Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-08-02
2003-09-30
Nguyen, Lee (Department: 2683)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S438000
Reexamination Certificate
active
06628949
ABSTRACT:
PRIORITY
This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from an application entitled METHOD OF PRIORITIZING HANDOFF REQUESTS IN MOBILE COMMUNICATION SYSTEM earlier filed in the Korean Industrial Property Office on Jul. 31, 1998, and there duly assigned Serial No. 98-31224.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of assigning a channel for a handoff in a mobile communication system, and more particularly, to a method of prioritizing handoff requests in a mobile communication system in order to assign free channels to queued handoff requests based on their priorities.
2. Description of the Related Art
In a mobile communication system, handoff is the procedure by which a call is switched to a new communication channel while a user is talking. Communication channel switching may be implemented within a particular cell at one base station or between different cells. During an inter-cell handoff, how to assign the limited number of available channels in each cell is a very significant consideration. The channels can be assigned by prioritizing handoff calls. More specifically, when there are no available channels in a base station associated with a corresponding cell, mobile handoff requests are prioritized and queued. Then, if there is a free channel, the free channel is assigned to the handoff call with the highest priority. A typical channel assignment procedure for handoff in a mobile communication system is depicted in
FIGS. 1 and 2
.
FIG. 1
is a block diagram of a base station
100
in a cell, which receives channel assignment requests from a plurality of mobile stations
105
a
,
105
b
, and
105
c
located within a handoff region.
Each mobile station
105
a
,
105
b
, and
105
c
sends an RF (Radio Frequency) signal, requesting a new channel assignment. Included in the RF signal is data indicative of the power level detected by the mobile station. Each mobile station
105
a
,
105
b
, and
105
c
samples the transmitted power level on a periodic basis and sends them to the base station
100
. The base station
100
receives the sampled power level readings through an antenna
130
and computes the rate of change of power level for the corresponding mobile station.
A demodulator
135
converts the received RF signal to digital data. The digital data includes voice and control data. A processor
140
receives the digital data and extracts the power level data from the voice and control data. The power level data is stored in a memory
145
. The voice data is transmitted to a mobile telephone switching office (MTSO)
150
which determines the destination of the data. The processor
140
retrieves the power level data for a particular mobile station and determines from it the cutoff time at which the mobile station will leave the handoff region
120
. Cutoff times are used to determine handoff priorities. For example, mobile stations having smaller cutoff times are assigned higher priority than mobile stations having larger cutoff times. Available channels are assigned to handoff requests based on their priorities within a queue in which the handoff requests are arranged according to their cutoff times. These assignments are communicated to the mobile stations using a modulator
170
and an antenna
165
.
FIG. 2
is a flowchart depicting a general handoff call process in a base station upon channel assignment requests from mobile stations. Referring to
FIGS. 1 and 2
, the foregoing will be described in more detail.
Upon receipt of a channel assignment request from a mobile station (step
201
), the processor
140
of the base station
100
then determines whether the call is a new call or a handoff call (step
203
). If the call is a new call, the base station
100
determines whether any of its channels are free (step
205
). If a free channel exists, the mobile station is assigned the channel (step
209
). If no channels are free, the processor
140
rejects the call (step
207
). If a handoff is requested (step
203
), the base station
100
again determines whether any of its channels are free (step
211
). If a channel is free, the mobile station is assigned the channel (step
209
). If no channels are available, the handoff request is placed in a queue (step
213
). If multiple handoff requests exist (step
215
), they are prioritized (step
217
). A determination is made whether any of the mobile stations requesting a handoff have left the handoff region
20
of
FIG. 1
(step
219
). If the mobile station has left the handoff region
120
and has not been assigned a new channel by the base station
100
, the call is aborted (step
221
). Once a channel is available (step
223
), the channel is assigned to the queued handoff request having the highest priority (step
225
).
However, since the movement of the mobile stations requesting a handoff cannot be accurately predicted, it is also impossible to exactly determine which handoff call will be first cut off. Yet the determination can be made with a relatively high accuracy by estimating the cutoff time of each call. The power level of the currently serving base station measured at the mobile station and the rate of change of the power level is used to make such an estimation. The power level is a significant factor which determines a cell boundary and defines a handoff region. From here the term ‘cutoff time’ is used to refer to the estimated cutoff time.
FIG. 3
is a flowchart depicting a conventional method of prioritizing handoff requests in step
217
of FIG.
2
.
Referring to
FIG. 3
, the minimum cutoff time MIN
t
is set to an initial value which is typically infinity (step
301
). Next, the base station
100
selects one of the queued handoff requests for which the cutoff time will be determined (step
303
). The cutoff time for a particular mobile station i is determined by
T
(
i
)=(
P
min
−P
l
(
i
))/
R
(1)
where T(i) is the cutoff time for a mobile station i; P
min
is the cutoff threshold power, P
l
(i) is the current power level detected by the mobile station i and transmitted to a base station, and R is the rate of change of power level for the mobile station i, given by
dP
l
(
i
)/
dt
(2)
The processor
140
retrieves the cutoff threshold power P
min
, the current power level P
l
(i), and the rate of change of power level R for the selected handoff request from the memory
145
(step
305
). Then, the processor
140
calculates the cutoff time T(i) using Eq.1 (step
307
). The processor
140
compares T(i) with MIN
t
(step
309
). If T(i)<MIN
t
, MIN
t
is set to T(i) (step
311
). Otherwise, it is determined whether there are any other queued handoff requests (step
313
). If there are additional queued handoff requests, steps
303
-
311
are repeated. If there are no additional queued handoff requests, the processor
140
sets the handoff call of the mobile station having the most critical state to the highest priority (step
315
). This corresponds to the mobile station i with T(i)=MIN
t
. This means that the highest priority is given to a handoff call having the smallest cutoff time among queued handoff requests and when a channel becomes available, the mobile station is assigned the channel.
The conventional method of prioritizing handoff requests is based on the assumption that all handoff calls need channels of the same capacity. Yet the channel capacities may vary in actual situations. For example, in areas where radio multimedia service is supported, it may be more appropriate to change the priority of a handoff according to service type. Therefore, there is a need for a new channel assignment scheme in which required channel capacity and service type are taken into account.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of prioritizing handoff requests in a mobile communication system supporting a multimedia service.
According to an embodiment of the present invention, when there ar
Dilworth & Barrese LLP
Nguyen Lee
Samsung Electronics Co,. Ltd.
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