A Channel Accessing Scheme with Joint Detection Receivers in Ad Hoc Networks

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1 A Channel Accessing Scheme with Joint Detection Receivers in Ad Hoc Networks Sumeeth Nagaraj, Christian Schlegel High Capacity Digital Communications Lab University of Alberta, Canada. Abstract In order to achieve high capacity packet transmissions in wireless ad hoc networks, the system throughput for a connectionless, distributed channel accessing scheme exploiting joint detection receivers is proposed and analyzed. Joint detection allows many concurrent, asynchronous packet transmissions to occur, thereby enhancing the capacity of a system. A novel packet format with physical layer header s containing an additional code identifier CID field as proposed in [2] is employed and CSMA is used for the physical layer header PLH of the packet. The data portion of each packet is spread by a unique random binary signature sequence identified by the code identifier field CID which allows joint detection of the asynchronous, overlapping packets. A comparison of using CSMA and Aloha multiple access for the PLH is made for a bimodal packet length distribution typical in TCP traffic. Further, an upper bound on the throughput capacity considering the proposed channel accessing schemes is given indicating a significant improvement of system and network throughput is possible with respect to conventional methods. I. INTRODUCTION An ad hoc network is a collection of nodes communicating with each other without a backbone infrastructure. These networks may have nodes that are immobile, mobile or a combination of the two. The speed of mobile nodes can vary depending on the application. An example for immobile nodes is a network of sensors in aircraft, monitoring the health of the wires and communicating it to the maintainer or the pilot [3]. Another example includes a network for controlling home appliances like televisions, computers, cell phones, toys and other devices from a distant place over the internet. Such appliances form an multihop ad hoc network to the gateway through other nodes in range. An application that involves nodes with high mobility is a multi hop network of Un-manned Airborne Vehicles UAV s for reconnaissance of terrain and battle field surveillance. The random movement of nodes and/or the inability to set up base stations at strategic points makes it extremely difficult to develop efficient protocols for such networks. The nodes communicate with each other locally and also assist in routing packets over multiple hops. The main challenge is to improve overall and per node throughput of such ad hoc networks. The low throughput in ad hoc networks as noted in [] is due to the average number of hops a packet travels before reaching the destination and also to the contention-based channel accessing method needed. Some of the issues in channel accessing schemes that need to be addressed for achieving better throughput are the well-known hidden terminal problem, the exposed node problem [4] and fairness issues. In the hidden node problem, different source nodes not in the carrier sensing range of one another sense the channel to be idle and transmit overlapping payloads causing collisions at all nodes that lie in the overlapping region of the two transmitting nodes. To avoid this, RTS/CTS control packets are exchanged between the transmitting and the receiving nodes [8]. The RTS/CTS control packet exchange scheme is used in the IEEE 802. [4] Wireless LANs. These packets are 20 6 byte control packets and are themselves vulnerable to the hidden terminal problem. In [5] it is shown that the number of RTS packets sent is twice as many as the CTS packets received and this behavior is attributed to the link loss or collisions of RTS packets. It was also observed that the per node throughput was very low, which was attributed to the number of hops a packet travels from source to destination and also to the high reliability required for the unicast delivery mechanism in IEEE 802. that consists of RTS-CTS exchanges for unicast packets. The channel accessing method for a single channel protocol suffers from inefficient usage of bandwidth due to control packet exchange, binary exponential backoff, and the exposed terminal problem. Due to node mobility, it is very difficult to maintain the channel usage information and hence very easy to cause packet collisions thereby further decreasing the throughput in mobile environment. In certain cases, a node refrains from transmission on receiving a RTS packet although transmission may not have caused collision at the receiving nodes. This behavior is called the exposed node problem. Figure explains the behavior of node B on receiving a RTS packet from node C. Although, transmission from node B to node A may not cause collisions at node A or D, node B waits till the packet transmission from node C to node D is complete. A further issue is fairness in channel accessibility. Any single node should not use excessive channel resources. Various fairness issues in the IEEE 802. WLAN standards have been studied specifically for ad hoc networks in [5] and [6] leading to similar conclusions on the unsuitability of IEEE 802. for ad hoc networks. Attempts to implement fairness increases the control information exchange [7] [9] and hence the complexity of the system. The network may suffer throughput degradation

2 A B C D packets over multiple hops to the destinations. Mobility of nodes adds additional complexities to the routing and medium access layer. Exchange of control packets and maintaining channel state information would not only further decrease the throughput of such networks but also makes the system extremely complex. The upper bound on the throughput capacity derived in [] would therefore be hard to attain. In the next section, we adopt a self-contained packet structure and propose a channel accessing scheme to suit mobility in ad hoc networks. Fig.. Exposed Node Problem. in achieving fairness. These issues are discussed in context with the novel packet structure for channel access in mobile ad hoc networks. In this paper, we propose and analyze system and network throughput of joint detection-based channel accessing schemes for ad hoc networks. A novel channel accessing scheme enabling joint detection of overlapping packets was proposed by Kota and Schlegel in [2]. In this paper, we introduce a variant of this method for mobile ad hoc networks that uses Carrier Sense Multiple Access CSMA for the Physical Layer Header PLH of a packet and compare its throughput with that of the original system. We show that using CSMA for the header achieves better throughput for a bimodal packet length distribution. Gupta and Kumar in [] proposed an abstract model for channel access in wireless ad hoc networks. Each immobile node in the model acts as a source and also assist in routing packets over multiple hops for various source destination pairs. An upper bound on the overall network throughput derived for such a network shows that as the number of nodes increases, the per node throughput decreases as n. This result shows that large ad hoc networks are inherently inefficient. The proposed channel access scheme gives a significant improvement in throughput. The organization of the paper is as follows: In Section II, we provide a detailed description of the new channel accessing scheme that has a self contained packet structure to accommodate mobility. In Section III, an equation for system throughput is obtained and the simulation results are discussed. This section also compares the throughput of the proposed system with and without CSMA. We also show that for a bimodal packet distribution the proposed system provides better throughput than the original system in [2]. In Section IV we review the upper bound on the network throughput capacity of [] and provide an upper bound on the network throughput for the channel accessing schemes proposed in this paper. A brief summary of the paper follows in Section V. II. MOBILE AD HOC NETWORKS A Mobile Ad Hoc network MANET is a collection of nodes communicating with each other without any infrastructure. Every node in the network act as a source and also relay A. Packet Structure and Channel Access The packet structure for channel access scheme is shown in Fig. 2. It consists of a Physical Layer Header PLH that has a Code Identifier CID field. The data packet from the MAC layer is spread with the code specified in the code identifier field. All packets use CSMA for the PLH. Since the PLH is spread by a common code, every node listens to this common PLH channel before a packet is transmitted. Figure 3 shows the common PLH channel and the data channel. If the channel is found idle, a random spreading code is used to modulate the data packet and is placed in the code identifier field of the PLH. If the medium is busy, the node sets a random backoff counter and retries when the backoff counter expires. Since the header duration is very short, the nodes need not wait for long before transmitting a packet. Due to this, fairness in channel accessibility and also the exposed terminal problem is not a major concern. However, in the proposed scheme, the fairness issue needs to be addressed from a different perspective in that choosing rate and power by each node transmitting a data packet must be balanced. CSMA for PLH reduces the number of header collisions and also the data packets lost during this process. Due to hidden terminals one PLH may be successfully detected while the other PLHs and the corresponding data packets are lost. On successfully receiving a packet that happens on successfully decoding the header and the payload, the node sends back an ACK to the source node. In case an ACK is not heard in the ACK-TIMEOUT period, the source node retransmits the packet. All broadcast and multicast packets use a common spreading sequence other than the one used for spreading the PLH. As in the unicast packets, the code identifier contains the code with which the packet is spread. For these packets, no acknowledgement is sent back from the receiving node. B. Advantages of the proposed system The fairness issues are less of a concern since the PLH including the preamble is approximately 200 bits in length and hence the nodes need not wait for the entire packet duration to access the channel. Collisions that lead to packet loss are limited to the PLH, as the data packets are modulated by a random spreading code. Since the packet structure contains the spreading code, no explicit distributed algorithm is required for code exchange. A code is not associated to a node but a packet itself. The

3 PLH 200bits MAC Data Packet Bytes CID 40 bits L d Fig. 2. Code Identifier field added to the PLH. Header Channel Common Spreading Code Data Channel Random Spreading Code Fig. 3. PLH channel and Data channel for the proposed system. probability that any two nodes in range of each other selecting the same code for a packet and transmitting nearly at the same time is very low. The exposed node problem is well handled, as every node uses a random code for every packet, and there is no explicit channel reservation. Various distributed code assignment schemes such as those proposed in [0] [] [2] can be completely avoided. III. SYSTEM THROUGHPUT The system throughput is calculated by taking into account the PLH and the data portion of the packet. The packets can be successfully detected by successful PLH detection and successful packet retrieval. A. CSMA for the Physical Layer Header For successful detection of the header, the PLH part of the packet from different nodes must not overlap. Overlapping PLHs are assumed to be destroyed and hence the packets from both the users are lost. However, under certain circumstances, it may be possible extract correct information from one of the overlapping PLHs if they are not bit synchronous. In [2], spread Aloha was considered for the header. In the Aloha system, nodes transmit without knowing if the channel is busy or not. In case of conflict, the node retransmits a packet again after waiting for a random amount of time. To avoid such repeated conflicts, CSMA is proposed instead of Aloha for the PLH. Note that this scheme is distributed and can take place at the node itself without requiring a feedback from any other node in the network. For PLH analysis, we consider an infinite number of users and in the form of a Poisson source with an aggregate packet generation rate of λ packets/sec. From [3], for non persistent CSMA, the probability of PLH success is given by, P P LH = G + 2a + where, G is the offered channel traffic and a is the ratio of propagation delay to packet transmission time. B. Joint Detection for the Data Packet For successful joint detection, assuming a joint detection capability of K, no more than K packets can exists in any given time slot of length 2L d. Considering a Poisson arrival of packets, we calculate the probability of K or fewer packets in this time slot as P d = k=k k=0 Q e 2λ L d 2λ L d k k! K 2λ L d L P LH 2λ 2 3 The Poisson distribution converges to a Gaussian distribution for large Kk 0 with mean and variance 2λ L d.

4 Probability of packet success: Overall system throughput: P P LH P d = S = λ G + 2a + G + 2a + Q K 2λ 2λ Q K 2λ 2λ 4 5 Number of packets and λ 2 is the arrival rate for L d2 = 536 byte-packets. λ new = λ + λ 2 = 0.66 L d λ L d2 λ Therefore, the overall system throughput, considering a bimodal packet length distribution, for PLH using Aloha and using CSMA is given by equations 6 and Packet lengthbytes Fig. 4. C. Overall System Throughput Histogram of data packets. The probability of successfully receiving the packet and the overall system throughput are given by equations 4 and 5 respectively. D. System Throughput for a Bimodal Packet Distribution Packets were collected from a TCP channel by using Ethereal [6], a packet sniffer. The packet distribution for a 00Mb download from an FTP server is shown in Fig. 4. From this distribution, we can see that a large number of packets are of length 50 60, or 530 bytes. This bimodal packet distributions was also noted in [7]. This distribution contains various network management packets including those involved for data transfer over the network. The throughput of the proposed system for a bimodal packet distribution can be found by modelling the system as two queues having different arrival rates, λ and λ 2. The sum of two poisson processes is a poisson process with mean equal to the sum of the means of the individual processes, i.e, P Y = K = λ + λ 2 K K! e λ +λ 2 where λ is the arrival rate for L d = byte-packets E. System Simulation Results The simulation of the system is based on the assumptions and the system description in Section III. Figure 5 shows the throughput for systems using CSMA and Aloha for PLH, for a joint detection capability of 20 and a payload of 500 bytes. The throughput of the joint detection system is plotted considering equation 3. The overall system throughput for both the proposed system and the one proposed in [2] is shown. It can be seen from Fig. 5 that the header collisions are avoided by sensing the physical layer header PLH channel and hence the overall system throughput is increased. The plot of CSMA and Aloha for the PLH in Fig. 5 is normalized to the packet length payload. For a system with joint detection capability of K = 0 and 50 respectively, the throughput is shown for different in Figures 6 and 7. It can be seen that the proposed system with CSMA for the header gives better performance than using spread Aloha for the header. The lines connecting the Aloha and CSMA points in the plot of Fig. 6 and 7 indicate that they are of the same packet lengths. The proposed system can also handle a higher packet arrival rate than spread Aloha for the same packet lengths. For large packet lengths, and low joint detection capability make the system joint detection capability limited and not PLH collision limited. However, with an increase in joint detection it can be seen that the system is PLH collision limited and not joint detection capability limited. Figure 8 shows the comparison of the throughput of the systems using Aloha and CSMA PLH for the bimodal packet length distribution. Figure 8 shows a considerable increase in throughput of the proposed system over using Aloha for the PLH for a bimodal packet length distribution. It can also be seen from the plot that as the joint detection capability increases, there is a significant increase initially but less further

5 S = λ new S = λ new e 2G K 2λnew Q 2λnew G + 2a + K 2λnew Q 2λnew 6 7 on. This is due to the header collision limitations. Therefore, we can conclude that for systems using Aloha or CSMA for the PLH, an increase in joint detection capability will not further increase the throughput after a certain point. The header collisions puts a limit on the requirement of joint detection capability for the system proposed in this paper. This gives a suitable value of K over which the proposed system throughput is limited by header collisions. Note, however, that the physical properties of the wireless channel are limiting traffic much before this saturation, essentially assuming that losses due to channel conditions are not a limiting factor. IV. AN ENHANCEMENT IN THROUGHPUT WITH JOINT DETECTION Let us review the results obtained in []. The nodes are arbitrarily placed in a disk of unit area. From the Protocol Model, a transmission from X i to X j is successful if for any other concurrent transmission from node X k over the same sub-channel, X k X j + X i X j, which means that there cannot be any other concurrent transmissions within the transmission radius of X i X j around the receiver X j. Using this Protocol Model, Gupta and Kumar in [] calculate 8 π W n an upper bound on the transport capacity λn L of bit-meters/sec, which makes an important assumption that only one source-destination pair within radio range to exchange packets. However, by employing joint detection, the overall network throughput can be enhanced. Let K be the number of concurrent packet transmissions in the one hop system then we have the following lemma. Lemma 4.: For ad hoc networks with a joint detection capability of K, the transport capacity λn L is bounded above by W K n πk+ bit-meters/sec. The proof of the lemma is given in [8]. As K tends to n π, the transport capacity of such a network is W n bit-meters/sec. For K =, as in the IEEE 802. WLAN s, the transport capacity is bounded by, W n 2π bit-meters/sec. V. CONCLUSION In this paper, we proposed a new channel accessing scheme to suit mobility in ad hoc networks. This channel access scheme is simple and does not include any control packets exchange overhead at the MAC layer and hence the nodes need not maintain any channel state information to transmit a packet. Due to the self contained packet structure, the nodes need not use any distributed spreading code assignment schemes for packet transmissions. It is shown that the proposed system achieves better throughput and packet arrival rates than the previously proposed system using spread Aloha for the PLH. The number of header collisions is decreased, and hence the throughput of the overall system increases. The system is distributed and hence very ideal for mobile ad hoc networks. With the proposed channel accessing scheme for ad hoc networks, an upper bound on the network throughput capacity is further enhanced by a factor of approximately K over the conventional spread spectrum systems. REFERENCES [] P. Gupta and P. R. Kumar, The capacity of wireless networks, IEEE Trans. on Information Theory, vol. IT-46, no.2, pp , March [2] P. Kota and C. Schlegel, A wireless packet multiple access method exploiting joint detection, Proc. ICC 03, vol. 4, pp , Anchorage, AK, USA May -5. [3] S. Nagaraj and C. Furse, An improved MAC for a network of sensors in aircrafts, unpublished manuscript. [4] ANSI/IEEE Std. 802., Part : Wireless LAN Medium Access Control MAC and Physical Layer PHY specifications, 999. [5] S. Xu and T. Saadwi, Does the IEEE 802. MAC Protocol Work Well in Multihop Wireless Ad Hoc Network?, IEEE Comm. Magazine, vol. 39, pages 30 37, June 200. [6] H.-Y. Hsieh and R. Sivakumar, IEEE 802. over Multi-hop Wireless Networks: Problems and New Perspectives. 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6 5 L d = 500 bytes and K = 20 Aloha Header CSMA Header Joint detection Data Header Aloha & Data JD Header CSMA & Data JD Header Aloha and Data Joint detection Header CSMA and Data Joint detection ratio = L d ratio = ratio = 20 ratio = ratio = 2 ratio = Arrival Rate in packets/packet duration Arrival Rate in packets/packet duration Fig. 5. Overall System throughput with joint detection. Fig. 7. System throughput comparison with a joint detection capability of Header Aloha and Data Joint detection Header CSMA and Data Joint detection ratio = L d ratio = Header Aloha & Data JD Header CSMA & Data JD 2.5 ratio = 20 ratio = ratio = 2 ratio = Arrival Rate in packets/packet duration Fig. 6. System throughput comparison with a joint detection capability of Joint detection JD capability Fig. 8. System throughput comparison for a bimodal packet length distribution and varying joint detection capability.