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5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.2.1 FEC for the PLPs | |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.2.1.1 FEC overview | The Forward Error-Correction (FEC) scheme in DVB-T2 is based on the concatenated BCH and LDPC code of DVB-S2, with the addition of a bit interleaver and demux for mapping bits to QAM constellation cells (see figure 66). ETSI ETSI TS 102 831 V1.2.1 (2012-08) 120 Figure 66: DVB-T2 FEC scheme The BCH outer code is include... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.2.1.2 BCH code | The shortened BCH codes, which are defined on GF(216) for the normal and GF(214) for the short LDPC codes, are exactly the same as in DVB-S2. The error-correction capability is 10 bits or 12 bits depending on the LDPC code rate. |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.2.1.3 LDPC code with multi-level constellations | |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.2.1.3.1 LDPC code | The LDPC codes of DVB-S2 have the following structure in their parity-check matrices: 1) Cyclic structure in information part. 2) Staircase structure in parity part. The first characteristic can be used for the implementation of encoder and decoder hardware based on partly parallel processing architecture. The second c... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.2.1.3.2 Bit Interleaver and demux | LDPC codes in DVB-T2 are irregular LDPC codes and the error-protection level of each code bit is not uniform, but depends on the column weight of the parity-check matrix. Table 25 and table 26 give the distribution of column weights, i.e. the number of columns having each weight, for the two FEC-block sizes. The protec... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.2.1.3.3 Constellation Mapping | The constellation mapping for QPSK, 16-QAM, 64-QAM is the same as for DVB-T. In addition, Gray mapping of 256-QAM is added in DVB-T2. Note that this is different to the QAM mapping in DVB-C, which is not Gray. |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.2.2 FEC for the L1 signalling | This clause gives an overview on protection of L1 signalling based on BCH and LDPC codes. In particular, this clause briefly introduces the shortening of the BCH information part and puncturing of the LDPC parity part according to the length of signalling information: these are L1-specific operations which are not perf... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.2.2.1 Shortening of BCH Information part | Both L1-pre- and post-signalling bits are protected by a concatenation of a BCH outer code and an LDPC inner code. The L1-signalling bits are first BCH-encoded and the encoded word is then further protected by a shortened and punctured 16 K LDPC code. For the protection of the L1-pre signalling, a BCH code with Kbch = ... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.2.2.2 Example for shortening of BCH information | Assume that the pure L1-post signalling consists of 7 971 bits, i.e. Kpost_ex_pad = 7 971 and it is transmitted using 64-QAM. Kpost_ex_pad denotes the number of information bits of the L1-post signalling excluding the padding field as described in clause 7.2.3 of [i.1]. Then, the number of LDPC blocks needed for the pr... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.2.2.3 BCH encoding | The Kbch information bits (including the Kbch - Ksig zero padding bits) are first BCH encoded according to clause 6.1.1 in [i.1] to generate Nbch = Kldpc output bits (i0… iNbch-1). |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.2.2.4 LDPC encoding | The Nbch = Kldpc output bits (i0… iNbch-1) from the BCH encoder, including the (Kbch - Ksig) zero padding bits and the (Kldpc - Kbch) BCH parity bits, form the Kldpc information bits I = (i0, i1, …, iKldpc-1) for the LDPC encoder. Note that for the protection of L1 signalling, the number of BCH parity bits, Nbch_parity... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.2.2.5 Puncturing of LDPC parity bits | 16 K LDPC codes with Kldpc = 3 240 and Kldpc = 7 200 for L1-pre- and post-signalling have 12 960 and 9 000 parity bits, respectively. Since the code length of 16 K LDPC code, Nldpc is 16 200, their effective LDPC code rates (Reff) are 1/5 and 4/9, respectively. All Nldpc - Kldpc LDPC parity bits, denoted by {p0, p1, …,... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.2.2.6 Example for Puncturing of LDPC parity bits | Assume that the pure L1-post signalling consists of 7 971 bits and is transmitted over 64-QAM. Then, Ksig is 3 986 as calculated in clause 9.2.2.2. Additionally, assume that the number of P2 symbols of a given FFT size, NP2 is 2. That is, the FFT size of the symbol in the T2 frame is 8 K as shown in table 44 in clause ... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.2.2.7 Removal of zero-padding bits | The (Kbch - Ksig) zero-padding bits are removed and are not transmitted. This leaves a word consisting of the Ksig information bits, followed by the 168 BCH parity bits and 9 000 - Npunc (= Nldpc - Kldpc - Npunc) LDPC parity bits without Npunc punctured bits, as illustrated in figure 75. 0th 1st 2nd 4th 5th 6th 7th 8th... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.2.2.8 Bit Interleaving and constellation mapping after shortening and puncturing | When 16-QAM or 64-QAM modulation is used for the L1-post signalling, the LDPC codeword of length Npost, consisting of Ksig information bits, 168 BCH parity bits, and (9 000 - Npunc) LDPC parity bits, are bit-interleaved using a block interleaver as described in clause 7.3.2.6 in [i.1]. Note that bit interleaving is not... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.2.3 Rotated constellations | The technique of rotated constellations consists of applying a rotation to the QAM constellation followed by a component-axes interleaving. After rotating, the projections of the constellation points on the I and Q channels carry the information regarding the m mapped bits. For a 16-QAM, instead of 2m/2=4 projections o... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.2.4 Cell Interleaver | The Cell Interleaver (CI) is a Pseudo-Random permutation of the cells in the FEC codeword, different for each FEC codeword of one Time-Interleaver Block, to ensure in the receiver an uncorrelated distribution of channel distortions and interference along the FEC codewords. The permutation-address-generation algorithm i... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.2.5 Time Interleaving | The concept of the Interleaving Frame was described in clause 6.2.3, and the choice of interleaving parameters was discussed in clause 5.9. This clause will consider the implementation of the Time Interleaver. It is most straightforward to consider the Time Interleaver as dealing with Interleaving Frames. This makes th... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3 Frame-building and OFDM generation | |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.1 Frame building | The function of the frame builder is to assemble the cells produced by the time interleavers for each of the PLPs and the cells of the modulated L1 signalling data into arrays of active OFDM cells corresponding to each of the OFDM symbols which make up the overall frame structure. The frame builder operates according t... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.2 Frequency Interleaving | To minimise memory use the frequency interleaver may be implemented with a single memory comprising of Nmax locations. Nmax is the maximum number of data cells in an OFDM symbol of the given FFT size. It is envisaged that practical implementations of DVB-T2 modulators should support all the FFT sizes included in [i.1].... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.3 MISO processing | The MISO processing in the modulator is fairly straightforward. The processing is applied to pairs of cells of adjacent cell index following the frequency interleaver, so the storage requirements are minimal. Note that the cells of a pair might not end up on adjacent carriers once pilots have been inserted. ETSI ETSI T... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.4 Pilot insertion | The choice of pilot pattern is dealt with in clause 5.4. |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.4.1 Purpose of pilot insertion | Pilots are inserted for several reasons: • The scattered, P2 and Frame-Closing pilots can be used for channel estimation and equalisation (see clause 10.3.2). • The continual, P2 and Frame-Closing pilots can be used for Common-Phase-Error correction (clause 10.3.2.1). • All of the pilots can potentially be used for Syn... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.4.2 Pilot locations | The pilot locations are described in detail in clause 9.2 of [i.1]. Particular care should be taken to note the following points: • The different types of symbol contain different combinations of the different pilot types. Table 27, reproduced from [i.1], summarizes this. • The Scattered Pilot pattern effectively begin... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.4.3 Number of pilot cells of each type | Implementing the various types of pilot is potentially error-prone given the number of combinations and the various additional pilots added under different circumstances. This clause presents a number of tables intended to provide a cross-check for implementers. |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.4.3.1 Pilots in normal symbols | Table 28 gives the total number of Continual Pilots (CPs) in each combination of FFT size and Scattered Pilot pattern. This includes CPs which are sometimes replaced by Scattered Pilots. The letters 'N' and 'E' after the FFT sizes indicate normal and extended-carrier modes respectively. Most but not all of the CPs are ... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.4.3.2 Pilots in P2 symbols | Calculating the number of pilots in P2 symbols is complicated by the extended carrier mode, MISO and tone reservation carriers. Table 31 details the various numbers of cells of different types for each FFT size, normal/extended carrier mode, and SISO/MISO where relevant. The first row gives the number of active carrier... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.4.3.3 Pilots in Frame-Closing Symbols | The pilots in the Frame-Closing symbols are relatively straightforward. There are Frame-Closing Pilots on all carriers which are a multiple of Dx, the Scattered Pilot-bearing carrier spacing, except for the edge carriers which are Edge Pilots. NOTE: In SISO mode there is no difference between edge, scattered and Frame ... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.4.4 Pilot boosting | The boosting of the Scattered Pilots is a function of the Scattered Pilot pattern, with more-sparse patterns having a boost value which is much greater than in DVB-T. ETSI ETSI TS 102 831 V1.2.1 (2012-08) 141 The boosting of the continual pilots is a function of the FFT size; the larger FFT sizes have greater boosting ... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.4.5 MISO pilots | Pilots in MISO are different from SISO in three ways: • Some pilots are inverted from transmitters in MISO group 2. This allows the sum and difference of the two channel responses to be estimated and the individual responses found. • Additional pilots are added in the P2 symbol to meet the constraints. • The Scattered ... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.4.5.1 Inversion of pilots | The following pilots are inverted from transmitters in group 2: • Scattered, Continual and Frame-Closing Pilots which fall on alternate Scattered Pilot-bearing carriers; • P2 pilots on carriers which are alternate multiples of 3 or 6; and • Edge Pilots on alternate symbols. The definition of the inversion rules and the... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.4.5.2 Additional P2 pilots | The additional pilots added in the P2 symbol are as follows: • The two carriers between the first normal and the first inverted pilot at each end of the normal carrier region of the spectrum are P2 pilots. These are padding pilots allowing the sum and difference estimates to be made without extrapolation. • The carrier... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.4.6 Use of the reference sequence in normal and extended carrier modes | In extended carrier mode, applicable for 8 K, 16 K and 32 K modes, the number of sub-carriers per OFDM symbol is increased, compared to normal mode. In extended mode, an equal number of sub-carriers is added on each side of the used carriers available in normal mode. This number is referred to as Kext. As a consequence... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.5 Dummy carrier reservation | In addition to the pilots, the cells reserved for the Tone Reservation method of PAPR reduction (clause 9.3.8.3) are initially set to zero and not allocated to data. The tone reservation algorithm itself is applied later, in the time domain but in such a way that all the energy of the added signal will end up in the re... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.6 Inverse Fast Fourier Transform (IFFT) | As for other COFDM systems, the Inverse Fast Fourier Transform (IFFT) algorithm may be used to implement the equations in clause 9.5 (and 9.8.2.4.1 for P1) of [i.1]. However, care is needed both in the exact IFFT implementation and in the mapping of the coefficients cm,l,k to the bins of the IFFT. The considerations ar... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.7 P1 generation | The P1 signal can also be generated using an Inverse Fourier Transform. The P1 carrier of index 426 corresponds to the centre carrier (k' = 0) for the other symbols, and should use the IFFT bin corresponding to the same frequency. Care should be taken with the normalisation of the IFFT coefficients and the gain of the ... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.8 PAPR reduction | |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.8.1 Overview and comparison of the two techniques | DVB-T2 provides two complementary techniques for reduction of PAPR, viz., ACE and Tone Reservation. ACE provides significant reduction in PAPR for lower order constellations (especially QPSK) while tone reservation provides greater benefit for higher order constellations. It is possible to use both ACE and TR simultane... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.8.2 ACE | The Active Constellation Extension (ACE) algorithm, which is described in clause 5.13.1 of [i.1], modifies the power distribution of time-domain signal samples so as to enable an improvement in power efficiency of subsequent power amplifiers, or in out-of-band emission levels. An additive signal is defined in such a wa... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.8.3 Tone Reservation | The basic idea of Tone Reservation is that some carriers are reserved to reduce PAPR. These reserved carriers don't carry any data information and are instead filled with a peak-reduction signal. Because data and reserved carriers are allocated in disjoint subsets of subcarriers, Tone Reservation needs no side informat... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.8.3.1 Reserved carriers and kernel | The kernel signal is defined as: ( ) ∑ ∈ − = l FFT c S k N K k n j TR n e N p π 2 1 where NFFT and NTR indicate the FFT size and the number of reserved carriers, respectively. KC is the index of the centre carrier. The kernel is the inverse Fourier transform of an (NFFT , 1) vector 1TR having NTR elements of ones at th... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.8.3.2 Peak-cancellation algorithm | Figure 84 shows the detailed block diagram of the peak-cancellation algorithm. The IFFT output (x) is fed into the peak-cancellation block, and the peak position and value of x are detected. Then the reference kernel, generated by the reserved carriers corresponding to the current OFDM symbol, is circularly shifted to ... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.8.3.3 Selection of the clipping level | The desired clipping level (in magnitude), Vclip, should be selected taking into account the requirements of the hardware modules. If it is too low, the cancelled peak re-grows owing to the impact of the imperfect kernel. Otherwise, if it is too high, the benefit from PAPR reduction scheme becomes marginal. Furthermore... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.8.3.4 Selection of the number of iterations | In general the higher number of iterations will give a lower PAPR. However in actual hardware implementations the number of iterations has to be limited to ensure the processing delay remains within reasonable limits. With typical hardware, the number of iterations is recommended to be kept to less than 10 iterations. ... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.8.3.5 Further TR examples | The following graphs plot the complementary cumulative distribution function (CCDF) with various values of Vclip and number of iterations. In these plots the CCDF shows the probability that the peak to average power (x-axis) of an OFDM symbol exceeds a threshold (y-axis), and thus gives us a measure of how much the PAP... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 9.3.8.3.6 SFN issues | Since the reserved tones are ignored by the receiver, the technique can be used in SFNs. It has been confirmed that the use of tone reservation does not exacerbate ICI problems, regardless of the SFN frequency offset. |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10 Receivers | 10.1 Overview This clause mainly concerns consumer receivers, which are required to decode and output the Transport Stream or other stream carried by one PLP, together with its common PLP if appropriate. Professional receivers may be designed which are capable of decoding more than one PLP, and possibly reproducing the... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.1.1 Receiver Model | The top-level receiver model shown in figure 92 will be assumed for the purposes of the following clauses. More detail of the "DVB-T2 demodulator" block is shown in figure 93. Figure 94 in turn gives the details of the chains for Bit Interleaved Coding and Modulation (BICM) for the PLPs and L1 signalling. The processin... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.1.2 Acquisition scenarios | The tasks of acquisition and synchronisation can take slightly different forms in a range of situations, for example: • when switched on for the very first time after purchase, or after a change of location; • when switched on again at the same location; • when a different service is selected which necessitates a chang... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.1.2.1 Initial scan | The initial scan is executed in order to find RF signals of DVB-T2 format at the current location and to determine their quality, i.e. if they are suitable for undisturbed reproduction or not. This requires determination of the modulation and coding parameters as well as synchronization to the RF signal in time and fre... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.1.2.2 Service-selection and reproduction | For service selection a programme guide listing all accessible services should be provided to the user. In order to set this up, Layer-2 signalling should also be processed. As soon as a new service is selected, changes to the settings of that service need to be monitored by the receiver. These changes might affect the... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.1.2.3 Hand-over (portable and mobile cases) | The word "handover" in DVB describes the change from one RF channel to another within the same delivery system for particular purposes as follows: a) The signal currently being received no longer gives undisturbed reproduction of the desired service, and the receiver should attempt to follow the service by switching to... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.1.2.3.1 Automatic service following | 10.1.2.3.1.1 Overview The functionality "automatic service following" is required for receivers that move, i.e. for portable and mobile devices. The purpose is to stay tuned to the same service although the multiplexes might change depending on the geographical region the terminal is located in. A lower level of this f... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.1.2.3.2 Announcements | For the accurate provision of announcements that are provided via a different RF channel than the tuned one, a-priori information needs to be available to the widest possible extent since a time-consuming search exercise would mean missing the initial part of the announcement. Hence, beside the announcement trigger, fr... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2 Acquisition and synchronisation | |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.1 Overview of synchronisation | The receiver needs to locate the received signal in both time and frequency before it can extract the transmitted information. Once initially achieved, this synchronisation should then be tracked continually throughout reception. The DVB-T2 signal contains many features that can be exploited for this purpose, leaving r... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.2 Use of P1 | |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.2.1 Introduction | The P1 symbol occurs once per frame, being inserted at the very beginning of each. It marks the beginning of each T2 frame and of each future-expansion-frame (FEF) part. The symbol has four main purposes: 1) To enable the receiver to determine quickly (e.g. during a band scan at a new location) whether a particular RF ... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.2.2 Structure of P1 | The length of the P1 is fixed, regardless of the FFT mode and guard-interval configurations of the payload OFDM symbols. This makes it easier to detect - there is only one thing to look for. The P1 signal is designed so that it can be detected despite the presence of a substantial frequency offset. This offset will occ... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.2.3 P1 features | The architecture of the P1 symbol has been designed to be robust, enabling the receiver to decode the P1 in challenging conditions, with the minimum overhead. P1 was therefore designed with the following features: • Protection against interference: - It is expected that the P1 symbol can be correctly received and decod... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.2.4 Overview of the use of P1 during channel scanning | The main use of the P1, when receivers can make use of the most of its capabilities, is the "initial scan" scenario. Assuming the receiver starts from scratch, it may know nothing about which (channelised) frequencies convey a T2 signal. In this situation, the first task would be the discovery of the T2 frequencies. ET... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.2.5 P1 detection | |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.2.5.1 Summary | The P1 detection can be achieved by correlating the two parts added at either side of the 1 K OFDM (see figure 95). The correlation will be based on two branches running in parallel, each branch looking for the maximum similarity of the respective part of the repetition. By adapting the correlation windows and offsets ... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.2.5.2 Goals | The P1 detection aims to achieve the following: • Detect the presence of a P1 symbol. • Set a reference for coarse time synchronisation. • Set an argument used to correct fractional (±0,5 carrier spacing) frequency offsets. ETSI ETSI TS 102 831 V1.2.1 (2012-08) 161 |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.2.5.3 Implementation | The implementation shown below performs the correlation for both parts of the P1 symbol, and has the merit of minimising the number of complex multipliers. Figure 97: Correlation for the P1 Detection (Block diagram) fSH is the frequency shift applied to the sections C and B of the P1 symbol at the transmitter, and corr... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.2.6 P1 validation | |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.2.6.1 Summary | The P1-validation step is needed to filter those false detections that match the previous step. The P1 symbol carries information in a subset of the available carriers of the symbol. This step should identify whether the incoming symbol matches with the distribution of the carriers. Even if the distribution is as expec... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.2.6.2 Goals | The P1 validation provides the following information: • It confirms that the 1 K symbol received is a P1 symbol. This implies that the power in the frequency domain is distributed in accordance with the distribution sequence specified for the P1 symbol. • It reports and corrects the integer number of carriers offset wh... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.2.6.3 Implementation issues (coarse frequency correction) | Since the active carriers in which pilots are conveyed have been distributed keeping a safety area on both sides of the centred carrier, the detection of the peak of the correlation is possible when the frequency of the transmitted signal is inside ±500 kHz range from the centre of the bandwidth (in the case of 8 MHz s... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.2.7 Decoding the P1 signalling | |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.2.7.1 Summary | Once the P1 structure is correctly detected and centred (i.e. the system knows which of the active carriers are used), the modulated and scrambled pilots are ready to be decoded. Before reaching the decoding process the received sequence extracted from the carrier distribution has to be unscrambled and then demapped. T... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.2.7.2 Goals | Signalling decoding provides basic transmission parameters that are needed to receive the rest of the preamble. The main features of the decoding are: • Firstly, the P1 identifies what type of preamble it is. Since it is possible to find FEF parts within the same channel carrying the T2 signal, the P1 will disclose whe... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.2.7.3 Implementation | In order to decode the signalling without any loss in the P1 robustness, the CSS-theory properties should be applied. The main two properties are described in clause 10.2.2.3, and also mentioned in [i.1]. In order to get the most from the sequence, the algorithm to decode each of the fields is the following: 1) Extract... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.2.8 Interpretation of P1 signalling fields | In order to validate the presence of the T2 signal, the signalling bits conveyed within the P1 should be decoded. Taking both S1 and S2 fields into account, three possibilities may occur: a) P1 signals a T2-preamble (S1=00X). In this case, the algorithm confirms the presence of the T2 signal. The scanning for the curre... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.2.9 The FEF loop | The FEF loop itself is entered only when the P1 that has been detected does not correspond to the T2 specification, but FEFs are mixed within the channel, i.e. case (c) above. In the other two cases, the FEF loop is not required. In order to identify whether the T2 signal is transmitted, the algorithm may take into acc... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.3 Determining the Guard Interval | Decoding the P1 signalling information tells the receiver what FFT size is in use and, in 8 K and 32 K a hint as to the guard interval, but the exact length of the guard interval (which can be chosen broadly independently of the FFT size) at this stage remains unknown. There are at least two possible approaches to dete... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.3.1 Determining the Guard Interval using P2 | In the 16 K and 32 K FFT modes, there is only one P2 symbol and it may therefore be possible to demodulate it and decode the L1-pre signalling (which signals the guard interval directly) even if the guard interval is not known. The L1-pre signalling can be decoded at an SNR of -4 dB in a Gaussian channel (see clause 14... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.3.2 Use of Guard Interval Correlation to determine the Guard Interval | In the FFT modes 8 K and below, there is more than one P2 symbol, making it more difficult to decode the L1-pre signalling without knowledge of the guard interval. In these modes, and as an alternative to decoding P2 in 16 K and 32 K, another way to determine the guard interval is to use so-called guard-interval correl... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.3.3 Use of the Guard-Interval hinting information from P1 | In some modes, the S2 signalling field of the P1 preamble gives a hint as to the guard interval. This indicates whether the guard interval belongs to the "DVB-T" set (1/32, 1/16, 1/8 or 1/4) or is one of the "new" guard intervals introduced in DVB-T2 (1/128, 19/256 or 19/128). This is helpful when performing guard inte... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.4 Time and Frequency synchronisation | The P1-symbol decoding process described in clause 10.2.2 can give an first estimate of the carrier frequency offset and the FFT window position. However, there is likely to be some error in both these estimates; in particular, the frequency- offset estimate is unlikely to be accurate enough for the 32 K-FFT mode, sinc... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.5 Use of Frame/Symbol level sequences for synchronisation | |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.5.1 Introduction | Clause 10.2.2 describes how the P1 preamble symbol can be used, among other things, to estimate frequency and time synchronisation. In addition to this process (or as an alternative), and to get a more robust synchronisation (providing clock sampling, symbol, frequency and frame synchronisations), a PN sequence has bee... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.5.2 Definition of the reference sequence | The pilots are modulated according to a reference sequence, rl,k, where l and k are the symbol and carrier indices as defined in [i.1]. The reference sequence is derived from a symbol-level PRBS, wk and a frame-level PN-sequence, pnl. This reference sequence is applied to all the pilots (i.e. Scattered, Continual, Edge... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.5.3 Synchronisation algorithm | In the synchronisation method described below, all the synchronisations can be achieved, such as frame, symbol, frequency, and clock recovery, either from Scattered Pilots only or using all types of pilots. This synchronisation algorithm can provide an alternative or additional method to that based on the use of P1 sym... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.5.4 Full synchronisation method | In order to minimize the complexity and to increase performance in the presence of a frequency offset, the cross-correlation is performed on N correlation fragments (N >= 16), that are afterwards added to deliver a fully reliable symbol peak independent of frequency offsets (within a given frequency range) and of the o... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.5.5 Evaluation of Synchronisation features | The main characteristics of the performance of this technique are as follows: • Maximal acquisition time in the worst propagation conditions is very low (less than 0,2 seconds). • Threshold for false detections is up to -12 dB of C/N in AWGN channel, 2 dB in SFN channel (Single Frequency Network, 0 dB echo) and 2 dB in... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.6 Use of P2 | |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.6.1 Initial Scan | After the P1 has been recovered, or frame synchronisation has been achieved by another method (see clause 10.2.5), the system may enter a set-up stage. Only basic parameters are known because either they were disclosed in the P1 symbol (i.e. FFT size, SISO/MISO) or they have been obtained from another process (i.e. gua... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.6.1.1 Reception of P2 symbols during initial scan | The P2 symbols come immediately after the reception of the P1. The number of P2 symbols is variable, but fixed for each FFT configuration. Thus, since the FFT size is already known, the number of P2 symbols that convey the L1 signalling is known. L1 signalling is interleaved throughout all the P2 symbols. Thus, the rec... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.6.1.2 TX Basic parameters verification | The signalling carried in the P1 as well as the guard-interval length of the transmission are repeated in the L1-pre signalling conveyed in the P2 symbols. These parameters are normally needed to correctly receive the P2 symbols. However, there may be scenarios in which receivers can correctly recover the rest of the p... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.6.1.3 Determining position in the super-frame | The P1 preamble sets a time reference for the beginning of every T2 frame. The P1 is also used to signal the beginning of the FEF parts embedded in a mixed-T2 channel. In order to complete the set-up of the system, some other synchronisation tasks are still pending. In particular, the position of the T2-frame within th... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.6.2 Channel-zapping | This clause will briefly enumerate the steps a receiver may follow when changing channel ("zapping"). The scenario is described as follows: the receiver is decoding a given PLP on the tuned frequency, and is instructed to change to a different PLP which is transmitted on a different frequency. |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.6.2.1 Assumptions | It is assumed that the receiver has already visited all the frequencies, so that their initial scan and set-up process has been achieved (see clause 10.2.2). In this stage, and also assuming that L1-pre signalling will not change, the receiver is in the following state: • Synchronisation: the receiver is perfectly sync... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.6.2.2 Requirements for zapping between two channels | A typical flow might be as follows: 1) The tuner is re-tuned to the other frequency (already known). The receiver is already aware about L1-pre signalling, which implies that the receiver has all basic TX parameters backed-up. 2) After its stabilisation, the receiver has to synchronise to the new frequency. In general,... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.7 Receiver Synchronisation/Zapping time | Owing to the complexity of the system as well as the wide variety of use cases, it is not possible to characterise the time needed to do the initial scan or zapping with just one formula. However, acquisition times in typical scenarios will be presented here. |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.7.1 Initial scan time | As explained in clause 10.2.2.4, the receiver can deduce that no T2 signal is present for a given frequency and channel bandwidth if no P1 is received in 250 ms. It might be sensible for a receiver to wait longer than this, in case it missed a P1 owing to poor channel conditions. The minimum scanning time for channels ... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.2.7.2 Zapping time | The zapping time depends on many factors, some of them listed below: • If the target PLP is in the same channel or a different one. • The type of the target PLP and its interleaving configuration. • The presence of FEF parts in within the T2 super-frame. • The status of the information in the configurable L1-post signa... |
5c408ecfef3e523a192afe3fd7249eaf | 102 831 | 10.3 OFDM Demodulation |
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