CA2002603C - Energy saving protocol for a tdm radio - Google Patents

Abstract

ENERGY SAVING PROTOCOL FOR A TDM RADIO

ABSTRACT of the DISCLOSURE
A battery powered TDM (or combined TDM/FDM) communication device (104,105) operates within a TDM
communication system (100) having a communication channel arranged into repetitive time frames (401) each having a plurality of slots. At least two of these slots (404) are allocated for communicating signalling and control information. To conserve energy, the battery powered communication device operates in a first mode (i.e., the energy saving mode) to shut down non-essential circuits for a predetermined time interval, awaking to monitor one of the two control slots. In this way signalling and control information may be received. When not operating in the energy saving mode, the battery powered communication device operates in a second mode (i.e., a fast access mode) to monitor at least two of the control slots (404).

Description

~2~3 F.N~RG~AVING PROTOCOL FOR A TDM RADIO

35 Tl~CHNICAL FIELD of the INVENTION
This invention relates generally to radio frequency comrnunication systems, and more specifically to battery powered radios capable of opera'dng in such systems, and is particularly directed toward a protocol for managing energy 40 consumption in battery powered radios thereby prolonging .. ~
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operation within a time divislon multiplex (IDM) (or combined TDM/FDM) radio frequency communication system.

B~C~ÇROU~LD of the I~VE~IIQN
Broadly ~tated, the goal of any energy saving system is to prolong the functional life of a device without degrading or disrupting the device's proper operation. To do this, the lifetime of the device's energy source (i.e., the battery) must be extended. Several energy saving techniques designed to accomplish battery life extension are known. The most widely known techniques involve the "Battery Savers" commonly used in the paging receiver arts. Typical of these battery saving techniques are methods to temporarily inactivate selected circuits durirg periods of non-activity. In more advanced battery savers, it is also known to reduce the operational speed (i.e., cloc~c rate) of mi~oprocessors and other digital circuits. III
this way the discharge rate of the battery is reduced thereby extending the pager's operational life between battery replacement or recharge.
The difficulty in extending the paging battery saving techniques to other applications arise fundamentally from the differences between a paging system and, for example, a two-way communication system. Paging systems operate in non-real time. That is, a message (or data) to be forwarded to page an individual is first recorded (or stored), and later played back (transmitted) when the pager has been awaken from a "sleeping" mode (i.e., the battery saver mode). Thus, a paging system provides essentially time delayed one-way communication in a communication system having only a rare need to effect system operational pararneter changes, and a system that need no~ be concerned wit~ access time, channel loading, or channel resource distribution.
Conversely, access time, channel loading, and channel resource distribution are initial and primarily concerns of two-way comn unication systems. Additionally, system operational parameter changes, channel grants, emergency calls, variation ~ , . .. ..

261~3 in talk-group affiliations, and the like must be rapidly effe~ed throughout a two-way conununication system to maintain the organi~ation of the subscribers operating within the system, and of the system itself. The real-time nature of cc>mmunication in 5 a two-way system complicates the process since messages cannot simply be stored until transmission is convenient for the communication system.

SUM~RY of th~ IMVENTI(:~N
Accordingly, it is an object of the present invention to provide an energy saving technique for use in a TDM (or combined ll~M/FDM) communication system that overcomes the deficiencies of the prior art.
Briefly, according to the invention, a battery powered 15 TDM (or connbined TDM/FDM~ communication device operates within a TDM communication systern having a communication channel arranged into repetitive time frames each having a plurality of slots. At least two of these slots are allocated for colrununicating signalling and control 20 information. To conserve energy, the battery powered communica~ion device operates in a first mode (i.e., the energy saving mode) to shut down non-essential circuits for a predetermined time interval, awaking to monitor one of the two control slots. In this way signalling and control 25 information may be received. When not operating in the energy saving mode, the battery powered communication device operates in a second mode (i.e., a fast access mode) in which each of the at least two control slots are monitored.

30 BRIEF DESC~IPllQ~ of the DR~WINGS
Figure 1 is a block diagram of a comrnunication system according to the invention;
Figure 2 is a block diagram of a repeater of Figure 1;
Figure 3 is a block diagram of a battery powered 35 communication device according to the invention;

~2~ 3 Figure 4 i5 an illustration of an arrangement of communication channels and slot timing suitable for use with the present invention;
Figure 5 is an illushation of an arrangement of outbound (i.e., central-to-communication units) information transmitted in a control slvt of Figure 4;
Figure 6 is an illustration of an arrangement of inbound (i.e., communication unit-to-central) information transmitted in a con~ol slot of Figure 4.
DleS(:RI~lON of gh~ PRl~E~RR~l:) lEM~ODIMENl~
Referring to Figure 1, a time division multiplexed ~TDM) communication system implementing the inv0ntion can be sean as gcnarally depicted by the numeral 100. The system 100 includ~s g~nerally a o~ntral controller 101, that controls the allocation o~ communication chann~ls (f~quencies and tims slots) to promota organized communication between various communication units. Th~ c~ntral controller 101 connects via known inter~onnections with a plurality of repeaters 102; th~re being one repeater 102 for each supported frequency (or frequency pair, depending upon the application). The central controller 101, controis the operation of the repeaters 102 and provides control channel information. Each repeater 102 may include a microprocessor and associated digital circuitry, in which case the controlling actions of ~he central controller 101 may take the form of data packets transferred over an interconnecting network 106.
Ths syst0m also includes a referencs timing gen~rator 103, and one or mor0 auxiliary interface units 107. The timing generator 103, which may include a high stability raference oscillator, provides various clock signals, including but not limited to a TDM frame clock, sl~t clock, and data symbol clock, to the central controller 101 and the repeater~ 102 to main~ain time synchronization throughout the systsm. The auxiliary interfaces 35 107 provide means ~o interconnect the system 100 with non-radio - .. - ~ ,.. . . . .

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, ,, z603 frequency (RF) communica~ion units, such as conventional telephone lines and dispatch consol0s.
Tha system also includes a plurality of communication units, including TD~UFDM capable units 104 and TDM only 5 communic~ion units 105. (As ussd herein, "communication unit"
is intended to refer to any of a variety of radio platforms, including but not limited to mobile radios, portable radios, and fixed loca~ion radios, including both one-way and two-way devices).
Referring to Figure 2, ths repeater 102 is shown to include 10 a r~c0iv~ and a transmit section (201 and 202 r~spactively). The receiver section 201 includas an RF receiv0r 2û3 that receives signals via an appropriate antenna structura 204 and that providas at its output a baseband signal. The baseband signal is passsd to both a voice recovery block 205 and a data recoYery 15 block 206. The voice recovery block 205 processes the recsived baseband signal to recover a voice information signal 207. This signal might consist of th~ original audio signal whcn receiving an FDM communication, or in the case of a TDM communication, the voice signal 207 may be a processed version of the original audio 20 signai (for exampla, voica processed via Sub-Band Coding or Linear Predictive Coding). Depending on the nature of the rep0atar int~rconnection network 106 and the type of communication (for example, an FDM communication (from unit 104) to a TDM unit (105), the voice recovery block 205 may 25 include an audio ~ncoder/dscoder to raformat the rqceived voic~
signal 207. In the case of the aforementioned audio processing, this coding ~unction couid bc accomplished with a digital signal processor (DSP), for example the Motorola DSP56001.
The data recovsry block operates in a known manner to 30 recovsr any data information (such as, for instanco, control slot inforrnation) included in the receiv~d signal, thus providing the racGivad data signal 208. The recovered voice signal 207 and d~ta signal 208 are passed ~o a repeater network interface 209, which communicates these si~nals to tha cen~ral controller 101 or 35 oth~r repeatars 10~ as is appropriat~ via the repeater interconnection n~twork 106. So con~igured, the repeater 192 '' ~' ' ' .
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receives TDM/FDM RF signals and properly processes them to recover both voic0 and data information ~hat may b~ pr~sent in the communication.
The transmitter section 202 also includes a network 5 interface 210 that rec~ives voice signals 211 and data signals 212. The voice signals 211 comprise received voice signals 207 from the r~pea~s receiv~r section 201 or from the receiver sections of other repeaters in the system. The data signals 212 include control channel informa~ion from the central controller 10 101. The voice signal 211 and data signal 212, are proc~ssed by a transmitter controller 213, to an appropriately modulated information signal at its output. As in the receiver sec$ion 201, the transmitter controller 213 may includ~ a DSP to r~format the voice signal 211 as appropriate for various receiving communication 15 units (such as, via Sub-Band Coding or Linear Predictive Codin~). The output of the transmit~er controller 213 is passed through a spla~ter filter 214 to an RF transmitter 215 that proparly processes the signai to provide a signal that may be appropriately radia~ed from an antenna device 216 to cause transmission of the 20 processed signal as desir~d.
Referring to Figure 3, a communication unit is shown to include an RF r~c~iver 301 that couples (via an antenna switch 303) to an appropriat~ antenna 302 to receive RF signals. The receiver 301 provides a synchronization signal 310, which 25 provides th~ information rsquired to establish frame and bit synchronization. The receiver 301 also provides a receiv~d baseband signal 304, which is digitized by an analog-to-digita (A/D) conv~rter 306. The output of ths A/D converter 306 is pass0d to a voice coder (vo-coder) 308 which may be embodied 30 as, for example, a Motorola DSP56001. The vo-codcr 308 providas th0 required coding/decoding function used in this ~mbodiment to properly encode (or decode) the audio information in a particular form. In the prsferred embodiment of the pr~sent invention, multi-level, digital sub-band coding comprises the 35 . pref~rred ancode/decode format. The vo-coder 308 oouples to a ~ilter and Cod0c 312, which when activatad (318) by a controllar ~;

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2Ei~3 320, rout~s d~coded signals ~o an appropriate sp~ak~r 314 to b~
rendered audible. Audio signals to be transmitted are routacl from a microphons 316 through ~h~ enabled (318) filtar and Cod0c to the vo-coder 308 for processing. The processed au~iio 5 information to be transmitt0d is pass~d from the vo-coder 308 to a digital-to-analog (D/A) convertor 322, to be appropria~ely converted before being applied ~o a transmi~ter 324. To conserve energy, both the transmitter 324 and the D/f~ 3 2 are deactivated (326) by the controller 320 unless the communica~ion unit is 10 transmitting. Of oourse, additional information appropriate to the TDM nature of the communications may be routed (328) ~o the vo-cod~r 308 by the con~roller 320 to be added prior to transmission.
Uitimately, the properly coded and slot and frame synchroniz~d information signal routed from the RF transceiver 324 to the 15 ant~nna switch 303, which is controlled (330) by th0 controllar 320 to selectiv~iy couple tha transmitt0r 324 to the antenna 302 so that th~ signal that may be appropriately radiated to cause transmission of thc processed si~nal as desired. Alt~rnately, a duplexer could be used in place of the antenna switch 303 as is 20 known in the art.
The controller 320 is the heart of the communication device and operates to control the many circuits comprising the communication devico. In a por~abls embodiment of the communication device, a ba~ery energy source (not shown) is 25 coupled to oach block of the communication unit to provide power. Accordin~ to the invention, the controller 320 periodically d~activates non-essential circuits to conservs ener~y. In tha preferr~d embodimsnt of the present invention, the controll~r 308 deactivat~s the rec2iver 301 and A/D 306 via the control linc 332;
30 tho vo~coder via communication bus 328; and a programmablo (336) synthesizer 334 via control linc 338. Of course, all of these circuits need not be activated for every embodiment, however, the pref~rr~d ~mbodiment deactivatas thcse circuits as they repr~sent the most significant consumcrs of energy. Following circuit 35 deactivation, the controller 320 triggers (346) a timer 340 and suspends (or greatly reduces th~ sp~ed ofl its own op~ration, ,;, . . . .. . . ..

,... . . . .. .

2~i~3 which drastically reduces tha current drawn by the con~roller. To recover from the energy saving state, a timer driven from a high stability (better than 1 n ppm) reference oscillator 342 re-activates (344~ th~ controllar 320. Aft~r re-activation, the controller reviv~s 5 the ~mporarily deaotivated cir~uits and opcrates to receive and d~code a~ least a por~ion of the nex~ control slot. The periodic deactivation/~-activation is precis~ly ~imed to insur~ ~hat the csmmunication unit r~-activates immediat01y b~fore a control slot so that signallin~ and csntrol information m~y be recaived and 10 act~d upon. Ueactivation may ba retriggered after ail or a por~ion of the control slots have been received. In this way, ths communication device may be deactivated for a minimurn of three quarters of a four slot TDM frame (less any wake-up time), awakening only to receive one of the two control slots used in the 15 prsf0rrsd embodiment of the present invention thereby keeping pace with system parameter changes, call assignmants or other signalling or control messa~es.
When not op~rating in the energy saving mode, the communication unit operates to receive both of the prefarably two 20 control slots. Comparsd to th~ en~rgy saving mode, receiving both control slots enabl0s the communication units to obtain system informa~ion twico as fast, thereby providing faster access to communication channels. In this way, the second operational mode improvss the communication efficiency of tha 25 communication d~vice. Of cou~se, this "Fast Access" moda consumes more energy, and ~hercfore, its use must be balanced against the operational life of the banery, and the communication unit itself.
Referring to Figure 4, the abov~ described system functions 30 in eith~r a TDM or an FDM/TDM operating environment. While certain lirnited applications could sffectively use a single channel, tho ~mainin~ discussion will describe the more general case wh~r~in at ~east two frequ~nci~s are available for the central 101 to assign for comrnunication purposes, and where each of ~he 35 frequeneies are subdivided into frames and slots. In this particular ~mbodiment, two chann~ls A and ~ ar~ d~picted, ~ach . .
.

, having frames 401 of 180 miiliseconds, with four slots 402 per frama (each slot being 45 milliseccnds). Each frame window 401 supports two control slots 403 and 404. In one embodiment, both control slots 403 and 404 may bQ on th~ same fr~qu~ncy. For 5 ~xample, channel A may support a control slot in slot 1 (403) and slot 3 (404) of each of its fram~s (401~. In another ~mbodim~n~, tha control slots may be on diff~nt frequ0ncies. For example, channel A couid support a control slot in slot 1 and channel B
could support a control slot in slot 3. Regardless of how the 10 control slots are apportioned among the available channels there will always be at least two control slots for each frame window.
A control slot supports communication control information.
Figure 5 depi~s the TDM outbound signalling information format (i.e., information sent by the central 1û1 to the communication 15 units (104 and 105)) that may be provided in a control slot for this particular embodiment. In the 4~ milliseconds provided for the slot, 1.25 milliseconds are us~d to prcvide 16 symbols that constitute a frams sync word 502 as is well understood in the art.
Next, a 4 symbol slot identification (ID) code 504 is transmitted 20 that identifles the slot's location within tha frame. Following this, the preferred embodimant transmits four outbound signalling packets (OSPs) 503 each comprising 139 symbols (10.86 ms) representing 58 data symbols that are error encoded up to the 139 symbol limit. The 58 data symbols ar~ allocated for a 25 communication uni~ identification (ID) code, a call type code (i.e., individual or group call), symbols for frequency assignment information, and symbols that represent the assigned slot number.
For example, on~ OSP could constitute a talk/request grant to a particular communication unit by identifying that communication 30 unit by its ID, and identifying the assigned fraquency and slot.
With reference to Figure 6, inbound signalling packets (ISPs) are also provided by the communication units (10~ and 105) to the central 101 Yia the control slo~s. (If the channel that supports th~ control slot actually comprises paired frequencies 35 . (one for transmitting and one for ~csiving) as oiten occurs in trunked communications, then ISPs and OSPs can be supported . ; : . , . . : -~, . . - .
, ~ . . ... . .
.. . , : .- ~ . . .. . .. . .
- . - . . . - . - . ~ . , 2~1~2~3 in a side-by-side manner. If the channel comprises a single frequency only, then ISP and OSP service must be staggercd to prevent contan~ion). For ISPs, the 180 milliseconds of the control slot are subdivided into 4 subslo~s 601.
In an FDM inbound transmission (from subsoriber 104), each subslot 601 includes a 4.06millisecond 13 symbol dotting pattern 6û2 (~o-allow-any transi3nt eff~c~s known to occur in the ~ransition from receive-~o-transmit to settle) followed by 5 millis~conds of 16 symbol frame sync word information (603).
33.13 milliseconds ar3 th~n allocat~d for 106 symbols of ISP
informa~ion (604). These 106 symbols include 44 data symbols that ara error ~ncoded up to the 106 symbols. The 44 data symbols include symbols aliocated for the communication unit ID
and call typa codes. Of course, some of the ID symbols or the call type request symbols can be allocatad in the system to identify whether a requesting unit is a TD~VFDM communication unit 104 or a TDM only communication unit 105. Finally, 2.81 ms (nine symbol) are resen/ed (605) to account for propagation delay.
TDM inbound transmissions (from eith~r subscriber 104 or 105) preferable employ the same communication format as illustrat0d in Figur~ 6, how~ver, the protocol differs in that ths slot timing discuss~d abov0 ar0 divided by four (4).
So configured, a communication unit (104 or 105~ can r~quest frequency and slot assi~nments via th~ two control slots provided in ~ach fram~ 401. The central can then identify the type of tho r0qu~sting communication unit (104 or 105) and the type of th~ communica~ion unit (104 or 105) with which the requesting unit desires to communicate. Further, the central 101 can assign a r~questing unit an appropriate frequency and slot assignment, via the two control slo~s provide in each frame 401. In an alternate embodiment, the control slots, via special ISPs and OSPs, could also be used to transfer data packets between communications units (104).
When making frequQncy and slot assignments, tha central 101 makes the assignmsnt in a way that will assure that the assigned communication unit 104 will still ba able to .

26~3 communizate on at least one of the controt slots. For example, with respe~ to Figure 4, a requesting unit could be assi~ned to transmit on slot 1 of channel B and to receive on slot 2 of channel A. This would allow the assigned communication unit (104) to 5 continu~ to monitor control information in the contrel slot (404) that occupies th~ third tim~ slot of chann~l A. In this way, important syst~m control information can be provided with assuranc~ to all TDM communication units (104), regardless of wh~ther such communication units (1043 ar~ currently ~ngaged in 10 communications with oth~r communication units or not.
Alternat~ly, th~ c~ntral (101 ) could dir~ct control information intsnded for the communication unit 104 pr0viously assigned to slots 1 and 2 to only the control slot 404 known to be monitored by the communicaticn unit 104. In y~ another 15 embodiment, communication units 104 may ba of differing types with differing constraints as to how ~uickly a change in mode (receiving or trans nitting3 or a change in frequency could be accommodated. Accordingly, the cantral 101 would assi~n units so as to guarantee sufficiant ~ime to switch from participating in 20 the assigned VoicQ communication to monitoring th~ availabla control slot.
According to the invention, the use of two control slots provides an advantage in that two distinct operating modes may be balanced between increased operational life (for portable 25 bat~ary pow~r~d units) and increas~d operationai performance.
In an ~n~rgy saving modc, non-~ssGntial circuits may b~
t~mporarily deactivated, being awakened to monitor one of the two control slots. This conscrves encrgy, whilo maintaining the communication d~vice operating within the flow of control 30 information being transmitted in th~ control slots. In a second mode, fast acc~ss to communication chann01s may b~ had since the communication device may ~ransmit and receive signailing and control information twice a rapidly as in the energy saver mode. In ~his way, op~rational psrfofmance may be balanced 35 against operational lifetime.
What is daimed is:

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Claims (24)

1. In a battery powered device operating within a time division multiplexed communications system having a communication channel arranged into repetitive time frames which include a plurality of slots, wherein each frame includes at least two control slots allocated for the communication of signalling and control information, wherein said signalling and control information include a slot identification code for identifying location of each control slot within the frame, a method for operating said battery powered device comprising the steps of:
(a) monitoring a portion of only one of said at least two control slots and detecting the slot identification code of said control slot when operating in a first mode; and (b) monitoring a portion of each of said at least two control slots and detecting the slot identification codes of the at least two control slots when operating in a second mode.

2. The method of claim 1, wherein step (a) includes the step of:
(a)(1) deactivating non-essential circuits in the battery powered device during each remaining control slot of said plurality of slots.

3. The method of claim 2, wherein step (a1) includes the step of:
deactivating the non-essential circuits during any remaining portions of said only one control slot monitored in step (a).

4. The method of claim 2, which includes the step of (a2) reactivating said non-essential circuits prior to a succeeding frame having saidslot monitored in step (a).

5. The method of claim 1, wherein step (b) includes the step of:
(b1) deactivating non-essential circuits in the battery powered device during each remaining slot of said plurality of slots.

6. The method of claim 5, wherein step (b1) includes the step of:
deactivating the non-essential circuits during any remaining portion of each of said at least two control slots monitored in step (b).

7. The method of claim 5, which includes the step of (b2) reactivating said non-essential circuits prior to a succeeding frame having saidat least two slots monitored in step (b).

8. The method of claim 1, wherein step (a) comprises monitoring a portion of only a selected one of said at least two control slots when operating in said first mode.

9. The method of claim 1, wherein step (a) comprises monitoring a portion of only an assigned one of said at least two control slots when operating in said first mode.

10. The method of claim 1, wherein step (a) comprises monitoring a portion of only one of said at least two control slots, and at least one of said plurality of slots other than said at least two control slots when operating in said first mode.

11. The method of claim 1, wherein step (b) comprises monitoring a portion of each of said at least two control slots, and at least one of said plurality of slots other than said at least two control slots when operating in said second mode.

12. The method of claim 1, wherein step (a) comprises monitoring a portion of only one of said at least two control slots while operating on at least one of said plurality of slots other than said at least two slots when operating in said first mode.

13. The method of claim 1, wherein step (b) comprises monitoring a portion of each of said at least two control slots while operating on at leastone of said plurality of slots other than said at least two control slots when operating in said second mode.

14. In a battery powered device operating within a time division multiplexed communication system having a communication channel arranged into repetitive time frames which include a plurality of slots, wherein each frame includes at least two control slots allocated for the communication of signalling and control information, wherein said signalling and control information include a slot identification code for identifying location of each control slot within the frame, a method for operating said battery powered device comprising the steps of:
(a) monitoring a portion of only one of said at least two control slots and detecting the slot identification code of said control slot while deactivating non-essential circuits during each remaining slot when operating in a first mode;
(b) monitoring a portion of each of said at least two control slots and detecting the slot identification codes of the at least two control slots while inactivating non-essential circuits during each remaining slot when operating in a second mode; and (c) adapting between said first mode and said second mode in response to a control signal.

15. A battery powered communication device being capable of conserving energy by functioning in one of two operational modes while operating within a time division multiplexed communication system having a communication channel arranged into repetitive time frames which include a plurality of slots, wherein each frame includes at least two control slots allocated for the communication of signalling and control information, wherein said signalling and control information include a slot identification code for identifying location of each control slot within the frame, said devicecomprising:
means for monitoring only a portion of one of said at least two control slots and detecting the slot identification code of said control slot when operating in a first mode; and means for monitoring a portion of each of said at least two control slots and detecting the slot identification codes of the at least two control slots when operating in a second mode.

16. The battery powered communication device of claim 15, wherein the first mode comprises a lower power mode or energy saving mode.

17. The battery powered communication device of claim 15, wherein the second mode comprises a higher power mode than the first mode.

18. The battery powered communication device of claim 15, which includes deactivation means for deactivating non-essential circuits during each remaining slot of the plurality of slots.

19. The battery powered communication device of claim 15, which includes reactivation means for reactivating any deactivated non-essential circuits prior to a succeeding frame having at least two slots allocated for thecommunication of signalling and control information.

20. The battery powered communication device of claim 15, wherein the particular one of the at least two slots monitored in the first mode is selectable by a mode selector means within the battery powered device.

21. The battery powered communication device of claim 15, wherein said means for monitoring a portion of each of said at least two control slots is responsive to said control and signalling information transmitted from said time division multiplexed communication system for assigning the particular one of the at least two slots monitored in the first mode.

22. The battery powered communication device of claim 15, which includes control means for controlling when the battery powered communication device operates in the first mode or the second mode in response to a control signal.

23. A battery powered communication device being capable of conserving energy by functioning in one of two operation modes while operating within a time division multiplexed communication system having a communication channel arranged into repetitive time frames which include a plurality of slots, wherein each frame includes at least two control slots allocated for the communication of signalling and control information, wherein said signalling and control information include a slot identification code for identifying location of each control slot within the frame, said devicecomprising:
receiving means for receiving information from one or more of the plurality of time slots;
means for monitoring only a portion of one of said at least two control slots and detecting the slot identification code of said control slot when operating in a low power or energy saving mode;

means for monitoring a portion of each of said at least two control slots and detecting the slot identification codes of the at least two control slots when operating in a higher power mode;
control means for controlling the battery powered communication device to operate in the energy saving mode or the higher power mode in response to a control signal.

24. In a battery powered device operating within a time division multiplexed communication system having a communication channel arranged into repetitive time frames which include a plurality of slots, wherein each frame includes at least two non-contiguous control slots allocated for the communication of signalling and control information, wherein said signalling and control information include a slot identification code for identifying location of each control slot within the frame, a method for operating said battery powered device comprising the steps of:
(a) monitoring a portion of only one of said at least two control slots and detecting the slot identification code of said control slot when operating in a first mode; and (b) monitoring a portion of each of said at least two control slots and detecting the slot identification codes of the at least two control slots when operating in a second mode.