Abstract:
In a wireless network, a primary synchronization signal and a secondary synchronization signal are sent at a predetermined bandwidth in a transmission. The predetermined bandwidth is a lowest operating bandwidth of the wireless network. Data is also sent in the transmission using an operating bandwidth greater than the lowest operating bandwidth.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/960,774 filed Dec. 6, 2010, which issued as U.S. Pat. No. 8,396,079 on Mar. 12, 2013, which is a continuation of U.S. patent application Ser. No. 12/033,824 filed Feb. 19, 2008, which issued as U.S. Pat. No. 7,848,353 on Dec. 7, 2010, which is a continuation of U.S. patent application Ser. No. 10/293,635 filed Nov. 13, 2002, which issued as U.S. Pat. No. 7,356,098 on Apr. 8, 2008, which claims the benefit of United Kingdom Patent Application Ser. No. GB 0127319.2 filed Nov. 14, 2001, the contents of which are all hereby incorporated by reference herein. 
    
    
     FIELD OF INVENTION 
     This invention relates to digital communication systems, and particularly to synchronisation in digital communication systems such as wireless cellular communication systems. The invention finds particular application in modern digital wireless communication to systems such as Universal Mobile Telecommunication Systems (UMTS). 
     BACKGROUND 
     It is known that synchronisation is an essential procedure in a modern digital communication system. It is the procedure used by a remote unit (often referred to as User Equipment, UE, in UMTS or Customer Premises Equipment, CPE) to identify valid transmissions from infrastructure equipment (often referred to as Node Bs in UMTS) and align the remote frequency reference and timing to that used by the infrastructure. 
     UMTS Terrestrial Radio Access (UTRA) Time Division Duplex (TDD) and Frequency Division Duplex (FDD) modes both provide a synchronisation channel (SCH) that is used by the UE to search for valid signals and perform the synchronisation procedure. The SCH transmission consists of one real valued Primary Synchronisation Code (PSC) and three complex Secondary Synchronisation Codes (SSC), all of length 256 chips. The PSC is common for all Node Bs, but the SSCs are Node B specific. The PSC and SSC are transmitted simultaneously from a given Node B at a specific fixed time offset (t offset ) from the start of time slot 0. The time offset is included to prevent the possible capture effect that would otherwise occur as a consequence of all Node Bs transmitting the common primary code at the same time. 
     The UE uses the PSC to search for and identify transmissions from Node Bs. The PSC is also used as a reference from which the UE is able to generate a correction that can be used to correct the frequency of the UE&#39;s reference oscillator. The SSC is included to signal the additional information required by the UE in order to achieve the full time-aligned synchronization and also to begin to demodulate system information broadcast on the Broadcast Channel (BCH) which is carried by the Primary Common Control Physical Channel P-CCPCH. 
     For single chip-rate systems where the chip rate used by the Node B and the UE is predetermined by the system design, the synchronization procedure briefly outlined above is sufficiently complete. 
     However, considering a network where multi-chip rates are supported, in an initial start-up condition, the UE will not be aware of the chip rate that is available; therefore, the receiver in the UE is unable to select the correct chip-rate. 
     In some known systems such as those using fixed line modems, the available bandwidth is negotiated in the initial data transfers between sender and receiver. This is done at a predetermined fixed rate, usually determined by the system design or backwards compatibility with early implementations. 
     Other possible schemes might transmit the whole timeslot in which SCH bursts are transmitted at the lower chip-rate (note that for a UMTS TDD system, the SCH is transmitted in every radio frame). 
     A plurality of subscriber terminals (or user equipment (UE) in UMTS nomenclature)  112 ,  114 ,  116  communicate over radio links  118 ,  119 ,  120  with a plurality of base transceiver stations, referred to under UMTS terminology as Node-Bs,  122 ,  124 ,  126 ,  128 ,  130 ,  132 . The system comprises many other UEs and Node Bs, which for clarity purposes are not shown. 
     The wireless communication system, sometimes referred to as a Network Operator&#39;s Network Domain, is connected to an external network  134 , for example the Internet. The Network Operator&#39;s Network Domain includes: 
     (i) A core network, namely at least one Gateway GPRS Support Node (GGSN)  144  and or at least one Serving GPRS Support Nodes (SGSN); and 
     (ii) An access network, namely: 
     (ai) a GPRS (or UMTS) Radio network controller (RNC)  136 - 140 ; or 
     (aii) Base Site Controller (BSC) in a GSM system and/or 
     (bi) a GPRS (or UMTS) Node B  122 - 132 ; or 
     (bii) a Base Transceiver Station (BTS) in a GSM system. 
     The GGSN/SGSN  144  is responsible for GPRS (or UMTS) interfacing with a Public Switched Data Network (PSDN) such as the Internet  134  or a Public Switched Telephone Network (PSTN)  134 . A SGSN  144  performs a routing and tunnelling function for traffic within say, a GPRS core network, whilst a GGSN  144  links to external. 
     However, the above known fixed initial rate negotiation scheme and the other possible schemes have the disadvantage that they are inefficient. 
     A need therefore exists for a synchronisation scheme for multi-rate communication wherein the abovementioned disadvantage may be alleviated. 
     STATEMENT OF INVENTION 
     In accordance with a first aspect of the present invention there is provided a method, for synchronisation in a multi-rate communication system, the method comprising: 
     receiving a signal having a synchronisation portion at a first, predetermined chip rate and containing an indication of chip rate used for a further portion; and 
     recovering the indication from the synchronisation portion at the first, predetermined chip rate; and 
     recovering information in the further portion at the chip rate indicated by the indication. 
     In accordance with a second aspect of the present invention there is provided a method, for synchronisation in a multi-rate communication system, the method comprising: 
     transmitting a signal having a synchronisation portion at a first, predetermined chip rate and containing an indication of chip rate used for a further portion, 
     whereby the indication may be recovered from the synchronisation portion at the first, predetermined chip rate; and information in the further portion may be recovered at the chip rate indicated by the indication. 
     In accordance with a third aspect of the present invention there is provided a multi-rate communication system comprising: 
     a transmitter having means for transmitting a signal having a synchronisation portion at a first, predetermined chip rate and containing an indication of chip rate used for a further portion; 
     a receiver having 
     means for receiving the transmitted signal, 
     means for recovering the indication from the synchronisation portion at the first, predetermined chip rate, and 
     means for recovering information in the further portion at the chip rate indicated by the indication. 
     In accordance with a fourth aspect of the present invention there is provided a communication unit, for use in a multi-rate communication system, the communication unit comprising: 
     means for receiving a signal having a synchronisation portion at a first, predetermined chip rate and containing an indication of chip rate used for a further portion; 
     means for recovering the indication from the synchronisation portion at the first, predetermined chip rate; and 
     means for recovering information in the further portion at the chip rate indicated by the indication. 
     In accordance with a fifth aspect of the present invention there is provided a communication unit, for use in a multi-rate communication system, the communication unit comprising: 
     means for transmitting a signal having a synchronisation portion at a first, predetermined chip rate and containing an indication of chip rate used for a further portion, 
     whereby the indication may be recovered from the synchronisation portion at the first, predetermined chip rate; and information in the further portion may be recovered at the chip rate indicated by the indication. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING(S) 
       One method, communication unit and communication system for synchronisation for multi-rate communication incorporating the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  shows a block diagram of a wireless communication system that can be adapted to support the various inventive concepts of a preferred embodiment of the present invention; 
         FIG. 2  shows a block diagram of a wireless communication unit that can be adapted to support the various inventive concepts of a preferred embodiment of the present invention; 
         FIGS. 3A and 3B  show block schematic diagrams illustrating SCH transmission and reception in a single chip rate system incorporating the invention; and 
         FIGS. 4A ,  4 B, and  4 C show block schematic diagrams illustrating SCH transmission and reception in a multi chip-rate system incorporating the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , a multi-rate cellular-based wireless telephone communication system  100  is shown in outline, in accordance with a preferred embodiment of the invention. Preferably, the cellular-based telephone communication system  100  is compliant with, and contains network elements capable of operating over, a UMTS air-interface. In particular, the invention relates to the Third Generation Partnership Project (3GPP) specification for wide-band code-division multiple access (WCDMA) standard relating to the UTRAN Radio Interface (described in the 3G TS 25.xxx series of specifications). 
     A plurality of subscriber terminals (or user equipment (UE) in UMTS nomenclature)  112 ,  114 ,  116  communicate over radio links  118 ,  119 ,  120  with a plurality of base transceiver stations, referred to under UMTS terminology as Node-Bs,  122 ,  124 ,  126 ,  128 ,  130 ,  132 . The system comprises many other UEs and Node Bs, which for clarity purposes are not shown. 
     The wireless communication system, sometimes referred to as a Network Operator&#39;s Network Domain, is connected to an external network  134 , for example the Internet. The Network Operator&#39;s Network Domain includes: 
     (i) A core network, namely at least one Gateway GPRS Support Node (GGSN)  144  and or at least one Serving GPRS Support Nodes (SGSN); and 
     (ii) An access network, namely: 
     (ai) a GPRS (or UMTS) Radio network controller (RNC)  136 - 140 ; or 
     (aii) Base Site Controller (BSC) in a GSM system and/or 
     (bi) a GPRS (or UMTS) Node B  122 - 132 ; or 
     (bii) a Base Transceiver Station (BTS) in a GSM system. 
     The GGSN/SGSN  144  is responsible for GPRS (or UMTS) interfacing with a Public Switched Data Network (PSDN) such as the Internet  134  or a Public Switched Telephone Network (PSTN)  134 . A SGSN  144  performs a routing and tunnelling function for traffic within say, a GPRS core network, whilst a GGSN  144  links to external packet networks, in this case ones accessing the GPRS mode of the system. 
     The Node-Bs  122 - 132  are connected to external networks, through base station controllers, referred to under UMTS terminology as Radio Network Controller stations (RNC), including the RNCs  136 ,  138 ,  140  and mobile switching centres (MSCs), such as MSC  142  (the others are, for clarity purposes, not shown) and SGSN  144  (the others are, for clarity purposes, not shown). 
     Each Node-B  122 - 132  contains one or more transceiver units and communicates with the rest of the cell-based system infrastructure via an I ub  interface, as defined in the UMTS specification. Each RNC  136 - 140  may control one or more Node-Bs  122 - 132 . Each MSC  142  provides a gateway to the external network  134 . The Operations and Management Centre (OMC)  146  is operably connected to RNCs  136 - 140  and Node-Bs  122 - 132  (shown only with respect to Node-B  126  for clarity). The OMC  146  administers and manages sections of the cellular telephone communication system  100 , as is understood by those skilled in the art. 
     In the preferred embodiment of the invention, at least one UE  112 ,  114 , and  116  and at least one Node-B  122 ,  124 ,  126 ,  128 ,  130 , and  132  have been adapted, to offer, and provide for, transmission, reception and processing of multi-rate high-speed signals generated in accordance with the approach discussed in detail below. 
     More particularly, in this embodiment the above elements have been adapted to implement the present invention in both transmitting and receiving modes of operation, such that in this embodiment the invention may be applied to both down-link and up-link transmissions. 
     It is also within the contemplation of the invention that such adaptation of the physical layer (air-interface) elements may alternatively be controlled, implemented in full or implemented in part by adapting any other suitable part of the communication system  100 . For example, equivalent parts in other types of systems may, in some circumstances, be adapted to provide some or all of the digital filtering implementation provided in this embodiment. 
     Further, in the case of other network infrastructures, implementation of the processing operations may be performed at any appropriate node such as any other appropriate type of base station, base station controller, etc. 
     Alternatively the aforementioned digital filtering operations may be carried out by various components distributed at different locations or entities within any suitable network or system. 
     Although the preferred embodiment of the invention is described with reference to a wireless communication system employing a UMTS air-interface, it is within the contemplation of the invention that the inventive concepts described herein can be applied to any multi-bandwidth/multi-data rate communication system-fixed or wireless. 
     Referring now to  FIG. 2 , a block diagram of a communication unit  200 , for example user equipment (UE)  112 , adapted to support the inventive concepts of the preferred embodiments of the present invention, is shown. However, it is within the contemplation of the invention that a similar block diagram would apply to a Node B element, say Node B  122 . Therefore, in the following description  FIG. 2  is described such that it also encompasses an implementation of a Node B baseband processing circuit, in broad principle, as would be appreciated by a person skilled in the art. 
     The UE  112  contains an antenna  202  preferably coupled to a duplex filter or circulator or switch  204  that provides isolation between receive and transmit chains within UE  112 . 
     The receiver chain includes scanning receiver front-end circuitry  206  (effectively providing reception, filtering and intermediate or baseband frequency conversion). The scanning front-end circuit  206  scans signal transmissions from its associated Node B. The scanning front-end circuit  206  is serially coupled to a signal processing function (processor, generally realised by a DSP)  208 . The final receiver circuits are a baseband back-end circuit  209  operably coupled to a display unit  210 , if the communication unit is a subscriber unit. 
     Alternatively, if the communication unit is a Node B, the final receiver circuits are a baseband back-end circuit  209  operably coupled to an interface port  210 , in order to forward the demodulated received signal to, say, a PC or a RNC. 
     In accordance with a preferred embodiment of the invention, the receiver chain, in particular the signal processing function  208 , coupled to the scanning baseband back-end circuit  209 , has been adapted for a receiving communication unit to receive and process multiple, high-speed signals of varying bandwidths. 
     A controller  214  is operably coupled to the scanning front-end circuitry  206  so that the receiver can calculate receive bit-error-rate (BER) or frame-error-rate (FER) or similar link-quality measurement data from recovered information via a received signal strength indication (RSSI)  212  function. The RSSI  212  function is operably coupled to the scanning front-end circuit  206 . A memory device  216  in the controller  214  stores a wide array of UE-specific data, such as decoding/encoding functions, timing details, neighbour and serving cell information relating to timing, channels, power control and the like, as well as link quality measurement information to enable an optimal communication link to be selected. 
     A timer  218  is operably coupled to the controller  214  to control the timing of operations, namely the transmission or reception of time-dependent signals, within the UE  112 . 
     In the context of the preferred embodiment of the present invention, timer  218  is used to synchronize the timing of the receiving communication unit  200  to be able to switch between two or more filter configurations, as will be described below, as well as to co-ordinate appropriate clocking of signals throughout the receiver. 
     For completeness, in broad terms, the transmit chain of the communication unit (either a UE or Node B) essentially includes an input device  220 , coupled in series through the processor  208 , transmitter/modulation circuitry  222  and a power amplifier  224 . The processor  208 , transmitter/modulation circuitry  222  and the power amplifier  224  are operationally responsive to the controller  214 , with an output from the power amplifier coupled to the duplex filter or circulator  204 , as known in the art. 
     The signal processor function  208  in the transmit chain may be implemented as distinct from the processor in the receive chain. Alternatively, a single processor  208  may be used to implement processing of both transmit and receive signals, as shown in  FIG. 2 . 
     Of course, it will be understood that the various components within the communication unit  200  can be realised in discrete or integrated component form, with an ultimate structure therefore being merely an arbitrary selection. 
     More generally, the digital filtering algorithms associated with the preferred embodiment of the present invention may be implemented in a respective communication unit in any suitable manner. For example, new apparatus may be added to a conventional communication unit (for example UE  112 , or Node B  122 ), or alternatively existing parts of a conventional communication unit may be adapted, for example by reprogramming one or more processors therein. As such the required adaptation may be implemented in the form of processor-implementable instructions stored on a storage medium or data carrier, such as a floppy disk, hard disk, PROM, RAM or any combination of these or other storage multimedia. 
     This invention, at least in a preferred form, implements a scheme where the SCH channel in the UTRA air-interface is transmitted at the lowest chip-rate supported by the system design. Note that only the SCH channel is always transmitted at the lower chip rate. 
     As the SCH is transmitted at the lower chip rate, the receiving UE will by default, select the receiver bandwidth appropriate to this lower chip-rate. In this configuration, the UE will be able to recover the SCH, irrespective of the chip rate used at the transmitting Node B. 
     The modulation of data onto the secondary SCH defined by the UTRA standard does not use all of the degrees of freedom available in the modulation scheme. Therefore, the mapping of the synchronisation specific data on to the SSC can be expanded to allow the additional signalling of the transmitting Node B chip rate to be added (see GB patent application no. 0122109.2, filed on 13 Sep. 2001 by the same applicant as the present application and entitled “ENCODER AND METHOD FOR EFFICIENT SYNCHRONISATION CHANNEL ENCODING IN UTRA TDD MODE”, the content of which is hereby incorporated herein by reference). 
     Simplified diagrams of the single chip-rate implementation of a preferred embodiment of the invention are shown in  FIGS. 3A and 3B . 
     In this example, the SCH is treated identically to the rest of the data burst. That is, the SCH is processed by the same transmit and receive filters as the physical channels used to transport the information having the same chip rate. 
     Thus, as shown in  FIG. 3A , in the transmit path of the transmitting Node B a combiner  310  combines SCH information  320  with the appropriate data burst construct  330 . The resultant data burst containing the SCH information is filtered in the digital low-pass transmit filter  340  (which may, for example, be of the ‘root-raised cosine’ type). The analogue section  350  of the transmitter is set to the bandwidth (narrowest) appropriate for the lowest chip rate, and the data burst is passed to the antenna for transmission. 
     Correspondingly, as shown in  FIG. 3B , in the receive path of the receiving UE the analogue section  360  of the receiver is set to the bandwidth (narrowest) appropriate for the lowest chip rate, and performs initial filtering of the data burst received at the antenna. The output of the analogue section  360  is then filtered in the digital low-pass receive filter  370  (which may, like the digital transmit filter  340 , be of the ‘root-raised cosine’ type). The output of the digital low-pass receive filter  370  is processed to recover the SCH information and (as will be explained in greater detail below) to decode the system chip rate information therefrom (as depicted at  380 ). Since (in this single chip rate case) the decoded system chip rate information does not indicate that the system chip rate is different than the chip rate used for the SCH information (i.e., it indicates that a single chip rate is used), the receive path digital filters remain configured for the single, lowest chip rate for subsequent processing of the data burst (as indicated at  390 ) and transport channel information as for the SCH information. 
     Referring now also to  FIG. 4A ,  4 B, or  4 C, in the case where a different chip-rate is available for the physical channel that is used to transport data, it is necessary to provide different filters (or to differently configure the filter(s)) for the SCH channel and the physical channels used to transport the data. Such different filters, or re-configuration of the same filter(s), may be implemented as in GB patent application no. 0118414.2, filed on 30 Jul. 2001 by the same applicant as the present application and entitled “DIGITAL FILTER FOR MULTI-RATE COMMUNICATION”, the content of which is hereby incorporated herein by reference. 
     Suppose the chip rate in a multi chip-rate system is given by
 
f c =nf b ; n=1, . . . , N
 
where f b  is the base chip rate and N is the number of available chip rates in the multi-chip rate system. When a UE is initialised it knows a priori that the chip-rate being used for the SCH is f b , but it does not know the system chip rate being used, f c . In the Node B transmitter, it is necessary to pass the SCH physical channel through a filter (typically a digital filter) optimised for f b . The physical channels transporting the data are filtered with a (digital) filter optimised for f c . In the analogue section of the Node B transmitter, the filter bandwidth is always equal to f c .
 
     In the receive section of the user equipment, the receiver bandwidth is set to f b  in both the analogue section and digital sections. In this configuration, the physical channels with chip-rate f c  may suffer severe inter-symbol interference when f c ≠f b  However, the SCH physical channel is received with minimal degradation. It is necessary to use a bandwidth of f b  in the analogue filter and the digital filter in order to apply maximum attenuation to potentially high-power adjacent channel interferers. 
     With a UE is in this configuration, it is possible to demodulate the SCH channel and decode the data transported by the SSC to determine f c . When initial synchronisation has been achieved, the analogue and digital filters are set to f c . 
       FIG. 4A ,  4 B, or  4 C shows the receiver/transmitter implementation of this multi-chip rate scheme. 
     Thus, as shown in  FIG. 4A , in the transmit path of the transmitting Node B a combiner  310  combines SCH information  320  (filtered by a digital low-pass filter  325  set to the low chip rate f b  so as to ensure that the SCH information can be recovered in the receiver by filtering at this chip rate) with the appropriate data burst construct  330 . The SCH information is encoded with the desired higher system chip rate f c , as explained in detail in the above-mentioned GB patent application no. 0118414.2. The resultant data burst containing the SCH information is filtered in the digital low-pass transmit filter  340  (now set for the desired high chip rate f c ). The analogue section  350  of the transmitter is set to a bandwidth (wider than in the case of  FIG. 3A ) appropriate for the higher chip rate, and the data burst is passed to the antenna for transmission. 
     Correspondingly, in the receive path of the receiving UE, in a first state, as shown in  FIG. 4B , the analogue section  360  of the receiver is set to the bandwidth (narrowest) appropriate for the lowest chip rate, and performs initial filtering of the data burst received at the antenna. The output of the analogue section  360  is then filtered in the digital low-pass receive filter  370 . The output of the digital low-pass receive filter  370  is processed to recover the SCH information and decode the system chip rate information therefrom. It will be appreciated that this initial stage of receive path processing is similar to that shown and described above in relation to the single chip-rate case shown in  FIG. 3A . As will be explained further below, at this stage (since the indicated system chip rate f c  is higher than the lowest chip rate f b  used for the SCH information) data burst processing is disabled (as indicated at  395 ). 
     In this multi chip-rate case, the system chip rate information decoded from the SCH information indicates the higher chip rate used for transport channel information. Since this indicated system chip rate f c  is higher than the low chip rate f b  used for the SCH information, the receive path is then configured into a second state, as shown in  FIG. 4C , in which the analogue section  360  and the digital low pass receive filter  370  are set to bandwidths appropriate for the higher chip rate f c . 
     In this second state, in the receive path of the receiving UE the analogue section  360  of the receiver performs (now at the higher bandwidth appropriate for the higher chip rate f c ) filtering of the signals received at the antenna. The output of the analogue section  360  is then filtered (now at the higher bandwidth appropriate for the higher chip rate f c ) in the digital low-pass receive filter  370 . The output of the digital low-pass receive filter  370  is then processed (i) to recover the data burst information (now enabled, as depicted at  390 ) and transport channel information at the higher chip rate, and (ii) to further process (after filtering by a digital low-pass filter  385  set to the low chip rate f b  so as to ensure that the SCH information can be recovered in the receiver by filtering at this chip rate) the SCH information (as depicted at  380 ). 
     It will be understood that the method, communication unit and communication system for synchronisation for multi-rate communication described above provides improved efficiency in supporting multi-chip rates.