We investigate the use of application layer FEC protection in DVB-T (Digital Video Broadcasting-Terrestrial) networks for the provision of mobile services. Mobile reception is characterized by variations of the received signal caused by fast fading and shadowing. DVB-T was originally designed for fixed and portable reception, and generally does not provide enough quality in mobile environments. The link layer protection mechanism MPE-FEC (Multi Protocol Encapsulation-Forward Error Correction) was standardized in DVB-H (Digital Video Broadcasting-Handheld) for the protection of mobile TV services. Although DVB-T itself does not incorporate any link or application layer protection mechanism, AL-FEC (Application layer Forward Error Correction) protection can be introduced in DVB-T in a backwards compatible way. By means of AL-FEC, it is possible to improve the robustness of DVB-T services for the provision of mobile TV. In this paper, we explain the concept of AL-FEC protection in DVB-T and evaluate its performance by means of laboratory measurements and dynamic simulations with shadowing. We study different configurations of AL-FEC and compare its performance with MPE-FEC. In this paper, we discuss some implementation aspects of AL-FEC in real scenarios and propose an implementation based on Raptor codes and hash sequences. We also present results obtained by a first AL-FEC prototype for DVB-T that demonstrates the feasibility of the approach.
Digital Terrestrial TV (DTT) networks are being deployed worldwide, and it is planned that DTT services completely replace analogue TV in many European countries by latest 2012. DVB-T (Digital Video Broadcasting-Terrestrial) [
The European digital mobile TV standard called DVB-H (Digital Video Broadcasting-Transmission System for Handheld Terminals) is a technological evolution of DVB-T, and was developed specifically for the provision of mobile TV services. DVB-H reutilizes the physical layer of DVB-T and introduces a set of enhancements in the link layer in order to adapt the transmission to mobile reception. These enhancements are aimed to reduce the terminal power consumption and counteract fast fading. A link layer protection mechanism called MPE-FEC (Multi Protocol Encapsulation-Forward Error Correction) [
AL-FEC (Application Layer Forward Error Correction) has been standardized in DVB-H for file delivery services. The advantage of AL-FEC is that it can spread the protection over large portions of information. AL-FEC takes advantage of the temporal diversity derived from user mobility by the use of extensive time interleaving (up to minutes or even hours), which increases the robustness of the transmitted information against fast fading and especially, against shadowing and signal outages. AL-FEC has been also proposed for DVB-H streaming services in the form of multi-burst protection [
Despite the fact that the physical layer of DVB-H is compatible with DVB-T, DVB-H encapsulates all the audio-visual information in IP (Internet Protocol) datagrams and generally simulcasts the services with lower quality than the MPEG-2 Transport Stream signal of DVB-T. Consequently, DVB-H requires the allocation of specific bandwidth for the transmission of the mobile TV content.
In some studies as well as even deployments, mobile reception of DVB-T has been verified. In order to enable mobile reception of current DVB-T services, antenna diversity techniques have been proposed. In [
In this paper, we study the use of AL-FEC protection in DVB-T in order to increase the robustness of the transmitted information and to achieve reception in mobile channels. Specifically, we introduce mechanisms that allow the transmission of the additional FEC data needed for error correction (similar to the MPE-FEC in DVB-H) in a backward compatible way, that is, in such a way that the legacy DVB-T receivers are not impacted by this additional FEC. We also evaluate the mobile performance of AL-FEC in DVB-T and provide direct comparisons with MPE-FEC. Although different codes may be used in order to provide AL-FEC protection for DVB-T services, in this paper we consider the use of Raptor codes [
The rest of the paper is organized as follows. In Section
Mobile channels are characterized by rapid variations of the received signal over time referred to as fast fading. Fast fading is caused by the Doppler shift of multiple propagation paths, which originate from the movement of the receiver with respect to the transmitter [
DVB-H reutilizes the physical layer of DVB-T, but integrates MPE-FEC at the link layer to repair the errors caused by mobile reception. MPE-FEC is an intra burst mechanism for which the protection is performed on a per burst basis [
The received signal in mobile channels is also characterized by slow variations known as shadowing. Shadowing results from the presence of large obstacles, such as buildings or hills that may block the line-of-sight between the receiver and the transmitter. Shadowing is generally modelled as a log-normal distributed variation of the received signal over the area of coverage [
For the integration of AL-FEC into DVB-T, the architecture according to Figure
Proposed system architecture.
The FEC stream generated by the FEC encoder consumes part of the bit rate capacity at the physical layer and thus, the number of services carried per MPEG-2 TS may have to be reduced in order to accommodate the FEC data. The main difference of the proposed approach with respect to DVB-H lies in the fact that the same multimedia content transmitted to fixed receivers is protected for its use by mobile users. Therefore, no additional content is needed for the transmission of mobile services and only the capacity required for carrying the FEC data must be taken into account to support mobile reception.
Based on this short system description and the presented requirements we will now further detail the different components of the proposed system, in particular the MPEG-2 Transport Stream protocol as well as the Forward Error Correction scheme and methods.
In DVB-T all the content is multiplexed in an MPEG-2 TS and transmitted as a sequence of TS packets [
Each ES is assigned a unique PID value inside the MPEG-2 TS. The associations between ESs and PID values are transmitted in the PSI/SI tables. DVB-T receivers parse the PSI/SI tables in order to identify the PID values of the ESs corresponding to the desired service. Then, the TS packets carrying the video, audio or data information of the service are demultiplexed by looking at the PID value of every TS packet.
FEC mechanisms are designed to cope with the loss of information by transmitting additional FEC data. Erasure codes are often used in FEC mechanisms as they can regenerate lost portions of information transmitted over an erasure channel. In order to do so, the information to be protected is partitioned into source blocks, each of them constituted by
The protection provided by FEC mechanisms depends on the code rate and the protection period. The code rate is the proportion of source data with respect to the total amount of information transmitted, accounting both source and repair data. The protection period refers to the duration of the information encoded in each source block. Figure
Example of the impact that the protection period holds on the error correction capabilities of FEC mechanisms.
The protection period has an impact not only in the protection provided by the FEC mechanism but also in the network latency and, most importantly, in the receiver latency and channel switching time. The network latency can be defined as the amount of time that passes from the moment the information enters the transmitter till the moment is delivered to the media decoders in the receiver. The channel switching time refers to the amount of time between the instant when the user switches to a new channel and the instant when the new content is displayed to the user. Although network latency is not essential for the majority of services, the channel switching time is considered as a crucial criterion in mobile TV user experience, and must not be increased beyond certain values. There is a trade off between the level of protection that can be offered in mobile reception and the channel switching time that the user experiences as disturbing. Increasing the protection period has also an impact on the memory requirements in the receiver, as at least the size of the source data contained in one protection period must be stored in order to perform the decoding.
The main idea of AL-FEC in DVB-T is to incorporate FEC protection by making use of erasure codes in a backward compatible way. In order to achieve this, the video and audio ESs must not be altered. Apart from the repair symbols, some additional information such as Source FEC Payload Identifier (ID) is necessary for the decoding process and must be passed to the receiver. For the purpose of connecting source and repair data, hash sequences of the source symbols are sent along with the repair symbols to provide this Source FEC Payload ID. By doing so, the source data is unmodified but the advanced receiver can still retrieve the necessary information by producing the same hash sequences. Moreover, the FEC data, including both the repair symbols and the hash sequences, must be encapsulated in a manner that ensures that legacy DVB-T receivers drop the TS packets carrying the FEC data without altering its proper operation. The MPEG-2 TS specification allows AL-FEC to be incorporated into the protocol stack of DVB-T in a transparent manner above the TS layer. The repair packets can be multiplexed into the MPEG-2 TS as another ES associated to the service, and will be discarded by the DVB-T receivers that do not incorporate AL-FEC. This is accomplished by assigning a specific new PID to the FEC elementary stream that is not recognized by the legacy receiver. By means of the TEI and the continuity counter fields in the TS packet header it is possible to detect erroneous or missing TS packets. As the source and repair packets are encapsulated in TS packets, the erasure of TS packets results in a symbol erasure channel. A source or repair symbol is considered erased if at least one of the TS packets carrying information of that symbol is lost. Longer source and repair packets are usually fragmented and encapsulated into multiple TS packets. As one erroneous or lost TS packet is sufficient to erase the entire source or repair packet, they tend to achieve a lower performance, especially in the presence of uncorrelated MPEG-2 TS packet errors.
In this paper we consider the use of Raptor codes [
In Figure
Comparison between the protection arrangement of MPE-FEC in DVB-H and AL-FEC in DVB-T.
Another limitation of MPE-FEC that can be overcome by AL-FEC is the dependency between code rate and protection period. Due to the nature of Reed Solomon encoding, in order to achieve different code rates other than the mother code rate (
The simulations have been performed assuming a DVB-T physical layer configuration of FFT 8K, guard interval
For the MPE-FEC evaluation, we have assumed a DVB-H service with a cycle time of 2 seconds where all the transmitted IP datagrams have a constant size of 512 bytes. The number of rows of the MPE-FEC frame has been configured to 512, and thus, each IP datagram fits exactly in one column of the MPE-FEC frame. The amount of columns of IP information and FEC data were configured according to each particular code rate.
The mobile performance has been evaluated by means of TU6 laboratory measurements. The laboratory measurement setup consists of a DVB-T modulator, a signal generator in charge of emulating the TU6 channel model and a DVB-T measurement system capable of recording the error data at the TS layer. By recording the error data at the TS layer it is possible to emulate the performance of FEC mechanisms in upper layers. The measurements were obtained in a range of CNR values from 0 to 30 dB and in a range of Doppler values from 5 to 80 Hz.
In order to study the influence of the shadowing, we have assumed a user moving at constant velocity across a log-normal CNR map defined by its standard deviation and correlation distance. The user velocity is given by the values of Doppler and frequency carrier configured in the simulations. The shadowing model outputs instantaneous CNR values that correspond to 100 millisecond time intervals. The instantaneous CNR values along with the Doppler determine the measured error data in each one of the 100 ms time intervals that will be passed to the link layer.
We have defined two different scenarios: a high diversity scenario with a correlation distance of 20 meters and a Doppler frequency of 80 Hz (which correspond to 144 km/h at 600 MHz), and a low diversity scenario with a correlation distance of 100 meters and a Doppler frequency of 10 Hz (which correspond to 18 km/h at 600 MHz). In both cases the standard deviation has been set to 5.5 dB which is the usual value employed in mobile TV planning for outdoor reception. The results have been averaged over 100 seeds of shadowing.
We have employed the PER (Packet Error Ratio) in order to measure the performance of both MPE-FEC and AL-FEC. The PER is defined as the percentage of packets in which there is at least one error. In the case of MPE-FEC, a packet corresponds to a MPE section (in which an IP datagram is encapsulated), whereas in the case of AL-FEC a packet corresponds to a source packet. Although this is not a one-to-one comparison it is a reasonable way to compare the performance of both mechanisms. We have considered a PER value of 1% as QoS criterion, which is more demanding than the MFER (MPE-FEC Frame Error Ratio) value of 5% generally employed in DVB-H evaluations.
First, we compare the mobile performance of DVB-T and DVB-H in Figure
Mobile Performance of DVB-T (a) and DVB-H (b) services with AL-FEC in a TU6 channel.
As can be seen in the figures, if no link or application layer protection is provided in DVB-H and DVB-T, both systems achieve a very similar performance. DVB-T and DVB-H employ the same physical layer, and the performance difference is due to the source packet size (528 bytes in the case of MPE-FEC and 184 bytes in the case of AL-FEC). As it is shown in the figures, the performance degradation due to the packet size is more severe for high Doppler values.
When MPE-FEC and AL-FEC are enabled, the performance of DVB-H and DVB-T increases considerably, achieving important gains in terms of CNR threshold. The gain obtained by MPE-FEC for the lowest code rate is up to 2 dB in the lower range of Doppler and up to 6 dB in the higher range. The performance difference between the lowest and highest code rates is around 2 dB. These values correspond quite well with the results presented in [
The gain obtained by AL-FEC is up to 6 dB in the entire range of Doppler. This represents an additional gain of 4 dB with respect to MPE-FEC in the case of low Doppler. This gain is originated by the longer effective protection period of AL-FEC. Despite the fact that MPE-FEC is configured with a cycle time of 2 seconds, the effective protection period is equal to the burst duration. With the configuration employed in the DVB-H service, the burst duration is approximately of 100 ms for the code rates
Figures
Performance of AL-FEC in a TU6 channel configured with 10 Hz (a) and 80 Hz (b) of Doppler.
Figure
In Figure
Performance obtained by AL-FEC (a) in the low diversity scenario (correlation distance of 100 m and Doppler of 10 Hz) and (b) in the high diversity scenario (correlation distance of 20 m and Doppler of 80 Hz).
MPE-FEC performance is not presented as the results are approximately the same as with an AL-FEC configuration of 0.1 seconds. The results show the little impact of the protection period in low diversity conditions. In the low diversity scenario, there is almost no gain when increasing the protection period, especially in the case of the less robust code rates. Increasing the protection period from 0.1 to 15 seconds only yields a gain between 1 and 2 dB depending on the code rate. On the contrary, increasing the protection period from 0.1 to 15 seconds provides a gain from 4 to 10 dB in the high diversity scenario.
In this paper we have evaluated so far the performance of an ideal AL-FEC implementation in DVB-T. In a practical implementation some additional issues need to be addressed. A main issue is the transmission of the additional information required in reception for the decoding process. As already mentioned, we apply a scheme using hash sequences along with the repair symbols. The hash sequences represent at most a 10% of the total FEC data, so some very minor additional overhead must be taken into account in the configuration of the AL-FEC mechanism when compared to the results from above.
Another issue is the practical size of the source packets. Depending on the limitations of the hash mechanism employed, mainly the maximum number of source and repair packets per source block, the size of the source packets may have to be larger than 184 bytes. In this case, the loss of a single MPEG-2 TS packet results in a drop of more information in the source block. The simulation results have shown how the presence of isolated erroneous TS packets is more common at high Doppler values whereas it is less problematic at the low and medium range of Doppler.
An AL-FEC prototype for the protection of DVB-T services has been developed as a result of the collaboration between the Universidad Politécnica de Valencia and Digital Fountain. The prototype employs Raptor codes integrated into an IPTV streaming framework and operates with a source packet size of 1288 bytes and a repair packet size of 1472 bytes that corresponds exactly to a payload of 7 and 8 MPEG-2 TS packets, respectively. Figure
Mobile Performance of DVB-T services with a practical implementation of AL-FEC in a TU6 channel
The implementation of long protection periods is problematic in terms of channel switching time and memory requirements. As it has been explained, longer protection periods involve higher requirements of memory in the user terminals along with an increase in the channel switching time experimented by the user. Assuming a DVB-T typical service of 2.5 Mbps and a protection period of 10 seconds, a minimum memory capacity of 3 MB is required in order to store all the information of one source block. On the other hand, the use of computationally efficient erasure codes like Raptor avoids the implementation of dedicated hardware and allows the incorporation of AL-FEC as a software update.
Longer protection periods also have an important impact in channel switching time. Long channel switching times can degrade the user experience, and must be taken into consideration. Values of less than 0.5 seconds are not perceived by the user whereas values up to 2 seconds are considered to be tolerable [
The available bandwidth for FEC data in DVB-T transmissions is also an important issue for the utilization of AL-FEC in existing DVB-T systems. The maximum amount of FEC data that can be multiplexed in an MPEG-2 TS depends on the available bandwidth and is limited. Existing DVB-T networks were not planned with mobile reception in mind and no specific bandwidth is assigned for the carriage of FEC data. Despite of this, it is possible to multiplex a limited amount of FEC data by means of the null TS packets generally present in an MPEG-2 TS. DVB-T transmitters insert null TS packets filled with stuffing data in the MPEG-2 TS in order to maintain a constant bit rate at the physical layer. We have performed preliminary studies in the MPEG-2 TS used in actual German and Spanish DVB-T transmissions to estimate the amount of null TS packets. The results reveal that depending on the multiplex, a percentage between 2% and 11% of the MPEG-2 TS corresponds to null TS packets that can be replaced by TS packets carrying FEC data.
DTT networks generally transmit several TV programs per MPEG-2 TS. Because of the limitation in the amount of available bandwidth to accommodate the FEC data, it may be desired to protect only few of the programs transmitted in an MPEG-2 TS. It is also possible to encode only a set of the video frames in order to increase the efficiency of the FEC data. Mobile reception in handheld terminals is generally performed with small displays that do not require the high frame rates of DVB-T services. The protection of Intra (I) and Predicted (P) frames only can increase the efficiency of the FEC data in a 33% without major penalties in the user experience (B frames usually represent
Our results have shown that the AL-FEC protection of DVB-T services is capable to obtain gains of 6 dB in mobile channels. However, other considerations apart from the fast fading must be taken into account in the planning of DVB-T networks for the provision of mobile TV.
While fixed reception is generally performed with high gain antennas located in the roof of the buildings, mobile reception is characterized by the reception at ground level and the use of low gain antennas. The added penalization of the height loss and the use of mobile antennas represent more than 20 dB if external antennas are used and more than 27 dB if integrated antennas are used instead [
On the other hand, DVB-T networks planned for portable reception like in Germany, take into account the penalization due to the height loss and lower gain antennas. However, in order to provide the necessary bit rate for DVB-T services, DVB-T networks normally operate with higher modulation orders and less robust physical layer code rates than mobile TV networks such as DVB-H. Additionally, portable reception in DVB-T networks is planned for a Rayleigh channel model, which is approximately between 5 and 10 dB less demanding than mobile channels like the TU6 channel model. The combined gain of AL-FEC and antenna diversity techniques can be used in DVB-T networks deployed for portable reception in order to provide mobile DVB-T services with a similar coverage area to that of fixed DVB-T services.
In this paper we have investigated the use of AL-FEC for the provision of mobile DVB-T services. AL-FEC can be implemented in a backward compatible way and can be used in existing networks and services to extend the mobile coverage of DVB-T services. AL-FEC protection can be used in conjunction with antenna diversity techniques and hierarchical modulations in order to further enhance the vehicular reception of DVB-T services in existing networks. At the same time, newly deployed networks can be planned for the simultaneously provision of fixed and mobile DVB-T services.
The protection provided by AL-FEC depends not only on the proportion of FEC data transmitted along with the service information but also on the protection period. Long protection periods take advantage of the temporal diversity derived from user mobility, and achieve a better protection in the presence of shadowing.
We have provided direct comparisons between the AL-FEC and MPE-FEC protection of DVB-T and DVB-H services in a TU6 channel. The results show that AL-FEC achieves an additional 4 dB gain with respect to MPE-FEC in the low range of Doppler. The additional gain at low Doppler values is motivated by the fact that AL-FEC can be configured with higher protection periods than MPE-FEC while having a little impact in the channel switching time. Long protection periods have been also investigated in TU6 channels affected by shadowing. The results show that the gain derived from long protection periods is heavily conditioned to the temporal diversity. A protection period of 10 seconds can provide a gain of 10 dB in high diversity scenarios. The use of longer protection periods involves higher memory requirements along with an increment in the channel switching time. The use of memory efficient decoding algorithms can solve the memory problems whereas fast channel switching techniques may decrease the channel switching time perceived by the user.
The implementation of AL-FEC in DVB-T presents a series of practical issues. The size of the source packets and the amount of additional information that must be transmitted have an important impact in the performance of the system. Current implementations of AL-FEC in DVB-T can loss about 2 dB with respect to the ideal implementation, especially at high velocities. The amount of bandwidth available in current DVB-T transmissions in order to accommodate FEC data is also an important issue. Several possibilities such as the protection of only a few services per MPEG-2 TS have been discussed in order to increase the efficiency of the transmitted FEC data.