Handover Overview
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There are three types of WCDMA handovers: intra-frequency, inter-frequency, and inter-system. Intra-frequency handovers occur within the same frequency and support soft, softer, and hard handovers. Inter-frequency handovers are between different frequencies within the same system and only support hard handovers. Inter-system handovers are between different systems, such as WCDMA to GSM, and only support hard handovers. Soft handovers connect the user equipment to multiple base stations simultaneously, while softer handovers occur within one base station but between different sectors. Hard handovers release the source before adding the new connection.
Handover Overview
- 1. Overview
- 2. WCDMA handovers can be categorized into three different types which support different handover modes Intra-frequency handover WCDMA handover within the same frequency and system. Soft, softer and hard handover supported Inter-frequency handover Handover between different frequencies but within the same system. Only hard handover supported Inter-system handover Handover to the another system, e.g. from WCDMA to GSM. Only hard handover supported Md. Joynal Abaden +880 1830 457 155
- 3. Soft handover Handover between different base stations Connected simultaneously to multiple base stations The transition between them should be seamless Downlink: Several Node Bs transmit the same signal to the UE which combines the transmissions Uplink: Several Node Bs receive the UE transmissions and it is required that only one of them receives the transmission correctly Md. Joynal Abaden +880 1830 457 155 UE1 BS 1 BS 2
- 4. Softer handover Handover within the coverage area of one base station but between different sectors Procedure similar to soft handover Md. Joynal Abaden +880 1830 457 155 UE1 BS 1 BS 2
- 5. Hard handover The source is released first and then new one is added Short interruption time Terminology Active set (AS), represents the number of links that UE is connected to Neighbor set (NS), represents the links that UE monitors which are not already in active set Md. Joynal Abaden +880 1830 457 155
- 6. Handover parameters Add window Represents a value of how much worse a new signal can be compared to the best one in the current active set in order to be added into the set Adding link to combining set can be done only if maximum number of links is not full yet (defined with parameter). Moreover a new link is added to the active set only if the difference between the best and the new is still at least as good after the ‘add timer’ is expired. Timer is started when the signal first reaches the desired level. Drop window Represents a value of how much poorer the worst signal can be when compared to the best one in the active set before it is dropped out Similarly to adding, signal which is to be dropped needs to fulfill the drop condition after the corresponding drop timer is expired. Md. Joynal Abaden +880 1830 457 155
- 7. Replace window Represents a value for how much better a new signal has to be compared to the poorest one in the current active set in order to replace its place Replace event takes place only if active set is full as otherwise add event would be applied Similarly to add and drop events, also with replace event there exist a replace timer Md. Joynal Abaden +880 1830 457 155
- 8. Exercises: Replace ‘Threshold_1’, ‘Triggering time_1’, etc with correct handover parameter names. Which event is missing from the example? Md. Joynal Abaden +880 1830 457 155 Received signal strength BS1 BS2 BS3 Threshold_1 Triggering time_1 Threshold_2 Triggering time_2 BS2 from the NS reaches the threshold to be added to the AS BS1 from the AS reaches the threshold to be dropped from the AS BS1 dropped from the AS