Chapter 1 Introduction
1.4 Thesis Organization
The following of this thesis is organized as follows. In chapter 2, OFDM will be introduced briefly and DVB-T/H technology will be discussed. Chapter 3 describes the effect of incorrect transmission/GI mode and detected boundary offset. Moreover, some algorithms, simulation and architectures for mode/GI detection and coarse symbol synchronization will be compared. A blind mode/GI and boundary detection scheme will be proposed. Algorithms, performance simulation and proposed scheme for scattered pilot synchronization, channel estimation and demapping will be discussed in Chapter 4. The architecture and hardware design will be presented in chapter 5. Finally, chapter 6 will give our conclusions and future works.
Chapter 2
OFDM and DVB-T/H Technology
2.1 Concept of OFDM
In mid 60s, frequency division multiplexing (FDM) was published. In FDM, multiple signals are sent at the same time with different sub-channels. OFDM [15] is based on this idea and uses orthogonal technique to overlap the sub-channels to send more signals. By overlapping sub-channels in orthogonal frequencies, OFDM is able to carry more information than FDM with the same bandwidth. The sub-channels in FDM and OFDM are shown in Fig. 2.1 and Fig. 2.2.
Fig. 2.1 Sub-channels in FDM modulation
Fig. 2.2 Sub-channels in OFDM modulation
OFDM with channel coding is called COFDM (Coded OFDM). The COFDM technique adopted by DVB-T, has the ability to reduce the ICI (Inter-Carrier Interference) and ISI (Inter-Symbol Interference) by inserting guard intervals (GI), a
copy of the last period of the same OFDM symbol, between successive OFDM symbols. Since the multipath effect will cause the ISI phenomenon, to insert a guard interval is able to prevent the damage from other sub-paths. With inaccurate symbol detection, system has a high probability to find a wrong boundary which may allocate near the correct one. Owing to the difficulty of finding accurate boundary with multipath effect, system must tolerate to the inaccurate symbol boundary location.
Therefore, the guard interval is filled by the copy of last period of a symbol. By using this characteristic, the detected boundary which locates before the correct one is able to decode correctly. Since the detected boundary locates before the correct one, according to OFDM mathematical function, the transmitted pattern looks like a cyclic shift without destroying the orthogonality of sub-carriers and will only cause a phase rotation. But if the detected boundary locates after the correct one, the sub-carriers will lose its orthogonality and lead to a wrong decoded answer. The received symbols with multipath effect are shown in Fig. 2.3.
symbol (n-1) symbol (n) symbol (n+1)
transmitted symbol (n)
symbol (n-1) guard interval symbol (n)
＋
guard interval symbol (n+1)main path
multipath 1
multipath 2
multipath 3
received data guard interval
symbol (n-1) symbol (n) symbol (n+1)
useable part useable part
destroyed part guard interval
＋
symbol (n-1) guard interval symbol (n) guard interval symbol (n+1)
＋
symbol (n-1) guard interval symbol (n)
＝
guard interval symbol (n+1)Fig. 2.3 Received OFDM symbols with multipath effect
2.2 DVB-T/H Technology
2.2.1 MPEG-2 Source Coding and Multiplexing
DVB-T is a broadcasting system based on OFDM modulation technology.
Generally, this system can be divided into two parts, transmitter and receiver. In Fig.
2.4, a DVB-T transmission system block diagram is presented. MPEG-2 source coding and multiplexing multiplexes video, audio and data into an MPEG-2 program stream. Pilots &
TPS source coding and multiplexing
Transport MUXes
To aerial
TERRESTRIAL CHANNEL ADAPTER Encoder
Fig. 2.4 Functional block diagram of DVB-T transmission system
2.2.2 Channel Coding
Channel coding includes randomizer, outer coder, outer interleaver, inner coder and inner interleaver. The MPEG-2 transport stream will be separated into high-priority (real line in Fig. 2.4) and low-priority (dotted line in Fig. 2.4). Therefore, in a small size monitor or weak signal environment, the receiver can switch a HDTV program into a SDTV program.
The MPEG-2 transport stream is organized as fixed length packets (118 bytes) and decorrelated by the block “MUX adaptation and energy dispersal”. Considering about the MPEG-2 Transport Stream have a probability to be a long 1 or 0 sequences, there will be some problem in synchronization. Doing Exclusive-OR operation with PRBS sequences can efficiently reach energy dispersal and randomize the MPEG-2 Transport Stream sequences. Fig. 2.5 illustrates the energy dispersal schematic diagram.
Fig. 2.5 Scrambler/descrambler schematic diagram
Outer coder uses a nonbinary block code, Reed-Solomon RS (240,188, t=8) shorten code, which have an ability to correct up to eight errors, with generator polynomial as Eqn. (2.1) is the usually used coding scheme recently.
1 )
(x = x8+x4+x3+x2+
p (2.1)
A convolutional byte-wise interleaving with depth I=12 is applied to make the transmitted data sequence being rugged to long sequences of errors. While transmission in air, a suddenly short period interference may occur because of many know or unknown reasons. Though RS have an ability to correct no more than 8 errors, the interference above is possibly lead to more than 8 errors in a RS package and unable to correct. At this time the outer interleaver opposites to this situation by interleaving the data in a symbol to other symbols. Fig. 2.6 is the conceptual diagram of the outer interleaver and deinterleaver.
≈
≈≈ ≈≈
Fig. 2.6 Conceptual diagram of the outer interleaver and deinterleaver
For the purpose to have better Bit-Error-Rate (BER), a punctured convolutional encoder is cascaded with five valid coding rates: 1/2, 2/3, 3/4, 5/6, and 7/8. Higher coding rate means lower correction ability. Fig. 2.7 is an example of 1/2 coding rate which means one of two bits is useful.
Fig. 2.7 The mother convolution code of rate 1/2
Finally, a block based bit-wise inner interleaver is used to against the Viterbi output burst errors.
2.2.3 Mapper & Frame
After two level channel coding, all data carriers in an OFDM symbol will be mapped onto QPSK, 16-QAM, 64-QAM, non-uniform 16-QAM or non-uniform
64-QAM by the mapper. DVB-T provides 2 transmission modes, 2K mode and 8K mode. In DVB-H a 4K transmission mode is specially provided. Each frame consists of 68 OFDM symbols and contains scattered pilot cells, continual pilot carriers and TPS carriers. Four frames constitute one super-frame.
2.2.4 Reference Signals
Reference signals can be classed as three types, scattered pilot cells, continual pilot carriers and TPS carriers. All of them are inserted in all OFDM symbols.
Scattered pilot cells do not really have a fixed position in a symbol but others do. The positions scattered pilot cells locates are as Eqn. (2.2) shows.
]
where k is the frequency index of the sub-carriers and l is the time index of the symbols. And it’s easy to seem that, scattered pilot cells’ location will be the same every four symbol. For different transmission mode, Kmax is different. Eqn. (2.3) shows how to generate scattered pilot cells’ value.
0
In summary, Fig. 2.8 shows the distribution of scattered pilots. According to regular fixed located characteristic, scattered pilots can be used for channel estimation.
By using timing interpolation, channel response can be reduced to 1/3.
Fig. 2.8 Distribution of scattered pilots
The locations of continual pilot carriers are listed in Table 2-1. The values of continual pilots are generated like that of Eqn. (2.3). The main function of continual pilots is to track carrier frequency offset.
Table 2-1 Continual pilot carrier position Continual Pilot Indices for Continual Pilot Carriers
2K mode
0 48 54 87 141 156 192 201 255 279 282 333 432 450 183 525 531 618 636 714 759 765 780 804 873 888 918 939 1137 1140 1146 1206 1269 1323 1377 1491 1683 1704
4K mode
0 48 54 87 141 156 192 201 255 279 282 333 432 450 183 525 531 618 636 714 759 765 780 804 873 888 918 939 1137 1140 1146 1206 1269 1323 1377 1491 1683 1704 1752 1759 1791 1845 1860 1896 1905 1959 1983 1986 2037 2136 2154 2187 2229 2235 2322 2340 2418 2463 2469 2484 2508 2577 2592 2622 2643 2646 2673 2688 2754 2805 2811 2814 2841 2844 2850 2910 2973 3027 3081 3195 3387 3408
8K mode
0 48 54 87 141 156 192 201 255 279 282 333 432 450 183 525 531 618 636 714 759 765 780 804 873 888 918 939 1137 1140 1146 1206 1269 1323 1377 1491 1683 1704 1752 1759 1791 1845 1860 1896 1905 1959 1983 1986 2037 2136 2154 2187 2229 2235 2322 2340 2418 2463 2469 2484 2508 2577 2592 2622 2643 2646 2673 2688 2754 2805 2811 2814 2841 2844 2850 2910 2973 3027 3081 3195 3387 3408 3456 3462 3495 3549 3564 3600 3609 3663 3687 3690 3741 3840 3858 3891 3933 3939 4026 4044 4122 4167 4173 4188 4212 4281 4296 4326 4347 4350 4377 4392 4458 4509 4515 4518 4545 4548 4554 4614 4677 4731 4785 4899 5091 5112 5160 5166 5199 5253 5268 5304 5313 5367 5391 5394 5445 5544 5562 5595 5637 5643 5730 5748 5826 5871 5877 5892 5916 5985 6000 6030 6051 6054 6081 6096 6162 6213 6219 6222 6249 6252 6258 6318 6381 6435 6489 6603
6795 6816
TPS pilots plays an important rule in DVB-T because of there is no any handshaking before communication. TPS pilots carry 68 different messages in a frame and the messages are listed in Table 2-2.
Table 2-2 TPS signaling information and format Bit Number Purpose/Content
S0 Initialization S1 ~ S16 Synchronization word
S17 ~ S22 Length indicator
S23 , S24 Frame number
S25 , S26 Constellation S27, S28, S29 Hierarchy information S30, S31, S32 Code rate, HP stream S33, S34, S35 Code rate, LP stream
S36, S37 Guard interval S38, S39 Transmission mode S40 ~ S47 Cell identifier
S48 ~ S53 Reserved
S54 ~ S67 Error protection
2.2.5 DVB-H Particular
For portable devices, time slicing technology is employed to reduce power consumption. For the purpose to improve the system performance in mobile environment, forward error correction for multiprotocol encapsulated data (MPE-FEC) is adopted with powerful channeling and time interleaving. Since 8K mode has better performance in large single frequency network (SFN) but worse in against Doppler Effect and 2K mode has better performance in against Doppler Effect but not suitable for large SFN, a comprised 4k mode is proposed. Overall, the specification of DVB-T/H is listed in Table 2-3.
Table 2-3 Specification of DVB-T/H Transmission mode 2K, 4K, 8K
Number of useful sub-carriers 1705, 3409, 6817 Number of continual pilots 45, 89, 177 Number of scattered pilots 141, 282, 564 Number of TPS pilots 17, 34, 68 Radio frequency (MHz) 45~860
Guard interval 1/4, 1/8, 1/16, 1/32 Bandwidth (MHz) 5, 6, 7, 8
Elementary period (us) 7/40, 7/48, 7/56, 7/64
Channel model Rayleigh, Ricean
Forward error correct Convolution code with puncturing Reed Solomon Code (204,188)
Constellation QPSK, 16QAM, 64QAM, non-uniform
16QAM, non-uniform 16QAM Required BER 2 X 10-4 after Viterbi decoder
Quasi Error Free after Reed Solomon
Chapter 3
Symbol Synchronization Algorithms
Fig. 3.1 illustrates the block diagram of DVB-T baseband inner receiver and all synchronization processes in digital domain. The block diagram contains mode/GI &
symbol boundary detection, carrier synchronization loop, sampling synchronization loop, frequency domain channel estimation/compensation and hard demapper. This thesis will design the architecture based on this block diagram and focuses on the highlighted blocks.
Fig. 3.1 Block diagram of DVB-T baseband inner receiver
Timing synchronization plays an important role in digital communication systems. Without accurate timing synchronization process, the systems will fail to work in the beginning or get unreliable outcome. Timing synchronization includes mode/GI detection, coarse symbol synchronization (CSS), scattered pilot synchronization (SPS), carrier frequency offset (CFO) and sampling clock offset (SCO) issues. The last two topics have been discussed in [14]. This chapter focuses on mode/GI detection and coarse symbol synchronization problems. These two jobs should be down first in receiver. The FFT window has to decide the window length and locations with the correct transmission mode, guard interval length and symbol
boundary information. Otherwise, the feedback loops will have no idea to recover CFO and SCO and so do those parts behind inner receiver.
The goal of mode/GI detection is to get precise mode/GI parameters of transmitted symbols. By using the detected transmission mode, the system is able to set a FFT window, which length equals to transmission mode. Transmission mode and guard interval parameters make the system being able to compute boundaries behind the first boundary detected by coarse symbol synchronization. After mode/GI detection, coarse symbol synchronization process will adopt the parameters from mode/GI detection to do more accurate symbol boundary detection for the purpose to reduce timing offset effects and avoid ISI effect. Moreover, the results from mode/GI detection and coarse symbol synchronization make the system have enough information to determine the FFT window locations.
3.1 Mode/GI Detection
It seems that the system can get the information of transmission mode and guard interval length from TPS pilots discussed in 2.2.4. But without these messages, how can the systems set a correct FFT window length and correct FFT window locations?
That means FFT has no idea to start to work and leads to no TPS information. This phenomenon will become a vicious cycle and system will never start to work. In order to make the FFT start to work, it’s necessary to do blind mode/GI detection before other processes of inner receiver.
3.1.1 Introduction to Mode/GI Detection
Mode/GI detection algorithms are usually similar to coarse symbol synchronization theorems. There are many methods to detect the mode/GI. For example, [16] proposed a blind transmission mode detection process, [17] modified a
mode/GI jointed detection process based on [16] and a two-stage mode/GI detection is discussed in [18].
a) Mode Detection
The basic idea of mode detection is to use the characteristic of the inserted guard intervals, a copy of tail in OFDM symbols. After surviving from fading channel, the guard interval part will still have a high correlation with symbol’s tail. Fortunately, other parts will have low or even no correlation with guard interval. Thus, for a 2K transmission mode, the mode detector shall find the correlation of r(n) and r(n-2K), where r(n) is the nth received signal. For the purpose to ensure the correlation result, to accumulate the correlation results between r(n)×r(n-2K) and r(n+64)×r(n-2K+64) is necessary. Therefore, 2K+2×64, 4K+2×64 and 8K+2×64 long moving windows are required to detect the 2K/4K/8K transmission modes for DVB-T/H mode detection.
Fig. 3.2 illustrates a simple diagram of the correlation results of 2K/4K/8K mode detection windows under 8K transmitted symbols. As the figure shows, only the 8K mode window is possibly located at the region of guard interval and symbol’s tail at the same time and will have prominent correlation results. Eqn. (3.1) presents the computation required in Fig. 3.2 mathematically.
guard
interval symbol (n) tail
2K correlation 4K correlation 8K correlation
guard interval
symbol (n-1) symbol (n+1)
8192 4096
2048
× 8K moving window
4K moving window 2K moving window
×
×
t
Fig. 3.2 2K/4K/8K correlation under 8K mode
∑
=where N is the delay-line length which value will be 2K, 4K or 8K according to different transmission mode, r(n) is the received nth signal. Eqn. (3.2) is a modified version of Eqn. (3.1). By using an integration length equals to the minimum guard interval length of each tested transmission mode, the detector is able to have a reliable correlation result.
∑
−=
−
−
×
−
=
32 1
0
*( ) ( )
) (
N
i
N i n r i n r n
x (3.2)
Fig. 3.3 (a) and (b) show the different correlation results under different transmission modes with 1/4 guard interval, 12dB SNR, CFO=23.33 and surviving from Rayleigh channel. In Fig. 3.3 (a) only 2K correlation results have apparent plateaus and Fig. 3.3 (b) has the same situation for 8K correlation results.
0 1000 2000 3000 4000 5000
0 2 4 6 8 10 12
Amplitude
Sample Index
2K Correlation 4K Correlation 8K Correlation 2K Correlation
4K/8K Correlation
(a)
0 0.5 1 1.5 2
8K Correlation 8K Correlation
2K/4K Correlation
(b)
Fig. 3.3 2K/4K/8K correlation results under (a) 2K (b) 8K transmission mode Even though Eqn. (3.1) or Eqn. (3.2) have the ability to distinguish the transmission mode, there is still some aliasing peaks occurred due to channel noise and may possibly lead to a wrong detected mode. For example, in Fig. 3.3 (a), there is an aliasing peak near sample index 3000 of 2K correlation results which is almost as high as the lowest plateau value near to sample index 2000 for 2K. Thus it is a problem to decide the peak or plateau threshold since there is no flat plateau and unitary plateau value.
To eliminate the effect from channel noises and ease the decision of the threshold, [17] used a normalized method to detect the transmission mode shown in Eqn. (3.3).
∑
The correlation result will be normalized to 1 theoretically by dividing the power
term. Fig. 3.4 (a) and (b) are the results of Eqn. (3.3), using the same pattern with Fig.
3.3. The plateau is ideally close to “1” and no other aliasing correlation results are higher than “0.707”. The characteristic of the normalized flat plateau can be also used to calculate the guard interval length and it will be discussed later.
0 1000 2000 3000 4000 5000
0 0.2 0.4 0.6 0.8 1
Amplitude
Sample Index 2K Correlation
4K Correlation 8K Correlation
2K Correlation
4K/8K Correlation
(a)
0 0.5 1 1.5 2
x 104 0
0.2 0.4 0.6 0.8 1
Amplitude
Sample Index 2K Correlation
4K Correlation 8K Correlation
8K Correlation
2K/4K Correlation
(b)
Fig. 3.4 2K/4K/8K normalized correlation results under (a) 2K (b) 8K mode
b) Guard Interval Length Detection
The guard interval length will be detected after transmission mode. With an incorrect guard interval length, FFT won’t get correct patterns and sub-carriers after FFT won’t be the same with transmitted. Fig. 3.5 illustrates the situations of incorrect guard interval length. In Fig. 3.5 (a), a smaller guard interval length is detected and the second FFT window has a probability to get signals form the ISI destroyed region.
The third FFT window gets some signals from the correct symbol, some from ISI destroyed region and others from previous symbol, this will lose the orthogonality of OFDM symbols and FFT outputs are absolutely incorrect. FFT windows in Fig. 3.5 (b) also get incorrect signals either.
(a)
(b)
Fig. 3.5 Detected guard interval length is (a) smaller (b) larger than transmitted [18] used the minimum guard interval length, which is 1/32 of transmission mode, and accumulate the results using different delay-line length windows, 1/32, 1/16, 1/8 and 1/4. Only the correct guard interval mode will have a maximum peak after different length accumulation. [17] adopted an simple and less delay-line method to calculate the guard interval length. This method just calculates the length of plateau period and that will approximate to “guard interval length subtracts mode/32”.
3.1.2 Proposed Mode/GI Scheme
The normalized method seems very easy to realize and compute the guard
interval length, but a divider wastes large power and area. Two-stage mode/GI detection proposed in [18] needs extra delay-lines which size will be from 2K to 8K.
This is another penalty. As a result, a modified method based on normalized mode/GI detection is proposed in this thesis. Since a threshold value is determined to define the plateau region, it implies that all x(n) which are bigger than the pre-defined threshold belongs to the plateau. Therefore, two key observations below can be found:
1) If the plateaus exist, the transmission mode will be the same with the tested mode.
2) The period of the plateau represents the guard interval length.
Using the two key observations above, if x(n) is bigger than the threshold means x(n) belongs to the plateau region. Now the threshold is defined as 0.707 and the derivation of using a subtractor to replace the divider is shown in Eqn. (3.4).
0
First, move the denominator of the left term to the right term. Thus, a divider is replaced by a multiplier. Second, as the result of calculating the absolute value for complex numbers is too complicated to implement, squaring both sides of the equation is used to replace the absolute value calculating. In fact as Fig. 3.6 shows, modified from Fig. 3.4, square operation eliminates the noises. Then move the right term to left, this action makes the comparator replaced by a subtractor. Finally, the square of threshold becomes “0.5” that means only a bit shift instead of a multiplier is able to accomplish this job. Overall, the transmission mode can be tested by observing whether the “result >=0” and guard interval length can be detected by computing the period of the “result >=0”.
0 1000 2000 3000 4000 5000
0 0.2 0.4 0.6 0.8 1
Amplitude
Sample Index 2K Correlation
4K Correlation 8K Correlation
2K Correlation
4K/8K Correlation
(a)
0 0.5 1 1.5 2 x 104 0
0.2 0.4 0.6 0.8 1
Amplitude
Sample Index 2K Correlation
4K Correlation
8K Correlation 8K Correlation
2K/4K Correlation
(b)
Fig. 3.6 2K/4K/8K squared normalized correlation under (a) 2K (b) 8K mode
3.1.3 Performance Simulation
Fig. 3.7 illustrates the error rate under different threshold of the proposed mode/GI detection. The simulation environment is 1000 2K transmission mode symbols with 1/4 guard interval, 12dB SNR, 23.33 sub-carriers CFO and surviving from Rayleigh channel. As the simulation shows, the error rate of the proposed mode/GI detection method under the threshold 0.5 to 0.8 is zero. That means the pre-defined threshold value 0.707 locates at the reliable region. Because of noise and channel effect, the normalized correlation results are closing to 0.9 instead of 1.
Therefore, the reliability decreases while the threshold is defined close to 0.9. For threshold smaller then 0.5 cases, the noise will influence the detection result.
0.4 0.5 0.6 0.7 0.8 0.9 0
0.1 0.2 0.3 0.4 0.5 0.6 0.7
Error Rate (%)
Threshold
Fig. 3.7 Error rate under different threshold of proposed mode/GI detection
3.2 Coarse Symbol Synchronization
Coarse symbol synchronization (CSS) is also named symbol boundary detection.
The goal of symbol boundary detection is to find out boundaries of transmitted symbols. Coarse symbol synchronization starts to work after finishing the mode/GI detection. With the information of transmission mode and guard interval length, symbol boundary detection has enough information to detect boundaries. After the first symbol boundary is detected, the system will use transmission mode, guard interval length and boundary location to derive the successive symbol boundaries.
In order to make the FFT windows locate on correct locations, symbol boundary detection needs to solve some problems. Since DVB-T signals are transmitted in SFN (Single Frequency Network), for a receiver there will have many signals from different transmitters with discordant delay time as Fig. 3.8 illustrates. The multipath effect leads to the same symbol with different delays overlaps and hard to detect the correct boundary of main path. Channel noises also reduce the reliability of coarse
symbol synchronization algorithms. The solution will be discussed in section.
≈≈≈≈
Fig. 3.8 Effect of multipath fading
3.2.1 Effect of Symbol Timing Offset
Before starting to introduce to coarse symbol synchronization algorithms, the effect of symbol timing offset must been derived. Symbol timing offset means the detected boundary does not locate on the true symbol boundary. This phenomenon is due to multipath effect, channel noise and aliasing. Two possible cases will occur, later or earlier than the true symbol boundary. Thanks to cyclic prefix, an earlier
Before starting to introduce to coarse symbol synchronization algorithms, the effect of symbol timing offset must been derived. Symbol timing offset means the detected boundary does not locate on the true symbol boundary. This phenomenon is due to multipath effect, channel noise and aliasing. Two possible cases will occur, later or earlier than the true symbol boundary. Thanks to cyclic prefix, an earlier