Method and apparatus for signal strength measurement and antenna selection in cellular radiotelephone systems

A method and apparatus for determining which sector antenna of a sectorized cellular radiotelephone system is receiving the strongest radio signal is disclosed. The signal strength of sector antennas is sequentially sampled, converted to digital representations, and stored according to which antenna received the signal. The digital representations are recalled from storage and the strongest signal along with an identification of the receiving antenna are determined. This determination may be used in the handoff process or in detecting when a remote unit requires a handoff.

BACKGROUND OF THE INVENTION 
Reference is made to three copending applications (U.S. patent application 
Ser. No. 830,166, "Scanning Receiver Allocation Method and Apparatus for 
Cellular Radiotelephone Systems", by Menich et al. Ser. No. 830,145, 
"Improved Cellular Radiotelephone Land Station", by Atkinson et al.; and 
Ser. No. 830,390, "Interface Method and Apparatus for a Cellular System 
Site Controller", by Menich et al.) filed on the same date Feb. 18, 1986 
as the present application and containing related subject matter. 
The present invention generally relates to the fixed equipment of 
radiotelephone communication systems and more specifically relates to a 
method and apparatus for determining which antenna of a sectorized 
cellular radiotelephone system receives the strongest radio signal. 
Mobile radiotelephone service has been in use for some time and 
traditionally has been characterized by a central site transmitting with 
high power to a limited number of mobile or portable units in a large 
geographic area. Mobile or portable transmissions, due to their lower 
transmission power, were generally received in previous systems by a 
network of receivers remotely located from the central site and the 
received transmission was subsequently returned to the central site for 
processing. In previous systems only a limited number of radio channels 
were available, thus limiting the number of radiotelephone conversations 
in an entire city to the limited number of channels available. 
Modern cellular radiotelephone systems have a comparatively large number of 
radio channels available which, further, can be effectively multiplied by 
reuse of the channels in a metropolitan area by dividing the radio 
coverage area into smaller coverage areas (cells) using low power 
transmitters and coverage restricted receivers. Such cellular systems are 
further described in U.S. Pat. Nos. 3,906,166--Cooper et al.; 
4,485,486--Webb et al.; and 4,549,311--McLaughlin, each assigned to the 
assignee of the present invention. The limited coverage area enables the 
channel frequencies used in one cell to be reused in another cell 
geographically separated according to a predetermined plan, such as a 
seven cell repeating omnidirectionally illuminated cell pattern shown in 
FIG. 1. In this pattern radio frequency energy is omnidirectionally 
transmitted from and received by a plurality of centrally located fixed 
stations and reuse of frequencies is accomplished in a pattern of cells 
such as that shown shaded in FIG. 1. 
An alternative cellular pattern, FIG. 2, depicts a corner illuminated cell 
system in which 120.degree. antennas are employed to illuminate the 
interior of a cell from three of the vertices of a hexagonal cell. 
(Although cell systems are conventionally shown as regular hexagonal 
patterns, such regularity is rarely achieved in practice). 
Another pattern, FIG. 3, depicts a center illuminated cell system in which 
the cells are further subdivided into sectors. The sectors are illuminated 
by 60.degree. antennas as illustrated in FIG. 3. A center illuminated 
sector cell system is further described in U.S. Pat. No. 
4,128,740--Graziano and assigned to the assignee of the present invention. 
Thus, a large number of channels can be made available in a metropolitan 
area and the service provided thereby can appear to be identical to a 
standard wire line telephone. 
A cell system typically utilizes one duplex frequency pair channel in each 
cell (a signalling channel) to receive requests for service from mobiles 
and portables, to call selected mobiles or portables and to instruct the 
mobiles or portables, to tune to another channel where a conversation may 
take place. This signalling channel is continuously assigned the task of 
receiving and transmitting data to control the actions of the mobile and 
portable radios. If the cell is sectorized as shown in FIG. 3, specialized 
receivers have been developed to enable the inputs from six 60.degree. 
antennas to be combined for instantaneous reception over the sectorized 
cell coverage area. One such specialized receiver is described in U.S. 
Pat. No. 4,369,520--Cerny, Jr., et al., assigned to the assignee of the 
present invention. 
Since the cells may be of relatively small size, the likelihood of a mobile 
or portable travelling between sectors or out of one cell and into another 
cell is high. The process of switching the established call from one 
sector or from one cell to another is known as handoff. Handoff previously 
has required specialized receiving equipment such as a "scanning" receiver 
which can be instructed to tune to any of the channels in use in any of 
the sectors of the cell to measure the signal strength of each active 
mobile or portable. If the measured signal strength is below a 
predetermined level, cellular control equipment determines the 
availability of other channels in other sectors of the same cell or in 
neighboring cells and composes an instruction to the mobile or portable 
commanding it to tune to the new channel. 
In order to determine which sector of a sectorized cell is receiving the 
best signal of the six sectors, traditional fixed site equipment included 
a plurality of specialized receivers which could be commanded to monitor 
one or more sectors at a particular frequency to determine whether a 
particular served remote unit signal was becoming too weak or whether a 
remote unit from another cell or another sector could be handed off to the 
particular sector being monitored. This traditional design required the 
specialized "scanning" receivers at additional cost and complexity. 
SUMMARY OF THE INVENTION 
Therefore, one object of the present invention is to reduce the cost and 
complexity of the fixed site equipment of a cellular radiotelephone system 
by reducing or eliminating specialized receiving equipment such as 
scanning receivers. 
It is a further object of the present invention to utilize common receiving 
equipment to measure the received signal strength of a served remote unit 
while providing a receiver for voice channel conversation. 
It is a further object to provide voice channel receivers with signal 
strength measurement capability such that adjacent sector antennas can be 
monitored for received signal strength or that six sectors of a sectorized 
cell can be monitored for received signal strength. 
Accordingly, these and other objects may be achieved with the present 
invention. Briefly, the present invention is the method and apparatus for 
determining which of three sector antenna receives the best signal 
strength from a served remote unit. This is realized by rectifying and 
digitizing an RF signal received in a primary sector antenna and storing 
the digital representation in a plurality of storage locations. Adjacent 
left and right sector antennas are sequentially sampled for the RF energy 
at the frequency being used by the served remote unit. The RF energy 
sample is then rectified and digitized into sequential digital 
representations. The sequential digital representations are cyclically 
stored in first and second storage locations which are associated with the 
adjacent sector antennas. Each storage location is then sampled and the RF 
signal with the greatest magnitude is determined along with the antenna on 
which that RF signal was received.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 4, there is illustrated a cellular radiotelephone 
communications system of the type which may particularly benefit from the 
invention herein described. The illustration of FIG. 4 shows three center 
illuminated sector cells of the type previously described in conjunction 
with FIG. 3 but with more detail regarding the type of equipment to be 
found in a sector cell system. Although the present invention will be 
described with particularity for the center illuminated sector cell 
system, it is obvious that a person skilled in the art may be able to 
apply the essence of the present invention to other types of cellular 
configurations such as those shown in FIG. 2 and FIG. 1. 
As illustrated in FIG. 4, the geographical area is subdivided into cells 
402, 404, and 406 which are illuminated with radio frequency energy from 
fixed site transceivers 408, 410, and 412, respectively. The fixed site 
transceivers are conventionally controlled by base site controllers 414, 
416, and 418 as illustrated. These base site controllers are each coupled 
by data and voice links to a radiotelephone control terminal 420 which may 
be similar to the terminals described in U.S. Pat. Nos. 3,663,762; 
3,764,915; 3,819,872; 3,906,166; and 4,268,722. These data and voice links 
may be provided by dedicated wire lines, pulse code modulated carrier 
lines, microwave radio channels, or other suitable communication links. 
Control terminal 420 is, in turn, coupled to the switched telephone 
network via a conventional telephone central office 422 for completing 
telephone calls between mobile and portable radiotelephones and landline 
telephones. 
The interconnection between control terminal 420 and the base site 
controllers (BSCs) is further shown in FIG. 5. The per-channel 
interconnection may be on a line per channel basis such as shown between 
control terminal 420 and BSC 416 or the interconnection may be on a PCM 
group basis such as shown between control terminal 420 and BSC 414. Either 
type of interconnection is well known in the art. A separate data line 
(which may be a standard telephone line or other communications link 
capable of carrying 4800 baud data) is extended between the control 
terminal 420 and each BSC under its control. 
Each of the fixed site transceivers 408, 410, and 412 includes a plurality 
of transmitters and receivers for operating on at least one duplex 
signalling channel and a plurality of duplex voice channels. One 
conventional system employs transmitters and receivers of the type 
described in Motorola Instruction Manual No.68P81060E30, published by 
Motorola Service Publications, Schaumburg, Ill., in 1982. Employing this 
equipment and spacing the channels in use at least 630 KHz from each 
other, enables the individual transmitters to be combined on a single 
antenna (which may be a 60.degree. directional antenna) as illustrated in 
FIG. 6. 
In FIG. 6, three transmitters 602, 604, and 606 are shown connected to an 
antenna 608 via combining cavities 610, 612, and 614 and harmonic filter 
616. Each transmitter consists of a voice channel exciter 618 for voice 
channels or a signalling channel exciter 620 for a signalling channel and 
a power amplifier 622. Each of the conventional transmitters shares a 
common reference oscillator 624, a supervisory audio tone (SAT) generator 
and failsafe circuit 626, and a distribution amplifier 628. 
The conventional receiving system is designed in modular groups of eight 
voice channel receivers, a signalling receiver, and a scanning receiver. 
Two possible system configurations are shown in FIGS. 7 and 8. Considering 
first the six sector receive system shown in FIG. 7, it can be seen that a 
set of broadband preselectors 702, signal splitters 704, and first mixers 
706 convert each sector antenna input to an intermediate frequency (IF) 
for use by the remainder of the receivers. Local oscillator input to first 
mixers 706 is provided by a common synthesizer 708 and distributed to each 
of the first mixers 706 by splitter 710. Two switch matrices, matrix 712 
and matrix 714 connect a pair of antennas corresponding to adjacent 
sectors to each of the voice receiver IFs 716 and the signalling receiver 
IF 718. A commutating RF switch 720 is connected to each of the signal 
splitters 704 and steps the scanning receiver 722 through each of the six 
sector antennas. 
An omnidirectional receive system is shown in FIG. 8 and is a conventional 
subset of the sector receive system of FIG. 7. Two omnidirectional 
antennas encompass the entire 360.degree. coverage area within the cell. 
The down converted signal from each of the antennas and output from first 
mixers 706 are split among each of the voice IF receivers 716 and the 
signalling receiver 718 by matrix splitters 812 and 814. Further, the 
scanning receiver 722 is toggled between the two antennas by commutating 
switch 820. 
The conventional base site controller (414, 416, or 418) is shown in more 
detail in the block diagram of FIG. 9. The base site controller (BSC) 
provides two-way subscriber remote unit (mobile and portable) signalling, 
voice communications, and complete control and performance monitoring of 
the fixed site equipment. The BSC consists of a site control processor 
module 902 which controls all aspects of the base site operation. The site 
control processor 902 contains memory storage buffers for communication 
with the peripheral processors 904, 906 and 908. The site control 
processor 902 also contains serial interface ports for communicating with 
other site control processors and for communicating with the control 
terminal 420 and an RS-232 port for connection to a maintenance terminal. 
A signalling channel controller peripheral 904 sends paging and overhead 
messages to subscriber units via the signalling channel by command of the 
site control processor 902. The signalling channel controller 904 also 
decodes and corrects data received from subscriber units. In systems using 
sector receive antennas, it uses information from the signalling channel 
receiver 718 to make an initial estimate of the subscriber unit's 
location. 
The scan processor peripheral 906 measures every active subscriber unit 
signal strength on each receive antenna. Method and apparatus for 
measuring signal strength on receive antennas is further described in U.S. 
Pat. No. 4,485,486--Webb et al., assigned to the assignee of the present 
invention. It also measures the supervisory audio tone frequency of 
subscriber units to verify that it is making measurements on the correct 
subscriber unit. The scan processor 906 is capable of directing the 
scanning receiver 722 to any subscriber frequency and measuring any of the 
three supervisory audio tone frequencies. The voice channel processor 
peripheral 908 controls up to eight voice channel IFs and the subscriber 
units using them. The voice channel processor 908 interfaces to each voice 
channel receiver through an audio conditioning board 910. The voice 
channel processor 908 sends messages to subscriber units by command of the 
site control processor 902 and further decodes and corrects data messages 
from subscriber units over the appropriate voice channel. The voice 
control processor 908 controls voice transmitters and voice channel 
receive antenna selection. The audio conditioning boards 910 are employed 
one for each voice channel in use at a fixed site. The audio conditioning 
boards 910 conditions and controls the audio for connection to both the 
receiver and transmitter radio equipment and the telephone lines to the 
control terminal 420. 
In order to reduce the amount of common equipment, provide for ease of 
expansion, and reduce the amount of intercabling and interconnection, the 
preferred embodiment of the novel invention of the present application may 
utilize the antenna system configuration shown in FIG. 10. The radio 
transceivers are connected to the sector antennas as shown. Especially 
note that each sector antenna is fed by a multicoupler (for example, RX 
multicoupler 1002) to the primary transceiver equipment dedicated to the 
particular sector (for example, transceivers 1004) and to both the 
adjacent sector transceiver equipment (for example, transceivers 1006 for 
sector 6 and transceivers 1008 for sectors 2). In addition, each sector 
antenna is coupled to a signalling receiver allowing the signalling 
receiver to have access to all six sector antennas. The transmitters of 
the primary transceiver equipment is coupled to the sector antenna via a 
duplexer (such as duplexer 1010). The duplexers may be similar to model 
ACD-2802-AAMO manufactured by Antenna Specialists Co., Cleveland, Ohio. 
The interconnection of the fixed site transceivers to the antenna system 
and to the base site controller is shown in FIG. 11. In this configuration 
a transceiver (such as transceiver 1102 or transceiver 1104) consists of a 
transmitter 1106 and 1107 respectively, diversity receivers 1110 and 1112 
(for transceiver 1102) and diversity receivers 1114 and 1116 for 
transceiver 1104. Each transceiver also comprises a microcomputer (1118 
and 1120, respectively) and a sector switch (1122 and 1124, respectively). 
Additionally, an identical transceiver may be used as a scan receiver by 
employing the diversity receivers and the microcomputer as shown for 
transceiver 1126. (The transmitter for transceiver 1126 is not used). 
Concentrating on the interconnections of transceiver 1102, it can be seen 
that transmitter 1106 and receiver (branch A) 1110 are coupled to the same 
primary antenna (via the duplexer 1010 and receiver multicoupler 1002 to 
antenna 1 as shown in FIG. 10). Receiver (branch B) 1112 is coupled to 
left and right adjacent sectors via sector switch 1122 (which from FIG. 10 
are antenna 6 and antenna 2). The output bus from the BSC 1108 is 
connected to each of the microcomputers of the transceivers at a cell 
site. In the transceivers of the present invention, the transmitter 1106 
input and receivers 1110 and 1112 output are connected to the switched 
network via the control terminal 420. Control of the interconnection to 
the control terminal 420 is achieved by microcomputer 1118 via control 
signals from the BSC 1108. 
A more detailed block diagram of the transceivers of the preferred 
embodiment is shown in FIG. 12. Transmitter 1106 and receivers 1110 and 
1112 from transceiver 1102 are shown in detail. Each of the other 
transceivers including the scanning transceiver 1126 may have an identical 
design. In a preferred implementation of the present invention, a 
synthesizer 1202 having a conventional VCO 1204 provides the local 
oscillator signal for both receivers 1110 and 1112 via a power splitter 
1206. The VCO 1204 also supplies a radio frequency signal to the 
transmitter 1106 via a buffer amplifier 1208 and power splitter 1210. A 
second output from power splitter 1210 is used as part of the conventional 
feedback of a frequency synthesizer and routed through pre-scaler 1212 to 
the phase detector and programmable dividers 1214 which, in turn, provides 
a correction signal to the charge pump and loop filter 1216 to place and 
hold the VCO 1214 at the proper frequency. Frequency selection is 
conventionally made by selecting the proper division ratio of the 
programmable dividers 1214 via the channel program bus and I/O circuit and 
buffers 1218. The channel program bus is coupled to the transceiver 
microcomputer (such as microcomputer 1118) which selects the proper 
digital signal for the bus to place the transceiver on a designated 
channel. The ultimate stability of the synthesizer 1202 is determined by 
the frequency reference generated by an input to the phase detector and 
programmable dividers 1214 via buffer 1220. This reference is generated by 
a reference oscillator located in the common rack front end cabinet. The 
transmitter 1106 accepts the synthesizer 1202 frequency output signal at 
buffer amplifier 1222 and filters the radio frequency signal by filter 
1224 before applying the radio frequency signal to mixer 1226. A second 
signal applied to mixer 1226 is generated by the sidestep VCO 1228 and 
conventionally angle modulated by voice signals from the switched network 
and data and supervisory audio tone (SAT) which are input to the sidestep 
VCO 1228 via transmit audio processing circuitry 1230 (which may be 
similar to model TRN9732 A, Audio/Control Board described in Motorola 
Instruction Manual No. 68P81071E17, published by Motorola Service 
Publications, Schaumburg, Ill., in 1985). The sidestep VCO 1228 is 
maintained at a frequency which is equal to or related to the spacing 
between the receiver and transmitter operating frequencies of the selected 
channel and the frequency chosen as the intermediate frequency of the 
receivers 1110 and 1112 by the conventional synthesizer control loop 
consisting of buffer amplifier 1232, prescaler and phase detector 1233, 
and loop filter 1234. Ultimate loop stability is controlled by the same 
frequency reference used for synthesizer 1202 via buffer amplifier 1236. 
(A similar method of sidestepping frequencies for duplex receivers and 
transmitters is further described in U.S. Pat. No. 3,825,830--O'Connor 
assigned to the assignee of the present invention). The output signal from 
mixer 1226, which in the present invention is equal to the output 
frequency of the transmitter on this selected channel is coupled via 
amplifier 1238 and filter 1239 to power amplifier 1240 where the 
transmitter signal is boosted in power to a useable transmission level. 
The output of the power amplifier 1240 is coupled through a power detector 
1242 and a filter 1244 before being coupled to a duplexer (such as 
duplexer 1010) or to an external power amplifier which may further 
increase the transmitter signal power. The output from power detector 1242 
is coupled to a conventional automatic output control circuit 1246 which 
fixes the level of the output of power amplifier 1240 at a constant level. 
The receiver local oscillator signal is coupled from the power splitter 
1206 to the mixer 1248 of receiver (branch A) 1110 via buffer amplifier 
1249 and filter 1250. (Identical circuit configuration exists for receiver 
(branch B) 1112 and an identical description for receiver (branch B) 1112 
is omitted here for brevity). A received signal from the primary antenna 
is input to the mixer 1248 via filters 1252 and 1253 and radio frequency 
preamp 1254. The intermediate frequency (IF) product of the two signals 
input to mixer 1248 is selected by filters 1256 and 1257 and amplified by 
IF amplifiers 1259 and 1260 before being applied to an IF 
limiter/discriminator circuit 1262. Two outputs are provided from the IF 
limiter/discriminator 1262, the first of which is the demodulated audio 
signal which is passed through audio switch 1264 and receive audio 
processing circuitry 1266 (which may also be similar to a model TRN9732A 
Audio/Control Board) where the voice signal is coupled to the switched 
network, the data is coupled to the BSC, and the supervisory audio tone 
(SAT) is detected by comparison to a locally generated tone and the 
detection is supplied to a microprocessor of the transceiver microcomputer 
1118. A second output from the IF limiter discriminator 1262 is a signal 
which corresponds to the signal strength of the received signal from the 
antenna and is known as the receive signal strength indicator (RSSI). The 
RSSI signal is coupled to a hysteresis comparator 1268 (an MC3302 or 
equivalent in the preferred embodiment) which compares the RSSI signal 
from receiver (branch A) 1110 and receiver (branch B) 1112. The result of 
the comparison causes the audio switch 1264 to pick the demodulated audio 
signal from either receiver 1110 or receiver 1112 depending upon which 
RSSI signal indicates a stronger received signal and allows that 
demodulated audio to be coupled to the received audio processing circuitry 
1266. In one implementation of the preferred embodiment, further 
processing of the RSSI signal is accomplished in a signal strength 
processing circuit 1270 and output to the transceiver microcomputer 1118 
for use by the BSC 1108 and control terminal 420. Such a receiver having 
the signal strength processing circuit 1270 may be used as a scanning 
receiver 1126. 
Referring to FIG. 13, it will be seen that the microcomputer 1118 and 1120 
of the transceivers may consist of a microprocessor 1302 (which may be an 
MC1468705G2 microprocessor available from Motorola, Inc. or equivalent) 
which is used to control the other submodules of the transceiver. The 
microcomputer 1118 has as major peripherals an A/D converter 1304, a 
supervisory audio tone (SAT) generator 1306, conventional RAM and ROM 
memory 1308, conventional data receivers, drivers and data mux (for 
selecting sources and destinations of land/radio data) 1310, and 
microprocessor clock and sanity timing device 1312. Control data and 
information is coupled between the microprocessor 1302 and the base site 
controller (BSC) via the data receivers/drivers 1310 which may have 
additional enable ports for external control. The analog to digital (A/D) 
converter 1304 for the received signal strength may be realized by a 
multiplexing A/D converter such as an MC145041 available from Motorola, 
Inc. or equivalent. Conceptually, a dual channel A/D converter 1304 can be 
considered individual A/D channel 1320 for receiver (branch A) and A/D 
channel 1322 for receiver (branch B). The digitized received signal 
strength is made available to microprocessor 1302 is needed. 
The supervisory audio tone (SAT) detector 1306 is realized in the preferred 
embodiment by generating a selected SAT frequency in a programmable SAT 
generator 1316 (which may conventionally be realized using a phase locked 
loop such as an MC14046 available from Motorola Inc. and standard 
programmable BCD/binary counters such as MC14569 available from Motorola, 
Inc.). The SAT output may then be coupled to a SAT detector circuit 1318 
which may be a conventional frequency comparison network. The detection 
may then be coupled to the microprocessor 1302. 
Because the transceiver of the preferred embodiment is equipped with a 
programmable frequency synthesizer 1202 for both receiver and transmitter 
(programmed by microprocessor 1302 via I/O circuit and buffers 1218), an 
A/D converter 1304, and a SAT generator 1306, the transceiver may be used 
interchangeably as a scan receiver, as a voice channel transceiver, or a 
signalling channel receiver. This fact allows the BSC 1108 to be relieved 
of the task of making and controlling the process of signal strength 
measurement and SAT detection thereby making possible the use of available 
voice channel transceivers as scanning receivers when a handoff 
measurement request is received from the control terminal 420. The 
transceiver takes cell site characteristics that are downloaded from the 
voice channel controlled of BSC 1108 via the transceiver interface 
communications link. The downloaded information is the cell type in which 
the transceiver is being operated (Omni, Sector) and what kind of function 
the transceiver is to perform in the system: voice channel transceiver, 
scanning receiver, or signalling transceiver. Also, the transceiver used 
for scanning is capable of queueing several handoff measurement requests, 
executing them, and queueing the results. 
Handoff measurement requests that come to a transceiver via the VCC are 
queued automatically and are run as soon as possible. The only reason that 
a handoff measurement request would not run immediately is that it would 
have to wait for a current request to finish execution. Included within 
the handoff measurement request are the channel frequency synthesizer 1202 
programming and the SAT generator 1316 programming. 
When a handoff measurement request comes to a selected transceiver, a flag 
is set that alerts the SSI measurement software to the fact that there is 
a measurement request that is waiting execution in the queue. When the 
measurement request software task runs, it pulls the request out of the 
queue, programs the SAT generator 1316, programs the frequency synthesizer 
1202, and then begins taking measurements. 
Results of the handoff measurement requests are queued in the RAM memory 
1308 of the transceiver microcomputer and await an opportunity to be sent 
uplink to the VCC. That opportunity comes when the VCC polls the 
transceiver for its status. Since handoff measurement responses have 
priority over all other outbound messages from the transceiver, the 
response will go uplink as soon as possible. 
FIG. 14 illustrates the basic block diagram of the BSC 1108 and its 
interconnection to the transceivers. The cell site controller (CSC) 1402 
is the highest level function and cordinates all of the activities at the 
cell site as well as providing the interface between the cell site and the 
telephone central office 422 (mediated by the control terminal 420). 
A more detailed block diagram of the cell site controller (CSC) is shown in 
FIG. 15. A microprocessor such as an MC6802 available from Motorola, Inc., 
is employed as a central processing unit (CPU) 1502 which coordinates all 
of the activities at the cell site in accordance with an operating program 
stored in RAM and EPROM 1504. Six data ports are used to exchange control 
and status messages with the control terminal 420 (via ADLC 1506), one or 
more voice channel controllers (VCC's) (via ADLC 1508), signalling channel 
receiver control (via ADLC 1510), redundant (slave) CSC and/or redundant 
(master) CSC (via ADLC 1512 and 1514 respectively), and a maintenance port 
(via ACIA 1516). All of the aforementioned serial ports in the preferred 
embodiment, except for the maintenance port, are bit-oriented synchronous 
serial data links using a version of the Advanced Data Communications 
Control Procedures (ADCCP) as the communications protocol. The maintenance 
port, used for maintenance and software loading, supports a standard 
asynchronous serial protocol. Additionally, a peripheral interface adapter 
(PIA 1518) supports auxiliary input/output which may be used as a local 
customer interface. 
A voice channel controller (VCC) 1404 may control up to 30 voice 
transceivers and one scanning receiver in the preferred emboidment. A 
redundant VCC 1408 may be employed to provide system redundancy down to 
the voice channel transceiver and double the transceiver capacity. This is 
possible because each transceiver has two communications ports with which 
to communicate with two VCCs. The communications ports on the transceivers 
are embodied within the transceiver microcomputer itself. A signal called 
"XCVREN" (transceiver enable) is used by the VCC to select the port on the 
transceiver that will be used for communications. One VCC will control one 
port and a redundant "partner" VCC will control the other port. The 
transceiver communicates with that VCC which is currently asserting the 
XCVREN signal. Thus it is possible for the second VCC to continue 
controlling a voice transceiver should the first VCC fail. It is even 
possible to reconstruct call activity as the call processing and 
maintenance state of the voice transceiver is continuously being updated. 
If the maintenance state of the channel is "in service", the call 
processing state can be taken from the transceiver and reconstruct the 
call. The call processing and maintenance states of a particular channel 
are stored within the transceiver by sending these states periodically in 
certain messages that are used to control the transceiver by the 
controlling VCC. When another VCC (the partner) takes control of the 
channel, it queries the transceiver as to its call and maintenance states. 
The maintenance sate is used to update a list and the call state is used 
to reconstruct the SAT detection algorithm in the VCC. From the call state 
it can be determined whether to be looking for positive or negative SAT 
detection on the channel. This corresponds to the channel being in a 
conversation state, a connect state, or a disconnect state. 
A detailed block diagram of a voice channel controller (such as VCC 1404) 
which may be employed in the present invention is shown in FIG. 16. The 
central processing unit (CPU 1602) may be a microprocessor such as an 
MC6809 available from Motorola, Inc. This CPU 1602 is used to control the 
cell site voice channel transceivers and scanning receiver(s) in 
accordance with programmed steps stored in RAM and EPROM 1604. In a 
nonredundant configuration of the present invention, VCC 1404 may control 
up to 30 channels of radio equipment and one scanning receiver employing 
SSDA 1606, voice channel transceiver interface 1608, and scanning receiver 
interface 1610. Signalling data encoding and decoding is controlled via 
SSDA 1612 and signalling encoding and recovery interface 1614. 
Transceivers are selected by select logic and line driver circuit 1616 
which is controlled by CPU 1602 via peripheral interface adaptors (PIA 
1618). In the redundant configuration requiring two VCC's, each 
transceiver (being dual-ported) can communicate with both VCCs. Under 
normal operating conditions, each VCC actively controls half of the 
channels while exchanging control and status messages with the other half. 
Interface with the master CSC (if present) via ADLC 1622. 
The signal strength detector and indicator of the present invention 
uniquely employs the two branch diversity receivers and the integral 
microcomputer of the aforementioned transceiver in a manner which enables 
the efficient use of radio equipment at a cellular fixed equipment site. 
This signal strength detector and indicator has previously been briefly 
discussed as the signal strength processing block 1270 of FIG. 12. and the 
microcomputer system of FIG. 13. A similar method of signal strength 
measurement is discussed in U.S. Pat. No. 4,485,486--Webb et al., assigned 
to the assignee of the present invention. 
Regrouping the functions previously described into a functional block 
diagram illustrating the received signal indicator (PSSI) system employed 
by the present invention, the block diagram of FIG. 17 results. This 
configuration in a cellular system is useful when a voice channel receiver 
is required to perform a signal strength measurement in the primary sector 
and its left and right adjacent sectors. The primary antenna is coupled to 
the receiver (branch A) 1110 which produces, inter alia, an analog signal 
representative of the signal strength of the radio frequency signal 
received by the primary (sector) antenna. This analog signal is digitized 
into one of 256 digital levels by A/D converter 1320 and output as an 
8-bit word to a memory location designated bin 0, upper RSSI. (Each sample 
of the A/D converter is stored in one of eight predetermined eight bit 
memory locations of RAM memory 1308 which are designated here bins 0 
through 7. For ease of understanding, the eight bins are mentally 
partitioned into six bins (0 through 5) labeled upper RSSI 1702 and two 
bins (6 and 7) labeled lower RSSI 1704. Although the preferred embodiment 
employs computer mediated storage of RSSI samples, the invention need not 
be so limited as other methods of storage and manipulation may be employed 
without deviating from the scope of the invention). At each pulse output 
from clock 1312, the A/D converter 1320 output is sequentially stored in 
the next bin. 
The radio frequency signal received by the left and right adjacent sectors 
is coupled to receiver (branch B) 1112 via sector switch 1122. The sector 
switch 1122 is toggled between the left and right antenna in synchronism 
with the clock 1312 so that the left and right antennas are alternately 
coupled to the receiver (branch B) 1112. The analog signal strength output 
of receiver (branch B) 1112 is coupled to A/D converter 1322 for 
digitization into one of 256 levels in a manner identical to that employed 
by A/D converter 1320. The output of A/D converter 1322 is subsequently 
coupled as an 8-bit word to bins 6 or 7 of lower RSSI 1704. The digital 
representation of the signal strength received by the right adjacent 
sector antenna is then stored in bin 6 of lower RSSI 1702 and the digital 
representation of the left adjacent sector antenna is stored in bin 7 of 
lower RSSI 1704. Bins 6 and 7 may be filled repetitively as the upper RSSI 
1702 is being filled, or they may be filled once during the filling of the 
six bins of upper RSSI 1702, or a process of signal strength selection may 
be employed without deviating from the scope of the present invention. The 
output of both upper RSSI 1702 and lower RSSI 1704 may be read by 
microprocessor 1302 at the time when all of the six bins of upper RSSI 
1702 have been filled with a signal strength value. The process of 
microprocessor 1302 interpretation of the signal strength processing 
circuit 1270 will be described later. 
A receiver which would make particularly beneficial use of this process is 
one which would be part of a voice channel transceiver. Uniquely, each 
voice transceiver would be able, in the present invention, to determine 
the signal strength of a remote unit being served by this transceiver (and 
the signal strength in the adjacent sectors) without requiring a special 
scan by another scanning receiver. 
An alternative to the sectorized signal strength described in FIG. 17 is an 
omnidirectional signal measurement with diversity antennas such as that 
illustrated in FIG. 18. A primary antenna 1802 is coupled to receiver 
(branch A) 1110 and the analog signal strength output is digitized by A/D 
converter 1320 and stored in the six bins of upper RSSI 1702 as described 
previously. Similarly, a secondary diversity antenna 1804 is coupled to 
receiver (branch B) 1112 and an analog signal strength output is coupled 
to A/D converter 1322 and stored in bins 6 and 7 of lower RSSI 1704. At 
the end of the bin filling cycle of upper RSSI 1702, the microprocessor 
may read the output of the bin storage locations and process the signal 
strength information. 
A third alternative may employ the signal strength measurement of the 
present invention to measure six sector antennas as shown in FIG. 19. Each 
of the six antennas may be sequentially coupled to receiver (branch A) 
1110 by a 6:1 multiplexor 1904. This multiplexor 1904 may be a 
conventional arrangement of PIN diodes or other switching elements which 
may be addressed by the microprocessor 1302. In the preferred embodiment 
of the present invention, three address lines are coupled to multiplexor 
1904 and switch antennas coupled to receiver (branch A) 1110 in 
synchronism with clock 13120. As described for previous alternatives, the 
signal strength output is digitized by A/D converter 1320 and coupled to 
upper RSSI 1702. The output from each particular antenna is stored in a 
predetermined bin so that it is possible for the microprocessor 1302 to 
correlate stored signal strength information to a particular antenna. The 
stored information may be read by microprocessor 1302 and processed as 
described later. 
The supervisory audio tone (SAT) detector 1306 compares the received SAT 
signal to a locally generated tone of a frequency determined by command of 
the microprocessor 1302 and control terminal 420, as described previously. 
If a match in SAT frequency is found, the SAT detector 1306 provides a 
detect output on the microprocessor 1302 bus which may be utilized as part 
of a signal strength measurement report. 
Referring now to FIGS. 20a and 20b, the process by which a signal strength 
measurement is made is shown in flowchart form. The frequency synthesizer 
1202 of the receiver system is programmed with the channel upon which the 
signal strength measurement is to be made and the SAT detector 1306 is 
programmed with the proper SAT frequency at step 2002. The pass count and 
starting antenna number are initialized at 2004. The process waits for a 
predetermined period of time (1 millisecond in the preferred embodiment) 
at 2006 for the receiver to settle before making an A/D conversion of the 
signal strength output from receiver (branch A) 1110 and storing the 
result in the upper RSSI bin indexed by the antenna number, at 2008. A 
similar measurement is made on the signal strength output from receiver 
(branch B) 1112 and stored in the lower RSSI bin indexed by the sum of 6 
and the least significant bit of the antenna number, at 2010. The antenna 
number is then incremented and the multiplexer 1904 is incremented (if 
used) at 2012. 
If the antenna number is not equal to six, as determined at 2014, the A/D 
conversion and storage is repeated for the next antenna. If the antenna 
number is equal to six (indicating that six antenna samples have been 
taken), the pass count is incremented and the antenna number is reset to 
zero at 2016. The number of passes is determined at step 2018 and the 
process is repeated if the pass number does not equal "N". "N" is 
determined by the system design and whether the receiver is a voice 
channel transceiver making a signal strength measurement on its assigned 
voice channel or whether the receiver is a voice channel transceiver 
assigned as a scanning receiver to a scan event. If the system has 
capacity for determining whether the stored signal strength in a 
particular bin is less than the currently measured strength, a plurality 
of passes enables the signal strength measurement to remove signal 
strength fluctuations caused by Rayleigh fading and provide a more 
reliable measurement of signal strength. In the preferred embodiment, "N" 
equals four for a voice channel receiver and two (for time considerations) 
in a scanning receiver. 
If the pass determination equals "N", a subroutine 2020 determines which 
antenna bin contains the strongest and second strongest signal strength. A 
determination that the measured signal strength is greater than zero is 
made at 2022. (If the signal strength is zero, a message so indicating is 
sent to the voice channel controller at 2024). To ensure that the proper 
remote unit is being measured, the SAT frequency is measured from the 
strongest antenna by setting the receiver system to the antenna having the 
strongest measured signal (at 2026). The process then waits a period of 
time at 2028 for the SAT detector 1306 to settle (which may be 60 
milliseconds in the preferred embodiment) before determining whether a SAT 
frequency match has occurred (at 2030). If a mismatch has occurred, the 
voice channel controller is notified at 2032; if a match has occurred, a 
message is sent to the voice channel controller at 2034 that the SAT is 
correct and providing the signal strength information. 
The strongest and second strongest antenna signal is determined by the 
process of FIG. 21. A determination of whether the receiver is a three 
sector receiver (such as that of FIG. 17), an omnidirectional diversity 
receiver (FIG. 18), or a six sector receiver (FIG. 19) is made at 2102. If 
the receiver is a six sector receiver the contents of lower RSSI bins 6 
and 7 are set to zero at 2104 (since they contain no useful information in 
this system configuration). If the receiver is a three sector or an 
omnidirectional receiver, the upper RSSI bins 1 through 5 are set to zero 
at 2106 since the information redundant with the contents of bin 0. The 
contents of each bin are compared by conventional comparison techniques to 
determine the bin having the strongest signal strength at 2108. The 
representation of the antenna receiving the strongest signal is also saved 
at 2110 and the corresponding bin is set to zero at 2112. The bin having 
the second strongest signal strength and the corresponding antenna 
representation is determined at 2114 before all bins are zeroed out at 
2116. Thus this process produces a record of the value of the strongest 
and second strongest received signal strengths and a record of which 
antennas received the strongest and second strongest signals. This 
information may be used by the voice channel transceiver to determine when 
the received signal strength may be weak enough to request a handoff from 
the CSC or central controller or the information may be queued and 
transmitted to the VCC for use in reporting the results of a handoff 
measurement request. 
In summary, then, the method and apparatus for measuring signal strength 
related to a particular sector antenna in a voice channel transceiver of a 
cellular radiotelephone system has been described. A novel voice channel 
transceiver may be employed as a temporary scanning receiver or as a 
signalling transceiver. In order that common equipment may be used for 
each function, a versatile signal strength measurement process capable of 
each type of cellular antenna configuration has been developed. Each 
transceiver has two receiver portions which may be coupled to two or more 
antennas. The signal strength received by each receiver portion is output 
in analog form to analog to digital converters which develop digital 
representations of the analog outputs. The digital representations are 
synchronously stored in memory location bins such that the contents of 
each bin may be correlated with the antenna which was used to receive the 
signal. The content of the bins may be read after all the antennas have 
been sampled by the receivers to determine the magnitude of the strongest 
and second strongest signal and the antennas which received the strongest 
and second strongest signals. Therefore, while a particular embodiment of 
the invention has been shown and described, it should be understood that 
the invention is not limited thereto since modifications unrelated to the 
true spirit and scope of the invention may be made by those skilled in the 
art. It is therefore contemplated to cover the present invention and any 
and all such modifications by the claims of the present invention.