Abstract:
A slave low power radio frequency transceiver participates in a network controlled by a master low power radio frequency transceiver and synchronises its timing to the network timing by receiving radio packets transmitted from the controlling low power radio frequency transceiver. The slave transceiver comprises timing means; detection means operable over an interval of time for detecting a predetermined portion of a received radio packet; and adjusting means for adjusting the timing means in response to the detection of a predetermined portion of a received radio packet. The slave low power radio frequency transceiver is arranged to controllably vary the duration of the interval of time.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the synchronization of a receiving transceiver to a transmitting transceiver. Particular embodiments of the invention relate to synchronization in a Bluetooth (trademark) piconet. 
     BACKGROUND OF THE INVENTION 
     In a communication network comprising a plurality of transceivers it may be necessary to keep the transceivers synchronized so that they use the same timing for communications. One transceiver may act as a master that defines the timing for the communication system. The other transceivers act as slaves and keep synchronized with the timing of the master. One such network is a Bluetooth (trademark) piconet as described in the Specification of the Bluetooth System v1.1, Part B (Baseband). 
     One way of maintaining synchronization within the network is for the master to transmit periodically radio packets. In the Bluetooth system the master transmits radio packet messages with an Access Code in the preamble of a packet header. This Access Code is detected by a slave receiver. The reception of a message sent from the master allows the slave to compare its timing with that of the master and to adjust its timing to maintain synchronization. This may be done by adding an offset value to the value of the native clock in the slave. 
     The Bluetooth Specification, revision 1.1 requires “The slave shall be able to receive the packets and adjust RX timing as long as the timing mismatch remains within the +/−10 μs uncertainty window”. 
     Consequently, in current implementations the slaves listen for a message in a listening window of fixed duration (20 μs) centred at the time a message is expected to be received. This means that if a master is transmitting more than 10 μs earlier or more than 10 μs later than the slave is expecting, the slave will not be able to receive the packet and will not be able to adjust its timing. Since both master and slave are using different native clocks that are allowed to drift around ±20 ppm, the slave may lose connection when it has not received a packet from the master within a time t conloss  where: 
     
       
         
           
             
               t 
               conloss 
             
             = 
             
               
                 
                   10 
                   * 
                   
                     10 
                     
                       - 
                       6 
                     
                   
                   ⁢ 
                   s 
                 
                 
                   
                     ( 
                     
                       20 
                       + 
                       20 
                     
                     ) 
                   
                   * 
                   
                     10 
                     
                       - 
                       6 
                     
                   
                 
               
               = 
               
                 250 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 ms 
               
             
           
         
       
     
     Therefore if the master has not polled the slave for more than 250 ms or the environment is disturbed for that time, in the worst case, the connection will be lost. 
     During a normal, undisturbed active connection between master and slave, this time is never reached since the master has to poll the slave within T Poll , that is 25 ms by default. However, if the master performs an Inquiry or Page, this continuous polling is interrupted for the time it takes to complete the Inquiry or Page, which is typically 5-10 s. As a consequence, it is very likely that all slaves that are connected to a piconet, will lose connection as soon as the master performs an Inquiry or Page. 
     One solution to this problem would be to increase the size of the fixed uncertainty window. This is unattractive because it is not required by the Bluetooth Specification and because it would result in a significant and permanent increase in power consumption by the receiver. 
     Another solution to the problem is provided in the Bluetooth Specification. It allows the master to force its slaves into a Hold mode for the page/inquiry time to prevent loss of synchronization. However, it is not mandatory for transceivers to support the Hold mode. In addition, the master has to separately force each slave to the Hold mode, which wastes air capacity. 
     The same problem of loss of synchronization can occur if the normal connection between the master and slave is disturbed. This may result from interference or multipath propagation causing degradation of the message such that it is not received in the listening window of the receiver. The use of Hold mode is not a solution to this problem. 
     It would be desirable to improve synchronization within a communications network. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention there is provided a low power radio frequency transceiver, operable to participate a network controlled by another low power radio frequency transceiver and to synchronise its timing to the network timing by receiving radio packets transmitted from the controlling low power radio frequency transceiver, comprising: timing means; detection means operable over an interval of time for detecting a predetermined portion of a received radio packet; and adjusting means for adjusting the timing means in response to the detection of a predetermined portion of a received radio packet, wherein the low power radio frequency transceiver is arranged to controllably vary the duration of the interval of time. 
     According to another aspect of the present invention there is provided a method of synchronising a first timing means of a first low power radio frequency transceiver, participating in a network controlled by a second low power radio frequency transceiver comprising a second timing means, to the second timing means, comprising the steps of: a) in the first transceiver, receiving data and performing a correlation over an interval to detect a predetermined data sequence in received data; and b) controlling the duration of the interval in dependence upon the time since the last detection of the predetermined sequence. 
     According to a further aspect of the present invention, there is provided a network comprising a first low power radio frequency transceiver comprising a first timing means synchronized to a second low power radio frequency transceiver comprising a second timing means, wherein the first transceiver is arranged to perform a correlation of controllable duration to synchronize its timing means to the timing means of the second transceiver. 
     According to another aspect of the present invention there is provided a method of joining an additional transceiver to a network of transceivers, wherein the network of transceivers comprises a Master and at least one Slave which synchronizes to the Master by performing a correlation to detect a pre-defined sequence transmitted by the Master, comprising the step of: extending the period of correlation while the Master joins a new transceiver to the network. 
     According to a further aspect of the present invention there is provided a receiver, for synchronising its timing to a transmitter by receiving radio packets therefrom, comprising: timing means; and detection means for detecting a predetermined portion of a received radio packet and for adjusting the timing means in response thereto, wherein the detection means is operable over an interval of time that is variable in dependence upon the time since the last detection of a predetermined portion. 
     The timing means of the receiving low power radio frequency transceiver may by provided by an offset (e.g. M_OFFSET) to its native clock. 
     Embodiments of the invention dynamically increase the synchronization window during an active connection state as long as no valid packet from the master has been received. The term “synchronization window” denotes a variable (non-fixed) window used for synchronization which is therefore different from the static (fixed) uncertainty window of the prior art. 
     Embodiments of the invention, may maintain synchronization when the master makes a page/inquiry without entering the Hold mode and maintain synchronization when the environment has disturbances. This is achievable without significantly and permanently increasing the power consumption of the receiving transceiver. For embodiments of the invention, the connection in the active state is extremely tolerant if the environment is disturbed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention and to understand how the same may be carried into effect reference will now be made to the accompanying drawings in which: 
         FIG. 1  illustrates a communications network including a master and slave units; 
         FIG. 2  illustrates the time frame of the communications network; 
         FIG. 3  illustrates a radio packet; 
         FIG. 4  illustrates a transceiver unit suitable for use as a master or slave; 
         FIG. 5  illustrates a process flow occurring in a slave according to one embodiment of the invention; and 
         FIG. 6  illustrates the variation of the length (T_swin_len) of the synchronization window with the time (t_not_synchronized) since the last synchronization. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a network  2  of radio transceiver units, including a master unit  4  and slave units  6 ,  8  and  10 , communicating by transmitting and receiving radio packets. The master unit is the transceiver unit which initiates the connection of a slave to the network. There is only one master in a network. The network operates in a time division duplex fashion. The transceiver units are synchronized to a common time frame determined by the master unit  4 . This time frame consists of a series of time slots of equal length (625 μs). Each radio packet transmitted in the network has its start aligned with the start of a slot and a single packet is transmitted in the network at a time. When the master unit is performing point-to-point communication a transmitted radio packet is addressed to a particular transceiver which replies to the master unit by transmitting a radio packet addressed to the master unit in the next available time slot. Any time misalignment between the master and a slave is corrected by adjusting the timing of the slave so that it is synchronized with the timing of the master. In the slave, an offset (M_OFFSET) is added to the native clock value of the slave to achieve the clock value of the master. Thus the timing of the slave and master are synchronized. 
     The transceivers transmit and receive, in this example, in a microwave frequency band, illustratively 2.4 GHz. The network reduces interference by changing the frequency at which each radio packet is transmitted. A number of separate frequency channels are assigned each with a bandwidth of 1 Mhz, and the frequency may hop at a rate of 1600 hops/s. The frequency hopping of the transceivers communicating in or joining the network is synchronized and controlled by the master unit. The sequence of hopping frequencies is unique for the network and is determined by a unique identification of the master unit. The position within the sequence of hopping frequencies is determined by the timing of the master i.e. the value of the native clock in the master. Each transceiver unit has a unique identification, the Unit ID, henceforth referred to as the Slave ID for the slave units and the Master ID for a master unit. 
     The network is a radio frequency network suitable for transmitting voice information or data information between transceivers. The transmissions made are of low power, for example 0 to 20 dBm, and the transceiver units can effectively communicate over the range of a few centimetres to a few tens or hundred of metres. 
     Referring to  FIG. 2 , a frame  20  is illustrated. This frame  20  is the common time frame used by the network  2  and controlled by the master unit  4 . The frame illustratively has slots  22  to  29 . The slots designated by even numbers are reserved. Only the master unit can begin transmitting a radio packet aligned with the start of the even numbered slots. The slots designated by odd numbers are reserved. Only radio packets transmitted by a slave, that is radio packets addressed for reception by the master unit can have their start aligned with the start of the odd numbered slots. In the communication mode, a slave can only transmit and must transmit in the slot immediately after that in which it was addressed. Each slot is allocated a different one of a sequence of hopping frequencies. A slot has a constant time period T_slot and is typically 625 microseconds. 
     Referring to  FIG. 3 , a typical radio packet  30  is illustrated. The radio packet  30  has a start  32  and contains three distinct portions: a first preamble portion contains an Access Code  34 , a second portion contains a Header  36  and a third portion contains a Payload  38 . 
     The Access Code is a series of symbols used in the network to identify the start of a radio packet. It has a fixed length. In a normal communication mode, the master and slave units use the Master ID as the Access Code. As there is only one master unit in a network, the Master ID identifies the network. 
     The header  36  may or may not be present. If present, it has a fixed length. The header contains control words. The local address (AM_ADDR) is a word uniquely identifying a slave within a network. The local address is assigned to a slave unit by the master unit when the master unit joins the slave to the network. 
     The payload  38  carries either transceiver control information or voice/data information. The payload is of variable length and may be absent. 
     Referring to  FIG. 4 , a schematic illustration of a transceiver unit  40  is shown. Only as many functional blocks and interconnections are shown in this diagram as are necessary to explain in the following how a transceiver unit and the communication network operates. The transceiver unit  40  contains a number of functional elements including: an antenna  42 , receiver  50 , controller  60 , memory  62 , clock  64 , and transmitter  66 . Although these elements are shown as separate elements they may in fact be integrated together and may be carried out in software or in hardware. 
     The controller  60  determines the content of the Access Code  34 , the Header  36  and the payload  38  of a packet to be transmitted. The payload content may be data supplied by data signal  71  to the controller  60  or control information generated in the controller  60 . The controller  60  provides the packet data to the transmitter  66  as packet data signal  61 . 
     The transmitter  66  modulates a carrier wave in dependence upon the packet data to produce the transmitted signal  63  supplied to the antenna  42  for transmission. The controller  60  provides a transmission frequency control signal  65  to the transmitter  66 , which controls the frequency of the carrier wave to be a correct one of a sequence of hop frequencies. 
     All packets transmitted in the communication mode in the network have an Access Code determined by the MASTER ID and a frequency determined by the master unit&#39;s frequency-hopping sequence. When the transceiver unit  40  is operating as a slave, the controller  60  emulates the clock of the master unit by combining its own native clock signal  67  and an offset value M_OFFSET stored in memory  62 . M_OFFSET represents the difference between the native clocks of the master and the slave. The controller  60  reproduces the master unit&#39;s frequency-hopping sequence using a value of MASTER ID stored in memory  62  and determines the correct position within the hopping sequence from the emulated master clock value. The transmission frequency control signal  65  is then produced by controller  60  to control the transmitter  66  to transmit at the correct hop frequency, which is determined from the master unit&#39;s frequency-hopping sequence and the emulated master clock value. Thus the master and slave synchronously hop frequencies. 
     The antenna  42  receives a radio signal  41  and supplies it to the receiver  50  which demodulates the radio signal  41  under the control of a reception frequency control signal  43 , from the controller  60 . Demodulation produces a digital signal  45 , which is provided to controller  60 . The reception frequency control signal  43  controls the receiver to receive at the correct hop frequency, which is determined from the master unit&#39;s frequency-hopping sequence and the emulated master clock value. 
     In this embodiment, the controller  60  is used to detect the Access Code of a received packet. In other embodiments, this function may be performed by a separate component. The controller  60  uses the digital signal  45  to synchronize the transceiver unit  40  to the time frame of the network. The controller determines the length of the synchronization window that should be used for the correlation process. The synchronization window is symmetrically positioned to when the access code transmitted by the master is expected to be received. A sliding correlation is then used by the controller  60  for the duration of the synchronization window to detect the presence of an expected Access Code in a received radio packet. 
     If the presence of the expected access code is detected, the controller resynchronizes the slave&#39;s emulated version of the master clock to the native clock of the master. The controller compares the clock value at which a message was expected to be received and the clock value at which the message was received and varies the value M_OFFSET stored in memory  62  by the difference. Thus the emulated master clock is resynchronised with the native clock of the master. 
     The controller  60  accepts those received radio packets with Access Codes which correspond to the expected Access Codes and rejects those received radio packets with Access Codes that do not correspond to the expected Access Code. If the radio packet is accepted, then the header of the packet is decoded. If the packet is addressed to the slave then the payload is obtained. The content of the payload may be provided to other circuitry (not shown) as output signal  73  by controller  60 . 
       FIG. 5  illustrates a process flow occurring in a slave according to one embodiment of the invention. For an understanding of this embodiment the following terms are defined: 
     T_swin_len is the length in microseconds of the synchronization window. 
     t_next_rx is when the access code of the next packet to be received is expected. 
     L_min is the minimum length of T_swin_len. 
     L_max is the maximum length of T_swin_len. 
     t_last_sync is the time at which synchronization between the slave and master last occurred. 
     n represents the number of Access Codes the detection of which has been missed. 
     Drift_slave and Drift_master are respectively the maximum drift in the slave native clock and the maximum drift in the master native clock. In active mode, each has a maximum drift of +/−20 ppm. This is fixed and a requirement of the Bluetooth Specification. 
     The slaves in active connection mode normally try and receive every 2 slots (1.25 ms), so t_next_rx is normally 1250 μs after the last Access Code was received. However, this may be interrupted for example if the slave transceiver conducts an inquiry scan for a different piconet (different master) or if the slave fails to receive an expected packet. It is to be expected, however, that the next Access Code will be received at some multiple of 1.25 ms after the last Access Code was received. 
     At step  100 , the controller  60  performs a correlation for the duration of the synchronization window or the detection of the Access Code, if sooner. The duration of the correlation is T_swin_len and the correlation window is centred on the expected time of arrival of the next Access Code (t_next_rx). Thus the correlation begins at t_next_rx−½×T_swin_len and continues until t_next_rx+½×T_swin_len. 
     If the Access Code is detected at step  100 , the process moves through steps  110 ,  112  and  114  before returning to step  100 . 
     At step  110 , the controller calculates the new value for M_OFFSET thereby bringing the slave&#39;s emulation of the master clock into synchronization with the master&#39;s native clock. The new value of M_OFFSET is equal to the old value plus the difference between the expected time of detection of the Access Code and the actual time the Access Code was detected in step  100 . 
     At step  112 , the value of t_last_sync is updated. It is made equal to the master clock time at which the Access Code was detected in step  100 . This is therefore the last time synchronization occurred with the master. 
     At step  114 , T_swin_len is set to the minimum value L_min and T_next_rx, which is the time at which the detection of the next Access Code is expected is made equal to the master clock time at which the Access Code was detected at step  100  (t_last_sync) plus the duration of a frame (1.25 ms). The variable n is set to 0. 
     If the Access Code is not detected at step  100 , the process moves through steps  120 ,  122 ,  124  and  126  before returning to step  100 . 
     At step  120 , it is checked that the synchronization window has finished and that the Access Code has not been detected. If this is the case the process proceeds to step  122 . If the synchronization window has not finished, correlation continues at step  100 . 
     At step  122 , the counter n is incremented one, as the detection of an Access Code has been missed. The process proceeds to step  124 . 
     At step  124 , the expected lime (t_next_rx) at which the next Access Code will be detected is increased by the duration of a frame (1.25 ms) from the last value of t_next_rx. This may be achieved by setting t_next_rx equal to t_last_sync plus, the Bluetooth period 1.25 ms multiplied by the number of missed detections n. The process then proceeds to step  126 . 
     At step  126 , the duration of the next synchronization window (T_swin_len) is calculated. Once calculated the process returns to step  100 , where the new values for the expected time of detection of the next Access Code and the length of the synchronization window are used. 
     The variation of the length (T_swin_len) of the synchronization window with the time (t_not_synchronized) since the last synchronization is illustrated in  FIG. 6 . The time since the last synchronization is the duration between the time at which the last Access Code was detected (t_last_sync) and the time at which the next detection of an Access Code is expected (t_next_rx) i.e. t_not_synchronized=(t_next_rx−t_last_sync). The length of the synchronization window is plotted in microseconds. The time since the last synchronization is plotted in microseconds. 
     The value of T_swin_len remains constant at L_min (20 μs) until over 250 ms have elapsed since the last successful synchronization. Thus during this period the size of the synchronization window is the same as the fixed uncertainty window of the prior art. 
     The value of T_swin_len then increases at a constant rate of, for example, 80×10 −6  until 15625 ms have elapsed since the last successful synchronization and T_swin_len has increased to 1.25 ms. The gradient of the graph is given by the sum of Drift_master and Drift_slave. Thus T_swin_len=(Drift_master+Drift_slave)*t_not_synchronized. 
     The value of T_swin_len then remains constant at L_max (1.25 ms) as the time since the last successful synchronization (t_not_synchronized) increases. L_max equals the Bluetooth frame (2 slots) as it should be possible to hear at every master TX slot. However, the value of L_max depends on the implementation. In a hardware implementation, 1.25 ms is best. But if a software implementation is used, for example, the preparation to receive again can consume time, so this preparation time must be subtracted from 1.25 ms. 
     The process of calculating the length of the synchronization window may proceed as follows.
     a) T_swin_len is set equal to (Drift_master+Drift_slave)×t_not_synchronized.   b) T_swin_len is then set equal to the greater of T_swin_len calculated at a) and L_min.   c) T_swin_len is then set equal to the lesser of T_swin_len calculated at b) and L_max.   

     The value of T_swin_len determined at c) is then used for step  100  in  FIG. 5 . 
     Embodiments of the invention may be implemented by updating the software of existing transceivers. 
     Although embodiments of the invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention. 
     Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.