This specification relates to a technique for identifying a symbol number, or symbol index, in a stream of data.

In data transmission systems, data may be conveyed in a series of frames, or packet or bursts. The frames include other fields, containing control information and so on. In Orthogonal Frequency Division Multiplexing (OFDM) systems, the transmission may be composed of multiple physical layer frames that include multiple OFDM symbols in which data content is preceded by a preamble. The preamble includes a symbol number field, which indicates the number of the first symbol in that frame conveying the desired data content.

Figure 1 depicts a data stream 1 including frames 2, 2'. The frames 2, 2' in this example may be configured in accordance with a digital video broadcasting (DVB) standard and the data stream 1 in this example is a broadcast stream. Each frame 2, 2' includes a preamble 3 and a data portion 4. The data portion 4 includes data symbols, which may correspond to a number of services. Each of the data symbols includes pilot cells. The pilot cells of the data symbols in the data portion 4 may be scrambled using a pseudo-random binary sequence. A pseudo-noise sequence is then used to scramble the pilot data at frame level, that is, all pilot cells of an OFDM symbol are multiplied by 1 or—1 depending on the frame level pseudo- noise sequence. Thus, there are two possible scrambling sequences for the OFDM symbols.

The preamble 3 of the frame 2 consists of signaling channels PI and P2, which carry layer one (LI) signaling. The symbols in the preamble 3 provide information such as the locations of the different services inside a data portion 4 of the frame 2. Frames 2, 2' may be configured differently from one another, requiring the preamble in each frame 3, 3' to be checked. Alternatively, a static frame

configuration may be employed for some or all of the frames 2, 2' in the broadcast stream, in order to reduce the amount of checking required. In order to reduce power consumption at a receiver, services may be transmitted in time slices or short bursts within a Time Division Multiple Access (TDMA) signal. This may allow a receiver to operate at a reduced level when bursts associated with services that are not required would otherwise be received. Depending on the configuration of the frames 2, 2' and broadcast stream 1, the receiver may remain in a sleep mode over several frames. Such power consumption considerations may be particularly important where the receiver is a battery-operated device, such as a handheld or mobile terminal. In order to receive a service, the receiver has to determine when to leave sleep mode and prepare for the next burst associated with that service. In the preambles 3, 3', the LI signaling indicates the symbol number, or index, of the first symbol in a burst, however the symbols themselves do not convey their own symbol number. Consequently, the receiver must expend power to accurately monitor time while it is in sleep mode so that it "wakes up" at the correct symbol.

A user may wish to change the service being received to another service carried by the broadcast stream 1, which may require the receiver to switch to receiving a signal at a new frequency. In order to obtain frame synchronization, the receiver must wait for the reception of a new preamble, potentially prolonging the time required for the changeover.

According to a first aspect, an apparatus comprises a multiplexer configured to multiplex data into a data stream as a plurality of data symbols, wherein each of the plurality of data symbols includes at least one pilot signal, a sequence generator configured to generate a reference sequence based, at least in part, on a symbol number of the data symbol in which the at least one pilot signal is to be included and a modulator configured to modulate the at least one pilot signal according to the reference sequence prior to multiplexing.

The data symbols may be orthogonal frequency division multiplexed symbols. The reference sequence may be generated as a pseudo-random binary sequence. Such a pseudo-random binary sequence may have an initial state based on a predefined value and the symbol number. The multiplexer may be configured to distribute the data symbols in the data stream in a series of bursts.

The at least one pilot signal may include one or more of a scattered pilot signal, an edge pilot signal and a closing pilot signal.

This aspect also provides a method including multiplexing data into a data stream, wherein the data stream includes a plurality of data symbols and each of the plurality of data symbols includes at least one pilot signal, the at least one pilot signal is modulated according to a reference sequence prior to said multiplexing and the reference sequence is based, at least in part, on a symbol number of the data symbol in which the at least one pilot signal is included.

This aspect also provides a computer program product comprising a computer readable medium on which is stored instructions that, when executed by a processing arrangement, cause such a method to be performed.

According to a second aspect, embodiments of the present invention may provide an apparatus comprising a receiver configured to receive a data stream, the data stream including a plurality of data symbols and each of the plurality of data symbols including at least one pilot signal, and a processing arrangement configured to determine a reference sequence corresponding to an expected scrambling sequence for at least one pilot signal in a data symbol having a predetermined symbol number and to identify a received data symbol corresponding to the predetermined symbol number based, at least in part, on the determined reference sequence.

The processing arrangement may be configured to determine a plurality of instances of time at which a series of bursts of selected ones of the data symbols in the data stream are due to be received, to enable reception of the data stream based on one of the determined instances of time that corresponds to one of the series of bursts and to disable reception of the data stream after the reception of said one burst until a determined instance of time corresponding to a second one of the series of bursts.

The data symbols may be orthogonal frequency division multiplexed symbols.

The at least one pilot signal may include one or more of a scattered pilot signal, an edge pilot signal and a frame closing pilot signal.

The second aspect also provides a method including receiving a multiplexed data stream, the data stream including a plurality of data symbols and each of the plurality of data symbols including at least one pilot signal, determining a reference sequence corresponding to an expected scrambling sequence for at least one pilot signal in a data symbol having a predetermined symbol number and identifying a received data symbol corresponding to the predetermined symbol number based, at least in part, on the determined reference sequence. This aspect also provides a computer program product comprising a computer readable medium on which is stored instructions that, when executed by a processing arrangement, cause such a method to be performed.

Embodiments of the invention may also provide a system including an apparatus according to the first aspect and an apparatus according to the second aspect and/ or a method of operating such a system including a method according to the first aspect and a method according to the second aspect.

In some embodiments of the first and second aspects, the data stream may be a multicast or broadcast stream. For example, the data stream may be a digital video broadcast stream, such as a Digital Video Broadcasting Second Generation

Terrestrial (DVB-T2) broadcast stream or similar. For example, the Digital Video Broadcasting New Generation Handheld (DVB-NGH) standard is currently being defined with features similar to DVB-T2 which would be compatible with the embodiments of the present invention.

The embodiments described in general terms above allow the scrambling of one or more pilot signals with codes based on symbol number. In this manner, the scrambling codes may be used by a device receiving the data stream to identify, and synchronize with, the symbol index from the pilot signals.

The use of the scrambling codes to convey symbol number allows a receiving device to synchronize its symbol index with incoming data and correctly identify the data symbols associated to be received and decoded, for instance, to provide a user with a required service. Since multiple pilot signals may be included in the data symbols, the symbol number is conveyed multiple times in the data stream. Therefore, the receiving device is not required to rely on information in a frame preamble for synchronization purposes and the time required to change between channels or services may be reduced.

A receiving device may be configured to receive bursts of selected data symbols and to conserve power by entering a sleep mode between such bursts. The

abovementioned embodiments permit a receiving device to determine a current symbol number when reawakening from sleep mode. Therefore, the prior need for accurate monitoring of time while in sleep mode may be avoided. This may reduce the power consumption of a receiving device when in sleep mode. Examples of embodiments of the invention will now be described, with reference to the accompanying drawings, of which:

Figure 1 depicts an exemplary series of DVB physical layer frames;

Figure 2 depicts an example of a communication system in which

embodiments of the present invention may be implemented;

Figure 3 is a block diagram of a stream generator apparatus according to an exemplary embodiment of the present invention;

Figure 4 is a block diagram of a receiving apparatus according to an exemplary embodiment of the present invention;

Figure 5 depicts data symbols of a frame in an OFDM system, in terms of the types of data conveyed by multiple subcarriers;

Figure 6 depicts a general structure of an exemplary sequence generator apparatus that may be implemented in the stream generator apparatus of Figure 3 and/ or in the receiving apparatus of Figure 4;

Figure 7 depicts the general structure of an exemplary sequence generator apparatus that may be implemented in the stream generator apparatus of Figure 3 and/ or the receiving apparatus of Figure 4 in another embodiment of the invention;

Figure 8 depicts an example of a sequence generator apparatus having the structure of Figure 7, configured to generate a 13 ^th order sequence;

Figure 9 is an exemplary flowchart of a method of sequence generation using the sequence generator apparatus of Figure 7;

Figure 10 depicts an exemplary sequence generator apparatus having the structure of Figure 7, configured to generate a 12 ^th order sequence;

Figure 11 is an exemplary flowchart of a method of receiving data performed by the receiving apparatus of Figure 4;

Figure 12 depicts an example of two scrambled OFDM symbols; and

Figure 13 is an exemplary flowchart of an alternative method of sequence generation using the sequence generator apparatus of Figure 7. Figure 2 depicts an example of a communication system 6 in which embodiments of the invention may be implemented. In the example system 6, a stream generator apparatus 7 may receive data 8, such as multimedia content, a file or other service, from one or more content sources 9 and generate a data stream 10 including the data for transmission to one or more receiving apparatuses 11a, l ib, 11c over a network 12. The data stream 10 may include multiplexed data 8 relating to a plurality of services or content items. In this particular example, the network 12 is a DVB-T2 (Digital Video Broadcasting Second Generational Terrestrial standard) network, as defined in European Standards Telecommunications Institute (ETSI) EN 302 755, and the data stream 10 is a broadcast stream transmitted to the receiving apparatuses 11a, l ib, 11c wirelessly by a transmitter 13. However, embodiments of the invention may be implemented in wired or wireless networks corresponding to other standards for unicast, multicast or broadcast transmissions. As shown in Figure 3, the exemplary stream generator apparatus 7 includes an input

14 capable of receiving the data 8 from the content source 9 and at least one output

15 through which the data stream 10 may be forwarded to one or more transmitters 13 via a network 16. The network 16 may, for example, be a local network or an external network such as the Internet. A processor 17 controls processing of the data 8 and the generation and onward transmission of the data stream 10. A multiplexer 18 is provided, which generates the data stream 10. A memory 19 is provided for storing software executed by the processor 17, together with a random access memory 20 for use in processing. The incoming data 8 may be stored in a cache 21 or, if required, in an external buffer, not shown. The various components of the stream generator apparatus 7 may be connected to a data bus 22 for communication there between. While the example shown in Figure 3 includes a processing arrangement that includes only one processor 17, in other embodiments, the stream generator apparatus 7 may include a processing arrangement having more than one processor.

An example of a receiving apparatus 11a is shown in Figure 4. The receiving apparatus 11a includes an antenna 23 and a receiver 24, operable to receive the data stream 10 from the network 12. The receiver 24 may be operable to process and extract the data 8 relating to a particular content item or service, under the control of a processor 25. Read-only and random access memory facilities 26, 27 are provided for storing software executed by the processor 25 and for use during processing respectively. While only one processor 25 is shown in Figure 4, in other embodiments, the receiving apparatus 11a may instead be provided with a multi processor arrangement.

A user interface is provided. In this example, the user interface includes a keypad 28, through which a user may input commands, along with a display 29 and a speaker 30 that may present information and received multimedia content under the control of the processor 25.

In this particular example, the receiving apparatus 11a is powered by a battery 31. Charging circuitry, not shown, may be included to facilitate recharging of the battery 31.

The receiving apparatus 11a may include features relating to other functionality. For example, the receiving apparatus 11a may be a mobile telephone handset, a smartphone or similar device, in which case other conventional features of mobile telephone handsets, such as a microphone, a second antenna and a transceiver configured to transmit and receive voice data over a telephone network may be included. The receiving device 11a may be a mobile terminal including other features depending on its required functionality. Such other features are omitted from Figure 3 for the sake of clarity.

The data stream 10 conveys a series of frames having a similar format to the series 1 of frames 2, 2' of the example shown in Figure 1. In this particular embodiment, the frames 2, 2' convey one or more services using respective pluralities of OFDM symbols, arranged into a preamble 3, 3', which includes control information, and a data portion 4, 4', which includes content.

In addition to data, the symbols include a plurality of pilot signals, including continuous pilot signals, edge pilot signals, scattered pilot signals and frame closing pilot signals. The distribution of the scattered pilot signals between the carriers 0 to k-1 varies from symbol to symbol. This is shown in Figure 5, which depicts a frame with ) OFDM symbols, numbered 0 to j-1, in terms of the type of signal carried in a plurality of k subcarriers, numbered 0 to k-1. The circles with horizontal shading indicate continuous pilot signals, the filled circles indicate scattered pilot signals and the open circles indicate symbol data. The circles with vertical shading,

corresponding to subcarriers 0 and k-1, denote edge pilot signals that are included in each symbol. The circles with diagonal shading in the final symbol j-1 of the frame denote the frame closing pilots, which are transmitted on all subcarriers that have been used to transmit scattered pilots in the frame.

The OFDM symbols are scrambled before transmission, using a sequence that is generated based, at least in part, on their symbol number. In this manner, the receiver 25 in a receiving apparatus 11a, l ib, 11c can identify a symbol having a particular symbol number from the sequence used to scramble one of the pilot signals in that symbol. For instance, the receiving apparatus 11a, l ib, 11c may generate an expected scrambling sequence for the first data symbol that is to be received, such as the first data symbol in, or preceding, a burst associated with a required service and compare the expected sequence with a scrambling sequence obtained from a pilot signal in a received symbol, in order to determine whether the received symbol is the first data symbol.

In this manner, a receiver 25 that begins to receive symbols in a frame 2 without having received the preamble 3 can synchronize its symbol index with the current symbol number, without having to wait for the preamble 3' of the next frame 2' to be received. This facility is particularly useful where the receiver 25 first begins to receive a particular service, for example, when changing between channels of streamed content, as it may reduce the time required to tune to a particular channel and/or to change between channels.

The capability to identify a current symbol number is also particularly advantageous where the processor 26 of the receiving apparatus 11a is arranged to deactivate or disable modules or functional parts of the receiver 25, or operate the receiver in a power saving mode, when data relating to a service required by a user is not scheduled to be received. For example, the processor 26 may be configured to activate the receiver 25 at a particular time to prepare to receive scheduled data in the data stream 10. Alternatively, or additionally, data symbols for various services may be arranged in the data stream 10 in respective series of time slices or bursts of data symbols. In this case, the processor 26 may be configured to put the receiver 25 into a sleep mode, in order to conserve power, and to reactivate the receiver 25 to receive only the bursts corresponding to the required service. In both cases, the processor 26 can identify a symbol having a particular symbol number from the scrambled pilot signals and, consequently, can correctly identify the data symbols to be processed by the receiver 25.

Figure 6 depicts an exemplary sequence generator 32 that may be used to generate scrambling codes based, at least in part, on the symbol number, for use in the multiplexer 18 of the stream generator apparatus 7 and/or the receiver 25 of a receiving apparatus 11 a, l ib, 11 c. In this particular example, the sequence generator 32 is a conventional linear feedback shift register (LFSR). A sequence generated by the LFSR may be divided up to provide unique sequences for each of the data symbols.

While the sequence generator 32 of Figure 6 can provide the required unique sequences, a more sophisticated technique would allow the generation of a scrambling code for a data symbol in the middle of a frame 2, 2' without requiring the generation of codes for the preceding data symbols. For example, Figure 7 depicts the general structure of an alternative exemplary sequence generator 33 that may be used in the stream generator apparatus 7 and/or the receiving apparatus 11 a, l ib, 11 c. The sequence generator 33 comprises parallel LFSRs 34, 35, having m- and n- bits respectively, connected to a logic network 36. The first LFSR 34, referred to as an initialization register, is arranged to receive a reset signal, while the second LFSR 35, referred to as the symbol index register, can be loaded with an initial symbol number j, allowing the generation of a sequence beginning with a symbol in the middle of a frame 2, 2'. The LFSRs 34, 35 produce m-bit and n-bit outputs respectively. The logic network 36 is used to generate a scrambling sequence r- _k based on those outputs and to provide respective feedback signals fl , f2 to the LFSRs 34, 35. In some embodiments, the logic network may consist of a small number of Exclusive OR (EOR) gates.

Figure 8 depicts a particular example of the sequence generator apparatus 33, configured as a Gold-based 13 ^th order sequence generator. Gold sequences, which are known, have suitable correlation properties for this application and may be generated using the two LFSRs 34, 35. In particular, the Gold sequences have low cross-correlation properties, which can allow different sequences to be distinguished from one another when compared by the receiving apparatus 11 a, l ib, 11 c.

The order of the symbol index register 35 is determined by the maximum number of symbols in a physical layer frame. In an example DVB-T2 system, the maximum frame duration may be limited to 250 ms, which for the highest channel bandwidth of 8 MHz means that the number of symbols per frame is limited to2098 symbols. However, subsequent standards, such as Next Generation Handheld Digital Video Broadcast (DVB-NGH), may support bandwidths up to 20 MHz, which may, in turn, increase the maximum number of symbols above 5000. A 13 ^th -order symbol index register can provide 2 ¹³ different stages, which is sufficient for generating sequences for such bandwidths. In order to generate a Gold sequence, the initialization register 34 and the symbol index register 35 have the same number of bits. Moreover, the polynomials describing the LFSR connections need to be a 'preferred pair' so that the combination of the two sequences yields a Gold sequence. In this example, both m and n equal 13, the initialization register 34 implements the polynomial D ¹³ + D ¹⁰⁺ D ⁹⁺ D ⁷ + D ⁵ + D ⁴ +l and the symbol register 35 implements the polynomial D ¹³ + D ⁴ + D ³ + D + l .

In this particular example, the initialization sequence is 'all ones' so that each bit in the initialization register 34 is set to 1 when an initial symbol index j is loaded into the symbol index register 35. Such a combination of this initialization sequence with the inverter at the output may cause each scrambling code to begin with a standard sequence of bits where the symbol number j appears as such. This may be beneficial under steady-state operation in good channel conditions, and as well for instances where multiple sequences are stored and handled in the memory 19, 20, 26, 27 of an apparatus 7, 11a, l ib, 11c.

The generated sequence is used to modulate scattered pilot signals and, optionally, edge pilot signals and/or frame closing pilot signals in OFDM data symbols. Edge pilot signals and frame closing pilot signals in a data symbol may be identified, based on the number of subcarriers for each data symbol and the number of data symbols in a frame 2, 2'. The locations of scattered pilot signals will vary between data symbols but may be predefined. In this particular example, in which the frame 2, 2' is a DVB-T2 frame, the scattered pilot signals ^ ^sc in a data symbol with symbol number j are conveyed by subcarriers k belonging to the set:

P ^c = {kmoA(D _x D _y ) = D _x (jmoAD _y )} (1) where D _x is the separation of pilot bearing carriers, D _y is the number of symbols forming one scattered pilot sequence, that is, the number of symbols after which the scattered pilot pattern is repeated, and j is the OFDM symbol index. Both D _x and D _y are defined in Table 1 and they specify the different scattered pilot patterns.

Table 1

In DVB-T2, the modulation of the scattered pilots carriers, ^with k belonging to the set defined by Equation (1), is given by translating the scrambling sequence to a real valued antipodal sequence:

R _Q {x _k \= 2A _SP (l/ 2 - r _{j t} )

Im{x }= 0 ⁽²⁾ where A _SP is the amplitude for the scattered pilots.

An example method of generating scrambling sequences for data symbols within a frame 2, 2' will now be described, with reference to the flowchart of Figure 9.

Beginning at step 9.0, a reset signal is fed to the initialization register 34 so that the initialization register 34 is set to a predefined initial value and a carrier index k, identifying a subcarrier for conveying part of the data symbol, is set to an initial value (step s9.1). A first n-bit symbol number j is loaded into the symbol index register 35 (step s9.2) and an initial carrier index k is set (step s9.3). For example, if the sequence generation is beginning with a first symbol in a frame, the symbol number j and the carrier index k may be set to zero.

In step s9.4, the LFSRs 34, 35 are shifted and a sequence of scrambling code bits r- _k , for scrambling a subcarrier k when carrying the data symbol with symbol number j, is generated.

In step s9.5, the sequence r _{j k} is output by being read and/or stored. Where the method is performed by the stream generator apparatus 7, the sequence r _{j k} is used to modulate scattered pilot signals and, optionally, the frame closing pilot signals and/ or edge pilot signals, in the relevant data symbol. Where the method is performed by a receiving apparatus 11 a, l ib, 11 c, the bits r _{j k} are stored and compared with scrambling sequences of pilot signals in received data symbols in order to identify a data symbol having a particular symbol number.

In certain other embodiments of the invention, step s9.4 and s9.5 may be performed in reverse order, so that sequence generated for a previous symbol j-1 and/or subcarrier k-1 are output (step s9.5) immediately before a new scrambling sequence r _{j k} is generated (step 9.6).

If there are further carriers to be scrambled for that data symbol (step s9.6), the carrier index k is updated (step s9.7), for instance, by increasing the index by one, so that k=k+l and scrambling code bits r- _{jk+ 1} for the next subcarrier are obtained and output (steps s9.4, s9.5). The sequence of steps s9.4 to s9.7 is repeated until it is determined that scrambling bits for the last subcarrier of the data symbol have been output (step s9.6) .

It is then determined whether scrambling codes for further data symbols are to be generated (step s9.8). For instance, in the apparatus 7, step s9.8 may determine whether the current symbol j is the last symbol of the frame 2. If so, the symbol index is updated, for example, by increasing the symbol index j to j + 1 (step s9.9). The initialization register 34 is reset (step s9.1), the updated symbol index j + 1 is loaded into the symbol index register 35 (step s9.2), the carrier index is reset (step s9.3) and steps s9.4 to s9.8 are repeated for the new data symbol.

If it is determined that there no further sequences are required (step s9.9), the procedure ends (step s9.10).

In the specific example shown in Figure 8, the sequence generator 33, shown in Figure 8, included two 13-bit LFSRs 34, 35, which may potentially accommodate bandwidths of up to 20 MHz and maximum symbol numbers up to 8192. If the maximum number of symbols for a system is less than this limit, a lower order sequence generator may be used in place of the 13 ^th order sequence generator 33 of Figure 8. Figure 10 depicts an example of 12 ^th -order sequence generator 37. In this particular example, the sequence generator 37 includes an initialization register 38 and an index symbol register 39, with both m and n equal to 12, and a logic network 40. Gold sequences are not available for such a 12 ^th -order arrangement. However, alternative sequences with suitable cross-correlation properties are available. The sequence generator 37 of Figure 10 implements the polynomial D ¹² +D ⁶ +D ⁴ +D + l in its initialization register 38 and the polynomial D ¹² +D ¹⁰ +D ⁶ +D ⁴ +D ³ +D + l in its symbol index register 39.

In the examples described above, the scrambling sequences are used to modulate the scattered pilots in the data symbols. In an OFDM system, such as DVB-T2, one scrambling code, inverted or not, may be used to modulate all pilot signals.

However, the continuous pilots may be used for initial synchronization and tracking during steady-state operation, before a receiving device obtains the scrambling codes. In embodiments of the present invention, the scrambling codes vary between data symbols. Therefore, some or all of the continuous pilot signals may be transmitted with a scrambling code that is not dependent on symbol number and is known to the receiving apparatus 11a, l ib, 11c, to allow the receiving apparatus 11a, l ib, 11c to use them without requiring knowledge of the scrambling codes.

Such considerations are less relevant for the edge pilots, scattered pilots and closing pilot signals, since they are used for functions such as channel estimation and so do not need to be decoded until the first data symbol of a burst is received.

Consequently, some or all of the edge, frame closing and scattered pilot signals may be scrambled.

As mentioned above, the use of scrambling codes that are dependent on symbol index may be used to reduce power consumption in the receiving devices 11a, l ib, 11c by reducing the need for accurate timekeeping in sleep mode. An example of a method of operating a receiving device 11a to receive a series of bursts of data symbols will now be described, with reference to Figure 11.

Starting at step si 1.1, the processor 26 may send a command to wake the receiver 25 from sleep mode, based on an announcement received in LI signaling and an internal clock signal. The internal clock may monitor time in a relatively coarse manner, since frame synchronization will be achieved through the scrambling codes used to scramble the data symbols. General time and frequency synchronization may be performed (step si 1.2) using a method such as guard interval correlation or similar.

Next, the receiver 25 generates an expected scrambling sequence for a first data symbol that is to be received, based on the corresponding symbol number (step si 1.3), for example by using the method of Figure 9. In some embodiments of the invention, scrambling sequences for one or more adjacent symbols may also be generated.

A data symbol is then received (step si 1.4) and signals from selected subcarriers are extracted in order to obtain a sequence that may, potentially, be a scrambling sequence used by the stream generator apparatus 7. In this example, the subcarriers are selected based on the set of subcarriers that are expected to carry scattered pilot signals in the first data symbol and are identified using Equation 1 above.

The obtained sequence is compared with the expected scrambling sequence generated in step sl l .3 (step si 1.5). An example method for correlating the obtained sequence and the expected scrambling sequence will be described below.

If the correlation between those sequences is acceptable, that is, if a correlation value based on the comparison of the two sequences exceeds a predefined threshold (step sl l .6), it is presumed that the received symbol is the desired first data symbol and the data symbol is decoded (step sl l .7). If the correlation value does not exceed the threshold (step sl l .6), one or more subsequent data symbols are received, sequences are obtained from them (step si 1.4) and the correlation is repeated (step si 1.5, sl l.6) until a data symbol having an obtained sequence that matches an expected scrambling sequence generated in step si 1.3 is identified.

In embodiments where multiple sequences are generated at step si 1.3, multiple correlations may be performed in step si 1.5. If the correlation indicates that the symbol received in step si 1.4 is the first data symbol or a subsequent data symbol in that burst (step sl l .6), step sl l.7 is performed. If, instead, it is determined at step sl l .6 that the symbol received in step si 1.4 precedes the first data symbol, step sl l .7 may be omitted. In such a case, the processor 26 may, optionally, use the determined symbol number to identify the first data symbol, omitting steps si 1.4 to sl l .6 for any remaining data symbols that precede the first data symbol. If there are further symbols remaining in the data burst (step si 1.8), the next data symbol is received (step si 1.9), the expected scrambling sequence for that data symbol is generated (step sl l.10) and used to decode the data (step sl l .7). In some embodiments of the invention, the order of steps sl l .9 and sl l .10 may be reversed. Steps sl l .7 to sl l .10 are repeated until the last data symbol in the burst has been received (step si 1.8) and the receiver 25 re-enters sleep mode (step si 1.11). A wake-up time for receiving the next burst is announced in LI signaling and the receiver 25 is woken from sleep mode (step si 1.1) according to the wake-up time and an internal clock. Steps sl l .2 to sl l .l l are then repeated for the next desired data burst.

Although the description of Figure 11 referred to a first data symbol, the first data symbol might not be the first data symbol in the burst that is to be received. It is likely that the signals received by the receiving device 11a will be affected by multipath propagation and other effects. For this reason, channel estimation and equalization may be performed before beginning the sequence correlation procedure of steps si 1.3 to si 1.6, in order ensure that the scrambling sequence detection process is robust. In order to achieve this, at least one symbol before the first data symbol of the burst should be received, so that a channel estimate may be obtained. The channel estimation may be based on reference information transmitted in the scattered pilots of a received data symbol. Although the locations of the scattered pilot signals change from symbol to symbol and will be unknown initially after wake-up (step sl l .l), the receiver may deduce potential scattered pilot locations from the index of the first data symbol and use those locations to obtain channel estimates from other data symbols.

An example of an algorithm that may be employed at the receiving apparatus 11a, l ib, 11 c to recognize a scrambling sequence (steps si 1.4, si 1.5 & si 1.6) will now be described. For simplicity, the example is given for a system where the channel estimate can be obtained from one data symbol. This is true for systems with short guard interval and mild frequency selectivity. However, the algorithm may be modified straightforwardly to allow for channel estimation based on more than one symbol.

In this example, the first data symbol of a burst has symbol number a and a channel estimate is calculated from symbol a - 1, based on the appropriate scattered pilot locations and scrambling sequence r _k For the scattered pilot symbols, or cells, the channel estimate can be calculated using the following equation: where y and x are the received and transmitted symbols, respectively. An estimate for the rest of the subcarriers is obtained using interpolation. The data symbol of index a is then equalized using the following channel estimate: sc

^X a,k - ~ (4)

H a-l,k

Once the receiver 25 has determined the scrambling sequence of the received data symbol j (in step si 1.4), it calculates the correlation to the expected scrambling sequence for symbol a (in step sl l .5). The normalized cross-correlation function of the scrambling sequence of received symbol j to the expected scrambling sequence for symbol a is given by the following equation: t ^X j,k-l ^X a,k (5)

' SP ^!± SP keP„ where x is the estimate of the transmitted symbol after equalization, P _a is the set of scattered pilots for symbol a, and N _SP is the number of scattered pilots in that symbol. In step si 1.6, the result of the cross-correlation is then compared with a predefined threshold to determine whether the expected scrambling sequence and obtained sequence match one another. The threshold may be based on the zero-lag (1=0) correlation, where the expected and obtained sequences match exactly and Rj (1) is equal to unity. For example, the threshold may be set to a predefined percentage of the maximum correlation, such as Rj(l)=0.7.

An example is now given for the case of an ideal channel, for which there is no need for channel estimation and no noise. Figure 12 shows a plurality of subcarriers, numbered p to p + 12, of two symbols 41, 42. The scattered pilot cells are indicated using dotted lines, while the data cells are shown using solid lines. Τη· continuous pilots, edge pilots and frame closing pilots are omitted from this example for the sake of simplicity.

In this particular example, the power levels of the pilot cells are boosted relative to the data cells to assist with channel estimation. Because of the boosting, the amplitude A _SP for the scattered pilots is 4/3. The data symbols have been taken from a QPSK constellation, but normalizing the power by the square root of 2 has been omitted for simplicity. Let's assume that symbol 42 is the first symbol a of desired burst and the receiver 25 wakes up from sleep mode to receive the preceding symbol 41 (a-1). The receiving device 11a generates an expected scrambling sequence for symbol 42 and assumes a pattern for the scattered pilots that data symbol. As shown in Figure 12, scattered pilots in data symbol 42 are carried by subcarriers p, p + 6 and p + 12. The receiving device 11a calculates the correlation between the expected scrambling sequence for symbol 42 and the sequence obtained from symbol 41. The receiver 25 extracts the sequence carried by subcarriers p, p + 6, and p + 12 from symbol 41, obtaining the sequence is [1 +i, ^— 1 ^— i, 1 ^— i] . The correlation of this sequence to the expected scrambling sequence is calculated using Equation (5) as follows:

= 0.25

If the result of the cross-correlation for symbol 41 is less than the threshold, the receiving device 11a determines that symbol 41 is not the first data symbol a. The receiver 25 then receives symbol 42 and extracts a sequence from subcarriers p, p + 6 & p + 12. The correlation between the sequence obtained from symbol 42 and the expected sequence for symbol a is then calculated using Equation (5):

Therefore, if the threshold is set to an appropriate value, the receiving apparatus 11a, l ib, 11c can correctly identify symbol 42 as data symbol a. In the embodiments described above, a scrambling sequence is calculated for each subcarrier k of a data symbol. Yet another embodiment of the invention may be configured to generate scrambling codes from a sequence generator 33, 37 for the scattered pilot cells only. An example of method that could be used in such an embodiment is shown in Figure 13.

The procedure shown in Figure 13 is similar to that of Figure 9, with steps sl3.0 to sl3.3 & sl3.5 to si 3.11 corresponding to steps s9.0 to s9.10 respectively. However, the method of Figure 13 includes an additional step, step sl3.4, which determines whether the carrier index k corresponds to a pilot cell for a current data symbol j. For example, the determination may be based on whether the current carrier index k corresponds to a subcarrier for a scattered pilot, that is, a subcarrier included in the subset defined by Equation (1), or a designated carrier for an edge pilot signal or frame closing pilot signal. If the determination is positive, the sequence is generated as described above (steps sl3.5 to 13.10). If the determination is negative, the carrier index k is increased (step sl3.5) and the procedure returns to step sl3.4. The identification and modulation of scattered pilot signals in the procedure of

Figure 13 may be implemented by the following pseudo code, where i runs through every integer from zero to N _SP — 1, and k runs through every scattered pilot index. i=0;

j— symbol index;

for (k = every scattered pilot)

generate r^

modulate scattered pilot x _jjk ⁼ A _SP (l /2- i=i+l ;

end

The embodiments described in detail hereinabove are examples that demonstrate how the present invention may be implemented. While those embodiments related to broadcast of data according to the DVB-T2 standard, the invention is not limited to such systems. The invention may be implemented in networks configured in accordance with other standards and/ or used for multicast or unicast data in addition to, or instead of, broadcast data. As noted above, the invention may be implemented in systems that transmit data symbols that include pilot signals over wired and/or wireless networks.

Furthermore, while a specific example in which the receiving apparatuses 11a, l ib, 11c are mobile telephone handsets was mentioned above, the invention is not limited to such receiving devices. The invention may be implemented using other receiving apparatuses that are mobile, such as portable computers, netbooks, personal digital assistants, personal music players and so on, receiving apparatuses that are provided in a movable environment, such as a receiving apparatus installed in a car, boat, aeroplane or other vehicle, or in fixed receiving apparatuses, such as desktop computers, set-top boxes, televisions and so on. Meanwhile, in certain embodiments of the invention, the stream generator apparatus 7 may be included in a head-end and/ or a transmitter arrangement.