SYNCHRONISATION IN A TELECOMMUNICATIONS NETWORK

Abstract: A telecommunications network comprises a plurality of cell sites and a station located in at least one of said cell sites, said cell sites being synchronised to transmit with reference to a common time reference, said station being arranged to monitor a predetermined transmission from at least two of said cell sites, said predetermined transmissions being transmitted at predetermined times within a first time period, wherein said predetermined transmissions are transmitted by said at least two cell sites at different times within said first time period.

Description: SYNCHRONISATION IN A TELECOMMUNICATIONS NETWORK Field of the Invention The present invention relates to a telecommunications network, and m particular to a synchronised telecommunications network Background to the Invention An area covered by a cellular telecommunications network is divided into a plurality of cells Each of these cells has a base station which is arranged to transmit signals no and receive signals from mobile stations located m the cell associated with the respective base station Mobile stations will be m active communication with the base station associated with the cell m which the mobile station is located However, mobile stations will also measure the signal strength of certain signals from base stations m neighbouring cells This information is used m handoff decision-making processes Handoff occurs when a mobile station moves from one cell into another and therefore changes the base station with which it is m active communication In the GSM (Global System for Mobile Communications) , the mobile station is arranged to monitor the frequency correction channel FCCH and the synchronisation channel SCH of the base stations m neighbouring cells More than one neighbouring base station may use the same frequency for the signals transmitted on the frequency correction channel and the synchronisation channel Accordingly, a base station identity code BSIC is allocated to each cell This code is not m itself unique and often the same code will be allocated to neighbouring cells However, no more than one of the neighbouring cells w ll have the same frequency and the same base station identity code In this way, it is possible for the mobile station to identify the base station, which it is monitoring. The base station identity code is transmitted on the synchronisation channel . The mobile station will use an idle slot in order to carry out the necessary measurements. This idle slot occurs once in every 26 slot cycles. In the GSM standard, the FCCH channel is transmitted regularly with a period of 51 x 8 burst periods whilst the idle slot occurs every 26 x 2 x 8 burst periods. This means that the idle slot will have 100% chance of being aligned with the frequency correction channel within 11 slot cycles. It has been proposed to provide synchronised networks . In other words, all of the base stations will use a common reference clock. This means that there will be synchronisation between the beginning of frames transmitted by adjacent base stations. A synchronised network has advantages in that the network may be spectrally more efficient, channel location may be easier and there may be a higher tolerance to noise. However, it has been appreciated by the inventor that the synchronisation of networks may lead to disadvantageous situations. In particular, in non- synchronised networks, the broadcast of the frequency correction channel and the synchronisation channel by each of the base stations are random with respect to each other. If the base stations were all to transmit the frequency correction channels and the synchronisation channels at the same time, this would increase the amount of time taken for a mobile station to obtain readings from each of the neighbouring base stations. It should be appreciated that in each idle slot, the mobile station attempts to obtain readings from one neighbouring cell. If the mobile station fails in that idle slot to obtain the required measurements, then the mobile station will attempt to obtain the readings in the subsequent idle slots . The mobile station will need to repeat this process for each of the neighbouring base stations, which it monitors. In some implementations, the mobile station may be given a prescribed order in which to measure the strength of the neighbouring cells. If all of the frequency correction channels and synchronisation channels are transmitted at the same time, this can increase the amount of time taken to measure the strength of the signals from the base stations m neighbouring cells Summary of the Invention It is therefore an aim of embodiments of the present invention to address the problem outlined hereinbefore According to one aspect of the present invention, there is provided a telecommunications network comprising a plurality of cell sites and a station located m at least one of said cell sites, said cell sites being synchronised to transmit with reference to a common time reference, said station being arranged to monitor a predetermined transmission from at least two of said cell sites, said predetermined transmissions being transmitted at predetermined times within a first time period, wherein said predetermined transmissions are transmitted by said at least two cell sites with a time difference within said first time period, wherein a plurality of said cell sites transmit the multiframe at the same time, each of said cell sites commencing at a different point m said multiframe structure. A group comprising a plurality of cell sites may be defined and within the group, tne predetermined transmissions may occur at different times. The network may comprise a plurality of groups of cell sites, wherein cell sites m different groups may transmit the predetermined transmission at the same times. The groups may also be overlapping, but within the cells of the same groups the predetermined transmissions occur at different times By ensuring that the predetermined transmissions, which are monitored by the station, are not transmitted at the same time, maximum efficiency can be obtained, m preferred embodiments of the present invention. In preferred embodiments of the present invention, each of the cell sites is arranged to transmit a multiframe including the predetermined transmission Preferably, one multiframe is transmitted m the first time period In other words, for each first time period, one multiframe is transmitted The multiframe may comprise a plurality of frames, the or each predetermined transmission bexng provided in one or more cf these frames In preferred embodiments of the present invention, the predetermined transmission occupies one frame each time it is transmitted Each cell site may use the exact same multiframe structure In other words, the same frames will occur m each multiframe used by each The or each predetermined transmission transmitted by each cell site is, m preferred embodiments in the same relative location m the multiframe The plurality of cell sites may transmit the multiframe at the same time but each of the cell sites may commence at a different point m the multiframe structure In this way, it is possible to ensure that the predetermined transmissions from different cell sites occur at different times The point of commencement for each cell may be selected so that all of the predetermined transmissions transmitted by each of the cell sites occur at different times The first time period may commence at the same time m each of the cell sites Each cell site may transmit the predetermined transmission at least three times m the time period, the predetermined transmissions being at regularly spaced time intervals m tne time period In alternative embodiments, two or more predetermined transmissions are provided at irregularly spaced time intervals m the time period Where the multiframe structure is used, the station may have at least one predetermined time m the second time period m which the station is able to monitor the predetermined transmission. Preferably, the first and second time periods are of different lengths so that the predetermined transmissions will occur m one of the at least predetermined time m the second time period, in at least one of the second time periods The first station may be provided with information identifying the predetermined times for the respective cell sites The first station may be arranged to identify when the predetermined transmissions occur with respect to the at least one predetermined time m the second time period so as to minimise the time taken for the first station to monitor the said at least two cell sites In this way, maximum efficiency may be obtained The predetermined transmission may comprise a synchronisation channel and/or a frequency correction cnannel The station may monitor only predetermined transmissions from cell sites surrounding the cell site m which the station is located However, m preferred embodiments, the station is also arranged to monitor the at least one predetermined transmission from the cell site m which the station is currently located Preferably, the network is a wireless network and the station is a mobile station. It should be appreciated that each cel_ site comprises one or more base stations According to a second aspect of the present invention, there is provided a cell site having at least one station associated therewith and being surrounded by a plurality of other cell sites, said cell site having means for transmitting with reference to a common time reference, said common time reference Deing used by said other cell sites, said cell site providing at least one predetermined transmission at least one predetermined t me within a first time period, wherein said at least one predetermined transmission is transmitted by said transmitting means at different times within said first time period to the transmission of corresponding predetermined transmissions transmitted by the plurality of other cell sites. Brief Description of the Drawings For a better understanding of the present invention and as to how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings in whicn: Figure 1 shows a network in whicn embodiments of tne present invention can be incorporated, Figure 2 shows a control channel multiframe structure, Figure 3a shows the frame number used by one cell, Figure 3b shows another frame number used by a different cell; Figure 4 shows a schematic view of a cellular network with a frame number distribution; and Figure 5 shows a cell site. Detailed Description of Embodiments of the Invention Reference is made to Figure 1, which shows a cellular telecommunications network 2 in which embodiments of the present invention can be implemented. The area covered by the network is divided into a plurality of cells 4. Each cell 4 has associated therewith a base transceiver station 6. The base transceiver stations 6 are arranged to communicate with mobile terminals 8 located m the cell for association with a given base station. In the schematic representation of Figure 1, one cell 4a is surrounded by six neighbouring cells 4. The base stations shown m Figure 1 are synchronised. In other words, all of the base stations use a common time reference. This means that the time slots w ll begin at the same time m each cell . The embodiment shown m Figure 1 is described as being m accordance with the GSM standard. Reference is made to Figure 2 which shows one multiframe consisting of 51 TDMA (time division multiple access) frames. As can be seen from Figure 2 the synchronisation channel occurs five times m each multiframe In particular, the synchronisation channels occur m positions 1, 11, 21, 31 and 41 m the multiframe The same multiframe structure is used r>y each base station In order to achieve orthogonality, the frame number 'FN) used oy adjacent cells is controlled In other words, the system will be most efficient if the different cells transmit the synchronisation channels at different times This means that tne amount of time required for a mobile station to measure the signal strengths from all the required neighbouring cells can be reduced Reference is made to Figures 3a and 3D With tne arrangement shown m Figure 3a the frame number is X + 0 This means that the first frame of the multiframe to be transmitted m the first slot is the zero frame. This same order is used m the seven remaining slots of the hyperframe Each hyperframe is made up of eight slots as shown m Figure 3 In the arrangement shown in Figure 3b, the frame number is X+20 In other words, the first frame within the first slot is frame number twenty. In order to transmit all of the multiframe m one slot, frame 0 will follow frame 50 so that the last frame transmitted m the first slot is frame 19 In principle, the first frame of the multiframe used at the beginning of each slot can be any of the fifty one frames making up the multiframe However as will be discussed hereinafter, there are advantages if the frame number of adjacent cells is controlled to minimise the possibility that synchronisation cnannels m adjacent cells are transmitted at the same time In the following analysis, cell A and cell B are synchronised adjacent cells having frame numbers FNA and FNB respectively the This analysis is performed using the modular arithmetic notation and m particular 51 modular arithmetic In 51 modular arithmetic, 52 would be expressed as 1. The notation mod(FNx,-51) is used. FNx is the frame number of cell x and mod(FNx;51) is the frame number of cell x expressed m 51 modular arithmetic If (FNA, 51) =mod (FNB , 51) , then all of the synchronisation channel broaαcasts by the different base stations w ll occur at the same time This s Because the same frame w__ll be transmitted at tine beginning of each slot and m a synchronised network, tne slots all start at tne same time If mod (FNA, 51) =mod (FNB, 51) +10 or mod (FNA, 51) =mod (FNB , 51 ) +41 then 80% of the synchronisation channel broadcasts transmitted by cell A w ll occur at the same time as those transmitted by cell B This is because the synchronisation channels are spaced apart by ten frames m tne multiframe structure Accordingly if the cell A starts with frame 0 at the beginning of the first slot and cell B starts with frame 10 at the beginning of the same slot, frames eleven, twenty-one, thirty-one and forty-one for cell A will occur at respectively the same times as frames one, eleven, twenty-one and thirty-one of cell B Only frame one of the cell A and frame forty one of cell B will not occur at the same time as a frame containing the synchronisation channel m the other cell The same can be demonstrated for the case where mod (FNA; 51) =mod ( FNB ; 51 ) +41. If mod (FNA, 51) =mod (FNB, 51) +20 or mod (FNA, 51) =mod (FNB) +31 then 60% of the synchronisation channel broaαcasts transmitted by cell A will occur at the same t me as synchronisation broadcasts transmitted by cell B Accordingly if cell A were to start with the frame 0 and cell B were to start with frame 20, frames twenty-one, thirty-one and forty-one for cell A would occur at the same time as frames one, eleven and twenty-one respectively Frames one and eleven of the first cell, cell A and frames thirty-one and forty-one of the second cell, cell B, do not occur at the same time as a frame containing the synchronisation channel m the other cell. The same analysis can be demonstrated for the case wnere mod (FNA; 51) =mod (FNA; 51) +31 If mod (FNA; 51) =mod (FNB; 51) +30 or mod (FNA, 51) =mod (FNB ; 51) +21, then 40% of the synchronisation channels broadcast by cell A will overlap with synchronisation channel broadcasts by cell B. If cell A were to start with frame 0 and cell B were to start witl frame 30 at the beginning of the first time slot, frames thirty- one and forty-one of cell A would occur at the same time as frames one and eleven of cell B. Tnus frames one, eleven ana twenty-one of cell A and frames twenty-one, thirty-one and forty- one of cell B would not occur at the same time as a frame containing the synchronisation channel m the other cell. Again, the same analysis can be performed for mod (FNA; 51) =mod(FNA;51) +21 If mod (FNA; 51) =mod (FNB; 51) +40 or mod (FNA; 51) =mod (FNB ; 51) +11 then 20% of the synchronisation channel broadcast by cell A will overlap with synchronisation channels broadcast by cell B. In other words, only one of the synchronisation channels of cell A will be broadcast at the same time as one of the synchronisation channels m cell B. If cell A were to commence with frame 0 at the beginning of the first slot and cell B were to commence with frame 40 at the same time, then frame forty-one of cell A would occur at the same time as frame one of cell B. In all other conditions, there will be no simultaneous broadcast of the synchronisation channels m cell A and cell B. This means that 42 out of the 51 possible combinations of frame number of cell A and cell B mean that the cell would will never broadcast the synchronisation channel bursts simultaneously. For example, if cell A were to commence with frame 0 and cell B were to commence with frame 1, none of the synchronisation channels from the cells would be transmitted at the same time as the synchronisation channels m the adjacent cells. The examples described hereinbefore have used the case where one of the cells starts with the first frame of the multiframe at the beginning of the first slot. However it should be noted that any frame can be used at the beginning of the first slot. The analysis described hereinbefore is relevant to the difference m the frame numbers of adjacent cells rather than the absolute values thereof The embodiment described hereinbefore has been m the context of the synchronisation channel However, t should be appreciated that the frequency correction channel will occur in tne 0, lOtn 20tn, 30th and 40th frames of the multiframe structure Accordingly, exactly the same analysis can be performed m respect of the frequency correction channel The embodiment described hereinbefore nas only considered the case wnere there are two cells which are monitored by the mobile station In practice, the mobile station will m a network where the mobile station will be monitoring a greater number of cells Reference is now made to Figure 4 which shows how the analysis set out hereinbefore can be used m a network. In the example shown m Figure 4 , each cell is immediately surrounded by six other cells. Additionally, there are two further cells which are not immediately adjacent to the current cell but nevertheless are close enough that the mobile station may monitor these additional cells In the embodiment shown m Figure 4, the current cell is referenced a, the six immediately adjacent cells are referenced b and the two further cells are referenced c It is therefore desirable that none of the nine cells m this group 50 have the same frame number It is also desirable that none of these cells transmit the syncnronisation channels at the same time as any of the other cells Accordingly, a subset of the frame number values are selected All of the values selected have the property which guarantees that the synchronisation channel as well as the frequency correction channel bursts will be transmitted at different times by the different base stations of the cells m this group A cluster of cells, for example group 50, could use this subset of frame number values which would ensure that the simultaneous broadcast of the frequency correction channel and the synchronisation channels would not occur at the same time within a cluster. Where the multiframe structure showr m Figure 2 is used, tne maximum group of frame numbers to have the property where simultaneous broadcast of the synchronisation channels or tne frequency correction channels is avoided is ten These frame numbers will have a the value zero one two, three four, five six, seven, eight or nine added to the basic frame number There are also equivalent cases wnere tnese frame numbers have multiples of 10 adαed thereto It s therefore possible to implement clusters of sites m which the shift m the frame number value (m terms of mod(FN,51)) varies from 0 to 9 It can be guaranteed that within the cluster no simultaneous broadcast of the synchronisation channels of the different base stations occur at the same time Likewise, there will be no simultaneous broadcast of the frequency correction channel by the base stations at the same time This means that it can be ensured that the time required for a mobile station to measure the strength of neighbouring cells is reduced compared to the case where the synchronisation channels are broadcast at the same time In some embodiments of the present invention, cell sectors are used Effectively, three base stations will be provided at the same location. Each of the base stations effectively covers 120° The base stations at the same location or site may share some elements The same principal described hereinbefore can also oe applied on a site Py site basis For example, 10 sites which effectively covers 30 cells can use the same frame number In the illustration shown m Figure 4, 9 cell sites define a set of frame number values This set of values can then be repeated with adjacent sets of cell sites The number next to each cell site m Figure 4 represents the number added to the basic frame number As can be seen m Figure 4, this network has a number of evenly distributed cell sites This makes the allocation of the numbers to be added to the frame number simple However as there are 10 possible options available in embodiments of the present invention, it is possible to use more complicated strategies for complicated arrangements such as those incorporating overlying cells or the like. Reference will now be made to Figure 5 which shows a schematic view of a base station which can be used with embodiments of the present invention The base station comprises a receiver 20 for receiving synchronisation information This receiver 20 will receive information defining the common reference ana information defining the offset (the numder to be added to tne basic frame number This may be proviαed by a iτiodiie switching services center (MSC) or the like using a processor 22, the processor determines the common reference and the offset and generates control signals These control signals are input to a transmitter 24 The transmitter 24 also receives the sιgna__ information m the form of frames to fie transmitted to tne mobile stations from a signal processor 26 Tne transmitter 24 transmits the frames m accordance with the control signals received from the processor 22 Whilst embodiments of the present invention have been described m the context of a GSM system, it should be appreciated that embodiments of the present invention can also be applied to other time division multiple access and/or frequency division multiple access systems. Embodiments of the present invention can be used with spread spectrum systems, such as code division multiple access Embodiments of the present invention can also be used with hybrid systems which comαine two or more access methods It should be appreciated that any length of multiframe can be used Embodiments of the present invention can be used m environments where the multiframe structure is not used but rather the synchronisation channels are broadcast at fixeα intervals from a reference point in time Embodiments of the invention can additionally or alternatively be used witn other types of channels other than the synchronisation channel or tne frequency correction channel For examples, embodiments of the present invention may be used with a reference or pilot channel or the like. Where multiframes are provided, it is of course possible for different numbers of frames to be incorporated m the multiframe. Additionally, the repeat of channels m the frame can be any suitable value and not just 10. The number of times that a channel s repeated may also take any suitable value. In embodiments of the present invention, the moαile station may monitor the synchronisation and frequency correction channels from the surrounding cell sites as well as the ceil site m which the mobile station is currently located. In alternative embodiments, the mobile station may only monitor the signals from the neighbouring cells and not ts current cell. The offsets m the frame numbers may be broadcast to the mobile station by its current base station and/or one or more of the base stations m the surrounding cells. This may assist the mobile station m using the most efficient strategy to measure the synchronisation channels of the various base stations. This is because in a synchronised network, the mobile station will be able to determine when its idle slots will occur with reference to the common time reference and hence when its idle slots will occur at the same time as synchronisation channels of the neighbouring cells. The mobile station can then determine the most efficient order in which to measure the strengths of the synchronisation channels. For example the mobile station may determine that its first idle slot coincides with the synchronisation channel of a first neighbouring station and its second idle slot coincides with the synchronisation channel of a second neighbouring station and so on. This means that mobile stations can avoid where ever possible wasting an idle slot needlessly. It should be appreciated that the mobile station is able to determine the cell site which has transmitted the signal which is to be measured based on one or more of the following: the frequency with which the channel is transmitted; and the base station identification code (BSIC) . In a code division multiple access system, the cell sites may be distinguished based on the spreading code used. This may be m addition or alternative to the use of an identification code. Tne mobile station may forward these measurements to the base station with wmcn it is m active communication The base station may pass this information onto another network element which uses th s information to decided if the mobile station should change the base station with which s currently m communication. The information can also be used to decide which base station should he the new base station with which the mobile station communicates. The decisions can alternatively be made by the current base station or by the mobile station. Alternatively, the mobile station and/or the base station can assist m the decision making progress. The measurements made by the mobile station can also be used to decide if the mobile station should go into soft handoff and if so with which base stations. Soft handoff is where a mobile station is m active communication with more than base station at the same time. It should be appreciated that the decision as to the frame number used by each cell site is made oy any suitable networ element. In a preferred embodiment of the invention, a network controller such as a radio network controller or a mobile services switching centre MSC upstream of the base stations will allocate the frame number .

Claims: CLAIMS 1 A telecommunications network comprising a plurality of ceil sites and a station associated with at least one of said cell sites, said cel_ sites being sync h ronised to transmit witin reference to a common time reference, sa_α station being arrangeα to monitor a predetermined transmission from at least two of said cell sites, saiα predetermined transmissions being transmitted at predetermined times within a first time period wherein at least one group comprising a plurality of substantially neighbouring cell sites is defined, and said predetermined transmissions are transmitted at different times by all the cell sites of the group within said first time period, wherein a plurality of said cell sites transmit the multiframe at the same time, each of said cell sites commencing at a different point m said multiframe structure . 2. A network as claimed m claim 1, wherein each of said cell sites is arranged to transmit a multiframe including said predetermined transmission. 3. A network as claimed m claim 2, wherein said one multiframe is transmitted m said first time period 4 A network as claimed m claim 2 or 3 , wherein said multiframe comprises a plurality of frames the or each predetermined transmission being provided m one or more of said frames 5 A network as claimed m claim 2, 3 or 4 wherein each cell site is arranged to use the same multiframe structure 6 A network as claimed m any of claims 2 to 5, wherein the or each predetermined transmission transmitted by each cell site is m the same location m the multiframe 7. A network as claimed m claim 6, wherein said point of commencement for each cell is selected so that all of said predetermined transmissions transmitted by each of the cell sites occur at different times 8 A network as claimed m any preceding claim, wnerem saiα first time period commences at the same time m eacn of said cel sites 9 A network as claimed m any one of the preceding claims, wherein each cell site transmits the predetermined transmission at least three times in said time period, said predetermined transmissions being at regularly spaced time intervals m said time period 10. A network as claimed m claim 10 when appended to claim 3, wherein the frames containing said predetermined transmissions are regularly spaced throughout said multiframe 11. A network as claimed m any preceding claim, wherein said station has at least one predetermined time m a second time period m which said station is able to monitor said predetermined transmissions . 12 A network as claimed m claim 11, wherein the first and second time periods are of different lengths so that the predetermined transmissions will occur m one of the at least one predetermined time m said second time period, m at least one of said second time periods 13 A network as claimed m any preceding claim, wherein said first station s provided with information identifying the predetermined times for the respective cell sites 14. A network as claimed m claim 13 , when appended to claim 11 or 12, wherein said first station is arranged to identify when the predetermined transmissions occur with respect to the at least one predetermined time m said second time period so as to minimise the time taken for the first station to monitor the sa d at least two cell sites. 15 A network as claimed m any preceding claim, wherein said predetermined transmission comprise a synchronisation channel and/or a frequency correction channel 16. A network as claimed in any preceding claim, wherein the station is arranged to monitor the at least one predetermined transmission from the cell s te in which tne station is currently located 17. A network as claimed m any preceding claim, wherein said network is a wireless network. 18. A network as claimed m claim 17, wherein said station is a mobile station. 19. A cell site arranged to have at least one station associated therewith and arranged to be surrounded by a plurality of other cell sites, said cell site having synchronisation means for determining a common time reference and transmitting means for transmitting with reference to a common time reference, said common time reference being used by said other cell sites, said cell s te providing at least one predetermined transmission at least one predetermined time within a first time period, wherein said at least one predetermined transmission is transmitted by said transmitting means within said first time period with a time difference to the transmission of corresponding predetermined transmissions transmitted by the plurality of other cell sites

Inventors: MELERO JUAN, MELERO, JUAN
Applicants: MELERO JUAN, NOKIA NETWORKS OY, MELERO, JUAN
Application year: 2000
Main classification: H04Q7
IPC codes: H04W56