High density cellular mobile radio communications

In a cellular mobile telecommunication system, a method and apparatus for controlling the process of locating a mobile unit from the cell sites. When a mobile unit goes beyond the radio range of its associated controlling cell site, that cell site sends a message to nearby cell sites to measure and report the strength of the received signal from that mobile on each of their directional antennas and to report radio channel availability. These reports are returned to the controlling cell site which compares the received signal strength mesurements against stored thresholds. The controlling cell site generates and transmits to the mobile telecommunications switching office (MTSO) a hand-off request message including a list of candidate hand-off cell sites and directional antennas. The MTSO then selects an available channel associated with one of the candidate cell sites and antennas and generates a sequence of messages to hand-off communications control from the controlling cell site to a candidate cell site.

TECHNICAL FIELD 
This invention relates to cellular mobile radio communication and, more 
particularly, to the control of the vehicle location and hand-off 
processes among cell sites. 
BACKGROUND OF THE INVENTION 
Mobile telecommunication systems provide communications among mobile units 
and between mobile units and land-based customer stations using a limited 
number of radio channels. In the Advanced Mobile Phone Service (AMPS) 
System described in the Bell System Technical Journal, (BSTJ), V. 58, No. 
1, pp. 1-269, Jan., 1979, a mobile unit is served by a different radio 
station, sometimes referred to as a cell site, as it moves from one cell 
area to another in a region. The radio stations are connected to a 
controlling mobile telecommunications switching office (MTSO) that 
provides access to land-based customer stations via the common-carrier 
telephone network. The cellular arrangement of radio stations makes it 
possible for distant mobile units to re-use the same radio channels 
without interference. To prevent a mobile unit from traveling so far out 
of one cell area that its signal could interfere with communications in a 
remote cell area using the same radio channel, the signal strength of a 
mobile unit is monitored by the serving radio station. A "vehicle 
location" operation is initiated when the signal strength falls 
sufficiently low to indicate that the mobile unit is leaving the boundary 
of the cell area. 
When the strength of the signals from the mobile unit drop below a 
threshold value, the serving radio station notifies the MTSO. The MTSO 
then requests nearby radio stations to measure the received signal 
strength of the mobile unit. In response to the received signal strength 
data from the group of nearby radio stations, the MTSO selects the radio 
station which would best be capable of taking over control of 
communications with the mobile unit and a "hand-off" of control is made to 
that radio station. 
With a limited number of radio channels and a growing number of mobile 
units to serve, it is necessary to re-use channels more frequently. This 
requires a decrease in the size of each cell area and/or a splitting of 
cell areas into sectors each sector associated with a different 
directional antenna; in either case, more frequent vehicle location and 
hand-off operations are required. Each such vehicle location and hand-off 
requires an intensive usage of the data processing facilities of the MTSO. 
As an MTSO serves more mobile units, the use of these data processing 
facilities increases both with the number of calls and with the greater 
number of hand-offs required per call. Ultimately, the data processing 
load on an MTSO could be expected to approach saturation. Because the use 
of multiple MTSO's in one region would be expensive and inefficient it 
would be advantageous to be able to permit an increase in the number of 
vehicle locations and handoffs processed by a system without greatly 
increasing the data processing load on the MTSO. 
SUMMARY OF THE INVENTION 
In accordance with our invention, the vehicle location process is both 
initiated and controlled by the cell site currently serving the mobile 
unit (the controlling cell site). When the signal from a specific mobile 
unit associated with a controlling cell site drops below a prespecified 
threshold, the controlling cell site itself selects the group of nearby 
cell sites which are to measure the signal strength in the radio channel 
used by the mobile unit. 
In accordance with one aspect of our invention, in one illustrative 
embodiment thereof, the controlling cell site itself receives and 
processes the return messages indicating the strength of the received 
signal at each nearby cell site. Based on the signal strength data 
supplied by the return messages, the controlling cell site generates and 
sends to the MTSO an ordered list of cell sites which are candidates for 
assuming control of communications with the specific mobile unit. The MTSO 
then effects the hand-off of the call from the controlling cell site to 
another cell site selected from the candidate list. 
In accordance with one aspect of our invention, in one illustrative 
embodiment thereof, the operation of the MTSO is simplified by the 
provision of packet switching apparatus which permits the MTSO to function 
primarily as a data switch during the signal strength measuring phase of 
call control. That is, the MTSO simply passes signal strength measurement 
request messages from the requesting cell site to each cell site in a list 
of nearby cell site addresses identified by the controlling cell site in a 
signal strength measurement request message. The addressed cell sites, in 
addition to replying with signal strength reports, also furnish data 
indicating the availability of channels in the channel groups associated 
with each antenna (antenna/channel groups) at each site. The controlling 
cell site uses the signal strength reports and antenna/channel 
availability data to generate the candidate list of hand-off cell sites. 
In accordance with another aspect of our invention, in one illustrative 
embodiment thereof, the controlling cell site further detects when the 
signal from a specific mobile unit drops below a second prespecified 
threshold. Only a limited list of nearby cell sites is measured if the 
signal is only below a first threshold. Candidate antenna/channel groups 
are selected whose reported signal strength is above an associated primary 
threshold. If the signal drops below a second threshold, a second group of 
nearby cell sites is reported. Candidates may be drawn from those 
antenna/channel groups of that second group of nearby cell sites whose 
reported signal strength is above an associated primary threshold. 
Candidates may be further drawn from antenna/channel groups in either 
group of cell sites whose reported signal strength is below an associated 
primary threshold but above an associated secondary threshold. 
Since the MTSO of our system has neither to generate the list of nearby 
cell sites for each signal strength request message nor to interpret the 
reply information to generate the list of optimum hand-off candidates, its 
workload is greatly reduced. Further, if the controlling cell site 
receives a sufficiently strong signal from the specific mobile unit on a 
different one of its antennas than that being used by the mobile unit and 
a channel associated with that alternate antenna is available, the 
controlling cell site directly generates a hand-off request data message, 
specifying the alternate antenna, to the MTSO. 
In accordance with another aspect of our invention, in one illustrative 
embodiment thereof, different groups of nearby cell sites are measured. 
The choice among the groups of cell sites is made on the basis of the 
antenna at the serving cell site which receives the strongest signal from 
the mobile unit. Advantageously, this arrangement makes it most likely 
that the group of cell sites being measured is in the correct direction.

GENERAL DESCRIPTION 
FIG. 1 shows a mobile telecommunications switching office MTSO 101 serving 
seven cell sites 111, 112, 113, 114, 115, 116, and 119 in a type of prior 
art cellular mobile telecommunication system described in the 
aforementioned BSTJ reference. The cell sites are located in cell areas or 
domains 111D, 112D, 113D, 114D, 115D, 116D, and 119D, respectively. MTSO 
101 is connected to the telephone network via transmission facility 199 
and to each of the cell sites by a respective two-way data and 
communication link 121, 122, 123, 124, 125, 126 and 129. MTSO 101 includes 
a circuit switch 195 to switch among these communication links and between 
these communication links and the transmission facility 199 connected to 
the telephone network. Each cell site contains a plurality of radio 
transmitters/receivers (transceivers), shown as transceiver (T/R) group 
155 in FIG. 1, each tuned to transmit to a mobile unit at the frequency 
associated with a particular transmit radio channel and to receive from 
that mobile over a corresponding receive radio channel. In addition, each 
cell has one or more tunable receivers 152 which can be used to measure 
the signal strength of a received signal associated with any mobile 
receive channel. 
For the sake of simplicity, only one mobile unit 102 is shown. Mobile unit 
102 receives signals from cell site 119 over transmit channel CHW and 
transmits signals to cell site 119 over a corresponding receive channel 
CHX. The communication is carried to MTSO 101 via transmission facility 
129T. 
Mobile unit 102 is shown sometime after it has left cell domain 119D and 
entered into cell domain 112D which is normally served by associated cell 
site 112. However, mobile unit 102 is still communicating with controlling 
cell site 119 over transmit/receive channel pair CHW/CHX. At cell site 
119, tunable receiver 152 detects a weakened signal from unit 102 and 
activates message control program 154 of cell site processor 150. When so 
activated, processor 150 seizes data link controller 160 to send weak 
signal strength report message 141 over data link 129D to MTSO 101 
indicating that the signal received on CHX is below threshold. 
At MTSO 101, processor 170, under the control of message control program 
185, accepts message 141 and, under the control of locate/select program 
176, analyzes message 141. Locate/select program 176 accesses data base A 
(172) and then generates "Measure CHX" signal strength measurement request 
messages 132. Under the control of message control program 185, data link 
controllers 190 send messages 132 to cell sites 111-116. In messages 132, 
llY represents the identifications of cell sites 111, . . . , 116. 
When each of cell sites 111-116 receives its respective signal strength 
measurement request message 132, it measures the strength of the signal 
received on channel CHX. Each of cell sites 111-116 then generates and 
transmits a signal strength report message 133 to MTSO 101. Messages 133 
are received at MTSO 101 and analyzed under the control of locate/select 
program 176 which accesses data base A (172). Locate/select program 176 
identifies a potential cell site candidate for a hand-off operation and 
passes the identification of this candidate to hand-off message generator 
program 180. This program accesses data base B (174) which contains 
control data determining availability of individual channels in each cell 
site. Hand-off message generator program 180 then generates the necessary 
sequence of messages to effect a hand-off of mobile 102 to another cell 
site. 
Because MTSO processor 170 is involved in controlling data link controllers 
190 to receive the weak signal strength report message 141, dispatching 
the measurement request messages 132, receiving the signal strength report 
messages 133, and accessing the A and B data bases 172, 174, processor 
occupancy rises rapidly as more mobile units must be served. 
FIG. 2 illustrates the changes necessary to implement the present invention 
which reduces processor occupancy in the MTSO. The system still includes 
two-way data and communication links 121-126, and 129, and a circuit 
switch 295 to switch among the communication links, and between the 
communication links and the transmission facility 299 connected to the 
telephone network. In FIG. 2, each cell site is advantageously equipped 
with three directional antennas, shown, for example, as 119A, 119B, and 
119C. This allows the system to handle more traffic with a given number of 
channels. Each antenna illustratively has a beam width of approximately 
120 degrees width and covers a preferred sector domain of approximately 
one-third of the cell area. An example of such cell division is shown in 
R. H. Frenkiel, U. S. Pat. No. 4,144,411, issued Mar. 13, 1979. The 
sectors for each cell of FIG. 2 are shown in FIG. 3. For ease of 
understanding, the sector numbers of FIG. 3 are those of the directional 
antennas shown in FIG. 2, which serve the sector domain, with the addition 
of the letter D. For example, cell domain 119D, associated with cell site 
119, is partitioned into cell sectors 119AD, 119BD, and 119CD which are 
served by directional antennas 119A, 119B, and 119C, respectively. Each 
directional antenna at a cell site has a different associated group of 
radio channels. Receive channel CHX is associated with one of the 
transceivers connected to directional antenna 119A covering sector 119AD. 
In a system with directional antennas, it is not enough to identify 
hand-off candidate cell sites; antennas and, more specifically, channel 
groups for a candidate cell site must also be identified. Some antennas 
are connected to high power and low power transceivers. These transceivers 
are in different channel groups, since the channels used by low power 
transceivers can be reused more frequently than the channels used by high 
power transceivers. Since a particular antenna may serve more than one 
channel group, the term antenna/channel groups is used to identify a 
specific channel group associated with a specific antenna. A candidate 
list of potential hand-off candidate channel groups in such a system 
consists of a list of cell site antenna/channel groups. For consistency, 
the term antenna/channel group is also used for the case in which a cell 
site has a single antenna and a single channel group associated with that 
antenna. The invention is also applicable to systems, some or all of whose 
call sites are equipped with only a single omnidirectional antenna. 
In this exemplary system, each antenna/channel group is served by a group 
of fixed frequency transceivers, each associated with one transmit/receive 
channel pair, and one communication channel to the MTSO. An 
antenna/channel group may be used for a communication if one of its 
transceivers is available. The low power antenna/channel group associated 
with a particular antenna is referred to in this description by the 
designation of the antenna followed by an L, e.g., 119AL. The high power 
antenna/channel group is referred to by the designation of the antenna 
followed by the letter H, e.g., 119AH. FIG. 2 shows transceiver groups AL, 
AH (256), BL, BH (257), and CL, CH (258) associated with antenna/channel 
groups 119AL, 119AH, 119BL, 119BH, 119CL, and 119CH. Within a transceiver 
group, subgroups of up to eight transceivers are connected to a voice 
channel controller. This voice channel controller included in boxes 256, 
257, and 258, is a microprocessor, used for monitoring and controlling the 
connected transceivers, and is connected to cell site processor 250. 
The vehicle location process for illustrative mobile unit 102 begins when 
the signal on receive channel CHX drops below a primary threshold. This 
condition is detected by the voice channel controller. The controller 
periodically compares the channel's signal strength to the primary 
threshold. When the signal strength drops below the primary threshold, the 
condition is reported to cell site processor 250. Before cell site 119 
makes a request for measurement by nearby cell sites, it first checks the 
received signal strength of channel CHX on its own alternate antennas 
119A, 119B and 119C as described below. Assume that these measurements 
indicate an inadequate signal. Locate/select program 276, corresponding to 
program 176 formerly executed in the MTSO 101 processor 170, is now 
executed by a cell site processor, such as cell site processor 250 in cell 
site 119. Data base A (172) of MTSO processor 170, has been partitioned 
and cell site 119 now contains a respective data base A portion 272. 
Packet switch 291 has been added to MTSO 101 to convey message data 
packets among data links 121-126 and 129D to reduce the workload on MTSO 
processor 270. 
As shown, mobile unit 102 communicating over channel CHX has moved into 
cell domain 112D, and, more specifically, into cell sector domain 112BD, 
apparently best covered by antenna 112B. Antennas 112C and lllA could also 
conceivably serve mobile unit 102. Assume that the signal transmitted by 
the mobile on channel CHX as received by antennas 112B, 112C and 111A are 
each of adequate strength. The process for forming the list of hand-off 
candidates, described below in detail with respect to FIGS. 5 and 6, will 
order these candidate cell sites and channel groups in order 112B and 
112C. If no channels are available in antenna/channel groups 112B and 112C 
and if the channel CHX signal at the serving antenna is below a secondary 
threshold, an attempt is made to employ secondary group cell site 111. 
The processes leading to the generation of data messages will now be 
discussed in more detail. A weak signal on channel CHX from mobile unit 
102 is detected by the voice channel controller connected to the 
communicating transceiver in 256 of controlling cell site 119. Under the 
control of locate/select program 276, cell site processor 250 first 
controls tunable receiver 252 to measure the signal strength of channel 
CHX as received by cell site 119's directional antennas 119A, 119B and 
119C. If the signal strength at antenna 119A is now above primary 
threshold, that measurement is averaged with the weakened signal as 
reported by the voice channel controller in 256. The averaging process 
allows for variations in the signal strength measurement apparatus, and 
thus permits more precise thresholds to be sent without generating 
excessive hand-off activity. If this adjusted signal is above primary 
threshold and is stronger than the signals from the other antennas, the 
call is retained using CHW/CHX. Otherwise if the received signal strength 
as thus adjusted, is adequate at one of the other antennas, say antenna 
119B, and one of the low power channels associated with that antenna is 
available, cell site 119 will immediately generate a hand-off request 
message, 134A, under the control of locate/select program 276 and 
candidate message generator program 279. This message is sent to MTSO 101 
and requests that the mobile currently associated with channel CHX be 
switched to a channel of antenna/channel group 119B. 
If the signal strength of channel CHX at none of the antennas of cell site 
119 is adequate, or if no low power channels are available associated with 
an antenna at cell site 119 receiving adequately strong signals from 
mobile unit 102, cell site processor 250 recognizes the need for a 
possible hand-off of communication with mobile 102 to another cell site. 
Cell site processor 250, under the control of locate/select program 276, 
consults its local cell site data base A 272. Cell site processor 250 
prepares and, under the control of message control program 254, transmits 
message 131 over two-way data link 129D to the packet switch 291. Message 
131 commands members of a list of nearby cell sites, stored in data base 
A, to measure the signal strength received on channel CHX. Packet switch 
291 forwards the text of message 131 as messages 135, 136 to each cell 
site on the list of destinations 112, 113 included in message 131. The 
sending of a single message to packet switch 291 reduces the data 
processing load of cell site processor 250, reduces the traffic over data 
link 129D, and reduces the data processing load of packet switch control 
program 286. 
Each cell site maintains channel availability data for each of its 
antenna/channel groups, as described further below with respect to FIG. 4, 
block 590. Every time a channel is seized or released, update program 277, 
responsive to seize or release messages (not shown) from message control 
254, updates data base A (272) to reflect the change in availability of 
channels in the associated group. 
Each of the cell sites 112, 113 upon receiving message 135, 136 makes 
measurements of the strengths of the signal received by each of its 
antennas from channel CHX and determines channel availability for each of 
its antenna/channel groups. Each of these cell sites 112, 113 then 
transmits its measurement data and corresponding channel availability data 
in a message 137, 138, which is returned via packet switch 291 to cell 
site 119. 
As each message 137, 138 is received at cell site 119, the measurements are 
compared under the control of locate/select program 276 against 
corresponding threshold data stored in data base A (272) of cell site 
processor 250. Locate/select program 276 generates an ordered list of 
antenna/channel group candidates for the hand-off of this mobile call. 
Candidate message generator program 279 then generates message 134, a 
hand-off request message containing an ordered list of hand-off candidate 
cell sites and antenna/channel groups, which is sent over data link 129D 
to MTSO 101. 
In MTSO 101, hand-off message generator program 280 receives message 134 
and generates the message sequence (not shown), directed to cell sites 119 
and the new controlling cell site, necessary to effect a hand-off. The 
hand-off message sequence and the subsequent actions by the cell sites and 
mobile unit are essentially the same as those for the prior art system 
described in the BSTJ articles cited above. 
DETAILED DESCRIPTION 
FIG. 3 is a geographic layout and state diagram showing how data base A 
(272) "views" the cell site 119's surrounding cell sites 111-116, for the 
purposes of generating an ordered list of hand-off candidates of 
antenna/channel groups. Each cell site sector is marked to denote the 
appropriate areas best served by a corresponding one of the directional 
antennas. 
Antenna/channel groups are formed into an initial or zeroeth group, and a 
first and second group. The first and second groups are further divided 
into subgroups. A candidate list of antenna/channel groups is an ordered 
list of such antenna/channel groups, the ordering being by group and 
subgroup. Within a subgroup, candidate antenna/channel groups are ranked 
in order of decreasing measured signal strength compared to the threshold 
associated with each antenna/channel group. In FIG. 3, group numbers are 
coefficients, subgroup numbers are subscripts, numbers outside parentheses 
refer to low power antenna/channel groups serving a sector domain, those 
in parentheses, the high power antenna/channel groups serving a sector 
domain. 
The choice of grouping is influenced by the direction of the current 
location of the mobile unit 102. This direction can be inferred from the 
initial measurements of signal strength at the controlling cell site 119. 
The mobile unit is likely to be in the direction covered by the 
controlling cell site directional antenna currently receiving the 
strongest signal from the mobile unit. In this example, the strongest 
signal is received on antenna 119A, and the state diagram of FIG. 3 
applies to the case of strongest signal received on antenna 119A. Similar 
state diagrams and data bases can readily be inferred for the case of 
strongest signal received on antenna 119B or antenna 119C. 
In this example, FIG. 3 shows that the low power antenna/channel groups 
associated with the serving cell site sectors 119AD, 119BD, and 119CD are 
each designated "0.sub.1 ". The low power antenna/channel groups 
associated with cell site sectors 112BD, 112CD, 113BD, and 113CD are each 
designated "1.sub.1 ". The high power antenna/channel groups associated 
with the serving cell sectors 119AD, 119BD, and 119D are designated 
"(1.sub.2)". The high power antenna/channel groups associated with cell 
site sectors 112BD, 112CD, 113BD, and 113CD, are designated "(1.sub.3)". 
The low power antenna/channel groups associated with cell site sectors 
111AD, 111CD, 114AD, and 114BD form are designated "2.sub.1 ". The high 
power antenna channel groups associated with 111AD, 111CD, 114AD, and 
114BD are designated "(2.sub.2)". 
The detailed data base A (272) entries required to support the candidate 
selection process are shown in FIG. 4. Block 490 contains the initial data 
required for processing the initial measurements at the controlling cell 
site. Byte 491 identifies the cell site and byte 492 indicates which 
directional antennas are equipped. Bytes 493 and 494 specify the primary 
and secondary signal strength thresholds associated with the first, i.e., 
the A antenna. Equivalent bytes are also provided for the B and C 
antennas. Data concerning nearby cell sites and high power antenna/channel 
groups in the controlling cell site is stored in one of three blocks 510, 
595, and 597. When the antenna with the strongest signal has been 
identified, one of these blocks is accessed via a respective one of the 
three pointers 495, 496 or 497. 
Block 510 shows the layout of data required for storing antenna/channel 
grouping and signal strength threshold data for the case of a mobile whose 
strongest signal strength measurement is from antenna 119A, identified in 
byte 511. Each of the blocks 540, 545, 550, 555, 560, 565, 570, 575, and 
580 stores data associated with either the high or the low power 
antenna/channel groups of one cell site. Byte 512 and 513 specify the 
number of blocks in the first and second group, respectively. 
Typical block 540 shows the data for the low power antenna/channel groups 
112BL and 112CL of cell site 112. The cell site is identified in byte 541. 
Box 532 is a descriptive layout of the second byte of each of the blocks 
such as block 540 and indicates that this byte contains three fields. 
Fields 522, 523, and 524 identify the use of the specific data entries 
542, 543, and 544 in typical block 540. Field 522 specifies which of the 
four antennas have an associated channel group of the correct power level 
which is a potential hand-off candidate. For block 540, this is antennas B 
and C (542). Field 523 specifies whether this block pertains to the high 
or low power channel group. Block 540 pertains to the low power channel 
group, hence the L in 543. Field 524 specifies a subgroup number, hence 
the data 1 in 544. The group number, 1, is deduced from the fact that 
block 540, is one of the first 5 blocks, the number 5 being specified by 
byte 512, hence the bracketed 1 shown next to field 544. These entries in 
block 540, associated with antenna channel groups 112BL and 112CL, 
correspond to the "1.sub.1 " shown in FIG. 3 in sector 112BD and 112CD. 
Similarly, the entries in block 545 correspond to antenna/channel groups 
113BL and 113CL. Byte 546 identifies the cell site 113, field 547 
identifies the B and C antennas, field 548 identifies the low power 
channel groups, the bracketed 1 next to field 549 indicates the group 
which is supplemented by the subgroup 1 in field 549. The contents of 
block 545 therefore correspond to the "1.sub.1 ", shown in sectors 113BD 
and 113CD of FIG. 3. In a similar way, block 550, pertains to the high 
power antenna/channel groups of cell site 119, shows that A, B and C 
antennas can be used, and shows that these channel groups are in the 
second subgroup of group 1. This corresponds to the "(1.sub.2)" in sectors 
119AD, 119BD and 119CD of FIG. 3. 
Skip now to block 565, the sixth block. This is the first block associated 
with the second group of antenna channel groups, hence the bracketed 2 
next to field 569. The cell site 111 (byte 566), antennas A and C (field 
567), low power channel groups (field 568) and first subgroup (field 569) 
are all specified in the block. This corresponds to the "2.sub.1 ", shown 
in sectors 111AD and 111CD of FIG. 3. The rest of the entries in block 510 
similarly correspond to the state diagram data of FIG. 3. 
Each block 540, 545, . . . , 575, 580 also stores primary and secondary 
thresholds associated with each of up to three antennas per block. These 
are shown in block 540 in exemplary bytes 530 and 531. 
Block 590 stores availability indicator for the antenna/channel groups of 
cell site 119. The indicator for each antenna/channel group is simply an 
up/down counter, initialized to the number of channels of an 
antenna/channel group, decremented whenever a channel is seized, and 
incremented whenever a channel is released. A positive count indicates 
that a channel of a group is currently available; a count of zero 
indicates that all channels are busy. 
A hand-off candidate list is generated by locate/select program 276 based 
on the grouping of antenna/channel groups described above, on measurement 
data received at the controlling cell site, and on thresholds stored as 
described above. Acceptable candidates within a subgroup are arranged in 
order of the excess of the measured signal strength compared to the stored 
threshold. Candidates are added to a candidate list until the list is 
"complete". The candidate list is complete when it contains an available 
candidate, i.e., an antenna/channel group associated with an acceptable 
signal strength measurement and a reported available channel, or when the 
list contains three candidates with acceptable signal strength 
measurements, whichever happens first. The number three was chosen as a 
reasonable maximum since the number of nearby cell site antennas which 
could handle a mobile that has wandered in any given direction from a 
controlling cell site sector is likely to be small. 
The details of the candidate selection process of FIG. 5 will now be 
described. When the voice channel controller in 256 detects that a 
received signal has dropped below the primary threshold, it notifies 
processor 250. This triggers process step 401 causing a test 402 to access 
data base A 272 to determine if the cell site has other (directional) 
antennas. If not, the group 1 cell sites are immediately queried; the 
serving cell site is, effectively, a group 1 cell site. If so, process 404 
measures the signal strength on all antennas of the serving cell site. As 
previously mentioned, the signal strength measurement on the serving 
antenna is averaged with the signal strength measurement from the voice 
channel controller. If the measured signal strength on any antenna exceeds 
the primary threshold, the associated antenna/channel group is entered on 
a candidate list to be examined by test 407 in order of excess above 
primary threshold. 
A candidate list is "complete" for the limited purposes of test 407 if a 
candidate with an available channel has been found in an alternate low 
power antenna/channel group. In that case, process 409 generates a 
hand-off request message 134A (FIG. 2). If the adjusted measured signal 
from the serving antenna was strongest, process 409 will simply allow the 
call to be retained on the serving antenna using the serving transceiver 
and channels. Otherwise, process 414 generates measurement request message 
131 (FIG. 2), directed to the first group of nearby cell sites. This 
process will also cause the serving cell site to make a signal strength 
measurement if it has only one antenna. Message 131 is sent to packet 
switch 291 at MTSO 101 which forwards the request to the group of cell 
sites identified in bytes 541 and 546 of FIG. 4. Each signal strength 
report message 137, 138 returned from this first group of nearby cell 
sites is compared in process 414 with the corresponding primary thresholds 
such as byte 530 (FIG. 4) for each directional antenna of the replying 
cell sites. Candidate antenna/channel groups with received signal strength 
above corresponding primary threshold are added to the candidate list. 
When test 420 detects that process 414 has generated a complete candidate 
list process, 414 is terminated and process 421 generates a hand-off 
request message 134 to MTSO 101. If process 414 is completed and test 420 
finds that the candidate list is not complete, process 424 tests whether 
the signal at the serving cell site and antenna is above a secondary 
threshold. lf so, process 427 tests whether the candidate list is empty. 
lf the list is not empty, process 428 generates a hand-off request message 
134 to MTSO 101. Otherwise, process 429 maintains the call on the present 
serving antenna and channel. Subsequently, in about five seconds, the 
voice channel controller in 256 (FIG. 2) may again detect that the 
received signal has dropped below the primary threshold, thus triggering 
process step 401 once more. 
If test 424 indicates that the signal level at the serving cell site has 
dropped below the secondary threshold, a test 434 (FIG. 6) is made to see 
if a second nearby cell site group exists. If so, process 445 causes a 
second measurement request message 131 (FIG. 2) to be sent, this one 
directed to the second group of cell sites. The second group of cell sites 
is identified by bytes 566 and 571 (FIG. 4). 
After signal strength report messages from the second group of cell sites 
have been received, process 445 generates a list, subgroup by subgroup, 
for all antenna/channel subgroups in the second candidate group whose 
signal strength is above the primary threshold. Process 448 makes a test 
for completeness of the list, and if a complete candidate list has been 
generated, process 449 generates a hand-off request message 134 which is 
sent to MTSO 101. If process 448 fails to find a complete list, it calls 
for the execution of process 437. Process 437 generates an ordered 
candidate list based on signals exceeding the secondary thresholds in both 
groups of antenna/channel groups. Process 437 is also executed directly if 
test 434 indicates that no second nearby cell site group exists. If test 
441 finds that the candidate list generated by process 437 is not empty, 
process 442 generates a hand-off request message 134 which is sent to the 
MTSO 101. If test 441 finds the candidate list generated by process 437 is 
empty, test 441 causes process 443 to insert a delay which will defer 
another attempted hand-off for a period substantially greater than the 
normal five seconds. 
FIG. 7 shows the layout of data base B 274, shown as block 600, maintained 
at MTSO 101. The data base is essentially the same as data base B 174 of 
FIG. 1, except that it contains data for a much larger number of antenna 
channel groups. This data base contains centralized control information 
used for the assignment of channels. The block of data 610 for cell site 
111 is shown in detail. It contains a section of data such as subblock 620 
for each antenna/channel group. Location 621 contains the identity of the 
antenna/channel group of section 620, and section 622 contains channel 
control data for each channel of the group. The control data 622 specifies 
which channels of the group are available. The members of a candidate list 
are examined in order by MTSO 101 to see if a channel is available on one 
of the antenna/channel groups on the list. 
If a channel is available, hand-off message generator process 280 (FIG. 2) 
generates the hand-off message sequence to transfer the call to that 
channel. Packet switch control process 286 controls packet switch 290 
which transmits the hand-off message sequence to the controlling cell site 
and the cell site which is to take over the call. 
FIGS. 8 and 9 show the layout of signal strength measurement request 
messages 131, 135, 136, signal strength report messages 137, 138, and 
hand-off request messages 134, 134A. The messages have been arranged in 
this system to allow standard types of items to be recognized and treated 
in a uniform manner. Each message includes a 12 byte header 300 (FIG. 8), 
optionally followed by a variable length data field 310. The first four 
bytes 301 of each header comprise a destination process identifier. The 
process identifier is used to refer the message to the MTSO or the 
appropriate cell site and to the appropriate program process. The next 
four bytes 302 convey similar information about the source process. In 
case a source and destination process communicate with each other in both 
directions, this arrangement makes it possible to interchange the source 
and destination process identifiers when preparing the header of a 
response message. 
The ninth and tenth bytes 304 of the header indicate the type of message. 
These bytes are examined first by packet switch control program 286 or the 
cell site message control program 254 (FIG. 2), in order to process other 
data in the message appropriately. Illustrative message types are signal 
strength measurement request, signal strength report, and hand-off 
request. The eleventh byte 305 indicates the priority of the message and 
is not further described herein. The twelfth byte 307 indicates the length 
in bytes of the subsequent data of the message data field 310. 
The detailed layout of the data field (FIG. 9) of a signal strength 
measurement request message 131 (FIG. 2) will now be described. The first 
byte 331 is an identification of the receiving (i.e., measuring) cell site 
task. The second byte 332 of the data field contains an identification of 
the requesting (i.e., control) cell site task. The next two bytes 334 
contain the channel number for which the measurements are to be taken. 
Finally, the message to MTSO 101 includes a list 335, . . . , 336 of the 
destination cell sites followed by the number 341 of these cell sites. 
List 335, . . . , 336 and number 341 are not transmitted to these cell 
sites. In the signal strength measurement request message 131, the data 
field length (field 307) is 5+n:4 bytes for the destination cell task, 
source cell task and channel number which will be transmitted from the 
MTSO to each of the cell sites which are to make measurements; 1 byte to 
indicate n, the number of cell sites, to be queried; and n bytes to 
specify the individual cell sites. 
Signal strength measurement request messages 135, 136 are commands to the 
receiving cell site that they perform signal strength measurements. 
Messages 135, 136 contain the 12 byte header (fields 301, 302, 304, 305, 
307) and the first four bytes of the data field (fields 331, 332, 334) of 
message 131. When packet switch 291 receives message 131, control program 
286 checks the type field 304 and recognizes that the first 16 bytes of 
the message are to be forwarded to each cell site on the list. Packet 
switch 291 initializes byte 307 to four, the number of bytes in the data 
field of messages 135, 136. Packet switch 291 then transmits signal 
strength measurement request messages 135, 136 to each of the cell sites 
on the list of message 131. 
Signal strength report messages 137, 138 are generated by a cell site in 
response to the reception of a signal strength measurement request message 
135, 136. A cell site, upon receiving a message 135 or 136, makes 
measurements of the received signal strength of the appropriate channel, 
indicated in the fifteenth and sixteenth bytes (field 334) of the signal 
strength measurement request message. A cell site equipped with 
directional antennas will make these measurements on each directional 
antenna. In signal strength report messages 137, 138 the source 301 and 
destination 302 process identification fields contained in the first eight 
bytes of the header of the signal strength measurement request message are 
simply interchanged. The type field 304 is appropriately set to indicate a 
signal strength report message and the length 307 is set to a standard 
value of eight. The first two bytes 351 and 352 of the data field of 
message 137, 138 are reversed from bytes 331 and 332 of the received 
signal strength measurement request message 135, 136 and now identify the 
task in the controlling and the measuring cell site, respectively. 
In signal strength report message 137, 138, one byte 354 is provided to 
indicate the order of strength of received signals from each of the three 
directional antennas. For convenience, the numbers 1, 2, and 3 represent 
the A, B and C directional antennas, respectively. If the received signal 
on one of these antennas is not sufficiently strong or the antenna is 
unequipped, a 0 is indicated. Byte 354 consists of three numbers 355, 356, 
and 357, each between 0 and 3. The first number 355 represents the 
directional antenna with the strongest received signal if it is above 
threshold, the second number 356 represents the directional antenna with 
the second strongest signal if it is above threshold, and the third number 
357 represents the directional antenna with the third strongest signal if 
it is above threshold. 
Next, the signal strength of channel CHX received in an omnidirectional 
antenna, and the A, B, and C antennas are indicated in standard locations 
359, 361, 363, and 365 in the next four bytes. 
Finally, the last byte 370 indicates availability of channels in each of 
the eight possible groups associable with a cell site. This information is 
conveyed in a standard pattern even though few or no cell sites will have 
all eight groups. These eight possible groups represent four possible 
groups associated with a low power transmitted signal on each of the three 
directional antennas and an omnidirectional antenna, plus four groups 
associated with a high power transmitted signal associated with each of 
these four possible antennas. Within byte 370, bit 371 represents 
availability of a low power channel on the omnidirectional antenna, bit 
372 represents availability of a low power channel on the A antenna, ..., 
bit 374 represents availability of a high power channel on the B antenna, 
and bit 375 represents availability of a high power channel on the C 
antenna. 
The signal strength report messages 137, 138 are sent from the various cell 
sites which have been requested to measure the signal intensity of channel 
CHX back to packet switch 291 and are then forwarded to the controlling 
cell site. The controlling cell site uses the destination cell task 
identifier 351 to associate the reports with channel CHX. The controlling 
cell site takes these measurements and, on the basis of the groupings of 
antenna/channel groups as stored in block 510 (FIG. 4), the relative 
signal strength compared to a stored threshold, and the availability of 
channels, generates an ordered list of candidate antenna/channel groups, 
as previously described with respect to FIGS. 5 and 6. 
The hand-off request message, such as message 134 (FIG. 2) has a data field 
format shown in FIG. 9. The message consists of a header, including source 
and destination process identifiers and the type of message, and a length 
of 4, 6, or 8 bytes depending on whether the candidate list is 1, 2, or 3 
entries long. The data field includes the controlling cell site task 
number in the first byte 381, the number of entries in the second byte 
383, and groups of two bytes (385, 386; 387, 388; 389, 390) indicating the 
cell site number and antenna/channel group (i.e., directional antenna 
identification plus a high or low power channel group identifier) for each 
of the candidates on the list. The controlling cell site task number 
identifies channel CHX to the MTSO. Hand-off message generator 280 in the 
MTSO uses this candidate list to find available channels and associated 
transceivers, accessing data base B 274 to check on channel availability. 
When an available channel is found, hand-off message generator program 280 
generates the conventional hand-off message sequence essentially in the 
same way as is done in the prior system. 
Because only the type field of a received message need be examined and the 
message can be readily forwarded to one or a plurality of destinations, 
less usage is made of the MTSO data processing resources in the system of 
FIG. 2 than in the system of FIG. 1. The more complex and time consuming 
locate/select program functions are carried out in the cell site. Thus, 
moving control of the vehicle location process to the controlling cell 
site substantially deloads the MTSO processor, especially during periods 
of peak traffic. 
In the exemplary system which has been described, no direct data links were 
shown among cell sites, all messages being sent via the MTSO acting as a 
message switch. In alternate configurations, it should be appreciated that 
it may be more economical to provide a limited number of direct data links 
between selected cell sites or to provide alternate facilities for data 
switching messages among cell sites and the MTSO. 
It is well-known that in hilly terrain, radio signal propagation patterns 
are irregular. In order to handle unusual situations, it may be desirable 
under some circumstances to select the nearby cell sites on the basis of 
the currently serving antenna instead of the antenna with the strongest 
received signal. Also, it may be desirable for certain cell sites to 
hand-off only to the antenna/channel groups associated with the strongest 
received signal. The changes necessary to the system as described in order 
to implement such functions are quite straightforward. Note further that 
the described process for selecting a hand-off antenna/channel group, 
based on measurements, thresholds, grouping and subgrouping can also be 
implemented in the MTSO if desired, and represents by itself an improved 
process for making such a selection. Mobile units can be mounted in 
vehicles or can be portable. 
It is to be understood that the above-described embodiment is merely 
illustrative of the principles of this invention; other arrangements may 
be devised by those skilled in the art without departing from the spirit 
and scope of the invention.