Handoff method of wireless local area network (LAN)

• US 20050030924A1
• Filed: 07/01/2004
• Published: 02/10/2005
• Est. Priority Date: 07/01/2003
• Status: Active Grant

• Current Assignee: Hitachi, Ltd.
• Original Assignee: Hitachi, Ltd.
• Inventors: Wang, Lan, Yano, Masashi, Niu, Zhisheng, Deng, Hui
• Granted Patent: US 7,289,472 B2
• Time in Patent Office: Days
• US Class Current: 370/332
• CPC Class Codes: H04W 36/302 due to low signal strength
• CPC Class Codes: H04W 36/362 Conditional handover
• CPC Class Codes: H04W 84/12 WLAN [Wireless Local Area Networks]

Abstract

A handoff method of wireless LAN comprises the steps of: 1) immediately starting by a mobile terminal to count time from a time point of entering the wireless LAN or building the new association, that is, when building association with a certain access point; 2) always tracking a signal-to-noise ratio (SNR) of the old access point by the mobile terminal, and once the mobile terminal starts getting weak, starting to search for a near access point and making a preparation for handoff, and issuing a probe request signal to a new access point, and always responding to a probe response message by the new access point after receiving the probe request signal; and 3) judging by the mobile terminal whether or not to associate the new access point according to a signal strength and an association time of each of the access points, and if a judging result is yes, issuing the reassociation request signal to the new access point. Accordingly, the new access point issues a reassociation response message to build the new association.

Claims

1. A handoff method of a wireless local area network (LAN), wherein each of access points in the wireless LAN periodically broadcasts a message of a time beacon, signal separation, an access point ID, and a traffic indication map, the handoff method comprising the steps of:

1) immediately starting by mobile terminal to count time from a time point of entering the wireless LAN or building a new association, that is, when building association with a certain another access point;

2) always tracking a signal-to-noise ratio (SNR) of old access point (1) by the mobile terminal, and once the mobile terminal starts getting weak, starting to search for a near access point and making a preparation for handoff; and

issuing a probe request signal to a new access point, and always responding to a probe response message by the new access point after receiving the probe request signal;

3) judging by the mobile terminal whether or not to associate the new access point according to a signal strength and an association time of each of the access points, and if a judging result is yes, issuing a reassociation request signal to the new access point.

2. The handoff method of a wireless LAN according to claim 1,

wherein when the handoff function is IST, it is expressed as follows:

IST(t)=SNR(t)*f_T(T) 

 

(1); and


f_T(T)=Exp(k₇T^k6), 

 

(2),

where SNR(t) is a signal-to-noise ratio, T is an association time between the mobile terminal (3) and the access point, and k₆ and k₇ are function parameters.

3. The handoff method of a wireless LAN according to claim 1,

wherein a handoff strategy comprising the steps of:

first, with respect to the old access point, setting two threshold values of H and h, wherein H is a high threshold value and h is a low threshold value;

judging the SNR of the received signal at the old access point, and when SNR>

H, indicating that the signal strength of the old access point is still strong and therefore handoff is not performed in this region or when SNR<

h, indicating that the signal strength of the old access point is rather weak and therefore the handoff must be performed; and

under the condition of h<

SNR<

H, using an IST value to decide whether or not the handoff is performed, that is, if a difference between the IST value of the new access point and the IST value of the old access point is greater than or equal to a threshold value Ts, performing the handoff, and if a difference between the IST value of the new access point and the IST value of the old access point is less than the threshold value Ts, not performing the handoff.

4. The handoff method of a wireless LAN according to claim 1, further comprising the step of:

automatically counting time after the mobile terminal receives the association response message by setting one timer for measuring the association time.

5. A handoff method of a wireless local area network (LAN), wherein each of access points in the wireless LAN periodically broadcasts a message of a time beacon, signal separation, an access point ID, and a traffic indication map, the handoff method comprising the steps of:

1) immediately starting by a mobile terminal to count time from a time point of entering the wireless LAN or building new association, that is, when building association with a certain another access point;

2) always tracking a signal-to-noise ratio (SNR) of an old access point by the mobile terminal, and once the mobile terminal starts getting weak, starting to search for a near access point and making a preparation for handoff, and issuing a probe request signal to a new access point, and always responding to a probe response message by the new access point after receiving the probe request signal;

3) judging by the mobile terminal whether or not to associate the new access point according to a signal strength, an association time, and system traffic load of each of the access points, and if a judging result is yes, issuing a reassociation request signal to the new access point (2).

6. The handoff method of a wireless LAN according to claim 5,

wherein a handoff function is ISLT, which is expressed by:

ISLT(t)=SNR(t)×

f_L(L)×

f_T(T) 

 

(3);


f_L(L)=Exp(k₂L^k1)+k₃;

 

 

(4); and


f_T(T)=Exp(k₅T^k4), 

 

(5),

where SNR(t) is a signal-to-noise ratio (SNR), L is the system load calculated, f_L(L) is a system load weight function, and k₁, k₃, and k₃ are function parameters.

7. The handoff method of a wireless LAN according to claim 5,

wherein a handoff strategy comprising steps of:

first, with respect to the old access point, setting two threshold values of H and h, wherein H is a high threshold value and h is a low threshold value;

judging the SNR of the received signal at the old access point, and when SNR>

H, indicating that the signal strength of the old access point is still strong and therefore handoff is not performed in this region or when SNR<

h, indicating that the signal strength of the old access point is rather weak and therefore the handoff must be performed; and

under the condition of h<

SNR<

H, using a ISLT value to decide whether or not the handoff is performed, that is, if a difference between the ISLT value of the new access point and the ISLT value of the old access point is greater than or equal to a threshold value Ts, performing the handoff, otherwise, if a difference between the ISLT value of the new access point and the ISLT value of the old access point is less than the threshold value Ts, not performing the handoff.

8. The handoff method of a wireless LAN according to claim 5, further comprising the step of:

automatically counting time after the mobile terminal receives the association response message by setting one timer for measuring the association time.

9. The handoff method of a wireless LAN according to claim 5,

wherein the system load (L) is a time period of system (channel) busy, in other words, it is a percentage of channel occupation of the access point, that is, a percentage of the buy times of the channel to a total measuring times.