Method and apparatus for performing a hand-off in a wireless communication system

• US 5,392,331 A
• Filed: 05/31/1994
• Issued: 02/21/1995
• Est. Priority Date: 08/25/1992
• Status: Expired due to Term

• Current Assignee: Motorola Mobility, Inc. (Lenovo Group Ltd.)
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Marko, Paul, Patsiokas, Stelios J., Krishna, Prathivadhi B., Wadin, Craig
• Primary Examiner(s): Kuntz, Curtis
• Assistant Examiner(s): Chernoff, Michael B.
• Application Number:
  US08/252,139
• Time in Patent Office: 266 Days
• Field of Search: 379/63, 379/58, 379/60, 379/59, 455/33.2, 455/34.1, 455/34.2, 455/51.1, 455/56.1, 455/54.1
• US Class Current: 455/442
• CPC Class Codes: H04W 36/18 for allowing seamless reselection, e.g. soft reselection
• CPC Class Codes: H04W 56/00 Synchronisation arrangements

Abstract

A method and apparatus in a digital wireless communication system (100) controls an inter-cell hand-off. First and second fixed units (102) receive a common synchronization signal from a controller (104), and a first wireless link is established between the first fixed unit and a portable unit (120) on a first frequency. The first wireless link is synchronized to the common synchronization signal, and frame synchronization is obtained between the fixed unit (102) and the portable unit (120) during establishment of the first wireless link. Frame synchronization is maintained only by maintaining bit synchronization while transmitting user data on the first frequency. A second wireless link is established between the second fixed unit (102) and the portable unit (120) on the first frequency if the first frequency is usable therebetween. The second wireless link is synchronized to the common synchronization signal. The first wireless link is disconnected after the second wireless link is established, thereby seamlessly completing the inter-cell hand-off.

Claims

1. A method in a digital wireless communication system for controlling an inter-cell hand-off of a wireless link from a first fixed communication unit to a second fixed communication unit, the system comprising a portable communication unit and a controller coupled to the first and second fixed communication units, the method comprising the steps of:

(a) receiving by the first and second fixed communication units a common master synchronization signal from the controller;

(b) establishing a first wireless link between the first fixed communication unit and the portable communication unit on a first radio frequency,

wherein the first wireless link is synchronized to the common master synchronization signal, and

wherein frame synchronization is obtained between the fixed communication unit and the portable communication unit during establishment of the first wireless link;

(c) maintaining said frame synchronization by maintaining bit synchronization while transmitting user data on the first radio frequency, wherein no frame synchronization information is transmitted while transmitting said user data;

(d) establishing, under direction of the controller, a second wireless link between the second fixed communication unit and the portable communication unit, the second wireless link established on the first radio frequency in response to the first radio frequency having been determined by the system to be usable therebetween, the second wireless link being synchronized to the common master synchronization signal; and

(e) disconnecting the first wireless link, after step (d), under direction of the controller, thereby seamlessly completing the inter-cell hand-off.

2. The method in accordance with claim 1, further comprising:

(f) commanding, by a link re-establishment message from the first fixed communication unit under direction of the controller, the portable communication unit to establish the second wireless link on a second radio frequency, thereby disconnecting the first wireless link, in response to the first radio frequency having been determined by the system to be unusable between the second fixed communication unit and the portable communication unit; and

(g) establishing, after step (f), under direction of the controller, the second wireless link on the second radio frequency, the second wireless link being synchronized to the common master synchronization signal, thereby minimizing resynchronization time to effect a nearly seamless inter-cell hand-off.

3. The method in accordance with claim 2, further comprising in the second fixed communication unit the steps of:

(h) receiving from the controller a request to monitor transmissions from the portable communication unit on the first radio frequency;

(i) performing in response to step (h) synchronous monitoring of the transmissions from the portable communication unit on the first radio frequency, the synchronous monitoring being synchronized by the common master synchronization signal;

(j) measuring a received signal strength of the transmissions from the portable communication unit monitored synchronously in step (i); and

(k) reporting to the controller the measured received signal strength in response to the measured received signal strength being greater than a pre-determined amount.

4. The method in accordance with claim 2, further comprising in the first fixed communication unit the steps of:

(h) periodically monitoring received signal strength from the portable communication unit with which the first wireless link is established; and

(i) transmitting a hand-off request to the controller in response to determining in step (h) that the received signal strength is less than a predetermined level.

5. The method in accordance with claim 4, further comprising in the first fixed communication unit the steps of:

(j) receiving a hand-off denial message from the controller in response to step (i); and

(k) sending an audible indication to the portable communication unit in response to step (j).

6. The method in accordance with claim 2, further comprising in the second fixed communication unit the steps of:

(h) receiving from the controller a command to select a radio frequency for accepting the hand-off;

(i) measuring a received signal strength on a plurality of radio frequencies in response to step (h);

(j) selecting one of the plurality of radio frequencies measured in step (i) as having the lowest received signal strength; and

(k) identifying to the controller the radio frequency selected in step (j).

7. The method in accordance with claim 2, further comprising in the second fixed communication unit the steps of:

(h) monitoring a radio frequency identified by the controller for a link request from the portable communication unit, in response to the radio frequency identified being different from the first radio frequency; and

(i) beginning radio transmissions at a specified subsequent fixed communication unit receive-to-transmit transition on the radio frequency identified by the controller, in response to the radio frequency identified being the first radio frequency.

8. The method in accordance with claim 2, further comprising in the controller the steps of:

(h) maintaining a current record of in-use radio frequencies and available radio frequencies for the first and second fixed communication units; and

(i) selecting from the available radio frequencies for the second fixed communication unit a radio frequency that minimizes interference with radio frequencies currently in use by the first fixed communication unit.

9. The method in accordance with claim 8, wherein step (i) comprises the steps of:

(j) avoiding radio frequencies currently in use by the first and second fixed communication units, except for the first radio frequency;

(k) avoiding radio frequencies adjacent to radio frequencies currently in use by the first and second fixed communication units; and

(l) avoiding radio frequencies that will produce interfering intermodulation products with radio frequencies currently in use by the first and second fixed communication units.

10. Apparatus for use in a digital wireless communication system for controlling an inter-cell hand-off of a wireless link from a first fixed communication unit to a second fixed communication unit, the system comprising a portable communication unit and a controller coupled to the first and second fixed communication units, the apparatus comprising:

first and second interface means for receiving, respectively, by the first and second fixed communication units a common master synchronization signal from the controller;

first wireless link establishment means in the first fixed communication unit, coupled to the first interface means, for establishing a first wireless link between the first fixed communication unit and the portable communication unit on a first radio frequency,

wherein the first wireless link is synchronized to the common master synchronization signal, and

wherein frame synchronization is obtained between the fixed communication unit and the portable communication unit during establishment of the first wireless link, and

wherein said frame synchronization is maintained by maintaining bit synchronization while transmitting user data, and

wherein no frame synchronization information is transmitted while transmitting said user data;

second wireless link establishment means in the second fixed communication unit, coupled to the second interface means, for establishing under direction of the controller a second wireless link between the second fixed communication unit and the portable communication unit, the second wireless link established on the first radio frequency in response to the first radio frequency having been determined by the system to be usable therebetween, the second wireless link being synchronized to the common master synchronization signal; and

first processor means in the first fixed communication unit coupled to the controller for disconnecting the first wireless link in response to establishment of the second wireless link on the first radio frequency, thereby seamlessly completing the inter-cell hand-off.

11. The apparatus in accordance with claim 10,

wherein the second wireless link establishment means commands, by a link re-establishment message from the first fixed communication unit under direction of the controller, the portable communication unit to establish the second wireless link on a second radio frequency, thereby disconnecting the first wireless link, in response to the first radio frequency having been determined by the system to be unusable between the second fixed communication unit and the portable communication unit, and

wherein the apparatus further comprises channel selector means coupled to the second wireless link establishment means for establishing, in response to the link re-establishment message, the second wireless link on the second radio frequency, the second wireless link being synchronized to the common master synchronization signal, thereby minimizing resynchronization time to effect a nearly seamless inter-cell hand-off.

12. The apparatus in accordance with claim 11,

wherein the second interface means receives from the controller a request to monitor transmissions from the portable communication unit on the first radio frequency;

and

wherein the apparatus further comprises in the second fixed communication unit;

synchronizer means coupled to the second interface means for performing synchronous monitoring of the transmissions from the portable communication unit on the first radio frequency, the synchronous monitoring being synchronized by the common master synchronization signal;

received signal strength measurement means coupled to the second wireless link establishment means for measuring a received signal strength of the transmissions from the portable communication unit monitored synchronously; and

second processor means coupled to the received signal strength measurement means for reporting to the controller the measured received signal strength in response to the measured received signal strength being greater than a pre-determined amount.

13. The apparatus in accordance with claim 11,

wherein the apparatus further comprises in the first fixed communication unit a received signal strength measurement means coupled to the first wireless link establishment means for periodically monitoring received signal strength from the portable communication unit with which the first wireless link is established, and

wherein the first processor means transmits a hand-off request to the controller in response to determining that the received signal strength is less than a pre-determined level.

14. The apparatus in accordance with claim 13,

wherein the first processor means receives a hand-off denial message from the controller in response to the hand-off request, and

wherein the first processor means sends an audible indication to the portable communication unit in response to the hand-off denial message.

15. The apparatus in accordance with claim 11,

wherein the second interface means receives from the controller a command to select a radio frequency for accepting the hand-off, and

wherein the apparatus further comprises in the second fixed communication unit a received signal strength measurement means coupled to the second wireless link establishment means for measuring a received signal strength on a plurality of radio frequencies in response to the command to select a radio frequency, and

wherein the channel selector means selects one of the plurality of radio frequencies measured as having the lowest received signal strength, and

wherein the second processor means identifies to the controller the radio frequency selected.

16. The apparatus in accordance with claim 11,

wherein the second wireless link establishment means monitors a radio frequency identified by the controller for a link request from the portable communication unit, in response to the radio frequency identified being different from the first radio frequency, and

wherein the second wireless link establishment means begins radio transmissions at a specified subsequent fixed communication unit receive-to-transmit transition on the radio frequency identified by the controller, in response to the radio frequency identified being the first radio frequency.

17. The apparatus in accordance with claim 11,

wherein the controller comprises a channel database for maintaining a current record of in-use radio frequencies and available radio frequencies for the first and second fixed communication units, and

wherein the channel selector means selects from the available radio frequencies for the second fixed communication unit a radio frequency that minimizes interference with radio frequencies currently in use by the first fixed communication unit.

18. The apparatus in accordance with claim 17, wherein the channel selector means comprises:

co-channel avoidance means for avoiding radio frequencies currently in use by the first and second fixed communication units, except for the first radio frequency;

adjacent-channel avoidance means for avoiding radio frequencies adjacent to radio frequencies currently in use by the first and second fixed communication units; and

intermodulation-channel avoidance means for avoiding radio frequencies that will produce interfering intermodulation products with radio frequencies currently in use by the first and second fixed communication units.