station and the base station.When the distance is greater than the given value, switch.
In fact, it is difficult to measure the carrier/interference ratio of the received signal during a call;However, it is sometimes difficult to ensure the distance accuracy when judging by distance.Therefore, the first one is commonly used.
process control
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There are three main types of process control:
① Cross zone switching controlled by mobile station
The mobile station continuously monitors the signal strength and quality of the current base station and several candidate base stations when they cross the zone. When certain handover criteria are met, the mobile station selects the best candidate base station with available traffic channels and sends a handover request.
DECT and other small systems are often used, which is easy to cause switching conflicts in large systems.
② Network controlled handover
The base station monitors the signal strength and quality from the mobile station. When the signal is below a certain threshold, the network starts to arrange handover to another base station.
Disadvantages: If MS loses contact, it will cause signal interruption.
The first generation simulation system adopts this method
The switching time is long, up to 10S.
③ Mobile station 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 stations. The network decides when to perform handover and which base station to switch to according to the test results.
The second generation system GSM and CDMA all adopt this method.
Features: fast time, switching process 1s~2s, signal interruption<1s.
matters needing attention
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In micro cells, high-speed mobile users only need to switch in a small amount of time, which puts too much pressure on the system,
An umbrella cell structure combining macro cell and micro cell. The macro cell channel can solve the above problem during handover.
Criteria for handover:
Criterion 1: relative signal criterion.
Select the base station with the strongest reception 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 provisions.
Only when the signal of the current BTS is weak enough (below a threshold) and the signal of the new BTS is stronger than the signal of the current BTS can the mobile user perform handover.
Criterion 3: Relative signal strength criterion with hysteresis margin.
Mobile users are 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 lag margin).
This technology can prevent the mobile station from repeatedly switching back and forth between two base stations due to signal fluctuations, that is, "ping-pong effect".
Criterion 4: relative signal strength criterion with hysteresis margin and threshold provisions.
The mobile user is only allowed to perform handover when the signal level of the current base station is lower than the specified threshold and the signal strength of the new base station is higher than a given lag margin of the current base station.
