Performance of the audio signals transmission over wireless networks with the channel interleaving considerations
© Mohamed El-Bendary et al; licensee Springer. 2012
Received: 27 October 2010
Accepted: 17 January 2012
Published: 17 January 2012
This article studies a vital issue in wireless communications, which is the transmission of audio signals over wireless networks. It presents a novel interleaver scheme for protection against error bursts and reduction the packet loss of the audio signals. The proposed technique in the article is the chaotic interleaver; it is based on chaotic Baker map. It is used as a randomizing data tool to improve the quality of the audio over the mobile communications channels. A comparison study between the proposed chaotic interleaving scheme and the traditional block and convolutional interleaving schemes for audio transmission over uncorrelated and correlated fading channels is presented. The simulation results show the superiority of the proposed chaotic interleaving scheme over the traditional schemes. The simulation results also reveal that the proposed chaotic interleaver improves the quality of the received audio signal. It improves the amount of the throughput over the wireless link through the packet loss reduction.
With increasing utilization of wireless devices, especially Bluetooth devices, there are two important factors for all wireless systems power efficiency and efficient throughput. Bluetooth is designed to transfer data using ACL packets and SCO packets for transferring audio streams [1, 2].
Bluetooth uses 64 kbps log PCM voice coding with either A-law or μ-law compression, it is the simplest. It uses an 8-bit logarithmic scale. Although it is simple to implement and it is useful in small applications. The log PCM is less robust than the continuous variable slope delta. It is a voice coding method, and is a delta modulation with variable step size. It is good noise immunity, where the damaged packets are discarded without impacting on sound quality [3–5]. In this study, the first voice coding log PCM is considered, for simplicity of the simulations.
There are variety of methods exists for optimizing the transmission of audio data, depending on the properties of the underlying network. In this article, the fixed and mobile Bluetooth networks are assumed to have bursty loss. This study is focused on the interleaving techniques to reduce the burst error effects on the audio signals through distributing the burst errors through a set of audio samples. Audio signals are transmitted as packets in many Bluetooth applications. A packet is considered lost when it is corrupted to the point that it cannot be used. When a packet is lost, a large block of the digital bit stream is missing. The article proposed using effective and powerful interleaver based on the chaotic Baker map. The proposed chaotic interleaver reduces the number of lost packets as shown in the results. So, the packet loss of the audio signal is decreased with the proposed scenarios. Also, over the communications link which permits retransmission the corrupted packets, using the proposed technique reduces the number of retransmission requirement [6–8].
The rest of the article is organized as follows. In Section 2, Bluetooth links are discussed. In Section 3, the proposed modifications are presented. In Section 4, the simulation assumptions are given. The simulation results are introduced in Section 5. Finally, the article is concluded in Section 6.
2. SCO, eSCO, and ACL links
Bluetooth supports both synchronous services such as voice traffic and asynchronous services such as bursty data traffic. The specifications define two different physical link types.
The SCO link is a symmetric, point-to-point link between the master and a single slave in the piconet. The SCO link is typically used to support time-bounded information like voice. The master can support up to three SCO links to the same slave or to different slaves. On the SCO links, the packets used include a CRC and are never retransmitted. The SCO packets have been designed to support 64 kbps speech. There are three pure SCO packets, namely, HV1, HV2, HV3, and one hybrid SCO (DV) packet [9, 10]. Also, the enhanced SCO (eSCO) link permits retransmitting the dropped voice packets. The packets of this link are EV3, EV4, and EV5 Also, the extended data rate (EDR) are eSCO, 2EV3, 2EV4, 2EV5, 3EV3, 3EV4, and 3EV5 packets .
The ACL link provides a packet-switched connection between the master and all active slaves in the piconet. A slave can send an ACL packet if it has been addressed by the master in the previous slot. The ACL packets can be retransmitted. Only a single ACL link can exist between a master and a slave. There are encoded ACL packets such as DM1, DM3, and DM5, which are protected with a rate of 2/3 FEC. Also, there are uncoded ACL packets such as DH1, DH2, and DH5, which are not protected. These ACL are called classic ACL packets. In the recent Bluetooth versions, there are other packets that are specified such as 2DH1, 2DH3, and 2DH5 or 3DH1, 3DH3, and 3DH5, these packets are called EDR packets .
The simulations of this article concentrate on the ACL packets and EDR eSCO packets through many scenarios. The proposed technique improves the quality of the received audio signals and reduces the dropped packets over the bursty channel.
3. Proposed modifications
In this article, two audio files are used, file-1 which meets the specification audio transmission over Bluetooth network. The second audio file-2 has different properties as shown in the simulation assumption section.
First, the size of audio file-1 is 32 kbytes. This file will be segmented to small packets. There are many classic (DH1) and EDR (2DH1 & 2DM1) packets used for transferring audio files. For example, in the DH1 packets, each one of these packets contains 30 bytes. The duration of this file is 4 s.
Number of packets (file-1) = 32 kbytes/30 bytes ≈ 1066 packets.
Number of audio portions (file-1) = 4 s/3.75 ms ≈ 1066 portion.
Second, the size of audio file-2 is 92 kbytes. This file will be segmented to small packets. Each one of these packets contains 30 bytes. The duration of DT audio file is 8 s.
Number of packets (file-2) = 92 kbytes/30 bytes ≈ 3066 packets.
Number of audio portions (file-2) = 8 s/3.75 ms ≈ 2133 portion.
As shown, audio file-1 matches Bluetooth specification in its properties. But, the second audio file-2 does not meet Bluetooth specification. It is used with the long packets simulations.
where B(r, s) are the new indices of the data item at (r, s), M i ≤ r < M i + n i , 0 < s < M and M i = n1 + n2 + ... +n i .
An M × M square matrix is divided into k rectangles of width n i and number of elements M.
The elements in each rectangle are rearranged to a row in the permuted rectangle. Rectangles are taken from right to left beginning with upper rectangles then lower ones. Inside each rectangle, the scan begins from the bottom left corner toward upper elements.
This technique is employed in the simulation as a data randomizing tool. It is employed in two forms; it is applied on the audio file to randomize the whole audio file. Also, it is applied on the packet-by-packet basis for randomize the encoded packets. The simulation results reveal that the proposed technique improves the quality of the received audio signals.
4. Simulation assumptions
In this section, the simulation environment used for carrying out the computer simulation experiments is described. An important assumption used in the simulation is that a packet is discarded if there is an error in the access code, The Header, or Payload field, was not corrected using the error correction scheme. This is a realistic assumption to simulate the real Bluetooth systems' operation . This assumption is employed to measure the number of corrupted packets, but the received packets are recollected after the decoding process for reconstruct the received audio signal.
Simulation audio signal (file-2) parameters
Audio sample size
Audio sample rate
Simulation audio signal (file-1) parameters
Audio sample size
Audio sample rate
5. Simulation results
In this section, the performance of the proposed transmission audio signal through ACL link over correlated and uncorrelated Rayleigh fading channels is studied through simulation. For the comparison purpose, using different scenarios, non-interleaving, bit-level interleaver, convolutional interleaver, and the proposed chaotic randomizing techniques are also simulated. In the case of correlated Rayleigh fading channel, the Jakes' model is used in the simulation.
In this section, there are many simulation scenarios devoted for evaluating the proposed scenarios of the audio signal transmission.
5.1. Short classic packets DH1
As shown in Figure 6, the bit-level interleaving scenario performs better than other scenarios in this simulation over an uncorrelated Rayleigh fading channel.
The result of this simulation reveals that the mobility of the transmitter increases the distortion of the received audio signal. Also, the audio signal transmission needs strong interleaving technique to spread the errors and reduce the bad mobility effects.
In the previous simulation, the performance of the audio signals transmission is studied over uncorrelated and correlated fading channels with different mobility velocities. These simulation scenarios indicated that there are bad effects of the mobility on the received audio signal waveforms. Also, it reveals that the interleaving techniques improve the audio waveforms. In the following section, the simulation evaluates the proposed interleaving technique which is the chaotic randomizing technique, based on the chaotic Baker map.
5.2. Randomizing technique
where P l is the payload length, PER is the packet error rate, x is the number of time slots occupied by the Bluetooth packet, and t is the duration of the Bluetooth time slots.
From the previous section, the following is concluded: Over mobile Bluetooth network, the quality of audio transmission is decreased with V c increasing. Traditional block interleaving of audio file is inefficient over mobile network. The proposed encoded EDR packets with block interleaving of encoded packet performs better than encoded packets (2DM1), which represents 2EV3 voice packets. The performance of standard packets (2DH1) is very bad compared with proposed cases.
As shown in the previous results, the randomize technique which is based on the chaotic Baker map concepts scenario performs better than other technique. From these results, the proposed chaotic interleaver packet by packet basis is presented as a novel efficient interleaver. The proposed technique is evaluated through two groups of simulations according to the type of packets which is employed for the proposed technique testing. These packets are the uncoded 2DH1 and encoded 2DM1 EDR packets.
5.3. The mobility effects on the audio transmission performance
5.4. Audio signal transmission using 2DH1 and 2DM1 packets
In the previous simulation, the mobility effect is studied on the audio signal transmission in a fixed and mobile communication channels in many of scenarios. Also, it evaluates the proposed chaotic interleaver on the whole audio file and packet-by-packet basis. The simulation results reveal that the ACL link improves the throughput of the audio signal. Also, the interleaving techniques enhance the received audio signals. The proposed data randomizing tool performs better than traditional interleavers over the mobile Bluetooth network. As shown in the results, the MSE and Cr of the received audio signals are improved by applying the proposed scenario of the chaotic randomizing technique.
In this article, the transmission of voice using ACL link is discussed. In the simulation two different audio files and different interleaver types were used. The simulations were carried out over Rayleigh correlated and uncorrelated fading channels. The simulation results reveal that over uncorrelated fading channel the best choice is bit-level interleaver in case of the classic uncoded packets, whereas over correlated fading channel, the convolutional interleaver is efficient. Also, the effectiveness of the interleaving techniques is inefficient with audio file that has higher bit rate than 64 kbps. The ACL link is an efficient for carrying voice unless there is a need for high-quality voice. Using this link for carrying voice also leads to improving the throughput. ACL link is a suitable choice for carrying voice specially with using interleaving technique. The article proposes a novel data randomizing tool for enhance the quality of the audio signal transmission over ACL link. The proposed chaotic technique performs better than the traditional interleaver over the mobile channel. It can be then applied on the EV packets and on the mobile Ad hoc network. It reduces the packet loss of the transmitted audio file and also it is an efficient tool for decreasing the mobility bad effects on the transmitted audio signals.
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