Performance Evaluation of Video Transmission over Ultrawideband WPAN Norazizah Mohd Aripin, Norsheila Fisal, Rozeha A. Rashid Faculty of Electrical Engineering Universiti Teknologi Malaysia Johor, Malaysia
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Abstract—Over the past few decades, unprecedented growth in wireless communications, networking and video-coding technologies have stimulate numerous applications such as video on demand and interactive video telephony. Ultrawideband technology (UWB) which offers high data rate with low transmit power, becomes a very promising wireless platform for video transmission. Recently, Cognitive Radio (CR) technology was introduced to fully utilized spectrum usage by sensing the spectral environment and use this information to opportunistically provide wireless links that meet the user communications requirements optimally. It is obvious that achieving more reliable and efficient multimedia communication over wirelesss UWB channel requires a cross design approach due to tight dependency among protocol layers. In this paper, proposed cross layer framework of video transmission over CRUWB will be presented. Then, a simulation study is carried out to evaluate the effect of resource allocation at the MAC layer to the video quality perceived at the receiver. Keywords – video transmission, ultra wideband, resource allocation, cognitive radio
I.
INTRODUCTION
Advancement in multimedia application for home networking has emerged tremendously and hence posed significant demand on the network resources such as bandwidth, data rate and also transmit power. UWB has gained importance for multimedia transmission as a consequence of its ability to support high data rates up to 480 Mbps over short distances with low emission power [1]. Cognitive Radio (CR) is defined as intelligent wireless communication system that is cognizant of its environment, learns from it, adapts its internal states to statistical variations in the incoming RF stimuli by making changes in certain operating parameters in real time with objectives of highly reliable communications whenever and wherever needed [2]. CR exists in two modes namely underlay and overlay. In underlay mode, cognitive user (CU) can co-exist together with the licensed user (LU) as long as the transmit power is not causing any harm to the LU. On contrary, in overlay mode, CU accesses the network using a portion of the spectrum that is not utilized by LU. Therefore, UWB is an attractive physical layer (PHY) candidate for high data rate Wireless Personal Area Networks (WPANs) as well as cognitive radio.
Two main standards have been developed for UWB networks namely IEEE802.15.3 and WiMEDIA MBOA, which are based on centralized and distributed WPAN. Wireless multimedia applications require significant bandwidth and often have to satisfy relatively tight delay constraints. Hence, integration of UWB and CR technology with multimedia application is another open research issue. Generally, transmitting multimedia content within certain QoS requirements such as delay, jitter and PSNR is very challenging due [3-6]: a) Resource constraint such as spectrum bandwidth, transmit power, data rate and time slot access. b) Dynamic network condition over time due to interference, shadowing and multipath fading. c) Video is encoded with different profiles packet and priorities. Loss of certain important frames may degrade the quality of received video significantly. d) Stringent delay constraint. Delays of less than 200 milliseconds are required for interactive applications, such as videoconferencing, surveillance etc., while for multimedia streaming applications delays of 1-5s are tolerable. Packets that arrive after their display time are discarded at the receiver side are, at best, can be used for concealing subsequently received multimedia packets. e) Upon arrival of LUs, CUs with multimedia traffic may be affected more than typical data traffic. Therefore, a cross layer design approach is needed to integrate appropriate sensing mechanism, spectrum access, spectrum adaptation, resource allocation and packet scheduling with the goal to find optimum solution in allowing multimedia transmission. In this paper, a proposed cross layer framework will be presented. This framework gives an overall overview on how communications and optimization among protocol layers are considered to meet our long term research objectives. However, particularly for this paper, we will emphasize our performance evaluation based on the interaction between APP and MAC layer only and leave the rest for future works.
The rest of this paper is organized as follows. Section II describes ultra wideband technology and its related standards. Section III explains the research challenges in resource allocation. Section IV introduces our proposed cross layer design framework. Simulation and results are presented in section V. Lastly, section VI concludes the paper. II.
ULTRA WIDEBAND TECHNOLOGY
A. Ultra Wideband Flavors Federal Communication Commission (FCC) in its report in 2002 [7] authorized the unlicensed use of UWB in 3.1-10.6 GHz and has restricted the minimum occupied bandwidth of each mono/multi band(s) to 500 MHz. Furthermore, it defined a spectral mask that specifies the power level radiated by UWB systems within this band to be near the thermal noise floor (-41.3 dBm/MHz). There are two commonly proposed means of implementing UWB namely single band and multiband OFDM. Single band UWB consists of Time-Hopping Pulse Position Modulation (TH-PPM) and Direct-Sequence UWB (DS-UWB). In single band UWB, very short pulses duration are occupying several GHz of bandwidth. Employing these UWB techniques over the whole allocated band has many disadvantages including need for high complexity rake receivers to capture multipath energy, high speed analog to digital converters (ADC) and high power consumptions [8]. In MB-OFDM, the available UWB spectrum is divided into several sub-bands, each one occupying approximately 500 MHz. By interleaving symbols across different sub-bands, UWB system can still maintain the same transmit power as if it was using the entire bandwidth. Narrower sub-band bandwidths also relaxes the requirement on sampling rates of ADCs and consequently enhancing digital processing capability. On top of that, it provides flexible data rate and interference mitigation by turning off some of the subcarriers. In [26], Tarokh et. al had developed UWB propagation model based on field measurement in indoor environment. Based on the measurement, UWB propagation model was derived based from a shadowing channel model. Tarokh’s UWB channel propagation model is represented as follows; PL(dB) = PL0 +10α log (d/d0) +S
(1)
Where PL0 = 47, path lossexponent, α = 1.7 and shadowing effect, S is equal to 4.4dB. B. MAC for Ultra Wideband WPA There are two distinct types of WPAN networks, which are centrally controlled and distributively controlled. Centralized WPAN is under consideration of IEEE802.15.3 [9], whereas WiMEDIA MBOA [10] is based on distributed WPAN. However, both are hybrid MAC protocols, which combine the best qualities of the random access and guaranteed access
protocols to achieve flexibility, efficiency and QoS provisioning. Since both of them possess different design requirement and are developed by different standardization bodies, we limit our simulation study to centralized WPAN only. In IEEE 802.15.3, the central controller, called the piconet controller (PNC), is responsible for allocating the radio resource to its members. The channel time is divided into superframes, with the structure shown in Fig. 1. Each superframe starts with a beacon period (BP), during which the PNC sends the beacon containing network synchronization and control message. Asynchronous data or commands can be sent using contention access period (CAP). Whereas, isochorous streams and asynchronous data can be sent thru channel time allocations period (CTAP) or also known as Guaranteed Time Slot (GTS) using TDMA discipline. Based on the successfully received requests from all devices, the PNC will schedule and allocate channel time in the CTAP to all devices. Although both commands and asynchronous data can be transmitted in the CAP, we anticipate that a better multimedia transmission quality will be achieved by minimizing the length of contention period. This is due to less protocol overheads and less probability of collisions. In addition, for multimedia applications, UWB devices may need channel times on a regular basis with a strict delay contraint. Furthermore, it is very difficult and costly for a UWB device to make frequent medium sensing with a very low transmit power (-41.3dBm) requirement set by the FCC. Hence, channel time requests will be made during the CAP to reserve isochronous channel time in the CTAP.
Figure 1.
Superframe architecture for IEEE 802.15.3 III.
RESOURCE ALLOCATION
Resource allocation in centralized WPAN network generally consists of three themes namely throughput maximization, fairness and QoS provision. Fortunately, IEEE 802.15.3 MAC specification introduces flexible parameters such as frame size, data rate, channel allocation and superframe durations. For throughput maximization, adaptive selection of parameters (ie: frame size, time slot) based on the instant channel status shall be considered. Optimal scheduling may also improve the throughput. If fairness is defined as equal share of resource, then trade off between fairness and efficiency need to be further investigated.
Optimum resource allocation and efficient packet scheduling are very crucial at the MAC layer in ensuring multimedia content can be sent within their delay bound. Numerous relevant research findings related to MAC-layer resource allocation and packet scheduling in IEEE802.15.3 and WiMEDIA MBOA has been carried out in the literature [13-25] and concluded as follows: a)
APP layer frames should be treated based on their priority. b) Admission control algorithm should consider time slot availability, interference margin, achievable transmission rate, deadlines and frame dependency. c) Contention-free access is more suitable for multimedia application in centralized WPAN. d) Management information required for scheduling such as queue size and deadline should be transmitted based on TDMA-like approach at the end of each super frame. e) Distributed MAC protocol is more suitable to avoid single point failure of PNC and minimal traffic congestion. f) Slot allocations should consider frame size and packet priority at the APP layer. g) MAC layer could help PHY layer in determining appropriate data rate and power to be used based on the channel SINR. Several existing scheduling mechanisms namely RoundRobin (RR), Shortest Remaining Processing Time (SRPT), Constant Bandwidth Server (CBS) and Idle Recovery Slot Allocation (IRSA) are also evaluated in this paper. In round robin, each user is allocated a fixed GTS at each specific interval. On the other hand, PNC that utilizes SRPT gives higher priority to the node that has least number of packet to transmit first, aiming to complete the shortest job. In CBS, PNC will guarantee timing requirements by reserving each task all the resources it needs for its execution. If such a guarantee cannot be done, the task is rejected. In addition, resource allocation is given to flows that need to be completed first. In IRSA, all sending nodes are given same amount of resources. Then, it allocates the remaining resources to nodes with large queue sizes. IV.
CROSS LAYER FRAMEWORK OF VIDEO TRANSMISSION OVER WIRELESS UWB
The literature in [13]-[25] shows that there exist strong dependencies among parameters across the protocol layers and thus cross layer design approach is necessary to further improve the performance. Unlike file transfers, real-time multimedia applications do not require a complete insulation from packet losses, but rather require the application layer to cooperate with the lower layers to select the optimal wireless transmission strategy that can maximize the multimedia performance. This is because multimedia applications can cope with a certain amount of packet losses up to 5% or more (depends on error concealement strategy).
Fig. 2 shows our proposed cross layer framework [11-12] which is based on MAC-centric approach, where optimization and decision are made at the MAC layer. This framework is developed based on CR MB-OFDM UWB at the PHY layer, IEEE802.15.3 MAC and MPEG video application. At the PHY layer, sensing mechanism and channel estimator provide information (as shown in the diagram) to the MAC layer. Apart from that, MAC layer also receives information from the APP layer such as frame rate and frame type. Based on all these information, MAC will optimize and decide the best action to suit requirements set by the APP layer and also changing channel conditions at the PHY layer. They cooperate together to determine the optimum data rate, transmission power, modulation level, spectrum usage and slot scheduling. Reconfiguration at the PHY layer will be done by the adaptation module, whereas optimal slots allocation and packet scheduling is done by the QoS aware scheduler module. In addition, appropriate co-existence technique will be handled by spectrum sharing module in the MAC layer. In this framework, it is assumed that there is only a single hop from source to destination.
Figure 2. Proposed cross layer framework
The above framework is developed to facilitate us in determining the required modules for the realization of the CR based UWB MB-OFDM system in our Telematic Research Group in UTM. However, as mentioned earlier, this paper will emphasize on performance evaluation based on the interaction between APP and MAC layers only (in dotted circle) and to continue the rest in our future works. The objective is to investigate the behaviours of several parameters at the APPMAC-PHY layer and their impact to the received video quality. V.
SIMULATION & RESULTS
Simulations were carried out using Network Simulator 2 (NS-2) to investigate the effect of time slot allocation, frame rate and no of flows. We also analyzed the performance of existing resource scheduling mechanisms. Throughput and Job
Failure Rate (JFR) are used as performance metrics. In this work, throughput is defined as total number of byte received per second. While JFR is defined as total number of failed packets over total packets transmitted. Table 1 shows the simulation parameters. SIMULATION PARAMETER
Parameter (unit)
Value
Video input
(a) Foreman.qcif (b) TES video traffic model
Data rate (Mbps)
100
Superframe size (µsec)
1-64000 (dynamic)
MTU size
2048
In order to evaluate the video performance from user-view perspective and investigate the frame details, foreman.qcif was used as input in our simulation. Fig. 3 shows the impact of GTS size to the JFR performance and Fig. 4 illustrates the instantaneous throughput over the simulation time. Those graphs clearly indicate that the received video quality at the APP layer and throughput performance at the PHY layer are closely dependent on the GTS allocation at the MAC layer. Based on packet level observation, we noticed that the impact is more obvious to I-frame. Without appropriate GTS allocation, more I-frames were lost and hence caused indirect lost to the P-B frames. Hence, appropriate optimization is needed to find optimal GTS allocation in accordance to frame type.
Figure 5. Subjective evaluation on the received video quality 60 50 40 JFR(%)
TABLE I.
Fig. 5 illustrates subjective comparison of the received video quality at frame 184 with various GTS allocation (100, 130 and 150µsec). We observed that the impact of GTS allocation is quite obvious at the user view-point.
5-flows 30 1-flow 20 10 0 8
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Video rate
Figure 6. JFR vs Video Rate
To simulate performance of various scheduling mechanism, video traffic model base on Time Expand Sample (TES) model was used. Fig. 6 depicts the effect of video rate to JFR when round robin scheduling mechanism was used. Video rate may be increased due to higher frame rate per second or due to higher quantization level during video encoding. Higher video frame rate per second also means that the video frame deadline become more stringent. When the number of video flows increased, superframe size also increased. Thus, more packets were dropped due to missing its deadline and represented by higher JFR.
Figure 3. JFR vs GTS size
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Figure 4. Throughput vs simulation time for various GTS allocation
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Fig. 7 and 8 illustrates the performance of various scheduling mechanisms under different channel BW and no of flows. Generally, both graphs indicate that the JFR is the least for SRPT, followed by CBS and IRSA. Whereas, RR obtained the highest JFR among all. As the flow count increases to more than 15, RR scheduler starts to perform better than IRSA and CBS due to fair resource allocations to all users. This is in contrast to CBS where as more resource allocation are demanded, some of the requests are rejected due to its policy of only accepting request that it can guaranteed. Whereas in IRSA, additional overhead time is needed to allocate the remaining idle resource to the requesting node.
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[8] Figure 7. Comparison of JFR performance vs channel BW for various scheduling mechanism [9]
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[14] Figure 8. Comparisons of JFR vs no of flow for various scheduling mechanisms [15]
VI.
CONCLUSIONS
This paper presented our initial performance evaluation of video transmission over UWB WPAN. The simulation showed that cross layer design approach is needed in finding optimal solution for resource allocation at the MAC layer, considering various parameters at the APP, PHY layer and time varying channel condition. It is believed that optimal cross layer strategy in allocating network resources with respect to varying channel conditions and video application requirement is vital.
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ACKNOWLEDGEMENT The authors would like to acknowledge Universiti Teknologi Malaysia, Ministry of Science & Technology Malaysia (MOSTI) and Islamic Development Bank (IDB) in supporting this research work.
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