GB2379134A

GB2379134A - Storage of base/transmitter station information

Info

Publication number: GB2379134A
Application number: GB0120435A
Authority: GB
Inventors: Gerald Williams; Diego Giancola; Jonathan Lucas; Andrew Papageorgiou
Original assignee: Ubinetics Ltd
Current assignee: Aeroflex Cambridge Ltd
Priority date: 2001-08-22
Filing date: 2001-08-22
Publication date: 2003-02-26
Also published as: GB0120435D0; WO2003019963A1

Abstract

A radiotelephone comprises a processor and non-volatile memory, and is arranged to store in (or write into) the non-volatile memory information concerning at least one transmitter/base station with which the radiotelephone has communicated. This storage/writing may take place in response to a power down request for the radiotelephone. This may allow the radiotelephone to later read the stored information and use it to search for signals from the stored transmitter/base stations in response to a power up condition. Information on the transmitter/base station may comprise identity, signal timing, clock drift rate and modulated code information. Stored information may also be compared to an age threshold and information is disregarded when the threshold is exceeded.

Description

A Radiotelephone and a Method of Operating a Radiotelephone

This invention relates to a radiotelephone, and to a method of operating a radiotelephone.

zu The proposed universal mobile telephone system (UMTS) will use the code division

multiple access (CDMA) communication technique. Each base station, or node B as it is known in UMTS, will modulate signals it transmits with a scrambling code not used by other node Bs in its geographical vicinity. The nature of the signals which will be transmitted by node Bs is such that radiotelephones, or user equipment (UEs) as they are called in UMTS, are such that significant amounts of time and processing resources are required to determine the slot and frame timing and scrambling code information needed to classify signals from a node B. Classification of signals is necessary before useful data can be decoded from that node B. There is a need for apparatus and methods which are able to reduce the time and processing resources required to classify signals from nodes B.

According to a first aspect of the invention, there is provided a radiotelephone comprising a processor and a non-volatile memory, the processor being arranged to write to the non-volatile memory information concerning at least one transmitter station with which the radiotelephone has communicated.

According to a second aspect of the invention, there is provided a method of operating a radiotelephone, the method comprising writing to a non-volatile memory information concerning at least one transmitter station with which the radiotelephone has communicated.

Using these aspects of the invention, the amount of time and the amount of processing resources required, for example on power-up or following an occasion on which no signals were classified, to classify signals transmitted by a node B may be substantially reduced in certain circumstances.

Embodiments will now be described, by way of example only, with reference to the accompanying drawing, which is a schematic diagram of a radiotelephone according to the invention operating in a UMTS system.

Referring to Figure 1, a radiotelephone 10 comprises a microprocessor 11, which is connected to receive input from a keypad 12 and has two-way connections to each of a random access memory (RAM) 13 and a non-volatile memory 14, such as electrically programmable read only memory (EPROM) or subscriber identity module (SIM). The non-volatile memory 14 is used to store information such as the identity number of the radiotelephone 10, as well as instructions used to program the processor 11 to operate the radiotelephone 10. These instructions may instead be stored in a separate ROM (not shown). The RAM 13 is used by the processor 11 to process the instructions stored in

the non-vo'la'Llie memor-t : l the non-volatile memory 14, and to temporarily store data during operation of the radiotelephone 10. Data stored in the RAM 13 is lost when the radiotelephone 10 is switched off, but the data stored in the non-volatile memory 14 is not. The radiotelephone 10 thusfar described is conventional.

In this example, the radiotelephone 10 is a UE configured to operate in a UMTS system. p The non-volatile memory 14 or the ROM (not shown) then also includes data representing 512 scrambling codes, any of which could be used by node Bs 15 from which signals could be transmitted to the UE. On power-up, the UE 10 searches for signals transmitted by node Bs (not shown) and classifies the strongest of these by determining the slot and frame timing and scrambling code information. The UE 10 then begins to demodulate data transmitted by one of the node Bs, and determines the identity of that node B from the data. A Node B 15 together with the area covered by it is often referred to as a cell. Node B and cell are used interchangeably in the following description.

The UE 10 maintains, as a background task, a list of cells, each of which is classified as one of the following sets of cells :- a camped cell, which is the cell with which the UE is primarily communicating; a set of active cells, which are cells with which the UE is communicating in a secondary manner, for example in soft handover situation; a set of

monitored cells, which are cells mentioned in a neighbouring cell list transmitted by Node Bs over a broadcast channel; and a set of detected cells, which are cells that the UE has classified and is observing but with which the UE is not communicating and which are not in any other set of cells. The list includes, for each cell, the identity of the node B, the scrambling code used by it, and, if available, the timing of signals transmitted by the node B relative to a reference (such as the timing of the camped cell) and the estimated clock drift of the reference clock of the node B.

In response to a request to power-down the UE 10, which may be initiated by a user through the keyboard 12 or by the UE itself (for example because of insufficient battery power), the UE stores certain information in the non-volatile memory 14. This information includes all of the information stored in the list, with the most up-to-date timing and clock drift information which is available, and the time of powering-down of the UE. The time of powering-down is taken from a real-time clock (not shown) included in the UE 10. This clock does not rely on any connection to the network including the node Bs 15.

On subsequent powering-up of the UE 10, the processor 11 reads the information stored in the non-volatile memory 14, which constitutes the last known information about the various cells. The UE 10 firstly determines the time passed between power-down and power-up. If this period of time is deemed to be too long for the relative timing information to be of use, the timing information for all of the cells is discarded. What determines a period of time which is too long is a matter specific to the implementation of the invention. In a UMTS system in which clock drift rate of clocks in node Bs is determined with reference to, and is assumed to be within 0.01 microseconds in one second of, time as used by satellites of the global positioning system (GPS), the threshold may be 8400 minutes, which is approximately six days.

The UE 10 typically then determines the slot timing characteristics of significant rays received by it using the primary synchronisation channel (PSCH). This is carried out for the sixteen strongest rays received. The UE 10 then attempts to determine the cell from which the strongest ray originates. This involves extracting the fifteen slot long

secondary synchronisation channel (SSC) sequence for the ray. The UE 10 then compares the SSC sequence of the received signal to the SSC sequence corresponding to the code group in which the scrambling code used by the camped cell falls. If a strong correlation is detected, the ray is deemed to be part-classified and no further SSC searches are performed for that ray. If the ray has not been classified the SSC sequence of that ray is then compared to the SSC sequences corresponding to the code groups in which the scrambling codes used by cells in the active set fall. Again, a strong correlation results in the ray being deemed to be part-classified. The same procedure is then carried out using the SSC sequences corresponding to cells in the monitored set and, if no part-classification results, the procedure is subsequently carried out using the SSC sequences corresponding to cells in the detected set.

If the above procedure fails 10 pan-classify The strongest ray, The procedure is repeated with the next strongest ray and so on until either a ray is part-classified or all rays have been tested against all cells stored in the list. Part-classification provides the UE 10 with frame timing information.

To classify a part-classified ray, the UE 10 correlates signals from a common pilot channel (CPICH) with the scrambling code it expects to be used by the cell. Detection of a high degree of correlation results in classification of that cell. A low correlation indicates that a different scrambling code is used by the cell, and no classification occurs.

This procedure is hereafter termed a CPICH check.

If the first ray to be classified corresponds to a cell which is in the detected set (rather than a higher priority set), it is assumed that the UE has moved a significant distance since it was powered-down. Accordingly, the UE attempts to classify the remaining rays without recourse to the list.

Classification of a ray provides the UE 10 with frame synchronisation information for that cell. The UE 10 then uses this information along with any relative timing information and clock drift estimates (calculated using the clock drift information stored in the list and the elapsed time between power-down and power-up) to estimate the

frame synchronisation of the other cells in the list. This estimation has a margin of error which may embrace plural slot timings.

The next action taken by the UE 10 is to attempt to part-classify the next strongest ray using the list and any frame synchronisation information estimated using the above described procedure. This is effected by correlating the SSC sequence associated with that ray with the SSC sequences associated with the code groups of the cells in the list. with a high degree of correlation resulting in part-classification of that ray.

Classification is then attempted using a CPICH check. This procedure is then repeated for the remaining rays, in decreasing order of strength. Rays which remain unclassified are subsequently classified without recourse to the list.

In one embodiment, the UE 10 is arranged to detect when a predetermined number of rays, for example three rays, are classified as being the camped cell or originating from cells in the monitored or active sets. Here, the UE assumes that it is very near to the location in which it was powered-down. Following that detection, rays from sets other than the detected set are deemed classified when they reach the part-classification stage described above; no CPICH checking is performed. This has the advantage of reduced classification time, but this occurs at the expense of an increased chance of false classification.

The invention is applicable also to situations when the UE 10 does not power-down, for example when the UE enters a tunnel or similar. In this case, the UE 10 can used the list stored in the non-volatile memory 14 to attempt to classify rays that it receives on exiting the tunnel. The invention may also be used where the UE 10 powers-down other than in a controlled manner, for example because its battery (not shown) becomes disconnected.

Although in the above described embodiments, cell classification involves comparing the SSC sequence of a received ray to an expected SSC sequence, this is not necessary. For example, cell classification could instead be carried out using any one of the techniques described in UK Patent Application No. 0108506.7, or any other suitable technique.

The above stated advantages of this invention arise when classification occurs without recourse to conventional classification techniques, since knowledge of the identity of the node B and or information concerning its scrambling code allows classification to occur with substantially reduced processing and in a shorter time. This is very likely to occur when the UE 10 is powered-up at the same or a similar location to one where it was powered-down.

Claims

Claims 1. A radiotelephone comprising a processor and a non-volatile memory, the processor being arranged to write to the non-volatile memory information concerning at least one transmitter station with which the radiotelephone has communicated.
2. A radiotelephone as claimed in claim 1, in which the information includes information concerning the identity of the at least one transmitter station.
3. A radiotelephone as claimed in either preceding claim, in which the information includes information concerning a code modulated onto signals by the at least one transmitter station.
4. A radiotelephone as claimed in any preceding claim, in which the processor is arranged to write the information in response to a request to power-down the radiotelephone.
5. A radiotelephone as claimed in any preceding claim, in which the processor is arranged to read the information from the non-volatile memory in response to a detection of a power-up condition or a signal reappearance condition, and to subsequently use the

information to search for signals transmitted by the at least one transmitter station.

ZD
6. A radiotelephone as claimed in claim 5, in which the information includes information concerning the timing of signals transmitted by at least two transmitter stations, and the processor is arranged to use this information to search for signals from at least one of the transmitter stations.
7. A radiotelephone as claimed in claim 6. in which the information includes information concerning the drift rate of a clock forming part of the at least two transmitter stations, and the processor is arranged to use this information to search for signals from at least one of the transmitter stations.

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8. A radiotelephone as claimed in claim 6 or claim 7, in which processor is arranged to compare the age of the information concerning the timing of signals to a

threshold, and to disregard the timing information if the threshold is exceeded.
9 9. A radiotelephone as claimed in any of claims 5 to 8, in which the processor is arranged to attempt to classify a received ray to plural transmitter stations in an order which is dependent on a hierarchy of the transmitter stations.
10. A radiotelephone as claimed in any preceding claim, in which the processor is

arranged to write to the non-volatile memory information concerning transmitter stations t which the radiotelephone has received signals from but not communicated with.
11. A method of operating a radiotelephone, the method comprising writing to a I L rl III %. IL t. 1 r,-. "I illvil III & LA.. non-volatile memory information concerning at least one transmitter station with which the radiotelephone has communicated.
12. A method as claimed in claim 11, in which the writing step includes writing information concerning the identity of the transmitter station.
13. A method as claimed in claim 11 or claim 12, in which the writing step includes writing information concerning a code modulated onto signals by the transmitter station.
14. A method as claimed in any of claims 11 to 13, in which the writing step is commenced in response to a request to power-down the radiotelephone.
15. A method as claimed in any of claims 11 to 14, further comprising detecting a power-up condition, and consequently reading the information from the memory and

using the information to search for signals transmitted by the at least one transmitter zz station.
16. A method as claimed in any of claims 11 to 15, further comprising, on detection of a power-up condition or a signal reappearance condition, reading the information from

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the non-volatile memory, and subsequently using the information to search for signals transmitted by the at least one transmitter station.
17. A method as claimed in claim 16, in which the information includes information concerning the timing of signals transmitted by at least two transmitters, the method comprising using the timing information to search for signals from at least one of the transmitter stations.
18. A method as claimed in claim 17, in which the information includes information concerning the drift rate of a clock forming part of the at least two transmitter stations, the method comprising using the drift rate information to search for signals from at least one of the transmitter stations.
19. A method as claimed in claim 17 or claim 18, comprising comparing the age of the information concerning the timing of signals to a threshold, and disregarding the timing information if the threshold is exceeded.
20. A method as claimed in any of claims 16 to 19, comprising attempting to classify a received ray to plural transmitter stations in an order which is dependent on a hierarchy of the transmitter stations.
21. A method as claimed in any of claims 11 to 20, comprising writing to the non-volatile memory information concerning transmitter stations which the radiotelephone has received signals from but not communicated with.
22. A radiotelephone substantially as herein described with reference and as shown in the accompanying drawings.
23. A method of operating a radiotelephone substantially as herein described with reference to the accompanying drawings.