Note: Descriptions are shown in the official language in which they were submitted.
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COMMUNICATION TRANSFER IN A
RADIOTELEPHONE SYSTEM
Field of the Invention
This invention relates generally to cellular
radiotelephone systems, and more particularly to cellular
radiotelephone systems requiring communication transfer
10 for call maintenance.
B ackground of the Invention
15 In cellular radiotelephone systems, handoff of a
mobile is required to maintain communication to the
mobile as it moves from cell to cell in the system. Handoff
is the process of transferring a call in progress from one RF
coverage area to another in coordination with the
2 0 movement of the mobile. It is also the process whereby a
call is transferred to another channel within a RF coverage
area because of interference within the coverage area.
The process of handing off a call in progress is one of the
most delicately balanced function related to cellular
2 5 radiotelephone systems because it requires a high level of
coordination among the various system processing
elements to ensure successful operation. Failure to hand a
call off at the proper time generally results in a reduction
in the call quantity, interference with neighboring
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coverage areas and even the undesired termination of the
call.
Current analog cellular radiotelephone systems
require that the system continuously monitor the quality
5 of every call which is operational on the system. The
system must recognize when the quality of a call falls
below a predetermined threshold in a particular coverage
area and must also determine what other coverage area
can satisfactorily handle the call. Once a more suitable
10 coverage area is identified, the system sends instructions
to the mobile directing it to another channel. The mobile
confirms that it is leaving its current channel, tunes to the
new channel, synchronizes to the new channel and begins
transmitting thereby confirming that it has arrived on the
15 new channel.
In digital cellular radiotelephone systems, the
procedure is modified somewhat in that the mobile is
capable of measuring other channels as instructed by the
system as well as its current channel and also that the
2 0 mobile reports this information back to the system. These
measurements consist of signal strength only and are
relative as the measured results are likely to vary
considerably due to varying environmental conditions.
Because of this and because the mobile cannot determine
2 5 if the measurement is that of an interferer or the correct
channel, the system must scan the cell selected by the
mobile.
Another factor to consider in the handoff process is
in current cellular radiotelephone systems, the
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radiotelephone switch receives a handoff request from the
source base-site and relays the message to potential target
base-sites. The target base-sites will monitor the quality
of tr~ncmi ssion of the mobile and if the required
5 conditions are met, the target base-site will respond back
to the radiotelephone switch to let it know it is a good
candidate. During this time, the radiotelephone switch
waits a predetermined period of time to receive additional
responses from all other potential target base-sites. If no
10 responses are received from any target base-sites, the
radiotelephone switch will relay a message to the source
base-site to increase the signal level level of the mobile
unit. The source base-site knows that a given signal level
step translates to a known signal level increase at the
15 mobile thus the source base-site sets the signal level step
accordingly. The source base-site again takes signal
quality measurements on the tr~nsmission of the mobile
and the entire process is repeated. This process of
constantly requesting a handoff when the handoff
2 0 condition has been met tends to overload the
radiotelephone switch since handoff information is
continuously being transferred throughout the system.
Still other systems have base-sites sending all
received signal strength measurements back to the
2 5 radiotelephone switch on a continuous basis to speed the
handoff process. This method, however, creates a large
amount of signal strength data traffic which the switch
must preserve.
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Thus, a need exists for a handoff procedure which
extends the time period that a target base-site has to
report back to the source base-site while maintaining
efficient throughput at the radiotelephone switch.
S~1mm~ry of the Invention
A radiotelephone system has a mobile unit
10 communicating to a source base-site wherein the mobile
unit requires a communication transfer from the source
base-site to a selected one target base-site. The
radiotelephone system obtains, at the source base-site, a
first signal quality value of a signal received by the source
15 base-site, provides a second signal quality value and
provides at least a first predetermined time period and a
second predetermined time period. The radiotelephone
system, at at least the selected one target base-site,
obtains a third signal quality value of a signal received by
20 at least the selected one target base-site. The
radiotelephone system, at at least the selected one target
base-site, compares the f*st signal quality value to the
third signal quality value during the first time period, and compares the
second signal quality value to the third signal quality value during the
2 5 second time period, and responsive to the comparison, transfers
communication from the source base-site to at least the selected one
target base-site during at least the first predetermined time period or the
second time period.
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Brief Description of the Drawings
FIG. 1 generally depicts an idealized geographic
layout of a digital radiotelephone system.
FIG. 2 depicts a mobile unit communicating to a
source base-site and moving toward two candidate target
base-sites .
FIGS. 3A-3C, together, is a flow diagram of the steps
the digital radiotelephone system takes to transfer
communication of the mobile unit in accordance with the
invention .
FIG. 4 depicts a block diagram of the source base-site
and the target base-sites.
Detailed Description of a Preferred Embodiment
A typical configuration in a cellular communication
system is shown in FIG. 1. A source cell 100 contains a
source base-site and is bounded by target cells 101
through 106 containing six separate target base-sites. The
source cell 100 contains the mobile that is traveling within
the boundary of the source cell 100.
FIG. 2 shows a magnified view of the source cell 100
and potential target cells 101 and 102. The source base-
site 200 and the target base-sites 205 and 210 are coupled
together through the base-site interface 220. The base-
site interface 220 typically has input lines from the public
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service telephone network. The mobile 225 is
communicating to the source base-site 200 throughout the
boundary of the source cell 100 and moves toward target
cells 101 and 102.
FIG. 3 is a flow diagram of the handoff process the
system undergoes as the mobile 225 moves away from the
source base-site 200 to which it is communicating toward
the target base-sites 205, 210. Referring to FIG. 3A, the
handoff process starts at 300 when the source base-site
1 0 200 measures at 301 a first (source) signal quality value
and stores the data. The signal quality value is a received
signal strength indication (RSSI) in the preferred
embodiment. A test is then performed at 303 to
determine if a first (source) signal quality value, RSSIS, is
1 5 greater than the transfer threshold. Although in the
preferred embodiment only one threshold is incorporated,
two or more thresholds may be incorporated to initialize
the handoff procedure before the absolute lower-level
threshold is reached by the source base-site 200. Use of
2 0 two thresholds would increase call quality since a handoff
could occur at higher source base-site 200 receiver
sensitivities. Continuing, if RSSIS is greater, the source
base-site 200 will continue to measure at 301 RSSIS and
store the data. If RSSIS is below the transfer threshold, a
second (forecasted) signal quality value, RSSIF, is
calculated at 306. RSSIF is a signal level value that the
source base-site 200 would expect to receive if the power
of the mobile were increased. The source base-site 200 is
capable of calculating RSSIF because it knows the
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correlation between the power increase value it would
send the mobile 225 and the would be power. A test is
then performed at 309 to see if the mobile 225, which has
been monitoring for viable target base-sites 205, 210, has
indicated any target base-sites 205, 210. If no viable
target base-sites 205, 210 were found by the mobile 225,
the power of the mobile 225 is increased at 312 and the
source base-site 200 continues to measure at 301 RSSIS
and store the data. If the mobile 225 does indicate viable
1 0 target base-sites 205, 210, a handoff request which
includes the target base-site 205, 210 information, RSSIS
and RSSIF are sent at 315 to the base-site interface 220
and the first predetermined time period (X) and second
predetermined time period (W) timers at the source base-
1 5 site 200 are reset. In the preferred embodiment, the X
time period is a 1-5 second time period the source base-
site 200 will wait for the base-site interface 220 to
respond; the base site interface 220 only responds if it
receives a response from a target base-site 205, 210. In
2 0 the preferred embodiment, the W time period is a 7
second time period the source base-site 200 will wait for
the base-site interface 220 to respond. The base-site
interface 220 receives at 318 the handoff request, sends
RSSIS and RSSIF to the viable target base-sites 205, 210
located by the mobile 225 and resets its own X and W
timers, which are both the same time period as the source
base-site 200 X and W timers but slightly delayed
compared to those of the source base-site 200. The target
base-sites 205, 210 receive at 321 the message and
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measure a third (target) signa~ quality value, RSSIT, of the
mobile 225.
Turning to FIG. 3B, a test is performed at 324 to
determine if the digital voice color code (DVCC) is valid for
5 the mobile 225. The DVCC is a binary code given to the
mobile 225 to distinguish it from other mobiles in the
same or surrounding cells; in the preferred embodiment, it
is used to ensure the target base-sites 205, 210 are
measuring RSSI of the same mobile 225 that the source
1 0 base-site 200 measured. If DVCC is valid for the mobile
225, a test is then performed at 326 to determine if RSSIT
is greater than RSSIS plus a hysteresis value. The
hysteresis value is a value predetermined by the system
designer which accounts for the different physical
1 5 locations of the source base-site 200 and target base-sites
205, 210, and can be positive, negative or zero. If RSSIT is
greater than RSSIS plus the hysteresis value, a response is
sent at 340 to the base-site interface 220 from the target
base-sites 205, 210 which have RSSIT greater than RSSIS
20 plus the hysteresis value. If, from above, DVCC was not
valid or RSSIT is not greater than RSSIS plus the hysteresis
value, the target base-sites 205, 210 re-measure at 330
RSSIT. During this time and essentially in the background,
the X timer at the source base-site 200 is timing the
25 process and will increase the power of the mobile 225 if
the X time period expires before the source base 200
receives a response from the base-site interface 220.
Continuing, the test at 324 to check for DVCC validity of
the mobile 225 is repeated. If DVCC is valid, a test is
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performed at 333 to determine if RSSIT is greater than
RSSIF. If RSSIT is greater than RSSIF, a response is sent at
340 to the base-site interface 220 from the target base-
sites 205, 210 which have RSSIT greater than RSSIF. If
5 RSSIT is not greater than RSSIF, a check is done at 336 to
determine if the W time period has expired at the source
base-site 200. If the W time period has not expired, the
re-measurement at 330 is repeated. If the W time period
has expired, the handoff process will end at 339 in the
1 0 source base-site 200.
Referring to ~IG. 3C, if a target base-site 205, 210
responds to the base-site interface 220 during either the X
or W time period at the base-site interface 220, a test is
performed at 341 to determined if the handoff process is
1 5 in the X or W time period. If the process is in the X time
period, the base-site interface 220 selects at 343 the best
target base-site 205, 210 after the X time period expires.
If the process is in the W time period, the base-site
interface 220 will assign at 348 a channel to the selected
2 0 target base-site 205, 210 and notify that target base-site
205, 210 of the channel. The selected target base-site 205,
210 then powers up at 357 to that channel and responds
back to the base-site interface 220. The base-site
interface 220 notifies at 360 the source base-site 200 of
2 5 the handoff response and the source base-site 200 re-
measures RSSIS at 362. A test is then performed at 363 to
determine whether RSSIT is still greater than RSSIS. If
RSSIT is below RSSIS, the source base-site 200 stops at 366
RSSIS the handoff process and sends a corresponding
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message to the base-site interface 220. If RSSIT is still
greater than RSSIS, the sourcé base-site 200 will send at
369 a handoff message to the mobile 225. The mobile 225
acknowledgement of the handoff message is received at
370 and the source base-site 200 sends a clear message to
the base-site interface 220. The mobile 225 tunes to the
new channel and the selected target base-site 205, 210
informs the base-site interface 220 at 372 that the mobile
225 is being received. Handoff of the mobile 225 from the
1 0 source base-site 200 to the selected target base-site 205,
210 has thus been completed.
FIG. 4 depicts a block diagram of the source base-site
200 and the target base-sites 205, 210. A conventional
interface 400 interfaces between a processor 405 and the
1 5 base-site interface 220. The processor, which is a
Motorola 68302 microprocessor in the preferred
embodiment, is coupled to conventional memory 420 and
also transmitter/receivers 410. The processor 405
controls the functions of the source base-site 200 and the
2 0 target base-sites 205, 210 while the transmitter/receivers
410 provide an interface to the external RF environment.
If both the X and W time periods expire before
communication transfer is required, the source base-site
200 may request a re-initialization of the process of FIG. 3.
Significant to the process of FIG. 3 is the fact that the
handoff process is given a greater chance to be completed
with the same amount of handoff requests sent by the
source base-site 200, resulting in an improved handoff
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method while maintaining efficient base-site interface 220
throu ghput .
What I claim is:
