A Cross-Layer Approach in Sensing and Resource Allocation for Multimedia Transmission over Cognitive UWB Networks
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We propose an MAC centric cross-layer approach to address the problem of multimedia transmission over cognitive Ultra Wideband (C-UWB) networks. Several fundamental design issues, which are related to application (APP), medium access control (MAC), and physical (PHY) layer, are discussed. Although substantial research has been carried out in the PHY layer perspective of cognitive radio system, this paper attempts to extend the existing research paradigm to MAC and APP layers, which can be considered as premature at this time. This paper proposed a cross-layer design that is aware of (a) UWB wireless channel conditions, (b) time slot allocations at the MAC layer, and (c) MPEG-4 video at the APP layer. Two cooperative sensing mechanisms, namely, AND and OR, are analyzed in terms of probability of detection ( Open image in new window ), probability of false alarm ( Open image in new window ), and the required sensing period. Then, the impact of sensing scheduling to the MPEG-4 video transmission over wireless cognitive UWB networks is observed. In addition, we also proposed the packet reception rate- (PRR-) based resource allocation scheme that is aware of the channel condition, target PRR, and queue status.
KeywordsMedium Access Control Cognitive Radio Medium Access Control Protocol Medium Access Control Layer Central Controller
Limited available spectrum and inefficient utilization of spectrum necessitate the use of Cognitive Radio (CR) approach to exploit the existing wireless spectrum opportunistically. Cognitive radio concept was first coined by J. Mitola  and can be defined as a radio that is capable of sensing its environment, learning about its radio resources and user/application requirements, and adapting behavior by optimizing its own performance in response to user requests . In CR networks, primary users (PUs) shall be protected while secondary or cognitive users (CUs) access the spectrum either in an overlay or an underlay mode. It is the responsibility of a CU to ensure that its existence is not felt by the PU. In overlay mode, CU uses higher transmission power. However, it is only applicable if the CU can ensure that the targeted spectrum is completely free of signals of other systems. While in underlay mode, CU is allowed to co-exist in the same spectral and temporal domains with the PU by lowering the amount of transmit power to avoid unintended interference.
Federal Communication Commission (FCC) in its report in 2002  authorized the unlicensed use of Ultra wideband (UWB) in 3.1–10.6 GHz and defined a spectral mask that specifies the power level radiated by UWB systems within this band to be near the thermal noise floor (i.e., Open image in new window 41.3 dBm/MHz). Thus, UWB device can easily coexist with PU using underlay mode. However, sensing is still vital to cognitive UWB (C-UWB) user in order to detect and avoid unnecessary interference to any PU. Most of existing research in cognitive radio had been mainly dedicated to the physical aspect of the cognitive radio design. Recently, only few research efforts were carried out to investigate the impact of sensing mechanism to the upper layer performance such as in [5, 6, 7, 8]. To the best of our knowledge, none of the existing works exploit a cross-layer strategy between APP, MAC, and PHY layers. Therefore, this paper proposes a novel MAC centric cross-layer design that is aware of the dynamic time-varying UWB wireless channel at the PHY layer and the target Quality of Service (QoS) for multimedia delivery, thus providing optimal sensing scheduling and adaptive resource allocation.
Consequently, a C-UWB node needs to consider several requirements simultaneously such as, user and application preferences, its own capabilities such as, battery status and channel conditions before any adaptation actions are taken. A compromise point, which can be regarded as optimization, is to be attained between these requirements. Hence, we believe that cross-layer design is the best suited approach for C-UWB.
Additionally,  introduced cognitive engine (CE) architecture that removes the distance between layers on the edges and allows parallel communications among layered protocol stacks, sensors, and memories. Each design approach has its pros and cons. Hence, the best cross-layer design solution is subject to the application requirements, used protocols, algorithms at each layer, and complexity.
Considering MPEG-4 video transmission at the APP layer, the impact of losing I-frame on the received video quality is more significant than P or B frames due to video frame dependencies. Thus, MAC should schedule the video packet optimally based on its priority, dependency, and delay deadlines. Furthermore, MAC shall also take advantage of the dynamic nature of wireless channel conditions at the PHY layer to adapt its action accordingly. For instance, more time slots are allocated to users with good channel conditions to improve the throughput. While at the APP layer, smaller quantization level (means coarser video) is assigned to user that experience bad channel conditions in order to reduce the bit error rates.
In view of that, we consider the MAC centric cross layer design which is aware of MPEG-4 QoS requirements and PHY channel conditions. AND and OR-rule cooperative sensing techniques are analyzed and the required sensing period for MAC layer scheduling is determined. Packet reception rate- (PRR-) based resource allocation is proposed to calculate the optimal time slot allocation for each user. Then, the impact of cross-layer design on MPEG-4 video transmission is evaluated.
The rest of this paper is organized as follows. Section 2 describes several related works on cross-layer design across APP, MAC, and PHY layer. Our proposed system design and approach is presented in Section 3. Results and analysis are given in Section 4. Finally, conclusion and future recommendations are drawn in Section 5.
2. Related Works
The allocation of system resources is constrained vertically across layers and horizontally among users for a system with cross-layer design. The bandwidth consumption for use in the application layer should not exceed the achievable capacity by the physical layer vertically, while allocating these resources to one user would horizontally affect the performances of the other users due to the limited amount of resources or interference of simultaneous usage. In addition, a dynamic temporal resource allocation should be adopted due to time-varying channel conditions and traffic source characteristics.
In the Time Division Multiple Access- (TDMA-) based MAC protocol, the main issue is the sharing of the time slots among the wireless users. Basically, scheduling algorithm is deployed in such networks and the wireless users will need to dynamically compete for transmission with each other. A game theoretic pricing mechanism resource allocation was considered in  where each user sends messages that represent their network-aware resource demands and corresponding prices to the Central Spectrum Moderator (CSM). Then the CSM will determine the suitable policy to divide the available resources among all users, while in , base station sets a price on the resource, and each mobile user determines its average resource request depending on the announced price and its own source utility characteristics.
Explicitly for C-UWB system, sensing activity is crucial to determine spectrum holes before any adaptation or management action can be taken. Sensing information can be a consideration for QoS requirements especially in multimedia application as it can assist C-UWB to dynamically allocate appropriate resources in accordance to the time-varying channel condition . In most cases, CR device has to postpone all its transmission during spectrum sensing. Thus, sensing activity should be scheduled accordingly and sensing period should be allocated appropriately to avoid any negative impact on video application that is more sensitive to delay. For instance, if a longer time is allocated for sensing, the overall throughput will decrease. Conversely, the probability of accurately detecting spectrum holes will be reduced if the sensing time is not sufficiently allocated.
In , digital fountain codes are used to distribute multimedia contents over unused spectrum and also to compensate the packet losses due to PU interference. Sensing activity is scheduled at the start of every group of picture (GOP). However, how the sensing activity is scheduled at the MAC protocol is not discussed in detail. Hong and Liang  proposed adaptive spectrum sensing that is aware of channel state information (CSI) and queue state information (QSI). Both CSI and QSI spectrum sensing is used to decide when to perform sensing and data transmission. However, the sensing time is fixed to 20%–50% of the super frame size. We argue that this allocated sensing period is too long and inappropriate for multimedia transmission.
To take into consideration the characteristics of multimedia traffic in the cross-layer design, Rhee et al.  carried out simulation studies on time slot allocation based on maximum I-P-B frame size. Then, the packets are transmitted based on FIFO scheduling. However, the maximum I-P-B frame size is fixed during the whole transmission without considering the varying channel conditions. In , the authors proposed a cross layer solution to jointly optimize the packet scheduling by explicitly considering varying channel condition and multimedia data characteristic. Though, the method of obtaining channel conditions is not clearly elaborated. Furthermore, sensing time is not taken into account in their cross-layer design.
From the literature, we observe that there is a significant research gap for a cross layer design between APP, MAC, and PHY layers especially for multimedia transmission over C-UWB network. Motivated from the above findings, the paper is devoted to linking the spectrum sensing at the physical layer with the optimal resource allocation to meet the QoS requirements set by the multimedia application. The cross-layer framework is similar to our previous work in [15, 16] which is aware of APP, MAC, and PHY layers parameters. The framework serves as a guideline to our overall research work in realizing a complete solution of the C-UWB system. However, the main contribution of this paper will be on the PRR-based resource allocation and the impact of sensing activity on multimedia transmission.
3. Proposed Cross Layer Design
3.1. System Model
whereL 0 is path loss at reference distance, Open image in new window is path loss exponent, and S is shadowing. In this study, L 0 of 50.5 dB, path loss exponent ( Open image in new window ), equal 1.7, and shadowing ( Open image in new window ) of 2.8 dB are used.
with Open image in new window being the number of video fragments.
Consequently, the proposed algorithm eliminates the need of dedicated channel time slot request from the C-UWB nodes to the central controller. Thus, the central controller will directly announce the allocated time slots and optimal data rate without having to wait for channel time request from C-UWB nodes (transmitter). Then, packet transmission will be performed based on optimal scheduling policy that resides at the MAC layer of C-UWB device (transmitter). For simplicity, we adopt a round robin scheduling policy, which allows C-UWB nodes to take turn in transmitting their multimedia traffic. Although it is a round robin mechanism, the MAC scheduling is improved by assigning a different time slot allocation to each C-UWB nodes depending on their target and instantaneous PRR and BER, queue status, and channel conditions of all C-UWB nodes in the network. Each C-UWB user is also assigned with an optimal sensing time to meet the target probability of detection ( Open image in new window ) and probability of false alarm ( Open image in new window ) during worst case channel conditions. Thus, the MAC scheduling is considered optimal in terms of the sufficient sensing time and the time slot allocation.
In the next section, we will provide more insights of the sensing mechanisms and PRR-based resource allocations adopted in our cross-layer design.
3.2. Sensing Mechanisms
where Open image in new window ; Open image in new window is the number of samples. The noise Open image in new window [n] is assumed to be AWGN with zero mean and variance Open image in new window . S[n] is the primary user's signal and is assumed to be a random Gaussian process with zero mean and variance Open image in new window .
Since we are interested in low SNR regime ( Open image in new window ), large number of samples should be used.
It can be seen that in bad channel condition (low SNR), P d is lower and the number of samples needed for PU detection increases, that is, the sensing time becomes longer. It is desirable to have a high Open image in new window for better PU protection. Meanwhile, a low Open image in new window is favorable for a better opportunistic access and higher achievable throughput for CU. Since these two magnitudes pose a trade-off on the sensing mechanism, an optimal sensing time needs to be determined such that some Quality of Service (QoS) is attained by both PU and CU.
It has been reported in  that cooperative spectrum sensing can greatly increase the probability of detection in fading channels. Multiple CUs can be coordinated to perform spectrum sensing cooperatively and the sensing information exchanged between neighbors is expected to have a better chance of detecting PU compared to individual sensing. A cooperative network of several CR-assisted systems can be modeled as an OR/AND-rule network.
where Open image in new window and P f are, respectively, the probability of detection and probability of false alarm of a stand-alone cognitive radio.
The performance of these cooperative sensing schemes will be compared to give an insight to the preferred one to be deployed.
3.3. PRR-Based Resource Allocation
Open image in new window is a scaling factor that depends on the selected modulation and coding scheme (MCS), N s is the number of subcarriers in a subband, and Open image in new window is the ratio of signal to interference and noise on the Open image in new window th subcarrier. Knowing the target PRR (PRRT) and the instantaneous PRR (PRRi) of each user i, central controller will compute the optimal time slot allocation. It is worth to note that the resource allocation is computed for each super frame. Let'us denote Open image in new window as superframe size, the PRR based resource allocation can be described as follows;
Optimization Problem: How to increase the throughput of each user (meaning maximizing Open image in new window ) while maintaining a target PRR.
Based on equation (29), all users will be assigned optimal time slot in accordance to their own channel condition as well as other users channel conditions. This may leads to two extreme cases. The first case occurs when users that experienced very bad channel condition may not be granted with any time slot allocations during that one superframe duration. The second case is when the time slot allocation is dominated by one user that has a very good channel condition. To overcome this issue, number of packets in queue is also considered in the algorithm. If there is no packet in queue, the user will be granted the shortest time slot, just enough for queue update. When there are packets in queue, the user will be given enough time slots according to the number of packets in queue, but limited to the maximum allowable Open image in new window . This to ensure that users are assigned with sufficient time slots according to number of packets in queue and subject to their channel condition. Additionally, the packet is also constrained with the delay deadline and retransmission limit. The packet is dropped if it is failed to be received by the central controller after the deadline expired.
4. Results and Analysis
In this section, simulation results of our proposed MAC-centric cross-layer design are presented. Simulations were carried out using MATLAB and Network Simulator 2 (NS-2).
(b) Time Expand Sampling (TES) video traffic model
Superframe size ( Open image in new window sec)
PHY header time
15 Open image in new window sec
10 Open image in new window sec
From Figure 13, we note that our proposed cross layer design outperformed the non-CLD and CLD-queue insensitive. However, the non-CLD performs better when more than 8 users share the limited resources. This is due to the fact that when more users are competing, the user that experiences bad channel condition will always get the minimum timeslot as compared to the fix timeslot allocation. Although the JFR is higher when more users are involved in the resource sharing, we observed that the received video quality is improved for users with quite stable and good channel conditions to compensate with users that experience very bad channel conditions.
By considering the findings in the preliminary investigation, SNR is considered as the main QoS metric at the PHY layer to determine the appropriate sensing time for cognitive users in our cross layer design. We propose that optimal time slot for optimal resource allocation to be assigned for sensing activity and data transmission at the MAC layer. The impact of sensing activity is minimal on the multimedia delivery and hence offer better cross layer strategy through PRR based resource allocations. We also recommend that cooperative sensing is implemented as it enhances decision making by collaborating CUs. OR-rule data fusion scheme is favored as from the comparison, it offers better PU protection. The proposed cross layer design will be further improved by considering the heterogeneous video traffic characteristics.
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