Technical Field
[0001]
The present invention relates to mobile communication devices and networks, particularly but not exclusively those operating according to the 3rd Generation Partnership Project (3GPP) standards or equivalents or derivatives thereof. The invention has particular although not exclusive relevance to addressing interference in the Long Term Evolution (LTE) of the Evolved Universal Terrestrial Radio Access Network (E-UTRAN).
Background Art
[0002]
In a mobile (cellular) communications network, (user) communication devices (also known as user equipment (UE), for example mobile telephones) communicate with remote servers or with other communication devices via base stations. In their communication with each other, communication devices and base stations use licensed radio frequencies, which are typically divided into frequency bands and/or time blocks.
[0003]
Traditionally, a given geographical area is divided into cells. Each cell of a network (thus each communication device within a cell) is served by a base station. The operation of the base stations is controlled by a base station controller, which might be implemented as part of the base stations or as a separate entity. The base stations are capable of dynamically adjusting their transmission power and/or direction to ensure optimum radio channel quality to the served communication devices (such as mobile telephones and other user equipment).
[0004]
Recent developments in telecommunications have seen a large increase in the use of machine-type communications (MTC) devices which are networked devices arranged to communicate and perform actions without human assistance. Examples of such devices include smart meters, which can be configured to perform measurements and relay these measurements to other devices via a telecommunication network. Machine-type communication devices are also known as machine-to-machine (M2M) communication devices.
[0005]
MTC devices connect to the network whenever they have data to send to or receive from a remote ‘machine’ (e.g. a server) or user. MTC devices use communication protocols and standards that are optimised for mobile telephones or similar user equipment. However, MTC devices, once deployed, typically operate without requiring human supervision or interaction, and follow software instructions stored in an internal memory. MTC devices might also remain stationary and/or inactive for a long period of time. The specific network requirements to support MTC devices have been dealt with in the 3GPP technical specification (TS) 22.368 version 13.1.0, the contents of which are incorporated herein by reference.
[0006]
For the Release 13 (Rel-13) version of the standards relating to MTC devices, support for a reduced bandwidth of 1.4 MHz in downlink and uplink is envisaged. Thus, some MTC devices will support only a limited bandwidth (typically 1.4 MHz) compared to the total LTE bandwidth and/or they may have fewer/simplified components. This allows such ‘reduced bandwidth’ MTC devices to be made more economically compared to MTC devices supporting a larger bandwidth and/or having more complicated components.
[0007]
The lack of network coverage (e.g. when deployed indoors), in combination with the often limited functionality of MTC devices, can result in such MTC devices having a low data rate and therefore there is a risk of some transmissions not being received by an MTC device (or not received correctly). In order to mitigate this risk, it has been proposed to increase the coverage of certain LTE transmissions (channels) to support such MTC devices (e.g. corresponding to 20dB for frequency division duplex (FDD) transmissions).
[0008]
One approach proposed for the enhancement of coverage for MTC devices is the repetition of the same information across multiple (e.g. two, three, four, or more) sub-frames. In other words, for coverage enhanced MTC devices, the base station duplicates the transmitted information in the time domain (the base station re-transmits the same information in one or more sub-frames subsequent to the sub-frame in which that information is first sent). Such a coverage enhanced MTC device can be configured to combine the multiple copies of the (same) information received in the multiple sub-frames, and after combining the received information, the coverage enhanced MTC device is more likely to be able to decode transmitted data successfully than based on a single copy of the information.
[0009]
In LTE systems, a bit-level scrambling code/sequence is currently applied for all downlink (DL) and uplink (UL) transmissions between E-UTRAN base stations and connected user equipment (including MTC devices). Specifically, scrambling is applied for transmissions over the Physical Broadcast Channel (PBCH), the Physical Downlink Shared Channel (PDSCH), the Physical Downlink Control Channel (PDCCH), the Physical Control Format Indicator Channel (PCFICH), the Physical Hybrid ARQ Indicator Channel (PHICH), and the Physical Uplink Shared Channel (PUSCH).
[0010]
The scrambling sequence currently specified by 3GPP is pseudo-random (created using a length-31 Gold sequence generator), with a periodicity of ten subframes (i.e. one radio frame), after which it is repeated indefinitely.
[0011]
The purpose of applying a scrambling sequence is to randomise interference that arises between transmissions in neighbouring cells (e.g. adjacent or overlapping cells), and thereby mitigate the adverse effects of such ‘inter-cell’ interference. This is possible, because when a receiving device (e.g. a UE / MTC device in case of DL transmissions) descrambles a received bitstream with a known cell-specific scrambling sequence, interference from other cells will be descrambled incorrectly and therefore only appear as uncorrelated noise (rather than actual user data).
[0012]
For PUSCH and PDSCH, the scrambling sequence is initialised, in each sub-frame, using the following formula (at both the transmitter and the receiver):
[Math. 1]

where c init is an initial scrambling code for a sub-frame; n RNTI is a temporary identifier associated with a communication device connected to a cell (operated by a base station); q is a codeword index; n s is a slot number;
[Math. 2]

is a unique identifier of that particular cell (a physical layer identity); and
[Math. 3]

is a flooring function (i.e. the largest integer not greater than ‘x’).
[0013]
The resulting scrambling sequence is referred to as the ‘legacy scrambling sequence’ or the ‘Rel-8 scrambling sequence’.
[0014]
However, the inventors have realised that in some cases the long repetition of information (over more than one radio frame) for MTC-specific coverage enhancement may have a number of disadvantages when using such a legacy scrambling sequence. For example, if the source of the inter-cell interference also uses a similar long repetition (coverage enhancement), then the interfering transmission may combine coherently with the affected transmission (over the course of multiple radio frames) and consequently any signal-to-interference-plus-noise ratio (SINR) gain of repetition combining may be reduced.
[0015]
This may result in a sub-optimal operation of the system (e.g. it may increase the need for re-transmissions, including repetitions), especially for MTC devices which are typically equipped with less sophisticated transceiver circuitry than conventional LTE user equipment (and hence such MTC devices may be more sensitive to inter-cell interference).
Summary of Invention
Technical Problem
[0016]
Accordingly, preferred exemplary embodiments of the present invention aim to provide methods and apparatus which overcome or at least partially alleviate the above issues.
Solution to Problem
[0017]
The invention provides communication apparatus for a cellular communication system comprising a plurality of cells, the communication apparatus comprising: means for communicating signals in a cell in a sequence of radio frames; and means for scrambling said signals communicated in said cell using a scrambling code; wherein said scrambling code is derived based on a parameter that changes from a given block of sub-frames to a subsequent block of sub-frames.
[0018]
Aspects of the invention extend to corresponding systems, methods, and computer program products such as computer readable storage media having instructions stored thereon which are operable to program a programmable processor to carry out a method as described in the aspects and possibilities set out above or recited in the claims and/or to program a suitably adapted computer to provide the apparatus recited in any of the claims.
[0019]
Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently (or in combination with) any other disclosed and/or illustrated features. In particular but without limitation the features of any of the claims dependent from a particular independent claim may be introduced into that independent claim in any combination or individually.
[0020]
Exemplary embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which:
Brief Description of Drawings
[0021]
[fig. 1] Figure 1 schematically illustrates a mobile telecommunication system of a type to which the invention is applicable;
[fig. 2a] Figure 2a illustrates a generic frame structure defined for use in the LTE communication network;
[fig. 2b] Figure 2b illustrates the way in which a slot illustrated in Figure 2a is formed of a number of time-frequency resources;
[fig. 3] Figure 3 shows a simplified block diagram of the main components of the communication device shown in Figure 1;
[fig. 4] Figure 4 shows a simplified block diagram of the main components of a base station shown in Figure 1;
[fig. 5] Figure 5 is a comparison of simulation results for various scrambling techniques that may be used in the system shown in Figure 1; and
[fig. 6] Figure 6 is an exemplary flowchart showing the initiation of a scrambling sequence in accordance with the present invention.
Description of Embodiments
[0022]

Figure 1 schematically illustrates a mobile (cellular) telecommunication system 1 in which a user of any of a plurality of communication devices 3 (such as mobile telephones, MTC devices, and/or the like) can communicate with each other and/or with remote servers via one or more of a plurality of base stations 5. Each base station 5 operates one or more associated cell 7. Specifically, in the system 1 shown in Figure 1, the first base station 5-1 operates a first cell 7-1 in which communication devices 3-1 and 3-3 are located (and served by the first base station 5-1) and the second base station 5-2 operates a second cell 7-2 in which communication devices 3-2 and 3-3 are located (albeit only communication device 3-2 is currently being served via the second cell 7-2). In the system illustrated in Figure 1, each base station 5 shown is an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) base station, which may also be referred to as an ‘eNodeB’ (or simply ‘eNB’) according to the commonly accepted 3GPP terminology.
[0023]
The base stations 5 are connected to each other via a so-called X2 interface, and to a core network 8 via an S1 interface. In LTE, such a core network 8 is referred to as an Enhanced Packet Core (EPC) network. The base stations 5 may be connected to each other and to the core network either directly or via a suitable gateway, such as a serving gateway, a small cell gateway, an X2 gateway, and/or the like. The core network 8 includes, amongst others, a mobility management entity (MME) 10, a serving gateway (S-GW) 11, and a packet data network (PDN) gateway (P-GW) 12. Although not shown in Figure 1, the core network 8 is typically also connected to an external network (e.g. The Internet) thereby providing the communication devices 3 access to the external network.
[0024]
Each base station 5 is configured to send and receive signals in a sequence of radio frames 13 (each typically 10ms in duration). Each radio frame 13 comprises a plurality of sub-frames 15 (typically 1ms in duration), each of which comprises a pair of slots 17 (typically 0.5ms in duration). Further details of the LTE frame structure will be given below with reference to Figures 2a and 2b.
[0025]
The radio frames 13 carry control and user data signals between the base stations 5 and the communication devices 3. The base station 5 dynamically allocates resource blocks to the communication devices 3 in dependence of current transmission needs, device capabilities, system conditions and other static or dynamic parameters.
[0026]
In order to provide coverage enhancement (e.g. for Rel-13 communication devices), when appropriate, the base stations 5 are configured to repeat downlink transmissions (of the same data) across multiple sub-frames 15 (and potentially across multiple radio frames 13). Similarly, when needed, the communication devices 3 are also configured to repeat their uplink transmissions for coverage enhancement purposes. The data that is transmitted repeatedly (in several successive sub-frames 15) can be combined at the receiving node (i.e. the receiving communication device 3 in case of downlink transmissions and the serving base station 5 in case of uplink transmissions), which is more likely to be able to decode the transmitted data successfully than based on a single copy of the information.
[0027]
In the example shown in Figure 1, the first communication device 3-1 located inside the coverage of the first cell 7-1 (e.g. when being served by the first base station 5-1) might experience strong inter-cell interference from the second cell 7-2 (e.g. from the second communication device 3-2 communicating with its own serving base station 5-2).
[0028]
Therefore, in this system, the communication devices 3 and the base stations 5 are configured to apply a bit-level scrambling sequence for their transmissions so that the effects of such inter-cell interference are mitigated (or avoided altogether). However, rather than using a legacy scrambling sequence that is repeated in every radio frame 13, the communication devices 3 and the base stations 5 are configured to use a parameter in the initialisation of their own scrambling sequence, which parameter changes from radio frame to radio frame. Beneficially, using such a parameter allows the communication devices 3 and the base stations 5 communicating with each other to derive a scrambling sequence that is longer than one radio frame 13 and that is different to the scrambling sequence used in any neighbouring cell 7. Therefore, the issue of an interfering transmission from a neighbour cell 7 combining coherently (over the course of multiple radio frames 13) can be avoided (or mitigated), when the repetitions for coverage enhancement spread over multiple radio frames 13. Consequently, by using this new type of parameter for deriving a scrambling sequence, it is possible to achieve a better signal-to-interference-plus-noise ratio (SINR) than using legacy scrambling sequences.
[0029]
In more detail, each communication device 3 and each associated serving base station 5 is configured to initialise, for each sub-frame 15, a scrambling sequence using the following formula:
[Math. 4]

where c init is an initial scrambling code for a sub-frame 15; n' RNTI is a parameter derived from a temporary identifier associated with a communication device 3 connected to a cell 7; q is a codeword index; n s is a slot number;
[Math. 5]

is a unique identifier of that particular cell 7 (a physical layer identity); and
[Math. 6]

is a flooring function (i.e. the largest integer not greater than ‘x’).
[0030]
In this example, the parameter n' RNTI is derived using the following formula:
[Math. 7]

where n RNTI is the temporary identifier associated with the communication device 3 and SFN is a radio frame number (i.e. a ‘system frame number’).
[0031]
Beneficially, since the parameter n' RNTI is dependent on the system frame number (SFN) associated with the current radio frame 13 (and hence it changes from one radio frame to another), the resulting c init for the same sub-frame 15 of a different radio frame 13 will also be different. Accordingly, the scrambling sequence in different radio frames 13 will also be different, which in turn reduces the risk of an interfering (repeated) transmission from a neighbour cell 7 combining coherently over multiple radio frames 13.
[0032]

Before describing detailed how the present invention can be put into practice, a brief description will be given of the access scheme and a general frame structure agreed for LTE communications. An Orthogonal Frequency Division Multiple Access (OFDMA) technique is used for the downlink to allow the communication devices 3 to receive data over the air interface from their respective serving base station 5. For each communication device 3, respective sub-carriers are allocated by the serving base station 5 (for a predetermined amount of time) depending on the amount of data to be sent to that communication device 3. These blocks of sub-carriers are referred to as physical resource blocks (PRBs) in the LTE specifications. PRBs thus have a time and frequency dimension. The serving base station 5 dynamically allocates PRBs for each communication device 3 that it is serving and signals the allocations for each sub-frame (TTI) to each of the scheduled devices in a control channel.
[0033]
Figure 2a illustrates one generic frame structure agreed for LTE communications over the air interface with the base station 5. As shown, one radio frame 13 is 10ms (‘msec’) long and comprises ten sub-frames 15 of 1ms duration (known as a Transmission Time Interval (TTI)). Each sub-frame or TTI comprises two slots 17 of 0.5ms duration. Each slot 17 comprises either six or seven OFDM symbols 19, depending on whether the normal or extended cyclic prefix (CP) is employed. The total number of available sub-carriers depends on the overall transmission bandwidth of the system. The LTE specifications define parameters for system bandwidths from 1.4 MHz to 20 MHz and one PRB is currently defined to comprise 12 consecutive subcarriers for one slot 17 (although this could clearly be different). The transmitted downlink signal comprises NBW subcarriers for a duration of Nsymb OFDM symbols. It can be represented by a resource grid as illustrated in Figure 2b. Each box in the grid represents a single sub-carrier for one symbol period and is referred to as a resource element (RE). As shown, each PRB 21 is formed from twelve consecutive sub-carriers and (in this case) seven symbols for each subcarrier; although in practice the same allocations are made in the second slot 17 of each sub-frame 15 as well.
[0034]

Figure 3 is a block diagram illustrating the main components of one of the communication devices 3 shown in Figure 1. The communication device 3 comprises a transceiver circuit 31 which is operable to transmit signals to, and to receive signals from, its serving base station 5 via at least one antenna 33. Although not necessarily shown in Figure 3, the communication device 3 may of course have all the usual functionality of a conventional mobile telephone (such as a user interface 34) and/or an MTC device and such functionality may be provided by any one or any combination of hardware, software and firmware, as appropriate.
[0035]
The operation of the transceiver circuit 31 is controlled by a controller 35 in accordance with software stored in memory 37. The software may be pre-installed in the memory 39 and/or may be downloaded via the telecommunications network or from a removable data storage device (RMD), for example. The software includes, among other things, an operating system 41, a communication control module 43, a scrambling module 45, and a coverage enhancement module 47.
[0036]
The communication control module 43 manages communication with the base stations 5 serving the cell 7, in which the communication device 3 is located. The communication control module 43 manages communications between the serving base station 5 and the communication device 3 according to the schedule determined by the serving base station 5 (including any repetition of transmissions for coverage enhancement purposes).
[0037]
The scrambling module 45 is responsible for scrambling data to be transmitted by the communication control module 43 to the serving base station 5 (and for de-scrambling data received by the communication control module 43 from the serving base station 5). When deriving (initialising) an appropriate scrambling sequence for a given sub-frame 15, the scrambling module 45 is configured to derive the scrambling sequence using a parameter that changes from radio frame to radio frame.
[0038]
The coverage enhancement module 47 is responsible for repeating transmissions of the same data (via the communication control module 43) in accordance with the type (or degree) of coverage enhancement needed for the communication device 3. The coverage enhancement module 47 is also responsible for combining (after an appropriate de-scrambling by the scrambling module 45) multiple copies of the (same) data received from the serving base station 5 to derive the transmitted data.
[0039]

Figure 4 is a block diagram illustrating the main components of one of the base stations 5 shown in Figure 1. The base station 5 comprises a transceiver circuit 51 which is operable to transmit signals to, and to receive signals from, the communication devices 3 via at least one antenna 53. The at least one antenna 53 may be configured as an antenna array forming part of an active antenna system (AAS) for controlled transmission/reception of signals in a specific direction.
[0040]
The base station 5 is also operable to transmit signals to and to receive signals from: the core network 8 via a network interface 54 (or ‘S1’ interface); and other base stations in the vicinity of the base station via a base station (or so called ‘X2’) interface 55.
[0041]
The operation of the transceiver circuit 51 is controlled by a controller 57 in accordance with software stored in memory 59. The software includes, among other things, an operating system 61, a communication control module 63, a scrambling module 65, and a coverage enhancement module 67.
[0042]
The communication control module 63 is operable to control communication with the communication devices 3 served by the base station 5 and to control communication with the core network 6 and other base stations via the transceiver circuit 51. The communication control module 63 manages communications between the base station 5 and connected communication devices 3 according to a schedule determined by the communication control module 63 (including any repetition of transmissions for coverage enhancement purposes, determined based on information provided by the coverage enhancement module 67).
[0043]
The scrambling module 65 is responsible for scrambling data to be transmitted by the communication control module 63 to connected communication devices 3 (and for de-scrambling data received by the communication control module 63 from connected communication devices 3). When deriving (initialising) an appropriate scrambling sequence for a given sub-frame 15, the scrambling module 65 is configured to derive the scrambling sequence using a parameter that changes from radio frame to radio frame.
[0044]
The coverage enhancement module 67 is responsible for repeating transmissions of the same data (via the communication control module 63) in accordance with the type (or degree) of coverage enhancement needed for the communication device 3 for which the transmission is intended. The coverage enhancement module 67 is also responsible for combining (after an appropriate de-scrambling by the scrambling module 65) multiple copies of the (same) data received from the communication devices 3 to derive the transmitted data.
[0045]
In the above description, the communication devices and the base stations are described for ease of understanding as having a number of discrete modules (such as the scrambling modules and the coverage enhancement modules). Whilst these modules may be provided in this way for certain applications, for example where an existing system has been modified to implement the invention, in other applications, for example in systems designed with the inventive features in mind from the outset, these modules may be built into the overall operating system or code and so these modules may not be discernible as discrete entities.
[0046]
A more detailed description of some of the novel aspects of the operation of the base stations 5 and of the communication devices 3 will now be given, with reference to Figures 5 and 6.
[0047]

Figure 5 is a comparison of simulation results for various scrambling techniques that may be used in the system 1 shown in Figure 1. Specifically, Figure 5 illustrates the performance difference between legacy (Rel-8) scrambling sequences and corresponding (extended/long) scrambling sequences according to exemplary embodiments of the present invention.
[0048]
The simulation assumptions are set in accordance with those specified in 3GPP R1-144513 for reference cases in MTC link-level performance evaluation. In more detail, the following two scenarios have been considered, both with the same average signal-to-interference-plus-noise ratio (SINR) of -3.0 dB but with different signal-to-interference ratios (SIRs) and interference-to-noise ratios (INRs):
　　　　1. noise dominated scenario: SNR=-2.96 dB; INR=-20 dB; SINR=-3.0 dB
　　　　2. interference dominated scenario: SNR=1.76 dB; INR=3.0 dB; SINR=-3.0 dB
[0049]
For both the noise dominated scenario and the interference dominated scenario, the legacy scrambling sequence and the above described extended/long scrambling sequence have been evaluated. Moreover, the simulations were repeated with and without frequency hopping.
[0050]
In Figure 5, the corresponding simulation results were divided into two groups: one group in which no frequency hopping was applied (simulation cases a to d); and another group in which frequency hopping, using 4 narrowbands was applied (simulation cases e to h).
[0051]
In both group, a distinction is made between interference dominated and noise dominated scenarios, and between applying the legacy scrambling sequence or the novel (extended/long) scrambling sequence.
[0052]
Accordingly, the group without frequency hopping includes four simulation cases:
　　　　a) interference dominated (legacy);
　　　　b) interference dominated (long scrambling sequence);
　　　　c) noise dominated (legacy); and
　　　　d) noise dominated (long scrambling sequence).
[0053]
Additionally, the group with frequency hopping also includes four simulation cases:
　　　　e) interference dominated (legacy);
　　　　f) interference dominated (long scrambling sequence);
　　　　g) noise dominated (legacy); and
　　　　h) noise dominated (long scrambling sequence).
[0054]
The performance of PDSCH transmissions with repetitions longer than ten sub-frames have been evaluated in link level simulation in the presence of inter-cell interference. In this case, the inter-cell interference is modelled in such a way that two communication devices 3 have transmissions at the same time in the same frequency bandwidth but each communication device 3 (i.e. with a different C-RNTI) is attached to a different cell 7 (i.e. with different Cell ID). One of the communication devices 3 is taken to be the desired signal and the other communication device 3 is assumed to be the interfering signal. Further details of the applied link level simulation assumptions are captured in Table 1 below.
[0055]
When no frequency hopping is applied, in the noise dominated scenarios (i.e. cases c and d in Figure 5), the difference in terms of an associated block error rate (BLER) between the performance of the legacy scrambling and the performance of the novel long scrambling is relatively small. However, in the interference dominated scenarios (cases a and b), the difference between the legacy scrambling sequence and the novel long scrambling sequence in terms of BLER performance is relatively large.
[0056]
When frequency hopping is applied, in noise dominated scenarios (cases g and h), the difference in terms of BLER between the performance of the legacy scrambling and the performance of the novel long scrambling is relatively small. However, in the interference dominated scenarios (cases e and f), the difference between the legacy scrambling sequence and the novel long scrambling sequence in terms of BLER performance is relatively large.
[0057]
Accordingly, there is a clear benefit that can be attributed to the above described extended scrambling sequence for long repetitions / coverage enhancement (at least in some cases). In other words, in the interference dominated case, the extended scrambling sequence has a significantly better performance for long repetitions (over multiple radio frames) than the legacy scrambling sequence regardless whether frequency hopping is applied or not (whilst the performance in noise dominated cases is also not adversely effected).
[0058]

Figure 6 is an exemplary flowchart showing the initiation of a scrambling sequence employed by the base stations 5 and of the communication devices 3 shown in Figure 1. Specifically, Figure 6 illustrates the procedure performed by the respective scrambling modules 45 and 65 of a communication device 3 and its serving base station 5 communicating with each other. This procedure is performed for every sub-frame 15 and for every cell 7 in which the communication device 3 and its serving base station 5 are communicating data for each other.
[0059]
Optionally, the procedure may start by performing a check (in step S01) whether or not the communication device 3 requires applying a legacy scrambling sequence (e.g. due to compatibility with only the Rel-8 scrambling technique). It will be appreciated that this check may be performed, for example, when the communication device 3 first connects to the base station 5, as part of an initial registration process (which typically involves exchanging compatibility information). Accordingly, in this case it is not necessary to perform/repeat this check during the procedure shown in Figure 6.
[0060]
If the communication device 3 and its serving base station 5 can establish that a legacy scrambling is not required (S01: ‘NO’), then they proceed to step S02.
[0061]
In step S02, the scrambling module 45/65 is configured to determine the parameter n' RNTI using the formula n' RNTI=(n RNTI+SFN) mod 2 16 (where n RNTI is the temporary identifier assigned to the communication device 3 by the serving base station 5; and SFN is a radio frame number for the current radio frame 13).
[0062]
Next, in step S03, the scrambling module 45/65 is configured to determine the scrambling code (c init) for the current sub-frame 15, using the following formula:
[Math. 8]

[0063]
Finally, as generally shown in step S04, the scrambling module 45/65 is configured to apply the scrambling code c init for its communications (with the corresponding base station/communication device) in the current sub-frame 15.
[0064]
Specifically, in cell 7-1 shown in Figure 1, the scrambling module 65 of the base station 5-1 uses, in the current sub-frame 15, the scrambling code c init (derived in step S04) for scrambling its downlink communications to the first communication device 3-1 (and uses this scrambling code c init for de-scrambling any uplink communications from the first communication device 3-1 in the current sub-frame 15).
[0065]
Similarly, the scrambling module 45 of the first communication device 3-1 applies, in the same sub-frame 15, the same scrambling code c init for its communications with the base station 5-1 (scrambling uplink transmissions and de-scrambling downlink transmissions).
[0066]
At the end of the current sub-frame, the procedure ends (but starts again for the next sub-frame, using the parameters specific to that sub-frame).
[0067]
Since each cell has a respective different associated cell identifier and each base station assigns different temporary identifiers to different communication devices, the procedure shown in Figure 6 will always yield a different c init value for each different combination of cell and communication device. Moreover, since the formula for deriving the value of c init includes the parameter n' RNTI, the procedure also yields a different c init value for different radio frames (for the same combination of cell and communication device). Beneficially, the risk of interfering transmissions from a neighbour cell combining coherently over multiple radio frames (when such transmissions are repeated for coverage enhancement) can be reduced.
[0068]

Detailed exemplary embodiments have been described above. As those skilled in the art will appreciate, a number of modifications and alternatives can be made to the above exemplary embodiments whilst still benefiting from the inventions embodied therein. By way of illustration only a number of these alternatives and modifications will now be described.
[0069]
It will be appreciated that n' RNTI may also be derived (in step S02) using the formula:
n' RNTI=(n RNTI+k) mod 2 16
where k is a sub-frame counter incremented with every repetition (i.e. with every re-transmission of the same data). It will be appreciated that the sub-frame counter k may be reset to an appropriate start value (e.g. the value ‘0’ or ‘1’) on each new transmission. Alternatively, the sub-frame counter k may be configured to increase even when initially transmitting new data. In this case, the sub-frame counter k may be configured to ‘wrap around’ (or reset to ‘0’) once it has reached its maximum value (e.g. determined by the number of bits used for the sub-frame counter k).
[0070]
Alternatively, the communication device and its serving base station may be configured to derive, for each sub-frame, a scrambling sequence using the following modified formula:
[Math. 9]

where c init is an initial scrambling code for a sub-frame; n' RNTI is a parameter derived from a temporary identifier associated with a communication device connected to a cell; q is a codeword index; n s is a slot number;
[Math. 10]

is a unique identifier of that particular cell (a physical layer identity); and
[Math. 11]

is a flooring function (i.e. the largest integer not greater than ‘x’).
[0071]
Advantageously, bit #30 of this modified formula is set to 1 (by including the value 2 30), so that even if the n' RNTI for a particular communication device in a cell (employing coverage enhancement) happens to match the n RNTI for another communication device in that cell (e.g. a legacy communication device employing a legacy scrambling sequence and not configured to derive an associated n' RNTI), these communication devices will still apply different scrambling sequences.

Claims
[Claim 1]
Communication apparatus for a cellular communication system comprising a plurality of cells, the communication apparatus comprising:
means for communicating signals in a cell in a sequence of radio frames; and
means for scrambling said signals communicated in said cell using a scrambling code;
wherein said scrambling code is derived based on a parameter that changes from a given block of sub-frames to a subsequent block of sub-frames.
[Claim 2]
The communication apparatus according to claim 1, wherein said parameter is derived based on at least one of: a system frame number associated with at least one of said sequence of sub-frames; and a counter.
[Claim 3]
The communication apparatus according to claim 1 or 2, wherein said parameter is derived using the following formula:
n' RNTI = (n RNTI+SFN) mod 2 16
where n' RNTI is said parameter; n RNTI is a temporary identifier associated with a mobile device connected to said cell and for which said scrambling code is applicable; and SFN is a system frame number associated with said at least one of said sequence of sub-frames.
[Claim 4]
The communication apparatus according to claim 1 or 2, wherein said parameter is derived using the following formula:
n' RNTI = (n RNTI+k) mod 2 16
where n' RNTI is said parameter; n RNTI is a temporary identifier associated with a mobile device connected to said cell and for which said scrambling code is applicable; and k is a sub-frame counter.
[Claim 5]
The communication apparatus according to claim 4, wherein said parameter is derived by incrementing said sub-frame counter upon each repetition of a particular signal.
[Claim 6]
The communication apparatus according to claim 4 or 5, wherein said sub-frame counter is reset to a default value (e.g. ‘0’) when at least one of the following occurs: said particular signal is being communicated for the first time; and said sub-frame counter reaches an associated maximum value.
[Claim 7]
The communication apparatus according to any of claims 1 to 6, wherein said scrambling code is derived using at least one of the following formulas:
[Math. 1]

and
[Math. 2]

where c init is the scrambling code; n' RNTI is the parameter; q is a codeword index; n s is a slot number associated with said sequence of sub-frames;
[Math. 3]

is a unique identifier for said cell; and
[Math. 4]

is a flooring function (i.e. the largest integer not greater than ‘x’).
[Claim 8]
The communication apparatus according to any of claims 1 to 7, wherein said scrambling means is configured to: scramble said signals using said scrambling code upon transmission by said communicating means; and/or de-scramble said signals using said scrambling code upon reception by said communicating means.
[Claim 9]
The communication apparatus according to any of claims 1 to 8, wherein each block of sub-frames comprises ten sub-frames.
[Claim 10]
The communication apparatus according to any of claims 1 to 9, comprising at least one of: a base station; a mobile (cellular) telephone; and a machine-type communication (MTC) device.
[Claim 11]
A system comprising the communication apparatus according to any of claims 1 to 10, the communication apparatus forming part of at least one of a base station and a mobile communication device.
[Claim 12]
A method performed by communication apparatus configured to communicate signals, in a sequence of radio frames, in a cell of cellular communication system comprising a plurality of cells, the method comprising:
deriving a scrambling code based on a parameter that changes from a given block of sub-frames to a subsequent block of sub-frames; and
scrambling signals communicated in a cell using a scrambling code.
[Claim 13]
A computer program product comprising instructions for causing a computer-programmable device to perform the method according to claim 12.