Performance Analysis of "WBC over DVB-H" Link Layer
© Zhanlin Ji et al. 2010
Received: 28 August 2009
Accepted: 15 December 2009
Published: 11 February 2010
This paper presents two novel smart cross-layer error-control coding schemes for improving the error protection of service advertisements that are broadcast to mobile terminals on wireless billboard channels (WBCs) established over a digital video broadcast-handheld (DVB-H) infrastructure. These are the smart section erasure (SSE) and smart transport stream erasure (STSE) schemes, which are jointly executed by the link layer and service layer cross-layer algorithms. The new schemes are analysed and compared to existing schemes. The solution enables the "WBC over DVB-H" system to operate with good flexibility, and in a more reliable way and with greater throughput efficiency than the standard IP datacasting supported in DVB-H.
Wireless billboard channels (WBCs) are novel infrastructural components of the emerging ubiquitous consumer wireless world (UCWW) [1–5]. UCWW is particularly matched to maximizing the efficient and economic use of the massive, and ever-growing, range of wireless communications interfaces, access networks' communications services and teleservices. It is particularly in harmony with the thrust to realize the user-driven always best-connected and best-served (ABC&S) communications paradigm [6, 7]. In it, the mobile user (MU) acts as a consumer and is identified by a personal, network-independent and location-independent, IPv6 address. The consumer is not constrained to any particular access network provider (ANP) and may use any available teleservice through any available access network that best matches consumer's needs at any time or place. For the use of services an MU pays through a trusted third-party authentication, authorization, and accounting service provider (3P-AAA-SP). The service providers (xSPs)—ANPs, value-added service providers (VASPs), and teleservice providers (TSPs)—can benefit from simplicity of having a single business agreement with 3P-AAA-SP and from the easier entry in the wireless communications market in order to succeed, and so forth.
In UCWW maximizing the volume of consumer wireless transactions, and not the number of subscriber contracts, is the primary business driver for service providers. The newly conceived WBC infrastructural component of UCWW aims to satisfy the requirement in this environment of facilitating the service providers "pushing" advertisements and information about their service offerings to potential consumers. Taking into account the potentially large number of services already available, efficient and easy-to-implement mechanisms for services' advertisement, discovery, and association (ADA), adapted to the terminal capabilities, user preferences, and user location, need to be developed for WBC.
A number of carrier technologies are suitable to carry the WBC dataset, for example, the digital audio broadcast (DAB), digital multimedia broadcasting (DMB), digital radio mondiale (DRM), digital video broadcast-handheld (DVB-H), multimedia broadcast/multicast service (MBMS), satellite DMB (S-DMB), digital audio radio satellite, and so forth. Among these technologies the DVB-H is an attractive solution due to its full support for IP datacasting (IPDC). The IPDC over DVB-H [8, 9] is a new European Telecommunications Standards Institute's (ETSI) standard which enables effective distribution of digital content to mass audiences. As commercial DVB-H services have already been rolled out, the market prospect of DVB-H devices seems very bright .
The rest of the paper is organized as follows. Section 2 provides an overview of the "WBC over DVB-H" link layer and describes the link layer's encapsulation, encoding, decapsulation, and decoding schemes. Section 3 explains the two new smart cross-layer encoding and decoding schemes. Section 4 focuses on the theoretical analysis, and Section 5 on a simulation to obtain performance results for "WBC over DVB-H" in a Rayleigh fading channel environment. Section 6 summarizes the conclusions.
2. "WBC over DVB-H" Link Layer
In 2004, the ETSI ratified the standard of DVB-H, which is an amendment of the digital video broadcasting-terrestrial (DVB-T) standard  and takes into account the specific properties of a typical portable mobile terminal (MT), such as limited battery-life, various signal environments, Doppler effect, propagation loss of indoor reception, limited antenna gain, and poor weather conditions. To enable an IPDC function, two new features were introduced into the DVB-H link layer, namely, time-slicing and MPE-FEC.
In DVB-T, all channels are multiplexed by means of an orthogonal frequency division multiplexing (OFDM) and broadcast in parallel. To receive data from a channel, MTs need to stay in active mode all the time, with no allowance made for the need to conserve battery power in the mobile unit. Responding to the normal MT requirement of efficient use of battery energy—maximizing battery inter-charge life— a time-slicing mechanism is used in DVB-H for arranging the data broadcasting in a burst mode.
With this mechanism MT remains most of the time in a sleep mode and becomes active only when a target burst is broadcast on air. A target burst is one likely to contain service description data of interest to the mobile user, as determined by his/her profile stored on the MT. The off-time between each burst is denoted by ETSI as (seconds) and the relative minimum time to the beginning of the next burst from the start of the present MPE (or MPE-FEC) section being received is delta-t. To successfully receive a data burst, MT should become active a little bit before the scheduled arrival of the target burst. This is to allow for variation in the actual arrival time from the scheduled arrival time due to the delta-t jitter (10 ms by default) and the synchronization-time (250 ms by default) .
The WBC burst duration is defined in  as
By comparison with a constant bit rate (CBR) channel, the power saving when employing the time-slicing mechanism could be calculated as in :
2.2. Multiprotocol Encapsulation-Forward Error Correction (MPE-FEC)
MPE-FEC is introduced in DVB-H to compensating for the performance degradations in wireless fading channels. The MPE-FEC frame is constructed as a table consisting of 255 columns and a number of rows (256, 512, 768, or 1024;  and Figure 2). Each cell in the table contains one byte. The 255 columns are divided in two parts: from 1st to 191st column–an application data table (ADT), and from 192nd to 255th column–a RS data table (RSDT). The IP packets are installed into the ADT column-wise, one by one. An RS code is used on each row of ADT for improving the link-layer error protection. Some parity columns are punctured to achieve different MPE-FEC code rates. Usually one DVB-H burst contains only one MPE-FEC frame. Each DVB-H burst carries a number of WBC segments.
2.3. Encapsulation and Encoding Algorithms
On the WBC service provider's (WBC-SP) side, the IP packets are cached on transition from the WBC service layer to the WBC logical link control (LLC) sublayer of the DVB-H link layer. When the number of IP packets in the cache becomes sufficient to fill the ADT, the IP packets are encapsulated into ADT in column-wise fashion, one by one. Than an RS (255,191) encoder runs on each row of ADT to generate 64 RS parity bytes, which are inserted into the corresponding FEC row. After the MPE-FEC frame is fully constructed, at the medium access control (MAC) sublayer, each IP packet in ADT and each column of RSDT are extracted for adding a MAC overhead, that is, 12B MPE/FEC header and 4B CRC-32 trailer (Figure 2). The resultant MPE/FEC data units are segmented into an MPEG-2 transport stream (TS) for broadcasting over the channel. Each TS packet consists of 4B header and 184B payload, and is encoded at the physical layer by using an RS (204, 188) code for further error protection.
Compared with the standard DVB-H link layer, our "WBC over DVB-H" link layer uses an additional 8B smart correct segment index table (CSIT), which is inserted at the end of the last WBC segment's padding area to help the decoder operate in an efficient way. CSIT includes the size of the erasure info table (EIT), used to store the decoding information, the number of WBC segments in one MPE-FEC frame, the advertisements delivery protocol's (ADP) parameters, and so forth.
Parameters of MPE/FEC and TS Headers.
2.4. Decapsulation and Decoding Algorithms
The encapsulation and encoding schemes are stated in the DVB-H standard . The design of the decoding method, however, is open.
In , an SE decoding scheme is suggested as follows. At theMAC sublayer, the MPE/FEC section is first generated from the received TS packets, and a CRC-32 code verifies it. If verification is successful, the MPE/FEC section is marked as "reliable"; otherwise it is marked for "erasure". Then the IP packet/parity section is extracted and together with the CRC verification result is sent to the LLC sublayer. At theLLC sublayer, the ADT/RSDT columns are generated from the received MPE/FEC sections one by one. The corresponding bits of EIT are filled with "0s" for "reliable" packets/columns and with "1s" for those marked for "erasure". When the MPE-FEC frame is fully reconstructed, the decoding algorithm checks the number of "1s" in the ADT part of EIT and the number of "0s" in the RSDT part of EIT. For any row, if the number of "1s" in the ADT part is smaller than the number of "0s" in the RSDT part, the MPE-FEC frame can be decoded with the RS algorithm; otherwise the decoding will fail.
To improve error protection, in  a TSE scheme is suggested. Compared with SE, at theMAC sublayer the TSE MPE/FEC section decoding does not depend on the CRC scheme. It uses a bit indicator—transport_error_indicator(TEI), located in the TS header—to identify whether the TS packet was or was not decoded successfully with the RS(204,188) at the physical layer. If TEI = 0, the decoding was successful. If TEI = 1, the decoding failed (this includes packet_IDentifier (PID) decoding failure and payload decoding failure). At theLLC sublayer, the corresponding TS packet position of EIT is filled with "0" or "1", and the decoding scheme is the same as in SE. As the TS payload is smaller than the MPE/FEC section's payload, the TSE scheme outperforms the SE scheme .
3. Smart Cross-Layer Encoding and Decoding Algorithms for "WBC over DVB-H"
3.1. Advertisements Delivery Protocol (ADP)
As the WBC segment is small in size, based on previous studies, the one-dimensional small-block RS codes are particularly well suited for ADP due to their characteristics, for example, good performance in correcting wireless burst errors, no "coupon collector problem", and so forth, .
The ADP decoding scheme includes packet erasure decoding (PED) and packet erasure plus byte error decoding (PE+BED) schemes.
The PE+BED scheme is explained in detail in .
3.2. Link Layer
Being not designed for WBCs with their specific characteristics in mind, the SE and TSE decoding processes are not efficient for direct use in WBCs. As one DVB-H burst (MPE-FEC frame) usually contains several WBC segments, even a single byte error in the MPE-FEC frame may cause a decoding failure. To improve the error protection in WBCs, the CSIT used by ADP for cross-layer decoding was developed.
The decapsulation process of the WBC smart cross-layer decoding is similar to that described in Joki and Paavola ; that is, it maps into a smart SE (SSE) and a smart TSE (STSE) schemes.
in the link layer:
if there are "1s" in corresponding EIT columns,
(a) extract i*x*n IP packets from the MPE-FEC
(b) decode with ADP in packet erasure mode.
if a WBC segment is decoded successfully,
(a) encode the recovered WBC segment
into IP datagrams with ADP packet
erasure scheme, and
(b) fix the corresponding MPE-FEC cells
and EIT table.
(c) try to run the RS algorithm on
the MPE-FEC frame.
if EIT contains errors,
if IP(good)<k, fill the ADP header reserved area (r)
forward IP packets to the service layer.
Comparing with SE and TSE decoding schemes, the new WBC smart cross-layer decoding schemes provide better error protection. On the other hand, due to the fact that the WBC smart decoding works with the ADP protocol, the time complexity is increased from to , where l is the number of lost packets. This, however, is acceptable because the complexity increases linearly.
3.3. Service Layer
To further improve the error protection, the PE+BED algorithm runs at the bottom of the service layer.
On the MU side, the decapsulation and decoding scheme is selected automatically by MT based on the MT's composite capability/preference profile (CC/PP) . MTs with very limited capabilities should select the SE decoding algorithm, whereas MTs with reasonable computing capabilities should select the STSE decoding algorithm.
4. IPER and SER Analysis
IPER and SER are important criteria for measuring the decoding performance for "WBC over DVB-H" link layer. In this section, the theoretical analysis of IPER and SER is performed by means of a one-state loss model (i.e., the bit/byte error rate follows a uniform distribution).
Total number of WBC segments in MPE-FEC frame
Number of RSDT columns in MPE-FEC frame
Bit error probability in binary symmetric channel (BSC)
Byte error probability in BSC
MPE column error probability in BSC
TS packet error probability in BSC
Number of rows of MPE-FEC frame
Number of columns of MPE-FEC frame being filled
To simplify the analysis, similarly to  the size of the IP packet is set to bytes (B) (i.e., close enough to the standard value for of 512). Each MPE-FEC column generates 3 TS packets with payloads of 171B, 184B, and 180B, respectively. The first TS packet includes the 12B MPE/FEC section's header and the 1B section start indicator, whereas the last TS packet includes the corresponding 4B CRC-32 code.
4.1. Smart Section Erasure (SSE)
The IPER after the SE decoding can be obtained as
Then the corresponding SER can be obtained as
After executing the smart cross-layer decoding algorithm (ADP PED) in the LLC sublayer, the link layer SER, IPER, and the corresponding byte error rate can be obtained, respectively, as
In the service layer, an ADP PE+BED algorithm is run if the received WBC segment contains errors. After completing the decoding process at the service layer, the service layer (SL) byte error rate can be obtained as
Then the corresponding IPER is
and the final SER can be obtained as
4.2. Smart Transport Steam Erasure (STSE)
With the SE decoding scheme, in order to reconstruct the MPE-FEC frame, at least TS packets must be received with no errors in the first 171 rows, next 184 rows, and in the final 180 rows, respectively. Then the MFER can be obtained as
The TS packet error rate (TSPER) after the SE decoding can be obtained as
Then IPER and corresponding SER can be obtained as
After running the smart cross-layer decoding algorithm (ADP PED) on top of the link layer, SER, TSPER, IPER, and the corresponding byte error rate can be obtained as
In the service layer, after completing the decoding process (PE+BED), the byte error rate is the same as in (9), and the corresponding TSPER is:
The final IPER and SER can be obtained as:
4.3. Comparison of Decoding Schemes
The analytical results show that the CRC-based decoding schemes (SE, SSE) are not better than the TS-based schemes (TSE, STSE) because the TS packet is shorter than the MPE/FEC section and thus more efficient for decoding. The smart cross-layer decoding schemes (SSE, STSE) are better than the ordinary decoding schemes (SE, TSE) because the number of erasure/error columns can be corrected by ADP PED at the LLC sublayer and by PE+BED at the service layer.
5. Simulation Analysis of "WBC over DVB-H" Link Layer in Rayleigh Fading Channel
5.1. Two-State Run Length Model (2SRL)
Previous studies have shown that errors in wireless channels occur in burst mode due to the multipath propagation [18–20]. In , a finite-state Markov model (FSMM) to represent Rayleigh fading channels in noninterleaving coding systems is introduced. In , a finite-state run length model for simulating the TS packet error behavior in a multipath channel environment is presented, but the model's parameters are obtained by unbiased estimators. In , a two-state run length model (2SRL) for analyzing the throughput in fading channels is proposed. However, the details of the fading channel and the manner of obtaining the parameters of the model are not explained. Despite the fact that the Rayleigh fading channel with a central chi-square distribution is known to be a flexible model that provides a definable number of degrees of freedom to experimental fading channel measurements for outdoor environments , there are very few studies for 2SRL over a central chi-square distribution Rayleigh channel. This subsection will focus on the received signal envelope having a central chi-square distribution and will use 2SRL to analyze the physical-layer TS packets outputting behavior.
The 2SMM model transition matrix is defined in  as
where b and g are the transition probabilities. The steady-state probability is
The average number of good states can be obtained as
whereas the average number of bad states can be obtained as
5.2. 2SRL Parameters in "WBC over DVB-H"
The probability density function of a central chi-square distribution is defined in  as
To get instantaneous SNR per symbol's PDF ,
Then PDF of SNR per symbol can be obtained as
The cumulative distribution function (CDF) of (24) is
Let be the maximum Doppler frequency, where is the speed of movement of the user terminal, is the carrier frequency, and is the speed of light. The expected number of times per unit interval (also known as the level crossing rate) that the received signal passes downward across level is given in  as
Since and are independent , could be obtained as
Finally, the error probabilities in state and were obtained as described in :
5.3. "WBC over DVB-H" PHY Simulation Testbed
PHY Testbed's Parameters.
Convolutional code rate
OFDM FFT size
Continual pilot carriers
5.4. Simulation Results
With the calculated values of 2SMM parameters, the run length probability distributions and of 2SRL can be calculated. Thus the 2SRL can easily represent the output behavior (good/bad TS packet output) for the "WBC over DVB-H" physical layer (i.e., "0" represents a good TS packet output and "1" represents a bad TS packet output). This way the data processing at the "WBC over DVB-H" link layer is reduced and the decoding algorithms running time is acceptable.
The Doppler Effect
As can be seen from the results, IPER/SER worsen when the Doppler frequency increases, especially for the SE scheme. To get IPER/SER below 1% (WBC datacasting requirement), the received signal value should be increased about 0.5–1 dB for TSE schemes and 1.5–2.5 dB for SE schemes for = 80 Hz comparing with = 10 Hz.
The Section-Based and TS-Based Effect
As the TS packet is smaller than the IP packet, a byte error in a TS packet will not mark the corresponding IP packet in error; thus the performance of TSE and STSE is better than SE and SSE. The results show that IPER/SER of the TS-based algorithms can gain 1-2 dB comparing with the section-based algorithms.
The Smart Cross-Layer Decoding Effect at the Link Layer
At the link layer, an ADP PED scheme is used on SE/TSE to improve reliability. The results show that, to get IPER/SER below 1%, the SSE/STSE gain is about 0.6-1 dB comparing with SE/TSE when = 10 Hz, and about 1-2 dB when = 80 Hz. The results also show that in order to get IPER/SER below 1%, the new STSE scheme can gain 2 dB comparing with SE when = 10 Hz and 3.5 dB when = 80 Hz.
The Smart Cross-Layer Decoding Effect at the Service Layer
With the use of the ADP PE+BED decoding scheme, the IPER/SER at the service layer can gain additional 0.5–1 dB comparing with the IPER/SER at the link layer.
The performance of the "WBC over DVB-H" system can benefit from DVB-H's time-slicing and multiprotocol encapsulation-forward error correction (MPE-FEC) features. This DVB-H's IPDC system design facilitates WBCs sharing bandwidth resources with other DVB-H services. This should help smooth the WBC standardization process. It also facilitates further actions to improve error protection of the WBC information, over and above that offered by MPE-FEC which itself is designed for particularly difficult reception situations.
Considering the commercial importance of the WBC services it is likely a high QoS will be required than the presently foreseen IPDC audio and video services. This paper has addressed techniques to improve error protection. Here the development of two novel smart cross-layer coding schemes based on the section erasure decoding and transport stream (TS) erasure decoding has been described. Based on an analysis of the error rate and on simulations using a wireless Rayleigh fading channel with received signal envelope having a central chi-square distribution, an approximation of the performance improvement has been obtained. A two-state packet-level run length model (2SRL) was introduced to evaluate the output statistics of MPEG-2 TS packets in a DVB-H fading environment. The results have confirmed that the newly proposed smart cross-layer decoding schemes can improve the error protection for the "WBC over DVB-H" system, over the MPE-FEC alone.
This publication has been supported by the Irish Research Council for Science, Engineering and Technology (IRCSET) and the Telecommunication Research Center, University of Limerick, Ireland.
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