Abstract:
Real-time location system and method, the system including a tag and a plurality of groups, each group including: a plurality of battery operated secondary technology base-stations, and a battery operated wireless timing beacon generator to provide a timing beacon to the group, such that the timing beacon generator for a group obtains timing information for the timing beacon wirelessly from a timing server, and such that each timing beacon generator transmits timing beacons with timing information to the plurality of base-stations in the group, and each secondary technology base-station transmits a secondary technology signal to the tag based on the received timing beacon.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 14/136,193, filed Dec. 20, 2013, the entire content of which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     Field of the Invention 
     Embodiments in accordance with the present invention relate to methods and systems for sensor data reporting with high positional accuracy and low power consumption. 
     Description of Related Art 
     Indoor Real-Time Location (RTL) Systems (RTLS) are popular in the healthcare industry for a variety of applications ranging from asset tracking through patient and staff tracking, environmental or patient sensing (e.g., temperature), hygiene compliance, elopement (i.e., a patient leaving a facility without authorization), theft prevention, and so forth. 
     Conventional RTL systems typically use radio frequency (RF) transmission to determine location. The RF-based methods may be augmented with infrared (IR) transmission as a localization method in order to improve accuracy to support room and sub-room level accuracies. An IR receiver typically is incorporated into a portable device (i.e., a tag) and IR transmitters are incorporated into base stations that are scattered in rooms and corridors within the enclosure. Typically, each the IR base stations transmits an identification (ID) to the portable devices, and the location of the portable devices is determined by their vicinity to a base station. 
     The basic advantages of synchronization of IR based systems are described in U.S. Pat. No. 8,139,945 (“the &#39;945 Patent”), which is hereby incorporated by reference in its entirety. Timing synchronization information is transmitted using a plurality of stationary IR base stations and a plurality of portable devices. Each IR base station is configured to receive the timing synchronization information and to transmit a corresponding IR location code in a time period based on the received timing synchronization information. 
     In the &#39;945 Patent, each portable device is configured: 1) to receive the timing synchronization information; 2) to detect the IR location codes from the IR base stations; and 3) to transmit an output signal including a portable device ID representative of the portable device and the detected IR location code. Each portable device is synchronized to detect the IR location code in the time period based on the received timing synchronization information. The &#39;945 Patent enables coexistence of multiple IR transmitters at the same physical space, the construction of virtual walls, as well as facilitating high tag update rate with extremely low power consumption for tags. 
     However, the &#39;945 Patent assumes that each infrared node will have its own unique infrared ID. The assumption does not apply to a configuration involving distributed IR transmissions. In such a distributed IR emitting system, all IR emitters transmit the same infrared ID at the same time. Conventionally, the only way to achieve transmitting the same infrared ID at the same time has been by use of wired connections among the independent emitters in order to allow the co-emissions of the same infrared ID from all the emitters. 
     Wired connections pose problems during installation and are more difficult to expand, compared to wireless connections. However, over the air super-synchronization has not been used because sufficient synchronization accuracies with low enough power consumption to support battery operated IR base-stations had not been possible or available because the timing accuracy needs to be much better than the high modulation rates (typically 30-40 kHz) of the IR signals. 
     Therefore, a need exists to provide a low power method to allow co-emission of the IR signals by multiple physically independent IR emitters, using the same ID from all the emitters. 
     SUMMARY 
     Embodiments in accordance with the present disclosure include a wireless infrared-aided location system, including: a portable tag to be located, the portable tag comprising an infrared (IR) receiver; and a plurality of base-stations, at least two of said base-stations including: a processor coupled to a memory; a first clock coupled to the processor, the first clock configured to be selectively activated and deactivated by the processor; a receiver coupled to the processor, the receiver configured to receive a periodic timing synchronization signal; and an infrared transmitter coupled to the processor, the infrared transmitter configured to transmit an IR signal to the portable tag, wherein the IR signal is substantially identical and in-phase with IR signals from other base-stations, wherein the processor is configured to synchronize said base-station to the periodic timing synchronization signal. 
     Embodiments in accordance with the present disclosure include a method to locate a wireless infrared-aided portable tag, including: operating a low-speed clock by a infrared (IR) base-station; activating a high-speed clock prior to an estimated time of arrival of a periodic timing synchronization signal; receiving the periodic timing synchronization signal; synchronizing the high-speed clock by use of the periodic timing synchronization signal; transmitting, by the IR base-station, an IR signal to the wireless infrared-aided portable tag at a time derived from the high-speed clock and the periodic timing synchronization signal, wherein the IR signal is substantially identical and in-phase with IR signals from other IR base-stations; and deactivating the high-speed clock after transmission of the IR signal. 
     The preceding is a simplified summary of embodiments of the disclosure to provide an understanding of some aspects of the disclosure. This summary is neither an extensive nor exhaustive overview of the disclosure and its various embodiments. It is intended neither to identify key or critical elements of the disclosure nor to delineate the scope of the disclosure but to present selected concepts of the disclosure in a simplified form as an introduction to the more detailed description presented below. As will be appreciated, other embodiments of the disclosure are possible utilizing, alone or in combination, one or more of the features set forth above or described in detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and still further features and advantages of the present invention will become apparent upon consideration of the following detailed description of embodiments thereof, especially when taken in conjunction with the accompanying drawings wherein like reference numerals in the various figures are utilized to designate like components, and wherein: 
         FIG. 1A  is a block diagram of a system for locating and identifying portable devices in an enclosure, according to an embodiment of the present invention; 
         FIG. 1B  is a configuration, according to an embodiment of the present invention; 
         FIG. 2  is a block diagram of a radio frequency (RF) base station, according to an embodiment of the present invention; 
         FIG. 3  is a block diagram of an infrared (IR) base station, according to an embodiment of the present invention; 
         FIG. 4  depicts timing of TSI from an RF base station, in accordance with an embodiment of the present invention; 
         FIG. 5  depicts timing of operations at an IR base station, in accordance with another embodiment of the present invention; 
         FIG. 6  depicts timing of TSI from an RF base station, in accordance with another embodiment of the present invention; and 
         FIG. 7  depicts timing of operations at an IR base station, in accordance with another embodiment of the present invention. 
     
    
    
     The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description or the claims. As used throughout this application, the word “may” is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). Similarly, the words “include”, “including”, and “includes” mean including but not limited to. To facilitate understanding, like reference numerals have been used, where possible, to designate like elements common to the figures. Optional portions of the figures may be illustrated using dashed or dotted lines, unless the context of usage indicates otherwise. 
     DETAILED DESCRIPTION 
     The disclosure will be illustrated below in conjunction with an exemplary communication system. Although well suited for use with, e.g., a system using a server(s) and/or database(s), the disclosure is not limited to use with any particular type of communication system or configuration of system elements. Those skilled in the art will recognize that the disclosed techniques may be used in any communication application in which it is desirable to utilize location sensors and other sensors (e.g., temperature and humidity) that communicate with a central monitor. 
     The exemplary systems and methods of this disclosure may also be described in relation to software, modules, and associated hardware. However, to avoid unnecessarily obscuring the present disclosure, the following description omits well-known structures, components and devices that may be shown in block diagram form, are well known, or are otherwise summarized. 
     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of embodiments or other examples described herein. In some instances, well-known methods, procedures, components and circuits have not been described in detail, so as to not obscure the following description. Further, the examples disclosed are for exemplary purposes only and other examples may be employed in lieu of, or in combination with, the examples disclosed. It should also be noted the examples presented herein should not be construed as limiting of the scope of embodiments of the present invention, as other equally effective examples are possible and likely. 
     As used herein, the term “Wi-Fi” or “conventional Wi-Fi” refers generally to a bi-directional radio communication technology that operates based on one or more of the Institute of Electrical and Electronics Engineers (IEEE) 802.11 family of standards, which are incorporated herein by reference. The IEEE 802.11 standards specify the RF and protocol characteristics of a bi-directional radio communication system. 
     As used herein, the term “module” refers generally to a logical sequence or association of steps, processes or components. For example, a software module may comprise a set of associated routines or subroutines within a computer program. Alternatively, a module may comprise a substantially self-contained hardware device or circuit device. A module may also comprise a logical set of processes irrespective of any software or hardware implementation. 
     As used herein, the term “transmitter” may generally comprise any device, circuit, or apparatus capable of transmitting an electrical, electromagnetic, infrared, ultrasonic, or optical signal. As used herein, the term “receiver” may generally comprise any device, circuit, or apparatus capable of receiving an electrical, electromagnetic, infrared, ultrasonic, or optical signal. As used herein, the term “transceiver” may generally comprise any device, circuit, or apparatus capable of transmitting and receiving an electrical, electromagnetic, infrared, ultrasonic, or optical signal. 
     The term “computer-readable medium” as used herein refers to any tangible storage and/or transmission medium that participates in storing and/or providing instructions to a processor for execution. Such a medium may take many forms, including but not limited to, non-volatile media and volatile media. Non-volatile media includes, for example, NVRAM, flash media, or magnetic or optical disks. Volatile media includes dynamic memory, such as main memory. When the computer-readable media is configured as a database, it is to be understood that the database may be any type of database, such as relational, hierarchical, object-oriented, and/or the like. 
     In embodiments in accordance with the present invention, a tag is customarily an active RFID tag. An active RFID tag uses an internal power source (e.g., a battery) within the tag to continuously power the tag and its RF communication circuitry. In contrast, passive RFID relies on RF energy transferred from a reader to the tag to power the tag. Active RFID allows relatively low-level signals to be received by the tag, and the tag can generate relatively high-level signals back to the access point. Active tags may also initiate communication with an access point or other tags. 
     RTL systems are known that include base stations that transmit IR signals (i.e., IR base stations) with their respective base station ID (BS-ID) to portable devices that are equipped with IR receivers. Upon reception of a BS-ID, the portable device transmits both the device ID and the received BS-ID to a reception device, for example, by radio frequency (RF) or IR transmission. The RTL system may include a server that associates the BS-ID with the device ID (received from the portable device by the reception device). In this manner, the position of the portable device may be displayed as the position of the IR base station. In exemplary implementations, both base stations and tags are battery operated. 
     In a distributed IR emitting system, all IR emitters transmit the same infrared ID at the same time. The number of such base-stations may be in some cases, six or more. All IR emitters with the same infrared ID are deemed to be a single location for position-locating purposes. For example, in order to create a virtual corridor between sets of bays, typically three IR emitters are used per bay, and the total number of IR emitters may exceed six. Using the same infrared ID at the same time avoids a problem of a tag mixing up the received signals, and the receiver not being able to decipher the signals. Although the infrared ID may be sent at different times, doing so may have several undesirable consequences. First, the tag response may be slower since a tag may have to wait N times the basic period to receive a signal. Second, embodiments may need to increase the basic rate and the tag will need to check for an IR signal N times more often, causing a corresponding increase in tag power consumption. 
     Referring now to  FIG. 1A , a block diagram is shown of a system  100  for determining a location and an identity of portable devices  108  in an enclosure  102 . System  100  may include a plurality of IR base stations (IR-BS)  106 , one or more portable devices  108  and at least one RF base station (RF-BS)  110 . In some embodiments, RF base station  110  may be a Wi-Fi access point (i.e., an RF access point). 
     RF base station  110  may transmit timing synchronization information (TSI) signal, for example, in a beacon, to IR base stations  106  and portable devices  108  that are each equipped with RF transceivers, by RF transmission. The TSI may be used by IR base stations  106  to transmit a corresponding BS-ID, i.e. an IR location code, in a period of time after receiving the TSI. The period of time for IR base stations  106  to transmit the respective BS-ID signals may be fixed or transmitted as a part of the information carried by the beacon. 
     Enclosure  102  may include a plurality of separate zones  104 , which typically coincide with individual rooms or zones within enclosure  102 . For example, zone  104 - 1  represents a corridor. Each room or zone may be provided with at least one IR base station  106 . For example, corridor  104 - 1  includes IR base stations  106 - 1 ,  106 - 2 ,  106 - 3 . 
     IR base station  106  typically transmits very short bursts of IR location signals from an IR transmitter (i.e. corresponding BS-IDs) at periodic intervals based on the TSI received from RF-BS  110 . Each IR base station  106  may transmit a BS-ID signal that may be identified at a central control as originating from a particular zone or room  104 . The BS-ID may, for example, be transmitted with an IR modulating frequency that is typically around 40 kHz that may be in the form of bursts of the order of about 0.5 milliseconds long. It is understood that any suitable frequency and duration of the IR burst may be used. Although IR base station  106  is described as including an IR transmitter, it is contemplated that IR base station  106  may also include an IR receiver. 
     Portable devices  108  may be provided for persons or apparatuses. The portable devices  108  may include an IR receiver and an RF transmitter or transceiver, which are coupled to each other. In this manner, the RF transceiver may receive the TSI and may transmit received BS-ID and its device ID at an RF carrier frequency to RF base station  110 . 
     The modulated carrier signal received by RF base station  110  may be decoded to reproduce the BS-ID and the device ID. 
     Although IR base stations  106  are described, it is contemplated that the base stations  106  may also be configured to transmit a corresponding BS-ID by an ultrasonic signal, such that base stations  106  may represent ultrasonic base stations. Accordingly, portable devices  108  may be configured to include an ultrasonic receiver to receive the BS-ID from an ultrasonic base station. If an ultrasonic interface is used, then a differential time of arrival method may be used for location detection by the tag. 
       FIG. 1B  illustrates a configuration  150  in accordance with an embodiment of the present disclosure. Configuration  150  may represent a deployment in which a plurality of RF base stations  110  provide coverage to a covered area (e.g., large room, corridor, etc.). Configuration  150  may include a plurality of RF base stations  110 - 1 ,  110 - 2  and  110 - 3 , along with a respective coverage region, as denoted by differently-shaded regions. As illustrated, some of the RF coverage regions may spatially overlap. Overlap of RF base station  110  coverage areas helps prevent dead zones within the covered area. 
     Configuration  150  may further include a plurality of IR base stations, e.g., IR base stations  106 - 1  through  106 - 29 . Configuration  150  is not limited to the positions or quantities of stations shown. Super-synchronization of each IR base station  106  is controlled by signals that IR base station  106  is able to receive from an RF base station  110 . Each IR base station  106 - n  is able to receive timing synchronization signals from at least one RF base station  110 - n . Some IR base stations, e.g., IR base stations  106 - 1  through  106 - 7  as illustrated, may be able to receive timing synchronization signals from only one RF base station (e.g., RF base station  110 - 1 ). Some IR base stations, e.g., IR base stations  106 - 8  through  106 - 12 , and  106 - 15  through  106 - 22  as illustrated, may be able to receive timing synchronization signals from more than one RF base station  110  (e.g., RF base stations  110 - 1  and  110 - 2 ). In order to avoid RF collisions, RF base stations whose coverage areas may overlap are configured to operate on different frequencies or to transmit at slightly different times. 
     In order to avoid excessive levels of interference to IR base stations  106 - n  in overlap regions (e.g., IR base stations  106 - 8  through  106 - 12 , and  106 - 15  through  106 - 22 ), adjacent RF base stations  110 - n  may transmit their timing synchronization signals at different times and/or frequencies. For example, RF base station  110 - 1  may be configured to transmit its timing synchronization signal at a first frequency or within a first timing slot. RF base station  110 - 2  may be configured to transmit its timing synchronization signal at a second frequency or within a second timing slot. Thus, although an IR base station  106  (e.g., IR base stations  106 - 8 ) may receive a first timing synchronization signal from a first source (e.g., from RF base station  110 - 1 ) and a second timing synchronization signal from a second source (e.g., from RF base station  110 - 2 ), the IR base station may use the received time slot and/or frequency information in order to distinguish between the different timing synchronization signals. 
       FIG. 2  illustrates a block diagram of RF base station  110 . RF base station  110  may include local area network (LAN) chip  202 , microcontroller  204 , RF transceiver  206 , and antenna  210  In one embodiment of the current invention, the RF base-stations  110  receive their timing synchronization via Ethernet. In another embodiment, the timing synchronization is transferred from one RF base-station  110  to another. 
     RF transceiver  206  may be configured to receive RF transmissions, for example, from portable device  108  ( FIG. 1A ) or from another RF base station  110 , from antenna  210 . RF transceiver  206  may also be configured to transmit the TSI, such as by transmitting an RF beacon that includes the TSI, via antenna  210 . 
     Microcontroller  204  may be configured to control LAN chip  202 , and RF transceiver  206 , for example, to transmit the TSI, communicate with other RF base stations and receive RF transmissions from portable devices  108  ( FIG. 1A ). 
       FIG. 3  illustrates a block diagram of IR base station  106 . IR base station includes RF receiver or transceiver  308  (henceforth collectively referred to as RF transceiver  308 ), antenna  312 , microcontroller  306 , IR LED driver board  302  and IR transmitter  304 . IR base station  106  may be powered by battery  314  or by an external power supply  316 . Synchronized IR base stations  106  may provide for a simple and low cost installation and allow for a coexistence of IR base stations  106  without dead regions (typically caused by an overlap in coverage). 
     RF transceiver  308  may be configured to receive RF transmissions, for example, beacons including the TSI from RF base station  110  ( FIG. 1A ) via antenna  312 . 
     Although not shown in  FIG. 3 , a BS-ID associated with IR base station  106  may be stored by IR base station  106 , for example, in a memory of microcontroller  306 . Driver board  302  may be configured to transmit the associated BS-ID to IR transmitter  304 . It is understood that IR transmitter may include any device suitable for transmitting an IR burst that includes the associated BS-ID. 
     Microcontroller  306  may be configured to control driver board  302 , and RF transceiver  308 . Based on the TSI received by RF transceiver  308  at time T 1  (see  FIG. 5 ), microcontroller  306  may control driver board  302  to transmit the BS-ID at time T 2  (see  FIG. 5 ), after the TSI is received. For example, referring to  FIG. 5 , at time T 0  the high speed clock  501  is activated. At time T 1  the IR base station  106  receives  503  the TSI  403 . At time T 2 , which is made as close as possible to T 1 , IR transmitter  304  (see  FIG. 3 ) may be controlled to transmit IR signal  505  including the BS-ID. High speed clock  501  stays on during transmission of IR signal  505 . When IR signal  505  transmission ends, the high speed clock  501  stops and the only active clock is a low-speed clock  502  (see  FIG. 5 ). 
     This process repeats with a period T K , i.e., at T K +T 1  another beacon  503  may again be received by IR base station  106  ( FIG. 3 ). Although IR transmission  505  is illustrated as occurring once every period T K , it is contemplated that an IR base station transmission  505  may be activated multiple times in time period T K . 
     IR base station  106  may use an optional time adjust module  318  in order to receive relatively high accuracy time estimates in order to correct a relatively lower-accuracy clock that is internal to IR base station  106 . Time adjust module may be local to IR base station  106  or may be remote but communicatively coupled to IR base station  106 . 
     Referring back to  FIGS. 1A, 1B and 3 , by synchronizing all of the nodes (e.g., the RF base station  110  and all IR base stations  106 ), portable device  108  and/or IR base stations  106  may be powered by battery  314 . Because base stations  106  are battery powered, if the nodes of system  100  were not synchronized, the IR transmitter  304  would need to transmit IR signals more often so the tag could quickly be able to “find” the transmissions. IR transmission may be power consuming and high rate transmissions may quickly deplete the battery of base stations  106 . 
     According to an exemplary embodiment, IR base station  106  may include a coarse, low-speed timer and a fine, high-speed timer, for example, as part of microcontroller  306 . The timers may be used by microcontroller  306  to begin IR transmission to portable devices  108 - n  at highly accurate times. 
       FIG. 4  illustrates periodic timing of the transmission of TSI  403  from RF base station  110  in accordance with an embodiment of the present disclosure. Depending upon the context of usage, reference item  403  may refer to either the TSI signal, or to the act of sending the TSI signal. RF base station  110  may maintain a high-speed clock  401 , which may be continuously powered if RF base station  110  is operated from a wired power source. RF base station  110  transmits a timing synchronization information (TSI)  403  having a period of T K  seconds, and sent at times T 1 , T K +T 1 , 2T K +T 1 , . . . nT K +T 1 , with “n” being a positive integer. Typically, the period T K  is in the range of 1≦T K ≦60 seconds. TSI  403  may be received by both the IR base stations  106  and portable devices  108 , and may be used to control timing of an IR signal transmitted by IR base station  106 . In some embodiments, IR base stations  106  may synchronize their clocks to TSI  403  as received from RF base station  110 . 
       FIG. 5  illustrates timing of operations at an IR base station  106 . All of the IR base stations  106  transmit a same IR-ID at a set delay from the time that each IR base station  106  received TSI  403 . As illustrated, IR base station  106  receives TSI  403  signals periodically with a period of T K , which is the same period as illustrated in  FIG. 4  for RF-BS  110 . In particular,  FIG. 5  illustrates reception  503  and usage of TSI  403 . Depending upon the context of usage, reference item  503  may refer to either the TSI signal received by IR base station  106 , or to the act of receiving the TSI signal by IR base station  106 . IR base station  106  may run a low-speed clock  502  substantially continuously. For example, low-speed clock  502  may run at about 32 kHz. 
     In some embodiments, IR base station  106  may further include a high-speed, high-accuracy clock  501 . In some embodiments, clock  501  may be an 8 MHz clock with 0.125 uS resolution and about 10 parts per million (ppm) accuracy. Timing accuracy between different IR base stations  106  should not differ by more than about 2 us. At predetermined times based upon low-speed clock  502 , IR base station  106  may activate clock  501 , clock  501  being activated at times to substantially ensure that clock  501  is active when TSI  403  is expected to be received  503  at IR base station  106 . This operation reduces power consumption by not powering clock  501  when it will not be needed. 
     Usage of periodically-activated high-speed, high-accuracy clock  501  in order to provide greater timing accuracy when needed, than can be provided by low-speed clock  502 , is referred to as super synchronization. 
     TSI  403  may include information that indicates a desired time delay between the receipt  503  of TSI  403  at IR base station  106 , and the transmission  505  of an IR beacon transmitted by IR base station  106 . Depending upon the context of usage, reference item  505  may refer to either the IR beacon transmitted by IR base station  106 , or to the act of sending the IR beacon by IR base station  106 . IR base station  106  may use the time delay indicated by TSI  403 , together with high-speed clock  501 , in order to more accurately control the time at which IR base station  106  transmits its IR beacon  505 , e.g., by setting T 2 =T 1 +TimeDelay. Embodiments in accordance with the present disclosure use time-division multiplexing (TDM) and frequency-division multiplexing (FDM) in order to control TimeDelay in order to coexist with other RF stations  110  that control different IR base-stations  106 . 
     As illustrated in  FIG. 5 , high-speed clock  501  may be activated at times T 0 , T K +T 0 , 2T K +T 0 , . . . nT K +T 0 , with T 0 &lt;T 1  and “n” being a positive integer. IR base station  106  may use high-speed clock  501  to denote the accurate time  503  of the RF base station clock for reception of TSI  403  (i.e., denote the occurrence of times T 1 , T K +T 1 , 2T K +T 1 , . . . nT K +T 1 ). At receipt  503  of TSI  403 , the desired time delay is decoded from TSI or a priori information given to the base-station  403 , and the decoded time delay is applied to high-speed clock  501  in order to accurately determine the times at which to transmit the IR beacon  505 , i.e., at times T 2 , T K +T 2 , 2T K +T 2 , . . . nT K +T 2 . High-speed clock  501  may be deactivated shortly after the end of transmission of IR beacon  505 , i.e., shortly after times T 3 , T K +T 3 , 2T K +T 3 , . . . nT K +T 2 . By deactivating high-speed clock  501  when it is no longer needed, power consumption by IR base station  106  may be reduced. Typically, high-speed clock  501  needs to be activated for only about 10 ms to about 200 ms within each period of T K  seconds. 
     In some embodiments, high-speed clock  501  may be activated or corrected based upon time information derived from a request for time from optional time adjust module  318 , and an associated response from time adjust module  318 . For example, IR base station  106  may transmit a request for time to time adjust module  318 . A response would be received and used to correct low-speed clock  502 . In this way, IR base station  106  is able to more accurately estimate the time at which high-speed clock  501  should be turned on, e.g., T 0 , . . . nT K +T 0 . 
       FIGS. 6-7  illustrate another embodiment of a method to provide accurate timing information, in which an IR base station  106  requests timing updates from RF base station  110 .  FIG. 6  illustrates timing of the periodic transmission of a timing synchronization information (TSI) signal  607  from RF base station  110  in accordance with an embodiment of the present disclosure. RF base station  110  transmits TSI  607  having a period of T K  seconds, and sent at times T 1 , T K +T 1 , 2T K +T 1 , . . . nT K +T 1 . Typically, the period T K  is in the range of 1≦T K ≦60 seconds. RF base station  110  may operate substantially continuously a high-speed clock  601 . 
     At some times, which may be unplanned or unpredictable by RF base station  110 , RF base station  110  may receive from IR-BS  106  a timing request  603  for timing information. The timing request  603  may be a request for a value of a time delay to a next beacon transmission time or the next IR transmission time. RF base station  110  may respond by transmitting a response  605  that provides coarse timing to the next transmission of TSI  607 . This is mainly needed in case the IR base-station  106  loses its coarse timing synchronization (e.g., by drifting too far) and does not accurately know when to initiate the high speed clock. This eliminates excessive power consumption especially in periods where the IR base-station  106  is not operational and there are no beacons and the IR base-station  106  needs to search for them. 
       FIG. 7  illustrates timing of operations at an IR base station  106 . An as illustrated, operation of IR base station  106  is periodic with a period of T K , which is the same period T K  as illustrated in  FIG. 6  for RF-BS  110 . In particular,  FIG. 7  illustrates periodic reception  707  of TSI  607 . 
     At some predetermined times, IR base station  106  transmits a timing request  703  to RF base station  110 . Reception of, and response to timing request  703  by RF base station  110  is illustrated in  FIG. 6 .  FIG. 7  illustrates reception  705  of the response from RF base station  110  with the coarse timing to the next reception  707  of TSI  607 . IR base station  106  may use the timing information within TSI  607  to adjust the coarse clock  701  within IR base station  106 . 
     In some embodiments, rough timing for the time location of the start of the high speed clock  501  is based on a second beacon which is transmitted less often and drives low speed clock  502  until the start time of high accuracy clock  501 . 
     In some embodiments, multiple RF base stations  110  that control a first set of IR base stations  106  may transmit their TSI  403  signals on different frequencies in order to avoid collisions with other RF base-stations  110  that control a different set of IR base-stations  106 . 
     In yet another embodiment in accordance with the present disclosure, multiple RF base stations  110  may transmit their TSI  403  signals in different time slots in order to avoid collisions. IR base station  106  are able to accommodate the different time slots, for example by provisioning time slots during setup of IR base station  106 , or through appropriate notification in the TSI  403  signals. 
     The disclosed methods may be readily implemented in software, such as by using object or object-oriented software development environments that provide portable source code that can be used on a variety of computer or workstation platforms. Alternatively, the disclosed system may be implemented partially or fully in hardware, such as by using standard logic circuits or VLSI design. Whether software or hardware may be used to implement the systems in accordance with various embodiments of the present invention may be dependent on various considerations, such as the speed or efficiency requirements of the system, the particular function, and the particular software or hardware systems being utilized. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the present invention may be devised without departing from the basic scope thereof. It is understood that various embodiments described herein may be utilized in combination with any other embodiment described, without departing from the scope contained herein. Further, the foregoing description is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. Certain exemplary embodiments may be identified by use of an open-ended list that includes wording to indicate that the list items are representative of the embodiments and that the list is not intended to represent a closed list exclusive of further embodiments. Such wording may include “e.g.,” “etc.,” “such as,” “for example,” “and so forth,” “and the like,” etc., and other wording as will be apparent from the surrounding context. 
     No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items. Where only one item is intended, the term “one” or similar language is used. Further, the terms “any of” followed by a listing of a plurality of items and/or a plurality of categories of items, as used herein, are intended to include “any of,” “any combination of,” “any multiple of,” and/or “any combination of multiples of” the items and/or the categories of items, individually or in conjunction with other items and/or other categories of items. 
     Moreover, the claims should not be read as limited to the described order or elements unless stated to that effect. In addition, use of the term “means” in any claim is intended to invoke 35 U.S.C. §112, ¶6, and any claim without the word “means” is not so intended.