Remote wireless unit having reduced power operating mode

A remote unit for a personal wireless area network includes a receiver, an AC power supply, a battery-backup power supply and a controller. The battery-backup becomes operative when the AC power supply fails and supplied power to the receiver. The controller detects when the AC power supply fails and controls the receiver and the battery-backup power supply by invoking a sleep mode of operation. The sleep mode of operation is periodically interrupted by the controller controlling the receiver and the battery-backup power supply to enter a standby mode of operation in which the receiver scans for a CONNECT message from a base station indicating an incoming call. The controller coordinates the sleep mode and the standby mode of operations based on a frame count that is generated from an identification number of the remote unit. A highly bandwidth-efficient communications method is employed in the base station to enable it to coordinate communication with the remote unit when it changes from the sleep mode to the standby mode.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The present invention relates to improvements to communications systems. 
More particularly, the present invention relates to wireless discrete 
multitone spread spectrum systems. 
The remote units are powered primarily from AC power sources and include a 
battery for providing battery backup power when AC power fails. To 
conserve battery power, an RU has a sleep mode of operation with periodic 
power-up modes for checking whether any calls are attempting to be 
connected to the RU. When an RU is in a sleep mode, it expedient that the 
system operate in such a way so that appropriate actions are taken for 
completing a call to a sleep mode RU. 
One approach for ensuring that calls are completed to a remote unit 
operating in a sleep mode is to maintain a database at a central location 
that stores the current operating mode of each remote in the system. When 
a remote unit enters a sleep mode of operation, the remote unit reports 
the change of operational status to the database. Similarly, the remote 
unit reports a change of status back to a standby operating mode. This 
approach has a drawback when a number of remote units recorded in the 
database experience frequent power outages. In such a situation, 
recording, managing and synchronizing power outage information in the 
database is particularly cumbersome when the database is large, perhaps 
holding status information for 3 to 4 thousand remote units. This drawback 
is further compounded when the database is duplicated multiple times 
throughout the system. When several thousand subscribers experience a 
power outage and AC power is restored before the database has completed 
recording the power outage, a database approach becomes unwieldy. Another 
complicated situation is when multiple remote units lose power at the same 
time. The affected remote units cannot all access the channel 
simultaneously for communicating their status to the database. A collision 
avoidance scheme must be implemented that spans a period of time and that 
is open for the possibility of power being restored before the database 
has been completely revised. 
This approach has another drawback in that a remote unit entering the sleep 
mode consumes system bandwidth in notifying the database. FIG. 4 shows an 
exemplary flow of internal messaging that occurs between various layers of 
a remote unit when loss of AC power is detected and a database is notified 
of the operational status change. Time is shown along the vertical axes of 
FIG. 4, with advancing time being indicated toward the bottom of FIG. 4. 
In FIG. 4, four layers of the remote unit operating system are shown: 
Health; OAM&P (Operations, Administration, Maintenance & Provisioning), 
MAC (Media Access Control) and physical. Only MAC layer of the base 
station is shown. At 40, AC power failure is detected by the Health layer. 
At 41, an EVENT message is sent from the Health layer to the OAM&P layer 
indicating that AC power has failed. The OAM&P layer sends an ACTION 
message to the MAC layer at 42. The MAC layer responds at 43 by sending an 
ACTION.sub.-- RSP message to the OAM&P layer indicating that base station 
notification is pending. At 44, the MAC layer waits a random length period 
of time before sending an unsolicited CAC message at 45 to the MAC layer 
of the base station indicating the need for the remote unit to enter the 
sleep mode. At 46, the MAC layer of the base station sends an 
acknowledgment message to the MAC layer of the remote unit acknowledging 
receipt of the unsolicited CAC message. In response, the MAC layer of the 
remote unit sends an EVENT message at 47 to the OAM&P layer that the 
notification is done. The OAM&P layer first sends an EVENT message to the 
MAC layer indicating that the sleep mode has been entered at 48, and then 
sends a message at 49 to the physical layer to power down. 
What is needed is a way for a PWAN system to be aware that a remote unit is 
operating in a sleep mode so that appropriate actions can be taken by the 
system so that calls can be completed to a remote unit operating in a 
sleep mode. 
SUMMARY OF THE INVENTION 
The present invention provides a method for reducing power consumption of a 
remote unit in a PWAN system. A remote unit is powered using a battery 
backup power supply when an AC power supply fails at the remote unit. A 
sleep mode of operation is entered at the remote unit that has a reduced 
power consumption for the battery backup power supply. The remote unit is 
synchronized to a TDD timing structure a predetermined period of time 
after entering the sleep mode of operation. A standby mode of operation is 
then entered at the remote unit in which a CONNECT message indicating an 
incoming call for the remote unit is scanned for by the receiver. When no 
CONNECT message is received, the remote unit reenters the sleep mode of 
operation. According to the invention, the predetermined period of time is 
a predetermined number of subframes after a boundary subframe of the TDD 
timing structure. Preferably, the predetermined number of subframes is 
based on an identification number of the remote unit. 
The present invention also provides a remote unit for a personal wireless 
area network that includes a receiver, an AC power supply, a 
battery-backup power supply and a controller. The battery-backup becomes 
operative when the AC power supply fails and supplies power to the 
receiver. The controller detects when the AC power supply fails and 
controls the receiver and the battery-backup power supply by invoking a 
sleep mode of operation. The sleep mode of operation is periodically 
interrupted by the controller controlling the receiver and the 
battery-backup power supply to enter a standby mode of operation in which 
the receiver scans a CONNECT message indicating an incoming call. The 
controller coordinates the sleep mode and the standby mode of operations 
based on a frame count that is generated from an identification number of 
the remote unit. 
In accordance with another aspect of the invention, a highly 
bandwidth-efficient communications method is disclosed for the base 
station to enable it to communicate with a remote unit that is in the 
sleep mode. The remote unit has a unique identification value that is 
different from the identification value of other remote units that may be 
communicating with the base station. The base station begins by 
establishing a periodic reference instant at the base station and at the 
remote station. Then the base station determines a delay interval 
following the periodic reference instant at the base station, the delay 
interval being derived from the unique identification value of the remote 
unit. The base station receives spread signals from the remote units with 
which it communicates, each comprising an incoming data traffic signal 
spread over a plurality of discrete traffic frequencies. The base station 
adaptively despreads the signals received it receives by using despreading 
weights. The base station attempts to initiate a communication with the 
remote unit that is currently in the sleep mode. If the attempting step 
fails to initiate communications with the remote unit, the base station 
concludes that the remote unit is in the sleep mode. In response to this, 
the base station waits for the delay interval following the periodic 
reference instant at the base station before transmitting to the remote 
unit. The base station then transmits to the remote unit a spread signal 
comprising an outgoing data traffic signal spread over a plurality of 
discrete traffic frequencies. The remote unit has simultaneously changed 
from the sleep mode to the standby mode and is able to receive and respond 
to the spread signal transmitted from the base station. 
In accordance additional aspects of the invention, the base station is part 
of a wireless discrete tone communications system. Further, the periodic 
reference instant is established by a beginning subframe count instant 
that is incremented by a packet count value at the base station and at the 
remote unit. In addition, the delay interval is determined by a value N of 
a quantity of M least significant bits of the unique identification value 
of the remote unit, the delay interval being an interval required for the 
occurrence of a plurality of N of the beginning subframe count instants. 
The resulting invention enables the base station to be aware that a remote 
unit is operating in a sleep mode so that appropriate actions can be taken 
by the base station to assure that calls can be completed to the remote 
unit.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows an architectural diagram of the personal wireless access 
network (PWAN) system described in the referenced Alamouti et al. patent 
application and which is the environment of the present invention. Two 
users, Alice and Bob, are located at a remote station unit, or remote unit 
(RU), X and wish to transmit their respective data messages to a base 
station Z. Remote unit X is positioned to be equidistant from each of 
antenna elements A, B, C, and D at base station Z. Two other users, Chuck 
and Dave, are located at a remote station unit Y and also wish to transmit 
their respective data messages to base station Z. Remote unit Y is 
geographically different from remote unit X and is not equidistant from 
each of antenna elements A, B, C, and D of base station Z. Remote units X 
and Y, and base station Z use a form of the PWAN protocol known as 
discrete tone which is used for providing efficient communications between 
base stations and remote units. The protocol is indicated in FIG. 1 as a 
discrete tone. 
In the protocol, a user data signal is modulated by a set of weighted 
discrete frequencies or tones. The weights are spreading weights that 
distribute the data signal over many discrete tones covering a broad range 
of frequencies. The weights are complex numbers having a real component 
that is used for modulating the amplitude of a tone and a complex 
component that is used for modulating the phase of the same tone. Each 
tone in the weighted-tone set bears the same data signal. Plural users at 
a transmitting station can use the same tone set for transmitting their 
data, but each of the users sharing the tone set has a different set of 
spreading weights. The weighted-tone set for a particular user is 
transmitted to the receiving station where it is processed with 
despreading weights that are related to the user's spreading weights for 
recovering the user's data signal. For each of a plurality of spatially 
separated antennas at the receiver, the received discrete tone signals are 
transformed from time-domain signals to frequency-domain signals. 
Despreading weights are assigned to each frequency component of the 
signals that are received by each antenna element. The values of the 
despreading weights are combined with the received signals for obtaining 
an optimized approximation of individual transmitted signals characterized 
by a particular discrete tone set and transmitting location. 
The PWAN system has a total of 2560 discrete tones (carriers) that are 
equally spaced in 8 MHZ of available bandwidth in the frequency range of 
1850 to 1990 MHZ, with a spacing between the tones of 3.125 KHz. The tones 
are used for carrying traffic messages and overhead messages between the 
base station and the plurality of remote units. The total set of tones are 
numbered consecutively from 0 to 2559, starting from the lowest frequency 
tone. The tones used for traffic messages are divided into 32 traffic 
partitions, with each traffic channel requiring at least one traffic 
partition of 72 tones. 
The overhead message tones are used for establishing synchronization and 
for passing control information between base stations and remote units. A 
Common Link Channel (CLC) is used by a base station for transmitting 
control information to remote units. A Common Access Channel (CAC) is used 
by a remote unit for transmitting messages to the base station. There is 
one grouping of tones assigned to each channel. The overhead channels are 
used in common by all remote units when control messages are exchanged 
with a base station. 
Transmission from a base station to a remote unit called "forward 
transmission" and transmission from a remote unit to a base station is 
called "reverse transmission". Time Division Duplexing (TDD) is used by 
base stations and remote units for transmitting data and control 
information in both directions over the same discrete tone frequency 
channel. The time between recurrent transmissions in either direction is 
called a TDD period which, is equal to 3 ms. For every TDD period, there 
are four consecutive transmission bursts in each direction. Data is 
transmitted during each burst using multiple tones. The base station and 
each remote unit synchronize and conform to a TDD timing structure and 
framing structure that has 1 frame equal to 8 subframes and 1 subframe 
equal to 2 TDD periods. A superframe is 256 subframes, or 1536 ms. All 
remote units and base stations are synchronized such that all remote units 
transmit simultaneously and then all base stations transmit 
simultaneously. When a remote unit initially powers up, it acquires 
synchronization from a base station so that control and traffic messages 
can be exchanged within the prescribed TDD time format. A remote unit must 
also acquire frequency and phase synchronization for the DMT-SS signals so 
that the remote unit is operating at the same frequency and phase as an 
associated base station. 
Selected tones within each tone set are designated as pilot tones that are 
distributed throughout the frequency band and carry known data patterns 
for enabling an accurate channel estimation. A series of pilot tones, 
having known amplitudes and phases, are spaced apart in frequency by 
approximately 30 KHz for providing an accurate representation of a channel 
response over the entire transmission band, that is, the amplitude and 
phase distortion introduced by the communication channel characteristics 
over the transmission band. 
FIG. 2 shows an architectural diagram of remote station X operating as a 
sender station. Alice and Bob each input data to remote station X. The 
data is sent to a vector formation buffer 202and also to a cyclic 
redundancy code generator. Data vectors are output from buffer 202 to a 
trellis encoder 206. The data vectors are in the form of a data message 
formed by concatenating a 64 K-bit data block with a serially assigned 
block number. CRC generator 204 generates LCC vectors that are output to 
trellis encoder 206. The LCC vectors are in the form of an error detection 
message formed by concatenating a CRC value with the serially assigned 
block number of the data block. The trellis encoded data vectors and LCC 
vectors are then output to a spreading processor 208. The resultant data 
tones and LCC tones are then output from processor 208 to a transmitter 
210 for transmission to the base station. 
The personal wireless access network (PWAN) system is described in the 
cross-referenced Alamouti et al. patent application. A base station 
transmits information to multiple remote units that are located in the 
base station's cell. 
FIG. 3 is an architectural block diagram of remote station X operating as a 
receiving station. Data tones and LCC tones are received by remote station 
antenna X and a receiver 610. Receiver 610 passes the data tones and the 
LCC tones to a spectral despreading processor 612 which despreads the data 
tones and LCC tones. The despread signals are then output from processor 
612 to a trellis decoder 614. Trellis decoder 614 generates data vectors 
from the despread signals. The data vectors are then output to a vector 
disassembly buffer 616. Data for Alice and data to Bob are output from 
buffer 616 to Alice and Bob, respectively. Data for Alice and Bob are also 
input to a CRC generator 618. CRC generator 618 computes a new CRC value 
for every 64 K-bit data block and outputs the new CRC value with the block 
number to a buffer within a CRC comparison processor 620. The receiving 
station buffers error detection messages that are received from the link 
control channel in CRC comparison processor 620so that the error detection 
messages are accessible by their block numbers N, N+1, N+2, etc. When the 
receiving station receives a data message on the traffic channel, it 
performs a CRC calculation on the data block in the message with CRC 
generator 618 for obtaining a resulting new CRC value. If the comparison 
determines that there is a difference in the values, then an error signal 
is generated by an error signal generator 622. The error signal can be 
processed and used in several ways by an error processor 630. For example, 
the error signal can initiate a negative acknowledgment signal that is to 
be sent from the receiving station back to the sender station requesting 
that the sender repeat transmission of the data block. The error signal 
can also initiate an update in spreading and despreading weights at the 
receiving station for improving the signal-to-interference and noise ratio 
of the traffic channel. Another use of the error signal is for initiating 
an alarm used for other real time control. Yet another use of the error 
signal is as part of a logging signal for compilation of a long term 
report relating to traffic channel quality. 
According to the invention, a remote unit includes a standby mode of 
operation and a sleep mode of operation. Normally, the standby mode is the 
mode in which a remote unit scans the CLC channel for a CONNECT message 
for the remote unit. The sleep mode of operation provides a reduced power 
consumption operating mode for extending remote unit battery runtime 
during an AC power outage condition. During the sleep mode of operation, 
the remote unit periodically switches between the standby mode and sleep 
mode, with the overall effect being a reduction in the average power 
required by the remote unit. 
Delivery of a CONNECT message to a remote unit operating in the sleep mode 
is scheduled so that the remote unit is in the standby portion of the 
sleep mode. That is, the remote unit is synchronized and ready for 
receiving data from the CLC when the base station begins transmitting on 
the CLC. In order to achieve synchronization, a system wide Packet Count 
(PKT.sub.-- CNT) is used. The basic unit of measure for synchronization is 
a mod[8] PKT.sub.-- CNT, which is called a subframe count (SUBFRM.sub.-- 
CNT). The SUBFRM.sub.-- CNT is incremented every 256 PKT.sub.-- CNTs, or 
every 6 ms. 
The base station and the remote unit both preferably use the least 
significant 8 bits of the remote unit ID for determining the particular 
SUBFRM.sub.-- CNT at which the CLC CONNECT message should be sent to the 
remote unit and, simultaneously, the appropriate time at which the remote 
unit should be in the standby portion of the sleep mode for receiving the 
CONNECT message. When the least significant 8 bits of the remote unit ID 
are used, the remote unit enters the standby mode once every 256 subframes 
and is ready for receiving an incoming call. The particular subframe that 
a remote unit will be ready for receiving an incoming call is called the 
N.sub.listen for the remote unit. 
To avoid using a remote unit power status database that is maintained at a 
central location, the sleep mode features of the present invention are 
preferably implemented as part of a standard terminating call retry 
mechanism. That is, when a terminating call request is received at the 
base station MAC Layer, the MAC Layer Access Manager proceeds normally 
through a terminating call setup procedure by transmitting a CONNECT 
message on the CLC to the target remote unit. In the situation when the 
target remote unit is operating in the sleep mode at the time of the 
CONNECT message transmission, the remote unit will generally be unable to 
process the message. The base station MAC Layer Access Manager will 
time-out and retry transmission of the CONNECT message. Preferably, a 
retry timer T.sub.r is nominally set to 72 ms. The base station MAC Layer 
Access Manager retries the CONNECT message for a predetermined number of 
tries that is set by a system manager. Preferably, the retry count is 2. 
When the number of retries equals the retry count, the base station MAC 
Layer Access Manager determines that the remote unit is in the sleep mode 
and, consequently, attempts to deliver the CONNECT message at a scheduled 
time that is based on the target remote unit ID. The scheduled time is a 
subframe occurring N.sub.listen subframes after the boundary subframe for 
the TDD timing structure. 
The base station MAC Layer Access Manager also reserves the CLC slot(s) 
required for completing the CLC CONNECT message transmission at the time 
the N.sub.listen subframe number is derived. That is, when the base 
station MAC Layer Access Manager has reached its retry count for a CONNECT 
message and has determined the N.sub.listen subframe, CLC slot 
availability is examined for reserving the appropriate CLC slot(s) for 
use. As an alternative, a remote unit can scan up to 3 CLC slots for a 
CONNECT message when in the sleep mode so that a base station can select 
from 3 CLC slots in case a specific slot is unavailable. 
FIG. 5 shows a message flow sequence for a terminating call for the 
situation when a target remote unit is operating in the sleep mode. The 
MAC Layer of the base station receives a terminating call request 50. At 
51, the MAC Layer of the base station sends a CLC CONNECT message to the 
target remote unit. Since the remote unit is in the sleep mode, it does 
not receive the CLC CONNECT message and, therefore, does not respond. 
Since there is no response from the target remote unit during the 
T.sub.retry period 52, the MAC Layer of the base station sends a second 
CLC CONNECT message to the target remote unit at 53. The remote unit does 
not respond during T.sub.retry 54, so the MAC Layer of the base station 
determines the N.sub.listen subframe for the remote unit using the least 
significant 8 bits of the remote unit ID and waits for the particular 
N.sub.listen subframe at 55. At N.sub.listen for the remote unit, the MAC 
Layer of the base station sends a CLC CONNECT message at 56. At 
N.sub.listen, the remote unit is in the standby mode and ready to receive 
the CLC CONNECT message at 57. In response, the remote unit MAC Layer 
sends a CAC.sub.-- ACK message to the base station at 58. 
The following definitions are used for describing the sleep mode of 
operation of the present invention: 
T.sub.sleep =the time that a remote unit is in a low-power mode (i.e., 
sleeping). 
T.sub.sync =the time required by a remote unit for re-acquiring 
synchronization when exiting the sleep mode. 
T.sub.scan.sbsb.--.sub.clc =the time that a remote unit is operating in a 
standby mode scanning the CLC for a CONNECT message. 
T.sub.standby =the total time a remote unit is running (i.e., T.sub.sync 
+T.sub.scan.sbsb.--.sub.clc) 
D.sub.sleep =T.sub.sleep +T.sub.standby)/T.sub.standby, that is, the 
definition of the duty cycle of the sleep mode duty cycle. 
Since the base station transmits the CONNECT message at the N.sub.listen 
subframe so that the call can be completed, and the remote unit therefore 
must be ready for receiving the messages on the CLC channel at the 
N.sub.listen subframe. The remote unit MAC Layer Access Manager is capable 
of deriving the N.sub.start.sbsb.--.sub.sync subframe number and insures 
that all hardware required for the remote unit synchronization and CLC 
scanning efforts are released from sleep mode at that time. This is done, 
for example, by using a programmable hardware counter 640 that is clocked 
in synchronism with the TDD subframe of the system, as shown in FIG. 3. 
Prior to entering the sleep mode, or at the time the sleep mode is 
entered, CPU 650 preferably uses the least significant 8 bits of the 
remote unit ID for determining the N.sub.listen subframe for the remote 
unit. CPU 650 loads counter 640 with a value related to N.sub.listen and 
synchronizes counter 640 using a Start Sync signal. Counter 640 provides 
an interrupt to CPU 650 once every 256 subframes, initiating a 
re-synchronization process. CPU 650 responds by controlling power supply 
660 to provide power 661 to the various components of remote unit used for 
receiving a CLC CONNECT message. CPU 650 also outputs an enabling signal 
to the despreading processor 612 to enable the remote unit to receive 
messages from the base station. 
The remote unit begins its re-synchronization effort at a subframe 
N.sub.start.sbsb.--.sub.sync that occurs some determined period of time 
prior to the occurrence of the N.sub.listen subframe. Simulations of the 
remote unit synchronization algorithms indicate that a remote unit 
acquires synchronization with a base station when exiting a period of 
sleep in a minimum time of 122 ms and a maximum time of 200 ms. The actual 
time additionally depends on hardware component tolerances, the ambient 
temperature and numerous other factors. For the purposes of this 
disclosure, a worst case synchronization acquisition time T.sub.sync of 
200 ms is used. This equates to approximately 34 subframes. Therefore, 
N.sub.start.sbsb.--.sub.sync =N.sub.listen -34 subframes. 
FIG. 6 is a timing diagram showing the sequence of events for a remote unit 
operating in the sleep mode. Each vertical line in FIG. 6 represents a 
subframe boundary. The time between each subframe boundary is 6 ms. A 
remote unit is shown as being in a sleep mode. At 
N.sub.start.sbsb.--.sub.synch, counter 640 sends an interrupt request to 
CPU 650 (FIG. 3). CPU 650 responds by controlling power supply 660 to 
provide power to the various components of the remote unit needed for 
receiving a CLC CONNECT message. In FIG. 6, the remote unit is in the 
sleep mode at 60. At 61, N.sub.start.sbsb.--.sub.sync occurs and the 
remote unit resynchronizes for a number of subframes. Preferably, about 34 
subframes are required for a remote unit to reacquire synchronization. At 
N.sub.listen , the remote unit scans the CLC channel for any CLC CONNECT 
messages for the remote unit. The remote unit scans for 2 subframes, as 
shown in FIG. 6 at 62. The remote unit can also be set to scan for a CLC 
CONNECT message over a different number of subframes other than 2 
subframes depending upon system requirements. If no CLC CONNECT message is 
received at N.sub.listen , the remote unit returns to the sleep mode at 
63.If a CLC CONNECT message is received, the call is established in a 
normal manner. 
As a first illustrative example of the timing aspects of the sleep mode of 
the present invention, the least significant 8 bits of a remote unit ID 
are used so that the N.sub.listen cycle time is 1536 ms (256.times.6 ms). 
The remote unit synchronization acquisition time N.sub.sync is estimated 
to be 34 subframes (204 ms), and a CLC scan time for 2 CLC subframes is 
chosen. It follows that, 
T.sub.sleep =220 subframe times=220.times.6 ms=1.320 s 
T.sub.sync =204 ms=34 subframes.times.6 ms 
T.sub.scan.sbsb.--.sub.clc =12 ms=2 subframes.times.6 ms 
T.sub.standby =212 ms 
Therefore, the total sleep mode/standby mode cycle time is 1536 ms, and the 
total remote unit power-on time is 212 ms. The overall duty cycle is 
7.25:1. For this example, the maximum delay for delivery of a CONNECT 
message is 1.530 seconds (1536 ms-6 ms). The nominal CONNECT message delay 
delivery time is about 0.766 seconds. 
Using a longer delay in CONNECT message delivery time permits the remote 
unit to be in the sleep mode for a greater period of time. As another 
example, the N.sub.listen subframe is determined by using the least 
significant 9-bits of a remote unit ID. Thus, the N.sub.listen interval is 
512 subframes. In this example, even though the sleep time is longer, the 
maximum synchronization acquisition time T.sub.sync remains the same. This 
is based on the fact that any temperature change of the remote unit is not 
sufficient for requiring a coarse TDD synchronization to be performed. It 
follows that, 
T.sub.sleep =476 subframe times=476.times.6 ms=2.856 s 
T.sub.sync =204 ms=34 subframes.times.6 ms 
T.sub.scan.sbsb.--.sub.clc =12 ms=2 subframes.times.6 ms 
T.sub.standby =212 ms 
The total sleep mode/standby mode cycle time is 3072 ms (512.times.6 ms), 
and the total remote unit power-on time is 212 ms. The overall duty cycle 
is 14.5:1. For this example, the maximum delay of delivery of a CONNECT 
message is 3.066 seconds (3072 ms-6 ms). The nominal time for delivery of 
a CONNECT message is about 1.536 s. 
Table I below summarizes various scenarios: 
TABLE I 
______________________________________ 
Nominal CLC Battery 
Sleep 
CONNECT Runtime 
Time message delay 
Synchronization 
Duty Cycle 
(approx. 
(ms) time (ms) 
Time (ms) 
hrs) 
______________________________________ 
1320 663 220 7.25:1 11.5 
1320 663 12 
2856 1428 150 
13.5 
2856 1428 200 
13 
2856 1428 300 
12 
______________________________________ 
The situation of a call originating from a remote unit that is operating in 
the sleep mode is straight forward compared to the situation when a call 
terminates at a sleeping remote unit. That is, the remote unit exits the 
sleep mode in response to a user command. The originating call delivery 
time, i.e., the time taken for delivering an ACCESS message on is delayed 
by approximately 200 ms since the remote unit must re-acquire 
synchronization before the ACCESS message may be transmitted. 
In normal system operation, a base station polls remote units at a periodic 
rate for determining status of each remote unit. Each remote unit responds 
to the Poll Request message with a Poll response message using the CAC 
channel. When a remote unit is in a sleep mode of operation, the Poll 
Request message will not be received and, consequently, the remote unit 
will not respond with a Poll Response message. The present invention 
provides two alternatives for handling such a situation from the system 
point of view. The first approach is to always schedule a Poll Request 
message to arrive at a remote unit during the N.sub.listen subframe for 
the remote unit whether the remote is in the standby or the sleep mode. 
The remote unit will receive the Poll Request message regardless of AC 
power status. A disadvantage associated with this approach is that the CAC 
channel is used by the remote unit for a Poll Response message, causing 
the remote unit transmitter to be used, effectively wasting battery power 
when in the sleep mode. 
The alternative approach is for a remote unit to ignore the Poll message 
from the base station during AC power outage situations and allow an OAM&P 
Layer at the base station to recognize that a non-responsive remote unit 
may possibly be in the sleep mode and, consequently, be aware of the power 
status of the remote unit in questions power. 
FIG. 7 is an exemplary graph showing Battery Operating Time, measured in 
hours, for Sleep Mode Duty Cycle (:1). From FIG. 7, it is apparent that 
the length of time that a remote unit is sleeping has a significant impact 
on the run time of the battery. Also, from FIG. 7, it is also apparent 
that the battery run time begins to flatten with duty cycle after about a 
10:1 ratio. Lab results for simulated sleep mode operation with a new, 7.2 
amp-hour battery installed in a prototype uniterruptabl power supply have 
yield runtimes between 12 hours, 12 minutes to 12 hours, 32 minutes under 
the conditions that the remote unit is at room temperature, the sleep mode 
period is set for 3 seconds, and the sleep mode duty cycle is 10:1 (0.3 s 
standby state and a 2.7 s sleep state). 
A remote unit operating in the sleep mode preferably provides the following 
characteristics: 
Sleep time=2856 ms 
RU Synchronization Time=200 ms 
Call delivery delay=1428 ms nominally 
RU CLC Scan time=36 ms (i.e., three slots for flexibility at Base MAC 
Layer) 
Total Cycle Time=3092 ms 
Standby Time=236 ms 
Duty Cycle=13:1 (approx.) 
Battery Operating Time=12.5 hours (approx.) 
FIG. 8 is an architectural diagram of the base station as a sender. The 
PSTN inputs data to base station Z. The data is sent to the vector 
formation buffer 502 and also to the cyclic redundancy code generator 504. 
Data vectors are output from buffer 502 to the trellis encoder 506. The 
data vectors are in the form of a data message formed by concatenating a 
64 K-bit data block with its serially assigned block number. The LCC 
vectors output from the CRC generator 504 to the trellis encoder 506 are 
in the form of an error detection message formed by concatenating the CRC 
value with the block number. The trellis encoded data vectors and LCC 
vectors are then output to the spatial spreading processor 508. The 
resultant data tones and LCC tones are then output from processor 508 to 
the transmitter 210 for transmission to the remote station. 
The base station transmits the CONNECT message at the N.sub.listen subframe 
so that the call can be completed to the remote unit. The base station 
knows to send the messages on the CLC channel at the N.sub.listen 
subframe. The base station's MAC Layer Access Manager is capable of 
deriving the N.sub.start.sbsb.--.sub.sync subframe number. This is done, 
for example, by using a programmable hardware counter 540 that is clocked 
in synchronism with the TDD subframe of the system, as shown in FIG. 8. 
When the base station wants to send a message to the remote unit, the CPU 
550 preferably uses the least significant 8 bits of the remote unit ID for 
determining the N.sub.listen subframe for the remote unit. CPU 550 loads 
counter 540 with a value related to N.sub.listen and synchronizes counter 
540 using a Start Sync signal. Counter 540 provides an interrupt to CPU 
550once every 256 subframes, initiating a re-synchronization process. CPU 
550 responds by outputting an enabling signal to the spatial spreading 
processor 508 to enable the base station to transmit messages to the 
remote unit when the remote unit is in its standby mode. 
Although the preferred embodiments of the invention have been described in 
detail above, it will be apparent to those of ordinary skill in the art 
that obvious modifications may be made to the invention without departing 
from its spirit or essence. Consequently, the preceding description should 
be taken as illustrative and not restrictive, and the scope of the 
invention should be determined in view of the following claims.