GB2452015A - Synchronizing communication between a mobile station and base station - Google Patents

Abstract

A synchronizer 300 for use in a digital mobile communication system (100 fig.1), e.g. Terrestrial Trunked Radio (TETRA), to synchronize a communication between a mobile station and a base station. The synchronizer 300 includes a memory 303 to store for each of a plurality of mobile stations (104-109 fig.1) served by a base station (101 fig.1) a record comprising an identity of the mobile station together with a synchronization location in time of a previous synchronization of the mobile station. The synchronizer 300 also includes a processor 301 to retrieve the synchronization location from the record for a selected mobile station and to establish current synchronization with the mobile station using the retrieved location. The synchronization location may be a time, relative to a precise reference instant in time provided by a timer (205 fig.2), at which the most recent synchronization was found. The synchronization location may be measured in units of data symbols, or fractions of data symbols, displaced away from the precise instant in time.

Description

TITLE: SYNCHRONIZER, BASE STATION, SYSTEM AND METHOD

FOR USE IN MOBILE COMMUNICATIONS

TECHNICAL FIELD

The technical field relates generally to mobile

communications. In particular, the technical field

relates to synchronizing communication between a mobile station and a base station in a digital mobile communication system.

BACKGROUND

In many digital mobile communication systems, a synchronization pattern is used to establish synchronization between a transmitting terminal and a receiving terminal so that a signal sent between the terminals can be sampled and decoded in the receiving terminal at the correct instant in time. The transmitting terminal sends a fixed, pre-defined pattern of symbols at a pre-defined position in the signal, and the receiving terminal receives the pattern and, in a processor known as a synchronizer, recognizes and uses the received pattern to determine an instant in time when the received pattern best matches the same fixed pattern held by the receiving terminal, thereby indicating synchronization.

For example, a system using such a synchronization procedure is a TETRA (Terrestrial Trunked Radio) system, that is a system operating according to the protocols specified in the TETRA standards as defined by ETSI (the European Telecommunications Standards Institute) . TETRA systems use a time slotted Time Division Multiple Access (TDMA) protocol in which communications are in allocated time slots on communication channels defined by different carrier frequencies. The time slots have a length of 14.16667 milliseconds. Four slots make up a frame and eighteen frames make up a multiframe, having a length of approximately one second. The data communicated in each allocated time slot is known as a data burst. In a TETRA system, the synchronization pattern normally employed is a pre-defined set of eleven fixed symbols located in the middle of each data burst sent within each appropriate time slot.

A common way of obtaining synchronization in a system in which a fixed synchronization pattern is used, such as a TETRA system, is to operate a correlation procedure or algorithm in the synchronizer of the receiving terminal to decide when the synchronization symbol pattern included in the incoming signal best matches the same synchronization pattern held by the receiving terminal. In noisy conditions, it may be difficult for the correlation procedure to find the correct match between the synchronization patterns.

The TETRA standards apply two conditions to assist the procedure. First, a terminal which is a mobile station is required to synchronize its uplink communications, that is communications sent from the mobile station to another terminal which is a serving base station, with its downlink communications, that is communications sent from the serving base station to the mobile station. Second, the maximum distance between the mobile station and its serving base station is limited to approximately 50 km. The maximum synchronization jitter (possible displacement in time from a correct instant in time required for synchronization) of an uplink communication is then also limited to about 167 microseconds which is equivalent to about three TETRA symbols. Each TETRA symbol is about 56 microseconds long; or 255 TETRA symbols make up one TETRA time slot.

In other words, the synchronization pattern in a TETRA system is always located within about three symbols from the correct synchronization location, which significantly increases the chance of finding the correct synchronization.

Further assistance in finding the correct instant in time to give synchronization can be obtained by using in the synchronizer a procedure known as a flywheel procedure. When a TETRA mobile station is in a circuit switched call on a voice communication channel, the mobile station transmits in every fourth slot, that is in the same slot of consecutive frames. If, say, the maximum speed of the mobile station is 250 kilometres per hour, the maximum displacement of a synchronization between the slots in adjacent frames is about ten nanoseconds, that is about 0.0002 TETRA symbols. In other words, the next synchronization is expected to be found very close to the current synchronization. The flywheel procedure operated by the synchronizer of the receiving terminal therefore moves the incoming synchronization pattern in very small steps in time relative to the pattern held by the receiving terminal to find a match between the patterns giving the correct synchronization.

Unfortunately, the flywheel procedure is not used in certain communications, e.g. transmissions on uplink TETRA control or data channels which result from a random access channel allocation procedure in which channel access requests may be sent by different mobile stations at the same time. As a result, the synchronization on such channels is more difficult to find and, in general, is less precise than on channels where the flywheel procedure is used. Loss of precision of synchronization leads to loss of performance in the receiving terminal.

Thus, there exists a need for a synchronizer, and a base station, a system and a method employing the synchronizer, for use in digital mobile communications, which addresses at least some of the shortcomings of past and present synchronizer techniques and/or procedures.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying drawings, in which like reference numerals refer to identical or functionally similar items throughout the separate views which, together with the detailed description below, are incorporated in and form part of this patent specification and serve to further illustrate various embodiments of concepts that include the claimed invention, and to explain various principles and advantages of those embodiments.

In the accompanying drawings: FIG. 1 is a block schematic diagram of an illustrative communication system for operation in accordance with embodiments.

FIG. 2 is a block schematic diagram of an illustrative layout of a base station of the system of FIG. 1.

FIG. 3 shows an illustrative form of a synchronizer coupled to a controller of a base station of the system of FIG. 1.

FIG. 4 is a flowchart of an illustrative method of operation embodying the invention in the system of FIG. 1.

FIG. 5 is a flow chart of an illustrative method of operation in a synchronizer of a base station of the system of FIG. 1 during operation of the method illustrated in FIG. 4.

FIG. 6 is a block schematic diagram of an illustrative layout of a mobile station of the system of FIG. 1.

Skilled artisans will appreciate that items shown in the drawings are illustrated for simplicity and clarity and have not necessarily been drawn to scale.

For example, the dimensions of some of the items may be exaggerated relative to other items to assist understanding of various embodiments. In addition, the description and drawings do not necessarily require the order illustrated. Apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the various embodiments so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Thus, it will be appreciated that for simplicity and clarity of illustration, common and well-understood items that are useful or necessary in a commercially feasible embodiment may not be depicted in order to facilitate a less obstructed view of these various embodiments.

DETAILED DESCRIPTION

Generally speaking, pursuant to the various embodiments to be described, there is provided a synchronizer for use in digital mobile communications, and a base station, a system and a method employing the synchronizer. The synchronizer is operable to synchronize a communication between a mobile station and a base station, especially an uplink communication sent from the mobile station to the base station. The synchronizer includes a memory operable to store for each of the mobile stations served by the base station a record comprising an identity of the mobile station together with a synchronization indication giving a synchronization time location of a previous synchronization with the base station. The synchronizer also includes a processor operable to retrieve the synchronization indication from the record of the memory for a selected one of the mobile stations and to establish current synchronization by the base station with the mobile station using the retrieved synchronization indication. Providing the synchronizer with a memory as described allows synchronization to be achieved more easily and more precisely by the synchronizer than by a comparable synchronizer that does not include such a memory. In particular, the memory allows the synchronizer to use a flywheel procedure to establish synchronization, e.g. particularly in situations in which such a procedure is

not used in the prior art.

Those skilled in the art will appreciate that these recognized advantages and other advantages described herein are merely illustrative and are not meant to be a complete rendering of all of the advantages of the various embodiments of the invention.

The embodiments to be described are applicable for example in TETRA systems. These TETRA systems include systems operating according to the well established TETRA standards known as TETRA 1' standards, which are used primarily for voice communication and provide limited slow data communication. The TETRA systems in which the embodiments to be described are applicable also include systems operating according to a newer, second generation of TETRA standards known as TETRA 2' standards. TETRA 2 standards are aimed at use in providing high speed data communication, for example for fast accessing of police databases, and for transfer of picture, image and video data and the like.

The embodiments to be described are also applicable in other systems in which a fixed pattern of symbols is sent to establish synchronization in the receiving terminal, such as an APCO 25 system, which is a system operating in accordance with the protocols of the APCO standards defined by the US Association of Public-Safety Communications Officials-International Inc. Referring now to the accompanying drawings, and in particular to FIG. 1, there is shown a block schematic diagram of an illustrative digital mobile communication system 100 for operation in accordance with embodiments. The system 100 operates in accordance with a pre-defined wireless communication protocol, e.g. providing speech and/or data communication. The operating protocol of the system 100 may for example be in accordance with the TETRA standards (TETRA 1 or TETRA 2) or other standards, such as the APCO 25 standards, in which a current synchronization for a comxnunication.between a transmitting terminal and a receiving terminal is established by the transmitting terminal sending a fixed pattern of symbols and the receiving terminal receiving, recognizing and using the received pattern to determine an instant in time when the received pattern matches the same fixed pattern held by the receiving terminal.

The system 100 includes a first base station (BS) 101 having wireless links with a plurality of user terminals in a service cell or site defined by the position of the BS 101. The user terminals include mobile stations and may also include at least one fixed terminal (not shown), e.g. used by a dispatcher or other operator sending and receiving operational control messages. Four of many possible mobile stations are shown linked to the ES 101, namely mobile stations (MSs) 104, 105, 107 and 109 having wireless links 110, 111, 113 and 115 respectively with the ES 101. The BS 101 thereby serves user terminals including the MS5 104, 105, 107 and 109 with wireless communications to and from other mobile stations either served by the BS 101 or by other base stations of the system 100 operably linked to the ES 101 or in other systems (not shown) operably linked to the system 100.

The system 100 may also include one or more further BSs each of which serves MSs within a coverage region or cell defined by the position of the ES. For example, the system 100 is shown in FIG. 1 as including a second base station (BS) 103 and further MSs 124, 125, 127 and 129 served by the ES 103 and having wireless links 130, 131, 133 and 135 respectively with the ES 103. The BS 103 is shown having a wireless link 117 with the first BS 101. The wireless link 117 is optional. It could be replaced by a fixed, e.g. cable or wired, link. The BS 103 has wireless links with a plurality of user terminals in a service cell or site defined by the position of the BS 103. The user terminals include the MSs 124, 125, 127 and 129 and may also include at least one fixed terminal (not shown), e.g. used by a dispatcher or other operator sending and receiving operational control messages. The ES 103 thereby serves user terminals including the MSs 124, 125, 127 and 129 with wireless communications to and from other mobile stations either served by the BS 103 or by other base stations of the system 100 operably linked to the BS 103, e.g. the BS 101, or in other systems (not shown) operably linked to the system 100.

Communications between the BS 101 and each of the MSs it serves, including the MSs 104, 105, 107 and 109 via the links 110, 111, 113 and 115 respectively, are made using the selected wireless communication protocol as discussed above. Similarly, communications between the ES 103 and each of the MSs it serves, including the MSs 124, 125, 127 and 129 via the links 130, 131, 133 and 135 respectively, are made by the same selected protocol.

The system 100 may include known infrastructure components or sub-systems in addition to the BSs 101 and 103. For example, the system may include one or more zone controllers (not shown) which provide co-ordination and control of the ESS in a given geographical zone or area.

FIG. 2 shows an illustrative layout 200 of one of the BS5 of the system 100. Any one or more of the BSs of the system 100, including each of the BS 10]. and the BS 103, may have the layout 200. As will be apparent to those skilled in the art, the layout of each of the BS5 may take one of many possible forms, and the layout 200 is therefore to be regarded as illustrative rather than definitive. Each ES of the system 100 may have more or less components than illustrated in the layout 200. In the layout 200, a controller 201 controls functional operations of the ES. A processor 202, e.g. a digital signal processor, operably connected to the controller 201 processes information sent in RF (radio frequency) signals to and from the ES.

The controller 201 and the processor 202 are operably connected to a timer 205, which is a clock providing operational timing, and to a memory 206 which stores data and programs needed in operation by the controller 201 and the processor 202. The processor 202 is operably connected to a plurality of RF transceivers two of which are shown, namely an RF transceiver 203 and an RF transceiver 207. Each of the RF transceivers 203 and 207 transmits and receives RF signals including signals carrying information sent to and from user terminals including MSs served by the ES. The signals are delivered over-the-air to and from an antenna 204 connected to the RF transceiver 203 and to and from an antenna 208 connected to the RF transceiver 207.

When the RF transceiver 203 receives via the antenna 204 an RF signal including information representing communicated speech or data, the signal is passed to the processor 202. Similarly, when the RF transceiver 207 receives via the antenna 208 an RF signal including information representing communicated speech or data, the signal is passed to the processor 202. The processor 202 converts each signal received by the RF transceiver 203 or the RF transceiver 207 into an electronic signal including communicated information. The communicated information includes system control information and user communicated information for onward delivery.

Where the communicated information comprises system control information the electronic signal produced by the processor 202 is passed to the controller 201. Where the electronic signal produced by the processor 202 comprises user communicated information for onward delivery it is delivered to a router 212 which routes the electronic signal toward its destination, e.g. via a wired or wireless link to another base station (such as via the link 117) or to a mobile station (other than the originator of the information) served by the ES via the processor 202.

Similarly, each incoming electronic signal received at the router 212 from a source other than the processor 202 which includes communicated user information to be sent to one of the user terminals including mobile stations served by the ES having the layout shown in the block diagram 200, is routed by the router 212 to the processor 202. The processor 202 processes each electronic signal which it receives from the router 212 into a form suitable for inclusion in an RF signal for transmission by the RF transceiver 203 via the antenna 204 or for transmission by the RF transceiver 207 via the antenna 208.

The processor 202 also prepares and receives system control messages and data received from the controller 201 to be sent to the mobile terminals served by the BS.

The BS having the layout 200 includes a power supply 211, e.g. from the main (mains) electricity supply, which provides a source of electrical energy for all active components of the BS.

Although the ES having the layout 200 is shown in FIG. 2 as having two RF transceivers connected respectively to two antennas 204 and 208, it could have one combination or alternatively more than two combinations of an RF transceiver and an antenna. In any event, the ES may operate in a full duplex manner.

Uplink communications from MSs to the BS may be sent in uplink channels, and downlink communications from the BS to MSs may be sent in separate downlink channels.

The uplink channels may for example use at least one carrier frequency different from at least one carrier frequency of the downlink channels.

The BS having the layout 200 also includes a resource scheduler 220 operably coupled to the controller 201. The resource scheduler 220 may be incorporated within the controller 201. The resource scheduler 220 is a processor, e.g. a digital signal processor, which operates a programmed algorithm to carry out functions within the BS relating to scheduling of uplink and downlink communications between the BS and MSs and other terminals (if any) served by the BS. In particular, the resource scheduler 220 computes, organises and specifies the allocation of the uplink and downlink communications. For example, where the operating protocol of the system 100 is a time slotted protocol and time slots in different channels are allocated to different communications according to a Time Division Multiple Access (TDMA) procedure, the resource scheduler 220 specifies which slots of each channel are to be used for the different communications. The resource scheduler 220 sends to MSs served by the BS advance notifications of resources, e.g. time slots in a given channel, to be used for a given communication.

A synchronizer 300 embodying the invention is operably coupled to the BS having the layout 200 and is thereby associated with the BS. The synchronizer 300 may be exclusively associated with the particular BS or may be associated with a number of BS5. As shown in FIG. 2, for example, the synchronizer 300 may be incorporated within the ES, e.g. coupled to the controller 201 of the BS and the timer 205.

FIG. 3 shows an illustrative arrangement of the synchronizer 300 coupled to the controller 201, showing the synchronizer 300 in more detail. The synchronizer 300 includes a synchronization processor 301 operably coupled to a memory 303. The memory 303 may be a dedicated part of the memory 206 (FIG. 2) or may be a separate memory. The synchronization processor 301 operates in conjunction with the memory 303 in a manner described later to establish synchronization for each uplink communication for which the resource scheduler 220 (FIG. 2) has allocated resource, e.g. in one or more specified slots of an uplink control channel or of an uplink traffic channel, e.g. a channel for delivering user information such as speech or packet data. The synchronization processor 301 may be a digital signal processor such as a microprocessor. The synchronization processor 301, or the complete synchronizer 300, may be incorporated together with the controller 201 (and optionally other components of the BS) in a combined microprocessor unit.

FIG. 4 is a flowchart of an illustrative method 400 of operation in the system 100 in accordance with an embodiment. The method 400 may be employed to establish a synchronized uplink communication from one of the MSs served by the BS incorporating the synchronizer having the form 300. For example, the method 400 may be employed to establish a synchronized uplink communication from the MS 104 to the BS 101 or from the MS 124 to the ES 103 (FIG. 1). In a preliminary step 401, the MS requests an uplink resource to send the required uplink communication to the ES. For example, the required resource may be one or more time slots of a control channel. Alternatively, the required resource may be one or more slots of a traffic channel, such as a packet data channel to send a series of data packets. The request may be made by a known random access procedure. In a step 403, the resource scheduler 220 (FIG. 2) allocates the required resource.

In a step 405, the resource scheduler 220 sends to the MS a notification of the resource allocation which has been made. This notification may for example provide an identification of the particular time slot(s), including the particular frame/multiframe in which the particular time slot(s) occur, and the particular channel carrier frequency to be employed by the MS for the uplink communication. The resource allocation and its notification may conveniently be made by the resource scheduler 220 in advance of the start of the communication, e.g. one or more time slots prior to the start of the communication. The number of time slots in advance depends on the current communication loading, e.g. traffic loading in the system 100, particularly involving the BS which includes the resource scheduler 220.

At substantially the same time as providing the notification in step 405, the resource scheduler 220 also provides in a step 407 a notification of the resource allocation to the synchronizer 300 of its BS.

This notification identifies the MS which is to make the uplink communication during the allocated resource.

For example, where the system 100 is a TETRA system, the MS may be uniquely identified by its Individual Short Subscriber Identity (ISSI) included in its signalling. In response, the synchronizer 300 refers to its memory 303 to retrieve a record relating to the identified MS. The record for each MS held in the memory 303 gives the synchronization time location, herein called synch location', of the most recent uplink communication synchronization between the identified MS and the BS. The synch location for the MS may be a time, relative to a precise reference instant in time provided by the timer 205, at which the most recent synchronization was found. The synch location may be measured in units of data symbols, or fractions of data symbols, displaced away from the reference instant in time in either a forward or reverse direction.

In a step 409, the synchronizer 300 retrieves by its synchronization processor 301 from the record for the identified MS in the memory 303 the synch location data specifying the most recent synchronization for an uplink communication from the identified MS.

In a step 411 which is carried out in response to the identified MS receiving the resource allocation notification sent in step 405, the MS sends in a first time slot of the resource allocation a data burst which contains a first fixed pattern of symbols to be received and identified by the BS to establish current synchronization with the MS. The first fixed pattern is in a relative position within the data burst which is the same for all (or most) data bursts and is known to the BS. For example, where the system 100 is a TETRA system, the first fixed pattern may be eleven fixed symbols at a known position in the middle of the data burst.

In a step 413 which follows receipt of the data burst sent in step 411, the synchronizer 300 begins a procedure to establish a current synch location of the MS using the data burst sent by the MS and received by the BS and the most recent synch location it has retrieved in step 409.

In a step 415, the synchronizer 300 retrieves from an internal memory, such as the memory 303 or the memory 206, a second fixed pattern of symbols intended to match the first fixed pattern and to be used in the procedure to establish the current synch location.

In a step 417, the synchronizer 300 applies by its synchronization processor 301 a correlation procedure to determine whether the first pattern of symbols matches the second pattern. In general, symbols of the first fixed pattern are required to correlate with symbols of the second fixed pattern. The synchronizer 300 looks first for a match between the two patterns using the synch location specified in the synch location data retrieved in step 409. The synchronization processor 301 may apply a known algorithm to determine when the best match is found between the symbols of the first and second fixed patterns. The algorithm may for example compare for each received symbol of the first fixed pattern in the received data burst an average root mean square (RMS) value of each of several samples of the symbol with a predicted R14S value of the symbol held in the second fixed pattern and determine that there is a match when the RMS and predicted NS values are sufficiently close to one another. The algorithm may further find whether all or a sufficient number of symbols of the first and second fixed patterns are sufficiently close to one another.

If no match between the first and second fixed patterns is found at the synch location retrieved from the memory 303 in step 417, the synchronizer 300 applies in a step 419 a flywheel procedure in which the synchronizer 300 moves the first fixed pattern relative to the second fixed pattern in small uniform steps until a match between the two patterns is found by the correlation procedure. The size of the steps applied may be selected to suit operational parameters in the system 100. Typically, such steps may have a size in the range of from about S/25 to about SilO, where S is the duration of a symbol.

The number of steps applied in the flywheel procedure of step 419 may be limited in a forward and a reverse direction starting from the synch location retrieved from the memory 303. For example, the number of steps applied to the first fixed pattern of symbols in each direction may be not greater than a number equivalent to ten symbols, especially a number equivalent to three symbols. The number of steps possible by moving the first fixed pattern in the forward direction may be greater than the number of steps possible in the reverse direction. The reason for this difference is as follows. If a MS is far from the 35, the sync location will be found late in the slot (forward direction). However, if a MS is near to the BS the synch location is very close to zero but still in the forward direction. The steps in the reverse direction are applied simply to allow for imperfect timing in the MS. For example, a maximum number of steps equivalent to seven symbols may be applied in the forward direction and a maximum number of steps equivalent to three symbols in the reverse direction may be found suitable in some embodiments.

In a step 421, a current synch location is found when the best match between the first and second fixed patterns of symbols is found by a combination of the flywheel and correlation procedures. Synchronization of the received data burst is thereby established. In a step 423 following step 421, the BS processes the received data using the established synchronization, e.g. by providing synchronized decoding of the data burst in the processor 202 or by providing synchronized delivery of the data burst to the router 212 for onward delivery. In a step 425, the synchronizer 300 updates its memory 303 in the record for the identified MS with the current synch location found in step 421.

Use of a synchronizer similar to the synchronizer 300 operating using a method similar to that of the method 400 has been found to show an improvement in the performance of a TETRA BS receiving an uplink communication. The co-channel rejection ratio, a known measure of uplink receiver performance, is improved compared with that obtained in a TETRA ES not able to operate by a flywheel procedure. Furthermore, the Bit Error Rate of the received signal is also improved.

FIG. 5 is a flow chart of an illustrative method 500 of operation in the synchronizer 300 during operation of the method 400. In a step 501, the synchronizer 300 receives from the resource scheduler 220 notification of the uplink communication from the particular identified MS. In a step 503, the synchronizer 300 refers to its memory 303. In a step 505 the synchronizer 300 determines whether or not there is a record relating to the identified MS already in the memory 303. If such a record does not exist, a NO' determination, the synchronization processor 301 of the synchronizer 300 creates in a step 507 a new record for the MS in the memory 303. At this stage the record contains data identifying the MS but no synch location data for the MS. If a record is found to exist in step 505, a YES' determination, the synchronizer 300 uses in a step 509 the synch location stored in the record in the manner of the steps 413 to 421 described earlier with reference to FIG. 4. Step 509 leads to an updated synch location being found for the MS.

Where a new record is created in step 507, the synchronizer 300 finds in a step 511 a current synchronization for the identified MS without the assistance of recorded synch location data from a recent synchronization, e.g. by a known procedure which does not include a flywheel procedure. In a step 513, the record for the identified MS in the memory 303 is updated with the current synch location found in either step 509 or in step 511.

Finally, in a step 515, the synch location data held in the memory 303 for the identified MS is deleted when a maximum age of the data is reached. The age of the data begins when the record for the identified MS is first updated with the synch location data for the identified MS in step 513. The age may be monitored by using the timer 205 to time stamp the recorded synch location data when it is first recorded in the memory 303 and by monitoring the time indicated by the timer 205 which has passed since the time stamp. The maximum age employed to trigger deletion in step 515 depends, for instance, on the number of MSs to be served (actually or potentially) by the BS incorporating the synchronizer 300 having the memory 303 in question and also the memory capacity available in the memory 303.

For example, a typical maximum age suitable for employment in a typical system, e.g. a typical TETRA system, is from about 0.1 seconds to about 10 seconds.

In general, the recorded synch location data becomes less useful as it ages since, if the identified MS is moving, the synch location will change with change of position of the MS. Therefore, deletion of the synch location data in the memory 303 for each served MS when the maximum age of the data is reached usefully keeps the total amount of data stored in the memory 303 within the available capacity of the memory 303 whilst maintaining the most useful data, in terms of age of the data, in the memory 303.

The maximum age employed in operation of step 515 may be a fixed age or a variable age, e.g. set dynamically according to a loading of the BS in terms of the number of MSs currently served by the BS, e.g. as notified periodically to the synchronizer 300 by the controller 201 of the BS.

When the deletion is applied in step 515, the complete record for the identified MS may be deleted from the memory 303. Alternatively, if capacity of the memory 303 permits, the record for the MS may be maintained but with no current related synch location data.

As will be apparent to those of ordinary skill in the art, the MS5 of the system 100 may take a number of different possible forms depending on an implementation of the MS, e.g. according to whether the MS is a portable or mobile radio, a mobile telephone, a personal digital assistant, a wireless enabled mobile computing device, or another known mobile terminal.

FIG. 6 shows a block diagram of an illustrative layout 600 of the MS illustrating the operational components present in the MS. Any one or more of the MS5 of the system 100, including the MS5 104, 105, 107, 109, 124, 125, 127 and 129, may have the layout 600.

In the layout 600, a controller 601 controls functional operations of the MS. A processor 602 operably connected to the controller 601 processes information sent to and from the MS. The controller 601 and the processor 602 are operably connected to a timer 605 which provides an internal clock for operational timing, and to a memory 606 which stores data and programs needed in operation by the controller 601 and the processor 602. The timer 605 and the controller 601 are further connected to a synchronizer 621, which provides synchronization operations.

The processor 602, which may for example comprise a digital processor, which may be included with the controller 601 in a common digital signal processing unit, is operably connected to a radio frequency (RF) transceiver 603 which transmits and receives RF signals including signals carrying information sent to and from the MS. The signals are delivered over-the-air to and from an antenna 617 connected to the RF transceiver 603.

When the RF transceiver 603 via the antenna 617 receives an RF signal including information representing communicated speech, the processor 602 extracts the speech information and delivers a signal including the extracted speech information to an audio output 610 which comprises a transducer such as a speaker which converts the signal to audio form to reconstruct the communicated speech for a user of the mobile station having the layout 600. The MS also includes an audio input 611 which comprises a transducer such as a microphone which converts speech of the user into the form of an electrical signal and delivers the signal to the processor 602 which processes the signal into a form suitable for inclusion in an RF signal for transmission by the RF transceiver 603 via the antenna 617.

When the RF transceiver 603 receives via the antenna 617 a signal representing communicated (non-speech) data, e.g. alphanumeric characters representing words or numerals or picture or video information, the processor 602 extracts information relating to the communicated data and delivers a signal including the extracted data to a data output 612. The data output may for example comprise a connection to an external data processing terminal (not shown), e.g. a personal computer.

A data input 613 provides an input signal from a user including data to be communicated. The data input 613 may for example comprise a connection to a data source, e.g. a personal computer (not shown). The signal provided by the data input 613 is delivered to the processor 602 which processes information included in the signal into a form suitable for inclusion in an RF signal to be transmitted by the RF transceiver 603 via the antenna 617.

The MS having the illustrative layout 600 also includes a user interface 614, e.g. a keypad and control buttons, which allows a user to enter instructions and data into the MS. The user interface 614 is operably connected to the controller 601 to receive signals representing instructions entered by a user at the user interface 614. The user interface 614 is also operably connected to the processor 602 to enable a signal representing data entered by the user at the user interface 614 to be delivered to the processor 602. The processor 602 processes data included in the signal into a form suitable for inclusion in an RF signal to be transmitted by the RF transceiver 603 via the antenna 617.

The MS having the layout 600 also includes a resource acceptor 620 operably coupled to the controller 601. The resource acceptor 620 may be incorporated within the controller 601. The resource acceptor 620 is a processor or part of a processor, e.g. a digital signal processor, which operates a programmed algorithm. The resource acceptor 620 carries out functions within the MS relating to scheduling of communications between the MS and the ES serving the MS in accordance with the defined protocol by which the MS and ES operate. In particular, the resource acceptor 620 accepts notification and allocation of times and channels made by the resource scheduler 220 (FIG. 2) of the serving ES. Thus, the resource acceptor 620 ensures that, under control of the controller 601, downlink transmissions from the serving ES are received, and uplink transmissions to the BS are sent, at the specified times and on the specified channels.

The synchronizer 621 establishes current synchronization between a downlink data burst from the serving BS by finding a match between a fixed pattern of symbols contained in the data burst and a fixed pattern of the same symbols held by the synchronizer 621. The synchronizer 621 operates a flywheel procedure in a manner similar to that described earlier with reference to FIG. 4 for operation of the synchronizer 300. However, operation of the synchronizer 621 can be simpler because normally the BSs do not move and their identity is always known to the MS. Therefore, the synchronizer 621 can operate the flywheel procedure without the need for a dynamic record similar to that provided by the memory 303 in the synchronizer 300.

The MS having the layout 600 includes an electro-optical display 609 operable to display information to a user in a known manner. The display 609 is driven by a display driver 607 under control of the controller 601.

The MS having the layout 600 includes a battery 616 which provides a source of electrical energy for all active components of the MS.

In the foregoing specification, specific

embodiments have been described. However, one of ordinary skill in the art will appreciate that various modifications and changes can be made without departing from the scope of the invention as set forth in the accompanying claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present teachings. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this patent application and all equivalents of those claims as issued.

Moreover in this document, relational terms such as first' and second', top' and bottom', and the like, may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions.

The terms comprises', comprising', has', having', includes', including', contains', containing' or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises, has, includes or contains a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element preceded by comprises...a', has 10...a', includes...a', or contains...a' does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises, has, includes, contains the element. The terms a' and an' are defined as one or more unless explicitly stated otherwise herein. The terms substantially', essentially', approximately', about' or any other version thereof, are defined as being close to as understood by one of ordinary skill in the art, and in one non-limiting embodiment the term is defined to be within 10%, in another embodiment within 5%, in another embodiment within 1% and in another embodiment within 0.5%, of a stated value. The term coupled' as used herein is defined as connected, although not necessarily directly and not necessarily mechanically. A device or structure that is configured' in a certain way is configured in at least that way, but may also be configured in ways that are not listed.

It will be appreciated that some embodiments may be comprised of one or more generic or specialized processors (or "processing devices") such as microprocessors, digital signal processors, customized processors and field programmable gate arrays (FPGAs) and unique stored program instructions (including both software and firmware) that control the one or more processors to implement, in conjunction with certain non- processor circuits, some, most, or all of the functions of the method and apparatus for synchronization in a digital mobile communication system as described herein. The non-processor circuits may include, but are not limited to, a radio receiver, a radio transmitter, signal drivers, clock circuits, power source circuits, and user input devices. As such, these functions may be interpreted as steps of a method to perform the synchronization in a digital mobile communication system as described herein.

Alternatively, some or all functions could be implemented by a state machine that has no stored program instructions, or in one or more application specific integrated circuits (ASICs), in which each function or some combinations of certain of the functions are implemented as custom logic. Of course, a combination of the two approaches could be used. Both the state machine and ASIC are considered herein as a processing device' for purposes of the foregoing discussion and claim language.

Moreover, an embodiment including a memory can be implemented as a computer-readable storage element having computer readable code stored thereon for programming a computer (e.g., comprising a processing device) to perform a method as described and claimed herein. Examples of such computer-readable storage elements include, but are not limited to, a hard disk, a CD-ROM, an optical storage device, a magnetic storage device, a RON (Read Only Memory), a PROM (Programmable Read Only Memory), an EPROM (Erasable Programmable Read Only Memory), an EEPROM (Electrically Erasable Programmable Read Only Memory) and a Flash memory.

Further, it is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such software instructions and programs and ICs with minimal experimentation.

The accompanying Abstract of the Disclosure is

provided to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.

In addition, in the foregoing Detailed Description, it can be seen that various features are grouped together in various embodiments for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter may lie in less than all features of a single disclosed embodiment. Thus the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separately claimed subject matter.

Claims (21)

1. A synchronizer for use in a mobile communication system to synchronize a communication between a mobile station and a base station, the synchronizer including a memory operable to store in respect of each of a plurality of mobile stations served by the base station a record comprising an identity of the mobile station together with a synchronization location in time of a previous synchronization of the mobile station with the base station and a processor operable to retrieve the synchronization location for a selected one of the mobile stations from the corresponding record of the memory and to establish current synchronization with the mobile station using the retrieved synchronization location.

2. A synchronizer according to claim 1 wherein the memory is operable to store in the record in respect of each of the served mobile stations the synchronization location in time of the most recent synchronization between the mobile station and the base station.

3. A synchronizer according to claim 1 or claim 2 wherein the synchronization location stored in each record of the memory is time stamped to indicate its age and the memory is operable to delete each synchronization location when its age reaches a maximum age.

4. A synchronizer according to claim 3 wherein the maximum age of the synchronization location is not more than about ten seconds.

5. A synchronizer according to any one of the preceding claims wherein the processor is operable to find a current synchronization of the selected mobile station by searching for a first fixed pattern of symbols in a signal received from the mobile station.

6. A synchronizer according to claim 5 wherein the processor is operable to apply a correlation procedure to correlate the first fixed pattern of symbols with a corresponding second fixed pattern of symbols held by the synchronizer.

7. A synchronizer according to claim 5 or claim 6 wherein the processor is operable to find the first fixed pattern of symbols of the received signal in individual time slots of a time slotted communication protocol employed by the base station and the mobile stations.

8. A synchronizer according to claim 6 or claim 7 wherein the processor is operable to apply a flywheel procedure using the retrieved synchronization location retrieved from the memory to find a current synchronization location in time.

9. A synchronizer according to claim 8 wherein in the flywheel procedure the processor moves from the retrieved synchronization location the symbols of the first fixed pattern relative to the symbols of the second fixed pattern in uniform steps to find a match between the first and second fixed patterns.

10. A synchronizer according to claim 9 wherein the processor in the flywheel procedure moves the symbols by a limited number of steps.

11. A synchronizer according to claim 9 or claim 10 wherein the processor in the flywheel procedure moves the symbols of the first fixed pattern in forward and reverse movements relative to the symbols of the second fixed pattern.

12. A synchronizer according to claim 11 wherein the processor limits the number of steps in the forward movement to a first maximum and limits the number of steps in the reverse movement to a second maximum, each of the first maximum and the second maximum being not greater than a number of steps equivalent to ten symbols.

13. A synchronizer according to any one of the preceding claims wherein the synchronizer is operable to establish the current synchronization to synchronize a received uplink communication from the mobile station.

14. A synchronizer according to claim 13 which is operable to establish the current synchronization to synchronize a received uplink data communication on a control channel or a traffic channel.

15. A base station including a transceiver for communication with a mobile station, a resource allocation processor operable to indicate a communication resource allocated for the communication with the mobile station and a synchronizer for synchronizing the communication in the allocated communication resource, wherein the synchronizer is a synchronizer according to any one of the preceding claims.

16. A base station according to claim 15 which is operable according to TETRA or APCO 25 standard procedures.

17. A mobile communication system including a base station according to claim 15 or claim 16 and a plurality of mobile stations each operable to communicate with the base station.

18. A method of operation in a mobile communication system including the steps of: (i) receiving at a base station a synchronization signal from a mobile station; (ii) retrieving by a processor of a synchronizer associated with the base station from a record relating to the mobile station in a memory of the synchronizer a synchronization location in time of a previous communication with the base station; (iii) and employing by the processor of the synchronizer the retrieved synchronization location and the received synchronization signal to establish current synchronization with the mobile station.

19. A method according to claim 18 wherein the synchronizer is a synchronizer according to any one of claims 1 to 14.

20. A synchronizer according to any one of claims 1 to 14 and substantially as herein described with reference to any one or more of the accompanying drawings.

21. A method according to claim 18 or claim 19 and substantially as herein described with reference to any one or more of the accompanying drawings.