Method for performing handover and roaming in a radio communication environment

• US 6,236,860 B1
• Filed: 04/16/1998
• Issued: 05/22/2001
• Est. Priority Date: 10/17/1995
• Status: Expired due to Term

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Jansen, Michel Godefridus, Van De Berg, Petrus Hubertus Gerardus, Hagting, Marco
• Primary Examiner(s): MAUNG, NAY AUNG
• Application Number:
  US09/051,661
• Time in Patent Office: 1,132 Days
• Field of Search: 455/403, 455/422, 455/426, 455/432, 455/434, 455/436, 455/450, 455/451, 455/462, 455/464, 455/509, 455/517, 455/552, 455/524, 455/525, 455/560, 455/561, 455/452, 370/329, 370/331, 370/337, 370/347, 370/465
• US Class Current: 455/436
• CPC Class Codes: H04W 36/00835 Determination of neighbour cell lists
• CPC Class Codes: H04W 36/0085 Hand-off measurements
• CPC Class Codes: H04W 36/18 for allowing seamless reselection, e.g. soft reselection
• CPC Class Codes: H04W 84/14 WLL [Wireless Local Loop]; RLL [Radio Local Loop]

Abstract

The invention relates to duplex radio communication systems comprising radio access units and at least one remote radio access units operating in a time asynchronous manner. The radio environment is scanned for radio links operating in a synchronous or an asynchronous manner to a particular radio communication unit. Handover to a synchronous link can be performed in a seamless manner using a single timing reference. There is also provided a method for initiating handover and roaming in an asynchronous radio environment. A call with a remote unit is handed over from a first radio access unit to a second radio access unit by suspending transmission of the remote unit at a first radio link while maintaining the transmission at this first radio link by the first radio access unit is established by the remote unit and the call is resumed at the second radio link after which the radio link is released by the first radio access unit.

Claims

1. A scanning method for initiating handover of a call or roaming in a duplex radio communication system comprising radio access units and at least one remote radio communication unit, said radio access units and the or each remote radio communication unit being arranged for establishing a call at a radio link between a remote radio communication unit and a radio access unit, which radio link is selected from a plurality of predetermined radio links, and for switching a call in progress at a first radio link between a remote radio communication unit and a first radio access unit to a second radio link between said remote radio communication unit and a second radio access unit, wherein each or clusters of radio access units are operated in a mutually time asynchronous manner forming an asynchronous radio environment and said remote radio communication unit is synchronised to a first radio access unit, said method comprises the following steps performed by said remote radio communication unit:

a) scanning said radio environment for radio access units which operate in a time synchronous manner with said first radio access unit using a scan window having a width and/or position in time such to essentially retrieve synchronisation data and control data exchanged during a predetermined time period at a radio link operating time synchronous with said first radio link,

b) scanning said radio environment for radio access units which operate in a time asynchronous manner with said first radio access unit using a scan window having a width and position in time such to retrieve synchronisation data and control data essentially exchanged outside said predetermined time period,

c) maintaining a list of synchronous and asynchronous radio links available for handover, and

d) evaluating said list on the basis of predetermined radio link criteria such to determine whether handover or roaming has to be initiated.

2. A method according to claim 1, wherein at step a) said remote communication unit synchronizes its timing reference to retrieved synchronisation data.

3. A method according to claim 1, wherein at step b) said scan window having a width and position in time such to include said predetermined time period, and wherein synchronous radio links and asynchronous radio links are distinguished on the basis of time difference between the receipt of synchronisation data and said predetermined time period.

4. A method according to claim 3, wherein said time difference is used by said remote communication unit to adjust its timing reference.

5. A method according to claim 1, wherein step a) and step b) are performed at different rates.

6. A method according to claim 5, wherein step a) is performed at a higher rate than step b) if a number of synchronous radio links adequately for handover is detected, said number and rate may be adoptively set subject to predetermined radio link criteria.

7. A method according to claim 1, wherein at step a) and step b) the number of radio links to be scanned is limited if a number of radio links adequately for handover is detected, said number may be adoptively set subject to predetermined radio link criteria.

8. A method according to claim 6, wherein said radio communication system operates in accordance with the Digital European Cordless Telecommunications (DECT) standard, and wherein said predetermined radio link criteria are formed by either one or a combination of the following criteria:

RF signal level (RSSI), burst synchronisation (SYNC) error, system information field test word (A CRC) error, data field test word (X CRC) error.

9. A method according to claim 1, further comprising the steps of:

e) suspending transmitting by said remote radio communication unit at said first radio link while maintaining transmitting by said first radio access unit at said first radio link,

f) establishing said second radio link,

g) resuming said call at said second radio link, and

h) releasing said first radio link by said first radio access unit.

10. A method according to claim 9, wherein said first and second radio access units are operated in a mutually time asynchronous manner, and wherein said first and second radio access units may form part of first and second time independently operated radio communication networks, respectively.

11. A method according to claim 9, wherein during steps e) and 0 said first radio access unit maintains transmitting said call at said first radio link, and said remote radio communication unit maintains receiving said call at said first radio link.

12. A method according to claim 9, wherein at the start of a handover said remote communication unit forwards a transmit suspension message to said first radio access unit.

13. A method according claim 12, wherein upon receipt of said transmit suspension message by said first radio access unit a signaling message is exchanged for the purpose of indicating that said call is subjected to handover, which message at a terminal side may take the form of but not limited to a voice signal, an audio tone signal or a visual display signal, and wherein said signaling message is released if said call is resumed.

14. A method according to claim 9, wherein if said call is resumed at said second radio link, said first radio link is released after a predetermined time period following suspension of transmitting by said remote communication unit.

15. A method according to claim 9, wherein if said call is resumed at said second radio link, a release message is forwarded to said first radio access unit via its corresponding radio communication network in order to release said first radio link.

16. A method according to claim 9, wherein if said second radio link cannot be established, said call is resumed at said first radio link.

17. A method according to claim 16, wherein if said call cannot be resumed at said first radio link, a third radio link between said remote communication unit and said first radio access unit is established and said call is resumed at said third radio link.

18. A method according to claim 9, wherein said call is resumed maintaining call control features particular to said first radio link.

19. A communication unit comprising:

a) means for scanning said radio environment for radio access units which operate in a time synchronous manner with said first radio access unit using a scan window having a width and/or position in time such to essentially retrieve synchronisation data and control data exchanged during a predetermined time period at a radio link operating time synchronous with said first radio link,

b) means for scanning said radio environment for radio access units which operate in a time asynchronous manner with said first radio access unit using a scan window having a width and position in time such to retrieve synchronisation data and control data essentially exchanged outside said predetermined time period,

c) means for maintaining a list of synchronous and asynchronous radio links available for handover, and

d) means for evaluating said list on the basis of predetermined radio link criteria such to determine whether handover or roaming has to be initiated.

20. The communication unit of claim 19, wherein said communication unit is a radio telephone.

21. The communication unit of claim 19, wherein said communication unit is a base station.