2009 Tenth International Conference on Mobile Data Management: Systems, Services and Middleware
Network Coding Based Data Distribution in WiMAX Ching-Chun Chou
Hung-Yu Wei
Department of Electrical Engineering, National Taiwan University Email:
[email protected]
Department of Electrical Engineering, National Taiwan University Email:
[email protected] There are several papers discussing combining the multicast/broadcast with the network coding in WiMAX. [9] provides a way to use network coding on error correction in multicast. However, it only increases the possibility of recover the corrupt packet, which could not ensure data reliability. [10] provides a different way of using network coding in multicast, and ACK/NACK are used to feedback the error packets. [11] proposes an environment of combining random network coding with multicast and uses mathematical analysis to prove its solvability. All these works focuses on performance of the network coding, which means the reduction of transmission times. Nevertheless, the signaling mechanism between BS and MS to ensure data reliability are not deeply investigated. Besides, signaling overhead will also affect the performance of network coding, because these control messages would cause additional medium resource consumption. In this paper we provide a network coding based data distribution design in WiMAX. It could ensure data reliability by MS sending signals to BS for error packets. It also utilizes common feedback channels and corresponding network coding schemes to reduce the signaling overhead. Different batch size operations are also proposed to adopt different use cases. Details are provided in later sections.
Abstract—This paper proposes a network coding based data distribution mechanism in WiMAX. Packets are sent by the BS and then a network coded packet is sent for the data reliability. Different schemes of coding and transmission method are also provided for the data distribution mechanism. Keywords: multicast, broadcast, network coding, feedback, retransmission
I.
INTRODUCTION
In the development of WirelessMAN [1], IEEE standard 802.16[2], which is also called WiMAX [3], is a promising standard. It provides high bandwidth and long distance radio link access. Besides, it could also accommodate a large wide range of applications. Therefore it is attracting large concentrations. In 802.16e [4], Multicast Broadcast Services (MBS) is provided in the standard. It allows the base station (BS) to send data packets to multiple mobile stations (MS) simultaneously. Such operation is well suitable for data distributions, while broadcast is the nature of wireless transmission. However, the MBS function in 802.16e lacks in details. The operation of the MBS is not clearly specified. First, the rate of transmitting data packet uses a fixed rate of BPSK. Second, the process of many BS-MS interactions is not stated concisely. Therefore the MBS in 802.16e is usually considered inapplicable. As a result, 802.16m [5], an amendment to provide mobility to the current 802.16 standard, adds a section called Enhanced MBS (EMBS). This section wishes to provide more capability, flexibility, and mobility for MBS. Therefore, additional functions are added in to the WiMAX system, and more options are provided for MBS operations. For example, introducing the idea of Multicast Broadcast Single Frequency Network (MBSFN) allows multiple BSs to form a larger multicast group serving more MSs. It also adds options for data transmission, including network coding. Network coding is a famous technique for data distribution. It is first proposed by [6]. There are several variants including random network coding [7], linear network coding [8], etc. Network coding utilizes information theory to encode several packets. If the receivers could receive sufficient number of coded packets, they could decode and restore those coded packets to the original one. Network coding could reduce the times of data transmissions, which is greatly helpful while there are scarce medium for transmission.
978-0-7695-3650-7/09 $25.00 © 2009 IEEE DOI 10.1109/MDM.2009.70
II. NETWORK CODING TRANSMISSION DESIGN A network coding retransmission scheme for MBS is proposed to improve data reliability. Data distribution is separated into two stages: (1) data transmission period and (2) retransmission period. A batch of data includes packet sent in stage (1) and (2). Fig. 1 gives a simple illustration of the two stages. In the first stage, data are sent by multicast/broadcast in sequence. After the multicast/broadcast, some packets may be corrupted, due to the varying channel states or other reasons. Then the second stage would handle the data recovery for these corrupt packets. Network coding is used for retransmission, in order to guarantee that data are successfully distributed to all the MBS users. The network coded packets are XOR coded, or coded with a set of coding coefficients. These coded packets are responsible for the correction of the previous corrupt packets within the batch. They must be able to fix all the corrupt packets for all the MBS subscribers. The size of the batch indicated the number of packets within the batch. With different design requirements, the size of the batch could be considered. The network coding
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scheme also varies along with the batch size. In the following sections, two schemes are proposed: fixed-sized batch and variable-sized batch. These two methods are provided in details.
could recover a specific number x of error packets if x coded packets are provided. An illustrative example is provided in Figure 3. It gives an example for a batch size=6. When there are two error packets, two set of coding coefficients are used and two coded packets are sent. This could recover any two error packets with MS1~MS3. More sets of coding coefficients and coded packets are needed if there are more error packets.
A. Variable-sized Batch In this scheme, the number of packets within the batch will vary. It depends on when the second erroneous packets occurs. The network coding scheme used here is the XOR coding. XOR coded packets are sent to recover those error packets in the batch. As XOR coded packet could recover only 1 error packet, the number of error packet within the batch should be limited to 1 or less. If a second error happens at one of the AMS, the AMS has to inform the ABS of the event. After the ABS receives the error signal, an XOR coded packet is sent. The content of XOR coded packet includes all the packets before the second error. This makes up the current batch, and the Second error packet will be covered in the next batch. Such process could guarantee that all the packets are received by the AMS, because they have at most 1 error packet. An illustrative example is provided as Fig. 2. In P1 (packet #1) through P5, there is at most 1 error packet for MS1~MS3. Once the second error occurs, which is P6, uplink feedback is sent to the BS. Therefore an XOR coded packet containing P1⊕P2⊕P3⊕P4⊕P5. This packet could
REFERENCES [1]
IEEE 802.16 Working Group on Broadband Wireless Access Standard, a unit of IEEE 802 LAN/MAN standards committee. [2] IEEE 802.16 Standard, P802.16 Rev2/D4 Apr 2008 [3] http://www.wimaxforum.org/ [4] , "IEEE Standard for Local and metropolitan area networks Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems Amendment 2: Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands and Corrigendum 1," IEEE Std 802.16e-2005 and IEEE Std 802.162004/Cor 1-2005 (Amendment and Corrigendum to IEEE Std 802.162004) , vol., no., pp.0_1-822, 2006 [5] http://wirelessman.org/tgm/ [6] Ahlswede, R.; Ning Cai; Li, S.-Y.R.; Yeung, R.W., "Network information flow," Information Theory, IEEE Transactions on , vol.46, no.4, pp.1204-1216, Jul 2000 [7] T. Ho, R. Koetter, M. Médard, D. R. Karger and M. Effros, "The Benefits of Coding over Routing in a Randomized Setting" 2003 IEEE International Symposium on Information Theory [8] Li, S.-Y.R.; Yeung, R.W.; Ning Cai, "Linear network coding," Information Theory, IEEE Transactions on , vol.49, no.2, pp.371-381, Feb. 2003 [9] Dong Nguyen; Tuan Tran; Thinh Nguyen; Bose, B., "Wireless Broadcast Using Network Coding," Vehicular Technology, IEEE Transactions on , vol.58, no.2, pp.914-925, Feb. 2009 [10] Xiao Xiao; Yang Lu-Ming; Wang Wei-Ping; Zhang Shuai, "A Wireless Broadcasting Retransmission Approach Based on Network Coding," Circuits and Systems for Communications, 2008. ICCSC 2008. 4th IEEE International Conference on , vol., no., pp.782-786, 26-28 May 2008 [11] Xiao Xiao; Luming Yang; Weiping Wang; Shuai Zhang, "A Broadcasting Retransmission Approach Based on Random Linear Network Coding," Young Computer Scientists, 2008. ICYCS 2008. The 9th International Conference for , vol., no., pp.457-461, 18-21 Nov. 2008
recover any single error in the current batch. The second batch would contain P6 and the packet thereafter. B. Fixed-Sized Batch In this scheme, the number of packets within the batch is fixed. That is, after transmitting a fixed number, n, of packets, coded packets are sent for the errors within the batch. As the size of the batch is fixed, it is possible that there are one or more error packets within the batch. Hence a coding scheme which could recover more packets is indispensable. A set of coding coefficients, which was defined according to Galois field, should be provided. It
Figure 1. The two stage scheme for network coded transmission.
Figure 2. Variable-sized batch using XOR coded data distribution.
Figure 3. Fixed-sized batch with 2 sets of coding coefficients.
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