Nomenclature
AbbreviationExpansionAC | Atenna Combination |
ACS | Adaptive Channel Selection |
AQAM | Adaptive Quadrature Amplitude Modulation |
AS | Antenna Set |
ASU | Antenna Selection Unit |
AVC | Advanced Video Coding |
BbB | Burst-by-Burst |
BER | Bit Error Ratio |
BL | Base Layer |
BLAST | Bell-Labs Layered Space-Time |
BPCU | Bit Per Channel Use |
CC | Convolutional Code |
CIF | Common Intermediate Format |
CND | Check Node Decoder |
CRA | Clean Random Access |
CRC | Cyclic Redundancy Check |
EEP | Equal Error Protection |
EL | Enhancement Layer |
e-PLR | Equivalent PLR |
EWF | Expanding Window Fountain |
FEC | Forward Error Correction |
FPS | Frame Per Second |
GOP | Group Of Pictures |
HEVC | High Efficiency Video Coding |
IDR | Instantaneous Decoding Refresh |
IL | interlayer |
IL-FEC | IL operation-aided FEC |
JCT-VC | Joint Collaborative Team on Video Coding |
LDPC | Low Density Parity Check |
LLR | Log-Likelihood Ratio |
Log-MAP | Logarithmic Maximum A Posteriori |
LSSTC | Layered Steered Space-Time Coding |
LT | Luby Transform |
MIMO | Multiple-Input Multiple-Output |
MPEG | Moving Picture Experts Group |
MS-STSK | Multi-Set STSK |
OFDM | Orthogonal Frequency Division Multiplexing |
PLR | Packet Loss Ratio |
PSNR | Peak SNR |
QCIF | Quarter CIF |
RA | Receive Antenna |
RF | Radio Frequency |
RSC | Recursive Systematic Convolutional |
SBC | Short Block Code |
SHVC | Scalability extension of HEVC |
SM | Spatial Modulation |
SNR | Signal-to-Noise Ratio |
SSK | Space-Shift Keying |
STBC | Space-Time Block Code |
STSK | Space-Time Shift Keying |
SVC | Scalable Video Coding |
TA | Transmit Antenna |
TC | Turbo Code |
UEP | Unequal Error Protection |
V-BLAST | Vertical BLAST |
VND | Variable Node Decoder |
Introduction
Driven by the emergence of immersive mobile telecommunication devices, the associated data rate demands have been soaring [1]. Nevertheless, flawless lip-synchronized video transmission in error-prone wireless environments remains a challenge [2]. Explicitly, the unpredictable wireless channel results in high packet loss ratios in wireless video transmission. To satisfy the challenging bit rate requirement of video transmissions, a number of video compression standards have been ratified, such as the H.261 [3], H.262 [4], H.263 [5], Moving Picture Experts Group (MPEG)-4 [6], and H.264/Advanced Video Coding (AVC) [7] standards, as well as the newest H.265/High Efficiency Video Coding (HEVC) [8] standard, which was developed by the Joint Collaborative Team on Video Coding (JCT-VC). The H.265 standard is gradually replacing the H.264 standard as the mainstream coding standard.
A. Scalable Video Coding
However, the afore-mentioned traditional video compression techniques fail to robustly operate under unreliable network conditions in the era of the Internet and of existing radio networks. More explicitly, the Internet based communications are devised to provide the best effort delivery for data, which, however, is incapable of guaranteeing the reliability of the link, hence leading to occasional packet loss due to jitter or network congestions [9]. These unpredictable network fluctuations may severely deteriorate the experience of the clients by requiring more buffering time or directly corrupting the reconstructed image quality. Scalable Video Coding (SVC) has fascinated researchers for more than 20 years, as a potential solution for enhancing the system’s spectral efficiency, ever since the H.262 standard was conceived [10]. However, the construction of scalable profiles has remained an open challenge due to their limited coding efficiency as well as owing to their considerable decoder complexity, until the H.264 scheme was developed, which finally significantly improved the video compression capability attained [7]. Furthermore, scalable video techniques are also employed by the HEVC standard, which are known as the Scalability extension of HEVC (SHVC) [11]. The scalability in SVC can be spatial, temporal and quality based, where the sub-layers of the bit stream in the spatial and temporal domains exhibit different spatial resolution and frame rate, respectively. On the other hand, the quality scalability usually relies on the same spatial and temporal configurations, while differing in terms of the associated image quality. The above-mentioned interlayer correlations become favourable, when the same source content is required by different clients at different resolution or frame rate for example.
A layered video scheme is shown in Figure 1, where the video sequences captured are encoded with the aid of one or more scalability functionalities into the four layers of Figure 1 with the aid of a scalable video encoder. The four layers seen in the example shown in Figure 1 include the Base Layer (BL) and three Enhancement Layers (EL). In Figure 1, the upper layer is encoded progressively with reference to the lower layers. For example,
B. Unequal Error Protection for Video Streaming
The afore-mentioned multilayer correlation may benefit from Unequal Error Protection (UEP), which offers a stronger protection for the BL than for the ELs [10]. The concept of UEP can be satisfied by carefully allocating resources, such as the coding rate of Forward Error Correction (FEC), the transmit power, the number of bits/symbol in modulation scheme etc. [12]. Furthermore, the first EL can be allocated a higher-quality coding technique than the second EL, assuming that the latter is coded depending on the first EL. Since the conception of UEP was proposed by Masnick and Wolf [13], numerous FEC based UEP capabilities have been investigated, relying for example Convolutional Codes (CC) [14] and Low Density Parity Check (LDPC) codes [15], just to have a few. Furthermore, UEP Turbo Codes (TC) [16], concatenated Short Block Codes (SBC) and Recursive Systematic Convolutional (RSC) Codes [17] as well as Fountain Codes [18] were also studied.
Conventionally, the process of communications is accomplished by the cooperation of all OSI layers [19], where the data sources, such as video and audio, are processed commencing from the Application Layer, progressing to the lower layers. Hence, the UEP scheme employed for transmitting the video stream can be implemented using a single layer or multiple layers, relaying on cross-layer operation, which may require explicit signaling from the upper ISO layers passing down information, such as the number of layers used for layered video streaming, in order to accomplish UEP.
Numerous UEP designs have been conceived for video transmissions in the higher layer with the aid of the FEC codes, such as Raptor codes [20]. Vukobratovic et al. [21] proposed a novel scalable multicast system using the Expanding Window Fountain (EWF) codes for transmitting scalable video streaming over hostile channels exhibiting packet loss events, where the ELs conveyed parity information embedded into them for protecting the more important BL. Ahmad et al. [22] advocated a Luby Transform (LT) coded UEP scheme for scalable video communications, which exhibited a lower Bit Error Ratio (BER) and a low overhead at the expense of an increased coding complexity. Hellge et al. [23] designed a Raptor coded cross-layer operation aided scheme for layered video transmission, which implants the bits of the BL into the ELs in order to recover the lost bits of the BL from the ELs. The enhanced Pro-MPEG COP3 codes were developed by Diaz et al. [24] for double-layer video transmission, where the BL packets are interspersed with the repair packets of the EL for improving the recovery capability of the protection scheme.
Additionally, a number of potent contributions employing UEP for improving the video quality have been conceived for the physical layer, including the channel coding and modulation schemes. Ha and Yim [25] proposed a metric for quantifying the layered video distortion in order to adaptively assign UEP and hence to minimize the error propagation effects imposed by the packet loss events. Moreover, with the aid of using different punctured TCs, Marx and Farah minimized the mean video distortion in order to enhance the quality of video transmission over wireless networks [26], where the redundancy imposed on the compressed stream is non-uniformly distributed between the consecutive video frames. The specific importance of the macroblocks and of the video frame type were taken into account in [27] to allocate different-rate CCs to the video stream. Nasruminallah and Hanzo [17] used RSC codes to achieve UEP for supporting data-partitioned aided AVC video streaming. Huo et al. [28] proposed a RSC coded interlayer (IL) operation-aided FEC (IL-FEC) technique that implants the bits of the BL into the ELs with the aid of taking their modulo-2 addition, where the iterative decoding is invoked for exchanging extrinsic information between two layers when the BL is not successfully decoded. In contrast to channel coding providing UEP, an adaptive hierarchical Quadrature Amplitude Modulation (QAM) based mapping algorithm was proposed in [29] to provide UEP for video streaming by Chang et al. [27], where UEP was achieved by varying the Euclidean distance of 16QAM constellation points according to the encoded video frame type. Xiao et al. [30] designed a cooperative Multiple-Input Multiple-Output (MIMO) scheme relaying on sophisticated power control aided SVC transmissions, where variable-rate LDPC codes and Space-Time Block Codes (STBCs) were employed for providing UEP. Li et al. proposed an Orthogonal Frequency Division Multiplexing (OFDM) assisted hierarchical QAM based UEP arrangement for transmitting appropriately partitioned AVC streams, which avoided mapping important information onto these OFDM sub-carriers, while experienced deep fading [31].
Furthermore, numerous cross-layer aided UEP schemes were also proposed for video streaming, which optimized the video quality with the aid of exchanging signaling across multiple OSI reference layers, including the source compression, channel coding and retransmission technique etc. For example, the data link layer provides both error control and flow control, while the network layer mainly deals with the routing issues. Hence, the collaboration of different layers may be expected to attain higher video quality improvements than a single layer does. Van Der Schaar et al. [32] designed an application/transport/MAC/physical layer based UEP scheme for transmitting the video streams, which dynamically adapts the parameters of the application-layer FEC scheme, the maximum MAC retransmission limit and the size of the packets in order to strike a tradeoff between the throughput, reliability and delay. Huusko et al. [33] proposed a cross-layer operation aided method for transmitting the control information and for optimizing the overall multimedia quality over both wireless and wired IP networks. An LDPC code assisted joint source and channel coding scheme was conceived in [34] for layered video streaming, where the source rate and the channel coding rate were optimally allocated according to the available bandwidth and to the average Packet Loss Ratio (PLR). Furthermore, an application/MAC/physical cross-layer structure was proposed by Khalek et al. [35], which optimized the perceptual quality of layered video streaming. Tseng and Chen [36] devised an optimized application/physical cross-layer allocation scheme for multi-user uplink transmission by designing an objective function to maximize the average Peak Signal-to-Noise Ratio (SNR) (PSNR).
One of the major contributions to FEC based SVC streaming techniques is the IL-FEC technique of [28]. The IL-FEC technique significantly improves the performance of a single IL-FEC protected layer,1 which however restricts the flexibility of its employment since the BL always enjoys priority, when invoking unequal protection. In other words, this technique cannot be readily applied to the ELs. One of our contributions is that we improve the BL performance and maximize the overall benefits of IL-FEC by exploiting the systematic bits of the reference layer.
Apart from the FEC scheme providing UEP, transceiver-based UEP has also been explored in the literature [30], [37]–[39]. Song and Chen [37] proposed an Adaptive Channel Selection (ACS) based MIMO system that transmits SVC signals, in which each bit stream is periodically switched between multiple antennas and the higher-priority video layer’s bit stream is mapped to higher-SNR channels. Additionally, a joint UEP scheduling scheme that considers both the FEC redundancy and the diversity gain of MIMO systems was proposed in [39]. Generally, transceiver based UEP techniques are usually realized by controlling the modulation mode or by selecting the best sub-channel for conveying the high-priority video bits in order to improve the attainable overall performance.
C. Multi-Set Space-Time Shift Keying
The popularity of transceiver-based UEP can also be attributed to the evolution of advanced wireless communications services, which fuelled the development of MIMO techniques for creating reliable high-rate links [40]–[42]. More explicitly, MIMO techniques are capable of enhancing the multiplexing gain by invoking the Bell Laboratories Layered Space-Time (BLAST) architecture [43], or the diversity gain by STBCs [44]. Alternatively, a combination of both gains can be attained by Layered Steered Space-Time Coding (LSSTC) [45] that combines the benefits of Vertical BLAST (V-BLAST) and STBCs techniques. Spatial Modulation (SM) advocated in [46] is capable of providing a high normalized throughput at the expense of low complexity. Since only a single antenna is activated, which is selected from multiple antennas, only a single Radio Frequency (RF) chain is required. A concept referred to as STSK was proposed in [47], where instead of activating the indexed antennas, one out of
In this treatise, we exploit for UEP video streaming that the three components2 of MS-STSK exhibit different BER performances.
D. Adaptive System for Video Streaming
It has been widely acknowledged that the radio channel imposes a severe challenge on reliable high-speed communication owing to its susceptibility to noise, interference, as well as dispersive fading environments [52]. For a fixed configuration wireless system, although the transmitters are expected to provide a sufficiently high signal level for the far-end receivers, due to fading the instantaneous received signal power fluctuates and routinely falls below the sensitivity of receivers. Hence the information cannot be decoded correctly [53].
In real-time video transmission, this discontinuity is even more obvious. Fortunately, numerous near-instantaneously adaptive techniques have been conceived for improving the robustness of the wireless system [54] by providing users with the best possible compromise amongst a number of contradicting design factors, such as the power consumption of the mobile station, robustness against transmission errors and so forth [55]–[57]. In order to allow the transceiver to cope with the time-variant channel quality of narrowband fading channels, the concept of Adaptive QAM (AQAM) modem was proposed by Steele and Webb, which provides the flexibility to vary both the BER and the bit rate to suit a particular application [58]. AQAM-aided wireless video transmission was conceived for example in [55] and [58], where the Burst-by-Burst (BbB) AQAM assisted system provides a smoother PSNR degradation when channel SNR is degraded. As a benefit, the subjective image quality erosion imposed by the video artefacts is eliminated and hence the dramatic PSNR degradation of the conventional fixed modulation modes is avoided by the adaptive system.
Furthermore, numerous adaptive systems have been devised for SVC streaming with the aid of UEP, which aim for judiciously allocating resources to strike the best compromise between the reconstructed video quality. Song and Chen [59] proposed a sophisticated power allocation scheme for attaining the maximum throughput. Li et al. [38] designed a scalable resource allocation framework for SVC streaming over MIMO OFDM wireless networks in a multi-user scenario, where the time-frequency resources, the modulation order and the power were adaptively allocated to the users in order to grant them at least a basic viewing experience. Additionally, an Historical Information Aware UEP scheme was conceived in [60] for SHVC video streaming, where the objective function of the current video frame was optimized based on the historical information of its dependent frames in order to adaptively adjust the coding rates of the RSC codes and hence to improve the reconstructed video quality.
As a further advance, Xu et al. [61] conceived an adaptive application/MAC layer based cross-layer protection strategy to convey the layered video stream by dynamically selecting the optimal combination of application-layer FEC and MAC retransmissions according to the near-instantaneous channel states, which resulted in a more graceful video quality degradation across a wide range of channel conditions. Zhang et al. [62] designed an application/MAC/physical layer based adaptive cross-layer video streaming scheme, where the resources were judiciously shared between the source and the channel coders with the aid of the minimum-distortion or minimum-power criterion according to the prevalent channel states. Xu et al. [63] designed link/physical cross-layer based adaptive rate allocation schemes for layered video transmission over wireless Rayleigh channels, which substantially improved the video throughput. A multimedia home gateway was put forward for three-screen television in [64], which dynamically controlled both the Raptor FEC overhead and the layer-switching aided SVC streaming.
E. Our Contributions
Against these backgrounds, we conceive a physical layer based powerful UEP scheme for transmitting SHVC streams over narrowband Rayleigh fading channels, where a novel MS-STSK transceiver is employed for the sake of attaining both multiplexing and diversity gains. Furthermore, the adaptive IL-FEC is designed for alleviating the image quality degradations imposed by the fluctuating wireless channel.
Inspired both by the fact that the EL becomes useless without successfully decoding all the layers it depends on and by the fact that MS-STSK provides different BER performances, we specifically design the MS-STSK transceiver of Figure 2 to conceive an UEP scheme for mitigating the BL corruption probability. Furthermore, an improved cross-layer design is conceived for adaptively adjusting the level of protection according to the near-instantaneous channel SNR for ensuring that every bit is likely to be error free. The novelty of this treatise can be summarized as follows:
We propose an adaptive IL-FEC aided MS-STSK system, which exploits for the first time the UEP MS-STSK sub-channels.
Furthermore, we provide design guidelines for beneficially configuring MS-STSK, which is achieved by carefully adjusting the number of bits input to the three different-sensitivity MS-STSK components, namely to the ASU, to the
QAM/PSK modulator and to the dispersion matrix generator. Explicitly, we show that at a given throughput the BER of the bits fed into the ASU is better than those of the classic modulator in the STSK block, while the bits conveyed by the choice of the dispersion matrices show the worst BER performance.L- We enhance the IL-FEC technique. Explicitly, instead of protecting the BL by implanting its systematic bits into the ELs as in the IL-FEC technique recently proposed in [28], we further improve it.
Finally, we demonstrate for a triple-layer scalable video scenario that our adaptive system exhibits a more graceful video quality degradation as the wireless channel degrades in comparison to the three fixed-mode constituent schemes. This prototype system may be readily extended to other IL-FEC MS-STSK scenarios.
System Architecture
In this section, we introduce our proposed MS-STSK-assisted adaptive system, which is amalgamated with our IL-FEC technique conceived for wireless layered video transmission. In Table 1 we define the notations used in the remainder of the paper. We continue by introducing the MS-STSK and IL-FEC principles followed by our proposed transmitter and receiver architecture.
A. MS-STSK
Again, the MS-STSK transmitter of Figure 2 consists of two basic components, namely the ASU and the STSK block, where the latter contains a dispersion matrix generator and a classic
Figure 4 shows the MS-STSK BER performance for various configurations, when transmitting over narrowband Rayleigh fading channels. The notation MS-STSK
BER performance of MS-STSK under various configurations at a fixed throughput of
In Figure 5, we show the BER performance difference of the MS-STSK transceiver components. Explicitly, Figure 5 depicts the BER performance of the three MS-STSK components using three different sets of configurations. It can be seen in Figure 5 that the ASU is capable of attaining a lower BER than the
B. Conventional IL-FEC
Figure 6(a) shows the encoder of the IL-FEC technique of [28], where three bit streams
At the decoder shown in Figure 6(b), there are six inputs, \begin{align*}&\hspace {-2pc}L(u_{1}\oplus u_{2}) \\=&\log \frac {1+e^{L(u_{1})}e^{L(u_{2})}}{e^{L(u_{1})}+e^{L(u_{2})}} \\\approx&sign(L(u_{1}))\cdot sign(L(u_{2}))\cdot \min (|L(u_{1})|,|L(u_{2}|) \tag{1}\end{align*}
under the additional rules of:\begin{equation*} L(u)\boxplus \pm \infty =\pm L(u) \qquad \quad L(u)\boxplus 0=0.\tag{2}\end{equation*}
Thus, in order to generate the soft information representing
Since the systematic bit streams
The soft information
is fed into VND 0 of Figure 6(b). Since at this stage no extrinsic information is provided by the CNDs,y_{s_{0}} is simply forwarded to VND 1 asy_{s_{0}} . Then FEC Decoder 0 generates the extrinsic informationL_{a}(s_{0}) , which also takes into account the received soft parityL_{e}(s_{0}) , which is passed back to VND 0 via VND 1.y_{p_{0}} At this stage, VND 0 of Figure 6(b) updates
with the aid of extrinsic information and feedsy_{s_{0}} to CND 0 and CND 1, hence enabling them to generate the a priori informationL_{e}(s_{0}) andL_{a}(s_{1}) by additionally taking into accountL_{a}(s_{2}) andy_{s_{01}} , respectively. FEC Decoder 1 and Decoder 2 of Figure 6(b) receive soft information ofy_{s_{02}} andL_{a}(s_{1}) as well as their associated parity streamsL_{a}(s_{2}) andy_{p_{1}} in order to generate the extrinsic informationy_{p_{2}} andL_{e}(s_{1}) , respectively, which is then returned to the CND 0 and CND 1 for updating and enhancing the a priori information ofL_{e}(s_{2}) .L_{a}(s_{0}) Assisted by the extrinsic information passed to it by CNDs, VND 0 of Figure 6(b) updates the a priori information furnished for
and therefore the confidence of the systematic bitss_{0} is enhanced, resulting in an improved BER performance, when the first iteration is completed. The iterative decoding process continues until the maximum number of iterations is reached.s_{0} When the affordable number of iterations is exhausted, the decoded bit streams
,\hat {l_{0}} and\hat {l_{1}} are generated by VND 1, VND 2 and VND 3 of Figure 6(b), respectively. To obtain\hat {l_{2}} , VND 1 of Figure 6(b) adds up\hat {l_{0}} gleaned from VND 0 andL_{a}(s_{0}) arriving from FEC Decoder 0, whileL_{e}(s_{0}) and\hat {l_{1}} are generated by VND 2 and VND 3 of Figure 6(b), respectively.\hat {l_{2}}
Huo et al. [28], [67] have proven that by iteratively repeating the above decoding phases, the BER performance can be significantly improved, which in turn dramatically improves the decoded image quality in terms of PSNR as well. The iterations are terminated as long as the BL is successfully decoded or the iterations reach the maximum number.
C. Proposed Transmitter Model
At the transmitter shown in Figure 8, the captured video source
Architecture of proposed MS-STSK aided adaptive system for scalable video streaming.
The (a) Encoder and (b) Decoder of the IL-FEC technique for
The protection modes conceived for the adaptive IL-FEC selection unit are described as follows:
: In order to protect the BL, the IL based protection applied toMode 0 in our system is identical to that proposed in [67] and [68], as shown in Figure 9(a), where the dotted line indicates that this implantation function is disabled, since the BL is independent of any other layers. It can be seen in Figure 9 that two copies of the systematic bit stream of the BLL_{0} are interleaved and implanted intos_{0} ands_{1} using the conventional XOR operation according tos_{2} ands_{01}^{k}=s_{0}^{k}\oplus s_{1}^{k} , respectively. This results in the mixed bit-streams ofs_{02}^{k}=s_{0}^{k}\oplus s_{2}^{k} ands_{01}^{k} seen in Figure 9(a). The outputs becomes_{02}^{k} ,s_{0} ands_{01} , complemented by the three corresponding parity bit streams.s_{02} : Figure 10 illustrates the enhancedMode 1 , whereMode 1 becomes the IL-FEC protected layer. Considering the dependency betweenL_{1} andL_{1} , apart from assigning IL-FEC protection toL_{0} , the robustness ofL_{1} is also taken into consideration. Thus, first the systematic bits ofL_{0} are interleaved byL_{0} and then implanted into\pi _{0} for the sake of guaranteeing the performance of the BL, yielding the mixed sequence ofs_{2} . Then, two copies of the systematic sequence of the protected layers_{02}=s_{0}\oplus s_{2} are interleaved bys_{1} and\pi _{1} , as shown in Figure 10, and then implanted into\pi _{2} ands_{0} , respectively. This operation results in generating two new sequences, namelys_{02} ands_{10}=s_{1}\oplus s_{0} , while the other copy of bit streams_{102}=s_{1}\oplus s_{02} is output directly. As shown in Figure 10, the IL-FEC-processed systematic bit streams becomes_{1} ,s_{10} ands_{1} .s_{102} : The process of assigning IL-FEC protection toMode 2 is quite similar to that of the enhancedL_{2} , with the IL-FEC protected layer becomingMode 1 instead ofL_{2} , as shown in Figure 11. Note that instead of guaranteeing the BER performance ofL_{1} as inL_{0} , the system provides extra protection forMode 1 by implanting the bit stream ofL_{1} into that ofs_{1} . Therefore, the system first interleavess_{0} and then implants it intos_{1} , hence resulting in a new bit sequence ofs_{0} . Then, as observed in Figure 11, two copies of the systematic bit streams_{10}=s_{1}\oplus s_{0} are interleaved and implanted intos_{2} ands_{10} , hence resulting in the new mixed streams ofs_{1} as well ass_{210}=s_{2}\oplus s_{10} , while the other copy remains unchanged. Finally, the new outputs representing the systematic bits becomes_{21}=s_{2}\oplus s_{1} ,s_{210} ands_{21} .s_{2}
The adaptive IL-FEC selection unit of Figure 8 selects the appropriate IL scheme to assign the most appropriate protection based on one of the above three modes by taking into account the estimated channel SNR \begin{equation*} IL-FEC = \begin{cases} Mode 0 & \gamma \leq f_{0}, \\ Mode 1 & f_{0}< \gamma \leq f_{1}, \\ Mode 2 & f_{1}< \gamma, \end{cases} \tag{3}\end{equation*}
\begin{equation*} P(L_{i})_{e}= \begin{cases} P(L_{0}) & i = 0, \\ \sum _{m=1}^{i}P(L_{m})\prod _{n=0}^{m-1} [1-P(L_{n})] \\ \qquad +P(L_{0}) & i > 0. \end{cases} \tag{4}\end{equation*}
We emphasize that
The remaining parity bits of all the three layers are then multiplexed with the newly generated systematic codes, leading to the three new bit streams of
Again, Figure 5 has demonstrated that in general the BER performance of the ASU of MS-STSK is better than that of the classic QAM/PSK sub-channel as well as that of the dispersion matrix-based sub-channel, hence resulting in the lowest BER among these components at a given SNR. Therefore, we feed
D. Proposed Receiver Model
In this section, we detail the decoding process of our proposed adaptive wireless video system. As illustrated in Figure 8, the MS-STSK transceiver first receives the signal symbols and translates them into the LLR representation of the MS-STSK codewords by the Logarithmic Maximum A Posteriori (Log-MAP) algorithm. Then, the soft MS-STSK codewords are forwarded to a demultiplexer to generate the three bit streams, namely
Figures 10 and 11 depict the enhanced IL-FEC
Here we present
The systematic bits of the IL-FEC protected layer, namely
in the example of Figure 10, is fed into VND 0 to generate the a priori information ofy_{s_{1}}^{k} with the aid of the extrinsic information gleaned from both CND 0 and CND 1. Furthermore,L_{a}(s_{1}^{k}) is generated by simply duplicating the soft value ofL_{a}(s_{1}^{k}) , since at the first iteration no extrinsic information is provided by the CNDs. Then,y_{s_{1}} is input to the RSC Decoder 1 of Figure 10(b) along with its corresponding parityL_{a}(s_{1}^{k}) , hence generating the extrinsic informationy_{p_{1}}^{k} . This extrinsic LLR is then fed back to VND 0 via VND 1 to produce extrinsic information for CND 0 and CND 1 with the aid ofL_{e}(s_{1}^{k}) , as seen in Figure 10(b).y_{s_{1}^{k}} CND 0 of Figure 10(b) receives the extrinsic information of
from VND 0 and then extractsL_{e}(s_{1}) fromy_{s_{0}}^{k} . The extrinsic bits obtained are then fed into VND 2 to generatey_{s_{10}}^{k} , which is equal toL_{a}(s_{0}^{k}) , because the bits iny_{s_{0}}^{k} have not as yet been processed, hence no extra information is provided by CND 1 for VND 2. The extrinsic informationL_{2} generated by RSC Decoder 0 is sent back to VND 2 and CND 0 for providing extra information both for CND 1 and VND 0.L_{e}({s_{0}^{k}}) Assisted by the output of VND 0 and VND 2, CND 1 becomes able to extract
fromL_{a}(s_{2}^{k}) and then feeds it to the RSC Decoder 2 of Figure 10(b) via VND 4 in order to generate the extrinsic informationy_{102}^{k} . As seen in Figure 10(b), CND 1 uses this extrinsic information together withL_{e}(s_{2}^{k}) and eithery_{s_{102}}^{k} orL_{e}(s_{1}^{k}) to generate feedback information for VND 0 and VND 2, respectively, in order to prepare for the next iteration.L_{e}(s_{2}^{k}) Then, the decoding process of Figure 10(b) starts again from VND 0. However, in contrast the procedure in Step 1), the extrinsic information gleaned from CND 0 and CND 1 is no longer zero, since the related soft information has been exchanged among the three RSC Decoders of Figure 10(b), hence improving the soft information
. Similarly, the a priori informationL_{a}(s_{1}^{k}) is enhanced by exploiting the extrinsic information of VND 2, hence resulting in an enhanced BER performance forL_{a}(s_{0}^{k}) . After two iterations, VND 1, 3 and 4 output the final LLR generated by considering bothL_{0} andL_{a}(s_{i}^{k}) , which is then hard-decoded toL_{e}(s_{i}^{k}) ,\hat {l_{1}} and\hat {l_{0}} .\hat {l_{2}}
The above process specifically details the philosophy of enhanced
System Performance
In this section, we present our simulation results for characterizing the proposed MS-STSK assisted adaptive IL-FEC aided system. Again, the SHVC reference software SHM is utilized for encoding the Foreman video clip. The Group Of Pictures (GOP) is set to 4 for all video simulations, which means that the Instantaneous Decoding Refresh (IDR)/Clean Random Access (CRA) frames are inserted every 4 frames. No B frames are used in our simulations due to the fact that they are prone to propagating inter-frame video distortions. Similarly, the bidirectional predictive B frames propagate video distortion and increase the latency, hence preventing flawless lip-synchronization. As a consequence, the video sequence in our simulations simply consists of I frames and P frames. Furthermore, we disable the spatial and temporal scalability functionalities, when encoding the video sequence into three different-quality layers, where the quality of the layers is controlled by setting the bit rate for each layer. The bit stream of each video frame is mapped to an MS-STSK packet, whose length is defined in Table 3. The receiver checks if the received packet has any bit errors using the associated Cyclic Redundancy Check (CRC). If the CRC detection fails, the corrupted frames are dropped and replaced by “frame-copy” based error concealment.
Apart from the above source configuration, the FEC-aided MS-STSK transceivers are configured as follows. The three RSC codecs are configured by the binary generator ipolynomials of [1101 1111]. Additionally, the MS-STSK(4, 2, 2, 2, 8, 4)|QAM and MS-STSK(8, 2, 2, 2, 16, 8)|PSK configurations are used by the MS-STSK transceiver. The bit allocations of two MS-STSK configurations are listed in Table 2. The MS-STSK transceiver configured as MS-STSK(4, 2, 2, 2, 8, 4)|QAM has 4 TAs and 2 RAs as well as 2 RF chains, hence resulting in a 6-bit
We first show the PLR and the PSNR versus channel SNR performance improvement attained by using the proposed UEP MS-STSK scheme in Figure 8. The associated configuration parameters can be found in Table 3, except that in order to highlight the improvement achieved by MS-STSK only, the RSC codecs were deactivated in this investigation and so was the channel’s shadow fading.
Figure 12 shows the e-PLR versus channel SNR of both the Equal Error Protection (EEP) and of the UEP schemes. A part of a video sequence, namely the Foreman sequence, which has 30 frames and is scanned at 30 Frame Per Second (FPS), is encoded into three layers, having bit rates of 126.7, 259.7 and 385.3 kbps respectively and using the MS-STSK configuration of MS-STSK(4, 2, 2, 2, 8, 4)|QAM, as shown in Table 3. In the EEP, the number of bits in each layer is split into three streams on average, which are then fed into the three modules of MS-STSK seen in Figure 2, while for the UEP the bits of different layers are fed into the three corresponding MS-STSK modules. Therefore, compared to EEP, the ASU of MS-STSK in UEP only contains the bits of the BL of the scalable video stream, namely
e-PLR versus channel SNR for the Foreman test sequence associated with MS-STSK(4, 2, 2, 2, 8, 4)|QAM.
Figure 13 provides the evidence that for the Foreman test sequence the image quality (PSNR) can be improved by feeding the BL bits to the ASU of Figure 2, where the first EL is fed into the modulator, while the second EL is used for dispersion matrix selection, in order to construct our basic UEP scheme for the layered video stream. Observe in Figure 13 that although both the UEP and EEP schemes yield an identical overall image quality (PSNR) at the channel SNR of 17 dB, it can be seen from Figure 12 that at this channel SNR the
PSNR versus channel SNR for the Foreman test sequence associated with MS-STSK(4, 2, 2, 2, 8, 4)|QAM.
In order to characterize our system, we compare the performance of our adaptive system that invokes the enhanced IL-FEC technique to that of the conventional adaptive system as well as to that of the UEP scheme and to that of the EEP scheme. Explicitly, the difference between two adaptive schemes is presented in the adaptive IL-FEC selection unit, as shown in Figure 8, while the other parameters set for the video sequences, the RSC codecs and the MS-STSK transceiver are identical. For the enhanced adaptive system, \begin{align*} P(L_{0})_{e}=&P(L_{0}), \tag{5}\\ P(L_{1})_{e}=&[1-P(L_{0})] \cdot P(L_{1})+ P(L_{0}).\tag{6}\end{align*}
The thresholds set for selecting modes are given in Table 4, where the terms enhanced and conventional represent the specific type of the IL-FEC technique applied for the adaptive system. Again, all other parameters specifying the systems can be found in Table 3. Note that no threshold value is set for the EEP and UEP schemes, since the IL-FEC mode is deactivated for both schemes.
Figure 14 depicts the probability density function of three enhanced modes versus channel SNR in the Foreman test scenario, where at a channel SNR of
Figure 15 compares the e-PLR and the image quality (PSNR) performances of our enhanced scheme to other counterparts for both the Foreman and Football clips.
The comparison between the enhanced and the conventional adaptive system for the Foreman (left column) and the Football (right column) test sequences, where the first, second and third rows present
Figures 15(a) and 15(b) depict the
Figures 15(e) and 15(f) show the image quality (PSNR) performance versus the channel SNR for two video sequences. Observe that the enhanced
Comparison of frames at the channel SNR of 11 dB for the Football sequence. The five columns (from left to right) represent the original video, the enhanced adaptive scheme, the conventional adaptive scheme, the UEP scheme and the EEP scheme, respectively.
Conclusions
In this paper, we proposed an MS-STSK assisted adaptive system for wireless video streaming, which adaptively selects the most appropriate enhanced IL-FEC schemes with the aid of the pre-recorded thresholds, as shown in Figure 8. Observe in Figures 4 and 5 that our MS-STSK transceiver is capable of providing UEP by mapping the video bit streams to different MS-STSK components, namely to the ASU, to the classic modulator and to the dispersion matrix generator, according to their importances.
Additionally, the enhanced IL-FEC technique of Figures 10 and 11 was conceived for protecting the ELs, which extends the philosophy of the conventional IL-FEC technique to multiple ELs, hence improving the PLR performance, as shown in Figure 15. Our simulation results shown in Figure 15 illustrate that with the aid the enhanced IL-FEC technique, our proposed adaptive system is capable of providing the best video quality.