Power and sub-channel optimization of JPEG 2000 image transmission over OFDM-based cognitive radio networks
Introduction
The rapid developments in technology have made all types of applications available in a single mobile handset, and multimedia applications are the most popular. The growing demand in wireless image and video transmission requires high data rate and reliable transmission of multimedia stream over severe channel conditions. However, such services are bandwidth intensive [1], and the time-varying wireless channels are shared by a huge number of users. These problems along with the total power consumption limitations impose challenges in maintaining high quality received videos and images.
Investigations have shown that a large portion of the licensed frequency spectrum is either entirely unused or seldom being utilized [2]. Naturally, a more effective utilization of the spectrum is desirable. Cognitive radio (CR) has emerged as a powerful scheme for unlicensed devices to use licensed bands under certain conditions, and the scheme was approved by the Federal Communication Commission (FCC) in 2002 [3].
Two dynamic spectrum access mechanisms have been proposed in literature based on the coexistence with respect to the primary user (PU): i) underlay spectrum access and ii) overlay spectrum access [4]. In the underlay spectrum access, the PU and secondary user (SU) can coexist in the same spectral band, while in the overlay CR the SUs can only access the unused licensed channel resources assigned to PUs, when the PU has no data to transmit [5]. In this paper the overlay CR is chosen as the access mechanism, which requires some flexibilities in the spectrum shape of the transmitted signals. Orthogonal frequency division multiplexing (OFDM) offers this kind of flexibilities by filling the spectral gaps without interfering with PUs [6]. However, a major problem with this kind of applications is the mutual interference between PU and SU. The interference to PU cannot exceed its tolerable collision level, which is dependent on the primary service type. This has been widely studied in [7], [8]. A considerable amount of literature has been published on subcarrier and power allocation in OFDM-based CR [8], [9], [10].
The above works, however, have mostly concentrated on the maximization of the throughput of the system under resource constraints with the assumption that all information bits in the transmitted bitstream are equally important. Applying these schemes to the transmission of scalable multimedia bitstreams results in poor resource utilization [11]. Therefore, several attempts have been made to address this issue. In [12], the authors optimized video streaming to exploit more channel resources for SU by developing a flexible sensing-transmission scheme. In [7], a cross-layer quality-aware resource allocation algorithm to optimize OFDM access-based (OFDMA-based) CR network performance was proposed, by considering the imperfect channel state information between the SU and PU. It also considers quality-aware resource allocation using the H.264/AVC standard. The study in [13] proposed a bit-error-rate-driven (BER-driven) resource allocation for scalable bitstreams over OFDMA systems, which can be applied to the CR networks with some changes in the considered constraints.
As suggested in [14], removing the redundancies via data compression leads to bandwidth saving, but it increases the sensitivity of the data to transmission errors. In [15], a new hyperspectral band ordering algorithm is presented that improves the compression performance. JPEG 2000 standard has a quality progression feature, which can improve the received image quality progressively as more data from different quality layers are received [16]. In order to improve the transmission performance, the important parts of the bitstream (lower layers) should be transmitted with more protection than the less important parts. This is known as unequal error protection [13], [17], [18], [19], [20], [21], [22], [23].
In this paper, we improve the received image quality in a CR system by taking advantage of the scalable bitstream and unequal power allocation in two stages. The first stage optimizes the power allocated to the JPEG 2000 bitstream at the coding pass (CP) level to minimize the total received distortion. The second stage employs subcarrier allocation, adaptive modulation, and power adjustment to meet the interference requirements, based on channel conditions, and at the same time keeps the same throughput for the system. This strategy is expected to enhance the image quality, since important parts of the image will be transmitted more reliably.
Section snippets
System model
The overall system block diagram of the proposed scheme is shown in Fig. 1, where the first block encodes the input image into the JPEG 2000 format. In the JPEG 2000 coding process, first the raw image is partitioned into a number of rectangular non-overlapping blocks which are referred to as tiles. Then a discrete wavelet transform (DWT) is applied to each tile to decompose it into sub-bands at different resolution levels. The first decomposition level has four sub-bands, , and .
Unequal power allocation
Eq. (1) is a non-linear optimization problem. Since the objective function in Eq. (1) is not a convex function, Gradient-based methods such as Lagrange multipliers, are not appropriate for finding the optimum solution of this problem and they may find the local optimums instead. Therefore, to solve the UPA problem, simulated annealing (SA) method is used to locate an acceptable approximation of the global minimum in a large search space, as suggested in [27]. Due to the large number of CPs and
Channel allocation and power adjustment
The purpose of the channel allocation block in Fig. 3 is to further protect the important JPEG 2000 bits by transmitting them over sub-channels with higher quality. On the channel allocation block, sub-channels with better conditions are assigned to the important layers of JPEG 2000 image. It can be seen from Eq. (7) that by increasing , the imposed interference on the PU increases. Therefore, the ratio between and can be an acceptable representation of the channel condition. This
Simulation results
To assess the proposed method, a grayscale image of Lenna, with the size of and 8 bit/pixel is used for transmission and the Kakadu software is utilized as the JPEG 2000 image coder. The image is processed with one level of decomposition and is divided into CBs and precincts. The final image has 3 layers with the same size. For the baseline scenario, 16 sub-carriers, one PU, and one SU are considered. Table 5 shows the selected values for the model parameters based on the
Conclusion
In this work, resource allocation algorithm is applied to enhance the quality of transmitted JPEG 2000 codec images over OFDM-based CR by considering the power and interference constraints. An optimization algorithm is used to allocate unequal power to the bits of image bitstream based on their impact on the image quality. The important bits in lower layers are further protected by applying subchannel allocation on top of UPA algorithm. The developed algorithm was observed to improve the SU’s
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