Handoff monitoring in cellular communication networks using slow frequency hopping

• US 5,448,569 A
• Filed: 04/12/1994
• Issued: 09/05/1995
• Est. Priority Date: 04/12/1994
• Status: Expired due to Term

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Natarajan, Kadathur S., Wetterwald, Michele M., Bauchot, Frederic, Huang, Chia-Chi, Bantz, David F., Narasimhan, Anand
• Primary Examiner(s): Hsu, Alpus
• Application Number:
  US08/226,724
• Time in Patent Office: 511 Days
• Field of Search: 370/13, 370/17, 370/18, 370/24, 370/29, 370/30, 370/50, 370/69.1, 370/77, 370/85.1, 370/94.1, 370/95.1, 370/95.3, 370/100.1, 379/58, 379/59, 379/60, 455/33.1, 455/33.2, 455/34.1, 455/34.2, 455/53.1, 455/54.1, 455/56.1, 455/67.1, 455/67.3, 455/68, 455/69, 455/70, 375/200, 375/202
• US Class Current: 370/332
• CPC Class Codes: H04B 1/713 using frequency hopping
• CPC Class Codes: H04W 24/00 Supervisory, monitoring or testing arrangements
• CPC Class Codes: H04W 36/08 Reselecting an access point
• CPC Class Codes: H04W 36/30 by measured or perceived connection quality data
• CPC Class Codes: H04W 84/12 WLAN [Wireless Local Area Networks]

Abstract

Methods for handoff in a wireless communication network including a local area network connected to a plurality of base stations, each of which has a geographic area, defined as a cell, within which mobile stations can communicate with a base station, with each such base station being capable of performing bidirectional wireless communication with one or more of said mobile stations under control of a controller, with communication between base stations and mobile stations being performed by frequency hopping, with there being n available frequency hopping channels in a fixed frequency band, with each channel being of a same fixed duration and having a fixed frequency different than another channel. The handoff method employs structure and techniques for determining by a mobile station that the quality of transmission between itself and a current base station has degraded below a predetermined performance level, structure and technique for the mobile station to listen to each of successive frequency channels for a duration substantially less than the fixed duration of a channel until it senses a transmission from another base station, and structure and techniques for transferring control of the mobile station to another base station in response to the sensed transmission.

Claims

1. A method for handoff monitoring in a wireless communication system including a local area network connected to a plurality of base stations, each of which has a geographic area, defined as a cell, within which a plurality of mobile stations having transceivers can communicate with a base station within each of a plurality of said cells, with each such base station being capable of performing bidirectional wireless communication with one or more of said mobile stations under control of a controller, with communication between base stations and mobile stations being performed by frequency hopping, with there being n available frequency hopping channels in a fixed frequency band, with each channel being of a same fixed time duration and having a fixed frequency different than another channel,

the base station in each of said cells having receivers each tuned to respective ones of the channels assigned to the cell for receiving signals of the mobile stations within the cell, a cluster of said cells having a network control system through which said mobile stations within said cells communicate with each other, said network control system operating in conjunction with hand-off apparatus for handing off a mobile station from a first of said base stations to a second of said base stations as the mobile station travels from the first cell to the second cell, said hand-off apparatus being responsive to the strength or quality of signals monitored at a mobile station,

said method for handoff monitoring comprising the steps of:

step 1;

determining for a given mobile station that the quality of transmission between said given mobile station and a current base station has degraded below a predetermined performance level;

step 2;

said given mobile station listening to each of successive ones of said n frequency channels for a time duration substantially less than said fixed time duration of a channel until said given mobile station senses a transmission from another of said base stations above said predetermined performance level; and

step 3;

transferring control of said mobile station by hand off to said another one of said base stations in response to said sensed transmission above said predetermined performance level.

2. A method according to claim 1 wherein said Step 1 includes each of said base stations in said wireless communication system generating and transmitting a wireless signal composed of a cyclic time sequence of frequencies hops in a superframe wherein each hop is of the same duration and length, wherein said transmitted wireless signal includes header information including Received Signal Strength Indicator data, Header Observation Rate data and Load Vector data.

3. A method according to claim 2 wherein, in said Step 2, said Received Signal Strength Indicator data is sampled by said mobile stations at multiple different transmitted frequencies and the mean value of said sampled samples is measured.

4. A method according to claim 2 wherein, in Step 2 said Header Observation Rate data is the number of transmitted headers received correctly by said mobile station divided by the total number of possible headers.

5. A method according to claim 4, wherein, in Step 2 said Load Vector data indicates the number of remote stations communicating with said base station.

6. A method according to claim 5 wherein, in Step 3 said mobile station selects and transfers control to a base station, referred to as a home base station, in accordance with the values of said Received Signal Strength Indicator data, said Header Observation Rate data and Load Vector data received by said mobile station.

7. A method according to claim 6 wherein, in said Step 3, said control of said mobile station is transferred by handoff from said home base station to another base station when said Received Signal Strength Indicator data and said Header Observation Rate data transmitted from said home base station decrease in strength and quality below a selected value.

8. A method according to claim 7 wherein the wireless signal composed of a cyclic time sequence of frequency hops in each cell of said plurality of cells is synchronized with the wireless signal composed of a cyclic time sequence of frequency hops in all of said other cells in said wireless communication system.

9. A method according to claim 7 wherein the wireless signal composed of a cyclic time sequence of frequency hops in each cell of said plurality of cells is loosely synchronized with the wireless signal composed of a cyclic time sequence of frequency hops of some or all of said other cells in said wireless communication system.

10. A method according to claim 7 wherein the wireless signal composed of a cyclic time sequence of frequency hops in each cell of said plurality of cells is not synchronized with any of the wireless signal composed of a cyclic time sequence of frequency hops of said other cells in said wireless communication system.