Abstract:
A mobile station in a spread spectrum communications system includes a matched filter that can be divided into segments. On initial acquisition, when a frequency deviation between the expected receiving frequency of the mobile station and the transmitting frequency of the base station is expected to be relatively large, the device can operate in a first synchronisation mode, in which the filter is used divided into segments. On searching for alternative cells, when the frequency deviation is expected to be smaller, the device can operate in a second synchronisation mode, in which the filter is used undivided. Thus, in the first mode, a reduced filter length avoids the difficulties caused by frequency deviation, while, in the second mode, an increased filter length allows faster acquisition.

Description:
This application claims priority under 35 U.S.C. §§ 119 and/or 365 to 0028870.4 filed in the United Kingdom on Nov. 27, 2000 and to 60/250,145 filed in the United States of America on Dec. 1, 2000; the entire content of which is hereby incorporated by reference. 

   TECHNICAL FIELD OF THE INVENTION 
   This invention relates to a mobile communications device, and in particular to a device for use in a spread spectrum communication system. 
   BACKGROUND OF THE INVENTION 
   In a Wideband Code Division Multiple Access (W-CDMA) cellular radio telecommunications system, for example as used in so-called 3rd Generation mobile communications systems, a mobile station (MS) is able to move around an area in which multiple cells are defined. Each cell is served by a base station. The base stations use the same carrier frequency for their transmissions, and so these transmissions are identified by means of code signals which are transmitted by the base stations. 
   In order to establish a connection with a base station, a mobile station must go through an acquisition procedure. This requires that the mobile station be synchronised to the base station. This synchronisation is achieved by means of a matched filter. The maximum length of this filter is set by the frequency deviation which may exist between the base station and the mobile station, and so the length of the filter is restricted. 
   When the mobile station has established a connection with a base station, it must then continue to make measurements on signals received from other base stations. Again, the mobile station must synchronise to the other base stations before making these measurements. However, at this stage, the restricted length of the matched filter increases the time taken to synchronise to the base stations. 
   EP-0884856 describes a system of this type, in which the speed of acquisition is sought to be increased by using multiple matched filters. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a spread spectrum communications system, in which a mobile station includes a matched filter which can be divided into segments. 
   In a first synchronisation mode, when a frequency deviation is expected to be relatively large, the filter is used divided into segments. In a second synchronisation mode, when the frequency deviation is expected to be smaller, the filter is used undivided. Thus, in the first mode, a reduced filter length avoids difficulties caused by frequency deviation, while, in the second mode, an increased filter length allows faster acquisition. 
   The first synchronisation mode can be used when the receiver is initially establishing a connection to a base station, while the second synchronisation mode can be used after a connection has been established, when detecting transmissions from other base stations. 
   According to another aspect of the invention, there is provided a method of controlling a receiver. 
   It should be emphasised that the term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic representation of a mobile communications network in accordance with the invention. 
       FIG. 2  is a block schematic diagram of a mobile communications device in accordance with an aspect of the invention. 
       FIG. 3  is a block schematic diagram of a matched filter in the mobile communications device shown in  FIG. 2 . 
       FIG. 4  is a flow chart showing a first synchronisation procedure in accordance with an aspect of the invention. 
       FIG. 5  is a flow chart showing a second synchronisation procedure in accordance with an aspect of the invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  shows a part of a cellular mobile communications network, operating in a Wideband Code Division Multiple Access (WCDMA) system.  FIG. 1  shows just four cells C 1 –C 4 , although it will be realised that these represent only a small part of a typical network. Each of the cells C 1 –C 4  includes a respective base station BS 1 –BS 4 . A typical mobile station (MS)  100  is also shown in the system. Again, it will be apparent that a real network will contain many such mobile stations. 
   Each base station BS transmits information to the mobile stations using the same nominal carrier frequency. These transmissions are spread using a Short Code. The mobile station is able to distinguish between the base stations because each base station also applies a respective Long Code to its transmissions. However, one part of each signal transmitted from a base station does not have the Long Code applied to it. This is the Long Code Masked symbol. 
   Although the invention is described herein with reference to a W-CDMA system, it will be apparent that it can be used in any system which uses a Long Code Masked symbol in this way, or, indeed, in any communication system in which a receiver must detect a code in a received signal. 
   When a mobile station  100  is switched on, it must establish a connection with one of the base stations. This requires it to synchronise to the transmissions from the base station. Firstly, the mobile station must detect the slot timings of transmissions from the base station. This is done by detecting the correlation between the known short code and a received signal, using a matched filter. Then, the long code can be detected. 
   A somewhat similar process carries on when a base station has been acquired. After acquisition, the mobile station detects transmissions from other base stations, to aid in determining whether it should handover communications to one of the other base stations. Similarly, the mobile station must detect the slot timings of transmissions from the other base station, by detecting the correlation between the known short code and a received signal using a matched filter. This allows the long code of the other base station to be detected. 
     FIG. 2  shows the relevant components of the mobile station  100 . The invention is described herein with reference to a mobile phone, but it is generally applicable to portable radio communication equipment or mobile radio terminals, such as mobile telephones, pagers, communicators, electronic organisers, smartphones, personal digital assistants (PDAs), or the like. It will be apparent that  FIG. 2  shows only those components of the mobile station  100  which are essential to an understanding of the present invention. 
   An antenna  102  detects radio transmissions from a base station BS. Front-end receiver circuitry  104  receives signals from the antenna  102 , and provides suitably filtered digital sample streams for the in-phase (I) and quadrature (Q) components thereof. The sample streams representing the in-phase and quadrature components I, Q are passed to respective matched filters  106 ,  108 . As discussed above, the matched filters  106 ,  108  detect the correlation between the known short code and the received signal components. Effectively, the filter slides over the signal stream received in a slot. Output filter values are supplied to an accumulator  110 , which sums the output values. When it is determined that the accumulated value exceeds a threshold, it is determined that this filter position corresponds to the slot boundary. As will be described in more detail below, the operation of the matched filter is controlled by control circuitry of the mobile station. 
   The result of the determination by the accumulator  110  is passed to a block  112  which, by using the determined slot position, is able to detect the long code applied to the transmissions, and the result is then used in demodulating the received signal, as is known to the person skilled in the art. 
     FIG. 3  shows the form of the matched filter  106  which receives the sampled in-phase signal (I), although it will be noted that the form of the matched filter  108  which receives the sampled quadrature signal (Q) is the same. 
   The filter  106  includes a shift register comprising 256 elements En, which is shown for convenience divided into four blocks, each having sixty-four elements, namely a first block  120  made up of elements E 0 –E 63 , a second block  122  made up of elements E 64 –E 127 , a third block  124  made up of elements E 128 –E 191 , and a fourth block  126  made up of elements E 192 –E 255 . Input received samples are applied to element E 255  and, as further samples are received, they in turn are applied to element E 255 , with previously received samples being shifted through the register. When 256 samples have been received, the first sample is in shift register element E 0 , while the most recently received sample is in shift register element E 255 . 
   At each stage, the value in each element En of the shift register is multiplied in a respective multiplier Mn by a corresponding coefficient Cn, which relates to a bit in the known short code discussed above. Thus, the value in element E 255  of the shift register is multiplied in multiplier M 255  by a coefficient C 255 , the value in element E 254  of the shift register is multiplied in multiplier M 254  by a coefficient C 254 , etc. 
   The outputs from the multipliers M 0 –M 63  associated with the first block  120  of the shift register are summed in an adder  128 , the outputs from the multipliers M 64 –M 127  associated with the second block  122  of the shift register are summed in an adder  130 , the outputs from the multipliers M 128 –M 191  associated with the third block  124  of the shift register are summed in an adder  132 , and the outputs from the multipliers M 192 –M 255  associated with the fourth block  126  of the shift register are summed in an adder  134 . 
   The outputs of each of the adders therefore represent the degree of correlation between the samples in the elements En of the corresponding shift register block, and the respective coefficient values Cn. 
   The outputs of the adders  128 ,  130 ,  132 ,  134  are connected to respective switches  136 ,  138 ,  140 ,  142  which can connect the respective adder outputs to respective alternative switch terminals A, B. The switch terminal A of each switch  136 ,  138 ,  140 ,  142  is connected to a respective block  144 ,  146 ,  148 ,  150 , which squares its received value to measure the power thereof. The switch terminal B of each switch  136 ,  138 ,  140 ,  142  is connected to an adder  152 , and the outputs of the blocks  144 ,  146 ,  148 ,  150  are also connected to the adder  152 . 
   The output of the adder  152  is connected to a further switch  154 , having alternative switch terminals A, B. The switch terminal A of the switch  154  is connected to the output of the filter  106 , and then to the accumulator  110  ( FIG. 2 ). The switch terminal B of the switch  154  is connected to a block  156  which squares its received value to measure the power thereof, and the output of the block  156  is also connected to the output of the filter  106 , and then to the accumulator  110 . 
   The operation of the filter  106 , and the corresponding filter  108 , will now be described in more detail with reference to  FIGS. 4 and 5 , which are flow charts illustrating the relevant parts of the synchronisation procedures carried out in the mobile station, under the control of control circuitry included in the mobile station. 
     FIG. 4  shows the synchronisation carried out when the mobile station is switched on. Thus, in step  200 , the acquisition procedure is started. In step  202 , the switches  136 ,  138 ,  140 ,  142 ,  154  in the filter  106  shown in  FIG. 3  (and the corresponding switches in the filter  108 ) are set to their respective positions marked A. The reason for this will be explained below. 
   In step  204 , based on the accumulated results from the filters  106 ,  108 , the slot synchronisation position is determined. Then, in step  206 , the long code of the base station is determined, these latter steps, and the subsequent steps which will not be described further, being generally conventional. 
     FIG. 5  shows the synchronisation carried out when the mobile station searches for transmissions from another base station. Thus, in step  220 , the cell search procedure is started. In step  222 , the switches  136 ,  138 ,  140 ,  142 ,  154  in the filter  106  shown in  FIG. 3  (and the corresponding switches in the filter  108 ) are set to their respective positions marked B. Again, the reason for this will be explained below. 
   In step  224 , based on the accumulated results from the filters  106 ,  108 , the slot synchronisation position is determined. Then, in step  226 , the long code of the base station is determined, these latter steps, and the subsequent steps which will not be described further, again being generally conventional. 
   When the mobile station is first switched on, there can be a relatively large frequency deviation, between the frequency at which the base station is transmitting, and the frequency at which the mobile station is expecting to receive transmissions, that is, the frequency at which samples are clocked through the shift register blocks  120 ,  122 ,  124 ,  126 . This frequency deviation can for example be up to +/−10 ppm, that is up to about 20 kHz if the carrier frequency is 2 GHz. This frequency deviation results in a phase rotation in every sample of the received sample stream. Since the performance of the matched filter is degraded severely if the total phase rotation over the length of the matched filter is too high, this effectively sets an upper limit on the maximum length of filter that can be used. 
   In this embodiment of the invention, where the frequency deviation can for example be up to +/−10 ppm, the maximum filter length is set at 64 elements. Thus, with the switches  136 ,  138 ,  140 ,  142 ,  154  at the positions A, the four blocks  120 ,  122 ,  124 ,  126  effectively act as four separate filters, each with 64 elements. 
   In this case, assuming that each of the four blocks  120 ,  122 ,  124 ,  126  produces a correlation amplitude value X, when these are squared in the blocks  144 ,  146 ,  148 ,  150 , and summed in the adder  152 , the output accumulation value is 4X 2 . 
   When the mobile station has established synchronisation with one base station, and is performing a cell search operation, as described in  FIG. 5 , the frequency deviation should not exceed +/−1 ppm, because the crystal oscillator in the frequency generator of the mobile station can be suitably compensated. Therefore, during this phase of operation, the possible frequency deviation does not effectively set any upper limit on the maximum length of filter that can be used. 
   In this embodiment of the invention, the switches  136 ,  138 ,  140 ,  142 ,  154  are set at the positions B, and the four blocks  120 ,  122 ,  124 ,  126  effectively act as a single filter, with 256 elements. 
   In this case, assuming that each of the four blocks  120 ,  122 ,  124 ,  126  produces a correlation amplitude value X, when these are summed in the adder  152 , and squared in the block  156 , the output accumulation value is 16X 2 , compared with an output accumulation value of 4X 2  when the four blocks  120 ,  122 ,  124 ,  126  effectively act as four separate filters. There is a corresponding increase of 6 dB in the signal-to-noise ratio of the output value. 
   If the slot boundary detection algorithm relies on accumulating the power from the matched filter until it reaches a threshold, then this increase in the output accumulation value allows the slot boundary to be found considerably more quickly. 
   This means that slot synchronisation can be achieved more quickly, that there is reduced power consumption because the algorithm runs for a shorter period, and hence that the battery life of the mobile station can be extended.