EP2798765A1

EP2798765A1 - Method and apparatus for repeat transmissions in a wireless communication system with ciphering

Info

Publication number: EP2798765A1
Application number: EP12798515.8A
Authority: EP
Inventors: David J. Chater-Lea
Original assignee: Motorola Solutions Inc
Current assignee: Motorola Solutions Inc
Priority date: 2011-12-29
Filing date: 2012-11-16
Publication date: 2014-11-05
Also published as: WO2013101355A1

Abstract

A method and apparatus are provided for data block re-transmission in wireless communication systems that encrypt slots on the basis of slot and frame number. A first data block is encrypted on the basis of the timing of a first transmission time slot, in which the first data block will be transmitted. In re-transmission, a second data block containing the same data as the first data block is provided. Encryption of the second data block is based on the timing of the first transmission time slot. The second data block is then transmitted in a second transmission time slot. A receiver uses information about the timing of the first transmission timeslot when decrypting the received version of the second block. Soft combining can therefore be used by a receiver.

Description

METHOD AND APPARATUS FOR REPEAT TRANSMISSIONS IN A WIRELESS COMMUNICATION SYSTEM WITH CIPHERING

FIELD OF THE DISCLOSURE

[0001] The present disclosure relates generally to repeat transmission of signals in a wireless communication system.

BACKGROUND

[0002] Existing wireless communication systems may use a succession of

transmission timeslots. Within each time slot, a transmitter includes encoded information. One or more receivers is synchronized with the time slots. The receiver(s) are able to decode the encoded information. The information may include, for example, voice calls, or data.

[0003] One example of such a wireless communication system is 'TETRA'. Some versions of TETRA include 'TEDS' (TETRA Enhanced Data Service). TEDS provides wideband high speed data communication.

[0004] A receiver may fail to be able to decode the signals received during one or more slots. There may be many causes of such failures. The receiver captures a stream of samples, each of which is a digitised value. Each digitized value may represent the amplitude and/or phase of a symbol of the received signal, at a different point in time. When too many of these samples do not allow the symbols of the signal to be decoded accurately, an entire slot may be considered as 'missing'. However, the receiver may still have received the signal sufficiently well that it has many samples from the slot that are in fact accurate.

[0005] The distance between the transmitter and receiver is one factor that determines how likely it is that a receiver will be able to decode the signals of a slot. When the distance is greatest, the proportion of missing slots tends to be greater than when the distance is not so great. The problem of 'lost' or 'missing' slots therefore provides a limit on the range at which some wireless communication systems can be used. CM14841

[0006] Some existing wireless communication systems re-transmit slots. This allows receivers to receive the same slot a second time. The receiver may be able to decode the second version of the slot, without difficulty. When that it is not possible, then 'soft combining' can be used. With 'soft combining', received samples taken from both received versions of the same slot are combined. This process of combining enhances the probability of the receiver being able to decode the slot successfully. Some 4G wireless communication systems, such as LTE, use re-transmission of slots with soft combining.

[0007] FIG 1 shows schematically the signals that may be transmitted and received in an existing wireless communication system that uses soft combining.

[0008] FIG 1 is divided into three rows, generally labeled A, B and C. Time progresses from left to right across FIG.1 Row A shows a transmitted slot of information 110, which may for example be transmitted by a base station. For the whole duration of slot 110, the transmitted information can be assumed to be error free, in this example. Slot 120 in row B shows the information received, for example by a mobile communication device to which slot 110 is transmitted. The reductions at points 122 or 124 indicate points in time when the transmitted information is received incompletely or inaccurately.

[0009] If the receiving device, such as the mobile communication device, cannot decode slot 120, it may request a re-transmission of the information that was in slot 110. Slot 130 in row A shows such a re-transmission. Slot 140 in row B shows the information received from re-transmitted slot 130, for example by the mobile communication device. There are also reductions at point 142 and point 144 in slot 140. Points 142 and 144 are points in time when the information is received inaccurately. In the example shown, point 142 in slot 140 occurs later than point 122 in slot 120. Also, point 144 in slot 140 occurs later than point 124 in slot 120.

[0010] Slot 150 in row C shows the result of soft combining of samples of the received slots 120 and 140. Points 152 and 154 correspond to points 142 and 144 in slot 140, respectively. Slot 150 has more complete or more accurate information at points 152 and 154, than either slot 120 or slot 140. Thus there is a greater probability that slot 150 can be successfully decoded and decrypted by the receiver than would be CM14841 possible with either slot 120 or slot 140 alone. Superimposed on slot 150, reference 156 shows the original envelope of slot 120, and reference 158 shows the envelope of slot 140.

[0011] Re-transmission of slots and soft combining has not been possible in all mobile communication systems. One reason for this is that some mobile

communication systems encrypt the slots with an algorithm that depends on the slot timing. This approach prevents re-transmission, because the second transmission of a slot will be done with encryption that is different from that used on the first transmission of the slot.

[0012] TETRA and TEDS use encryption that depends on slot timing. One way of measuring slot timing depends on the slot and frame number, which is the approach used in TETRA. As a consequence, if known re-transmission approaches were applied to TETRA, the information transmitted in the first and second slots would differ. So the samples of the second transmission received by the receiver would differ from the samples that were received the first time that the slot was transmitted. Such differing samples cannot be combined, and 'soft combining' cannot therefore be used. This situation may place a limit on the range at which TETRA and TEDS communications can be reliably received. Considering FIG. 1 again, in the TETRA example, the symbols transmitted in slot 130 would not be an exact replica of the symbols transmitted in slot 110, even though the useful data that both slots carry is the same.

[0013] One type of transmission in a conventional wireless communication system involves transmission of slots on a 'downlink'. This type of transmission originates from the infrastructure of the wireless communication system. For example the downlink transmissions may be from a base station, which is communicating with mobile communication devices within a cell of the system. Such a transmission may be from the base station to one mobile communication device. Alternatively, such a transmission may be to a group of mobile communication devices. If the wireless communication system provides repeat transmissions of slots, these re-transmissions may be provided as a result of a request from a mobile communication device. Where the transmission is to a group of mobile communication devices, it is possible that a CM14841 significant number of the mobile communication devices will be unable to decode at least one slot. However, the exact slot that is missing may be different, for each mobile communication device of the group. If each member of the group requests retransmission of the particular slot that it was unable to decode, then the wireless communication system may receive requests for re-transmission of many different slots. Supplying each requested re-transmission may place significant demands on the transmission capacity of the mobile communications system. So, in conventional systems where re-transmission is possible, the demand for slot re-transmissions when the infrastructure is addressing a group of mobile communication devices may become very large.

[0014] Wireless communication systems may also have particular difficulty in providing sufficient re-transmissions when higher bit rates are in use. This is because higher bit rates may lead to more missing frames or slots than lower bit rates. As a consequence of there being more missing frames or slots, there will be more requests for re-transmission. This problem may be in addition to the 'range' problem identified above, and to the demands resulting from each mobile communication device separately requesting re-transmissions.

[0015] One particular scheme for repeat transmissions and soft combining is that used with known 4G technologies. These may use 'Hybrid Automatic Repeat reQuest' (Hybrid ARQ or HARQ) techniques, which can incorporate soft combining. Where soft combining is used, the receiver receives a data block from the air interface. The data block comprises a set of samples, which are derived by from the receiver. The set of samples may be a set of digitised values representing the amplitude and/or phase of the received signal at different points in time. The samples therefore correspond to the data symbols. The receiver attempts to decode the samples. If decoding of any received block fails, then the receiver can store the samples derived from the received data block, and request a repeat transmission of the data block.

[0016] When this repeat transmission arrives, there are two possible outcomes. If the repeated (re-transmitted) block can be decoded, then it can be used as a replacement for the original block, which can then be discarded. If the re-transmitted block cannot be decoded, then the receiver can attempt to combine the repeated data block with the CM14841 original data block. The receiver may find that the improved signal to noise ratio of the combined sets of symbols, from the two data blocks, is sufficient to decode the data correctly. In this case, soft combining has delivered information to the receiver that was not obtainable from just the first or second transmission of the block alone.

[0017] The HA Q with soft combining is a form of time diversity, which is applied to the received signals. The simplest form of combination uses maximal ratio combining of the original and repeated data block. However, other forms of combination are possible. The technique could be applied over more than one repeat of the same information. So a receiver may request that a given slot be re-transmitted a second time. The receiver would then have three copies of the same block, from which the receiver may be able to decode the block. If the transmitter is required to re-transmit the original block twice, then there is clearly an additional load on the transmitter. There will be correspondingly less transmission time available for other signal transmissions.

[0018] In the 4G example, signal encryption is performed in a higher signaling layer than the Layer 1/Layer 2 soft combining (and HARQ) process. This feature of the system design leads to data samples, at the air interface, being the same for each corresponding location within both the first transmission of any block and the retransmission of the block. Since the sets of samples that the receiver has to process are the same, then soft combining can be used to enhance the probability of successfully decoding the block. However this cannot be applied where encryption is carried out at Layer 1 or Layer 2, as is the case in TETRA and TEDS.

[0019] Accordingly, there is a need for a method and apparatus for improved repeat transmission of signals in a wireless communication system where encryption is carried out at Layer 1 or Layer 2, dependent on the timing of the timeslots.

BRIEF DESCRIPTION OF THE FIGS

[0020] The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views, together with the detailed description below, are incorporated in and form part of the specification, and serve to CM14841 further illustrate embodiments of concepts that include the claimed invention, and explain various principles and advantages of those embodiments.

[0021] FIG. 1 is a schematic diagram of signals in a known wireless communication system.

[0022] FIG. 2 provides an illustrative flowchart of a method of encrypting a repeat transmission in a wireless communication system.

[0023] FIG. 3 provides an illustrative flowchart of a known method of encrypting a repeat transmission in a wireless communication system.

[0024] FIG. 4 is a schematic diagram of signals encrypted in accordance with the flowchart of FIG 2.

[0025] FIG. 5 is a schematic diagram of signals encrypted in accordance with the flowchart of FIG 3.

[0026] FIG. 6 is a method of transmission in accordance with embodiments.

[0027] FIG. 7 is a wireless communication system in accordance with embodiments.

[0028] Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.

[0029] The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.

DETAILED DESCRIPTION

[0030] A method of data block re-transmission in a wireless communication system comprises providing a first data block, the first data block comprising first data. The first data block is encrypted, to provide a first encrypted data block. Encryption of the CM14841 first data block is based on the timing of a first transmission time slot, and the first encrypted data block is transmitted in the first transmission time slot. The method further comprises providing a second data block, the second data block also comprising the first data. The second data block is encrypted, to provide a second encrypted data block, the encryption of the second data block being based on the timing of the first transmission time slot. The second encrypted data block is transmitted in a second transmission time slot, the second transmission time slot occurring after the first transmission time slot.

[0031] Thus, within for example TET A/TEDS, the encryption process can be made identical in both the original transmission timeslot, and in one or more timeslots used for repeat transmissions. The encryption keystream is the same in each of these timeslots, and therefore the encryption synchronization will now be the same for these timeslots. This is a change from the current TETRA encryption process.

[0032] Encryption of the first transmission of information uses the encryption keystream generated from the initial vectors appropriate to the first transmission timeslot, or from vectors which would be expected for the timeslot or timeslots in which the first transmission of information is sent. Then, when a retransmission is sent, the same keystream is used to encrypt the re-transmission, as was used for the initial transmission. This may apply to the re-transmissions of each of the slots that comprise a message, when the message was first transmitted over more than one slot, and at least two slots need to be re-transmitted.

[0033] However, in known systems, the initial vectors held by a receiver, for the keystream generated to encrypt the re-transmission, would not now be correct. The receiver would have values appropriate to the timing of the re-transmission slot.

These are not now the values that will be used for the air interface timeslots in which the repeated information is being sent. In order that the receiver can use the correct initial vectors, the transmitter will transmit an indication of the correct initial vectors that the receiver must use for decryption. This indication may be transmitted in the channel allocation message from the transmitter, which informs the receiver which timeslots will carry the repeated information. CM14841

[0034] Thus the same sets of symbols are transmitted for both the first transmission and the re-transmission of any slot. If all symbols were to be received perfectly, without distortion or loss of any symbols, then the receiver would receive exactly the same sets of symbols at the air interface for the first transmission and the retransmission. However, when there is so much loss or distortion of symbols that the receiver cannot decode the first transmission, the receiver can perform soft combining of the two sets of symbols that it receives. This soft combining aids in decoding the message.

[0035] FIG. 2 provides an illustrative flowchart of a method of encrypting a repeat transmission in a wireless communication system. Although FIG. 2 and the detailed description below focus on approaches to re-transmitting one slot, the re-transmission may include multiple slots that were each part of a 'first' transmission.

[0036] At block 210, current values of a slot number and a frame number are provided, in a wireless communication system. The current slot and frame numbers are incremented for each successive transmission slot. The actual mechanism to provide this may, for example, be a counter. The counter counts timeslots within a frame, then frame numbers. The frame numbers may be counted in a sequence of frame - multiframe - hyperframe. The provision of current values of the slot and frame numbers provides timing, based on a numbering system that rises with each successive slot.

[0037] At block 220, stored values of a slot number and a frame number are provided. These stored values correspond to previous values of a slot number and frame number.

[0038] Reference 230 shows, symbolically, a branch in the flowchart. This indicates that either the current slot and frame numbers from block 210, or the stored slot and frame numbers from 220, may be supplied to block 240.

[0039] Block 240 provides encryption synchronization for a current transmission time slot, in which the wireless communication system is about to transmit

information. Block 250 provides a stream cypher based on the synchronization information from block 240. CM14841

[0040] The wireless communication system provides raw blocks of data, forming a stream. These are indicated at block 260. Each block of data may be part of a voice or data transmission. The transmission may be from a base station to one or more mobile communication devices. Alternatively, the transmission may be from a mobile communication device to a base station, or in a 'peer to peer' communication with other mobile communication devices.

[0041] Operator 270 indicates encryption and encoding of each block of data, with encryption and encoding appropriate to the transmission timeslot. For any given transmission timeslot, if block 210 supplies the 'current' slot and frame number as the input to blocks 240, 250 and operator 270, then the block of data will be encrypted on the basis of the current slot and frame number. However, if block 220 supplies a stored slot and frame number as the input to blocks 240, 250 and operator 270, then the block of data will be encrypted on the basis of the stored slot and frame number.

[0042] Block 280 indicates a stream of encrypted, encoded blocks of data, ready for transmission. The encryption synchronization of an encrypted, encoded block of data may be based on values from block 210. However, if it is a re-transmission, the encryption synchronization of the block will be based on stored frame and timeslot values from block 220.

[0043] The flowchart illustrated in FIG. 2 therefore provides the possibility of retransmitting a slot, or multiple slots. Encryption of the first transmission of the block of data in a slot will be based on the current slot and frame value from block 210. When re-transmission of the block of data in the slot occurs, encryption will be on the basis of stored slot and frame numbers from block 220. The stored slot and frame number values will be the values that were used when the block of data was first transmitted. If the same block of data is transmitted a third time, then the same stored slot and frame numbers from block 220 will be used again, i.e. those that were current for the first transmission and were used for the second transmission.

[0044] FIG. 3 is a flowchart using similar terminology to that in FIG.2. FIG. 3 is provided to illustrate how re-transmission is enabled in known systems that have synchronization based on slot and frame numbering, in contrast to FIG.2. Blocks 340, 350, 360 and 370 correspond to the similarly numbered blocks of FIG.2. CM14841

[0045] In FIG. 3, the input to block 340 is always from block 310. In the known method illustrated in FIG. 3, therefore, encryption synchronization is only ever based on the current slot and frame numbers.

[0046] Block 380 indicates a stream of encrypted, encoded blocks of data, ready for transmission. However, the stream of encrypted, encoded speech or data in block 380 differs from that in block 280 of FIG. 2. Encrypted, encoded blocks in the stream represented by block 380 will have been encrypted on the basis of the current frame and slot number of their transmission frame, even if they are re-transmitted blocks. A receiver that received those re-transmitted blocks could not use time diversity, such as soft combining, to improve the possibilities of successfully decoding the retransmitted block.

[0047] FIG. 4 is a schematic diagram of signals processed in accordance with the flowchart of FIG 2.

[0048] Blocks of data 'Datal ', 'Data2', 'Data3', 'Data4' and 'Datal ' are indicated at 410. Each of these blocks of data is transmitted, in the order 'Datal ', 'Data2', 'Data3', 'Data4' and 'Datal ' . The first four blocks are different blocks of data. As can be seen at reference 412, the final block of data is a re-transmission of first block of data 'Datal '.

[0049] Operator 420 indicates an 'XO ' operator. In row 430, there are five key stream values. These are, in succession, 'Keystreaml ', 'Keystream2',

'Keystream3', 'Keystream4' and 'Keystreaml '. Each of the blocks in row 410 is XORed with the keystream value immediately below it. As can be seen at reference 432, the final keystream value 'Keystreaml ' is the same as the first keystream value in row 430.

[0050] Arrow 440 indicates the result of the XOR operation, which is shown in row 450. Cipher text values 'Cipher Textl ', 'Cipher Text2', 'Cipher Text3', 'Cipher Text4' and 'Cipher Textl ' are shown in row 450. As can be seen at reference 452, the final cipher text value 'Cipher Textl ' is the same as the first cipher text value in row 450. CM14841

[0051] The method of the flowchart of FIG. 2 results in the situation shown in FIG.4. When block of data 'Datal ' is to be re-transmitted, 'Keystreaml ' is the keystream value generated. As a consequence, the cipher text value 'Cipher Textl ' for the retransmission of block of data 'Datal ' is identical to the cipher text value for the first transmission of block of data 'Datal '.

[0052] FIG. 5 is a flowchart using similar terminology to that in FIG.4. FIG. 5 is provided to illustrate the results of the known method of FIG. 3, in contrast to the results shown in FIG. 4.

[0053] In each of rows 510, 530 and 550, the leftmost four blocks correspond to those in FIG. 4. Reference 520 is the XOR operation. Reference 540 shows the result of the XOR combination of each data block in row 510 and corresponding keystream value in row 530.

[0054] Reference 512 shows re-transmission of block of data 'Datal '. The

corresponding keystream value in row 530 is a value 'Keystream5', see reference 532. 'Keystream5' is based on the actual slot and frame number of the re-transmission slot of 'Datal ', in contrast to the situation in FIG. 4. As a result, the cipher text block at reference 552 has value 'Cipher Text5'. This is different from cipher text value 'Cipher Textl ' in the corresponding location 452 in FIG. 4.

[0055] FIG. 5 shows that different symbols will be transmitted in the timeslot in which block of data 'Datal ' is re-transmitted, in comparison to the original transmission of block of data 'Datal '. As illustrated in FIG. 5, with known

approaches, a synchronization scheme that uses the frame and slot numbering of the transmission time slot as an 'Initial Vector' leads to re-transmissions being

'scrambled' differently, at the physical layer, in comparison to the first transmission of the same data block. As a result, the first and second received symbol streams will be different, even though they each contain within them the same datablock 'Datal '. Any conventional radio using slot and frame based encryption, therefore, would receive a re-transmission of 'Datal ' to which it could not apply soft combining.

[0056] The approach of FIGs 2 and 4 can be used to add soft combining to

TETRA/TEDS transmissions, in order to improve the probability of reception. Where CM14841 a transmission is addressed to a group, there may be particular benefits. The greater probability of successful re-transmission of a datablock may reduce the number of repeat requests received by a base station transmitting to a group address. The transmission range may be improved.

[0057] FIG. 2 showed an approach wherein encryption of a block for transmission preceded encoding. This is the approach used in TET A. Firstly, a data block is encrypted, then it is encoded. Here the encoding comprises adding 'forward error correction'. Then the block is transmitted. The received block must be handled in the opposite order. First an attempt is made to decode the forward error correction of the received block. If that is successful, then the block is decrypted.

[0058] In contrast to the encoding followed by encryption use in TETRA, the approach in GSM is to carry out encoding of a block for transmission, before encryption. In mobile communication systems such as GSM, therefore, a block for transmission is encoded, then encrypted, then transmitted. When the block is received at a receiver, it is firstly decrypted, and then an attempt is made to decode it.

[0059] A receiver will take samples of the blocks that are transmitted to it. These samples are digitised values, which represent the amplitude and/or phase of the received signal at different points in time. The sampling rate used will be equal to or greater than the channel modulation rate used for the transmission. Samples from the second encrypted, encoded data block will be combined with stored samples of the first encrypted, encoded data block. The soft combining may comprise applying maximal ratio combining of these samples, i.e. those from the first and second transmissions.

[0060] The methods of FIGs 2 and 4 may be employed in the infrastructure of a mobile communication system, such as a base station. In this case, the receiver will be either one mobile communication device, or a group of mobile communication devices. However, transmissions in accordance with the methods of FIGs 2 and 4 may alternatively, or in addition, be employed in a mobile communication device. In this case, the receiver is the infrastructure. In either case, there are various possible ways of allocating re-transmission slots. CM14841

[0061] A mobile communication device may initially be allocated two timeslots on an uplink, i.e. one for transmission, and one for a re-transmission. Alternatively, the second timeslot, for the re-transmission, may only be allocated when the

infrastructure requests a retransmission. In this case, the infrastructure will signal to the mobile communication device, in order to tell the mobile communication device which timeslot to use for the re-transmission. The signaling may be part of channel allocation signaling. It may tell the mobile communication device the initial vector of the timeslot to use. Alternatively, it may send an offset value. The offset value allows the mobile communication device to determine the timing difference between the first transmission timeslot and the re-transmission timeslot that the mobile communication device is to use.

[0062] FIG. 6 illustrates an example of a method in a transmitter. The method in FIG. 6 corresponds generally to the method explained in connection with FIGs. 2 and 4.

[0063] In block 610, the transmitter provides a first block of data that is to be transmitted. In block 620, the first block is encrypted, to provide a first encrypted data block. The encryption is based on the timing of the first transmission slot, in which the first encrypted data block is to be transmitted. This transmission then occurs in block 630. In block 640, a second block of data is provided. The second block of data contains the data of the first block of data. The second block of data may be provided in response to a request for a re-transmission, or may be sent without a request being received. In block 650, the second block of data is encrypted, based on the timing of the first transmission slot, to provide a second encrypted block. In block 660, the second encrypted data block is transmitted, in the second transmission timeslot.

[0064] FIG. 7 shows a wireless communication system 700. System controller 710 is linked to one or more base stations of the wireless communication system 700. Link 712 connects system controller 710 to base station 720. Link 714 connects system controller 710 to base station 722. Base stations 724 and 726 of the wireless communication system 700 are also linked to system controller 710. However, the links have not been shown, in order to keep FIG. 7 uncluttered. System controller 710 may be linked via physical or wireless connections to the base stations 720, 722, 724, 726. CM14841

[0065] First mobile communication device 730 is in wireless communication with base station 724. Second mobile communication device 732, third mobile

communication device 734 and fourth mobile communication device 736 are also in communication base station 724. The four mobile communication devices shown may form part of a single talkgroup. Each may also be capable of direct mode

communication with other mobile communication devices, without communications passing through any of the base stations of the wireless communication system 700

[0066] Wireless communication system 700 may, for example, be a TETRA/TEDS system. Wireless communication system 700 may allow retransmission of data blocks, and allow a receiver to soft combine first and second transmitted data blocks.

[0067] Wireless communication system 700 may allow several modes of operation. In one mode of operation, first mobile communication device 730 may transmit on an uplink to base station 724. When first mobile communication device 730 is

transmitting to base station 724, the timing of the first transmission time slot may be indicated by an initial vector. The initial vector may be a number, which is based on a count of timeslots in a frame, and frame numbers in a sequence of frames. As explained with reference to FIG. 4, first mobile communication device 730 may use an encryption keystream to encrypt a first data block for transmission, in a first transmission timeslot. The encryption keystream is generated from an initial vector of the first transmission timeslot, the initial vector therefore being based on the timing of the first transmission timeslot.

[0068] Any re-transmission of the data, as a second data block, will encrypt the second data block on the basis of the encryption keystream generated from the initial vector of the first transmission timeslot. Encrypting the first and second data blocks may comprise applying air interface encryption, prior to channel coding.

Synchronization of the air interface encryption uses the frame number and slot number of the first time slot, as the initial value of the first transmission time slot.

[0069] First mobile communication device 730 may transmit the second data block in response to a request for re-transmission from the cellular mobile communication system 700, the request for re-transmission being received after transmission of the first encrypted data block. When first mobile communication device 730 transmits the CM14841 second data block in response to a retransmission request, the cellular mobile communication system 700 may assign the second transmission timeslot to first mobile communication device 730, after receipt of the first data block by the cellular mobile communication system. Alternatively, first mobile communication device 730 may transmit the second data block in a pre-scheduled second transmission timeslot. In this case, the second transmission timeslot is assigned to the mobile

communication device by the cellular mobile communication system, prior to receipt of the first data block.

[0070] Base station 724 may therefore allocate timeslots to a first mobile

communication device 730, for an initial transmission, and may then also immediately grant further timeslots for a repeat of the information. The grant may be achieved by sending a specifically allocated second transmission timeslot. However, grant of a second transmission timeslot may rely on an offset value that was included in the original signaling from bases station 724, which allocated the first transmission timeslot. First mobile communication device 730 will send one or more repeated transmission(s) using the initial vector that applied for the original transmission, i.e. the first transmission timeslot. Base station 724 will combine and then decrypt the transmission appropriately, since it knows the relationship between first and second transmission timeslots.

[0071] In another mode of operation, base station 724 of cellular mobile

communication system 700 may transmit to first mobile communication device 730, on a downlink. Base station 724 of cellular mobile communication system 700 may be in communication with all of mobile communication devices 730-736, as part of a talkgroup of a cellular mobile communication system 700.

[0072] Base station 724 may transmit any re-transmissions in a pre-scheduled second transmission timeslot. Use of a pre-scheduled second transmission timeslot may reduce the amount of signaling required to members of the talkgroup, informing them when a re-transmission will occur.

[0073] Alternatively, base station 724 may inform one or more mobile

communication devices when a re-transmission will occur. This may be in response to one or more requests for re-transmission. Such requests for re-transmission will have CM14841 been received after transmission of the first encrypted data block, from members of the talkgroup. Base station 724 may transmit the initial vector of the first transmission timeslot, as part of channel allocation signaling, when informing a mobile

communication device that a re-transmission request has been granted. A receiving mobile, such as first mobile communication device 730, can then use the initial vector of the first transmission time slot, when decrypting the second encrypted data block received in the second transmission timeslot.

[0074] Instead of transmitting the initial vector, base station 724 may transmit an indication of the initial vector of the first transmission timeslot, as part of channel allocation signaling. When decrypting the second encrypted data block received in the second transmission timeslot, first mobile communication device 730 can retrieve or generate the initial vector of the first transmission time slot, based on the indication of the initial vector. The indication transmitted by bases station 724 may be an offset. More than one offset scheme can be used. For example, the offset may be an offset to a later value of the initial vector, the later value of the initial vector being a value appropriate to the second transmission time slot. First communication device 730 can then use the offset to identify the second transmission timeslot.

[0075] A further method of operation of cellular mobile communication system 700 involves first mobile communication device 730 in 'peer-to-peer' communication with another mobile communications device, such as one of mobile communication devices 732, 734 or 736. This may be direct mode communication. In this case, the device that is transmitting at any point in the direct mode call has control of the timing of any re-transmissions.

[0076] If first mobile communication device 730 is transmitting as part of 'peer-to- peer' communication with second mobile communication device 732, then second mobile communications device 732 may request a retransmission. The initial vector for synchronizing the encryption process may be based on timing such as by slot and frame number, or the initial vector may be transmitted as a separate indication, for example as is done in TET A DMO transmissions. First mobile communication device 730 will decide whether to grant the request for a re-transmission. First mobile communication device 730 may also signal to second mobile communications device CM14841

732, to indicate which slot, i.e. second transmission timeslot, will be used for the retransmission, particularly where the slot and frame timing is used to synchronize the encryption. The indication may use any of the approaches for channel allocation described above in connection with downlink transmission from base station 724.

[0077] In this peer to peer or DMO environment, the indication of the correct initial vector may take a number of forms. For example, the indication could be a repeat of the complete initial vector of the original transmission first timeslot. This would mean that there was a repeat of the complete original transmission, using the original initial vector. However, alternatively, an offset can be transmitted to a receiver. This may happen, for example, when part of the transmission was sent with a different initial vector, or when the timing was determined from an external source. Timing from an external source is possible where a gateway synchronizes timing with an

infrastructure. With this approach, the channel allocation or timing indication or initial vector indication for the repeated transmission includes an offset from the expected vector in the allocated timeslot, to the earlier initial vector that was used to encrypt the original message.

[0078] As explained previously, the indication of the correct initial vector in either the infrastructure or peer to peer mode may take a number of forms. Three forms (i)- (iii) are considered in the following: (i) The indication could be a complete initial vector of the original transmission first timeslot. (ii) However for reasons of efficiency, an offset value can be transmitted to a receiver. With this approach, the channel allocation for the repeated transmission includes an offset from the expected vector, in the allocated timeslot, to the earlier initial vector that was used to encrypt the message. As an example, an original timeslot initial vector might be numerically equal to 12345678. In this case, if the repeated timeslot would be expected to have initial vector of 12345698, then the offset would be sent as '-20'. This offset indicates that the initial vector should be taken from that used 20 slots earlier, (iii) An alternative approach would be to send the originally allocated initial vector, and include in this an offset to the later initial vector value(s), where the information would be repeated. The same allocation message could be sent, even after the transmission of the original message. In this case, the receiver would work out in CM14841 which slot it could expect the repeated information to be contained, i.e. the identity of the 'second transmission timeslot'. The receiver could then apply the initial vector that was relevant to the original slot sent in the channel allocation message, to both the original and the repeats of the message.

[0079] The method could include implicit allowance for 'reserved frames'. One example of a reserved frame is the 18^th frame used as 'Slow Associated Control Channel' in TET A. Such allowance would be used if the original allocation used timeslots that spanned over the 18^th frame. One example would be when the first transmission used timeslots 1-4 of frames 17, 1 and 2 in TETRA. The timeslot encryption of the repeated timeslots would then allow for this. The receiver would know the original channel allocation and could calculate the point where the initial vector advanced, due to the 18^th frame. The receiver could thus calculate the appropriate initial vector values to apply, including allowance for any further spanning of the 18^th frame during the repeat transmission of the information.

[0080] In the foregoing specification, specific embodiments have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present teachings.

[0081] The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.

[0082] Moreover in this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms "comprises," "comprising," "has", "having," "includes", "including," "contains", "containing" or CM14841 any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises, has, includes, contains a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by "comprises ...a", "has ...a", "includes ...a", "contains ...a" does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises, has, includes, contains the element. The terms "a" and "an" are defined as one or more unless explicitly stated otherwise herein. The terms "substantially", "essentially", "approximately", "about" or any other version thereof, are defined as being close to as understood by one of ordinary skill in the art, and in one non-limiting embodiment the term is defined to be within 10%, in another embodiment within 5%, in another embodiment within 1% and in another embodiment within 0.5%. The term "coupled" as used herein is defined as connected, although not necessarily directly and not necessarily

mechanically. A device or structure that is "configured" in a certain way is configured in at least that way, but may also be configured in ways that are not listed.

[0083] It will be appreciated that some embodiments may be comprised of one or more generic or specialized processors (or "processing devices") such as

microprocessors, digital signal processors, customized processors and field programmable gate arrays (FPGAs) and unique stored program instructions (including both software and firmware) that control the one or more processors to implement, in conjunction with certain non-processor circuits, some, most, or all of the functions of the method and/or apparatus described herein. Alternatively, some or all functions could be implemented by a state machine that has no stored program instructions, or in one or more application specific integrated circuits (ASICs), in which each function or some combinations of certain of the functions are implemented as custom logic. Of course, a combination of the two approaches could be used.

[0084] Moreover, an embodiment can be implemented as a computer-readable storage medium having computer readable code stored thereon for programming a computer (e.g., comprising a processor) to perform a method as described and claimed herein. Examples of such computer-readable storage mediums include, but are not limited to, CM14841 a hard disk, a CD-ROM, an optical storage device, a magnetic storage device, a ROM (Read Only Memory), a PROM (Programmable Read Only Memory), an EPROM (Erasable Programmable Read Only Memory), an EEPROM (Electrically Erasable Programmable Read Only Memory) and a Flash memory. Further, it is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such software instructions and programs and ICs with minimal experimentation.

[0085] The Abstract of the Disclosure is provided to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, it can be seen that various features are grouped together in various embodiments for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separately claimed subject matter.

Claims

CM14841 CLAIMS We claim:

1. A method of data block re-transmission in a wireless communication system, the method comprising:

providing a first data block, the first data block comprising first data;

encrypting the first data block to provide a first encrypted data block, whereby encryption of the first data block is based on the timing of a first transmission time slot;

transmitting the first encrypted data block in the first transmission time slot; providing a second data block, the second data block comprising the first data; encrypting the second data block, to provide a second encrypted data block, whereby encryption of the second data block is based on the timing of the first transmission time slot; and

transmitting the second encrypted data block in a second transmission time slot, the second transmission time slot occurring after the first transmission time slot.

2. A method of data block re-transmission in accordance with claim 1 , further comprising:

encrypting the first data block using encryption synchronization information comprising frame and slot numbering of the first transmission timeslot. CM14841

3. A method of data block re-transmission in accordance with claim 1, wherein:

the timing of the first transmission time slot is indicated by a number, the number based on a count of timeslots in a frame and frame numbers in a sequence of frames.

4. A method of data block re-transmission in accordance with claim 1, further comprising:

using an encryption keystream to encrypt the first data block, the encryption keystream being generated from an initial vector of the first transmission timeslot, the initial vector based on the timing of the first transmission timeslot; and

encrypting the second data block on the basis of the encryption keystream generated from the initial vector of the first transmission timeslot.

5. A method of data block re-transmission in accordance with claim 4, further comprising:

transmitting the initial vector of the first transmission timeslot, as part of channel allocation signaling;

whereby, when decrypting the second encrypted data block received in the second transmission timeslot, a receiver can use the initial vector of the first transmission time slot. CM14841

6. A method of data block re-transmission in accordance with claim 4, further comprising:

as part of channel allocation signaling, transmitting an indication of the initial vector of the first transmission timeslot, to be used for decryption of the second encrypted data block;

whereby, when decrypting the second encrypted data block received in the second transmission timeslot, a receiver can retrieve or generate the initial vector of the first transmission time slot.

7. A method of data block re-transmission in accordance with claim 6, wherein:

the indication is an offset.

8. A method of data block re-transmission in accordance with claim 4, further comprising:

transmitting the initial vector of the first transmission timeslot;

transmitting an offset to a later value of the initial vector, the later value of the initial vector being a value appropriate to the second transmission time slot;

whereby a receiver can use the offset to identify the second transmission timeslot. CM14841

9. A method of data block re-transmission in accordance with claim 1, wherein:

the second data block is transmitted in response to a request for retransmission, the request for re-transmission being received after transmission of the first encrypted data block.

10. A method of data block re-transmission in accordance with claim 1, wherein: encrypting the first and second data blocks comprises applying air interface encryption, prior to channel coding; and

synchronization of the air interface encryption uses a frame number and a slot number of the first time slot as an Initial Value (IV) of the first transmission time slot.

11. A method of data block re-transmission in accordance with claim 1 , wherein: the first and second data blocks comprise data transmitted in a TET A

Enhanced Data Service (TEDS) wireless communication system.

12. A method of data block re-transmission in accordance with claim 1, wherein: the first and second data blocks are transmitted as part of a one-to-many transmission; and

the second data block is transmitted in a pre-scheduled second transmission timeslot. CM14841

13. A method of data block re-transmission in accordance with claim 12, wherein: the first and second data blocks are transmitted to mobile communication devices in a talkgroup of a cellular mobile communication system.

14. A method of data block re-transmission in accordance with claim 1, wherein: the first and second data blocks are transmitted to a mobile communication device, as part of a one-to-one transmission on a downlink from a network of a cellular mobile communication system; and

the second data block is transmitted in a pre-scheduled second transmission timeslot.

15. A method of data block re-transmission in accordance with claim 1, wherein: the first and second data blocks are transmitted by a mobile communication device on an uplink to a network of a cellular mobile communication system; and the first and second transmission timeslots are pre-scheduled timeslots, assigned to the mobile communication device by the cellular mobile communication system prior to receipt of the first data block.

16. A method of data block re-transmission in accordance with claim 1, wherein: the first and second data blocks are transmitted by a mobile communication device on an uplink to a network of a cellular mobile communication system; CM14841 the mobile communication device transmits the second data block, in response to a retransmission request from the cellular mobile communication system, using an initial vector assigned by the cellular mobile communication system for transmission of the first data block.

17. A wireless communication system comprising an infrastructure and mobile communication devices, wherein the infrastructure and/or at least one mobile communication device is operable to:

provide a first data block, the first data block comprising first data;

encrypt the first data block, to provide a first encrypted data block, whereby encryption of the first data block is based on the timing of a first transmission time slot;

transmit the first encrypted data block in the first transmission time slot;

provide a second data block, the second data block comprising the first data; encrypt the second data block, to provide a second encrypted data block, whereby encryption of the second data block is based on the timing of the first transmission time slot; and

transmit the second encrypted data block in a second transmission time slot, the second transmission time slot occurring after the first transmission time slot.

18. A wireless communication system employing time-diversity reception for encrypted, encoded data blocks, the wireless communication system comprising an CM14841 infrastructure and mobile communication devices, wherein the infrastructure and/or at least one mobile communication device is operable to:

receive a first encrypted, encoded data block in a first transmission timeslot; decode the first encrypted, encoded data block to provide a first decoded datablock, and subsequently decrypt the first decoded data block, if the first encrypted, encoded data block can be decoded;

if the first encrypted, encoded data block cannot be decoded:

store samples of the first encrypted, encoded data block; receive a second encrypted, encoded data block, in a second transmission timeslot;

if the second encrypted, encoded data block can be decoded, then decode the second encrypted, encoded data block to provide a second decoded data block, and subsequently decrypt the second decoded data block with encryption synchronization information appropriate to the first transmission time slot;

if the second encrypted, encoded data block cannot be decoded, then: soft combine samples of the second encrypted, encoded data block with the stored samples of the first encrypted, encoded data block, to provide a third encoded, encrypted datablock; and decode the third encoded, encrypted datablock to provide a third decoded data block, and subsequently decrypt the third decoded data block with encryption synchronization information appropriate to the first transmission time slot. CM14841

19. A method of time-diversity reception for encrypted, encoded data blocks, received in a wireless communication system, the method comprising:

receiving a first encrypted, encoded data block in a first transmission timeslot; if the first encrypted, encoded data block can be decoded, then decoding the first encrypted, encoded data block to provide a first decoded datablock, and subsequently decrypting the first decoded data block;

if the first encrypted, encoded data block cannot be decoded, then storing samples of the first encrypted, encoded data block;

receiving a second encrypted, encoded data block, in a second transmission timeslot;

if the second encrypted, encoded data block can be decoded, then decoding the second encrypted, encoded data block to provide a second decoded data block, and subsequently decrypting the second decoded data block with encryption

synchronization information appropriate to the first transmission time slot;

if the second encrypted, encoded data block cannot be decoded, then:

soft combining samples of the second encrypted, encoded data block with the stored samples of the first encrypted, encoded data block, to provide a third encoded, encrypted datablock; and

decoding the third encoded, encrypted datablock to provide a third decoded data block, and subsequently decrypting the third decoded data block with encryption synchronization information appropriate to the first transmission time slot. CM14841

20. A method of time-diversity reception in accordance with claim 19, wherein:

encryption synchronization information appropriate to the first transmission time slot comprises frame and slot numbering of the first transmission timeslot, and the frame and slot numbering of the first transmission timeslot are used for decrypting the decoded datablocks.

21. A method of time-diversity reception in accordance with claim 19, further comprising:

if the first encrypted, encoded data block cannot be decoded, then requesting re-transmission of the first encrypted, encoded data block;

whereby the second encrypted, encoded data block in the second transmission timeslot carries a data payload corresponding to a datapayload of the first encrypted, encoded data block.

22. A method of time-diversity reception in accordance with claim 20, wherein: decoding comprises recovering original data using forward error correction data from the first, second and/or third data blocks; and

the first, second and/or third data blocks are TEDS transmissions in the TET A cellular mobile communications system. CM14841

23. A method of time-diversity reception in accordance with claim 19, wherein:

soft combining samples of the second encrypted, encoded data block with the stored samples of the first encrypted, encoded data block, comprises applying maximal ratio combining of the samples.

24. A method of time-diversity reception in accordance with claim 19, wherein:

the samples of the of the first and/or second encrypted, encoded data blocks are digitised values representing the amplitude and/or phase of the received signal at different points in time, the sampling rate being equal to or greater than a rate of channel modulation.

25. A method of time-diversity reception for encoded, encrypted data blocks, received in a wireless communication system, the method comprising:

receiving a first encoded, encrypted data block in a first transmission timeslot; if the first encoded, encrypted data block can be decrypted, then decrypting the first encoded, encrypted data block to provide a first decrypted datablock, and subsequently decoding the first decrypted data block;

if the first encoded, encrypted data block cannot be decrypted, then storing samples of the first encoded, encrypted data block;

receiving a second encoded, encrypted data block, in a second transmission timeslot; CM14841 if the second encoded, encrypted data block can be decrypted, then decrypting the second encoded, encrypted data block with encryption synchronization

information appropriate to the first transmission time slot, to provide a second decrypted data block, and subsequently decoding the second decrypted data block; if the second encoded, encrypted data block cannot be decrypted, then:

soft combining samples of the second encoded, encrypted data block with the stored samples of the first encoded, encrypted data block, to provide a third encoded, encrypted datablock; and

decrypting the third encoded, encrypted datablock with encryption synchronization information appropriate to the first transmission time slot, to provide a third decrypted data block, and subsequently decoding the third decrypted data block.

26. A method of time-diversity reception in accordance with claim 25, wherein:

encryption synchronization information appropriate to the first transmission time slot comprises frame and slot numbering of the first transmission timeslot, the frame and slot numbering of the first transmission timeslot being used for decrypting the encoded, encrypted datablocks.

CM14841

27. A method of time-diversity reception in accordance with claim 25, further comprising:

if the first encoded, encrypted data block cannot be decrypted, then requesting re-transmission of the first encoded, encrypted data block;

whereby the second encoded, encrypted data block in the second transmission timeslot carries a data payload corresponding to a datapayload of the first encoded, encrypted data block.