Method and system for performing a handoff in a wireless communication system
A system for maintaining wireless communication with a plurality of base stations in a communication network is disclosed. A portable terminal having a communication unit is configured to measure signal strength of a transmission signal for neighboring base stations in a frequency band being used by the communication unit, and create a first handoff candidate list in which one or more handoff candidates in a same frequency band are recorded. A handoff using the first handoff candidate list is performed by the portable terminal. Thereafter, the portable terminal measures signal strength of one or more base stations included in a second handoff candidate list. The result of the signal strength measurements are reported to the currently communicating base station, and a handoff is performed based on a response to the report at the portable terminal.
1. A portable terminal device which maintains wireless communication with a plurality of base stations in a communication network, comprising:
a communication unit communicable in a plurality of frequency bands and configured to select one frequency band from the plurality of frequency bands to perform wireless communication; and
a control unit configured to change the frequency band for use in the wireless communication by the communication unit to perform the wireless communication, wherein
the control unit includes
a first handoff processing unit which, when a neighboring base station list including information of one or more neighboring base stations is received from a currently communicating base station, measures a signal strength of a transmission signal for each of the one or more neighboring base stations in the frequency band being used by the communication unit, creates a first handoff candidate list in which one or more handoff candidates in a same frequency band are recorded in decreasing order of signal strength, and performs a handoff using the first handoff candidate list, and
a second handoff processing unit which, when a second handoff candidate list including information for switching the communicating frequency band and one or more different frequency band handoff candidates is received from a currently communicating base station, measures signal strength of one or more base stations included in the second handoff candidate list, reports the result of the signal strength measurement to the currently communicating base station, and performs a handoff based on a response to the report upon receipt of the response from the communication unit, wherein
the second handoff processing unit determines whether one or more of the one or more base stations included in the second handoff candidate list is also included in the first handoff candidate list, and measures first the signal strength of the one or more base stations included in both the first handoff candidate list and the second handoff candidate list.
2. The portable terminal device according to claim 1 , wherein
the first handoff processing unit is a soft handoff processing unit which performs a handoff using the first handoff candidate list without switching the used frequency band, and
the second handoff processing unit is a hard handoff processing unit which switches the communication unit into a frequency band based on the switching information to measure signal strength, and reports the result of the signal strength measurement to the currently communicating base station using the previous frequency band before switching.
3. The portable terminal device according to claim 2 , wherein
the hard handoff processing unit is configured to measure signal strength of the one or more handoff candidates included in the second handoff candidate list sequentially starting from the handoff candidate having the best signal strength in decreasing order of the handoff candidates in the first handoff candidate list.
4. The portable terminal device according to claim 3 , wherein
when one handoff candidate whose signal strength satisfies a predetermined condition is extracted from the one or more handoff candidates included in the second handoff candidate list, the hard handoff processing unit is configured to report the extracted handoff candidate to the currently communicating base station.
5. The portable terminal device according to claim 2 , wherein
the control unit is configured to measure the signal strength based on a pilot signal of a handoff candidate base station received by the communication unit.
6. A handoff method for maintaining wireless communication in a wireless communication system comprising a plurality of base stations and a portable terminal device configured to perform wireless communication with one or more of the plurality of base stations, comprising:
receiving a neighboring base station list including information on one or more neighboring base stations from a currently communicating base station;
measuring signal strength of a transmission signal for each of the one or more neighboring base stations in a currently used frequency band;
generating a first handoff candidate list in which one or more handoff candidates of the same frequency band as the currently used frequency band are recorded in decreasing order of signal strength determined by the measuring the signal strength of the transmission signal for each of the one or more neighboring base stations;
performing a handoff using the first handoff candidate list without switching the used frequency band;
receiving a second handoff candidate list including switching information of the used frequency band and one or more different frequency band handoff candidates from the currently communicating base station;
switching communication to a frequency band based on the switching information, and measuring a signal strength of the one or more handoff candidates included in the second handoff candidate list;
reporting the result of the measured signal strength of the one or more handoff candidates to the currently communicating base station using the original frequency band;
receiving a response to the reporting at the portable terminal; and
performing a handoff based on the response, wherein
the measuring the signal strength of the one or more handoff candidates included in the second handoff candidate list comprises:
determining whether one or more of the one or more base stations included in the second handoff candidate list is also included in the first handoff candidate list, and measuring first the signal strength of the one or more base stations included in both the first handoff candidate list and the second handoff candidate list.
7. The handoff method according to claim 6 , further comprising:
measuring the signal strength of the one or more handoff candidates included in the second handoff candidate list sequentially starting from the handoff candidate having a best signal strength in decreasing order of the handoff candidates in the first handoff candidate list.
8. The handoff method according to claim 6 , wherein
when one handoff candidate whose signal strength satisfies a predetermined condition is extracted from the one or more handoff candidates included in the second handoff candidate list, the reporting step reports the extracted handoff candidate to the currently communicating base station.
9. The handoff method according to claim 6 , wherein
the measuring the signal strength of the transmission signal for each of the one or more neighboring base stations and the measuring signal strength of the one or more handoff candidates included in the second handoff candidate list perform one or more signal strength measurements based on a pilot signal of a handoff candidate base station received by the portable terminal.
10. The handoff method according to claim 6 , wherein
the neighboring base station list is transmitted by the currently communicating base station.
11. The handoff method according to claim 6 , wherein
the switching information and the second handoff candidate list are transmitted by the currently communicating base station.
12. The handoff method according to claim 6 , wherein
the currently communicating base station determines whether a handoff of the portable terminal device is permitted to a handoff candidate upon receipt of the reporting from the currently communicating portable terminal device, and transmits a result of the determining to the portable terminal device as a response to the reporting.